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1.
A general method for the synthesis of beta-aryl/alkylarylidene malonates is reported. The key step involves the coupling of an arylboronic acid to a beta-chloroalkyl/arylidene malonate, in the presence of K2CO3 and 1 mol % of the air-stable palladium catalyst (POPd) under microwave irradiation, to afford beta-aryl/alkylarylidene malonates in good yields. The combination of mild reaction conditions, air stable catalyst, microwave-enhanced chemistry, and high levels of functional group compatibility make this an attractive synthetic approach to this class of compounds. 相似文献
2.
The enantioselective ring opening of aziridines using a latent source of HF is described. A combination of two Lewis acids, (salen)Co and an achiral Ti(IV) cocatalyst, provided optimal reactivity and enantioselectivity for the trans β-fluoroamine product. The use of a chelating aziridine protecting group was crucial. Acyclic and cyclic meso N-picolinamide aziridines underwent fluoride ring opening in up to 84% ee, and the kinetic resolution of a piperidine-derived aziridine was performed with krel=6.6. The picolinamide group may be readily removed without epimerization of the fluoroamine. Preliminary studies revealed a bimetallic mechanism wherein the chiral (salen)Co catalyst delivers the nucleophile and the Ti(IV) cocatalyst activates the aziridine. 相似文献
3.
Xin He Pengchen Ma Yuhai Tang Jing Li Shenyu Shen Martin J. Lear K. N. Houk Silong Xu 《Chemical science》2022,13(43):12769
We herein report a phosphine-catalyzed (3 + 2) annulation of cyclopropenones with a wide variety of electrophilic π systems, including aldehydes, ketoesters, imines, isocyanates, and carbodiimides, offering products of butenolides, butyrolactams, maleimides, and iminomaleimides, respectively, in high yields with broad substrate scope. An α-ketenyl phosphorous ylide is validated as the key intermediate, which undergoes preferential catalytic cyclization with aldehydes rather than stoichiometric Wittig olefinations. This phosphine-catalyzed activation of cyclopropenones thus supplies a versatile C3 synthon for formal cycloadditon reactions.A phosphine-catalyzed (3 + 2) annulation of cyclopropenones with a wide variety of electrophilic π systems has been developed, in which an α-ketenyl phosphorous ylide is validated as the key intermediate used as a versatile C3 synthon.The development of effective strategies to construct cyclic molecular architectures has attracted long-standing interest from the chemistry community.1 In this regard, phosphine catalysis2 has emerged as a powerful and versatile approach for the construction of various carbo- and heterocyles. Lu''s (3 + 2),3 Kwon''s (4 + 2),4 and Tong''s (4 + 1)5 annulations represent seminal advances in this field, from which a plethora of reactions6 and asymmetric variants2b have been inspired. Since phosphine-catalyzed reactions are usually initiated by the conjugate addition of a phosphine to a polar double or triple bond to generate reactive zwitterionic intermediates, the prevalent substrates of phosphine catalysis rely almost entirely on electron-deficient alkenes, alkynes, allenes, and their derivatives2a (Fig. 1a). These substrate entities serve as effective C1 to C4 synthons for generating various ring systems. Alternatively, we envisaged that the integration of the C–C bond activation of strained carbocycles within phosphine catalysis would significantly expand the scope. In 2018, we disclosed that electron-deficient vinylcyclopropanes (VCPs) undergo phosphine-catalyzed activation to generate zwitterions A that triggers the rearrangement of vinylcyclopropylketones to cycloheptenones (Fig. 1b, up).7 Very recently, an elegant phosphine-catalyzed enantioselective (3 + 2) annulation of electron-deficient vinylcyclopropanes with N-tosylaldimines with a zwitterion B as the key intermediate has been developed by Lu and co-workers8 (Fig. 1b, down). In the meantime, we have established that electron-deficient alkylidenecyclopropanes (ACPs) also readily undergo phosphine-catalyzed substrate-controlled rearrangements to afford polysubstituted furans and dienones.9Open in a separate windowFig. 