Rhodium‐Catalysed Coupling of Allylic, Homoallylic, and Bishomoallylic Alcohols with Aldehydes and N‐Tosylimines: Insights into the Mechanism

The isomerisation of alkenols followed by reaction with aldehydes or N‐tosylimines catalysed by rhodium complexes has been studied. The catalytically active rhodium complex is formed in situ from commercially available (cyclooctadiene)rhodium(I) chloride dimer [Rh(COD)Cl]2. The tandem process affords aldol and Mannich‐type products in excellent yields. The key to the success of the coupling reaction is the activation of the catalysts by reaction with postassium tert‐butoxide (t‐BuOK), which promotes a catalytic cycle via alkoxides rather than rhodium hydrides. This mechanism minimises the formation of unwanted by‐products. The mechanism has been studied by 1H NMR spectroscopy and deuterium labelling experiments.