A 13C n.m.r. study of 5-cyano-, 5-methoxycarbonyl-, 5-carbamoyl-, and 5-acetyl-3-nitro-2-X-thiophenes: substituent effects and their relation to the charge distribution in corresponding 2,2-dimethoxy Meisenheimer adducts

Abstract

A ¹³C n.m.r. study in (CD₃)₂SO has been carried out on 5-cyano-(5), 5-methoxycarbonyl-(6), 5-carbamoyl-(7), and 5-acetyl-3-nitro-2-X-thiophenes (8) in order to investigate the 2-X-substituent effect on the C(α) chemical shifts of the different 5-probes. The results obtained show that, unlike the case of the acetyl group, the α-carbon chemical shifts of the cyano, methoxycarbonyl, and carbamoyl groups are not appreciably affected by through-conjugation with the 2-X-substituents, π-polarization being the more important outcome of the substituent effect on the probe group. The analysis of both the C(5) and C(α) chemical-shift variations in the series (5)–(8), by means of a gradual modification of the electron-releasing power of the substituents, reveals a trend which has been interpreted as a useful indicator of the electronic effects in play on the distribution of the π-electron densities in the corresponding Meisenheimer adducts (1)–(4).