A Sigmatropic Shift

Woodward-Hoffmarm rules require a sigmatropic shift of this type to proceed with inversion of configuration. The orbitals involved in a [1,4] sigmatropic shift are shown in Fig. 13.7. 

As an extension of this work, Atkinson and co-workers (123) prepared l-dibenzylamino-l,2-dihydro-2-quinolone (78) and 1 -(/V-benzy l-N-carboxy-methyl)amino-l, 2-dihydro-2-quinolone (79). The benzylic protons of 78 showed an AB quartet that did not coalesce up to 180°C, and 79 was resolved into optical isomers. The E, for racemization was 26.2 0.4 kcal/mol. Various attempts were made to elucidate the possible pathways for isomerization in these quinolone derivatives (123). Radical dissociation, a sigmatropic shift followed by rotation, and restricted rotation about the S—N bond were excluded. The aforementioned authors (123) also excluded the possibility of nitrogen inversion and preferred restricted rotation about the N—N bond as an explanation for the existence of stereoisomers. They supported this explanation by examining the steric effects... 

Although evidence for thermally induced migrations points to a sigmatropic shift of the substituent, as discussed above, the photochemical conversion of 2-cyanopyrrole into 3-cyanopyrrole proceeds by a [1,3] sigmatropic walk of the heteroannular atom by route (a), as shown in Scheme 2. 2-Cyano-3-methylpyrrole and 2-cyano-4-methylpyrrole undergo similar phototranspositions to produce 3-cyano-4-methylpyrrole (by route a) and 4-cyano-2-methylpyrrole (by route a), respectively (75CC786), whilst 2-cyano-5-methylpyrrole produces... 

Frontier orbitals can help to explain the results seen when a sigmatropic shift involves migration of carbon rather than hydrogen. A [1,3] migration of carbon involves Att electrons. We can think of the reaction as migration of a carbon radical across the tt system of an allylic radical. In this case, overlap of the p orbital of the carbon radical with the allylic v system can be visualized as shown in the following diagram. 

Two pericyclic reactions a sigmatropic shift and a cycloaddition in one reaction scheme. Suggested solution... 

Polyiodinated diphenyl ethers are important thyroid hormones and in order to discover the relationship between activity and planarity of the two benzene rings, iodinated dibenzofurans have been synthesized photochemically. Pyridine iV-oxides react with alkynes to give furopyridines via a sigmatropic shift the product varies with the substituents on the alkynes. 3,S-Dihalopyridine oxides give moderate yields of furo[3,2-A]pyridines as the major product smaller amounts of the [3,2>c] isomers are also formed. 2,4-Disut tituted pyridine oxides produce the (3,2-c] isomers in better yields than for most of the 3,5-isomers. 

In contrast to dihydropyran rearrangements, the rearrangement of 1,4-dioxins is believed to proceed via a chairlike transition state with the substituents in axial positions. Dihydro-1,4-dioxins 10 and 11, obtained by acid-catalyzed double-bond isomerization of 9. on heating in a sealed tube, undergo a sigmatropic shift to give dihydropyrans 12 and 13322 323. 

Interestingly, the reaction of 3-pyrrolidino-l, 2,4-triazine 4-oxide with ammonia has been established to give 5-amino-l,2,4-triazine 4-oxide, due to the te/e-substitution process with elimination of pyrrolidine (Scheme 25) [51 ]. As for the mechanism of this amino-dehydrogenation reaction, a sigmatropic shift of hydrogen has been postulated, substantiated by registration of the key intermediates. 

The rearrangement step is a ground-state thermal process, and may be classified as a [l,4]-sigmatropic shift of carbon across the face of a 2-oxybutenyl cation. The Woodward-Hoffmann rules require a sigmatropic shift in this system to proceed with inversion of configuration. The symmetry properties for 1,4-shifts with inversion and retention are shown in Fig. 11.6. 

MAA-EGDMA copolymers were imprinted using a chiral oxazine-based TSA, and selectivity for the TSA over both the substrate and the product was observed in both aqueous and nonpolar media. A 15-fold rate enhancement (fecat 0.1 s ) was obtained using the L-form of the TSA, and enantioselective preparation of phenylalanine was observed (32% 4% ee) for reactions carried out in an aqueous buffer system. This study was the first example of a sigmatropic shift in aqueous media catalyzed by MIPs. It was also one of a number of early examples that displayed only modest rate enhancements when compared to enzymes. 

A sigmatropic shift is defined as a reaction wherein a cr bond migrates over one or more ir systems. Numbers in brackets are used to define the movement of the cr bond. An [i,j] sigmatropic rearrangement is the migration of a o bond flanked by one or more it systems to a new location i-1 and j-1 atoms away. Like many concepts in this chapter, sigmatropic shifts are... 

CONCERTED ACTIONS In the concerted reaction called a sigmatropic shift, one part of the molecule flies about, eventually being caught at one speficic position within the same molecule. 

Antarafacial motion (Section 20.5) Migration of a group from one side of a Jl system to the other in a sigmatropic shift. 

Attempts to cause the conversion of cyclic sulphides, e.g. (58), into carbocycles, e.g. (59), by an anionic [2,3]-shift of the derived enolate anions failed. After 5-methylation, a product of a sigmatropic shift could be obtained only in the case of the six-membered ring (58), but Stevens 1,2-migrations and fragmentations to... 

Classify each of the following pericychc reactions as an electrocychc reaction, a cycloaddition, or a sigmatropic shift. 

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