Spiro transition-state mechanism

The epoxidation of a,y3-unsaturated esters by dimethyldioxirane (DMDO) in dried acetone was less sensitive to steric effects and the rate was slower than that found for simple alkenes. DFT calculations based on a spiro transition-state mechanism were in good agreement with the observed relative rates except for compounds with cis- 3-substituents. The reaction of DMDO with 5,7-dimethoxyflavones produced the... 

Other advantages include a mechanism that allows one to rationalize and predict the stereochemical outcome for various olefin systems with a reasonable level of confidence utilising a postulated spiro transition state model. The epoxidation conditions are mild and environmentally friendly with an easy workup whereby, in some cases, the epoxide can be obtained by simple extraction of the reaction mixture with hexane, leaving the ketone catalyst in the aqueous phase. 

The spiro transition state is now generally accepted as the mechanism in operation during both dioxirane- and oxaziridine-mediated epoxidation. This conclusion is supported by theoretical and computational studies [31-33]. 

The mechanism for the conversion of the A -oxide (94) to the o-methylaminophenylquinoxaline (96) involves an initial protonation of the A -oxide function. This enhances the electrophilic reactivity of the a-carbon atom which then effects an intramolecular electrophilic substitution at an ortho position of the anilide ring to give the spiro-lactam (98). Hydrolytic ring cleavage of (98) gives the acid (99), which undergoes ready dehydration and decarboxylation to (96), the availability of the cyclic transition state facilitating these processes. ... 

One mechanistic matter that has caused quite a bit of general consternation about a decade ago concerns the experimental evidence for the involvement of diradical intermediates (proposed as sources for the observed radical products) in dioxirane epoxidations, which were thought to be formed through induced peroxide-bond homolysis by the alkene. Nonetheless, rigorous experimental and high-level theoretical work disposed such radical chemistry in the epoxidation of alkenic substrates. The latter computations unequivocally confirm the established concerted mechanism, in which both CO single bonds in the incipient epoxide are concurrently formed by way of an asynchronous, spiro-structured transition state for the oxygen transfer. 

Various new observations, including the higher reactivity of cis-alkenes, isotope effects, and syn stereospecificity, confirm a concerted mechanism with a transition state of spiro geometry. Theoretical studies provided support to this view.1184-1188 Radical pathways were shown by computational methods and experiments to also be possible.1189,1190... 

A concerted, spiro-structured, oxenoid-type transition state has been proposed for C-H oxidation by dioxiranes (Scheme 5). This mechanism is based mainly on the stereoselective retention of configuration at the oxidized C-H bond [20-22], but also kinetic studies [29], kinetic isotopic effects [24], and high-level computational work support the spiro-configured transition structure [30-32], The originally proposed oxygen-rebound mechanism [24, 33] was recently revived in the form of so-called molecule-induced homolysis [34, 35] however, such a radical-type process has been experimentally [36] and theoretically [30] rigorously discounted. 

Many mechanisms have been proposed for this reaction, such as the epoxidation via initial attack of a hydroxyl cation, 1,3-Dipolar Cycloaddition of a hydroxycarbonyl oxide to an olefinic double bond, and the commonly accepted planar butterfly transition state, " by which the n HOMO orbital of the olefin approaches the terminal oxygen of perbenzoic acid and interacts with the a LUMO of the 0-0 bond at 180". The planar butterfly transition state is further extended by Sharpless to a spiro-trunsition state, which has been consolidated by many other investigators. An illustrative mechanism from mCPBA epoxidation is provided here. 

Having shown earlier that in 4-vinylcyclohexene, the main product from the dimerization of butadiene, rearrangement of the vinyl group into the ring on thermal automerization is faster than the loss of optical activity, Doering and Brenner now report rate constants for the three processes involved, after resolution of products into optical antipodes and analysis of isotope transpositions in each. Stereo-electronic effects guide the cycloaddition of dichloroketen to 3,3-dimethylcyclohexene and l-methyl-5,5-dimethylcyclohexa-l,3-diene ° whereas steric effects predominate in the additions to 3,3-dimethylpentene and the spiro-diene (56). Evidence was obtained for a non-parallel transition state for addition as required by the [2s + 2a] or [2s + 2s + 2s] mechanisms this was also found in additions of dichloroketen to methylenecyclohexenes. 

The redistribution of free voliunes also influences the sub-glass transition temperatures Tp and T observed for photoisomerization reactions in polymer solids. T, Tp and T are frequency-dependent, and the response of any process to the transitions at these temperatures depends on the time scale. The time scale of photoprocesses may not be equal to those of DSC or dynamic mechanical methods, which are of the order of 10 to 1( Hz. However, for photodecoloration of the merocyanine form of spiro-bepzopyran in polycarbonate film under steady-state irradiation of 560 nm light after laser-single-pulse induced coloration, it was found that the Arrhenius plot of the apparent rate coefficient broke at T (150 °C), Tp (20 C), and T (—120 °Q of the matrix polycarbonate these temperatures are the ones determined by dynamic mechanical measurements. The excited state lifetime of the merocyanine form in polycarbonate was 1.8 ns . Hence, the decolorating isomerization during the lifetime proceeded only in a small fraction of the molecules surrounded by a sufficient amount of free volume. Thus, it is likely that the temperature dependence of the apparent rate coefficient reflecting the relative quantum yield is controlled by the frequency of redistribution of free volumes, which may be comparable with the frequency determined by dynamic mechanical measurements. 

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