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

Abstract

AbstractExperimental 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 (σI) of the substituent directly attached to the coupled carbon atom is correlated with the 1J(CH) coupling. This is also borne out by the good correlations between 1J(CH) values in the allenes and 1J(CH) values in various other classes of chemical compounds. Theoretically calculated 1J(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 1J(CH) for these types of compounds. In disubstituted allens and ethenes the substituent effects on 1J(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 4J(CH) and 5J(CH) in the allenes and butatrienes. These large values are ascribed to σ‐π interactions. For geometrically equivalent couplings a constant ratio of nJ(CH)/nJ(HH) is found in the ethenes (0.65; n = 3) and in the allenes and butatrienes (0.55; n = 4 and 5, respectively). 1J(CH) and 3J(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.