ON THE ORIGIN OF SUBSTRATE DIRECTING EFFECTS IN THE EPOXIDATION OF ALLYL ALCOHOLS WITH PEROXYFORMIC ACID

The reactant cluster and transition state for epoxidation of allyl alc ohol with peroxyformic acid have been located at the MP2/6-31G(d) leve l of theory, The free energy of activation (Delta(278)double dagger = 19.8 kcal/mol) predicted at the MP4 parallel to MP2/6-31G(d) level is quite comparable with experimental data for epoxidation of 3-hydroxycy clohexene (Delta G(278)double dagger = 19.7 kcal/mol), A spiro transit ion state (TS) was found where the plane defined by the peroxyacid moi ety is oriented at 89 degrees to the C-C bond axis. Intrinsic reaction coordinate analysis suggests that after the barrier is crossed a 1,4- hydrogen migration of the peroxyacid hydrogen to the carbonyl oxygen t akes place in concert with O-O bond cleavage affording the epoxide of allyl alcohol hydrogen bonded to the neutral formic acid leaving group , The activation parameters calculated at the B3LYP/6-311G(d,p) level are ill excellent agreement with the MP4 parallel to/MP2 values, The t ransition structure with the allyl alcohol O-C-C=C dihedral angle of 1 6.4 degrees is 2.1 kcal/mol lower in energy than a transition structur e with a dihedral angle of 134.3 degrees. The directing effect of the hydroxyl group is attributed initially to a primary hydrogen bonding i nteraction between the relatively more acidic peroxy acid proton and t he oxygen of the allyl alcohol. In both the reactant complex 1 and the transition structure (TS-2) for oxygen atom transfer the alcohol rema ins hydrogen bonded to the more basic carbonyl oxygen of the peroxyaci d, The G2 proton affinities (PA(298)) of the carbonyl oxygen and the p roximal peroxo oxygen of peroxyformic acid are 177.1 and 153.3 kcal/mo l, respectively.