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1.
Guoqiang Wang Xiaoshi Su Liuzhou Gao Xueting Liu Guoao Li Shuhua Li 《Chemical science》2021,12(32):10883
Here, we describe simple B(C6F5)3-catalyzed mono- and dihydrosilylation reactions of terminal alkynes by using a silane-tuned chemoselectivity strategy, affording vinylsilanes and unsymmetrical geminal bis(silanes). This strategy is applicable to the dihydrosilylation of both aliphatic and aryl terminal alkynes with different silane combinations. Gram-scale synthesis and conducting the reaction without the exclusion of air and moisture demonstrate the practicality of this methodology. The synthetic utility of the resulting products was further highlighted by the structural diversification of geminal bis(silanes) through transforming the secondary silane into other silyl groups. Comprehensive theoretical calculations combined with kinetical isotope labeling studies have shown that a prominent kinetic differentiation between the hydrosilylation of alkynes and vinylsilane is responsible for the chemoselective construction of unsymmetrical 1,1-bis(silanes).A B(C6F5)3/silane-based system enables the chemoselective dihydrosilylation of terminal alkynes. Using a combination of different types of hydrosilanes, a series of unsymmetrical or symmetrical 1,1-bis(silanes) could be constructed. 相似文献
2.
Pd-catalyzed C(sp3)–H oxygenation has emerged as an attractive strategy for organic synthesis. The most commonly proposed mechanism involves C(sp3)–H activation followed by oxidative addition of an oxygen electrophile to give an alkylpalladium(iv) species and further C(sp3)–O reductive elimination. In the present study of γ-C(sp3)–H acyloxylation of amine derivatives, we show a different mechanism when tert-butyl hydroperoxide (TBHP) is used as an oxidant—namely, a bimetallic oxidative addition-oxo-insertion process. This catalytic model results in an alkoxypalladium(ii) intermediate from which acyloxylation and alkoxylation products are formed. Experimental and computational studies, including isolation of the putative post-oxo-insertion alkoxypalladium(ii) intermediates, support this mechanistic model. Density functional theory reveals that the classical alkylpalladium(iv) oxidative addition pathway is higher in energy than the bimetallic oxo-insertion pathway. Further kinetic studies revealed second-order dependence on [Pd] and first-order on [TBHP], which is consistent with DFT analysis. This procedure is compatible with a wide range of acids and alcohols for γ-C(sp3)–H oxygenation. Preliminary functional group transformations of the products underscore the great potential of this protocol for structural manipulation.Alkoxypalladium(ii) species lead to γ-C(sp3)–H acyloxylation and alkoxylation products using tert-butyl hydroperoxide as the oxidant. 相似文献
3.
Qifeng Jiang Alexandra F. Gittens Sydnee Wong Maxime A. Siegler Rebekka S. Klausen 《Chemical science》2022,13(25):7587
Main group organometallic compounds can exhibit unusual optical properties arising from hybrid σ,π-conjugation. While linear silanes are extensively studied, the shortage of methods for the controlled synthesis of well-defined cyclic materials has precluded the study of cyclic conjugation. Herein we report that Ru-catalyzed addition of cyclosilanes to aryl acetylenes (hydrosilylation) proceeds with high chemoselectivity, regioselectivity, and diastereoselectivity, affording complex organosilanes that absorb visible light. We further show that the hydrosilylation products are useful building blocks towards novel conjugated polymers.Hybrid σ,π-conjugated cyclosilanes were synthesized via highly selective hydrosilylation and have shown great potentials as building blocks to construct novel conjugated polymers with control of tacticity. 相似文献
4.
Magdalena Stosur Teresa Szymaska-Buzar 《Journal of molecular catalysis. A, Chemical》2008,286(1-2):98-105
Photochemically activated [Mo(CO)6] and [Mo(CO)4(η4-nbd)] have been demonstrated to be very effective catalysts for hydrosilylation of norbornadiene (nbd) by tertiary (Et3SiH, Cl3SiH) and secondary (Et2SiH2 and Ph2SiH2) silanes to give 5-silyl-2-norbornene, which under the same reaction conditions transform in ring-opening metathesis polymerization (ROMP) to unsaturated polymers and to a double hydrosilylation product, 2,6-bis(silyl)norbornane. The yield of a particular reaction depends very strongly on the kind of silane involved. The reaction products were identified by means of chromatography (GC–MS) and 1H and 13C NMR spectroscopy. In photochemical reaction of [Mo(CO)4(η4-nbd)] and Ph2SiH2 in cyclohexane-d12, η2-coordination of the SiH bond to the molybdenum atom is supported by 1H NMR spectroscopy due to the detection of two equal-intensity doublets with 2JHH = 5.4 Hz at δ 6.12 and −5.86 ppm. 相似文献
5.
