Radical Cyclizations of Acylgermanes. New Reagent Equivalents of the Carbonyl Radical Acceptor Synthon

An in depth study of the capability of acylgermanes to function as acceptors in radical cyclizations is reported. Radicals add to acylgermanes, and rapid fragmentation of the resulting α-germylalkoxy radicals provides ketones and germyl radicals. The germyl radicals in turn propagate the chain by addition or abstraction, so the reaction occurs by a unimolecular chain transfer (UMCT) process. In contrast, acylsilanes also function as radical acceptors, but they do not participate in UMCT processes because a “radical-Brook” rearrangement intervenes. Cyclizations in 5-exo and 6-exo modes show good to excellent scope, and rate constants for cyclization can be varied over 2 orders of magnitude by changing the germanium substituents. Acyltriarylgermanes are among the best radical acceptors yet identified, and this quality makes them superior reagent equivalents of the carbonyl radical acceptor synthon. Parent cyclizations in the 4-exo and 7-exo modes fail. Attempted 3-exo cyclization results in a 1,2-acyl shift, which can be conducted alone or in tandem with a subsequent cyclization to the rearranged acylgermane. The results provide a foundation for future synthetic applications of radical cyclizations to acylgermanes.