Hydride shift defined

We have seen that 1,2-H migrations in singlet carbenes may be affected by (e.g.) the participation of carbene precursor excited states, QMT, stabilization of the hydride shift transition state by polar solvents, and temperature. Here, we consider our third principal theme, the effect of substituents on the kinetics of carbenic rearrangements. We first examine the influence of bystander and spectator substituents (as defined in Eq. 22) on 1,2-H rearrangements of alkyl, alkylchloro, and alkylacetoxycarbenes. 

The epimerization at both the ring stereogenic centers causes the diastereomeric interconversion 22< >30. Since the rates of [l,5]-hydride shifts in these two diastereomers are comparable, some of the product formed from the pyrolysis of 22 must actually arise from 30 and vice versa. After making corrections for the concurrent double epimerization by following an established procedure [9, 10], it was shown that the mechanistically significant ratio of the products /i,Z-31 and Z,Z-31 formed directiy from 22 was >220 1. Very clearly, the orbital overlap controlled the reaction of the stereochemically well defined reactant 22 and caused one symmetry-allowed pathway to take prominence over the other symmetry-allowed pathway. 

The substrate requires a hydride acceptor proximal to a C—H bond serving as hydride donor, and the reaction is initiated by a hydride shift (or related H-atom-transfer step), which formally oxidizes the carbon donor and reduces the hydride acceptor. The new C-X bond will be formed at the hydride-donor atom, after the hydride shift takes place. The defining characteristic for these reactions is the functionalization of a C-H bond concurrent with a hydride shift. The names proposed by Sames ( HT-cyclization ) and Akiyama ( Internal Redox Cascade ) would seem more appropriate if focusing more on the unique hydride-shift mechanism that draws together a diverse group of substrates, at least for intramolecular examples. 

Side chains. The speed of the hydride, methyl and ethyl shift reactions is the basis of the lumping by number of carbon atoms and by number of branches. Consequently, as soon as a compound with a given number of branches is formed, the other isomers with the same number of branches are formed immediately. This is also true for the side chains of an activated complex, enabling us to define the notion of equivalent side chains. 

Conventional Fiiedel-Crafts o-complexes have likewise been generated under stable ion conditions. Olah and Kuhn generated alkylation and formylation of o-complexes [70], both isolated as solids with well-defined ionic character (conductance studies), and heating of the solids produced the S Ar products (Scheme 1.24). A subsequent NMR study involving the low-temperature ethylation of 1,3,5-tiiethylbenzene also revealed the presence of o-complex intermediates [65a] however, isomeric species were formed rapidly by hydride/alkyl shifts. Indeed, this aspect of cyclohexadienyl cation chemistry has made the study of o-complexes difficult, as the barrier for isomerization is often quite low. 

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