Nonconcerted process

Is the reaction concerted As was emphasized in Chapter 11, orbital symmetry considerations apply only to concerted reactions. The possible involvement of triplet excited states and, as a result, a nonconcerted process is much more common in photochemical reactions than in the thermal processes. A concerted mechanism must be established before the orbital symmetry rules can be applied. 

Cyclobutanes can also be formed by nonconcerted processes involving zwitter-ionic intermediates. The combination of an electron-rich alkene (enamine, enol ether) and an electrophilic one (nitro- or polycyanoalkene) is required for such processes. 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of the 3-hydroxyimi-nomethyl-l,2,5-oxadiazole has been carried out by using semi-empirical modified neglect of diatomic overlap (MNDO) and ab initio Hartree-Fock procedures. Different transition structures and reactive pathways were obtained in the two cases. Semi-empirical treatment shows asymmetrical transition states and nonconcerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous processes involving symmetrically located transition states <1998JMT(452)67>. 

Diphenylketene and more reactive haloketenes will undergo cycloaddition with allenes such as 1,1-dimethylallene giving regioisomeric mixtures of cyclobutanones. Yields of these reactions are only modest5-7 and distributions of isomers are solvent dependent, which is indicative of a nonconcerted process involving ionic intermediates.8... 

The mechanism of the disproportionation reaction is currently receiving attention. Two basic pathways have been suggested. The first proposes the concerted transformation via a symmetrical intermediate, variously described as quasi-cyclobutane (11), tetramethylene (12), etc., of one pair of olefins into another. The second mechanism postulates a nonconcerted process involving metallocyclopentane intermediates (13). Experimental evidence for both mechanisms has been presented, but the issue is still unresolved. 

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

In the first stereochemical study, Brook has observed nonstereospecific addition of methanol to certain isolable silenes65,66. Although the precise stereochemistry of the products was not established, a 1/3 mixture of syn/anti or anti/syn isomers was obtained (equation 13). The results indicate a nonconcerted process for the addition of alcohol to silenes. Wiberg has proposed a two-step mechanism involving an initial formation of a silene-alcohol complex, in accord with the formation of nucleophilic adducts, followed by proton migration from the alcohol to the carbon of the silene (equation 14)59,61. This mechanism may be compatible with the results obtained by Brook and coworkers, if rotation around the silene s Si—C bond occurs faster than the proton migration. 

Fig. 2. Schematic classification of reaction mechanisms in terms of partitioning of modified More O Ferrall diagrams. Line a represents an asynchronous but concerted process, line b a nonconcerted process...

Fig. 2. Plots of bond indices for two new bonds formed in a cycloaddition for synchronous and simultaneous concerted reactions, and for a nonconcerted process...

Fig. 3. Schematic molecular orbital level diagrams for concerted and nonconcerted process involving N bonding orbitals in the reactant and product...

A stepwise mechanism for the formation of Binor-S was offered by Katz, a strong proponent for nonconcerted processes 9). Katz based his position on doubts regarding the actual structure of Binor-S stemming from the apparent absence of some ir bands in the Raman spectrum. He therefore suggested the anti structure 57. Such a structure would, of course, preclude the more symmetrical involvement of metal centers proposed by... 

The comparison of the walk rearrangements in the vinylogous systems bi-cyclo[2.1.0] pentene, norcaradiene, and bicyclo[6.1.0] nonatriene is particularly instructive in the context of the borderline between concerted and nonconcerted processes. Each rearrangement turns out to occur with inversion at the migrating carbon atom. In the case of bicyclopentene and bicyclononatriene, the stereochemical course is in accord with the predictions of orbital symmetry (4), but this is not so for the norcaradiene. Obviously stereochemistry is no reliable criterion for the distinction between a concerted and nonconcerted process in these cases. 

S. ferax and related sterol-producing Oomycetes synthesize lanosterol rather than cycloartenol (10) in Stage II. The mechanism of squalene-oxide cyclization to lanosterol involves a nonconcerted process in which a terminal intermediate,... 

When j8-fluoropropionyl-CoA is incubated in the presence of HC03, ATP, and Mg(II) and added to the enzyme, ADP is formed and F" is released. However, the rate of F" release is 6 times that of ADP formation and no evidence for the formation of fluoromethylmalonyl-CoA was obtained. The release of an F" ion is an indication of an abstraction of an a-proton of the substrate, and the formation of ADP corresponds to the formation of biotin-C02. Therefore, these results indicate that hydrogen abstraction can occur without concomitant CO2 transfer from biotin-C02 to the substrate. The concerted mechanism proposed earlier is thus not applicable when propionyl-CoA carboxylase acts on j8-fluoropropionyl-CoA. The authors also suggest that the nonconcerted process should occwr with normal substrates and that there must be a group at the active site of the enzyme which functions as proton acceptor. Furthermore, it was shown that this site does not exchange protons with the solvent. 

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