The preparations of 5,6-dimethylidene-2exo-bicyclo[2.2.2]octanol (8), its endo isomer 9, 5,6-dimethylidene-2-bicyclo[2.2.2]octanone (10) and 2exo,3 exo-epoxy-5,6dimethylidenebicyclo[2.2.2]octane (11) are described. The kinetics of their cycloaddition to tetracyanoethylene has been measured in toluene at 25" together with those of 2,3-dimethylidenebicyclo[2.2.2]octane (7) and 5,6-dimethylidenebicyclo-[2.2.2]oct-2-ene (12). The effects of remote substitution on the Diels-Alder reactivity of 2,3-dimethylidenebicyclo[2.2.2]octanes are compared with those observed in the 2,3-dimethylidenenorbornane series (1-6).Introduction. -The spectroscopic [2] and chemical properties of an exocyclic s-cis-butadiene moiety grafted onto a rigid skeleton can be modified by remote substitution [3-91. For instance, the carbonyl group in 5,6-dimethylidene-2-norbornanone (4) causes a significant rate retardation effect [7] in the DieZs-Alder addition compared of the behavior of 2,3-dimethylidenenorbornane (l)4). The effect is larger than that introduced by hydroxyl groups (e. g. 2 and 3) [7], but not as large as that observed for the 2exo,3exo-epoxy-5,6-dimethylidenenorbornane (5) [8] [9] relative to the parent diene 1.
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: k1 = (255 + 5) 10−4 mol−1 · s−1, ΔH≠ = (12.2 ± 0.5) kcal/mol, ΔS≠ = (−24.8 ± 1.6) eu.; 7 + TCNE → 9, k2 = (0.7 ± 0.02) 10−4 mol−1 · s−1, ΔH≠ = (14.1 ± 1.0) kcal/mol, ΔS≠ = ( −30 ± 3.5) eu.; 2 + TCNE → 8: k1 = (1.5 ± 0.03) 10−4 mol−1 · s−1, ΔH≠ = (14.8 ± 0.7) kcal/mol, ΔS≠ = (−26.4 ± 2.3) eu.; 8 + TCNE → 10; k2 = (0.004 ± 0.0002) 10−4 mol−1 · s−1, ΔH≠ = (17 ± 1.5) kcal/mol, ΔS≠ = (−30 ± 4) eu. The possible origins of the relatively large rate ratios k1/k2 are discussed briefly.
The photoelectron spectrum of the title compound is reported and assigned by correlation with the photoelectron spectra of related molecules.The photoelectron spectra of 2-methylidene-(1) and 2,3-dimethylidene-bicyclo-[2.2.2]octane (2) have been recorded and discussed by Klessinger et al. [ 11. Recently, we have reported the photoelectron spectrum of 2,3,5,6-tetramethylidene-bicyclo-[2.2.2]octane (3) [ 2 ] . In Figure I is shown the He(1u) photoelectron spectruni of the title compound (4), the synthesis of which will be described elsewhere [3].Fig. I. He(1a) photoelectron spectrum of [2.2.2]Hericene Band maxima positions If": 0 8.38 eV: 0 8.75 eV; 0 10.2, eV; 8 -10.7, eV.
I )The general name [I.m.n]hericene is proposed for bicyclo[l.m.n]alkanes with I + m + n methylidene groups, after the latin name hericeus for hedgehog. Thus the title compound would be called [2.2.2]hericcne. On leave of absence from the Peking
The first permethylenated bicyclic, the title compound (1), was synthesized in several steps from (2). Compound (1) forms colorless crystals. The hedgehog‐shaped molecule suggests the name “hericenes” (Latin name for hedgehog = hericeus) for such polycyclics.
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