Synthesis and Diels‐Alder Reactivity of 2,3,5,6‐Tetramethylidenenorbornane

Pd‐catalyzed double carbomethoxylation of the Diels‐Alder adduct of cyclo‐pentadiene and maleic anhydride yielded the methyl norbornane‐2,3‐ endo ‐5, 6‐ exo ‐tetracarboxylate ( 4 ) which was transformed in three steps into 2,3,5,6‐tetramethyl‐idenenorbornane ( 1 ). The cycloaddition of tetracyanoethylene (TCNE) to 1 giving the corresponding monoadduct 7 was 364 times faster (toluene, 25°) than the addition of TCNE to 7 yielding the bis‐adduct 9 . Similar reactivity trends were observed for the additions of TCNE to the less reactive 2,3,5,6‐tetramethylidene‐7‐oxanorbornane ( 2 ). The following second order rate constants (toluene, 25°) and activation parameters were obtained for: 1 + TCNE → 7 : k 1 = (255 + 5) 10 −4 mol −1 · s −1 , Δ H ≠ = (12.2 ± 0.5) kcal/mol, Δ S ≠ = (−24.8 ± 1.6) eu.; 7 + TCNE → 9 , k 2 = (0.7 ± 0.02) 10 −4 mol −1 · s −1 , Δ H ≠ = (14.1 ± 1.0) kcal/mol, Δ S ≠ = ( −30 ± 3.5) eu.; 2 + TCNE → 8 : k 1 = (1.5 ± 0.03) 10 −4 mol −1 · s −1 , Δ H ≠ = (14.8 ± 0.7) kcal/mol, Δ S ≠ = (−26.4 ± 2.3) eu.; 8 + TCNE → 10 ; k 2 = (0.004 ± 0.0002) 10 −4 mol −1 · s −1 , Δ H ≠ = (17 ± 1.5) kcal/mol, Δ S ≠ = (−30 ± 4) eu. The possible origins of the relatively large rate ratios k 1 / k 2 are discussed briefly.