Abstract:The twofold bridged sesquinorbornenes 2 and 6 were prepared using sequential [4 + 2] cycloadditions of benzoquinone with 1,5‐dihydropentalenes 1 and 5. These syntheses were improved using dilution conditions or a more reactive substituted benzoquinone. Results from semiempirical and ab initio DFT calculations indicated remarkably high pyramidalization angles (ϕ = 46‐47°) for the central C–C double‐bond atoms. The chemical reactivity with triplet and singlet oxygen, dimethyldioxirane and N‐methyl‐1,2,4‐triazoli… Show more
“…In order to examine the dependence of the 13 C NMR chemical shift of non-isolable, highly pyramidalized alkenes on the level of theory, calculations were performed for (i) two highly pyramidalized but isolable compounds: 1,16-dodecahedradiene (pyramidalization angle, 1a Φ = 39.9Њ), 1, 3a and hexacyclo-[6.5.1.0 2,7 .0 4,12 .0 5,10 .0 9,13 ]tetradec-9(13)-ene-3,6-dione (Φ = 46.5Њ), 2; 20 (ii) the less pyramidalized alkenes tricyclo[3.3.3.0 3,7 ]undec-3(7)-ene (Φ = 28.1Њ), 3, 8 and bicyclo[3.3.0]oct-1(5)-ene (Φ = 5.9Њ), 4, 21 and (iii) the non-pyramidalized but highly strained bicyclo[2.2.0]hex-1(4)-ene, 5, 22 (Fig. 1).…”
“…In order to examine the dependence of the 13 C NMR chemical shift of non-isolable, highly pyramidalized alkenes on the level of theory, calculations were performed for (i) two highly pyramidalized but isolable compounds: 1,16-dodecahedradiene (pyramidalization angle, 1a Φ = 39.9Њ), 1, 3a and hexacyclo-[6.5.1.0 2,7 .0 4,12 .0 5,10 .0 9,13 ]tetradec-9(13)-ene-3,6-dione (Φ = 46.5Њ), 2; 20 (ii) the less pyramidalized alkenes tricyclo[3.3.3.0 3,7 ]undec-3(7)-ene (Φ = 28.1Њ), 3, 8 and bicyclo[3.3.0]oct-1(5)-ene (Φ = 5.9Њ), 4, 21 and (iii) the non-pyramidalized but highly strained bicyclo[2.2.0]hex-1(4)-ene, 5, 22 (Fig. 1).…”
“…Deviations from planarity in pyramidalized alkenes can result from a combination of strain and unsymmetric steric effects, as in the syn -sesquinorbornenes 2 and related alkenes . Such deviations from planarity are typically smaller, and the compounds are, in many cases, stable and isolable.…”
The highly pyramidalized alkene, pentacyclo[4.3.0.0(2,4).0(3,8).0(5,7)]non-4-ene (9), has been generated via treatment of 4,5-diiodopentacyclo[4.3.0.0(2,4).0(3,8).0(5,7)]nonane (12) with n-butyllithium and tert-butyllithium. The title alkene has also been trapped as its Diels-Alder adduct with 1,3-diphenylisobenzofuran, 2,5-dimethylfuran, and spiro[2.4]hepta-4,6-diene. Products resulting from alkyllithium addition to the pyramidalized double bond of 9 have been isolated and fully characterized spectroscopically. The geometry, olefin strain energy, heat of hydrogenation, and relative HOMO/LUMO energies of 9 have been obtained by ab initio calculations at the MP2 and B3LYP levels using the 6-31G* basis set.
“…Holthausen and Koch 2c demonstrated from their calculations on various norbornene derivatives that hyperconjugation as well as torsional effects play important roles in determining the extent of the nonplanarity of the double bonds. As an alternative, density functional theory (DFT) has been used in studying the geometries of pyramidalized alkenes. − …”
[reaction: see text] endo- and exo-2,3,4,7-tetrahydro-1H-1,4-methanobenzocycloheptene-7-carboxylic acid ethyl esters have been synthesized, and their Diels-Alder cycloaddition reactions with maleic anhydride, dimethyl acetylenedicarboxylate and singlet oxygen have been investigated. The X-ray analysis of four adducts indicated the pyramidalization of the central double bond. Density functional theory calculations on the isolated products and model compounds showed excellent agreement between the experimental and theoretical determined butterfly angles. Furthermore, it has been shown that a cyclopropyl group fused to [2.2.2] system decreases significantly the degree of the pyramidalization which is attributed to the steric interactions between the cyclopropyl group and ethano bridge of the norbornene systems. Due to the instability of the bicyclic endoperoxides, their X-ray analysis could not be carried out. DFT calculations on model compounds showed increased bending in the case of the product obtained by the addition of singlet oxygen to endo-2,3,4,7-tetrahydro-1H-1,4-methanobenzocycloheptene-7-carboxylic acid ethyl ester.
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