NMR studies on derivatives of heteroaromatic compounds: exceptionally small carbon-carbon couplings, 1J(CC), in 2-lithiothiophene, 2-lithio-N-methylpyrrole and 2-lithiofuran

Spin-spin carbon-carbon coupling constants across one bond and carbon proton coupling constants across one, two and three bonds have been measured for a large series of derivatives of five-membered heterocyclic compounds. This included 2-methyl and 2-lithio derivatives of furan, N-methyl pyrrole and thiophene and a series of 2-R-substituted thiophenes where R = O-t-Bu, Cl, Br, I, Si(CH3)3, MgBr and MgTh. For the long-range C-H couplings their signs have been determined in several compounds by the use of modern 2D NMR techniques, and in all the cases they have been found to be positive. A good linear dependence upon electronegativity of the substituent has been observed for 1J(CC), 2J(C2H3), and 3J(C2H4). Very small 1J(CC) couplings of ca. 30 Hz only have been found in all three lithio compounds; they belong to the smallest couplings of this type reported up to now. They are accompanied by very large and positive two-bond couplings, 2J(C2H3) of ca. +20 Hz, which in turn are the largest couplings of this type reported so far. In both cases the changes observed are interpreted in terms of the changes in the Fermi contact contribution.