Degenerate rearrangement of bicyclo

Degenerate rearrangement of bicyclo[3.1.0]hex-2-ene (Chart 2) has a PES, in which four degenerate products are separated through four degenerate TSs with the common energy plateau on the surface.9 Here, four compounds are identical except for the position of deuterium. The rearrangement from 4-exo isomer (6x) is expected to afford 4-endo (6n), 6-exo (7x), and 6-endo (7n) isomers in equal amount if the reaction follows statistical reaction theory (TST). Thus, this reaction provides a situation previously presented by Carpenter to predict nonstatistical product distribution due to dynamics effect.1... 

Houk and co-workers reported a theoretical study of the degenerate rearrangement of bicyclo[3.1.0]hex-2-ene labeled with deuterium in the 4-exo position (113 in Figure 11.113) that offers additional insight into the nonsta-tistical dynamics model of reactivity. Breaking the Cl—C5 bond of 113 leads to a diradical (114) that can reclose to 113 with retention of configuration at both Cl and C5 (the rr path), can close suprafacially to 115 with retention at C5 (the rs path), can close to 116 with inversion at both Cl and C5 (the ii path), or can close antarafacially to 117 with inversion at C5 (the ia path). Baldwin and Keliher had reported that the activation parameters for formation of 115, 116, and 117 are identical and that the ratio of relative rate constants k s ku ki is 48 36 16. ... 

The bicyclic vinylcyclobutene (323) undergoes a ready gas-phase thermolysis at 200 °C to give (325) and the cis,trans,cis-cycloheptatriene (324) is a probable intermediate. Another vinylcyclobutene rearrangement which is known is the degenerate rearrangement of bicyclo [4,2,0] octa-2,7-diene (326). The... 

The Cope rearrangement of bicyclo[5.1.0]octa-2,5-diene (3,4-homotropylidene) (5) leads to a product that is identical with the substrate however, six of eight carbon atoms have changed places. The rearrangement is degenerate . As a consequence NMR and C NMR spectra... 

For the non-substituted bicyclo[3.1.0]hexenyl cation the degenerate rearrangement proceeds far more slowly and it could only be observed by labeled atoms The quantum chemical analysis of this process is made in Another rearrangement of bicyclo[3.1.0]hexenyl cations is the C,—C5 bond rupture to the benzenium ions 254-256.277,767.771.774,. 

The 2-bicyclo[2.1.1]hexyl cation [113] or [114] shows only three carbon signals, one for C-4 and two averaged signals for C-l/C-2 and C-3/C-5/C-6 respectively. The fast dynamic process leading to the observed symmetry could be either a Wagner-Meerwein shift between trivalent cations (83) or a three-fold degenerate rearrangement of hypercoordinated cations [114] as shown in (84). 

The Cope rearrangement of hexa-l,5-diene does not allow for differentiation of starting material and product this is called a degenerate Cope rearrangement. Another example is the automerization of bicyclo[5,l,0]octa-2,5-diene 7 ... 

It was pointed out earlier that a Cope rearrangement of 1,5-hexadiene gives 1,5-hexadiene. This is a degenerate Cope rearrangement (p. 1380). Another molecule that undergoes it is bicyclo[5.1.0]octadiene (105). At room temperature the NMR... 

The 3-homonortricyclyl cation 70 was prepared by the isomerization of bicyclo[3.2.1]oct-3-en-2-yl cation 71 at 20 °C in SbFs/SC ClF solution83. The ion shows a threefold degenerate rearrangement between -85 °C to 20 °C. At 20 °C the C4, C6, C8 and Cl, C3, C7 carbons become equivalent with an average of 36.19 ppm and 135.8 ppm, respectively (equation 43). Below -80 °C the cation is a static secondary cyclopropylcarbinyl cation with the cationic center chemical shift at <5I3C 234.1. 

A search for possible degenerate behavior in bicyclo[3.3.0]octa-2,6-dienes164 has not produced clear results due to structural misassignments.126 The normal Cope rearrangement of 112 can be redirected through coordination to rhodium (I) to... 

Ion [160], prepared by ionization of bicyclo[4.1.0]hepyl-2-ol [163], underwent a degenerate two-fold rearrangement with A(7 = 8.5 0.5 kcal mol" at —85°C (105). A puckered cyclobutyl cation was considered as the most probable intermediate. The 1-methyl substituted alcohol [164] also gave a... 

The bicyclo[3.1.0]hex-3-en-2-y[ cation [167] and related ions constitute another group of carbocations capable of undergoing degenerate rearrange-... 

The Marcus equation provides a nice conceptual tool for understanding trends in reactivity. Consider for example the degenerate Cope rearrangement of 1,5-hexadiene and the ring-opening of Dewar benzene (bicyclo-[2,2,0]hexa-2,5-diene) to benzene. Figure 15.29. The experimentally observed activation energies are 34 kcahmol and 23 kcal/mol, respectively. The Cope reaction is an example of a Woodward-Hoffmann... 

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