The Lithium Effect in Ketenyl Anion Chemistry

Abstract

Ketenyl lithium compounds of type [RC(Li)CO] (with R = Ph2P(E), E = O, S, Se) were found to exhibit lower thermal stabilities than their potassium analogues due to the stronger coordination of the oxygen of the ketene moiety to the harder metal cation, resulting in a more pronounced ynolate character. Using additional ligands allows manipulation of the O-Li interaction, thereby influencing the stability and reactivity of the ketenyl anions. Ketenes are important intermediates in synthetic organic chemistry, offering access to a variety of carbonyl containing compounds.1 In general, ketenes are highly reactive species due to their polarized cumulated double bond.2 Therefore, they are often generated in situ and directly converted to the desired products. Over the years, several synthetic methods to ketenes have been developed, including the Wolff rearrangement of diazoketones,3 photolytic or pyrolytic reactions of suitable precursors,4 or the carbonylation of carbenes or metal carbenes.5 However, many of these methods have drawbacks, such as requiring harsh conditions or unstable starting materials, resulting in low yields and poor selectivity, or having limitations regarding substrate scope. Ketenyl anions, [RC=C=O]−, offer an alternative entry into ketene chemistry. In the past, these anions were synthesized by carbonylation of diazomethanides (e.g. I, Figure 1A) and used without isolation.6,14 Recently, we developed a novel synthetic route to ketenyl anions through a mild PPh3/CO exchange at the ylidic carbon center in α-metalated ylides.7,8 This method enabled the first isolation and structure elucidation of a ketenyl anion, revealing an intermediate bonding situation in the anion between the ketenyl and ynolate form (Figure 1C). Using this synthetic pathway, we isolated the complete series of phosphinoyl-substituted ketenyl anions V,12 the tosyl-ketenyl anion (VI),9 and the cyano-substituted ketenyl anion (VII).10 Furthermore, the Liu group reported the isolation of ketenyl anion IIX using the N2/CO exchange in diazomethanides.11