13C, 13C coupling constants and 13C chemical shifts of aromatic carbonyl compounds. Effects of ortho‐ and peri‐interactions involving the carbonyl substituent

Abstract

Carbon‐13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives ¹³C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non‐hindered molecules and molecules in which the carbonyl substituent is subject to ortho‐ or peri‐interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range ¹³C,¹³C coupling constants involving the carbonyl carbon, in particular ²J and ³J, on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. ²J(s‐t)>²J(s‐c) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s‐t and s‐c refer to the orientation of the CO group relative to the aromatic bond in question with respect to the connecting single bond). For ketones ³J(t,s‐c)<³J(t,s‐t), and both of these ³J(t) values decrease with increasing angle of twist, θ, about the single bond, whereas ³J(c,s‐c) increases with θ. For acids and acid derivatives no similar regularity was found. (The initial t and c refer to the geometry of the three‐bond coupling path). Generally it is found that ³J(t)>³J(c) and ³J(t)>²J, confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.