Reduction of allenes and acetylenes by catalytic and chemical methods: ‘molecular queueing’ effects in competitive hydrogenation

Abstract

Reductions of but-2-ynoic, but-3-ynoic, and buta-2,3-dienoic acid have been studied over the complete range, but with special attention to semi-reduction. Similar results are reported for but-2-yn-1-ol, but-3-yn-1-ol, and buta-2,3-dien-1-ol. Heterogeneous catalytic (Lindlar Pd, Pd–BaSO₄, Pt–C, Rh–C, and a Raney-type Ni), homogeneous catalytic [Rh(PPh₃)₃Cl and Pt–SnCl₂ complex], and chemical reductants (Zn–Cu couple, CrSO₄, and di-imide) have been employed for each of the six substrates: selectivities, stereoselectivities, and regioselectivities are reported and compared. Various competitive hydrogenations have been examined. In the case of a but-3-ynoic–buta-2,3-dienoic acid system (Pd catalyst), ‘molecular queueing’ among five unsaturated species is observed.