In Crystallo Wolff Rearrangement of a Metalated Diazoester: Structural Confirmation of the Singlet Carbene Wolff-Intermediate

Abstract

The Wolff rearrangement (WR) is widely used for the synthesis of ketenes from diazoketones and -esters. Stepwise WR reactions are proposed to proceed through transient carbonylcarbene (R–C–C(O)–R′) intermediates, which so far have evaded structural characterization. Here, a Wolff metallocarbene (Pt^II–C–C(O)–OEt) is reported as a fleeting intermediate in the photoinitiated fragmentation of a diazoester ligand. Frozen solution and crystal matrix isolation experiments enabled the spectroscopic, magnetic, crystallographic, and computational characterization of this highly reactive species. All methods confirmed a singlet ground state for the WR metallocarbene, which is stabilized by π interactions with the carboxyl substituent, thus complementing computational and transient spectroscopy studies for classic organic WR reactions.