Lewis Acid-Catalyzed Selective [2 + 2]-Cycloaddition and Dearomatizing Cascade Reaction of Aryl Alkynes with Acrylates

Combined
Lewis acid, consisting of two or more Lewis acids, sometimes
shows unique catalytic ability, and it may promote reactions which
could not be catalyzed by any of the Lewis acids solely. On the other
hand, the development of efficient methods for the facile synthesis
of cyclobutenes and densely functionalized decalins is an attractive
target for synthetic chemists due to their versatile synthetic utilities
and widespread occurrence in natural products. Herein, we wish to
report an efficient method for the assembly of cyclobutenes and densely
functionalized decalin skeletons through In­(tfacac)3-TMSBr
catalyzed selective [2 + 2]-cycloaddition and dearomatizing cascade
reaction of aryl alkynes with acrylates with high chemo- and stereoselectivity.
The obtained cyclobutene could be easily converted into cyclobutane
as well as synthetically useful 1,4- and 1,5-diketones with high chemo-
and stereoselectivity. On the basis of mechanistic studies, plausible
reaction mechanisms were proposed for both the [2 + 2]-cycloaddition
and the dearomatizing cascade reaction. Finally, the computational
studies of reaction mechanisms were conducted, and the results suggest
that the combined Lewis acid could efficiently promote both reactions.