Mechanism Bordwell

The Sn2 mechanism as shown above involves the simultaneous movement of three pairs of electrons. However, Bordwell and co-workers contended that there is no evidence requiring that this bond making and bond breaking be in fact concerted, and that a true Sn2 mechanism is a myth. There is evidence both for and against " this proposal. There is also a review of the Sn reaction. ... 

In a few instances, anti or nonstereoselective elimination has been found this behavior is generally ascribed to the intervention of other mechanisms. For example, see Bordwell,... 

In a few instances anti or nonstereoseicctive elimination has been found this behavior is generally ascribed to the intervention of other mechanisms. For example, see Bordwell Landis J. Am. Chem. Soc. 1958, 80. 2450. 6383 Briggs Djcrassi 3. Org. Chem. 1968, 33. 1625 Smissman Li Creese J. Org. Chem. 1970,35. 1352. 

The relations become complex when all of scheme (216) must be considered. When 2-arylnitrocyclohexanes are involved, K = (117)/(118) 100, and the deprotonation rate ratio (118)/ (117) 200 (Bordwell and Vestling, 1967). Again, near-equality within a factor of two is indicated in the transition-state free energies. To assign a probable mechanism, we must investigate the energy relations implicit in (216). 

The latter mechanism has been termed Sn2 (intermediate) (Bentley and Schleyer, 1976), and the intermediate [34] has been referred to as an ion-sandwich (Bordwell et al., 1975 see also, Doering and Zeiss, 1953). The former term emphasizes the Sn2 character of the reaction, whereas the latter emphasizes the ionic... 

Bordwell, P. G., Cooper, G. D. The mechanism of formation of olefins by the reaction of sodium hydroxide with a-halo sulfones. J. Am. Chem. Soc. 1951, 73, 5187-5190. 

It is now widely accepted that a-halo sulfones are converted into alkenes under the basic conditions of the Ramberg-Backlund reaction via intermediate thiirane 1,1-dioxides (Scheme 7). Experimental data in support of this mechanism were first put forward by Bordwell in 1951, and extensive studies carried out since that time, particularly by Bordwell and Paquette in the late 1960s and early 1970s, have substantiated this overall scheme and have provided further insight into the mechanisms of the individual steps. Very little additional work concerning the mechanism of the reaction has been published since 1975. Consequently, only a summary of the essential features is presented here, and readers are referred to the detailed reviews, written by the leaders of the key research groups themselves, for the full data which support the following broad outline. ... 

By contrast, cis- and trans-2-h itene episulfides react smoothly with this phosphorus reagent to give essentially quantitative yields of cis- and trans-2-huteneSj respectively. The stereospecific nature of this latter reaction rules out any mechanism involving nucleophilic attack by phosphorus on carbon to give intermediates 45 and 46. This path, which is favored for the comparable reactions of epoxides (286), would lead to the opposite stereochemical result. Direct abstraction of sulfur by phosphorus is favored by Neureiter and Bordwell (239,240). 

Bordwell and coworkers reported the stereospecific quantitative desulfurization of episulfides with organolithiums.39) The mechanism of this reaction, however, remained undefined. In addition to a disrotatory concerted sulfurane decomposition [i.e. Eq. 9), p elimination from a 2-alkylthioalkyl-lithium may be envisioned (see Eq. 10). However, independent stereo-... 

This work was part of a thorough investigation into the mechanism of the Favorskii rearrangement by F. G. Bordwell and group, J. Am. Chem. Soc, 1970,92,2172. 

For a discussion, see reference 28. Also see Bordwell, F. G, Mecca, T. G. /. Am. Chem. Soc. 1972,94, 2119. It may also be possible that the ion pair forms during, and not before, attack of the base on the substrate. Saunders, W. H., Jr. Acc. Chem. Res. 1976, 9, 19 discussed the approaches used to distinguish concerted and nonconcerted elimination mechanisms. 

Significant advances have been made to the understanding of the RBR mechanism by Bordwell, Paquette, Taylor and others.Aspects of the mechanism have been studied theoretically and the results agree with experimental outcomes.The release of halide ion was found to be first order in both hydroxide ion and sulfone, lending credence to the proposed mechanism Deuteration experiments have shown that deprotonation is reversible and that all possible a-anions form fScheme 8.14). ... 

The formation of a-alkoxyketones according to Bordwell s mechanism (Scheme 8) requires a protic medium in fact 2-chloro-6-phenylcyclohexanone (24) (Tables) gives a-methoxyketones in both protic and aprotic media. Similarly, 2-chlorocyclopentanones yield solely a-alkoxyketones whatever the experimental conditions, whereas the a-halocycloalkanones from C 7 to C12 as also give essentially ring contraction. Finally, in the cyclic series, the... 

Bordwell" has proposed an alternative mechanism to account for the rate increase called frangomeric assistance in some (or perhaps all) cases. ... 

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