The participation of alkynylboronates in inverse electron demand [4 + 2] cycloadditions: a mechanistic study.
The participation of alkynylboronates in [4 + 2] cycloadditions has been investigated using both kinetic and DFT studies. Kinetic studies of the cycloaddition of tetrazine 1 with alkynylboronate 2 strongly suggest that a concerted cycloaddition mechanism is in operation. This mechanism has been confirmed by DFT calculations; moreover, a highly synchronous transition state appears to operate in this process. The experimentally observed poor reactivity of electron-rich dienes with alkynylboronates has also been confirmed by theoretical studies by analyzing the transition states of the cycloadditions with bis-2,5-trimethylsilyloxyfuran. The surprising conclusion has been made that alkynylboronates are relatively electron rich and have a cycloaddition reactivity that resembles that of acetylene. In contrast, the related dichloroalkynylborane cycloaddition reactivity resembles that of dimethylacetylene dicarboxylate.
Gomez-Bengoa
Enrique
E
Departamento de Química Orgánica I, Facultad de Química, Universidad del País Vasco, San Sebastian, Spain. enrique.gomez@ehu.es
Helm
Matthew D
MD
Plant
Andrew
A
Harrity
Joseph P A
JP