Concerted fragmentation

Studies on the UV irradiation of the 1-phthalimidoaziridine system have been carried out (71JCS(C)988). When the photolysis was performed in the presence of an alkene, an exchange reaction was observed. Evidence was presented which showed that the reaction involved concerted fragmentation of the phthalimidoaziridine (114) to an alkene and... 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

Malonic acid or cyanoacetic acid can also be used as the nucleophile. With malonic acid or cyanoacetic acid as reactants, the products usually undergo decarboxylation. This may occur as a concerted fragmentation of the adduct.214... 

The reaction can occur by a concerted fragmentation process initiated by a two-electron oxidation. 

The key cyclization in Step B-2 was followed by a sequence of steps that effected a ring expansion via a carbene addition and cyclopropyl halide solvolysis. The products of Steps E and F are interesting in that the tricyclic structures are largely converted to tetracyclic derivatives by intramolecular aldol reactions. The extraneous bond was broken in Step G. First a diol was formed by NaBH4 reduction and this was converted via the lithium alkoxide to a monomesylate. The resulting (3-hydroxy mesylate is capable of a concerted fragmentation, which occurred on treatment with potassium f-butoxide. 

Concerted fragmentation of the transition state in the peroxy radical recombination yields carbonyl compound molecules in the excited triplet state, alcohol in its singlet ground state, and oxygen in its triplet ground state, in fulfilment of the spin selection rules. 

Such decay is known as concerted fragmentation. Peroxides have the weak O—O bond and usually decompose with dissociation of this bond. The rate constants of such decomposition of ROOR into RO radicals demonstrate a low sensitivity of the BDE of the O—O bond to the structure of the R fragment [4], Bartlett and Hiat [8] studied the decay of many peresters and found that the rate constants of their decomposition covered a range over 105 s-1. The following mechanism of decomposition was proposed in parallel with a simple dissociation of one O—O bond [3,4] ... 

The following peculiarities characterize the concerted fragmentation in comparison with the decay of a molecule with dissociation of one bond [3,4] ... 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

In the concerted fragmentation, the breaking of the a—c and b—d bond are going on at the same time. The overall reaction does not involve an intermediate. There is only one energy barrier (Fig. 1) in which both o bonds are partially broken and new n a = b and c = d are partially formed. 

Several pathways may be possible for the Beckmann fragmentation reaction (equations 68 and 69). Stepwise processes may occur (equation 68), but stereospecific concerted fragmentations are also common (equation 69). Stepwise processes may follow different routes, but in most cases the fragmentation may have the same intermediate as the Beckmann rearrangement the nitrilium ion. 

The reactions of 1,3-dihaloadamantanes with various carbanions in DMSO have been studied.18 For example, potassium enolates of acetophenone and pinacolone and the anion of nitromethane react with 1,3-diiodoadamantane (19) under photo-stimulation a free-radical chain process forms a 1-iodo monosubstitution product (20) as an intermediate, which undergoes concerted fragmentation to yield derivatives of 7-methylidenebicyclo[3.3.1]nonene (21). These and other results were interpreted in terms of the Srn1 mechanism. The work has been extended to the reactions of 1- and 2-halo- and 1,2-dichloro-adamantanes, examples of the SrnI mechanism again being found.19... 

Evidence in support of a stepwise thermal fragmentation of oxadiazolines in benzene at 110 °C has implicated carbonyl ylide intermediates and oxiranes as unstable precursors of the elimination product, ketene acetals, rather than concerted fragmentation to N2, acetone and carbene.26... 

In the solvolytic decarboxylation of potassium trans /9-halocin-namates (230) and acrylates in aqueous ethanol, both the ketones (226) and the acetylenes (227) (e.g. 12% acetophenone from (230, Ar = Ph) were found. The cis isomers (231) gave only acetylenes by an assumed concerted fragmentation (Grob1964). The intermediate formation... 

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