Mechanism and Stereochemistry of General Acid Catalyzed Additions to Bicyclobutane

The electrophilic addition of MeOH across the central bond of 3-X-bicyclobutanecarbonitriles (X = H, Me, Cl) was found to be a general acid catalyzed reaction with Bronsted α = 0.96. The ρ⁺ value of these reactions is -7.1 ± 1.3. These two coefficients are interpreted as an indication of an unbalanced transition state. In the case of X = H and Me, the addition reaction is of syn stereochemistry. This is probably also the stereochemistry in the case of X = Cl. However, the initially formed adduct in this case reacts further to give ketal 3a. In the presence of NaC1O₄, anti addition products are also observed and reaction rates increase linearly with the concentration of the salt. Ab initio calculations (3-21G) show that protonated bicyclobutane has two stable geometries (C_s symmetry), one highly puckered and the other nearly planar. The first structure is more stable by 9.5 kcal/mol It is concluded that with strong nucleophiles, the syn stereochemistry results from a concerted attack by the proton and the nucleophile from the equatorial directions. With weaker nucleophiles, the first step is protonation of the substrate which is followed by nucleophilic attack on the puckered cation. Both attacks take place from an equatorial direction leading therefore ultimately to the observed stereospecificity.