1Substrates of phosphine-catalyzed annulation reactions. (a) Commonly used substrates of phosphine catalysis. (b) The use of electron-deficient vinylcyclopropanes (VCPs) as substrates in a phosphine-catalyzed rearrangement reaction (up), and (3 + 2) annulation with N-tosylaldimines (down). (c) This work describes the use of cyclopropenones as a versatile C3 synthon for annulation reactions under phosphine catalysis.As part of ongoing studies, we hypothesized that cyclopropenones, as triggered by phosphines, would serve as C3 synthons for possible (3 + n) annulations (Fig. 1c). Mechanistically, the nucleophilic addition of a phosphine to cyclopropenones followed by ring cleavage would generate an α-ketenyl phosphorus ylide C.10 Prescher and co-workers11 have previously employed such ylides to react with nucleophiles, e.g. primary amines, for applications in bioorthogonal ligations. By virtue of its amphiphilic structure bearing both a nucleophilic ylide and an electrophilic ketene moiety, we proposed that it might be used as a 1,3-dipole surrogate for annulation reactions with unsaturated electrophiles (Fig. 1c).As a subclass of “non-benzenoid aromatic compounds”, cyclopropenones12 are strained, highly unsaturated, and readily available building blocks which have drawn tremendous interest in contemporary organic synthesis due to their unique and versatile reactivities.13 The activation of these strained compounds is typically achieved through transition metal catalysis, via oxidative addition to the C–C single bond14 to bring about various transformations,13b especially annulation reactions.15 Wender and co-workers15b pioneered the Rh-catalyzed (3 + 2) cycloaddition of cyclopropenones with alkynes to build cyclopentadienones, whereas Li and co-workers15f developed a Ni-catalyzed (3 + 2) annulation of cyclopropenones with α,β-unsaturated ketones/imines to access butenolides and lactams. Gleiter and co-workers15k,l also demonstrated an interesting Co-mediated dimerization of cyclopropenones to form Co-capped benzoquinones. Other metal complexes involving Pd,15c,i Ru,15a,16 Ag,17 and so forth,18 are also known to facilitate a range of annulations with cyclopropenones. Compared to transition metal-catalyzed methods, however, the organocatalytic activation of cyclopropenones toward practical transformations remains far less explored.19 Stemming from our interest in Lewis base catalysis,7,9,20 we now report the phosphine-catalyzed activation of cyclopropenones as a new subset of C3 synthons that are capable of undergoing (3 + 2) annulations with various unsaturated electrophiles (vide infra).Initially, we examined the phosphine-catalyzed reaction of diphenylcyclopropenone 1a with several activated alkenes such as acrylates and maleates. These attempts were unsuccessful; however, the employment of benzaldehyde 2a as the reaction partner led to the anticipated (3 + 2) annulation to afford a butenolide product 3a (21 Another point of note is that the aforementioned α-ketenyl phosphorus ylide C does not undergo the usual Wittig reaction with aldehydes but enters into a catalytic cycloaddition pathway (see mechanism discussions below).Survey on conditionsa
Open in a separate windowaReaction conditions: 1a (0.30 mmol), 2a (0.20 mmol), and catalyst (0.02 mmol, 10 mol%) were stirred in the solvent (2.0 mL) at r. t. under N2 atmosphere.bYield of isolated product.c0.20 mmol 1a was used.d5 mol% of PMe3 was adopted.e2 mol% of PMe3 was used.f0.1 mol% of PMe3 was adopted.It was found that PPh3 did not promote the reaction, whereas PBu3 and PMe3 catalyzed the reaction with yields of 22% and 30% of 3a, respectively (entries 1–3). Nitrogen-containing Lewis bases such as DABCO, DMAP, and DBU were inefficient catalysts for the reaction (not shown). Interestingly, the addition of 4 Å molecular sieves (4Å MS) improved the yield to 73% in a shorter time (entry 4), suggesting the progress of the reaction to be water sensitive. Increasing the amount of 1a to 1.5 equivalents led to quantitative conversion, and halving the catalyst loading to 5 mol% still furnished an excellent yield of 92% in 2 h (entries 5 and 6). Further reducing the catalyst loading to 2 mol% gave 78% yield over 24 h, while 0.1 mol% of catalyst