共查询到20条相似文献,搜索用时 15 毫秒
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《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(33):10038-10041
Discovered by Hawthorne in 1965, dicarbollide ions are an intriguing class of nido ‐carboranes that mimic the behavior of the cyclopentadienyl anion. Herein, we show that it is possible to directly link the dicarbollide ion to an N‐heterocyclic carbene (NHC) to form an isolable N‐dicarbollide‐substituted NHC dianion. This molecule can be accessed by the sequential double deprotonation of a mono‐nido ‐carboranyl imidazolium zwitterion. As revealed by a single‐crystal X‐ray diffraction study, the first deprotonation leads to a monoanionic dicarbollide ion that adopts a bis(dicarbollide) structure in the solid state. Subsequent deprotonation of this monoanion leads to the first N‐dicarbollide NHC, which was fully characterized by multinuclear NMR spectroscopy as well as single‐crystal X‐ray diffraction. 相似文献
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Leming Wang Xinhang Jiang Jiean Chen Yong Huang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(22):7488-7492
In contrast to well‐established asymmetric hydrogenation reactions, enantioselective protonation is an orthogonal approach for creating highly valuable methine chiral centers under redox‐neutral conditions. Reported here is the highly enantio‐ and diastereoselective hydrofluorination of enals by an asymmetric β‐protonation/α‐fluorination cascade catalyzed by N‐heterocyclic carbenes (NHCs). The two nucleophilic sites of a homoenolate intermediate, generated from enals and an NHC, are sequentially protonated and fluorinated. The results show that controlling the relative rates of protonation, fluorination, and esterification is crucial for this transformation, and can be accomplished using a dual shuttling strategy. Structurally diverse carboxylic acid derivatives with two contiguous chiral centers are prepared in a single step with excellent d.r. and ee values. 相似文献
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Xuan B. Nguyen Yuji Nakano Nisharnthi M. Duggan Lydia Scott Martin Breugst David W. Lupton 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(33):11607-11614
Direct polarity inversion of conjugate acceptors provides a valuable entry to homoenolates. N‐heterocyclic carbene (NHC) catalyzed reactions, in which β‐unsubstituted conjugate acceptors undergo homoenolate formation and C?C bond formation twice, have been developed. Specifically, the all‐carbon (5+1) annulations give a range of mono‐ and bicyclic cyclohexanones (31 examples). In the first family of annulations, β‐unsubstituted acrylates tethered to a divinyl ketone undergo cycloisomerization, providing hexahydroindenes and tetralins. In the second, partially untethered substrates undergo an intermolecular (5+1) annulation involving dimerization followed by cycloisomerization. While enantioselectivity was not possible with the former, the latter proved viable, allowing cyclohexanones to be produced with high levels of enantiopurity (most >95:5 e.r.) and exclusive diastereoselectivity (>20:1 d.r.). Derivatizations and mechanistic studies are also reported. 相似文献
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《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(10):2774-2778
N‐Heterocyclic carbene (NHC) catalysis has been widely used for the umpolung of aldehydes, and recently for the umpolung of Michael acceptors. Described herein is the umpolung of aldimines catalyzed by NHCs, and the reaction likely proceeds via aza‐Breslow intermediates. The NHC‐catalyzed intramolecular cyclization of aldimines bearing a Michael acceptor resulted in the formation of biologically important 2‐(hetero)aryl indole 3‐acetic‐acid derivatives in moderate to good yields. The carbene generated from the bicyclic triazolium salt was found to be efficient for this transformation. 相似文献