《Journal of organometallic chemistry》2003,665(1-2):1-6
The photoactivated (350 nm) hydrosilylation of alkynes by silanes catalyzed by platinum(II) bis(acetylacetonato) has been studied. Platinum(II) bis(acetylacetonato) is an efficient catalyst. High yields of adducts (>98% for terminal alkynes) can be obtained in 2–3 h after a short induction period with a catalyst–reactant molar ratio of 10−3/1. The reaction rate depends on the choice of silane, irradiation time and the concentration of catalyst. The major product is the β-trans adduct. Minor products are the α isomer with a trace of β-cis isomer. Comparisons of hydrosilylation reactions of alkynes with hydrosilylation reactions of alkenes are reported. 相似文献
6.
Sarah V. Maifeld 《Tetrahedron letters》2005,46(1):105-108
Hydrosilylation of terminal alkynes with a variety of silanes catalyzed by Cl2(PCy3)2RuCHPh (1) affords mainly the Z-isomer via trans addition in excellent yields. The presence of a hydroxyl group in close proximity to the triple bond was observed to exert a strong directing effect, resulting in the highly selective formation of the α-isomer. Intramolecular hydrosilylation of a homopropargylic silyl ether was demonstrated to give the cis addition product. 相似文献
7.
Hiroki Yamagishi Kenshiro Hitoshio Jun Shimokawa Hideki Yorimitsu 《Chemical science》2022,13(15):4334
Silylcoppers function as convenient and effective sources of silicon functional groups. Commonly used precursors for those species have been limited to certain symmetric disilanes and silylboranes. This fact renders the development of silylcopper precursors desirable so that more diverse silyl groups could be efficiently delivered. Here we extend the utility of sodium silylsilanolates as competent precursors of silylcoppers. A silanolate unit operates as an auxiliary to transfer a variety of silyl groups to the copper centre, which was demonstrated in the copper-catalysed hydrosilylation of internal alkynes, α,β-unsaturated ketones, and allenes. Our mechanistic studies through DFT calculation suggested that a copper silylsilanolate undergoes intramolecular oxidative addition of the Si–Si bond to the copper centre to generate a silylcopper, in contrast to the typical formal σ-bond metathesis mechanism for conventional disilanes or silylboranes and copper alkoxides. Accordingly, sodium silylsilanolate has been established as an expeditious precursor of a variety of silylcopper species.Sodium silylsilanolates are demonstrated as useful silylating reagents for copper-catalysed hydrosilylation of unsaturated bonds via the formation of reactive silylcopper species that can deliver a series of silyl groups. 相似文献
8.
Wojciech Duczmal Elzbieta Śliwińska Beata Maciejewska Bogdan Marciniec Hieronim Maciejewski 《Transition Metal Chemistry》1995,20(5):435-439
Summary Trisubstituted silanes, HSiR3-n
X
n
(R = Me or Et, X = Cl, OEt, or Ph; n = 0–3) oxidatively add to the complex [RhCl(cod)(1-hexene)] (cod = cycloocta-1,5-diene) to yield [RhCl(cod)(1-hexene)(H)(SiR3)] [(1)]. Subsequent steps of hydrosilylation follow, i.e. cis-insertion of the alkene (- rearrangement) and then reductive elimination of the product, according to the general Chalk and Harrod scheme. A quantitative correlation between the second order rate constant, k
1, of the oxidative addition (followed spectrophotometrically) at 20°C in benzene solution and the structure of the trisubstituted silane represented by Stereoelectronic parameters , and E' for the SiR3-n
X
n
groups was established. The maximal hydrosilylation rate followed by g.l.c., is strongly retarded by highly electronegative substituents X on silicon and results from the elimination rate of the hydrosilylation product from (1) and the maximal concentration of (1) in solution.Dedicated to Professor K. Rühlmann on the occasion of his 65th birthday. Part XXVII in the series Catalysis of Hydrosilylation; for Part XXVI see Polish J. Appl. Chem., 38, 169 (1994). 相似文献
9.
Palle J. Pedersen 《Tetrahedron letters》2008,49(43):6220-6223
Regio- and stereoselective hydrosilylation of terminal alkynes on solid support using diisopropyl hydrosilanes yielding β-(E)-vinyl silanes with excellent selectivity is reported. The hydrosilylation is catalyzed by Pt(DVDS)/P(iBuNCH2CH2)3N (DVDS = 1,3-divinyl-1,1,3,3-tetramethyl-disiloxane), in which the bulky proazaphosphatrane ligand plays a key role for the selectivity. The immobilized products are characterized with gel phase13C NMR and 1H high resolution magic angle spinning NMR. 相似文献
10.
Annika C. S. Page Spencer O. Scholz Katherine N. Keenan Jessica N. Spradlin Bridget P. Belcher Scott M. Brittain John A. Tallarico Jeffrey M. McKenna Markus Schirle Daniel K. Nomura F. Dean Toste 《Chemical science》2022,13(13):3851
Photoaffinity labeling (PAL) is a powerful tool for the identification of non-covalent small molecule–protein interactions that are critical to drug discovery and medicinal chemistry, but this approach is limited to only a small subset of robust photocrosslinkers. The identification of new photoreactive motifs capable of covalent target capture is therefore highly desirable. Herein, we report the design, synthesis, and evaluation of a new class of PAL warheads based on the UV-triggered 1,2-photo-Brook rearrangement of acyl silanes, which hitherto have not been explored for PAL workflows. Irradiation of a series of probes in cell lysate revealed an iPr-substituted acyl silane with superior photolabeling and minimal thermal background labeling compared to other substituted acyl silanes. Further, small molecule (+)-JQ1- and rapamycin-derived iPr acyl silanes were shown to selectively label recombinant BRD4-BD1 and FKBP12, respectively, with minimal background. Together, these data highlight the untapped potential of acyl silanes as a novel, tunable scaffold for photoaffinity labeling.Irradiation initiated 1,2-photo Brook rearrangement of acyl silanes generated α-siloxycarbene intermediates that were used for photoaffinity labeling. Optimization of the acyl silane group produced a probe capable of capturing small molecule–protein interactions. 相似文献
11.