resulted in a substantially lower yield (entries 7 and 8). Examination of common solvents indicated dichloromethane to be optimal, although toluene gave comparable results (entries 9–13).With optimized conditions in hand, the scope of the (3 + 2) heteroannulation of cyclopropenones with aldehydes was investigated first (Fig. 2). A series of benzaldehydes with electron-donating groups (–Me, –tBu, –OMe, –OCF3), halogens (–F, –Cl, –Br), or electron-withdrawing groups (–CO2Me, –CF3, –NO2), substituted at either para, ortho, or meta position, all proceeded smoothly producing the corresponding adducts 3b–3r in 55–96% yields. While naphthalene formaldehyde produced butenolide 3s in 88% yield, heteroaryl aldehydes such as 2-furaldehyde, 2-thienaldehyde, and 3-indole aldehyde, yielded their respective annulated products 3t–3v in 94–99% yields. The structure of 3v was confirmed by single-crystal X-ray analysis. Notably, aliphatic aldehydes, such as butyraldehyde and pentanal, were also highly efficient substrates, providing adducts 3w and 3x in 89% and 87% yields, respectively. Even paraformaldehyde was found to undergo the (3 + 2) annulation with 1a to give butenolide 3y in 82% yield. To explore the scope of cyclopropenones, fluoro- and methyl-substituted diphenylcyclopropenones (1b and 1c) were reacted with 4-methylbenzaldehyde, which produced the adducts 3z and 3aa in 91% and 93% yields, respectively. When cyclopropenones with unsymmetric substituents (R1 = aryl, R2 = methyl) were adopted, the annulated products 3ab–3ad were obtained in 89–92% yields with excellent regioselectivity, possibly due to the preferential attack of the phosphine catalyst to the less sterically hindered side of the cyclopropenone. However, when a bigger ethyl is incorporated in the cyclopropenone (R1 = phenyl, R2 = ethyl), the annulated product 3ae was obtained in 51% yield with a poor regioselectivity (1.5 : 1). It was then found that 1,2-dibutylcyclopropenone failed in the annulation (not shown), probably due to its less electrophilicity retarding the nucleophilic attack of the phosphine catalyst. Among aldehyde substrates, it is noteworthy that salicylic aldehyde reacted differently to form the enolate ester 4, presumably via phenolate addition to a protonated ketenyl phosphonium intermediate.22 Besides aldehydes, it was found that the ketoester 5 also underwent (3 + 2) annulation readily with representative cyclopropenones to afford fully-substituted butenolides 6a–6c in 91–98% yields (Fig. 2, bottom left). Normal ketones like acetone and benzophenone, however, were ineffective under the current reaction conditions. More intriguingly, N-tosylimine 7 was also found to be an efficient partner for (3 + 2) annulation with 1, which produced the butyrolactams 8a–8c in 71–88% yields (Fig. 2, bottom right).Open in a separate windowFig. 2Scope of PMe3-catalyzed (3 + 2) annulation with electrophilic C = X partners. (a) Reaction with aldehydes. (b) Reaction with ketoester. (c) Reaction with imines.As C O and C N bonds can be both successfully integrated into annulations, we next examined the reaction of isocyanates possessing cumulated C O and C N bonds. Under optimized conditions (see ESI for details†), the phosphine-catalyzed (3 + 2) annulation of cyclopropenones with isocyanates 9 occurred exclusively at the C N bond to provide the maleimide derivatives 10 in high yield (Fig. 3). The scope of the reaction was therefore found to be broad. Aryl isocyanates with varied electron properties substituted at either para, ortho, or meta position typically reacted well to produce 10a–10k in good yields. A trend can be discerned, such that groups with increased electron-withdrawing ability on the benzene ring decreased the productivity. It was found that both alkyl and allyl isocyanates also readily coupled with cyclopropenones to provide N-substituted maleimides 10l–10q in 60–83% yields. The structure of 10e was confirmed by single-crystal X-ray analysis. Substitution of the phenyl groups of cyclopropenones was tolerated, as shown by the formation of 10r–10u in 72–81% yields. Bis-isocyanates were also found efficient, which annulated with two molecules of 1a to form adducts 10v and 10w in excellent yields. It