Steric perturbation of the conjugated triene chromophore. Conformational analysis of (E)- and (Z)-3-methyl-1,3,5-hexatriene and (Z)-3-tert-butyl-1,3,5-hexatriene

A theoretical and NMR-spectroscopic conformational analysis is presented of the 3-methyl-1,3,5-hexatrienes and of (Z)-3-tert-butyl-1,3,5-hexatriene. It is shown that (E)-3-methyl-1,3,5-hexatriene exists mainly as the tEt rotamer and (Z)-3-methyl-1,3,5-hexatriene as the tZt rotamer. Indications are found for steric strain involving the Me group of the E isomer. In contrast to earlier theoretical predictions, this strain does not lead to detectable amounts of s-cis rotamers. Ab-initio calculations reported here agree with this experimental finding, but MMP2 calculations underestimate the energy of the s-cis form. 1H NMR relaxation rates and Nuclear Overhauser Enhancements (NOE) are shown to allow a reliable determination of the predominant ground-state conformation of alkyl-substituted hexatrienes. NOE experiments on (Z)-3-tert-butyl-1,3,5-hexatriene reveal that a tert-Bu group changes the preferred conformation from tZt to cZt but, surprisingly, is unable to completely counteract the strong intrinsic preference for a spatially extended s-trans,s-trans conformation. We estimate the rotamer population as cZt/tZt = 80/20 (±5).