Mechanisms and stereochemistry of acid-induced ring opening of optically active 1,2-propene oxides in the gas phase

Article abstract of DOI:10.1002/chem.19970031223

The acid-induced ring opening of (S)-(-)-1,2-propene oxide (1S) and (R)-(+)-1,2-propene oxide (1R) has been investigated in gaseous CH₄ and CH₃F at 720 torr and in the presence of a nucleophile, NuOH (Nu = H or CH₃). The mechanism of the ring-opening reaction has been assessed by modulating the composition of the gaseous mixture. Two reaction pathways are operative in the gas phase, both proceeding through complete inversion of configuration of the reaction center. A first process is detectable only in the CH₃F/H₂O systems and takes place within a persistent proton-bound complex generated by interaction of the epoxide with the CH₃OH₂/⁺ ion, formed by methylation of H₂O with (CH₃)₂F⁺. Such an intracomplex ring-opening pathway proceeds through proton transfer from the CH₃OH₂/⁺ ion to the epoxide followed by motion of the neutral CH₃OH moiety around the 1-H-oxonia-2-methyl-cyclopropane structure (H-1R or H-1S) (k<10⁸ s^-1) before attacking the ring carbons from the rear. In all the other systems with added CH₃OH, this intracomplex pathway is preceded by a faster 'extracomplex' pathway involving the attack of an external CH₃OH molecule on the proton-bound adduct. The regioselectivity of the intracomplex process is similar to that of the extracomplex pathway. Both are characterized by a slight preference for the Cβ center of H-1 R (or H-1S) (extra-complex path regioselectivity: α/β = 0.72 ± 0.05; intracomplex path regioselectivity: α/β= 0.71 ± 0.05). The regioselectivity of H-1 R (or H-1S) is substantially different from that of the 1-Me-oxonia-2-methyl-cyclopropanes (Me-1 R or Me-1S) toward the same nucleophile NuOH (α/β = 4.13 ± 0.35 (Nu = H); 2.28 ± 0.16 (Nu = CH₃)). This difference is attributed to a transition structure wherein the Cα-O bond rupture increases from H-1 R (or H-1S) to Me-1R (or Me-1S) and in passing from CH₃OH to H₂O. The regio- and stereoselectivity of the gas-phase acid-induced ring opening of 1 S and 1 R are compared with those of related reactions carried out in solution.