Preparation and Properties of Centrally Bridgehead-Substituted Hexacyclo[4.4.0.02,1.03,5.04,8.0 7,9]]decanes ("Diademanes") and Related (CH)10 Hydrocarbons

Abstract

6-Trimethylsilyl- (1b), 6-hydroxymethyl- (1e), and 6-methyldiademane (1f) have been prepared by irradiation of the corresponding snoutene derivatives, in 23, 2.8, and 17% yields, respectively, together with the isomeric 1-trimethylsilyl- (10b) and 1-methyldiademane (10f) (8 and 2% yields, respectively). The starting 4-trimethylsilyl- (9b) and 4-(trimethylsilyloxymethyl)-snoutene (9d) were prepared from the correspondingly substituted cyclooctatetraenes 4b and 4c in several steps in 20 and 8% overall yields, respectively. Upon heating, as well as under the conditions of gas-chromatographic separation, diademanes 1b, 10b, 1f, and 10f rearranged into the corresponding C10-and C1-substituted triquinacenes 3b, 3f, 11b, and 11f, respectively. Rough kinetic measurements of these rearrangements indicate some acceleration of the reaction caused by the presence of a methyl substituent and retardation by that of a trimethylsilyl substituent, relative to the parent diademane 1a. At this insufficient precision, however, the activation energies (Ea) of 29.0 and 28.1 kcal mol-1, respectively, are essentially the same as that reported for 1a (28.3 kcal mol-1). An X-ray crystal structure analysis of trimethylsilylsnoutene 9b revealed a significant lengthening of the distal (with respect to the substituent) bond (1.534 versus 1.505 Å) in the unsubstituted cyclopropane ring. In the substituted cyclopropane ring, the two proximal bonds are lengthened (1.530 Å) and the distal bond is slightly shortened 1.492 Å). This indicates a small, but significant electron-withdrawing effect of the trimethylsilyl group in 9b. An X-ray crystal structure analysis of 6-hydroxymethyldiademane 1e showed pronounced alternation of the bond lengths in the six-membered ring, with 1.494(4) between and 1.539(4) Å within the three cyclopropane moieties, in close agreement with computations at different theoretical levels. This structural feature corroborates a predisposition of the tris-σ-homobenzene skeleton of this molecule in the ground state to undergo the facile [σs2 + σs2 + σs2] cycloreversion to the triquinacene skeleton observed for the parent diademane 1a, its derivative 1b and 1f, as well as for other tris-σ-homobenzene derivatives.