Sigmatropic Migrations

Lithium aluminum hydride reduction of 2,3,4-triphenylisoxazolin-5-one yielded 1,2,3-triphenylaziridine and dibenzylaniline. The reaction was considered to proceed by a concerted [l,3]-sigmatropic migration of the N to a C atom. HOMO-LUMO calculations show this type of concerted reaction is possible (Scheme 68) (80JA1372). 

Sigmatropic migration involving alkyl group shifts can also occur ... 

The thermal rearrangements of methyl-substituted cycloheptatrienes have been proposed to proceed by sigmatropic migration of the norcaradiene valence tautomer. The first step is an electrocyclization analogous to those discussed in Section 11.1. 

The actual reported results bear out this analysis. Thus a thermal [1,3] migration is allowed to take place only antarafacially, but such a transition state would be extremely strained, and thermal [1,3] sigmatropic migrations of hydrogen are unknown." On the other hand, the photochemical pathway allows suprafacial [1,3] shifts, and a few such reactions are known, an example being " ... 

Sigmatropic migrations of alkyl or aryl groups" are less common than the corres-... 

FIGURE 18.5 Hypothetical orbital movement for a thermal [1,5] sigmatropic migration of carbon. To move from one — lobe to the other — lobe, the migrating carbon use only its own — lobe, retaining its configuration. 

L8.6 Hypothetical orbital movement for a thermal [1,3] sigmatropic migration of carbon. The migrating carbon moves from a — to a + lobe, requiring it to switch its own bonding lobe from — to -f, inverting its configuration. 

In general four types of transition states can be envisaged for sigmatropic migrations of the order (/, /), two in which the migrating group moves suprafacially (a) and (b) and two in which it means antarafacially (c) and (d)... 

If both ortho positions are substituted, the allyl group undergoes a second sigmatropic migration, giving the / ara-substituted phenol ... 

The reaction involves a [1,3] sigmatropic migration of the carbon originating at C2 with transformation of the three-membered ring C3 C4- C5 to an allylie system. For that reason, the methylenecyclobutane system undergoes thermal automerization. 

Electrocyclic rearrangements of cyclobutenes and cyclohexadienes 8-31 [ 1,/] Sigmatropic migrations of hy-... 

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