Sanghyup Seo Joonho Jung Prof. Dr. Hyunwoo Kim 《Angewandte Chemie (International ed. in English)》2023,62(28):e202303853
Novel P,O-type ligands, N-disulfonyl bicyclic bridgehead phosphorus triamides, were synthesized and utilized in Pd-catalyzed hydrosilylation involving tertiary silanes, unactivated alkenes, and conjugated dienes. The ligand displayed a remarkable level of reactivity for alkene hydrosilylation with tertiary silanes and its use resulted in a significant improvement in the regioselectivity of diene hydrosilylation towards 1,2-hydrosilylation. X-ray crystallographic analysis confirmed the bidentate nature of the ligand, with coordination of phosphorus and oxygen. Control experiments provided evidence for the formation of Pd0 species and the reversibility of Pd−H insertion in the reaction mechanism. Density functional theory (DFT) computations supported the importance of the hemilabile P,O ligand in stabilizing both the rate-determining transition state of Pd−H insertion and the transition state of reductive elimination that determines the regioselectivity. 相似文献
12.
Elusive Silane–Alane Complex [SiH⋅⋅⋅Al]: Isolation,Characterization, and Multifaceted Frustrated Lewis Pair Type Catalysis
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Dr. Jiawei Chen Prof. Dr. Eugene Y.‐X. Chen 《Angewandte Chemie (International ed. in English)》2015,54(23):6842-6846
The super acidity of the unsolvated Al(C6F5)3 enabled isolation of the elusive silane–alane complex [Si? H???Al], which was structurally characterized by spectroscopic and X‐ray diffraction methods. The Janus‐like nature of this adduct, coupled with strong silane activation, effects multifaceted frustrated‐Lewis‐pair‐type catalysis. When compared with the silane–borane system, the silane–alane system offers unique features or clear advantages in the four types of catalytic transformations examined in this study, including: ligand redistribution of tertiary silanes into secondary and quaternary silanes, polymerization of conjugated polar alkenes, hydrosilylation of unactivated alkenes, and hydrodefluorination of fluoroalkanes. 相似文献
13.
Jiawei Chen Eugene Y.‐X. Chen 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(23):6946-6950
The super acidity of the unsolvated Al(C6F5)3 enabled isolation of the elusive silane–alane complex [Si H⋅⋅⋅Al], which was structurally characterized by spectroscopic and X‐ray diffraction methods. The Janus‐like nature of this adduct, coupled with strong silane activation, effects multifaceted frustrated‐Lewis‐pair‐type catalysis. When compared with the silane–borane system, the silane–alane system offers unique features or clear advantages in the four types of catalytic transformations examined in this study, including: ligand redistribution of tertiary silanes into secondary and quaternary silanes, polymerization of conjugated polar alkenes, hydrosilylation of unactivated alkenes, and hydrodefluorination of fluoroalkanes. 相似文献
14.
