σ‐Bond Stretching: A Static Approach for a Dynamic Process

Ring stretching: The reaction profile corresponding to the inversion of bicyclo[1.1.0]butane derivatives, including the cyclobutane-1,3-diyl transition state, has been modeled by the preparation of a series of boron–phosphorus-containing analogues (see picture: P red; B blue). These differ only in the substituents around the B2P2 core, yet a variation of 40 % in the boron–boron separation is observed.