MECHANISTIC INVESTIGATION ON THE REACTIVITY OF THE SILICON SILICON DOUBLE-BOND WITH EPOXIDES - REACTION OF TETRAMESITYLDISILENE WITH CIS-STILBENE AND TRANS-STILBENE OXIDES

Starting from either cis- or trans-stilbene oxide, the products of reaction with tetramesityldisilene, 1, in benzene at 100-degrees-C were (a) an E/Z mixture of the disilyl enol ethers, 1,2-diphenyl-1-[(1,1,2,2-tetramesityldisilanyl)oxy]ethylene, 4, (b) the five-membered ring, 1,1,2,2-tetramesityl-trans-4,5-diphenyl-3-oxa-1,2-disilacyclopentane, 5, and (c) the products of deoxygenation, stilbene and 1,1,2,2-tetramesityl-3-oxa-1,2-disilacyclopropane, 3. From the cis-epoxide the ratio of 4:5:stilbene was 5.25 (E:Z, 21:1):3.28:1(c:t, 1.6:1), and from trans-stilbene oxide, 1.12 (E:Z, 5.9:1):1:1.25 (E only). A mechanism is proposed for formation of 4 and 5 involving a ring-opened zwitterionic intermediate, allowing carbon-carbon bond rotation and the observed stereoselectivity. Two pathways are considered for deoxygenation. The stereochemistries of Z4 and 5 were determined by X-ray crystallography. Both crystallized in the triclinic space group P1BAR with Z = 2. Lattice constants for Z4 were a = 11.3064(14) angstrom, b = 12.4175(8) angstrom, c = 14.9490(11) angstrom, alpha = 88.477(6)-degrees, beta = 79.070(6)-degrees, and gamma = 79.470(10)-degrees, and for 5, a = 12.0524(12) angstrom, b = 12.0670(9) angstrom, c = 15.5685(9) ansgtrom, alpha = 86.663(9)-degrees, beta = 70.44(4)-degrees, and gamma = 73.255(9)-degrees.