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1.
The synthesis of a series of hydrosilylboronates via the selective iridium- or nickel-catalyzed monoborylation of dihydrosilane Si–H bonds is described. The synthesized silylboronates, which bear a single Si–H bond, can be used as novel silicon nucleophiles in the presence of transition-metal catalysts or bases. The first 29Si{1H} NMR spectroscopic evidence for the formation of (t-Bu)2HSiLi, generated by the reaction of (t-Bu)2HSi–B(pin) with MeLi, is reported as the first example of a dialkylhydorosilyl lithium species.Monoborylation of a dihydrosilane Si–H bond can be achieved in the presence of iridium- or nickel-based catalysts, yielding novel hydrosilylboronates that bear a hydrogen atom at the silicon center. 相似文献
2.
Sebastian Kaufmann Frederic Krtschmer Ralf Kppe Thorben Schon Christoph Schoo Peter W. Roesky 《Chemical science》2020,11(46):12446
The synthesis of a 1,2,3,4-tetramethylcyclopentadienyl (Cp4) substituted four-membered N-heterocyclic silylene [{PhC(NtBu)2}Si(C5Me4H)] is reported first. Then, selected reactions with transition metal and a calcium precursor are shown. The proton of the Cp4-unit is labile. This results in two different reaction pathways: (1) deprotonation and (2) rearrangement reactions. Deprotonation was achieved by the reaction of [{PhC(NtBu)2}Si(C5Me4H)] with suitable zinc precursors. Rearrangement to [{PhC(NtBu)2}(C5Me4)SiH], featuring a formally tetravalent silicon R2C Si(R′)–H unit, was observed when the proton of the Cp4 ring was shifted from the Cp4-ring to the silylene in the presence of a Lewis acid. This allows for the coordination of the Cp4-ring to a calcium compound. Furthermore, upon reaction with transition metal dimers [MCl(cod)]2 (M = Rh, Ir; cod = 1,5-cyclooctadiene) the proton stays at the Cp4-ring and the silylene reacts as a sigma donor, which breaks the dimeric structure of the precursors.A cyclopentadienyl functionalized silylene or its derivatives can be coordinated in all three forms: silylene (A), anion (B), and sila fulvene (C). 相似文献
3.
Adedamola A. Opalade Joshua D. Parham Victor W. Day Timothy A. Jackson 《Chemical science》2021,12(38):12564
While alkylperoxomanganese(iii) (MnIII–OOR) intermediates are proposed in the catalytic cycles of several manganese-dependent enzymes, their characterization has proven to be a challenge due to their inherent thermal instability. Fundamental understanding of the structural and electronic properties of these important intermediates is limited to a series of complexes with thiolate-containing N4S− ligands. These well-characterized complexes are metastable yet unreactive in the direct oxidation of organic substrates. Because the stability and reactivity of MnIII–OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new MnIII–OOR complexes using a new amide-containing N5− ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H6Medpaq) ligand, we generated the [MnIII(OOtBu)(6Medpaq)]OTf and [MnIII(OOCm)(6Medpaq)]OTf complexes through reaction of their MnII or MnIII precursors with tBuOOH and CmOOH, respectively. Both of the new MnIII–OOR complexes are stable at room-temperature (t1/2 = 5 and 8 days, respectively, at 298 K in CH3CN) and capable of reacting directly with phosphine substrates. The stability of these MnIII–OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [MnIII(OOCm)(6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the MnIII–OOR complexes by O–O bond homolysis. In contrast, direct reaction of [MnIII(OOCm)(6Medpaq)]+ with PPh3 provided evidence of heterolytic cleavage of the O–O bond. These studies reveal that both the stability and chemical reactivity of MnIII–OOR complexes can be tuned by the local coordination sphere.A pair of room-temperature-stable MnIII–alkylperoxo complexes were characterized and shown to oxidize PPh3. Thermal decomposition studies provide evidence of both homolysis and heterolysis of the MnIII–alkylperoxo O–O bond. 相似文献
4.
