Strain-Inducing Positional Alkene Isomerization.

Small, strained ring systems are important pharmacophores in medicinal chemistry and versatile intermediates in organic synthesis. However, the kinetic and thermodynamic instability of many strained organic molecules renders them challenging to prepare. Here, we report a strain-inducing positional alkene isomerization reaction that provides mild and selective access to cyclobutene building blocks from readily obtained cyclobutylidene precursors. This endergonic isomerization relies on the sequential and synergistic action of a decatungstate polyanion photocatalyst and cobaloxime co-catalyst to store potential energy in the form of ring strain. The versatility of the cyclobutene products is demonstrated through diverse subsequent strain-releasing transformations. Mechanistic studies reveal a steric basis for strain-selective product formation.