The Methylene-Transfer Reaction: Synthetic and Mechanistic Aspects of a Unique C−C Coupling and C−C Bond Activation Sequence

Abstract

Oxidative addition of aryl iodides ArI (Ar = (a) C6H5, (b) C6H4CF3, (c) C6H3(CF3)2, (d) C6H4CH3, (e) C6H4OCH3), to the PCP-type complex Rh(PPh3)[CH2C6H(CH3)2(CH2PPh2)2] (1), yields the complexes Rh(Ar)[CH2C6H(CH3)2(CH2PPh2)2](I) (2a−e). Compounds 2a−e undergo intramolecular methylene transfer from the bis-chelating ligand to the incoming aryl under mild conditions (room temperature) giving Rh(CH2Ar)[C6H(CH3)2(CH2PPh2)2](I) (3a−e). The methylene transfer, which is a unique sequence of sp2−sp3 C−C bond reductive elimination and sp2−sp3 C−C bond activation, was investigated kinetically (reaction 2a → 3a), yielding the activation parameters ΔH⧧ = 17 ± 3 kcal/mol, ΔS⧧ = −23 ± 4 eu. The rate-determining step of this reaction is the C−C reductive elimination rather than the C−C activation step. X-ray structural analysis of 2a and 3b demonstrates that the Rh atom is located in the center of a square pyramid with the aryl (2a) and the benzyl (3b) trans to the vacant coordination site. Reaction of the complex Rh(CH...