Rearrangement of metal-stabilised carbocations containing cyclopropyl and other alkyl groups to give cyano- and isocyano-complexes: reactions of Group VI metal isocyano-complexes with nucleophiles

Abstract

The preparation of a series of cyclopropylcarbene complexes [(CO)₅Cr{C(X)C₃H₅}](X = OH, OMe, or NH₂) is reported. The spectroscopic properties (particularly proton n.m.r.) of these complexes show no evidence of stabilisation of the carbocationic site by the cyclopropyl group. Formation of acetoxy-carbene complexes in the reaction between the salts [(CO)₅Cr{C(O–NMe₄⁺)Y}](Y = CH₂SiMe₃ or C₃H₅) and acetyl chloride is inferred from their further reaction with hydrogen azide which gives rise to [(YNC)Cr(CO)₅] and [(YCN)Cr(CO)₅]. The mechanism of the reaction is discussed and it is suggested that migration of the group Y from the carbon to the nitrogen atom in a common intermediate [(CO)₅MC(N)Y] will occur most readily when there is little or no stabilisation of the carbocation, through a vertical process, by the group Y. The complex [(cyclopropyl isocyanide)Cr(CO)₅] has been fully characterised. The preparation and characterisation of [(acetyl isocyanide)Cr(CO)₅] is described and the presence of extensive conjugation between the isocyano- and the carbonyl group is established. The acetyl isocyanide ligand is rapidly solvolysed by methanol giving [(HNC)Cr(CO)₅] and methyl acetate. When the complex [(Me₃CNC)Cr(CO)₅] is treated with n-butyl-lithium and subsequently alkylated the sole product is the complex cis-[(CO)₄(Me₃CNC)Cr{C(OEt)Bu}] resulting from nucleophilic attack at a carbonyl rather than the isocyano-ligand.