Ke Yang Zhi Li Chong Liu Yunjian Li Qingyue Hu Mazen Elsaid Bijin Li Jayabrata Das Yanfeng Dang Debabrata Maiti Haibo Ge 《Chemical science》2022,13(20):5938
The transient directing group (TDG) strategy allowed long awaited access to the direct β-C(sp3)–H functionalization of unmasked aliphatic aldehydes via palladium catalysis. However, the current techniques are restricted to terminal methyl functionalization, limiting their structural scopes and applicability. Herein, we report the development of a direct Pd-catalyzed methylene β-C–H arylation of linear unmasked aldehydes by using 3-amino-3-methylbutanoic acid as a TDG and 2-pyridone as an external ligand. Density functional theory calculations provided insights into the reaction mechanism and shed light on the roles of the external and transient directing ligands in the catalytic transformation.Aliphatic aldehydes are among the most common structural units in organic and medicinal chemistry research. Direct C–H functionalization has enabled efficient and site-selective derivatization of aliphatic aldehydes.Simple aliphatic functional groups enrich the skeletal backbones of many natural products, pharmaceuticals, and other industrial materials, influencing the utility and applications of these substances and dictating their reactivity and synthetic modification pathways. Aliphatic aldehydes are some of the most ubiquitous structural units in organic materials.1 Their relevance in nature and industry alike, combined with their reactivity and synthetic versatility, attracted much attention from the synthetic organic and medicinal chemistry communities over the years (Fig. 1).2 Efficient means to the functionalization of these molecules have always been highly sought after.Open in a separate windowFig. 1Select aliphatic aldehyde-containing medicines and biologically active molecules.Traditionally, scientists have utilized the high reactivity of the aldehyde moiety in derivatizing a variety of functional groups by the means of red-ox and nucleophilic addition reactions. The resourceful moiety was also notoriously used to install functional groups at the α-position via condensation and substitution pathways.3 Although β-functionalization is just as robust, it has generally been more restrictive as it often requires the use of α,β-unsaturated aldehydes.4,5 Hence, transition metal catalysis emerged as a powerful tool to access β-functionalization in saturated aldehydes.6 Most original examples of metal-catalyzed β-C–H functionalization of aliphatic aldehydes required the masking of aldehydes into better metal coordinating units since free unmasked aldehydes could not form stable intermediates with metals like palladium on their own.7 Although the masking of the aldehyde moiety into an oxime, for example, enabled the formation of stable 5-membered palladacycles, affording β-functionalized products, this system requires the installation of the directing group prior to the functionalization, as well as the subsequent unmasking upon the reaction completion, compromising the step economy and atom efficiency of the overall process.8 Besides, some masking and unmasking protocols might not be compatible with select substrates, especially ones rich in functional groups. As a result, the development of a one-step direct approach to the β-C–H functionalization of free aliphatic aldehydes was a demanding target for synthetic chemists.α-Amino acids have been demonstrated as effective transient directing groups (TDGs) in the remote functionalization of o-alkyl benzaldehydes and aliphatic ketones by the Yu group in 2016.9 Shortly after, our group disclosed the first report on the direct β-C–H arylation of aliphatic aldehydes using 3-aminopropanoic acid or 3-amino-3-methylbutanoic acid as a TDG.10 The TDG was found to play a similar role to that of the oxime directing group by binding to the substrate via reversible imine formation, upon which, it assists in the assembly of a stable palladacycle, effectively functionalizing the β-position.11 Since the binding of the TDG is reversible and temporary, it is automatically removed upon functionalization, yielding an efficient and step-economic transformation. This work was succeeded by many other reports that expanded the reaction and the TDG scopes.12–14 However, this system suffers from a significant restriction that demanded resolution; only substitution of methyl C–H bonds of linear aldehydes was made possible via this approach (Scheme 1a–e). The steric limitations caused by incorporating additional groups at the β-carbon proved to compromise the formation of the palladacycle intermediate, rendering the subsequent functionalization a difficult task.12Open in a separate windowScheme 1Pd-catalyzed β-C–H bond functionalization of aliphatic aldehydes enabled by transient directing groups.Encouraged by the recent surge in use of 2-pyridone ligands to stabilize palladacycle intermediates,15,16 we have successfully developed the first example of TDG-enabled Pd-catalyzed methylene β-C–H arylation in primary aldehydes via the assistance of 2-pyridones as external ligands (Scheme 1f). The incorporation of 2-pyridones proved to lower the activation energy of the C–H bond cleavage, promoting the formation of the intermediate palladacycles even in the presence of relatively bulky β-substituents.17 This key advancement significantly broadens the structural scopes and applications of this process and promises future asymmetric possibilities, perhaps