Radical bond formation from

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

Redox reactions with FAD involve the transfer of two hydrogen atoms to the part of the molecule shown in green. Typical reactions of FAD involve dehydrogenations—as in double bond formation from single bonds. Of course, one of the H atoms can be transferred to FAD as a proton—only one need be a hydride ion H , though both could be transferred as radicals (H ). 

It is apparent that much resourceful, imaginative experimentation has been done to resolve the question of C-C bond formation from methanol. Although the answer remains elusive, these experiments tell us at least what is probably not involved in the bond formation, particularly in the presence of zeolite catalysts. The Stevens rearrangement of oxonium ylide can be ruled out, as well as the carbocationic route invoking hypervalent carbon transition states. Not excluded are surface-bound species such as carbenoids and ylides. Again there seems to be a consensus that surface methoxyls are precursors to these reactive C- intermediates, which seems somehow to be "coming full circle", since surface methoxyls were early shown to be intermediates in the formation of DME, which is itself an intermediate in hydrocarbon formation. Finally, if the free radical character of the initiation step proves correct, the implications to zeolite catalysis will be far-reaching. 

With unsymmetrical alkenes, there are two regioisomeric meta adducts,1154 1160 1161 which are easily understood as the consequences of bond formation from the diradical intermediates 8.138 and 8.141, with the isolated radical centre forming a bond to either end of the allyl radical with little selectivity stemming from the presence of the substituent on the alkene. The low degree of selectivity is equally accommodated by a concerted reaction in which there is no intermediate. The main effect of having an electron-donating group, as in the reaction with ethyl vinyl ether 8.76, is that the ortho adduct 8.131 is the major product. The meta adducts 8.142-8.145 (R = OEt) are minor products (ortho.meta 65 35). With the rather less effective donor substituent, as in the reaction with vinyl acetate, the meta adducts 8.142-8.145 (R = OAc) are the major products (meta ortho 88 12). They show some selectivity in favour of the endo adducts (64 36 for R = OEt), and the major product 8.142... 

Virtually every possible reactive C intermediate has been invoked to explain the crucial step of initial C-C bond formation from methanol/DME. Proposed mechanisms can be broadly classified as carbenic, carbocationic, ylide, and free radical. In some proposals several of these categories are combined. 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

In the singlet state of Jt-type 1,3-diradical (e.g., TM, 2), there may also exist the through-space interaction between radical centers, i.e., p...q interaction (Fig. 9), in addition to the previously addressed cyclic -p-o -q-o- orbital interactions (Fig. 6). The through-space interaction is indispensable for the bond formation between the radical centers. The corresponding delocalization of the a-spin electron is shown in Fig. 9a. Clearly, the involvement of the through-space p... q interaction gives rise to two cyclic orbital interactions, -p-o -q- and -p-o-q-. From Fig. 9, one can find that the cyclic -p-o -q- orbital interaction can satisfy the phase continuity requirements for the a-spin electron the electron-donating radical orbital, p (D) can... 

With radicals of the benzyl type, 11 through 18, the dimerization equilibrium depends on the relative magnitudes of the energy gain arising from the C-C bond formation and of the ir-electron energy loss which results from a reduction of the conjugated system. 

By contrast, addition-elimination mechanisms in their simplest form begin with formation of an addition complex resulting from a well on the PES, followed by dissociation of the complex, yielding products. Both the entrance to and exit from the well may be hindered by barriers on the PES. Addition mechanisms are uncommon in radical -b saturated closed-shell reactions due to the difficulty of bond formation with the saturated species (ion-molecule reactions are exceptions). By contrast, additions are more common in radical -b unsaturated closed-shell species, where the double or triple bond allows a low barrier or barrierless pathway for addition of the radical into the 7i-bond of the stable species, such as the reaction... 

Even for the Cl oxidation process on the Ru02/Ti02 electrode, for which the individual electron transfer processes are sufficiently fast for thermodynamic equilibrium to be maintained, at least close to E°, it is highly unlikely that two electrons are transferred simultaneously from two CP ions that are exactly the right distance apart for Cl2 bond formation to take place. It is far more likely that some kind of radical intermediate is involved which is stabilised by complexation on the surface. 

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