Rearrangements versus substitution

A [3,3]-sigmatropic cyanate-isocyanate rearrangement has been identified as a route to a-isocyanato allylboronic esters that can be trapped with nucleophiles (Scheme 2). A DFT (density functional theory) investigation of [3,3]-sigmatropic rearrangement versus a [2 4-2]-cycloaddition of a formal 5 2 substitution of imido metal complexes with allylic electrophiles has been reported (Scheme 3). ... 

Substituting the observed passage into Eq. (20-60) and rearranging yields Eq. (20-61). A plot of the LHS versus J data yields the mass-transfer coefficient from the slope, similar to the Wilson plot for heat transfer-coefficient determination ... 

The ratios of nucleophilic substitution versus [3,3]-sigmatropic rearrangement for the collapse of allenyl(aryl)iodine(III), generated from the reaction of aryliodanes with propargylsilanes in the presence of Bp3.0Et2 in alcohols, have been determined. The mechanism proposed by the authors ° involves the generation of propargyl cations from the allenyliodine (III) via a unimolecular pathway. 

On the another hand, over the modified zeolite, the rearrangement of the substituted epoxide is slightly slower than the styrene oxide isomerization (vQ = 0.2.10- versus 0.3.10 3 mol.mn. g"1). 

Dolbier Jr., W. R. Palmer, K. W. Effect of terminal flnorine substitution on the Cope rearrangement boat versus chair transition state. Evidence for a very significant fluorine steric effect, J. Am. Chem. Soc. 1993,115,9349-9350. 

This experiment, which combines well anticipated and totally unexpected results, was designed to compare the thermal versus the Lewis acid-catalyzed rearrangement of a doubly activated cyclopropane. This twofold activation was understood in terms of the electron donating effect of the p-anisyl substituent at C-2 that is complementary to the familiar electron withdrawal of the 1,1 -diacyl substitution, which closely resembles Problems 15 and 23. This combination, called captodative cyclopropanes, considerably facilitates some of the various ring unraveling pathways open to the trimethylene ring. 

In a mechanistic study on the Lewis acid catalyzed addition of ethyl diazoacetate to ketones a similar profile of rearrangement to the 3-keto esters was observed (Scheme 9). In the same reaction with acetophenones, substitution on the benzene ring was found to only slightly affect the otherwise 90 10 preference for migration of aryl versus methyl. ... 

The kinetics for the Christmas rearrangement of 9-benzyl- and of variously 4-substituted 9-phenyl-8-azapurine-6-thiones were found to be first order, and the rate constants and equilibrium constants were recorded. A plot of log forward component versus Hammett s sigma values for the 4-phenyl substituents was rectilinear. The 8-azapurine isomer was disfavored by electron-withdrawing substituents and by increased temperatures. In dimethyl sulfoxide, the equilibrium favored the 8-azapurine isomer more than in alcohols. Ultraviolet monitoring of the reaction, in both directions, detected no buildup of any acyclic intermediate." ... 

The factors that affect azepine formation versus production of open-chain rearranged compounds by cycloadditions to vinyl-aziridines (discussed in last years report) have been further assessed. The reaction of tetrahydronaphthalene tertiary alcohols with HNg-HgSC has been found to provide a high-yield regiospecific method for the synthesis of tetrahydro-2,3,4,5-benzo[b]-lH-azepines substituted at position 2... 

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