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

✍ Scribed by A. M. Brouwer; L. Bezemer; H. J. C. Jacobs

• Publisher: Elsevier Science
• Year: 2010
• Tongue: English
• Weight: 614 KB
• Volume: 111
• Category: Article
• ISSN: 0165-0513
• DOI: 10.1002/recl.19921110305

✦ Synopsis

Abstract

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 methyl 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.

^1^H 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‐ter‐butyl‐1,3,5‐hexatriene reveal that a tert‐butyl 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).