Carbene, chiral heterocyclic

The creation of all-carbon quaternary chiral centers by asymmetric conjugate addition is a challenging task. A chiral heterocyclic carbene 199 has been used as a ligand for this reaction. Chiral 3,3-disubstituted cyclohexanones 200 were obtained by this method with up to 85% ee (equation 126) . ... 

N-heterocyclic salt such as chiral imidazolium salt 2.87 is the precursor of N-heterocyclic carbene, chiral imidazol-2-ylidene ° 2.88. 

It was also reported that a chiral heterocyclic carbene rhodium complex catalyzed the reduction of acetophenone with diphenylsilane giving 32% ee and 90% yield [47]. 

Heterocyclic carbenes. 9. Chiral heterocyclic carbenes in asymmetric homogeneous catalysis. Angew. Chem., Int. Edt. Eng. 35 2805-2807 Lapert, M., 1988, The coordination chemistry of electron-rich alkenes (enetetramines). J. Organomet. Chem. 358 185-187. 

Metal cyanides and heterocyclic carbenes are commonly employed catalysts for the benzoin condensation. Symmetrical acyloins can be prepared by dimerization of a wide range of aromatic and aliphatic aldehydes. Chiral heterocyclic carbene catalysts have also been developed that provide symmetrical acyloin products with high levels of enantiomeric purity. 

The use of chiral carbenes as asymmetric organocatalysts has attracted the interest of more and more research groups over the last years [88, 100]. Noteworthy, the use of chiral heterocyclic carbenes for asymmetric benzoin condensations dates back to 1966 when Sheehan et al. used the thia-zolium salt 206 to obtain benzoin 207 with an optical purity of around 20% [101] (Scheme 6.34A), and other groups introduced more powerful chiral thiazolium catalysts later... 

Scheme 2-61. Asymmetric copper-catalyzed 1,4-addition mediated by a chiral heterocyclic carbene.

Synthesis of Chiral AJ-heterocyclic Carbenes and of Their Complexes. .. 194... 

M-heterocyclic carbenes are neutral compounds with a divalent carbon atom located between the two nitrogens. The four types of stable diaminocarbenes used for the synthesis of chiral complexes are listed below (Fig. 2) ... 

AT-heterocyclic carbene complexes of Pd(II) or Pd(0) were extensively used in various reactions and several groups have reported syntheses of chiral complexes [5]. However, only a few examples of asymmetric catalysis are... 

Zhang YR, He L, Wu X, Shao PL, Ye S (2008) Chiral N-heterocyclic carbene catalyzed Staudinger reaction of ketenes with imines highly enantioselective synthesis of W-Boc P-lactams. Org Lett 10 277-280... 

The protocol of the allylic alkylation, which proceeds most likely via a c-allyl-Fe-intermediate, could be further improved by replacing the phosphine ligand with an M-heterocyclic carbene (NHC) (Scheme 21) [66]. The addition of a ferf-butyl-substituted NHC ligand 86 allowed for full conversion in the exact stoichiometric reaction between allyl carbonate and pronucleophile. Various C-nucleophiles were allylated in good to excellent regioselectivities conserving the 71 bond geometry of enantiomerically enriched ( )- and (Z)-carbonates 87. Even chirality and prochirality transfer was observed (Scheme 21) [67]. 

Gabriele B, Salerno G, Costa M (2006) Oxidative Carbonylations. 18 239-272 Gade LH, Bellemin-Laponnaz S (2006) Chiral N-Heterocyclic Carbenes as Stereodirecting Ligands in Asymmetric Catalysis. 21 117-157 Gade LH, see Kassube JK (2006) 20 61-96 Gandon V, see Aubert C (2006) 19 259-294 Garcia JI, see Fraile JM (2005) 15 149-190... 

Rogers MM, Stahl SS (2006) N-Heterocyclic Carbenes as Ligands for High-Oxidation-State Metal Complexes and Oxidation Catalysis. 21 21-46 Roland S, Mangeney P (2005) Chiral Diaminocarbene Complexes, Synthesis and Application in Asymmetric Catalysis. 15 191-229 Roll R, see Behr A (2008) 23 19-52... 

In asymmetric ketone hydrosilylation, axially chiral bidentate N-heterocyclic carbene ligands derived from BINAM proved to be moderately effective when bound to iridium, but less so with rhodium (Scheme 3.19) [44]. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

This review will focus on the use of chiral nucleophilic A-heterocyclic carbenes, commonly termed NHCs, as catalysts in organic transformations. Although other examples are known, by far the most common NHCs are thiazolylidene, imida-zolinylidene, imidazolylidene and triazolylidene, I-IV. Rather than simply presenting a laundry list of results, the focus of the current review will be to summarize and place in context the key advances made, with particular attention paid to recent and conceptual breakthroughs. These aspects, by definition, will include a heavy emphasis on mechanism. In a number of instances, the asymmetric version of the reaction has yet to be reported in those cases, we include the state-of-the-art in order to further illustrate the broad utility and reactivity of nucleophilic carbenes. 

It should be noted that asymmetric acyl transfer can also be catalyzed by chiral nucleophilic A-heterocyclic carbenes [27-32] and by certain chiral Lewis acid complexes [33-37] but these methods are outside the scope of this review. Additionally, although Type I and Type II tr-face selective acyl transfer processes have been reported to be catalyzed by some of the catalysts described in this review, these also lie outside the scope of this review. 

A striking example of the power of A -heterocyclic carbene (NHC)-bearing catalysts with sterically demanding substrates was disclosed by Chavez and Jacobsen, " who presented a route to several iridoid natural products, exemplified by the enantio- and diastereoselective synthesis of boschnialactone 31 outlined in Scheme 5. Chiral aldehyde 27, available from citronellal by Eschenmoser-methylenation in a single step, reacted despite the presence of an isoprenyl moiety and a gi OT-disubstituted double bond, in the presence of catalyst C smoothly to form... 

Gade, L.H. and Bellemin-Laponnaz, S. (2007) Chiral N-heterocyclic carbenes as stereodirecting ligands in asymmetric catalysis. Top. Organomet. Chem., 21, 117-157. 

In 2001, Grubbs and co-workers reported a class of Ru catalysts [(cf. 90, Eq. (2)] [30] that bear a chiral monodentate N-heterocyclic carbene ligand [31], The re-... 

V-Heterocyclic carbenes were demonstrated to be efficient catalysts for the Staudinger reaction of ketenes with /V-aryl-, /V-alkylcarbonyl imines [121]. Chiral /V-heterocyclic carbenes gave the corresponding < /.v-(3-lactams in good yields with good diastereoselectivities and excellent enantioselectivities (ee>99%). 

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