Substituent Effects on the Structure of Spiropentane
In order to investigate the influence of substituents on the bond lengths of the spiropentane framework, the molecular structures of the spiropentane derivates 1–7 have been determined by X-ray analysis. The π acceptor substituents (CO, CO2R, Ph) shorten the distal bond of the substituted three-membered ring by δ = -0.02 Å. The vicinal bonds are lengthened by δ/2. The π donor-s̀ acceptor groups have the opposite effect [δ(Cl2) = 0.019, δ(F2) = 0.056 Å]. The influence of several substituents is additive. The longer peripheral bond [1.530(14) Å] is varied to the same degree as the shorter central bond [1.482(9) Å]. The second three-membered ring of the spiropentane system is not affected. These results are explained qualitatively by considering the valence MO′s of a distorted spiropentane. Quantum chemical calculations using a 3–21 G basis set confirm this picture.