- Dihalocarbene insertion reactions into C-H bonds of compounds containing small rings: Mechanisms and regio- and stereoselectivities
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(Chemical Equation Presented) Novel insertion reactions of dichloro- and dibromocarbene into carbon-hydrogen bonds adjacent to cyclopropane rings are reported. It is found that the predominant isomers formed in the reactions with bicyclo[4.1.0]heptane result from insertion into the endo carbon-hydrogen bonds alpha to the three-membered ring. In the reactions of bicyclo[3.1.0]hexane, however, the exo dihalocarbene insertion products are formed as the major isomers. In some compounds cyclopropane rings "activate" adjacent carbon-hydrogen bonds, whereas other systems containing three-membered rings do not. Moreover, the influence of various substituents (methyl, geminal dimethyl, phenyl, methoxy, and ethoxy) attached to bicyclo-[3.1.0]hexane and bicyclo[4.1.0]heptane in dihalocarbene reactions has been studied. The findings can be explained by the concept of maximum orbital overlaps of Walsh orbitals of the cyclopropane rings and the α carbon-hydrogen bonds. In stark contrast, selective insertion into the tertiary carbon-hydrogen bonds of the cyclobutane ring in bicyclo[4.2.0]octane is observed.
- Brinker, Udo H.,Lin, Guoying,Xu, Linxiao,Smith, William B.,Mieusset, Jean-Luc
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p. 8434 - 8451
(2008/02/13)
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- 6. 1. 0.
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The enthalpies of formation of cis- and trans-1,2-diethylcyclopropane were determined by oxygen bomb calorimetry. The cis-diethyl isomer was 1. 1 kcal/mol less stable than the trans isomer. Alkyl groups were found to stabilize a cyclopropane ring by the same amount as for a carbon-carbon double bond. The enthalpies of formation cis- and trans-bicyclo left bracket 6. 10 right bracket nonane also were determined by oxygen bomb calorimetry. The two bicyclo left bracket 6. 1. 0 right bracket nonanes have essentially the same enthalpies of formation, in contrast to cis- and trans-cyclooctene. The introduction of a trigonal center makes the trans-bicyclononane 3 kcal/mol less stable than the cis isomer. Molecular mechanics calculations are reported for a series of cis- and trans-bicyclo left bracket n. 1. 0 right bracket alkanes. In order to better estimate the strain energy of trans-bicyclo left bracket 4. 1. 0 left bracket heptane, the difference in energy between it and its cis isomer was calculated via ab initio molecular orbital theory.
- Wiberg,Lupton Jr.,Wasserman,de Meijere,Kass
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p. 1740 - 1744
(2007/10/02)
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- PHOTOCHEMISTRY OF ALKYL HALIDES - VII. CYCLOPROPANATION OF ALKENES
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The previously observed cyclopropanation of alkenes by irradiation of diiodomethane (1) in their presence has been studied in more detail and found to be a synthetically useful procedure which is significantly less subject to steric effects than the traditional Simmons-Smith method.The results from photocyclopropanation of a variety of alkenes are summarized in Tables 1 and 3-4.In a number of cases the photochemical procedure afforded improved results over the Simmons-Smith method, particularly with sterically congested alkenes.Cycloalkenes showed relative rates of photocyclopropanation as a function of ring size similar to those of the Simmons-Smith method (Table 5).However, the photocyclopropanation reaction exhibited steadily increasing relative rates with increasing substitution about the double bond-in contrast with the Simmons-Smith method (Table 6), in which steric effects offset increasing nucleophilicity of the alkene with increasing substitution.The α-iodocation 2 is suggested as the methylene transfer species.In the presence of lithium bromide cation 2 was trapped to afford bromoiodomethane.
- Kropp, Paul J.,Pienta, Norbert J.,Sawyer, Joy A.,Polniaszek, Richard P.
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p. 3229 - 3236
(2007/10/02)
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