Carbomagnesiation alkynes

II. CARBOMAGNESIATION REACTIONS OF ALKYNES A. Intramolecular Addition to Simple Alkynes... 

Chromium salts can catalyze the carbomagnesiation reactions of 1,6-enynes with cy-clization (Scheme 12) . This reaction is probably initiated by carbometalation of the alkyne unit. The resultant organomagnesium species 16 undergoes further functionalization upon treatment with various electrophiles (Scheme 12) . ... 

The carbomagnesiation reaction proceeds with a range of organomagnesium compounds (Scheme 30) °. Not only aryl, alkenyl and aUcynyl groups, but also alkyl groups were found to add across a triple bond. The enhanced reactivity of cyano-substituted alkynes is worthwhile, and this may be due to accelerated alkyl transfer from 53 with activation of the cyano group by MgX2 (Scheme 29). 

Chelation is essential for this carbomagnesiation. A control experiment, in which the THP-protected cyanoalkyne 55 is exposed to n-BuMgCl, leads to 90% recovery of unchanged alkyne (Scheme 31). In addition, homopropargyUc alcohol 56 does not undergo carbomagnesiation . 

Hydroxymethylated cyclopropenes, which can be readily prepared by Rh-catalyzed reaction of diazoesters and alkynes, are good snbstrates for nncatalyzed and Cu-catalyzed carbomagnesiation. For example, a range of substitnted cyclopropanes 82 can be synthesized in a regio- and stereoselective fashion by the Cu-catalyzed addition of Grignard reagents to (3-hydroxymethyl)cyclopropenes 81 (Scheme 57) . 

Terminal alkynes are readily deprotonated by Grignard reagents, and no further addition occurs to al-kynylmagnesium halides. In the presence of transition metal complexes of titanium,70 iron,70 rhodium,71 nickel,70 72 palladium70 or copper,73 the carbomagnesiation takes place in moderate yields. The regio- and stereo-selectivity of the additions are variable. In the presence of a copper(I) salt, however, only the syn... 

Scheme 10.90 Scope of the chromium-catalyzed carbomagnesiation of unfunctionalized disubstituted alkynes [73].

Scheme 10.96 Proposed catalytic cycle for the silver-catalyzed regioselective carbomagnesi-ation of alkynes.

Somewhat related to cross-coupling reactions, the carbomagnesiation of terminal alkenes was shown to proceed efficiently in the presence of [(IMes)AgCl] and 1,2-dibromoethane, which was used as an oxidant. The hydroboration of terminal alkynes was also achieved using [(NHC)Ag] catalysts, and with higher regioselectivity compared to copper catalysis conditions. 

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