Pairing mechanism

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Tri- and pentacoordinate phosphoms compounds often react by electron-pair mechanisms as demonstrated by the nucleophilic reactivity of the lone pair electrons in trivalent compounds, and the electrophilicity of the phosphoms atom in the pentavalent compounds. Some compounds also react by free-radical mechanisms. The theoretical and synthetic aspects of the chemistry of phosphoms compounds have been described (6—9). 

Kinetic studies of the addition of hydrogen chloride to styrene support the conclusion that an ion-pair mechanism operates because aromatic conjugation is involved. The reaction is first-order in hydrogen chloride, indicating that only one molecule of hydrogen chloride participates in the rate-determining step. ... 

In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

The other mechanism which has been advocated58 is that known as the radical-pair mechanism , in which two cation radicals are thought of as intermediates held in a solvent cage so preserving the intramolecularity of the reaction, viz. 

The ion pair mechanism initially suggested by Darwish and McLaren28 (equation 2) has received further support from related studies conducted by several other investigators38-42. For example, Fava and coworkers38 have reported that during isomerization in acetic acid, optically active benzhydryl p-toluenesulfmate loses optical activity at a rate which is about two and a half times faster than the rate of sulfone formation, thus indicating that return from an ion-pair species is occurring (equation 3). 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

An interesting sulfone to sulfinate rearrangement has been observed by Schank and Schmitt100 in the thermal fragmentation of a-alkoxysulfones, and is believed to involve an ion-pair mechanism (equation 32). 

Recently, a study of this rearrangement has been repeated and extended in order to determine the influence of a- and y-substitution on the position of the 68-69 equilibrium in the presence of silica, and the utility of this reaction for a novel and convenient synthesis of highly substituted a, / -unsaturated ketones, by subsequent treatment with CuCl2 in methanol-water136. An ion-pair mechanism can also be suggested for the facile rearrangement of sulfone 70 to 71, a key intermediate in the Hoffmann-La Roche Sulfone Route to Vitamin A137. 

SCHEME 6. Suggested radical/anion-radical pair mechanism for the reaction of a-sulfonyl carban-ions with perhaloalkanes (CC14 as the example)553, R" = R3S02CR1R2, k2> > k. ... 

The occurrence of characteristic by-products can be taken as indicative of a radical pair mechanism in rearrangements. Thus the observation... 

The equilibrium between propargyl- and allenyl-tin compounds is not spontaneous, but it occurs in the presence of Lewis acids or coordinating solvents, and an ion-pair mechanism has been proposed (159). Substitution by iodine, or addition to chloral, occurs with propargyl/al-lenyl rearrangement (160, 161), analogous to the allylic rearrangement already mentioned. 

The preparation and chemistry of nickel trithiocarbonate complexes have been studied in detail, and both [NKCSs) ] and [Ni(CS4)2 have been isolated (379). Shul man and co-workers (379) reprepared the known [Ni(NH3)3(CS3)]and [Ni(en)3]CS3, and the kinetics of the reaction of Ni(II) with the trithiocarbonate ion in methanol was studied the results confirmed the previously proposed ion-pair mechanism (380). 

Sneen et al. formulated an intermediate-mechanism theory. The formulation is in fact very broad and applies not only to borderline behavior but to all nucleophilic substitutions at a saturated carbon. According to Sneen, all SnI and Sn2 reactions can be accommodated by one basic mechanism (the ion-pair mechanism). The substrate first ionizes to an intermediate ion pair that is then converted to products ... 

These data have also been explained as being in accord with the ion-pair mechanism Sneen, R.A. Larsen, J.W.. Am. Chem. Soc., 1969, 91, 6031. 

The mechanism as we have pictured it can lead only to an ortho product. However, a small amount of para product has been obtained in some cases. A mechanism in which there is a dissociation of the ArC—N bond (similar to the ion-pair mechanism of the Stevens rearrangement, p. 1419) has been invoked to explain the para products that are observed. 

The El reactions can involve ion pairs, just as is true for S l reactions (p. 398), This effect is naturally greatest for nondissociating solvents it is least in water, greater in ethanol, and greater still in acetic acid. It has been proposed that the ion-pair mechanism (p. 400) extends to elimination reactions too, and that the S l, Sn2, El, and E2 mechanisms possess in common an ion-pair intermediate, at least occasionally. ... 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

A recent investigation by Macomber (100) of the system where Rj = CH3, 126 (R2 and all other R = H), sheds additional light on the mechanism of homoallenic participation in solvolysis. In this case, the value of the integrated rate constant increased significantly throughout acetolysis, implying rearrangement to a system with comparable reactivity. The results were interpreted by means of an ion-pair mechanism shown in Scheme VII. 

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