Carbon cyanosilylation reactions

Air-stable and easy-to-prepare imidazolium-2-carboxylates (122) and (benz)imidaz-olium hydrogen carbonates (123) have been evaluated for their potential as organic precatalysts for diverse NHC-mediated reactions.Both (122) and (123) proved efficient as NHC sources and have been shown to catalyse benzoin condensation as well as transesterification and cyanosilylation reactions. Interestingly, the catalytic activity of (122) has been found to be higher than for (123). 

Stereoselective formation of carbon carbon and carbon heteroatom bonds remains an important goal in synthetic chemistry. Very recently lanthanide alkoxides were successfully utilized in enantioselective C-C bond forming reactions. Catalysis of aldol, cyanosilylation, nitroaldol and Michael reactions has been ascribed to the basic character of lanthanide alkoxides [158, 250, 251]. Ln3(OfBu)9 was successfully employed in test runs and subsequently optically active bidentate ligands were used (Fig. 35) [250a]. 

Cyanosilylations of carbon-oxygen and carbon-nitrogen double bonds with cyanosilanes are very important synthetic reactions since the products, cyanohydrin silyl ethers and a-amino nitriles, serve as synthetic intermediates for a variety of natural products. A number of studies on these subjects have been reported in the last decade however, this review does not deal with carbonyl and imine cyanosilylations due to the availability of recent reviews and limited... 

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. 

Nitrites. Allylic cyanides are formed from the acetates or carbonates in (PhsPjaPd-catalyzed transformations. Intramolecular cyanosilylation of alkynes provides a way of controlled functionalization of such compounds using a tether tac- a-Cyano selenides are products from SnCU-catalyzed reaction of diselenoacetals with MeiSiCN. Seleno-ortho esters also exchange one seleno group for the cyanide. ... 

Polymer-supported, carbon dioxide-protected NHCs were also prepared for catalysis reactions [150]. These supports were prepared using DMN-H6 with 3-(bicyclo[2.2.1]hept-2-ene-5-ylmethyl)-l-(2-propyl)-3,4,5,6-tetrahydropyrimidin-l-ium-2-carboxylate and a Schrock catalyst. They were later used for the trimerization reaction of isocyanates and for the cyanosilylation of carbonyl-containing compounds. Various metals could be immobilized through the NHC hgands including rhodium(I), iridium(l), and palladium(II). The resulting monoUths polymerized phenylacetylene and were successful catalysts for Heck-type couplings [150]. 

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