CRYSTAL-STRUCTURE CORRELATIONS IN THE PHOTOCHEMISTRY OF DIMETHYL 9,10-DIMETHYL-9,10-DIHYDRO-9,10-ETHENOANTHRACENE-11,12-DICARBOXYLATE

The photochemistry of the title 9,10-dimethlydi-benzobarrelene-11,12-diester (1) in solution and in the solid state exhibits unique differences as a function of reaction medium. In acetone the normal di-pi-methane reaction leads to a semibull-valene derivative, dimethyl 4b,8b-dimethy-4b,8b,8c,-8d-tetrahydrodibenzo[a,f]cyclopropa[calpentalene-gc,8d-dicarboxylate (2), while in acetonitrile an additional cyclooctatetraene (COT) photoproduct, dimethyl 5,11-dimethyldibenzo[a,e]cyclooctene-6,12-dicarboxylate (3), is obtained. In the solid state a new pentalene, dimethyl 5, 10-dimethyl-5, 10-dihydro-indeno[2,1-a]indene-5,10-dicarboxylate (4), is formed as the major product. Crystal data for (1), (3) and (4): T = 294 K, Cu Kalpha, lambda = 1.5418 angstrom, C22H20O4, M(r) = 348.40; (1), triclinic, P1BAR, a = 8.695 (1), b = 13.633 (1), c = 8.273 (1) angstrom, a = 98.51 (1), beta = 113.34(1), gamma = 80.97(1)-degrees, V = 885.2(1) angstrom3, Z = 2, D(x) = 1.307 g cm -3, mu = 6.9 cm -1, F(000) = 368, R = 0.057 for 3046 reflections; (3), monoclinic, P2(1)/n, a = 9.124 (2), b = 14.392 (2), c = 14.328 (1) angstrom, beta = 94.30 (1)-degrees, V = 1876.1 (5) angstrom3, Z = 4, D(x) = 1.233 g cm-3, mu = 6.5 cm-1, F(000) = 736, R = 0.048 for 2289 reflections; (4), monoclinic, I2/a, a = 19.020 (6), b = 11.963 (1), c = 17.083 (4) angstrom, beta = 105.15 (1)-degrees, V = 3752 (2) angstrom3, Z = 8 (two independent half-molecules per asymmetric unit), D(x) = 1.233 g cm-3, mu = 6.5 cm-1, F(000) = 1472, R = 0.072 for 1574 reflections. The structure of the solid-state photoproduct (4) provides an important clue in deriving a mechanism for its formation, involving a biradical intermediate with a pentalene-like skeleton. This mechanism also accounts for the formation of COT (3), whose structure is not that expected on the basis of previous mechanistic studies.