Wei W. Chen Nahiane Pipaon Fernndez Marta Díaz Baranda Anton Cunillera Laura G. Rodríguez Alexandr Shafir Ana B. Cuenca 《Chemical science》2021,12(31):10514
A stepwise build-up of multi-substituted Csp3 carbon centers is an attractive, conceptually simple, but often synthetically challenging type of disconnection. To this end, this report describes how gem-α,α-dimetalloid-substituted benzylic reagents bearing boron/silicon or boron/tin substituent sets are an excellent stepping stone towards diverse substitution patterns. These gem-dimetalloids were readily accessed, either by known carbenoid insertion into C–B bonds or by the newly developed scalable deprotonation/metallation approach. Highly chemoselective transformations of either the C–Si (or C–Sn) or the C–B bonds in the newly formed gem-Csp3 centers have been achieved through a set of approaches, with a particular focus on exploiting the synthetically versatile polarity reversal in organometalloids by λ3-aryliodanes. Of particular note is the metal-free arylation of the C–Si (or C–Sn) bonds in such gem-dimetalloids via the iodane-guided C–H coupling approach. DFT calculations show that this transfer of the (α-Bpin)benzyl group proceeds via unusual [5,5]-sigmatropic rearrangement and is driven by the high-energy iodine(iii) center. As a complementary tool, the gem-dimetalloid C–B bond is shown to undergo a potent and chemoselective Suzuki–Miyaura arylation with diverse Ar–Cl, thanks to the development of the reactive gem-α,α-silyl/BF3K building blocks.This work explores divergent reactivity of the benzylic gem-boron–silicon and boron–tin double nucleophiles, including the arylation of the C–B bond with Ar–Cl, along with a complementary oxidative λ3-iodane-guided arylation of the C–Si/Sn moiety. 相似文献
5.
Li Wang Qi Zhong Youliang Zou Youzhi Yin Aizhen Wu Quan Chen Ke Zhang Jiachen Jiang Mengzhen Zhao Hua Zhang 《Chemical science》2021,12(45):15104
Selective carbon–carbon bond activation is important in chemical industry and fundamental organic synthesis, but remains challenging. In this study, non-polar unstrained Csp2–Csp3 and Csp2–Csp2 bond activation was achieved by B(OMe)3/B2pin2-mediated fragmentation borylation. Various indole derivatives underwent C2-regioselective C–C bond activation to afford two C–B bonds under transition-metal-free conditions. Preliminary mechanistic investigations suggested that C–B bond formation and C–C bond cleavage probably occurred in a concerted process. This new reaction mode will stimulate the development of reactions based on inert C–C bond activation.Non-polar unstrained Csp2–Csp3 and Csp2–Csp2 bond activation was achieved via B(OMe)3/B2pin2-mediated fragmentation borylation, in which C–C bond activation occurred regioselectively at the C2-position in various substituted indoles. 相似文献
6.
Indole 2,5-diketopiperazines (DKPs) are an important type of metabolic cyclic dipeptides containing a tryptophan (Trp) unit possessing a range of interesting biological activities. The intriguing structural features and divergent activities have stimulated tremendous efforts towards their efficient synthesis. Herein, we report the development of a unified strategy for the synthesis of three Trp-containing DKPs, namely tryprostatin A, and maremycins A and B, via a sequential C–H activation strategy. The key Trp skeletons were synthesized from the inexpensive, readily available alanine via a Pd(ii)-catalyzed β-methyl C(sp3)–H monoarylation. A subsequent C2-selective prenylation of the resulting 6-OMe-Trp by Pd/norbornene-promoted C–H activation led to the total synthesis of tryprostatin A in 12 linear steps from alanine with 25% overall yield. Meanwhile, total syntheses of maremycins A and B were successfully accomplished using a sequential Pd-catalyzed methylene C(sp3)–H methylation as the key step in 15 linear steps from alanine.Indole 2,5-diketopiperazines (DKPs) are an important type of metabolic cyclic dipeptides containing a tryptophan (Trp) unit possessing a range of interesting biological activities. 相似文献
7.
The conversion of metal nitrides to NH3 is an essential step in dinitrogen fixation, but there is limited knowledge of the reactivity of nitrides with protons (H+). Herein, we report comparative studies for the reactions of H+ and NH3 with uranium nitrides, containing different types of ancillary ligands. We show that the differences in ancillary ligands, leads to dramatically different reactivity. The nitride group, in nitride-bridged cationic and anionic diuranium(iv) complexes supported by –N(SiMe3)2 ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic –N(SiMe3)2 ligands promote the N–H heterolytic bond cleavage of NH3, yielding a “naked” diuranium complex containing three bridging ligands, a nitride (N3−) and two NH2 ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by –OSi(OtBu)3 ligands, the nitride group is easily protonated to afford NH3, which binds the U(iv) ion strongly, resulting in a mononuclear U–NH3 complex, where NH3 can be displaced by addition of strong acids. Furthermore, the U–OSi(OtBu)3 bonds were found to be stable, even in the presence of stronger acids, such as NH4BPh4, therefore indicating that –OSi(OtBu)3 supporting ligands are well suited to be used when acidic conditions are required, such as in the H+/e− mediated catalytic conversion of N2 to NH3.Ancillary ligands alter the reactivity of U-nitrides with H+, relevant to N2 conversion to NH3. The amides lead to complete ligand loss and NH3 activation, while for siloxides, the nitride is protonated to NH3 leaving the ancillary ligands intact. 相似文献
8.
