Open concerted mechanism

Scheme 4 Transfer of the phosphoryl group between pyridine nucleophiles - an open concerted mechanism shown also is the charge distribution in the transition structure and that expected for two putative intermediates...

Characteristic of these models of active sites is the appearance of the OZn O Zn(0)Et species. Considering the structural properties of the discussed catalysts as well as the polymer chain microstructure and the structure of the end groups of poly(propylene oxide) obtained with the PhOZnOCeHi ( -Bu)OZnEt catalyst, a concerted mechanism of epoxide ring-opening polymerisation with catalysts containing multinuclear species, including those with condensed zinc atoms, has been postulated [65,74] ... 

Los and Simpson reported that the catalytic constant for the mutarotation of D-glucose is strongly dependent on the D-glucose concentration, and they suggested that this effect is occasioned by the incursion of a concerted mechanism for the formation of the open-chain form (see XLI — XLII). Such a mechanism was proposed by Swain and Brown ... 

Barrett has reported a systematic study on the cleavage of multiple cyclopropanes (see e g. equation 20). Interestingly, it was found that while the mercury-mediated ring opening of monocyclopropanes usually proceeds via a concerted mechanism, in the case of multiple cyclopropanes, it occurs through a stabilized free carbocation. Furthermore, the presence of an adjacent cyclopropane increases the rate of the ring opening of the first cyclopropane. 

Diels-Alder reactions of Ceo are generally believed to proceed via a thermally allowed concerted (suprafacial) process or a photochemical concerted (antarafacial) process [283-286]. However, an alternative stepwise (open-shell) mechanism for the Diels-Alder reaction has recently merited increasing attention [287-294], Along this line several reports describe an electron transfer with the formation of radical ion pairs as primary step of the Diels-Alder reactions, followed by a stepwise bond formation [295-301], The photochemical Diels Alder reaction of Ceo with an-... 

For both cycles, a concerted mechanism is suggested in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of 2. It has been inferred, from experimental data, that the system may involve a spiro arrangement [3, 5 a]. The selectivity toward epoxides can be enhanced by the addition of Lewis O- or Wbases such as quinuclidine, pyridine, pyrazole or 2,2 -bipyridine to the system [3, 6d, lOg-k]. Lewis acids catalyze ring-opening reactions and diol formation. These reactions are suppressed after the addition of Lewis bases. An... 

The discrimination between these mechanisms in both 1,1- and 1,2-insertions depends on the electronic characteristics of the element-hydride compound. Unsaturated E-H compounds, (i.e., hydrides of elements with an unfilled valence orbital) typically proceed via a concerted mechanism, because of their open pathway for the unsaturated substrate to approach the element-hydride a bond. Substrates without an empty orbital typically proceed nonconcertedly, with the distinction between radical and heterolytical mechanisms depending on the polarity of the E-H bond. 

Zipse has suggested on the basis of computational work that both the ipso and cine processes might take place via open-shell concerted mechanisms [26, 41]. 

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