Heats of Hydrogenation, IV. - Homoaromaticity in Norcaradiene and Cycloheptatriene
By comparison of the heats of hydrogenation of fixed norcaradienes with analogous cyclobutane derivatives and from the difference of experimental heats of formation with values calculated by force-field methods the homoaromatic stabilization of norcaradienes and cycloheptatrienes are calculated and compared with the equilibrium position of the respective valence isomers.Key Words: Homoaromaticity / Resonance energy / Calculations, force-field / Heats of hydrogenation
Roth, Wolfgang R.,Klaerner, Frank-Gerrit,Siepert, Gerd,Lennartz, Hans-Werner
p. 217 - 224
(2007/10/02)
PHOTOCHEMISTRY OF ALKYL HALIDES - VII. CYCLOPROPANATION OF ALKENES
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.
p. 3229 - 3236
(2007/10/02)
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