A dinickel(0)–N2 complex, stabilized with a rigid acridane-based PNP pincer ligand, was studied for its ability to activate C(sp2)–H and C(sp2)–O bonds. Stabilized by a Ni–μ–N2–Na+ interaction, it activates C–H bonds of unfunctionalized arenes, affording nickel–aryl and nickel–hydride products. Concomitantly, two sodium cations get reduced to Na(0), which was identified and quantified by several methods. Our experimental results, including product analysis and kinetic measurements, strongly suggest that this C(sp2)–H activation does not follow the typical oxidative addition mechanism occurring at a low-valent single metal centre. Instead, via a bimolecular pathway, two powerfully reducing nickel ions cooperatively activate an arene C–H bond and concomitantly reduce two Lewis acidic alkali metals under ambient conditions. As a novel synthetic protocol, nickel(ii)–aryl species were directly synthesized from nickel(ii) precursors in benzene or toluene with excess Na under ambient conditions. Furthermore, when the dinickel(0)–N2 complex is accessed via reduction of the nickel(ii)–phenyl species, the resulting phenyl anion deprotonates a C–H bond of glyme or 15-crown-5 leading to C–O bond cleavage, which produces vinyl ether. The dinickel(0)–N2 species then cleaves the C(sp2)–O bond of vinyl ether to produce a nickel(ii)–vinyl complex. These results may provide a new strategy for the activation of C–H and C–O bonds mediated by a low valent nickel ion supported by a structurally rigidified ligand scaffold.A structurally rigidified nickel(0) complex was found to be capable of cleaving both C(sp2)–H and C(sp2)–O bonds. 相似文献
9.
Adrian W. Markwell-Heys Michael Roemelt Ashley D. Slattery Oliver M. Linder-Patton Witold M. Bloch 《Chemical science》2021,13(1):68
Using metal–organic cages (MOCs) as preformed supermolecular building-blocks (SBBs) is a powerful strategy to design functional metal–organic frameworks (MOFs) with control over the pore architecture and connectivity. However, introducing chemical complexity into the network via this route is limited as most methodologies focus on only one type of MOC as the building-block. Herein we present the pairwise linking of MOCs as a design approach to introduce defined chemical complexity into porous materials. Our methodology exploits preferential Rh-aniline coordination and stoichiometric control to rationally link Cu4L4 and Rh4L4 MOCs into chemically complex, yet extremely well-defined crystalline solids. This strategy is expected to open up significant new possibilities to design bespoke multi-functional materials with atomistic control over the location and ordering of chemical functionalities.A new strategy to design atomically precise multivariate metal–organic frameworks is presented. This is achieved by linking two preformed metal–organic cages via a precisely tuned Rh–aniline interaction. 相似文献
10.
We report the reactivity between the water stable Lewis acidic trioxatriangulenium ion (TOTA+) and a series of Lewis bases such as phosphines and N-heterocyclic carbene (NHC). The nature of the Lewis acid–base interaction was analyzed via variable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction, UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such as PMe3, led to the formation of a Lewis acid–base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P(tBu)3. The TOTA+–P(tBu)3 FLP was characterized as an encounter complex, and found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation, dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of the C–Br bonds, and interception of Staudinger reaction intermediates. Moreover, TOTA+ and NHC were found to first undergo single-electron transfer (SET) to form [TOTA]·[NHC]˙+, which was confirmed via electron paramagnetic resonance (EPR) spectroscopy, and subsequently form a [TOTA–NHC]+ adduct or a mixture of products depending the reaction conditions used.Frustration at carbon! Herein, we present a frustrated Lewis pair system derived from a water stable carbon-based Lewis acid, trioxatriangulene (TOTA+), and a variety of Lewis bases, which successfully promotes bond cleavage and molecule fixation. 相似文献
11.
