MECHANISM AND STEREOCHEMISTRY OF ACID-INDUCED RING-OPENING OF OPTICALLY ACTIVE 1,2-PROPENE OXIDES IN THE GAS PHASE

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 compn. of the gaseous mixt. 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 mol. 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 C.beta. center of H-1 R (or H-1 S) (extracomplex path regioselectivity: .alpha./.beta. = 0.72 0.05; intracomplex path regioselectivity: .alpha./.beta. = 0.71 0.05). The regioselectivity of H-1R (or H-1S) is substantially different from that of the 1-Me-oxonia-2-methylcyclopropanes (Me-1R or Me-1S) toward the same nucleophile NuOH (.alpha./.beta. = 4.13 0.35 (Nu = H); 2.28 0.16 (Nu = CH3)). This difference is attributed to a transition structure wherein the C.alpha.-O bond rupture increases from H-1 R (or H-1 S) to Me-1 R (or Me-1 S) 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 soln.