title: 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.
abstract: 6-Trimethylsilyl- (1 b), 6-hydroxymethyl- (1 e), and 6-methyldiademane (1 f) have been prepared by irradiation of the corresponding snoutene derivatives, in 23, 2.8, and 17 % yields, respectively, together with the isomeric 1-trimethylsilyl- (10 b) and 1-methyldiademane (10 f) (8 and 2 % yields, respectively). The starting 4-trimethylsilyl- (9 b) and 4-(trimethylsilyloxymethyl)snoutene (9 d) were prepared from the correspondingly substituted cyclooctatetraenes 4 b and 4 c in several steps in 20 and 8 % overall yields, respectively. Upon heating, as well as under the conditions of gas-chromatographic separation, diademanes 1 b, 10 b, 1 f, and 10 f rearranged into the corresponding C10- and C1-substituted triquinacenes 3 b, 3 f, 11 b, and 11 f, 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 1 a. At this insufficient precision, however, the activation energies (E(a)) of 29.0 and 28.1 kcal mol(-1), respectively, are essentially the same as that reported for 1 a (28.3 kcal mol(-1)). An X-ray crystal structure analysis of trimethylsilylsnoutene 9 b revealed a significant lengthening of the distal (with respect to the substituent) bond (1.534 versus 1.505 A) in the unsubstituted cyclopropane ring. In the substituted cyclopropane ring, the two proximal bonds are lengthened (1.530 A) and the distal bond is slightly shortened 1.492 A). This indicates a small, but significant electron-withdrawing effect of the trimethylsilyl group in 9 b. An X-ray crystal structure analysis of 6-hydroxymethyldiademane 1 e showed pronounced alternation of the bond lengths in the six-membered ring, with 1.494(4) between and 1.539(4) A within the three cyclopropane moieties, in close agreement with computations at different theoretical levels. This structural feature corroborates a predisposition of the tris-sigma-homobenzene skeleton of this molecule in the ground state to undergo the facile [sigma(2)(s) + sigma(2)(s) + sigma(2)(s)] cycloreversion to the triquinacene skeleton observed for the parent diademane 1 a, its derivative 1 b and 1 f, as well as for other tris-sigma-homobenzene derivatives.
authors.affiliation: Institut für Organische Chemie, der Georg-August-Universität Göttingen, Tammannstrasse 2, 37077 Göttingen, Germany. armin.demeijere@chemie.uni-goettingen.de
authors.forename: Armin
authors.initials: A
authors.lastname: de Meijere
authors.forename: Chih-Hung
authors.initials: CH
authors.lastname: Lee
authors.forename: Bengt
authors.initials: B
authors.lastname: Bengtson
authors.forename: Ehmke
authors.initials: E
authors.lastname: Pohl
authors.forename: Sergei I
authors.initials: SI
authors.lastname: Kozhushkov
authors.forename: Peter R
authors.initials: PR
authors.lastname: Schreiner
authors.forename: Roland
authors.initials: R
authors.lastname: Boese
authors.forename: Thomas
authors.initials: T
authors.lastname: Haumann