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.