Au-Catalyzed Intermolecular [2+2] Cycloadditions between Chloroalkynes and Unactivated Alkenes

The
[2+2] cycloaddition is a versatile strategy for the synthesis
of strained cyclobutenes of high synthetic value. In this study, two
efficient intermolecular [2+2] cycloadditions between two different
types of chloroalkynes and unactivated alkene are realized with gold
catalysis. Of significance is that the reaction works with challenging
monosubstituted unactivated alkenes, which is unprecedented in gold
catalysis and scarcely documented in other metal-catalyzed/promoted
reactions; moreover, the reaction exhibits excellent regio­selectivities,
which are much better than those reported in literature. With 1,2-disubstituted
unactivated alkenes, the reaction is largely stereospecific. The cyclobutene
products can be prepared in nearly gram scale and readily undergo
further reactions including various cross-coupling reactions using
the C­(sp2)–Cl and/or C­(sp2)–SPh
bond, which in turn substantially broaden the scope of accessible
cyclobutenes and enhance the synthetic utility of this bimolecular
reaction.