Preparation and Diels‐Alder Reactivity of 2,3,5,6,7,8‐Hexamethylidenebicyclo [2.2.2]octane (‘[2.2.2]Hericene’). Force‐field calculations of exocyclic dienes as a moiety of bicyclic skeletons

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Abstract
The [2.2.2]hericene (6), a bicyclo[2.2.2]octane bearing three exocyclic s‐cis‐butadiene units has been prepared in eight steps from coumalic acid and maleic anhydride. The hexaene 6 adds successively three mol‐equiv. of strong dienophiles such as ethylenetetracarbonitrile (TCE) and dimethyl acetylenedicarboxylate (DMAD) giving the corresponding monoadducts 17 and 20 (k1), bis‐adducts 18 and 21 (k2) and tris‐adducts 19 and 22 (k3), respectively. The rate constant ratio k1/k2 is small as in the case of the cycloadditions of 2,3,5,6‐tetramethylidene‐bicyclo [2.2.2]octane (3) giving the corresponding monoadducts 23 and 27 (k1) and bis‐adducts 25 and 29 (k2) with TCE and DMAD, respectively. Constrastingly, the rate constant ratio k2/k3 is relatively large as the rate constant ratio k1/k2 of the Diels‐Alder additions for 5,6,7,8‐tetramethylidenebicyclo [2.2.2]oct‐2‐ene (4) giving the corresponding monoadducts 24 and 28 (k1) and bis‐adducts 26 and 30 (k2). The following second‐order rate constants (toluene, 25°) and activation parameters were obtained for the TCE additions: 3+TCE→23: k1 = 0.591±0.012 mol−1·l·s−1, ΔH≠=10.6±0.4 kcal/mol, and ΔS≠ = −24.0±1.4 cal/mol·K (e.u.); 23+TCE→25: k2=0.034±0.0010 mol−1·l·s−1, ΔH≠ = 10.6±0.6 kcal/mol, and ΔS≠ = −29.7±2.0 e.u.; 4+TCE→26: k1 = 0.172±0.035 mol−1·l·s−1, ΔH≠ 11.3±0.8 kcal/mol, and ΔS≠ = −24.0±2.8 e.u.; 24+TCE→26: k2 = (6.1±0.2)·10−4 mol−1·l·s−1, ΔH≠ = 13.0±0.3 kcal/mol, and ΔS≠ = −29.5±0.8 e.u.; 6+TCE→17: k1 = 0.136±0.002 mol−1·l·s−1, ΔH≠ = 11.3±0.2 kcal/mol, and ΔS≠ = −24.5±0.8 e.u.; 17+TCE→18: k2 = 0.0156±0.0003 mol−1·l·s−1, ΔH≠ = 10.9±0.5 kcal/mol, and ΔS≠ = −30.1 ± 1.5 e.u.; 18+TCE→19: k3=(5±0.2) · 10−5 mol−1 mol−1 ·l·s−1, ΔH≠ = 15±3 kcal/mol, and ΔS≠ = −28 ± 8 e.u. The following rate constants were evaluated for the DMAD additions (CD2Cl2, 30°): 6+DMAD→20: k1 = (10±1)·10−4 mol−1 · l·s−1; 20+DMAD→21: k2 = (6.5±0.1) · 10−4 mol−1 ·l·−1; 21+DMAD→22: k3 = (1.0±0.1) · 10−4 mol−1 ·l·s−1.The reactions giving the barrelene derivatives 19, 22, 26 and 30 are slower than those leading to adducts that are not barrelenes. The former are estimated less exothermic than the latter. It is proposed that the Diels‐Alder reactivity of exocyclic s‐cis‐butadienes grafted onto bicycle [2.2.1]heptanes and bicyclo [2.2.2]octanes that are modified by remote substitution of the bicyclic skeletons can be affected by changes inthe exothermicity of the cycloadditions, in agreement with the Dimroth and Bell‐Evans‐Polanyi principle.Force‐field calculations (MMPI 1) of 3, 4, 6 and related exocyclic s‐cis‐butadienes as a moiety of bicyclo [2.2.2]octane suggested single minimum energy hypersurfaces for these systems (eclipsed conformations, planar dienes). Their flexibility decreases with the degree of unsaturation of the bicyclic skeleton. The effect of an endocyclic double bond is larger than that of an exocyclic diene moiety.

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