Singlet state reactivity mechanisms

The lack of any difference in the rate of isomerization between fluoro-sulfonic acid solutions of 34 which had been thoroughly degassed, and those which were saturated with oxygen, suggests that the reaction does not proceed via a triplet mechanism. In fluorosulfonic acid no unproton-ated acid is detected, ruling out the possibility of n,7r excitation. Thus, there is little doubt in this case that it is the Tr,Tr singlet state which is the reactive species. Experiments carried out with a variety of methyl-substituted protonated cydohexadienones have likewise ruled out the... 

From examination of Fig. 11, it is inferred that the zn-n state is less reactive, and a biradical mechanism should be the major reaction pathway. The degenerate stabilizing perturbation of the bonding levels is missing, and concerted pathways are not likely if stabilized only by much smaller secondary interactions. If the hi-n singlet state could be intercepted in some way, the all-suprafacial concerted mechanism would be favored [K(ji ) -0(jr )] relative to the suprafacial-antarafacial mechanism [O(tt) - -K( i )]. 

The di-ji-methane reaction results in a 1,2-shift in a 1,4-diene unit, but such shifts sometimes occur in monoalkenes (2.44, and the mechanism must be different. The substrates are usually tetra-substituted ethylenes, and it is suggested that the reactive excited state is a Rydberg singlet state, which rearranges initially to give a carbene. Support for such a mechanism comes from the structures of products obtained from 1,2-dimethy(cyclohexene (2.45), which are... 

Secondly, we will look at some of the factors that govern autoxidation by examining its well-established mechanism. A molecule in its triplet state is a biradical, and an organic chemist expects a radical to be highly reactive. In fact, triplet oxygen does react very fast only with radicals, whereas the vast majority of organic molecules are in the singlet state, and the reaction... 

However, this model can also be generalized to other cydicalkenes. The ISC reactive conformation of the intermediary triplet biradicals is important for stereoselectivity, as the rate constant for ISC, which is controlled by a spin-orbit-coupling (SOC) mechanism, is heavily dependent on the orientation of the two spin centers. The importance of the ISC process was reasonably proved by the low mdo-sclcctivity in PB reactions with naphthaldehydes or aliphatic aldehydes, in which the excited singlet states may react with alkenes [34]. The stereoselectivity observed in the PB... 

Reactivity with Diazo Oxides and Azides. Diazo oxides, e.g. 2,6-di-f0rf-butyl-4-diazo-2,5-cyclohexadiene-l-one (184, R = terr-butyl) or 2,6-diphenyl-4-diazo-2,5-cyclohexadiene-l-one (184, R = phenyl) are thermolabile and photolabile. Car-benes 185 are formed from 184 as reactive intermediates. The process proceeds in a one-step mechanism in which the carbene species reacts in the singlet state [238], The attachement of 185 to saturated hydrocarbon chains proceeds via insertion in C—H bond during processing. LDPE and PP were modified in this way [239, 240]. Poly(oxymethylene) was modified similarly. 

The TTyir singlet states of the cyclopropenes (101) are reactive and lead to re2irrangement products by way of a carbene (102) mechanism as outlined above. Typical examples of the reaction are shown in the Scheme 8, where it can be seen that the rearrangement affords the isomeric substituted furans (103) and (104) from (101, X = 0) and pyrroles (105) and (106) from (101, X =NMe). This latter reaction also yields the diene-substituted pyrrole (107), which is good evidence for the proposed carbene mechanisms. The thiophene derivative (101, X = S) yields a single product (108) on photoexcitation. 

Singlet methylene is much more reactive than triplet methylene. In fact, methylene is more stable in its triplet ground state by some 9 kcal mol than its singlet state." " Apart from the well-known reaction of olefins, methylene indiscriminately inserts into C-H bonds, " From the time of the original discovery of this latter reaction by Meerwein et al." and Doering and coworkers," a question has remained whether the mechanism of the transformation is a direct concerted reaction [Eq. (6,126)] or a two-step abstraction-recombination pathway [Eq. (6.127)] ... 

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