Bu3SnH-mediated pinacol coupling of 1,5- and 1,6-dicarbonyl compounds:: Synthetic and mechanistic studies

A new method is described for the intramolecular pinacol coupling of 1,5- and 1,6-dicarbonyl compounds, employing Bu3SnH as the stoichiometric reductant. The key steps in this pinacol cyclization are the addition of a tin ketyl to a carbonyl group and a subsequent intramolecular S(H)2 reaction. The isolation of 1,3-dioxa-2-stannolanes, along with other product and labeling studies, provides strong support for the proposed homolytic substitution step, which distinguishes the pinacol cyclization from other reductive cyclizations of tin ketyls, all of which proceed through abstraction of hydrogen from Bu3SnH in the final step. An interesting consequence of the S(H)2 pathway is very high cis selectivity in the cyclization of 1,5-dicarbonyl compounds. Mechanistic studies furnish evidence that the steps that precede homolytic substitution, including C-C bond formation, are reversible under the reaction conditions.