is noteworthy that the convenient synthesis of polysubstituted maleimides by our current strategy stands in sharp contrast with transition-metal catalyzed ones, for example, as reported by Kondo and co-workers16 through ruthenium-catalyzed (2 + 2 + 1) cocyclization of isocyanates, alkynes, and CO. To further demonstrate the generality of our phosphine-catalyzed annulation method, two commercially available carbodiimides 11 were employed as annulation partners with representative cyclopropenones (Fig. 3, bottom). These reactions smoothly generated the iminomaleimides 12a–12f in excellent yields (81–91%; single-crystal X-ray structure confirming 12a unequivocally).Open in a separate windowFig. 3Scope of PMe3-catalyzed (3 + 2) annulation with cumulated X = C N partners. (a) Reaction with isocyanates. (b) Reaction with carbodiimides.Collectively, our findings clearly indicate that the phosphine-catalyzed (3 + 2) heteroannulation of cyclopropenones is general for a broad range of C X substrates including aldehydes, ketoesters, imines, isocyanates and carbodiimides. Notably, the products butenolide, butyrolactam, maleimide, and iminomaleimide are of high biologically relevance23 and synthetic utility,24 which can now be readily generated in high efficiencies under mild conditions. This annulation strategy also constitutes a highly attractive alternative to transition metal-based variants.15f,iA31P NMR tracking experiment was conducted in order to detect any essential intermediates in the PMe3-catalyzed (3 + 2) annulation (See ESI for details†). When mixing cyclopropenone 1a, isocyanate 9a with PMe3 in CDCl3 for 3 h, it was found that, with the disappearance of PMe3, several new species with signals at 5.8, 15.6, 22.9, and 38.6 ppm appeared in the 31P NMR spectrum. This result supports the involvement of the phosphine in the catalysis, and implies that free phosphine is not the resting state of the catalytic cycle. In addition, when the reaction mixture was subjected to HRMS, a peak at 283.1248 (C18H19OP [M + H]+) corresponding to the adduct of 1a and PMe3 was detected, which may also support the formation of the proposed α-ketenyl ylide intermediate (See ESI for details†).To further probe the reaction mechanism and the origins of chemoselectivity toward the formation of 3a over Wittig-based pathways to 3a*, density functional theory (DFT) calculations were performed as shown in Fig. 4 (see ESI for details†). The reaction of cyclopropenone with PMe3 has a 24.0 kcal mol−1 energy barrier to form the α-ketenyl phosphorus ylide IM1. The reaction involves concerted P–C bond formation and C–C cleavage, and no stable intermediate resulting from the phosphine addition on the cyclopropenone was found. The ketene and the phosphorus ylide are not conjugated, as the ylide C and P lie in a plane perpendicular to the plane of the ketene and its substituents. IM1 was shown to computationally undergo a concerted cycloaddition with benzaldehyde 2a to form IM2, via a five-membered ring transition state TS2 with a 24.9 kcal mol−1 barrier. This may be a pseudo-pericyclic reaction25 and does not involve a cyclic delocalized 6-electron transition state. Instead, the nucleophilic carbon of the ylide attacks the electrophilic aldehyde π system, while the oxygen of the aldehyde attacks the highly electrophilic π system of the ketene, in the plane of the forming lactone ring. The cyclization is more favorable than the Wittig-type attack of the aldehyde oxygen at the ylide phosphorus via a four-membered ring transition state TS2*, which is higher in energy than TS2 by 3.7 kcal mol−1, even though the product 3a* is more stable by 2.4 kcal mol−1. The adduct of the cycloaddition (IM2) is unstable, which readily undergoes 1,4-elimination to form product 3a. These pathway calculations are in accord with the fact that only product 3a is observed experimentally.Open in a separate windowFig. 