Sigmatropic Hydrogen and Alkyl Shifts

Thermal [1,3]-suprafacial hydrogen shift is orbital symmetry forbidden process, but [1,3]-suprafacial alkyl shift is symmetry allowed process with inversion of configuration of migrating alkyl carbon. For example, the thermal rearrangement of bicyclo-[3.2.0]-heptene 1 to bicyclo-[2.2.1]-heptene 2 [4]. 

In compound 1, deuterium is trans to the acetoxy group, while in the product 2, it is cis. This indicates that the inversion of configuration at C-7 occurs during this rearrangement via the TS. 

Similarly, bicydo[2.1.1]-hexenes 3 give bicyclo[3.1.0]-hexenes 4 on heating through a favored TS [5]. 

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Several important types of sigmatropic reactions are listed in Scheme 10.9. We first discuss shifts of hydrogen and alkyl groups, concentrating on [1,3]-, [1,5]-, and... 

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

Benzosultams 450 undergo a tandem alkylation-sulfanylation on treatment with 4-bromobutyl thiocyanate under phase transfer conditions. Extrusion of SO2 occurs on thermolysis of the resulting spiro-tetrahydrothiopyranobenzo-sultams, generating an azaxylylene and thence 6-aryl-3,4-dihydro-2//-thiopyrans by a [l,5]-sigmatropic hydrogen shift (Scheme 155) <2005T8848>. 

The possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

Piers and Ruediger have smartly used this basic reaction (equation 168) for a synthesis of the sesquiterpenoid -himachalene ". However, loading the systems to be rearranged with alkyl groups can considerably decelerate the Cope process with the result of competing homo[l, 5]-sigmatropic hydrogen shift in the trans compounds (equation... 

The product 16 is identical with the one expected from direct [1,5] methyl migration [path (A)]40 42 (see also refs 41 and 42). For 5,5-diethyl-l,3-cyclohexadiene it is concluded that no more than 3% of the reaction can result from a direct [1,5] alkyl migration42. However, in 5,5-disubstituted 1,3-cyclohexadienes, hydrogen and other substituents, such as the formyl35 and acetyl groups35, should migrate by the [1,5] sigmatropic shift pathway. 

The cycle size plays an important role in the transition state during elimination of simple molecules (HHal, H20, H2S) from the M+ of monofunctional alkyl derivatives. Five-member transition state (1,4-sigmatropic shift of a hydrogen atom) is preferential for the elimination of HC1 and HBr. For thiols this process gives 40% of the eliminating H2S, while the preferential one (60%) involves 1,4-elimination (1,5-sigmatropic shift). The latter process (1,4-elimination) completely dominates in the case of alcohols [14]. Another example of steric effects involves a McLafferty-type... 

A short route to 5-monosubstituted oxazoles is based on the flash vacuum pyrolysis of azolides of 1,2,4-triazole (169) <89H(29)103>. 5-Aryloxazoles are obtained in excellent yield, but the formation of ketene is a major side reaction for alkyl analogues with an a hydrogen. The rearrangement is believed to proceed by [1,5] sigmatropic shift of the acyl substituent followed by loss of nitrogen and cyclization of a diradical intermediate (Scheme 79). 

Most examples of this class of reaction involve hydrogen shifts, but reactions are known in which alkyl, aryl, and other groups undergo sigmatropic migrations. ... 

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