Radical, Stevens rearrangement

Under similar profiles of raising in temperature, it was shown that the selectivity favoring 1,2 Stevens rearrangement is exemplified under the action of microwaves. A tentative explanation can be to consider that, under the action of radiation, the more polar mechanism (1,2 ionic shift) is favored when compared to less polar one (2,3 radical shift). Maybe this result is indicative of a competition between ionic and radical pathways. 

The other major reaction pathway for oxonium ylide is [l,2]-shift (Stevens rearrangement). Compared with [2,3]-sigmatropic rearrangement, which is an orbital symmetry-allowed concerted process, the [l,2]-shift has higher activation barrier, [1,2]-Shift is generally considered as stepwise process with radical pair as possible intermediates. 

Eq. (25) 5 . Various products have been reported in the photocatalyzed oxidation of dimethyl sulfide depending on the initial concentration of thioether. On TiO2, CdS, or ZnSe, a Stevens rearrangement occurs, Eq. (26) The key intermediate appears to be a dimethylsulfide dimer cation radical, and the reaction is only efficient in protic solvents. 

Scheme 2.6 1,2-Migration via a radical pair in a Stevens rearrangement of ylid 13.

Finally, the ammonium ylide derived from 21 gave rise to amine 22 in 95% ee. This example also illustrates the radical pair nature of the Stevens rearrangement in that diamine 23 and alkane 24 were isolated as... 

While [2,3]-onium ylide rearrangements are proceeding most often by concerted pathways (review [388]), [l,2]-onium ylide rearrangements (Stevens rearrangements) often involve radical intermediates (recent reviews [389, 390]). A study aimed at the comparison of Rh(II) and Cu(II)-catalyzed oxonium ylide reactions of oxygenated diazoketones using 3 mol% of the former and 15 mol% of the latter showed that the copper-catalyzed process provided better yields and selectivities for [1,2]-rearrangement products (see Part 2, Sect. 6, Fig. 107) [391, 392]. 

Some other stepwise reactions seeming to disobey the rules, are the 1,2-shifts of ylids, like the Stevens rearrangement 6.107 >6.109. The symmetry-allowed reactions, suprafacial-with-inversion 6.74 or antarafacial-with-retention 6.75, are unreasonable—there is no flexibility for migration across only two atoms, and yet reactions like this take place easily. It is now clear that these reactions are stepwise, taking place by homolytic cleavage 6.107 —> 6.108, followed by rapid radical recombination 6.108 —> 6.109. 

A third mechanism is also proposed in which ion pairs are formed instead of radical pairs. A variant of the Stevens rearrangement is the rearrangement of sulfur ylides. 

If the Stevens rearrangement is a concerted reaction, it is a symmetry-forbidden process based on the Woodward-Hoffmann rules. Indeed, it was shown to occur via an intramolecular hemolytic cleavage-radical pair recombination process, which explains the lack of crossover products and the observed retention of configuration at the migrating... 

Baldwin, J. E., Erickson, W. F., Hackler, R. E., Scott, R. M. Simultaneous observation of a radical pathway and retention in a Stevens rearrangement of a sulfonium ylide significance for a general theory of ylide rearrangements. J. Chem. Soc., Chem. Common. 1970, 576-578. 

It is apparent that much resourceful, imaginative experimentation has been done to resolve the question of C-C bond formation from methanol. Although the answer remains elusive, these experiments tell us at least what is probably not involved in the bond formation, particularly in the presence of zeolite catalysts. The Stevens rearrangement of oxonium ylide can be ruled out, as well as the carbocationic route invoking hypervalent carbon transition states. Not excluded are surface-bound species such as carbenoids and ylides. Again there seems to be a consensus that surface methoxyls are precursors to these reactive C- intermediates, which seems somehow to be "coming full circle", since surface methoxyls were early shown to be intermediates in the formation of DME, which is itself an intermediate in hydrocarbon formation. Finally, if the free radical character of the initiation step proves correct, the implications to zeolite catalysis will be far-reaching. 

The yield and ratio of the two Stevens rearrangement products are dependent on both the base and the solvent however, the ortho-rearrangement product was only detected when butyllithium was used in hexane. Variation of the halide ion had only a small effect on the ratio of rearrangement products. Several mechanisms were considered 93) as routes to the formation of the rearrangement products, 1) ion-pairs, 2) cationic rearrangement, 3) carbenoid, 4) dimetallated intermediates, 5) free-radical, 6) predissociation of the ammonium salt and, 7) sigmatropic shift. 

Proton magnetic resonance studies of the reaction showed stimulated emission and enhanced absorption of the benzylic protons on N-methyl-N-( -phenylethyl)-aniline. This observation suggests that the benzyl ylid untergoes a Stevens rearrangement by a free radical pair methyl migration 94 9 >. 

In an nmr study of the reaction of N,N-dimethylbenzylamine with benzyne, Lepley 98> observed a free radical intermediate (path 8) which is formed from the ylid and which rearranges to N-methyl-N-(a-phen-ethyl)aniline via a Stevens rearrangement. Prior to this report 8 ") radical intermediates had not been observed in the Stevens rearrangement but their presence had been suggested m>. This radical path must be considered in all reactions of nitrogen ylids for it may be the route by which many rearrangement reactions occur. 

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