Transmission of substituent effects through furan and thiophen rings. Rates of reaction of substituted 2-benzoyl-furans and -thiophens with sodium borohydride

Abstract

Rates of reduction by sodium borohydride in propan-2-ol have been measured for the 2-furyl and 2-thienyl ketones 2-(XC₆H₄CO)C₄H₂YE (E = O or S) for a range of X groups with Y = H, and of Y groups with X = H. The results permit direct comparisons to be made of transmission of substituent effects through the various aromatic rings to the same reaction centre, and it is concluded that the substituent effects are transmitted to virtually identical extents through the two heterocyclic rings, but markedly less effectively through the benzene ring. Charge distributions have been calculated by an ab initio method for furan, thiophen, and benzene and for the carbanions, 2-furylCH₂^–, 2-thienylCH₂^–, and PhCH₂^–. The change in the π-electron density at the 5-position on introduction of the 2-CH₂^– groups is smaller for furan than for thiophen, and the change in the latter case is slightly smaller than that induced at the 4-position on introduction of the 1-CH₂^– group for benzene, suggesting that transmission of conjugative effects between p-positions in the benzene ring is greater than that between the 5- and 2-positions in the heterocyclic compounds. The corresponding changes in total electron density, however, are effectively the same for all three rings. The electron density on the CH₂^– group is virtually identical in all three carbanions, suggesting that, contrary to the usual assumption, the π-systems of furan and thiophen rings do not interact more readily than those of benzene with a substituent capable of conjugation. Some of the data have been used to derive σ_5,2 and σ_4,2 constants for thiophen and furan systems.