1,2-Sigmatropic shifts

groups A and B at C become endo and exo in therolysis in 1, 3, 5-cycloheptatriene like systems in a series of suprafacial, 1, 5-shift with retention of configuration at C7. 

An example of thermal 1, 3-sigmatropic rearrangement (isomerization) is depicted in the following figure  

Analysis of sigmatropic rearrangement is also possible by PMO method and conclusion about their feasibility are same as by other methods. For instance, 1, 3-sigmatropic suprafacial shift occurs via transition state with 0 nodes and 4 electrons (antiaromatic) and hence it is thermally forbidden. But [1, 3]-antarafacial sigma migration occurs through a transition state with 1 node and four electrons (aromatic T.S.) and is therefore thermally allowed. 

Selection rule for sigmatropic rearrangement of the order [1, /] (PMO-method) 

Number of electrons involved (i+ f) Number of nodes Aromaticity of T.S. Mode of migration 

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Recently, the 1,3-sigmatropic shift in 2-cyanopyrrole was studied by using the SINDOl semiempirical method (90JOC2288). This study showed that the reaction occurred via a transition and that some biradical intermediates are probably involved in the reaction. 

Unlike many other metal-allyl systems, allylsilanes are rcgio-stable at inormal temperatures, with 1,3-sigmatropic shifts occurring at a significant Irate only at temperatures in excess of 300°C. 5-Trimethylsilylcyclo-Ipentadiene provides an exception to this generalization, but it can still be lhandled quite readily. 

Fig. 17. a) Some interesting molecules that undergo 1-3 sigmatropic shifts, b) Clockwise (CW) and counterclockwise (CCW) rotation at the migrating center, c) Mechanisms and parameters for the EHT study of the rearrangements of bicyclo [3.2.0] heptenes with and without inversion of configuration. 

The latter reacts with a second DMAD molecule to give cycloadduct (726). The reaction proceeds via opening of the aziridine ring to give compound (725). The cyclo-adduct (726) undergoes a 1,3-sigmatropic shift and aromatization to give compound (723) (Scheme 2.302) (816). 

The secondary process of amine fragmentation (1,3-sigmatropic shift) may be used as an example of a process initiated by the charge center (Scheme 5.18). 

Woodword-Hoffman theory states that there is high critical energy for the 1,3-sigmatropic shifts. However, theoretical calculations [28] and experimental data... 

This view also explains satisfactorily why a [1, 3] sigmatropic shift of a hydrogen in 1, 3 pentadiene will not be possible. In this case, a [1, 3] shift would have to be antarafacial and such a rearrangement would be structurally prohibited. 

For example, CdS can mediate a symmetry forbidden [1,3] sigmatropic shift of hydrogen, Eq. (43) In the absence of oxygen, these cyclic derivatives of diphenyl-cyclobutene underwent a reversible double bond migration. Since the reaction was... 

Irradiation of 3,3-diphenylthiooxindole gives 9-phenylthioxanthene (72T597). It is likely that this transformation involves successive cheletropic extrusion of CO, electrocyclization and a photolytic 1,3-sigmatropic shift of hydrogen (Scheme 152). The last step can also be brought about by base catalysis. 

Diphenylpyrrolidine (77) catalyses the enantioselective cy-chlorination of aldehydes.299 Mechanistic and computational studies suggest that - in contrast to pre- viously proposed mechanisms involving direct formation of the carbon-electrophile bond - iV-chlorination occurs first, followed by a 1,3-sigmatropic shift of chlorine to the enamine carbon. The product iminium ion is then hydrolysed in the ratedetermining step. 

Quinolylimino-l,2,3-dithiazoles 62 undergo by thermolysis an unusual rearrangement to give imidazoquinoli-nethiones 63 (Equation 10) delivery of a sulfur atom to the quinoline 2-position appears to be intramolecular and possibly involves a [1,3] sigmatropic shift of a carbon-sulfur bond <2000J(P1)555>. 

However, the recently reported reaction of phthalimidonitrene with acetylenes probably offers the clearest example of the 2-azirine ring system actually being a reaction intermediate.4 The oxidation of iVT-aminophthalirnide (17) with lead tetraacetate produces nitrene (18) which in the presence of 3-hexyne gives 1-azirine (20). This product is most easily rationalized in terms of 2-azirine (19) as an unstable intermediate which undergoes a 1,3-sigmatropic shift of NR2 giving the product (20). 

Fig. 7. The [1,3] sigmatropic shift of vinylcyclopropane and two transition structures involved in the reaction. Left transition structure for the concerted process right transition structure for the bond closure in the diradical pathway...

In certain systems — especially in those carrying more than one donor or acceptor group — a ring expansion to five-membered heterocycles 11 occurs at relatively low temperatures. This process can be classified as a 1,3-sigmatropic shift and is most conveniently explained by the intermediacy of the 1,3-zwitterion 10. 

The efficient formation of dihalocyclopropanes could be extended to allyl alcohol adducts 74, which are oxidized to 75. The resulting ketones are very useful synthetic building blocks. Reaction with alkoxide, for instance, affords dihydrofuran derivatives 76 via alkoxysubstituted cyclopropyl ketones 43). This ring enlargement might be a purely thermal process (1,3-sigmatropic shift), but other mechanistic possibilities could also be conceived (cf. Scheme 2). 

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