Conformational studies by dynamic nuclear magnetic resonance. Part 30. Exchange kinetics between the rotamers of 2-acyl-furans and -thiophenes

Abstract

Low-temperature ¹³C n.m.r. spectra allowed the observation of two different groups of signals, corresponding to the O,O-anti- and O,O-syn-rotamers, of a number of 2-acylfurans with the following alkyl groups bonded to carbonyl: Me, Et, Prⁱ, Bu^t, pentan-3-yl. The ratios of these rotamers in dimethyl ether were thus obtained, and the barriers for the syn-anti interconversion were found to decrease with increasing bulkiness of the alkyl groups. Although separate ¹³C signals could not be detected for the rotamers of the corresponding 2-acylthiophenes, line-broadening effects nonetheless allowed measurement of the interconversion barriers. Lanthanide-induced shift experiments carried out on the latter derivatives (at both ¹H and ¹³C frequencies) showed that in all the 2-acylthiophenes investigated the S,O-syn rotamers are predominant.