In Situ Observation of Elusive Dirhodium Carbenes and Studies on the Innate Role of Carboxamidate Ligands in Dirhodium Paddlewheel Complexes: A Combined Experimental and Computational Approach.

Abstract

Carboxamidates as equatorial ligands in dirhodium paddlewheel catalysts are widely believed to increase selectivity at the expense of reactivity. The results of the combined experimental and computational approach described in this paper show that one has to beware of such generalizations. First, 103Rh NMR revealed how strongly primary carboxamidates impact the electronic nature of the rhodium center they are bound to; at the same time, such ligands stabilize donor/acceptor carbenes by engaging their ester carbonyl group into peripheral interligand hydrogen bonding. This array benefits selectivity as well as reactivity if maintained along the entire reaction coordinate of a catalytic cyclopropanation. In settings where the hydrogen bond needs to be distorted for the reaction to proceed, however, it constitutes a significant enthalpic handicap. Representative examples for each scenario were analyzed by DFT; in both cases, the cyclopropanation step rather than carbene formation was found to be turnover-limiting. While this conclusion somehow contradicts the literature, it implied that the direct observation of highly reactive dirhodium carbenes in truly catalytic settings might be possible, even though the intermediates carry olefinic sites amenable to intramolecular cyclopropanation. Such in situ monitoring by NMR is without precedent, yet it was successful with the homoleptic catalyst [Rh2(OPiv)4] as well as with its heteroleptic sibling [Rh2(OPiv)3(acam)] comprising an acetamidate (acam); in the latter case, the carbene bound to the rhodium atom at the [O3N]-face was observed, which concurs with the computational data that this species is stabilized by the forecited interligand hydrogen bonding.