Umpolung reactivity of strained C-C σ-bonds without transition-metal catalysis.

Umpolung is an old and important concept in organic chemistry, which significantly expands the chemical space and provides unique structures. While, previous research focused on carbonyls or imine derivatives, the umpolung reactivity of polarized C-C σ-bonds still needs to explore. Herein, we report an umpolung reaction of bicyclo[1.1.0]butanes (BCBs) with electron-deficient alkenes to construct the C(sp)-C(sp) bond at the electrophilic position of C-C σ-bonds in BCBs without any transition-metal catalysis. Specifically, this transformation relies on the strain-release driven bridging σ-bonds in bicyclo[1.1.0]butanes (BCBs), which are emerged as ene components, providing an efficient and straightforward synthesis route of various functionalized cyclobutenes and conjugated dienes, respectively. The synthetic utilities of this protocol are performed by several transformations. Preliminary mechanistic studies including density functional theory (DFT) calculation support the concerted Alder-ene type process of C-C σ-bond cleavage with hydrogen transfer. This work extends the umpolung reaction to C-C σ-bonds and provides high-value structural motifs.