Mechanism solvent-cation donors

However, the well-known ability of organolithium compounds to form associated species or to form complexes with electron donor compounds (240—242) provides strong support for mechanisms involving cationic attack by the lithium cation on the monomer prior to an anionic addition. With three orbitals available for coordination, a monomeric lithium alkyl should be able to complex both double bonds of a diolefin to provide the orientation for making cis-1,4 polymer and still have an orbital available for forming associated species in hydrocarbon solvents. The lithium orbitals are presumed to be directed tetrahedrally. Looking at the top of a tetrahedron with the fourth lithium oibital above and normal to the plane of the paper, the complex could have structure A below. In the transition state B for the addition step, the structure... 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

At very long separations, for example, transfer to the biphenyl cation radical over 34 A in 1C)2 s, electronic interactions seemed to be propagated by ion states of the solvent (25), although quantum mechanical tunnelling may be important when diffusion is blocked by steric factors or by immobilization of the reagents (26). Perhaps most convincing are Miller and Closs demonstration of intramolecular electron transfer between donor-acceptor pairs separated by a rigid steroid spacer (27, 28). In 1, for example,... 

Because systematic variations in selectivity with reactivity are commonly quite mild for reactions of carbocations with n-nucleophiles, and practically absent for 71-nucleophiles or hydride donors, many nucleophiles can be characterized by constant N and s values. These are valuable in correlating and predicting reactivities toward benzhydryl cations, a wide structural variety of other electrophiles and, to a good approximation, substrates reacting by an Sn2 mechanism. There are certainly failures in extending these relationships to too wide a variation of carbocation and nucleophile structures, but there is a sufficient framework of regular behavior for the influence of additional factors such as steric effects to be rationally examined as deviations from the norm. Thus comparisons between benzhydryl and trityl cations reveal quite different steric effects for reactions with hydroxylic solvents and alkenes, or even with different halide ions... 

Triorganostannates such as LiSnRs are sufficiently electron rich to be potential candidates for electron transfer to halides [52,126]. As for the silicon analogues MSiRa [115], the electron transfer mechanisms for the reduction by such at-complexes may be quite complicated because of the solvent-induced separation between the SiRJ donor anion and the a acceptor cation M. ... 

Another mechanism for the 2n -f 27t ) photocycloaddition of alkenes via electron transfer is the reaction that proceeds via a triplet state which is produced by a back-electron transfer from a radical anion of the electron acceptor to a radical cation of the electron donor. The triplet state alkenes generated by this way can undergo the cyclodimerization (Scheme 22). Farid showed that the DCA-sensitized (2n 3- 2n) photocyclodimerization of 1,2-diphenylcyclo-propene-3-carboxylate occurs via the triplet state of the cyclo-propene in acetonitrile [84]. In this photoreaction, two types of the (An -(- 27t) photocycloaddition reactions take place between DCA and the cyclopropene depending upon solvents. One type of the cycloadduct is produced in benzene via exciplex and the other type of the photocycloadduct is produced in... 

There are indications, however, that the reduction mechanism in aprotic solvents may be different for cations in which one of the substituents possesses a-hydrogens. The reduction of a series of lO-methyl-lO-arylphenoxarsinium salts of the general structure 1 has been studied in DMF by polarography in the absence and in the presence of added proton donors. ... 

Gutmann 9 has rejoined that the dichloroiron(lll) ion, [FeC (solvent)], is not found in dilute solutions in phosphorus oxychloride but only in concentrated solutions or those to which a strong acid such as SbCl5 has been added. In such cases the chloride donor ability of the solvent has been exceeded and chloride ions are abstracted from the iron(lll) chloride. This point was made earlier- by the observation that the controversy is at least partly a semantic one. The only characteristic property of the solvo-cations and solvo-anions in the solvent system autoionization is that they are the slronnest acids and bases that can exist in that particular solvent without hei/if leveled. In trielhyl phosphate (a nonleveling solvent) the dichloro-iron(lll) ion is perfectly stable. In phosphorous oxychloride a mechanism for leveling exists, namely ... 

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