Structure and Thermal Behaviour of Methylcyclopentadiene Dimers

The mixture of isomeric dimethyl‐endo‐tricyclo[5.2.1.02,6]deca‐3,8‐dienes (A) resulting from Diels‐Alder reactions of 1‐, 2‐, and 5‐methylcyclopenta‐1,3‐dienes (i–iii, respectively) at 20° was shown by GLC analysis to consist of at least 10 components (Table 1). The structures of the six major isomers 1–6, representing 96% of the total mixture, were established by 1H‐ and 13C‐NMR spectroscopy. Whereas on heating up to 110° the proportions of 1, 2, 4, and 6 remain nearly unaffected (±2 %), the dimers 3 and 5, formed in 22 % and 24 % yield, respectively, at 20°, isomerise above 70° reversibly via [3,3]‐sigmatropic rearrangement and equilibrate at 110° to a ca. 10:1 ratio.