Conformational analysis of organic carbonyl compounds. Part 7. Conformational properties of aroyl derivatives of furan and benzo [b] furan studied by X-ray crystallography, n.m.r. lanthanide-induced shifts, and ab-initio MO calculations

Abstract

The conformational properties of 2- and 3-p-anisoyl derivatives of furan, compounds (1b) and (2b), and of benzo [b] furan, compounds (3) and (4), were studied in the solid state and in solution. X-Ray analysis was performed and the crystal data are: (1b) space group P2₁2₁2₁, a= 11.001(3), b= 22.266(5), c= 4.034(1)Å, Z= 4, R= 0.0387 for 1 504 observed reflections; (2b) space group P2₁2₁2₁, a= 24.526(3), b= 10.094(1), c= 4.058(1)Å, Z= 4, R= 0.0478 for 1 393 observed reflections; (3) space group P , a= 11.236(4), b= 9.110(3), c= 6.302(2)Å, α= 76.59(2), β= 100.52(2), γ= 88.89(1)°, Z= 2, R= 0.0426 for 1 679 observed reflections; (4) space group P2₁, a= 12.588(2), b= 10.141(2), c= 4.885(1)Å, β= 95.83(2)°, Z= 2, R= 0.0433 for 1 097 observed reflections. In all the molecules examined the rings are distorted with respect to the carbonyl plane; the angle of twist in the 2-substituted compounds is higher for the p-anisoyl than for the heterocyclic ring while an almost equal degree of twist is observed in those 3-substituted. The orientation of the carbonyl and heterocyclic oxygens is of the O,O-trans type except in derivative (3), where packing forces provide a probable explanation for the adoption of the more polar O,O-cis conformation. The conformational properties in solution were studied with the n.m.r. lanthanide-induced shifts (LIS) method (¹H and ¹³C nuclei were employed). In compounds (1b) and (3) a chelate complex is formed with the lanthanide atom and conformational information is thus not representative of these molecules; nevertheless the n.m.r. chemical shifts indicate that a situation close to that of the corresponding aldehydes, with an equilibrium between the O,O-cis and O,O-trans forms, should be present. For compounds (2b) and (4) the O,O-trans form is the preferred and the exclusive conformation in solution, while the rings are distorted from the carbonyl plane. MO ab-initio calculations in the minimal STO-3G basis set were also performed on the 2- and 3-benzoyl derivatives of furan, chosen as model compounds, and the conformations of minimum energy were located on the potential energy surface. The structural characteristics of these conformations were compared with those derived experimentally for compounds (1b) and (2b).