Highly Substituted 1,3‐Dienes, IV. An Experimental and Theoretical Study of the Stereochemical Properties of 2‐<i>tert</i>‐Butyl‐1,3‐butadiene and 2,3‐Di‐<i>tert</i>‐butyl‐1,3‐butadiene

Abstract

The molecular structure and conformation of 2‐ tert ‐butyl‐1,3‐butadiene ( 1 ) and of 2,3‐di‐ tert ‐butyl‐1,3‐butadiene ( 2 ) have been studied by the gas electron diffraction method. The monosubstituted compound is observed to have a gauche orientation (φ = 32°) of the conjugated C=C bonds. The forces determining the preferred conformation of this molecule ( syn or gauche ) appear to be similar to those operative in the case of the high‐energy conformer of unsubstituted 1,3‐butadiene. The disubstituted compound has an approximately perpendicular orientation of the C=C–C=C system (φ = 101.5°), and in this case the preferred orientation is described as a result of minimalization of the nonbonded interactions. – Theoretical calculations of the structure and conformation of the two molecules have been carried out by molecular mechanics (MM3) and by ab initio (HF/3–21G) methods. The results from both sets of calculations are in good agreement with the observed structures of the two compounds.