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

Abstract

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 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 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−CC 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.

Function-oriented investigations of a peptide-based catalyst that mediates enantioselective allylic alcohol epoxidation.

“…Henbest and others have demonstrated that hydroxyl-groups are capable of directing peracids with stereoselectivity by themselves. 5,25 The influence of the Henbest-type mechanism in the present case is unclear, but catalyst 6 (or its analogues) may adopt conformations that favor this mode of epoxidation (Figure 4, A ). It is also possible that the peptide directs the epoxidation exclusively through other interactions (C) or using a combination of these interactions (B).…”

Regio- and Diastereoselective Catalytic Epoxidation of Acyclic Allylic Alcohols with Methyltrioxorhenium: A Mechanistic Comparison with Metal (Peroxy and Peroxo Complexes) and Nonmetal (Peracids and Dioxirane) Oxidants

“…Both m -CPBA and Ti-β/H 2 O 2 associate with the substrate by hydrogen bonding with an optimal dihedral angle (O−C−CC) of ca. 120°, , while DMD prefers ≥130° 7g. Unfortunately, the methyl derivatives 1b − i do not allow a differentiation between these two dihedral arrangements for the rhenium oxidants; presumably the steric interactions are not sufficiently severe.…”

Analysis of the nitrile and methyl torsional vibrations of n‐propyl cyanide in its S0 electronic state

“…Additionally, the energy values are calculated at each grid point at the MP4(SDQ)/aug‐cc‐pVTZ//MP2/aug‐cc‐pVTZ level 15, in short, MP4//MP2‐aug‐cc‐pVTZ. MP4//MP2 calculations have been carried out in previous works, finding a good agreement with experimental data such as, enthalpies and entropies 17, energy barriers 17–19, and activation energy data 20. On the other hand, to obtain the VAC contribution, harmonic frequencies at each grid point are computed at the MP2/aug‐cc‐pVTZ level.…”