1,1-DIMETHYLSILA-, -GERMA-, AND -STANNACYCLOHEPTATRIENES FULLY ANNELATED WITH BICYCLE [2.2.2]OCTENE - SYNTHESES, STRUCTURES, AND PROPERTIES

摘要

The titled compounds, i.e., a series of 1,1-dimethyl-2,3:4,5:6,7-tris(bicyclo[2.2.2]octeno)metallacyclo- heptatrienes (metallepins) containing silicon (2a), germanium (2b), and tin (2c), were synthesized by dilithiation of the terminal dibromide of the linear trimer of bicyclo[2.2.2]octene (3) followed by the reaction with (CH3)(2)MCl(2) (M = Si, Ge, Sn). The X-ray structure determination was conducted for the first time for these metallepins containing Group 14 elements and indicated that the central seven-membered ring is in a boat-form for all of 2a-c. As the effective size of the Group 14 metal atom increases, the dihedral angle between the base plane and the stern part of the boat form was found to increase, i.e., the boat structure becomes more deeply folded, for release of the inner angle strain. Variable-temperature NMR measurements indicated that the metallepin ring of 2 is rapidly inverting in solution, and the line-shape analyses gave the activation parameters for ring-inversion at 25 degrees C as follows: 2a, Delta H double dagger = 13.4 +/- 1.3 kcal mol(-1), Delta S double dagger = 3 +/- 5 cal K-1 mol(-1) Delta G double dagger = 12.4 +/- 0.2 kcal mol(-1); 2b, Delta H double dagger = 16.0 +/- 1.8 kcal mol(-1), Delta S double dagger = 7 +/- 6 cal K-1 mol(-1), Delta G double dagger = 14.0 +/- 0.1 kcal mol(-1); 2c, Delta H double dagger = 21.3 +/- 3.1 kcal mol(-1), Delta S double dagger = 10 +/- 9 cal K-1 mol(-1), Delta G double dagger = 18.4 +/- 0.5 kcal mol(-1). Thus, the energy barrier for ring inversion increases as the carbon-metal bond is elongated, reflecting the increasing instability of the planar transition-state structure due to the inner-angle strain. The larger energy barrier for inversion of 2a than that of the corresponding cycloheptatriene 6 implies that the cyclic (p-d)pi delocalization would not be operating in a planar transition-state structure or negligibly small if present. The UV absorption of 2a, 2b, and 2c (lambda(max) in n-hexane. 276, 275, and 268 nm, respectively) did not show much difference from that of the C-unsubstituted metallepins 1a, 1b, and 1c in spite of annelation with three bicyclo[2.2.2]octene units, probably because of the more folded boat structure of the central seven-membered ring and less pi-conjugation in 2a-e than in 1a-c. [References: 38]