We report here a sequential enantioselective reduction/C–H functionalization to install contiguous stereogenic carbon centers of benzocyclobutenols and cyclobutanols. This strategy features a practical enantioselective reduction of a ketone and a diastereospecific iridium-catalyzed C–H silylation. Further transformations have been explored, including controllable regioselective ring-opening reactions. In addition, this strategy has been utilized for the synthesis of three natural products, phyllostoxin (proposed structure), grandisol and fragranol.We report here a sequential enantioselective reduction/C–H functionalization to install contiguous stereogenic carbon centers of benzocyclobutenols and cyclobutanols.Molecules with inherent ring strain have gained considerable interest in the synthetic community.1 Among them, four-membered ring molecules have been recognized as powerful building blocks in organic synthesis.2 Driven by ring strain releasing, the reactions of carbon–carbon bond cleavage have been extensively studied in recent years.3 Meanwhile, cyclobutane motifs represent important structural units in natural product and bioactive molecules as well (Scheme 1).4 Therefore, a general and robust method to constitute four-membered ring derivatives is of great value, especially in an enantiomerically pure form.5Open in a separate windowScheme 1Representative cyclobutane-containing bioactive molecules.[2 + 2]-Cycloaddition6 and the skeleton rearrangement reaction7 are two primary methods to prepare chiral cyclobutane derivatives. Recently, the precision modification of four-membered ring skeletons to access enantioenriched cyclobutane derivatives has attracted emerging attention. Several strategies have been developed, including allylic alkylation,8 α-functionalization,9 conjugate addition10 and C–H functionalization11 of prochiral or racemic cyclobutane derivatives (Scheme 2a).12 However, the enantioselective synthesis of chiral benzocyclobutene derivatives is still underdeveloped.13 Although two efficient palladium-catalyzed C–H activation strategies have been developed by Baudoin14 and Martin15 groups via similar intermediate five-membered palladacycles, no enantioenriched benzocyclobutene derivative has been prepared by employing the above two methods. In 2017, Kawabata reported an elegant example of asymmetric intermolecular α-arylation of enantioenriched amino acid derivatives to afford benzocyclobutenones with tetrasubstituted carbon via memory of chirality (Scheme 2b).16 In 2018, Zhang reported an iridium-catalyzed asymmetric hydrogenation of α-alkylidene benzocyclobutenones in good enantioselectivities (3 examples, 83–88% ee).12c To the best of our knowledge, there is no report on enantioselective synthesis of benzocyclobutene derivatives with all-carbon quaternary centers.Open in a separate windowScheme 2Asymmetric synthesis of cyclobutanes and their derivatives. (a) Enantioselective functionalization of four-membered ring substrates. (b) Synthesis of chiral benzocyclobutenone via memory of chirality. (c) This work: sequential enantioselective reduction/C–H functionalization.In line with our continued interest in precision modification of four-membered ring skeletons,9d,10c,12a we initiated our studies on the synthesis of chiral benzocyclobutenes via enantioselective functionalization of highly strained benzocyclobutenones. It is well known that benzocyclobutene derivatives are labile to undergo a ring-opening reaction to release their inherent ring strains.17 Therefore, it is a challenging task to modify the benzocyclobutenone and preserve the four-membered ring skeleton at the same time. We envisioned that a carbonyl group directed C–H functionalization18 of the gem-dimethyl group could furnish enantioenriched α-quaternary benzocyclobutenones (Scheme 2c). This could be viewed as an alternative approach to achieve the alkylation of benzocyclobutenone, which was otherwise directly inaccessible using enolate chemistry through the unstable anti-aromatic intermediate.19 In addition, a highly regioselective C–H activation would be required to functionalize the methyl group instead of the aryl ring. Here we report our work on sequential enantioselective reduction and intramolecular C–H silylation to provide enantioenriched benzocyclobutenols and cyclobutanols with all-carbon quaternary centers. The excellent diastereoselectivity and regioselectivity of silylation were attributed to rigid structural