A Combination of Computational and Experimental Studies to Correlate Electronic Structure and Reactivity of Donor–Acceptor Singlet Carbenes

Abstract

Most of the reactivities of donor-acceptor (D-A) singlet carbenes are similar to metal carbenoids. However, the lone pair at the carbenoid carbon, coordinated with metal, is free in D-A carbene thereby making it nucleophilic as well. Herein, DFT optimized structural features of D-A carbene has been investigated and is compared with rhodium-carbenoid. It was observed that, when a D-A carbene reacts with cyclic-1,3-diones in different ethereal solvents, it is the lone pair at the sp2 orbital of the carbene abstracts the proton from the enol form (of the cyclic-1, 3-diones) to form a benzylic carbocation and an enolate. Subsequently the carbocation undergoes nucleophilic attack by O of the ether solvents and then by the enolate to afford the desired ether-linked products. Accordingly, herein the reaction in THF, which otherwise had failed to work as a substrate in reported amino-etherification reactions, worked well. DFT calculated orbital energy levels and reaction profile support this reverse reactivity of singlet carbenes. Furthermore, HOMO-LUMO calculations indicated that electron-rich arenes in D-A carbene stabilises LUMO and destabilises HOMO which increases yield. Additionally, a library of 37 enol-ether and 39 ether-linked compounds of potential medicinal relevance have been synthesized with good to excellent yields using numerous cyclic-1,3-diones.