via the use of a chiral TDG or external ligand or both. Notably, a closely related work from Yu''s group was published at almost the same time.18We commenced our investigation of the reaction parameters by employing n-pentanal (1a) as an unbiased linear aldehyde and 4-iodoanisole (2a) in the presence of catalytic Pd(OAc)2 and stoichiometric AgTFA, alongside 3-amino-3-methylbutanoic acid (TDG1) and 3-(trifluoromethyl)-5-nitropyridin-2-ol (L1) at 100 °C (ii) sources proved Pd(OAc)2 to be the optimal catalyst, while Pd(TFA)2, PdCl2 and PdBr2 provided only moderate yields (entries 10–12). Notably, a significantly lower yield was observed in the absence of the 2-pyridone ligand, and no desired product was isolated altogether in the absence of the TDG (entries 13 and 14). The incorporation of 15 mol% Pd catalyst was deemed necessary after only 55% yield of 3a was obtained when 10 mol% loading of Pd(OAc)2 was instead used (entry 15).Optimization of reaction conditionsa
Open in a separate windowaReaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Pd source (15 mol%), AgTFA (0.3 mmol), L1, TDG1, solvent (2.0 mL), 100 °C, 12 h. Yields are based on 1a, determined by 1H-NMR using dibromomethane as an internal standard.bIsolated yield.cPd(OAc)2 (10 mol%).To advance our optimization of the reaction conditions, a variety of 2-pyridones and TDGs were tested (Scheme 2). Originally, pyridine-2(1H)-one (L2) was examined as the external ligand, but it only yielded the product (3a) in 7% NMR yield. Similarly, other mono- and di-substituted 2-pyridone ligands (L3–L10) also produced low yields, fixating L1 as the optimal external ligand. Next, various α- and β-amino acids (TDG1–10) were evaluated, yet TDG1 persisted as the optimal transient directing group. These amino acid screening results also suggest that a [5,6]-bicyclic palladium species is likely the key intermediate in this protocol since only β-amino acids were found to provide appreciable yields, whereas α-amino acids failed to yield more than trace amounts of the product. The supremacy of TDG1 when compared to other β-amino acids is presumably due to the Thorpe–Ingold effect that perhaps helps facilitate the C–H bond cleavage and stabilize the [5,6]-bicyclic intermediate further.Open in a separate windowScheme 2Optimization of 2-pyridone ligands and transient directing groups.With the optimized reaction conditions in hand, substrate scope study of primary aliphatic aldehydes was subsequently carried out (Scheme 3). A variety of linear primary aliphatic aldehydes bearing different chain lengths provided the corresponding products 3a–e in good yields. Notably, relatively sterically hindered methylene C–H bonds were also functionalized effectively (3f and 3g). Additionally, 4-phenylbutanal gave rise to the desired product 3h in a highly site-selective manner, suggesting that functionalization of the methylene β-C–H bond is predominantly favored over the more labile benzylic C–H bond. It is noteworthy that the amide group was also well-tolerated and the desired product 3j was isolated in 60% yield. As expected, with n-propanal as the substrate, β-mono- (3k1) and β,β-disubstituted products (3k2) were isolated in 22% and 21% yields respectively. However, in the absence of the key external 2-pyridone ligand, β-monosubstituted product (3k1) was obtained exclusively, albeit with a low yield, indicating preference for functionalizing the β-C(sp3)–H bond of the methyl group over the benzylic methylene group.Open in a separate windowScheme 3Scope of primary aliphatic aldehydes. Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), Pd(OAc)2 (15 mol%), AgTFA (0.3 mmol), L1 (60 mol%), TDG1 (60 mol%), HFIP (1.8 mL), HOAc (0.2 mL), 100 °C, 12 h. Isolated yields. aL1 (60 mol%) was absent and yields are given in parentheses.Next, substrate scope study on aryl iodides was surveyed (Scheme 4). Iodobenzenes bearing either an electron-donating or electron-withdrawing group at the para-, meta-, or ortho-position were all found compatible with our catalytic system (3l–3ah). Surprisingly, ortho-methyl- and fluoro-substituted aryl iodides afforded the products in only trace amounts. However, aryl iodide with ortho-methoxy group provided the desired product 3ac in a moderate yield. Notably, a distinctive electronic effect pattern was not observed in the process. It should be mentioned that arylated products bearing halogen, ester, and cyano groups could be readily converted to other molecules, which significantly improves the synthetic applicability of the process. Delightfully, aryl iodide-containing natural products like ketoprofen, fenchol and menthol were proven compatible, supplying the corresponding products in moderate yields. Unfortunately, (hetero)aryl iodides including 2-iodopyridine, 3-iodopyridine, 4-iodopyridine and 4-iodo-2-chloropyridine failed to produce the corresponding products. Although our protocol provides a novel and direct pathway to construct β-arylated primary aliphatic aldehydes, the yields of most examples are modest. The leading reasons for this compromise are the following: (1) aliphatic aldehydes are easily decomposed or oxidized to acids; (2) some of the prepared β-arylated aldehyde products may be further transformed into the corresponding α,β-unsaturated aldehydes.Open in a separate windowScheme 4Scope of aryl iodides. Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), Pd(OAc)2 (15 mol%), AgTFA (0.3 mmol), L1 (60 mol%), TDG1 (60 mol%), HFIP (1.8 mL), HOAc (0.2 mL), 100 °C, 12 h. Isolated yields.Density functional theory (DFT) calculations were performed to help investigate the reaction mechanism and to elucidate the role of the ligand in improving the reactivity (Fig. 2). The condensation of the aliphatic aldehyde 1a with the TDG to form imine-1a was found thermodynamically neutral (ΔG° = −0.1 kcal mol−1). As a result, it was permissible to use imine-1a directly in the calculations. According to the calculations results, the precatalyst [Pd(OAc)2]3, a trimeric complex, initially experiences dissociation and ligand metathesis with imine-1a to generate the Pd(ii) intermediate IM1, which is thermodynamically favorable by 21.9 kcal mol−1. Consequently, the deprotonated imine-1a couples to the bidentate ligand to form the stable six-membered chelate complex IM1. Therefore, IM1 is indeed the catalyst resting state and serves as the zero point to the energy profile. We have identified two competitive pathways for the Pd(ii)-catalyzed C–H activation starting from IM1, one of which incorporates L1 and another which does not. On the one hand, an acetate ligand substitutes one imine-1a chelator in IM1 to facilitate the subsequent C–H activation leading to IM2, which undergoes C(sp3)–H activation through concerted metalation-deprotonation (CMD) viaTS1 (ΔG‡ = 37.4 kcal mol−1). However, this kinetic barrier is thought to be too high to account for the catalytic activity at 100 °C. On the other hand, the chelate imine-1a could be replaced by two N-coordinated ligands (L1) leading to the Pd(ii) complex IM3. This process is endergonic by 6.4 kcal mol−1. To allow the ensuing C–H activation, IM3 dissociates one ligand (L1) producing the active species IM4, which undergoes TS2 to cleave the β-C(sp3)–H bond and form the [5,6]-bicyclic Pd(ii) intermediate IM5. Although this step features an energy barrier of 31.2 kcal mol−1, it is thought to be feasible under the experimental conditions (100 °C). Possessing similar coordination ability to that of pyridine, the ligand (L1) effectively stabilizes the Pd(ii) center in the C–H activation process, indicating that this step most likely involves a manageable kinetic barrier. This result explicates the origin of the ligand-enabled reactivity (TS2vs.TS1). Additionally, we considered the γ-C(sp3)–H activation pathway viaTS2′ which was found to have a barrier of 35.5 kcal mol−1. The higher energy barrier of TS2′ compared to that of TS2 is attributed to its larger ring strain in the [6,6]-bicyclic Pd(ii) transition state, which reveals the motive for the site-selectivity. Reverting back to the supposed pathway, upon the formation of the bicyclic intermediate IM5, it undergoes ligand/substrate replacement to afford intermediate IM6, at which the Ar–I coordinates to the Pd(ii) center to enable oxidative addition viaTS3 (ΔG‡ = 27.4 kcal mol−1) leading to the five-coordinate Pd(iv) complex IM7. Undergoing direct C–C reductive elimination in IM7 would entail a barrier of 29.6 kcal mol−1 (TS4). Alternatively, iodine abstraction by the silver(i) salt in IM7 is thermodynamically favorable and irreversible, yielding the Pd(iv) intermediate IM8 coordinated to a TFA ligand. Subsequently, C–C reductive coupling viaTS5 generates the Pd(ii) complex IM9 and concludes the arylation process. This step was found both kinetically facile (6.1 kcal mol−1) and thermodynamically favorable (30.7 kcal mol−1). Finally, IM9 reacts with imine-1avia metathesis to regenerate the palladium catalyst IM1 and release imine-3a in a highly exergonic step (21.0 kcal mol−1). Ultimately, imine-3a undergoes hydrolysis to yield the aldehyde product 3a and to release the TDG.Open in a separate windowFig. 2Free energy profiles for the ligand-promoted Pd(ii)-catalyzed site-selective C–H activation and C–C bond formation, alongside the optimized structures of the C–H activation transition states TS1 and TS2 (selected bond distances are labelled in Å). Energies are relative to the complex IM1 and are mass-balanced. 相似文献
Entry | Pd source | L (mol%) | TDG1 (mol%) | Solvent (v/v, mL) | Yield (%) |
---|---|---|---|---|---|
1 | Pd(OAc)2 | L1 (30) | TDG1 (40) | HFIP | 30 |
2 | Pd(OAc)2 | L1 (30) | TDG1 (40) | AcOH | <5 |
3 | Pd(OAc)2 | L1 (30) | TDG1 (40) | HFIP/AcOH (1 : 1) | 28 |
4 | Pd(OAc)2 | L1 (30) | TDG1 (40) | HFIP/AcOH (9 : 1) | 47 |
5 | Pd(OAc)2 | L1 (30) | TDG1 (40) | HFIP/AcOH (1 : 9) | <5 |
6 | Pd(OAc)2 | L1 (30) | TDG1 (60) | HFIP/AcOH (9 : 1) | 50 |
7 | Pd(OAc)2 | L1 (30) | TDG1 (80) | HFIP/AcOH (9 : 1) | 25 |
8 | Pd(OAc)2 | L1 (60) | TDG1 (60) | HFIP/AcOH (9 : 1) | 70(68)b |
9 | Pd(OAc)2 | L1 (75) | TDG1 (60) | HFIP/AcOH (9 : 1) | 51 |
10 | Pd(TFA)2 | L1 (60) | TDG1 (60) | HFIP/AcOH (9 : 1) | 60 |
11 | PdCl2 | L1 (60) | TDG1 (60) | HFIP/AcOH (9 : 1) | 52 |
12 | PdBr2 | L1 (60) | TDG1 (60) | HFIP/AcOH (9 : 1) | 54 |
13 | Pd(OAc)2 | — | TDG1 (60) | HFIP/AcOH (9 : 1) | 9 |
14 | Pd(OAc)2 | L1 (60) | — | HFIP/AcOH (9 : 1) | 0 |
15c | Pd(OAc)2 | L1 (60) | TDG1 (60) | HFIP/AcOH (9 : 1) | 55 |
15.
In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si–H and B–H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E–H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η1 or η2) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.In this Perspective we compare the ability of transition metals and p-block Lewis acids to activate electrophilically hydrosilanes and hydroboranes. The mechanistic similarities and dissimilarities in different catalytic transformations are analyzed. 相似文献
16.
