Dissociation-recombination pathway

Rautenstrauch and colleagues have shown the rearrangement of vinyl ethers 47 to 48 (equation 26) and proposed an ionic dissociation-recombination pathway as the mecha-... 

Warpehoski, M. A., Chabaud, B., Sharpless, K. B. Selenium dioxide oxidation of endocyclic olefins. Evidence for a dissociation-recombination pathway. J. Org. Chem. 1982, 47, 2897-2900. 

Rearrangements of allylic ethers can follow several pathways (Scheme 1). The 2,3- and 1,4-routes are symmetry allowed concerted processes, whereas the 1,2- and 3,4-rearrangements proceed in a stepwise manner dirough radical dissociation and recombination (equation 2). In fact, all four pathways have been experimentally observed with bis-7,7-(dimethyl)allyl ether (24 equation 8). Anions derived from allyl vinylcyclopropylmethyl ethers (e.g. i) rearrange by competing, 1,2, 1,4, homo-2,5, homo-4,5 and radical dissociation-recombination pathways. Nakai has shown that 1,4-rearrangement proceeds with retention of stereochemistry in the cyclohexenyl system (29 equation 9), as required by orbital symmetry considerations. ... 

HCN is formed in interstellar clouds through one of two major pathways via a neutral-neutral reaction (CH2 + N HCN + H) and via dissociative recombination (HCNH + e HCN + H). The dissociative recombination pathway is dominant by 30% however, the HCNH must be in its linear form. Dissociative recombination with its structural isomer, H2NC produces hydrogen isocyanide (HNC), exclusively. 

Molecular ions are efficiently destroyed in reactions of dissociative recombination with electrons and negative ions. These processes are especially fast at low temperatures, with typical rates of about 10-7cm-3 s-1 at 10 K and negative temperature dependence, fl —0.5 (Woodall et al. 2007). For nearly all observed species, dissociative recombination is an important formation pathway (e.g. water and hydrocarbons). Often, at later evolutionary times, > 105 years, dissociative recombination is balanced by protonation reactions, e.g. CO + H+ -> HCO+ + H2 followed by HCO+ + e - CO + H. Dissociative recombination rates are not difficult to derive but branching ratios and products are not easily predictable (e.g. Bates Herbst 1988 Spanel Smith 1994). 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Pathway I was observed for all the 02 complexes studied, strained or unstrained, as well as for the unstrained CO-complexes. This particular pathway is the same one observed in the photodissociation of the natural heme complexes (3,4) (HbCO, MbCO, HbO and MbO ) with the exception that there is no detectable geminate recombination to the limit of our experiment, 50 ps. Pathway II, observed for the strained-CO complexes, reveals the presence of a fifth intermediate X found early in the dissociation that is either absent or undetectable in the natural or synthetic heme complexes following pathway I. The kinetics associated with the evolution of these intermediates will be discussed shortly. First, it is appropriate to examine in some detail the experimental AA difference spectra of two representative complexes, 1 -CO and 1-ET-CO. A discussion of 1-ST-CO and l-ET-O is also included for comparative purposes. ... 

Cis-trans isomerization can take place either photochemically or in the dark, but the reaction pathways are quite different. In the light-induced process the reaction goes through a tetrahedral intermediate formed from the triplet excited state, whereas the dark reaction involves a dissociation of the complex, followed by recombination. In the latter case the presence of free glycine is demonstrated by the use of radioactive tracers no free glycine appears in the photochemical reaction. 

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