4Calculated reaction profiles. The (3 + 2) annulation reaction is in black; Wittig olefination reaction is in blue. Energies are in kcal mol−1 and distances are given in Å.The frontier molecular orbitals (FMOs) of the reactants are shown in Fig. 5a. The nucleophilic carbon terminus of the phosphorus ylide, HOMO of IM1, interacts with the large LUMO coefficient at C1 of 2a. These orbitals differ in energy by 6.42 eV. Hirshfeld charges of corresponding atoms are shown in red in Fig. 5b. From the perspective of molecular charge reorganizations, these charges are very complementary to the transition state of the observed reaction. The two steps of the observed reaction have similar barriers, so that substituents that influence the rate of either step can have an effect on the overall reaction rate. Interestingly, the normally good dienophiles and dipolarophiles, acrylates and maleates, are not reactive in these cases. The low reactivity of acrylates as compared to aldehydes is likely due to the necessity for strong electrostatic interactions between the heteroatom of the electrophile and the central carbon of the ketene. In addition, it is known2a,26 that these Michael acceptors would react with PMe3 catalysts to form off cycle intermediates thereby deactivating the desired reaction mode.Open in a separate windowFig. 5The frontier molecular orbitals (FMOs) and Hirshfeld charges. (a) FMOs interactions stabilizing TS2 (see Fig. 4). (b) Hirshfeld charges of 2a and IM1.In summary, we report the development of a phosphine-catalyzed (3 + 2) heteroannulation of cyclopropenones with an extensive range of electrophilic C X π systems including aldehydes, ketoesters, imines, isocyanates, and carbodiimides. This valuable alternative to transition metal-based methods not only provides efficient access to highly substituted sets of butenolides, butyrolactams, maleimides, and iminomaleimides, but also highlights the versatility and generality of the organocatalytic (3 + 2) annulative approach. Computational mechanistic investigations confirmed that an α-ketenyl phosphorus ylide is formed as a key intermediate. This species then undergoes a cycloaddition with aldehydes in a catalytic manner, rather than a stoichiometric Wittig olefination pathway, thus showcasing a unique and interesting reactivity. The organocatalytic activation of cyclopropenones also expands the scope of phosphine catalysis by supplying a new subset of 1,3-dipole surrogates that complements existing well-studied synthons, for example, allene substrates. Reaction development based on new modes of phosphine-catalyzed C–C bond activations is being explored in our laboratory. 相似文献
| Entry | catalyst | Additive | Solvent | Time | Yieldb (%) |
|---|---|---|---|---|---|
| 1c | PPh3 | — | CH2Cl2 | 3 h | Trace |
| 2c | PBu3 | — | CH2Cl2 | 3 h | 22 |
| 3c | PMe3 | — | CH2Cl2 | 3 h | 30 |
| 4c | PMe3 | 4Å MS | CH2Cl2 | 30 min | 73 |
| 5 | PMe3 | 4Å MS | CH2Cl2 | 30 min | 99 |
| 6d | PMe3 | 4Å MS | CH2Cl2 | 2 h | 92 |
| 7e | PMe3 | 4Å MS | CH2Cl2 | 24 h | 78 |
| 8f | PMe3 | 4Å MS | CH2Cl2 | 5 d | 20 |
| 9 | PMe3 | 4Å MS | THF | 1 h | 88 |
| 10 | PMe3 | 4Å MS | CH3CN | 1 h | 35 |
| 11 | PMe3 | 4Å MS | Toluene | 1 h | 95 |
| 12 | PMe3 | 4Å MS | Cyclohexane | 1 h | 69 |
| 13 | PMe3 | 4Å MS | DMF | 1 h | 44 |
4.
Yamagiwa N Qin H Matsunaga S Shibasaki M 《Journal of the American Chemical Society》2005,127(38):13419-13427
The full details of a catalytic asymmetric aza-Michael reaction of methoxylamine promoted by rare earth-alkali metal heterobimetallic complexes are described, demonstrating the effectiveness of Lewis acid-Lewis acid cooperative catalysis. First, enones were used as substrates, and the 1,4-adducts were obtained in good yield (57-98%) and high ee (81-96%). Catalyst loading was successfully reduced to 0.3-3 mol % with enones. To broaden the substrate scope of the reaction to carboxylic acid derivatives, alpha,beta-unsaturated N-acylpyrroles were used as monodentate, carboxylic acid derivatives. With beta-alkyl-substituted N-acylpyrroles, the reaction proceeded smoothly and the products were obtained in high yield and good ee. Transformation of the 1,4-adducts from enones and alpha,beta-unsaturated N-acylpyrroles afforded corresponding chiral aziridines and beta-amino acids. Detailed mechanistic studies, including kinetics, NMR analysis, nonlinear effects, and rare earth metal effects, are also described. The Lewis acid-Lewis acid cooperative mechanism, including the substrate coordination mode, is discussed in detail. 相似文献
5.