Regioselective catalytic multi-functionalization reactions enable the rapid synthesis of complexed products from the same precursors. In this communication, we present a method for the regiodivergent borocarbonylation of benzylidenecyclopropanes with aryl iodides. Various γ-vinylboryl ketones and β-cyclopropylboryl ketones were produced in moderate to good yields with excellent regioselectivity from the same substrates. The choice of the catalyst is key for the regioselectivity control: γ-vinylboryl ketones were produced selectively with IPrCuCl and Pd(dppp)Cl2 as the catalytic system, while the corresponding β-cyclopropylboryl ketones were obtained in high regioselectivity with Cu(dppp)Cl, [Pd(η3-cinnamyl)Cl]2 and xantphos as the catalytic system. Moreover, γ-vinylboryl ketones and β-cyclopropylboryl ketones were successfully transformed into several other value-added products.A novel procedure for regiodivergent borocarbonylation of benzylidenecyclopropanes has been developed. A variety of valuable γ-vinylboryl ketones and β-cyclopropylboryl ketones can be obtained selectively in excellent yields. 相似文献
17.
Dr. Zhaoyang Cheng Minghua Li Xu-Yang Zhang Yue Sun Qing-Lei Yu Prof. Dr. Xing-Hong Zhang Prof. Dr. Zhan Lu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2023,135(1):e202215029
Double hydrosilylation of alkynes represents a straightforward method to synthesize bis(silane)s, yet it is challenging if α-substituted vinylsilanes act as the intermediates. Here, a cobalt-catalyzed regiodivergent double hydrosilylation of arylacetylenes is reported for the first time involving this challenge, accessing both vicinal and geminal bis(silane)s with exclusive regioselectivity. Various novel bis(silane)s containing Si−H bonds can be easily obtained. The gram-scale reactions could be performed smoothly. Preliminarily mechanistic studies demonstrated that the reactions were initiated by cobalt-catalyzed α-hydrosilylation of alkynes, followed by cobalt-catalyzed β-hydrosilylation of the α-vinylsilanes to deliver vicinal bis(silane)s, or hydride-catalyzed α-hydrosilylation to give geminal ones. Notably, these bis(silane)s can be used for the synthesis of high-refractive-index polymers (nd up to 1.83), demonstrating great potential utility in optical materials. 相似文献
18.
Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Oxidant-free Au-catalyzed reactions are emerging as a new synthetic tool for innovative organic transformations. Still, a deeper mechanistic understanding is needed for a rational design of these processes. Here we describe the synthesis of two Au(i) complexes bearing bidentated hemilabile MIC^N ligands, [AuI(MIC^N)Cl], and their ability to stabilize square-planar Au(iii) species (MIC = mesoionic carbene). The presence of the hemilabile N-ligand contributed to stabilize the ensuing Au(iii) species acting as a five-membered ring chelate upon its coordination to the metal center. The Au(iii) complexes can be obtained either by using external oxidants or, alternatively, by means of feasible oxidative addition with strained biphenylene Csp2–Csp2 bonds as well as with aryl iodides. Based on the fundamental knowledge gained on the redox properties on these Au(i)/Au(iii) systems, we successfully develop a novel Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid. The oxidative addition of the aryl iodide, which in turn is allowed by the hemilabile nature of the MIC^N ligand, is an essential step for this transformation.A novel hemilabile MIC^N ligand-based Au(i)-catalytic procedure for the synthesis of γ-substituted γ-butyrolactones through the arylation-lactonization reaction of the corresponding γ-alkenoic acid is presented. 相似文献
19.
Subir Panja Salman Ahsan Tanay Pal Simon Kolb Wajid Ali Sulekha Sharma Chandan Das Jagrit Grover Arnab Dutta Daniel B. Werz Amit Paul Debabrata Maiti 《Chemical science》2022,13(32):9432
The Fujiwara–Moritani reaction is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation. However, the need for superstoichiometric amounts of toxic chemical oxidants makes the reaction unattractive from an environmental and atom-economical view. Herein, we report the first non-directed and regioselective olefination of simple arenes via an electrooxidative Fujiwara–Moritani reaction. The versatility of this operator-friendly approach was demonstrated by a broad substrate scope which includes arenes, heteroarenes and a variety of olefins. Electroanalytical studies suggest the involvement of a Pd(ii)/Pd(iv) catalytic cycle via a Pd(iii) intermediate.The Fujiwara–Moritani reaction using electric current is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation.Transition metal-catalysed C–H functionalisation reactions have increasingly gained importance over the last few decades since they allow direct and rapid installation of functionality. Regardless of the undeniable synthetic value of such transformations, the need for superstoichiometric quantities of expensive and hazardous oxidants (e.g., silver and copper salts) remains a major drawback from a sustainable chemistry perspective.1,2 Additionally, chemical oxidants often lead to the formation of by-products, hindering purification and decreasing atom economy. Nevertheless, very few reports were also reported in the literature wherein mild oxidant such as molecular oxygen can also serve as the oxidising agent.2j To make chemical processes and transformations intrinsically sustainable, organic chemists re-discovered synthetic electrochemistry as an environmentally friendly approach.3–6 In the domain of synthetic