Competition between peroxy acid oxygens as hydrogen bond acceptors in B3LYP transition structures for epoxidations of allylic alcohols with peroxyformic acid

The transition structures (TSs) for the epoxidation of 8-propen-1-ol and 2- cyclobuten-1-ol with peroxyformic acid have been located with the B3LYP met hod using three basis sets (i.e., 6-31G*, 6-311G**, and 6-311+G**). Syn att acks on these alcohols by peroxy acid lead to syn TSs in which hydrogen bon ding is operative. The allylic OH group always acts as hydrogen-bond donor while either the carbonyl oxygen or the peroxo oxygens of the peroxy acid c an play the role of hydrogen-bond accepters. In the case of propenol, the t wo syn TSs (O-C-C=C dihedral angles: 135.0 degrees and 16.0 degrees, respec tively) with the peroxo oxygens involved in hydrogen bonding have free enth alpies comparable with those of their counterparts (O-C-C=C dihedral angles : 134.0 degrees and 16.3 degrees, respectively) with hydrogen bonding to ca rbonyl oxygen. Basis set extension as well as electrostatic solvation effec ts favor the former TSs over the latter ones. In the case of cyclobutenol t he syn TS (O-C-C=C dihedral angle = 124.2 degrees) with hydrogen bonding to peroxo oxygens is significantly more stable (by greater than or equal to 1 .8 kcal/mol(-1)) than the corresponding TS (O-C-C=C dihedral angle = 128.0 degrees) with carbonyl oxygen involved in hydrogen bonding. Anti TSs are in herently less stable than syn TSs but this difference is strongly reduced i n solution. The Sharpless qualitative TS model for peroxy acid epoxidation of allyic alcohols is discussed in the light of computational data.