Selectivity Control of the Ligand Exchange at Carbon in α-Metallated Ylides as a Route to Ketenyl Anions.

α-Metallated ylides have recently been reported to undergo phosphine by CO exchange at the ylidic carbon atom to form isolable ketenyl anions. Systematic studies on the tosyl-substituted yldiides, R P=C(M)Ts (M=Li, Na, K), now reveal that carbonylation may lead to a competing metal salt (MTs) elimination. This side-reaction can be controlled by the choice of phosphine, metal cation, solvent and co-ligands, thus enabling the selective isolation of the ketenyl anion [Ts-CCO]M (2-M). Complexation of 2-Na by crown ether or cryptand allowed structure elucidation of the first free ketenyl anion [Ts-CCO] , which showed an almost linear Ts-C-C linkage indicative for a pronounced ynolate character. However, DFT studies support a high charge at the ketenyl carbon atom, which is reflected in the selective carbon-centered reactivity. Overall, the present study provides important information on the selectivity control of ketenyl anion formation which will be crucial for future applications.