Discrete intermediates

CO3) XH2O and Ce202(C03) may also be present. Thermal decomposition causes loss of crystallization water followed by the formation of ill-defined hydroxy- and oxy-species and eventually, at C, Ce02. Discrete intermediates are not seen because of the concomitant oxidation of Ce(III) to Ce(IV). 

The evidence for the involvement of 1,3-dipoles as discrete intermediates includes the observation that the reaction rates are independent of dipolarophile concentration. This fact indicates that the ring opening is the rate-determining step in the reaction. Ring opening is most facile for aziridines that have an electron-attracting substituent to stabilize the carbanion center in the dipole. 

These reactions, while not very important for high-energy donors in the absence of azulene, do help define what the potential energy diagrams should look like, that is, the fram-stilbene triplet is a discrete intermediate while the cij-stilbene triplet is a high vibrational form of the phantom triplet. Herkstroeter and McClure(13> have been able to observe the fro/w-stilbene triplet via flash photolysis using low-temperature glasses however, no similar triplet was observed for cfr-stilbene. Only when they studied cfr-stilbene-like... 

The multistep radical elimination may involve the generation of discrete intermediates, which for instance could be formed by a cyclization process7) such as 15- 16- 17e). Alternatively, there may be no intermediate involved in the elimination sequence, but the actual transition states 19, 22 are substantially lower in energy due to the anchimeric assistance of suitable functional groups9,10) (4). 

As a 3-step mechanism, the electron-transfer paradigm provides a pair of discrete intermediates [D, A] and D+, A for the prior organization and the activation, respectively, of the donor and the acceptor. The quantitative evaluation of these intermediates would allow the overall second-order reaction (k2) to be determined. Although the presence of [D, A] does not necessarily imply its transformation to D+, A-, a large number and variety of donor/ acceptor couples showing transient charge-transfer absorptions associated with [D, A] have now been identified. In each case, the product can be predicted from the expected behavior of the individual ion radicals D+ and A-. Consider for example, the labile 1 1 benzene complex with bromine that has been isolated at low temperatures and characterized crystallographically (Chart 9).256... 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

A re-evaluation of the Hofmann rearrangement in electron-deficient systems has been undertakenA detailed study of the discrete intermediates, and the sensitivity of the intermediates and products to reagents and to each other in the Hofmann rearrangement of A-a-tosylasparagine, has led to a process that produces 2-(S)-(tosylamino)-/ -alanine on a large scale." ... 

The non-classical ion may exist as edge or face protonated nortricyclene, represented by stmcture [9], as an alternative. This species is not a resonance stmcture of [7] or [8]. If it is a discrete intermediate, one might expect to be able to detect the displaced proton via a suitably designed exchange experiment. 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

Phenonium ions as discrete intermediates in certain Wagner-Meer-wein rearrangements. J. Amer. chem. Soc. 86, 3767 (1964). 

It is obvious that much further work is needed to unravel all the mysteries of enone rearrangements. Not the least of the problems to be explained is the retention of absolute configuration about C-9 in the rearrangement of 35. The shift of C-l from C-9 to C-10 and the shift of C-9 from C-I to C-4 must occur almost synchronously, and the existence as discrete intermediates of either biradical or charge-separated species with a planar C-9 seems unlikely. 

Kinetic experiments show that the rate of thermal decomposition of 4-benzyl-5-tosylimino-AM,2,3,4-thiatriazoline is not influenced by the presence of enamines (the first-order rate constant is 42 x 10 5 s l in CC14 at 60°) excluding attack of alkene on the thiatriazoline with simultaneous loss of nitrogen.65 Instead, this result indicates formation of a discrete intermediate, which apparently is either the thiaziridine (19) and/or the 1,3-dipolar species (20) [Eq. (24)]. 

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