Hydrogen atom 1,5-sigmatropic shift

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

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... 

So it is the number of electrons and not the number of atoms which determines the selection rule. Therefore, the selection rules for hydrogen migration in thermal sigmatropic shifts can be summarized as follows as given in the table ... 

Ene reaction (Hydro-allyl addition)-A reaction of an allylic compound with an olefin which resembles both a cycloaddition and a [1, 5]-sigmatropic shift of hydrogen. In this reaction an alkene having an allylic hydrogen atom reacts thermally with a dienophile (enophile) C = C, C = 0, N = N, N = 0, C = S... 

In addition to the migration of hydrogen atoms in sigmatropic rearrangements, alkyl shifts also take place. A large number of such reactions occur... 

A [1,5]-sigmatropic shift of hydrogen from an A-methyl group to the carbonyl carbon atom in protonated 3-(A,A-dialkylhydrazono)-l,l,l-trifluoroalkan-2-ones has been found to be a key step for the acid-catalysed cyclization of these ketones to 6-trifluoromethyl-3,6-dihydro-2//-[l,3,4]oxadiazines.58 Gas-phase kinetic studies on interconversions of monodeuterocyclohexadienes have given activation parameters in reasonable agreement with previous experimental and theoretical data.59... 

A frontier orbital analysis of the ene reaction is shown in Fig. 8.62. It displays many features of the [3,3]-sigmatropic shift. Thus, the hydrogen atom transfer from an allylic site to a double bond is seen to be symmetry allowed with respect to the HOMO of an... 

Woodward and Hoffmann define a sigmatropic change of order [i, j] the migration of a cr-bond, flanked by one or more ir-electron systems, to a new position whose termini are i-1 and j-1 atoms removed from the original loci, in an uncatalyzed intramolecular process (8). The transformation XXIX-> XXX, for example, involving the shift of a hydrogen atom from c-j to c-1 is described [1, j]. 

There are two possible concerted mechanisms for such a rearrangement, namely a 1,3-sigmatropic shift of the hydroxy group (317318320) and a 1,3-sigmatropic shift of one of hydrogen atoms (317 319 320). According to the calculations, the... 

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