Transition metal carbene chemistry. 3:Nucleophilic attachment of 4-nitrophenyl acetonitrile and benzoyl nitromethane anions to Fischer carbene complexes^†

Abstract

Rate constants for the nucleophilic attachment of 4-NO₂- PhCH(CN)^- and PhCOCHNO₂^- to Fischer carbene complexes of<i> </i>the type (CO)₅M=C(SMe)C₆H₅ with M = Cr and W in 50%

MeCN-50% water (<i>v/v</i>)<i> </i>at 25^oC are reported. The acetonitrile anions PhCH(CN)^-show much higher reactivity towards these carbene complexes than OH^-, primary aliphatic amines (e.g., <i>n</i>- butylamine) or secondary alicyclic amines (e.g., piperidine) but are less reactive than thiolate ions (e.g .,HOCH₂CH₂S^-) and CH(CN)₂^-. The carbanion PhCOCHNO₂^- is more reactive than OH^-but less reactive than the above-mentioned ions. The reactivity of carbanions towards a particular carbene complex follows the order: CH(CN)₂^- > 4-NO₂-PhCH(CN)^- > PhCOCHNO₂^- for both the carbene complexes. The possible explanation for it comes from the delocalization of negative charge on the carbanion which follows the reverse order. The higher reactivity of <b>1-W</b> over <b>1-Cr</b> for all the carbanions fits a pattern observed previously and arises mainly clue to lowering of intrinsic barrier of the tungsten carbene complex than corresponding chromium one.