The Non‐planarity of the Bicyclo[2.2.1]hept‐2‐ene Double bond. Crystal Structures of Bicyclo[2.2.2]oct‐2‐ene, Bicyclo[2.2.1]hept‐2‐ene, and Bicyclo[2.1.1]hex‐2‐ene Systems

The crystal structures of (1R,4R,5S,8S)‐9,10‐dimethylidentricyclo[6.2.1.02,7]undec2(7)‐ene‐4,5‐dicarboxylic anhydride (3), (1R,4R,5S,8S)11‐isopropylidene‐9,10‐dimethylidenetricyclo[6.2.1.m2,7]undec‐2(7)‐ene‐4,5‐dicarboxylic anhydride (6), (1R,4R,5S8S)‐9,10‐dimethylidenetricyclo[6.2.2.02,7]dodec‐2(7)‐ene‐4,5‐dicarboxylic anhydride (9), (1R4R5S8S)‐TRICYCLO[6.2.2.02,7]dodeca‐2(7), 9‐diene‐4,5‐dicarboxylic anhydride (12) and (4R,5S)‐tricyclo[6.1.1.02.7]dec‐2(7)‐ene‐4,5‐dicarboxylic acid (16) were established by X‐ray diffraction. The alkyl substituents onto the endocyclic bicyclo[2.2.1]hept‐2‐ene double bond deviate from the C(1), C(2), C(3), C(4), plane by 13.5°4 in 3 and by 13.9° in 6, leaning toward the endo‐face. No such out‐of‐plane deformations were observed with the bicyclo[2.2.2]oct‐2‐ene derivatives 9 and 12. The exocyclic s‐cis‐butadiene moieties in 3, 6 and 9 do not deviate significantly from planarity. The deviation from planarity of the double bond n bicyclo[2.2.1]hept‐2‐ene derivatives and planarity in bicyclo[2.2.2]oct‐2‐ene analogues is shown to be general by analysis of all known structures in the Cambridge Crystallographic Data File. The non‐planarity of the bicyclo[2.2.1]hept‐2‐ene double bond cannot be attributed only to bond‐angle deformations which would favour rehybridizatoin of the olefinic C‐atoms since the double bond in the more strained bicyclo[2.1.1]hex‐2‐ene drivative 16 deviates from planarity by less than 4°.