Rhodium(I)-catalyzed carbonyl allenylation versus propargylation via redox transmetalation across tetragonal tin(II) oxide.

[reaction: see text] A reagent combination of beta-SnO and catalytic [Rh(COD)Cl](2) in THF-H(2)O promotes the reaction of propargyl bromides and aldehydes and directs the regioselectivity toward the formation of either allenic alcohols or homopropargylic alcohols. This highly regioselective either/or transformation proceeds via a transmetalation from rhodium to tin, in which metallotropic rearrangement between a propargylmetal and allenylmetal is arrested.