2-Acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles and related compounds: a question of hypervalent S … O interactions

Abstract

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 new 2-acylidene-3-(2,4-dibromophenyl)-5-phenyl-2,3-dihydro-1,3,4-thiadiazoles (11) and thioacylidene analogues (12) are described. The crystal structures of 11a, 11b, 11c, and 11d reveal a hypervalent interaction, ca. 2.45–2.7 Å long, between the sulfur and carbonyl oxygen atoms. The dibromophenyl ring is nearly perpendicular to the thiadiazole ring plane in the crystal structures, and NMR data suggest that this conformation is maintained in solution. Molecular mechanics calculations show that the S … O interaction need only be a few kilocalories in order to stabilize the observed acylidene side chain configuration over other possible isomers. Crystals of 11a are triclinic, [Formula: see text] with cell dimensions a = 12.937(2), b = 13.429(2), c = 13.489(2) Å, α = 60.14(1)°, β = 74.59(1)°, γ = 58.70(1)°, Z = 4, and R = 0.044 for 3287 observed reflections. Crystals of 11b are monoclinic, P21/c, with cell dimensions a = 10.512(1), b = 12.084(2), c = 16.268(4) Å, β = 96.91(1)°, Z = 4, R = 0.050 for 1938 observed reflections. Crystals of 11c are monoclinic, P21/c, with cell dimensions a = 17.492(4), b = 16.979(1), c = 14.962(1) Å, β = 106.46(1)°, Z = 8, R = 0.057 for 3112 observed reflections. Crystals of 11d are monoclinic, P21/c, with cell dimensions a = 11.749(1), b = 8.533(1), c = 22.670(4) Å, β = 100.17(1)°, Z = 4, and R = 0.059 for 2265 observed reflections.