Preparation and properties of centrally bridgehead-substituted hexacyclo[4.4.0.02,1.03,5.04,8.07,9]decanes (\"diademanes\") and related (CH)10 hydrocarbons.
6-Trimethylsilyl- (I; R = TMS), 6-hydroxymethyl- (I; R = CH2OH), and 6-methyldiademane (I; R = Me) have been prepd. by irradn. of the corresponding snoutene derivs., e.g. II (R = TMS), in 23, 2.8, and 17% yields, resp., together with the isomeric 1-trimethylsilyl- (III; R = TMS) and 1-methyldiademane (III; R = Me) (8 and 2% yields, resp.). The starting 4-trimethylsilyl- (II; R = TMS) and 4-(trimethylsilyloxymethyl)- snoutene (II; R = CH2OTMS) were prepd. from the correspondingly substituted cycloocta-tetraenes in several steps in 20 and 8% overall yields, resp. Upon heating, as well as under the conditions of gas-chromatog. sepn., diademanes I (R = TMS), III (R = TMS), I (R = Me), and III (R = Me) rearranged into the corresponding C10- and C1-substituted triquinacenes, e.g. IV. Rough kinetic measurements of these rearrangements indicate some acceleration of the reaction caused by the presence of a Me substituent and retardation by that of a trimethylsilyl substituent, relative to the parent diademane. At this insufficient precision, however, the activation energies (Ea) of 29.0 and 28.1 kcal mol-1, resp., are essentially the same as that reported for the parent diademane (28.3 kcal mol-1). An X-ray crystal structure anal. of II (R = TMS) revealed a significant lengthening of the distal (with respect to the substituent) bond (1.534 vs. 1.505 .ANG.) in the unsubstituted cyclopropane ring. In the substituted cyclopropane ring, the two proximal bonds are lengthened (1.530 .ANG.) and the distal bond is slightly shortened (1.492 .ANG.). This indicates a small, but significant electron-withdrawing effect of the trimethylsilyl group in II (R = TMS). An X-ray crystal structure anal. of 6-hydroxymethyldiademane (I; R = CH2OH) showed pronounced alternation of the bond lengths in the six-membered ring, with 1.494(4) between and 1.539(4) .ANG. within the three cyclopropane moieties, in close agreement with computations at different theor. levels. This structural feature corroborates a predisposition of the tris-s-homobenzene skeleton of this mol. in the ground state to undergo the facile [s2s + s2s + s2s] cycloreversion to the triquinacene skeleton obsd. for the parent diademane, its deriv. I (R = TMS) and I (R = Me), as well as for other tris-s-homobenzene derivs.
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