Steric Influence on Reactions of Benzyl Potassium Species with CO.

Reactions of benzyl potassium species with CO are shown to proceed via transient carbene-like intermediates that can undergo either dimerization or further CO propagation. In a sterically unhindered case, formal dimerization of the carbene is the dominant reaction pathway, as evidenced by the isolation of ((Ph3 SiO)(PhCH2 )C)2 2 and PhCH2 C(O)CH(OH)CH2 Ph 3. Reactions with increasingly sterically encumbered reagents show competitive reaction pathways involving intermolecular dimerization leading to species analogous to 2 and 3 and those containing newly-formed five-membered rings tBu2 C6 H2 (C(OSiR3 )C(OSiR3 )CH2 ) (R=Me 6, Ph 7). Even further encumbered reagents proceed to either dimerize or react with additional CO to give a ketene-like intermediates, thus affording a 7-membered tropolone derivative 14 or the dione (3,5-tBu2 C6 H3 )3 C6 H2 CH2 C(O))2 15.