Unsaturated N-heterocyclic carbenes

A PS-supported Ru Catalyst with Unsaturated N-Heterocyclic Carbene Ligation... 

N-Heterocyclic carbenes form intriguingly stable bonds with the majority of metals [12,21,29]. Whereas for saturated and unsaturated N-heterocyclic carbenes of comparable steric demand very similar bond dissociation energies have been observed, phosphines generally form much weaker bonds (Table 2) [21]. As a result, the equilibrium between the free carbene and the carbene metal complex lies far more on the side of the complex than... 

Jahier-Diallo C, Morin MST, Queval P, Rouen M, Artur I, Querard P, Toupet L, Crevisy C, Basle O, Mauduit M (2015) Multicomponent synthesis of chiral bidentate unsymmetrical unsaturated N-heterocyclic carbenes copper-catalyzed asymmetric C-C Bond formation. Chem Eur J 21 993-997... 

Addition of acyl anions generated from acylsilanes to a,(3-unsaturated ketones using N-heterocyclic carbenes (NHCs) derived from thiazolium salts as catalyst produced 1,4-diketones, which cyclized to form the corresponding furans in good yields under an acidic condition <06JOC5715>. 

One application of this catalytic generation of homoenolate type intermediates is in the stereoselective formation of y-butyrolactones 64 from a,/ -unsaturated aldehydes 62 and their reaction with aldehydes or ketones 63 [60]. (For experimental details see Chapter 14.19.2). Glorius [60a] and Bode [60b] almost simultaneously published their results utilizing a N-heterocyclic carbene generated from a bisar-ylimidazolium salt 65 (IMes). The corresponding disubstituted y-butyrolactones... 

The postulated catalytic cycle is shown in Scheme 9.19. The N-heterocyclic carbene catalyst 11 (l,3-dimesityl-imidazolin-2-ylidene, IMes) attacks the a,/ -unsaturated aldehyde 62, generating the zwitterionic substance 66. After... 

Analysis of the 13C NMR spectra of a series of N-heterocyclic carbenes (NHCs) with general structures 144, 145 and 146 suggests that the chemical shift of the carbene carbon (S = 211-244) correlates with the N(l)-C(2)-N(3) bond angle (101-106°) in the solid state <2003AGE5243>. The 13C carbene shifts of the unsaturated systems 144 (<5 = 205-220) are 15-25ppm upfield from the saturated systems 145 (see also Table 40, Section 2.4.4.2.5). 

Abstract. N-Heterocyclic carbenes (NHC) have become an important class of organocatalysts and class of ligands for transition-metal catalysis. In organocat-alyzed umpolung reactions, thiazolium salt-derived NHC have been used successfully for decades. Even so, during recent years there has been an increased interest in NHC-catalyzed transformations and many new reactions have been developed. This article focuses on the use of NHC in the conjugate umpolung of ,f>-unsaturated aldehydes. 

Total Synthesis of (—)-7-Deoxyloganin Exploiting N-Heterocyclic Carbene Catalysis with a,p-Unsaturated Enol Esters... 

Later, these authors reported the synthesis of chiral cis-l,3,4-trisubstituted cyclopentenes based on an enantioselective N-heterocyclic carbene-catalyzed domino annulation of a,p-unsaturated aldehydes and 4-oxoenoates [236]. The domino products were achieved in low to high yields (25-93%), in moderate to... 

In 2013, the first successful generation of ds-homoenolate equivalents from ds-enals under the catalysis of N-heterocyclic carbene D4 was developed by the Chi group. The ds-homoenolate intermediates I undergo effective reaction with a,p-unsaturated imines to afford chiral cyclic ketones 62. Compared to the trans-enals, ds-enals show different stereoselectivities and new reactivity patterns (Scheme 20.29). 

In 2004, the N-heterocyclic carbene-catalysed internal redox reaction of epo Q aldehydes and a-haloaldehydes were developed by Bode et al. and Rovis et al., respectively. In 2012, Rovis group reported that NHC catalysed [4 + 2] hetero-Diels-Alder reactions of simple aliphatic aldehydes with a,p-unsaturated ketimines and ketones under oxidative conditions, giving the fra/2s-lactams and c/s-lactones in high yields with high enantioselectivities (Scheme 20.61). ... 

In 2011, Ye and coworkers found that N-heterocyclic carbene C7 was an efficient catalysts for the cyclisation reaction of a,p-unsaturated p-methylacyl chloride with activated trifluoromethyl ketones or isatin. The spirocyclic oxindole-dihydropyrones 198 and trifluoromethyldihydropyrones 199 were obtained in good yield with good to high enantioselectivity (Scheme 20.84). 

Arylalkylketenes react with P,y-unsaturated a-ketophosphonates with chiral catalysis by N-heterocyclic carbenes giving enantioselective formation of products of formal [4 + 2] cycloaddition (Eqn (4.103)). ° These ketenes react similarly with acylcarboxylates forming the corresponding esters and amides with high enantioselectivity (Eqn (4.104)). ° A similar lactone synthesis by [4 + 2] cycloaddition is shown in Eqn (4.99), and such ketene reactions catalyzed by N-heterocycHc carbenes have been recently reviewed. 

Conejero S, L6pez Serrano J, Paneque M, et al. Tautomerisation of 2-substituted pyr-idines to N-heterocyclic carbene hgands induced by the 16e-unsaturated... 

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