Regiospecific hydride abstraction from metallacycles: conversion of metallacyclopentanes to cationic π-allylic complexes

Abstract

The rhoda- and irida-cyclopentane complexes [[graphic omitted]H₂)(η⁵-C₅Me₅)(PPh₃)][M = Rh (1) or Ir (2)] react with the trityl cation [CPh₃]⁺ to give the n³-1-methylallyl derivatives [M(η³-CH₂CHCHMe)(η⁵-C₅Me₅)(PPh₃)][BF₄][M = Rh (3) or Ir (4)]. Deuterium-labelling studies show that in these cases as well as in the previously reported palladacyclopentane →(η³-1-methylallyl) palladium complex transformations, the trityl cation abstracts regiospecifically one of the β-hydrogen atoms of the metallacyclic moiety. The involvement of a σ-3-butenyl intermediate which rearranges to a η³-1-methylallyl derivative is confirmed by reacting the palladium and rhodium dihalides, [PdCl₂(Ph₂PCH₂CH₂PPh₂)] and [Rh(η⁵-C₅Me₅)(PPh₃)I₂], with 3-butenylmagnesium bromide. In the case of palladium a σ-3-butenyl complex is obtained which, by reacting with AgBF₄, gives the η³-1-methylallyl derivative [Pd(η³-CH₂CHCHMe)(Ph₂PCH₂CH₂PPh₂)][BF₄]. In the case of rhodium the PPh₃ ligand is lost and the η³-1-methylallyl compound [Rh(η³-CH₂CHCHMe)(η⁵-C₅Me₅)I] is obtained directly. By reacting [Rh(η⁵-C₅Me₅)(PPh₃)I₂] with 3-pentenylmagnesium bromide, the η³-1,3-dimethylallyl complex [Rh(η³-MeCHCHCHMe)(η⁵-C₅Me₅)(PPh₃)][BF₄] is obtained. Mechanistic implications are discussed along with the significance of the reactions studied in connection with the role of transition-metal metallacyclopentane derivatives in organometallic chemistry and in catalysis.