Strain-Release-Driven [4 + 1] Annulation: Leveraging Bicyclo[1.1.0]butanes as C1 Synthons via Rh(III) Carbenoid

In this paper, we describe a Cp*Rh(III)-catalyzed C–H
activation/[4
+ 1] annulation of sulfoxonium ylides and N-carbamoylindoles
utilizing BCBs as C1 synthons for ring construction via a strain-release
protocol. Through Cp*Rh(III)-catalyzed C–H activation, an organometallic
precursor is formed, which is readily tied with strained bicyclobutanes
(BCBs) acting like carbene species to achieve 1,1-difunctionalization
of BCBs. This protocol reveals a rare transformation pathway for BCBs
in transition metal catalysis and shows the potential of BCBs as C1
synthons to facilitate an unconventional reaction process. This reaction
is characterized by a broad substrate scope, ample functional group
tolerance, scalability, and versatile synthetic transformations. Furthermore,
DFT calculations indicate that the intermediates of rhodium carbenoid
and metallocyclobutane species were involved in the reaction process.
Notably, the heterocyclic compound 5q exhibits promising
anticancer activity, highlighting the value of this methodology.