- Effect of a methoxy substituent on the vinylcyclobutane carbon migration
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(Chemical Equation Presented) Over the temperature range 250-300°C, 8-exo-methoxybicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-exo- and 5-endo-methoxybicyclo[2.2.2]oct-2-enes, 2a and 2b, respectively, with a clear preference for the si product: si/sr = 3.2. Both 1a and its 8-endo epimer 1b experience appreciable epimerization and fragmentation. A long-lived intermediate with weakly interacting diradical centers, one of which is stabilized by a methoxy substituent, can account for all such observations.
- Leber, Phyllis A.,Lasota, Celina C.,Strotman, Neil A.,Yen, Gloria S.
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p. 912 - 919
(2007/10/03)
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- The bicyclo[2.2.2]octyl carbene system as a probe for migratory aptitudes of hydrogen to carbenic centers
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A series of tosylhydrazone derivatives of exo-6-substituted bicylo[2.2.2]octan-2-ones have been prepared. Thermal decomposition of the sodium salts of these tosylhydrazones gives carbene-derived products from 1,3-migration of either the C6 hydrogen (perturbed) or the C7 hydrogen (unperturbed), along with smaller amounts of alkenes derived from 1,2-hydrogen migration. The exo-6-substituent strongly activates 1,3-hydrogen migration in the case of SiMe3 and weakly activates it in the case of CH3 substitution. Thiomethoxy and carbomethoxy are weakly deactivating, while cyano and methoxy groups are strongly deactivating. B3LYP/6-31G* calculations on these substituted carbenes and transition states are in qualitative agreement with the ease of 1,3-hydrogen migration of perturbed vs unperturbed hydrogen. These experimental results and computational studies suggest carbene stabilization due to the exo-6-silyl group. They also suggest a reactant-like transition state for 1,3-hydrogen migration in which the inductive effect influences ease of migration. In the case of the exo-6-methoxy group, the inductive effect overwhelms any potential resonance-stabilizing effects.
- Creary,Butchko
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p. 1569 - 1578
(2007/10/03)
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- Corner Attack on endo- and exo-Tricyclo2,4>oct-6-ene by Deuterium and Mercuric Ions: The Effect of Electrophile on Reaction Course
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Reaction of endo-tricyclo2,4>oct-6-ene (1) with mercuric acetate in methanol gave 2b as the major product of reaction after stereospecific reduction with sodium-mercury amalgam in sodium deuteroxide, along with 3b and 4b.In contrast 4-endo-deuterio-2-endo-methoxybicyclooct-6-ene (2b) is formed as a minor product from acid-catalyzed reaction of endo-tricyclo2,4>oct-6-ene (1) with methanol-d1.Methoxymercuration of exo-tricyclo2,4>oct-6-ene (8) occurs at the double bond to give 7-exo-(acetoxymercurio)-6-exo-methoxy-exo-tricyclo2,4>octane (9c; 89percent).Acid-catalyzed addition of methanol to exo-tricyclo2,4>oct-6-ene (8) gave 6-exo-methoxy-exo-tricyclo2,4>octane (9a; 15percent) and 6-exo-methoxytricyclo2,7>octane (11) in a process involving corner attack of the electrophile.This tricyclic compound rearranges to 5-exo-methoxybicyclooct-2-ene (10a; 55percent).The preference for corner attack by mercuric ion and deuteron with C2-C4 bond cleavage in the reactions of 1 and 8 is rationalized and the competition between reaction at cyclopropane and addition to the double bond discussed.
- Coxon, James M.,Steel, Peter J.,Whittington, Barry I.
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p. 3702 - 3709
(2007/10/02)
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