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