Hua Huang Qing-Song Dai Hai-Jun Leng Qing-Zhu Li Si-Lin Yang Ying-Mao Tao Xiang Zhang Ting Qi Jun-Long Li 《Chemical science》2022,13(9):2584
The Suzuki–Miyaura cross-coupling of C(sp3)-hybridised boronic compounds still remains a challenging task, thereby hindering the broad application of alkyl boron substrates in carbon–carbon bond-forming reactions. Herein, we developed an NHC/photoredox dual catalytic cross-coupling of alkyl trifluoroborates with acid fluorides, providing an alternative solution to the classical acylative Suzuki coupling chemistry. With this protocol, various ketones could be rapidly synthesised from readily available materials under mild conditions. Preliminary mechanistic studies shed light on the unique radical reaction mechanism.An acylative Suzuki-type cross-coupling of alkyl trifluoroborates and acid fluorides was developed by merging NHC organocatalysis with photoredox catalysis. A broad spectrum of ketones could be facilely synthesised under mild reaction conditions. 相似文献
6.
Hongqiang Ye He Zhao Shujian Ren Hongfeng Ye Dongping Cheng Xiaonian Li Xiaoliang Xu 《Tetrahedron letters》2019,60(18):1302-1305
Utilizing Lewis base/photoredox dual catalysis, carbon radicals generated from either alkylboronic acids or esters were coupled with Baylis–Hillman derivatives under visible light irradiation. This protocol provides a mild and operationally simple method for the synthesis of a variety of α,β-unsaturated carbonyl compounds in a broad scope of the substrates. The mechanism of Lewis base activation and reductive quenching cycle was probably involved. 相似文献
7.
Satoshi Kikuchi 《Tetrahedron letters》2006,47(12):1973-1975
We found that the combination of Ph3P and TiCl4 was the excellent promoter for reductive Mannich-type reaction of S-2,4,6-triisopropylphenyl 2-bromopropanethioate with several imines and that the corresponding products were obtained in good yields with high anti-selectivity. 相似文献
8.
9.
Nickel/Lewis acid binary catalysis is found effective to direct regioselective alkenylation of imidazoles through C-H bond activation and stereoselective insertion of alkynes. Use of P(t-Bu)3 as a ligand allows exclusive regioselective C(2)-alkenylation, while PCyp3 is found effective for C(5)-alkenylation of C(2)-substituted imidazoles. The reaction demonstrates a broad scope of imidazoles and internal alkynes to give trisubstituted ethenes highly regio- and stereoselectively in modest to good yields. 相似文献
10.
The development of a visible light-mediated atom transfer radical addition (ATRA) of perfluoroalkyl iodides to potassium 1-penten-5-yl-, vinyl- and allyltrifluoroborates using the reductive quenching of [Ru(bpy)3]Cl2 or [Ru(bpy)3](PF6)2 is described. Using an operationally simple and mild protocol, the corresponding potassium trifluoroborates containing perfluoroalkyl groups were obtained in moderate to high yields. In the case of potassium allyltrifluoroborate, the use of acetone as the solvent resulted in allylboration followed by ATRA of perfluoroalkyl iodides to the formed homoallyl alcohol. A one-pot protocol was developed for the synthesis of a series of fluorinated alcohols using potassium allyltrifluoroborate, perfluoroalkyl iodides and selected aliphatic ketones. 相似文献
11.
12.
The combination of photoredox catalysis and enamine catalysis has enabled the development of an enantioselective aerobic oxidative cross-dehydrogenative coupling between glycine derivatives and simple ketones or aldehydes, which provides an efficient approach for the rapid synthesis of enantiopure unnatural α-alkyl α-amino acid derivatives in good yield with excellent diastereo- (up to >99 : 1) and enantioselectivities (up to 97% ee). This process includes the direct photoinduced oxidation of glycine derivatives to an imine intermediate, followed by the asymmetric Mannich-type reaction with an enamine intermediate generated in situ from a ketone or aldehyde and a chiral secondary amine organocatalyst. This mild method allows the direct formation of a C–C bond with simultaneous installation of two new stereocenters without wasteful removal of functional groups.A visible-light-induced enantioselective aerobic oxidative cross-dehydrogenative coupling between glycine derivatives and simple ketones or aldehydes is achieved. 相似文献
13.
