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 46. Several new 2-acylidene-3-(2,4-dibromophenyl)-5-phenyI-2,3-dihydro-1,3,4-thiadiazoles (11) and thioacylidene analogues (12) are described. The crystal structures of l l a , l l b , l l c , and l l d reveal a hypervalent interaction, ca. 2.45-2.7 A 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 pcssible isomers. Crystals of l l a are triclinic, P1, with cell dimensions a = 12.937(2), b = 13.429(2), c = 13.489(2) A, a = 60.14(1)", P = 74.59 (1)

Carbon-13 chemical shifts of alkene carbons in 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles and related benzothiazoles and -selenazoles, and their relationship to other push-pull alkenes

“…However, competing steric effects can also arise from a substitution. These lead to some angle widening in the push-pull system to accommodate the a substituent, a shortening of the S...O distance, and some variation in C=C bond lengths, all of which will affect the resonance system (6).…”

“…The shifts for C=O, Ca, and CP are virtually the same for l a and lb, but in l c small shifts to lower frequency are noted for both C=O and C a . Changes in bond length within the C=C-C portion of the push-pull systems are likely to be involved (6), resulting in a change in the resonance description of lc. The similar but more marked effect seen in I d can be ascribed to resonance of the ester group and the resulting decreased ability of this group to lower electron density at Ca.…”

“…The main features of the four com ounds studied (6) angles of approximately 165" and 1 00°, respectively, and (c) evidence of delocalization within the C(5)-C=C-C=O structural unit, but with C=O bond lengths close to normal. The energy associated with the "nonbonded" or "closed shell" S...O interaction need be no more than a few kcallmol to account for the overwhelming predominance of the s-cis conformer (6)(7)(8).…”

“…Recently, from spectroscopic and X-ray crystallographic studies, it has been possible to assign (Z) configurations to these compounds and to confirm that they exist in the s-cis conformation (shown) as a result of stabilization through an S...O interaction (6). The main features of the four com ounds studied (6) angles of approximately 165" and 1 00°, respectively, and (c) evidence of delocalization within the C(5)-C=C-C=O structural unit, but with C=O bond lengths close to normal. The energy associated with the "nonbonded" or "closed shell" S...O interaction need be no more than a few kcallmol to account for the overwhelming predominance of the s-cis conformer (6)(7)(8).…”

“…It was of interest to us in that it may indicate any effects of modification resulting from heteroatom "nonbonded" or "closed shell" interactions, which can result in conformational preference (6)(7)(8)21). We now report I3c NMR data for the series of compounds 1 and 2 and for the related benzothiazoles 3 and benzoselenazoles 4 and 5 (7).…”