electrochemistry, the Lei group achieved a significant milestone and installed C–C bonds through a different cross-coupling strategy.1k,2f–h Electroorganic synthesis utilizes electric current to realize redox processes and thereby avoids the use of dangerous, expensive, and polluting chemical oxidising or reducing agents. Precise control of electrochemical reaction parameters often leads to commendable reactivity and chemoselectivity and hence to an improved atom economy. In addition, electrochemical processes fulfil the expectations of sustainability since electricity can be generated from renewable energy sources, such as wind, sunlight or biomass. Recent efforts in the field of electrochemical C–H activation resulted in significant progress towards efficient C–C and C–heteroatom bond formations.7–10 Hence, the utilization of electric current as an alternative oxidant in Pd-catalysed C–H functionalisations is emerging as an attractive alternative to stoichiometric reagents.11–13The Fujiwara–Moritani reaction is one of the earliest known examples of Pd-catalysed oxidative C–H functionalisations for C–C bond formation.14 This extraordinary C(sp2)-H alkenylation reaction avoids the use of prefunctionalised starting materials; however, it suffers from the drawbacks of regioselectivity, reactivity and use of excess arenes.15 Since its development, a number of modified strategies have been reported by different research groups to address the issue of reactivity and selectivity.16–21 In recent times, the ligand assisted oxidative C–H alkenylation of arenes without directing substituents has been established as one of the major strategies to overcome the reactivity issue and to elaborate the substrate scope.However, regioselectivity for most of the sterically and electronically unbiased arenes is still not up to the mark. The most recent studies on the non-directed oxidative C–H olefination of arenes were reported independently by Yu and van Gemmeren (Scheme 1). The Yu group employed electron-deficient 2-pyridone as an X-type ligand for the olefination of both electron-rich and electron-poor arenes including heteroarenes as the limiting reagent (Scheme 1a).18 The pyridone ligand improves the selectivity in a non-directed approach as compared to the directed C–H olefination reaction by enhancing the influence of steric effects. On the other hand, the van Gemmeren group utilizes two complementary ligands N-Ac–Gly–OH and a 6-methylpyridine derivative in a 1 : 1 ratio to accomplish the non-directed olefination reaction of arenes (Scheme 1b).20 Despite the indisputable advances made by these research groups in the area of non-directed oxidative C–H olefination of arenes, the use of superstoichiometric amounts of toxic and waste-generating oxidants (Ag salts) deciphers into a strong call for an environmentally responsive and atom-economic protocol. To address these shortcomings, we recently introduced Pd-photoredox catalysed olefination of non-directed arenes with excellent site selectivity under oxidant free conditions.21Open in a separate windowScheme 1Recent approaches to sustainable C–H alkenylation reactions.In 2007, Jutand reported the directing group assisted Pd-electracatalysed ortho-olefination of acetyl protected aniline in a divided cell by utilizing catalytic amounts of benzoquinone as a redox mediator (Scheme 1c).22a A Rh-catalysed ortho-C–H olefination of benzamide was developed through an electrooxidative pathway by the Ackermann group (Scheme 1d).22b Simple arenes that bear no directing groups are cheap, easily available and very desirable starting materials. However, the use of such arenes is significantly more challenging for selective functionalisation as transformations often result in the formation of complex product mixtures. With no report of an electrooxidative Pd-catalysed C(sp2)-H alkenylation of simple arenes present, we wish to present such a variant of the Fujiwara–Moritani reaction (Scheme 1e). The developed method proceeds through a non-directed pathway and is controlled by stereoelectronic factors. This protocol does not require additional chemical oxidizing agents and is executed using an operator-friendly undivided cell setup.To start our study, naphthalene was chosen as a challenging substrate because of its ability to form α- and β-products. We examined various reaction conditions for the desired Pd-catalysed electrooxidative C–H alkenylation in a simple undivided cell setup (†) with n-butyl acrylate as the coupling partner. After rigorous optimisation, we found that naphthalene reacts with n-butyl acrylate in dichloroethane (DCE) in the presence of Pd(OAc)2 (10 mol%), ligand L1 (20 mol%), and the electrolyte tetra-n-butylammonium hexafluorophosphate (TBAPF6, 0.5 equiv.) while employing a graphite felt anode and a platinum cathode maintaining constant current electrolytic conditions (j = 2.5 mA cm−2, Entry Alteration from standard conditions Yield of 1b (%) Selectivity (β : α) 1 None 70 >25 : 1 2 Co(OAc)2·4H2O instead of Pd(OAc)2 9 1 : 1 3 [Ru(p-cymene)Cl2]2 instead of Pd(OAc)2 NR 4 Pd(OAc)2·(5 mol%) 51 >25 : 1 5 Pd(OAc)2·(20 mol%) 71 >25 : 1 6 L2 instead of L1 45 8 : 1 7 L3 instead of L1 59 20 : 1 8 L4 instead of L1 19 5 : 1 9 L5 instead of L1 8 1 : 1 10 Benzoquinone (10 mol%) 68 >25 : 1 11 PivOH (1.0 equiv.) 61 20 : 1 12 Ni foam instead of Pt 64 >25 : 1 13 GF instead of Pt 49 15 : 1 14 Steel instead of Pt 31 13 : 1 15 6 mA cm−2 instead of 2.5 mA cm−2 27 11 : 1 16 24 h reaction time 47 20 : 1 17 12 h reaction time 56 21 : 1 18 No electricity NR — 19 No Pd(OAc)2 NR —