Abstract
An NMR study of ketones 5β12 was undertaken to gain insight into the low electrophilicity of the carbonyl moiety of butenones 9β11. Initial IR studies on compounds 9β12 indicated that there is relatively strong double bond character (and hence low electrophilicity) in the carbonyl of saturated and unsaturated cyclobutyl ketones. The ^13^C chemical shifts confirm that the carbonyl moiety is highly conjugated with the fused benzene ring in 9, and with the olefinic linkage in 10 and 11. Partial positive charge is distributed away from the carbonyl carbon, which is also expected to lower the electrophilicity of the carbonyl carbon atoms of 9β11. Oneβbond carbonβproton coupling constants (^1^J~CH~) depend directly on carbon hybridization. In the fourβmembered ring ketones 9β12 the experimental values are larger than in cyclobutane, probably as a result of the additional strain of the extra trigonal centers in the ring. A similar trend is seen in the case of the olefinic CH in 10 and 11 (ca 175 Hz), for which the coupling constant is larger than for the corresponding carbon in cyclobutene. ^1^J~CC~ values between the ring carbon atoms of the cyclobutenones are some 20% lower than in the modelsβa bigger difference than in cyclobutanes, again indicative of the increased ring strain. The very low 42.4 Hz coupling between Cβ1 and Cβ2 in 9 might well indicate a measure of bond localization. ^2^J~CC~ and ^3^J~CC~ values are also discussed. Copyright Β© 2003 John Wiley & Sons, Ltd.