CYLIDENE-3,5-DIARYL-2,3-DIHYDRO-1,3,4-THIADIAZOLES AND RELATED-COMPOUNDS - A QUESTION OF HYPERVALENT S...O INTERACTIONS

A new synthesis of 1 by selective deacetylation of 2 is reported. The acylation step implied in earlier syntheses of 1 and related compounds is exemplified by direct acylations of 3 to give 4a and 4b. Several n ew omophenyl)-5-phenyl-2,3-dihydro-1,3,4-thiadiazoles (11) and thioacy lidene analogues (12) are described. The crystal structures of 11a, 11 b, 11c, and 11d reveal a hypervalent interaction, ca. 2.45-2.7 angstro m long, between the sulfur and carbonyl oxygen atoms. The dibromopheny l ring is nearly perpendicular to the thiadiazole ring plane in the cr ystal structures, and NMR data suggest that this conformation is maint ained in solution. Molecular mechanics calculations show that the S... O interaction need only be a few kilocalories in order to stabilize th e observed acylidene side chain configuration over other possible isom ers. Crystals of 11a are triclinic, P1BAR, with cell dimensions a = 12 .937(2), b = 13.429(2), c = 13.489(2) angstrom, alpha = 60.14(1)degree s, beta = 74.59(1)degrees, gamma = 58.70(1)degrees, Z = 4, and R = 0.0 44 for 3287 observed reflections. Crystals of 11b are monoclinic, P2(1 )/c, with cell dimensions a = 10.512(1), b = 12.084(2), c = 16.268(4) angstrom, beta = 96.91(1)degrees, Z = 4, R = 0.050 for 1938 observed r eflections. Crystals of 11c are monoclinic, P2(1)/c, with cell dimensi ons a = 17.492(4), b = 16.979(1), c = 14.962(1) angstrom, beta = 106.4 6(1)degrees, Z = 8, R = 0.057 for 3112 observed reflections. Crystals of 11d are monoclinic, P2(1)/c, with cell dimensions a = 11.749(1), b = 8.533(1), c = 22.670(4) angstrom, beta = 100.17(1)degrees, Z = 4, an d R = 0.059 for 2265 observed reflections.