organization of the 4/5 fused ring. Furthermore, this strategy has been utilized to accomplish the total synthesis of natural products phyllostoxin (proposed structure), grandisol and fragranol.We commenced our studies with enantioselective reduction of readily prepared dimethylbenzocyclobutenone 1a (Scheme 3).15,20 Surprisingly, enantioselective reduction of the carbonyl group of cyclobutanone derivatives received little attention. The first reduction of parent benzocyclobutenone was studied in 1996 by Kündig using chlorodiisopinocamphenylborane21 or chiral oxazaborolidines (CBS reduction),22 and only moderate enantioselectivity (44–68% ee) was obtained.23 Although copper-catalyzed asymmetric hydrosilylation of benzocyclobutenone 1a using CuCl/(R)-BINAP gave the benzocyclobutenol ent-2a in 88% ee, optimization of ligands gave no further improvement (Scheme 3a, see Tables S1–S4† for details).24 Gladly, excellent enantioselective reduction could be achieved in 94% yield and 97% ee under Noyori''s asymmetric transfer hydrogenation conditions (Scheme 3b, conditions A, RuCl[(S,S)-Tsdpen](p-cymene)).25 The product 2a showed remarkable stability and no ring-opening byproduct 2a′ was observed. The reduction of parent benzocyclobutenone was examined under conditions A, and benzocyclobutenol was obtained in 90% yield and 81% ee. Apparently, the steric influence imposed by the α-dimethyl group enhanced the enantioselectivity of the reduction. Similarly, the CBS reduction ((S)-B–Me) of benzocyclobutenone 1a gave better results compared with parent benzocyclobutenone, affording the product 2a in 86% yield and 92% ee (Scheme 3c).Open in a separate windowScheme 3Enantioselective reduction of benzocyclobutenone 1a. (a) Copper hydride reduction. (b) Ru-catalyzed asymmetric transfer hydrogenation. (c) CBS reduction.We then examined the substrate scope of the reduction reaction (26 was chosen to improve the yield and enantioselectivity. Besides, benzocyclobutenol 2g with nitro substitution could be obtained in 96% yield and 93% ee. Treatment of pyrrolidinyl substituted benzocycobutenone 1h with catalyst (S,S)-Ts-DENEB afforded desired product 2h in 49% yield and 89% ee, together with ring-opening product 2h′ (18%).Enantioselective reduction of benzocyclobutenonesa
Open in a separate windowaConditions A: 1a (0.5–2.0 mmol), RuCl[(S,S)-Tsdpen](p-cymene) (1–2 mol%), HCOOH/Et3N (5/2), rt. All results are corrected to the (S)-catalyst. The ee values were determined by HPLC analysis; see the ESI for more details.b(S,S)-Ts-DENEB (1–2 mol%) was used, rt or 60 °C.3,3-Disubstituted cyclobutanones were also explored (l-selectride gave cis-4i as a single product in 99% yield and 96% ee. The reaction of 3j gave similar results, and enantioenriched cyclobutanols cis-4j could be furnished in 78% yield and 97% ee from ent-trans-4j (98% ee) following the above oxidation–reduction procedure. The absolute configurations of 2a, ent-2j and trans-4i were unambiguously determined by single-crystal X-ray diffraction analysis of their corresponding nitrobenzoate derivative.27Enantioselective reduction of cyclobutanones 3a
Open in a separate windowaConditions B: 3a (1.0–5.0 mmol), (S)-B–Me (10 mol%), BH3·Me2S (0.6 equiv.), THF, rt.b(S)-B–Me (20 mol%), BH3·Me2S (1.0 equiv.).c(−)-Ipc2BCl (1.2 equiv.), THF, −20 °C. (−)-Ipc2BCl = (−)-diisopinocampheylchloroborane.Inspired by powerful and reliable directed C–H silylation chemistry pioneered by Hartwig,28 we envisioned that the transition-metal catalyzed intramolecular C–H silylations of the above alcohols would provide a single diastereomer owing to rigid structural organization. The challenges here are the control of regioselectivity in the cyclization step and inhibition of the ring-opening pathway. Benzocyclobutenol 2a was chosen as a model substrate to study this intramolecular C–H silylation. The transition-metal catalyst system and alkene acceptors were screened (Scheme 4, see Tables S5–S9† for details). Acceptor norbornene (nbe) derivative A gave the optimal yield in the cyclization step (63% NMR yield), and other phenanthroline ligands gave inferior results. The reaction showed remarkable regio- and diastereoselectivity; no silylation of the arene was detected.With optimal intramolecular silylation conditions in hand, sequential hydroxysilylation/C–H silylation/phenyllithium addition reaction of 2a provided desired product 5a in 56% overall yield without any obvious erosion of enantiomeric purity (