Organized surface functional groups: cooperative catalysis via thiol/sulfonic acid pairing 总被引：3，自引：0，他引：3
Margelefsky EL Zeidan RK Dufaud V Davis ME 《Journal of the American Chemical Society》2007,129(44):13691-13697
The synthesis and characterization of heterogeneous catalysts containing surfaces functionalized with discrete pairs of sulfonic acid and thiol groups are reported. A catalyst having acid and thiol groups separated by three carbon atoms is ca. 3 times more active than a material containing randomly distributed acid and thiol groups in the condensation of acetone and phenol to bisphenol A and 14 times more active in the condensation of cyclohexanone and phenol to bisphenol Z. Increasing the acid/thiol distance in the paired materials decreases both the activity and selectivity. This work clearly reveals the importance of nanoscale organization of two disparate functional groups on the surface of heterogeneous catalysts. 相似文献
14.
15.
[reaction: see text]. We describe a superior procedure for the catalytic, asymmetric synthesis of beta-lactams using a bifunctional catalyst system consisting of a chiral nucleophile and an achiral Lewis acid. 相似文献
16.
A practical and efficient protocol for Ag/Ru-cocatalyzed regioselective C-H amination of 8-hydroxyquinoline esters with pyrazoles was developed, This reaction proceeded smoothly via a photoredox-mediated direct C-H/N-H oxidative coupling process. The remarkable features of this reaction include the wide substrate scope, mild reaction conditions and high regioselectivity at the C4 site of the quinolinyl moiety. 相似文献
17.
A series of unsymmetric salen ligands derived from 1,2-diaminocyclohexane bearing an appended Lewis base on the three-position of one aromatic ring were synthesized by the reaction of various functional salicyaldehydes with the condensation product of 1,2-diaminocyclohexane mono(hydrogen chloride) and 3,5-di-tert-butylsalicylaldehyde. These ligands in conjunction with Ti(O(i)Pr)(4) exhibited excellent activity in catalyzing the cyanosilylation of aldehydes with trimethylsilyl cyanide (TMSCN) at mild conditions. The highest activity was observed in the catalyst system with regard to the salen ligand bearing a diethylamino group, which proved to be active even at a high [aldehyde]/[catalyst] ratio up to 50000. In a low catalyst loading of 0.05 mol%, the quantitative conversion of benzaldehyde to the corresponding cyanosilylation product was found within 10 min. at ambient temperature. An intramolecularly cooperative catalysis was proposed wherein the central metal Ti(IV) is suggested to play a role of Lewis acid to activate aldehydes while the appended Lewis base to activate TMSCN. 相似文献
18.
《合成通讯》2012,42(1):112-122
AbstractLewis acid ZnCl2 promoted cyclization protocol to 4H-chromenes is accomplished, using readily available phenols and acetophenones as starting materials. Interestingly, the process is feasible under the solvent free environment. Synthesis of a variety of 4H-chromenes have been accomplished using this strategy. In addition, this concept is extended to the synthesis of ortho-benzylphenols by treating phenols either with styrenes or secondary benzylic alcohols. 相似文献
19.
Cycloadditions of arylidene imines of α-amino acid esters to a range of dipolarophiles show substantial rate enhancements in the presence of Bronsted and Lewis acids. For Bronsted acids the rate is related to the pKa of the acid and cycloadditions to reactive dipolarophiles occur at room temperature. For the Lewis acids studied the rate acceleration decreases in the order Zn(OAc)2 > AgOAc> LiOAc> MgOAc2 but is also anion dependent with LiBr> LiOAc and AgOAc> AgOTs. The Lewis acid catalysed processes are believed to be examples of cycloadditions of metallo-1,3-dipoles. In both Bronsted and Lewis acid catalysed processes the cycloadditions are regio- and stereo-specific. 相似文献
20.
Vermoortele F Vandichel M Van de Voorde B Ameloot R Waroquier M Van Speybroeck V De Vos DE 《Angewandte Chemie (International ed. in English)》2012,51(20):4887-4890
Functionalized linkers can greatly increase the activity of metal-organic framework (MOF) catalysts with coordinatively unsaturated sites. A clear linear free-energy relationship (LFER) was found between Hammett σ(m) values of the linker substituents X and the rate k(X) of a carbonyl-ene reaction. This is the first LFER ever observed for MOF catalysts. A 56-fold increase in rate was found when the substituent is a nitro group. 相似文献