Conformational analysis of organic carbonyl compounds. Part 8. Conformational properties of aroyl derivatives of thiophene and benzo[b]thiophene studied by X-ray crystallography, nuclear magnetic resonance lanthanide-induced shifts spectroscopy, and MO ab initio calculations

Abstract

An investigation of the conformational properties in the solid state and in solution of 2- and 3-p-anisoyl derivatives of thiophene and of benzo[b]thiophene, compounds (1b) and (2b), and (3) and (4) respectively, is reported. X-Ray analysis of the compounds gave the following crystal data: (1b) space group Pbca, a= 18.479(2), b= 11.450(1), c= 9.874(1)Å, Z= 8, R= 0.0438 for 1 611 observed reflections; (2b) space group Pbca, a= 46.245(25), b= 7.393(4), c= 6.195(3)Å, Z= 8, R= 0.0435 for 1 153 observed reflections; (3) space group P2₁2₁2₁, a= 14.039(2), b= 12.469(2), c= 7.457(1)Å, Z= 4, R= 0.0697 for 1 871 observed reflections; (4) space group P2₁/a, a= 11.138(5), b= 14.698(6), c= 7.764(5)Å, Z= 4, R= 0.0556 for 1 983 observed reflections. In each of the molecules the rings are twisted with respect to the carbonyl plane, the heterocyclic ring is less twisted than the phenyl ring, and this deviation from planarity is particularly evident in the 2-substituted derivatives (1b) and (3). The orientation of the carbonyl group is of the S,O-cis and S,O-trans type in the 2-substituted and 3-substituted compounds; the former derivatives show the presence of attractive interactions between the sulphur and oxygen atoms. The conformational properties in chloroform were studied with the n.m.r. lanthanide-induced shifts (LIS) method (¹H and ¹³C nuclei were employed). The preferred conformation (ca. 70%) found for derivatives (1b) and (3) is of the S,O-cis type, with the thiophene ring nearly coplanar with the carbonyl plane and the phenyl ring twisted. For derivative (2b) the preferred conformation in the equilibrium mixture is the S,O-trans type, yet in derivative (4) this conformation is almost exclusive. The heterocyclic and aryl rings show nearly the same degree of twist relative to the carbonyl plane. The conformational situation of derivatives (1)–(4) is compared with that of the corresponding furan and benzo[b]furan derivatives, and close conformational behaviour is found for the 3-substituted compounds. MO Ab-initio calculations in the minimal STO-3G basis set were also performed on the 2- and 3-benzoyl derivatives of thiophene, chosen as model compounds of those under investigation, and the conformations of minimum energy were located in the potential energy surface. For the 2-substituted compound (1a) the conformation of S,O-cis type was found to be the more stable conformer and a lower degree of twisting was found for the heterocycle than for the phenyl ring, in agreement with the experimental findings relative to compound (1b). For the 3-substituted derivative (2a) the results of calculations showed that the two conformers of the S,O-cis and S,O-trans type differ only slightly in energy content, the former being predicted to be more stable, in contrast with the experimental behaviour found for derivative (2b).