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On the Origin of Substrate Directing Effects in the Epoxidation of Allyl Alcohols with Peroxyformic Acid

1998; American Chemical Society; Volume: 120; Issue: 4 Linguagem: Inglês

10.1021/ja9717976

1943-2984

Robert D. Bach, Carlos M. Estévez, Julia Winter, Mikhail N. Glukhovtsev,

Oxidative Organic Chemistry Reactions

The reactant cluster and transition state for epoxidation of allyl alcohol with peroxyformic acid have been located at the MP2/6-31G(d) level of theory. The free energy of activation (Δ = 19.8 kcal/mol) predicted at the MP4//MP2/6-31G(d) level is quite comparable with experimental data for epoxidation of 3-hydroxycyclohexene (Δ = 19.7 kcal/mol). A spiro transition state (TS) was found where the plane defined by the peroxyacid moiety is oriented at 89° to the CC 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 takes 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 in excellent agreement with the MP4//MP2 values. The transition structure with the allyl alcohol O−C−CC dihedral angle of 16.4° is 2.1 kcal/mol lower in energy than a transition structure with a dihedral angle of 134.3°. The directing effect of the hydroxyl group is attributed initially to a primary hydrogen bonding interaction between the relatively more acidic peroxy acid proton and the oxygen of the allyl alcohol. In both the reactant complex 1 and the transition structure (TS-2) for oxygen atom transfer the alcohol remains hydrogen bonded to the more basic carbonyl oxygen of the peroxyacid. The G2 proton affinities (PA298) of the carbonyl oxygen and the proximal peroxo oxygen of peroxyformic acid are 177.1 and 153.3 kcal/mol, respectively.