Mechanism and Origins of Chemo- and Regioselectivities of Pd-Catalyzed Intermolecular σ‑Bond Exchange between Benzocyclobutenones and Silacyclobutanes: A Computational Study

The
palladium/isocyanide catalyst is able to facilitate the intermolecular
σ-bond exchange between benzocyclobutenones and silacyclobutanes.
This reaction cleaves the C–C bond of the benzocyclobutenones
and the C–Si bond of the silacyclobutanes, providing a unprecedented
access to the eight-membered silacycles with remarkable chemo- and
regioselectivities. We studied the mechanism and origins of the chemo-
and regioselectivities with density functional theory (DFT) calculations.
The reaction proceeds via two sequential oxidative additions (first
with benzocyclobutenone and second with silacyclobutane) and two subsequent
reductive eliminations (first, C–Si bond formation; second,
C–C bond formation). The oxidative addition abilities of the
substrates and the trans effect of silyl group synergistically
control the chemoselectivity toward the heteroexchange. The homoexchange
of benzocyclobutenones is unfavorable because the oxidative addition
ability of benzocyclobutenone is not strong enough to facilely generate
the Pd­(IV) intermediate. For the homoexchange of silacyclobutanes,
the strong trans effect of the silyl group increases
the energy of the Pd­(IV) intermediate, leading to the high overall
barrier of the subsequent reductive elimination step. The regioselectivity
of the C–C bond activation of benzocyclobutenone is controlled
by the interaction between substrate and palladium, the favorable
aryl–palladium interaction directs the catalyst to selectively
cleave the C­(aryl)–C­(carbonyl) bond of benzocyclobutenone.