Carbon–proton coupling constants in allenes, ethenes and butatrienes. Application to conformational analysis of allenyl and vinyl alkyl ethers and thioethers

Abstract

Abstract Experimental carbon–proton coupling constants are reported for 36 monosubstituted allenes, 20 ethenes and 5 butatrienes. The data for the allenes cover a range of substituents in which either the first atom of the substituent (all Group IV–VII elements of the second and third row are covered) and/or the substitution of that atom is varied. The electronegativity (σ I ) of the substituent directly attached to the coupled carbon atom is correlated with the 1 J (CH) coupling. This is also borne out by the good correlations between 1 J (CH) values in the allenes and 1 J (CH) values in various other classes of chemical compounds. Theoretically calculated 1 J (CH) values correctly reflect the substituent effects on the experimental values in the allenes, ethenes and butatrienes spearately, but fail to give a satisfactory description of the differences in 1 J (CH) for these types of compounds. In disubstituted allens and ethenes the substituent effects on 1 J (CH) values are additive. The experimental and calculated values differ by less than 1 Hz. Two‐ and three‐bond carbon–proton couplings are also discussed in terms of electronegativity, substituent and hybridization effects and mutual relationships. Large values, up to 6 Hz, are found for 4 J (CH) and 5 J (CH) in the allenes and butatrienes. These large values are ascribed to σ‐π interactions. For geometrically equivalent couplings a constant ratio of n J (CH)/ n J (HH) is found in the ethenes (0.65; n = 3) and in the allenes and butatrienes (0.55; n = 4 and 5, respectively). 1 J (CH) and 3 J (CH) coupling constants are used for the conformational analyses of vinyl, allenyl and butatrienyl ethers and thioethers. At room temperature the methyl and ethyl ethers are predominantly in an s ‐ cis conformation, whereas the iso‐propyl allenyl ether is a mixture of s ‐ cis and s ‐ trans ; the tert ‐butyl allenyl ether exists mainly in the s ‐ trans conformation. The thioethers are all in the s ‐ cis conformation.