Concerted reaction pathways

Offer an explanation for the facility of the reaction, as compared to the vinylcyclopropane rearrangement of hydrocarbons, which requires a temperature above 200°C. Consider concerted reaction pathways which would account for the observed stereospecificity of the reaction. 

It follows that a proper description of the stepwise and concerted reaction pathways requires a three-dimensional representation as illustrated by Figs 8a and 8b. 

Solvolysis and isomerization may either proceed through a common ion-pair reaction intermediate, or the isomerization reaction may proceed by a separate concerted reaction pathway that avoids formation of this intermediate kcon Scheme 10). Hammett reaction constants of = —4.9 and pjt = —5.5 for reactions of X-4-0(S)CPb were calculated from the data in Table 1. The larger negative value... 

Alkyl migration in [Mn(R)(CO)5] species is assumed to follow a concerted reaction pathway, namely, concomitant bond breaking and bond formation (3). Calderazzo and Cotton investigated the reaction of CO with [Mn(CHj)(CO)5] and obtained an activation energy of 61.9 kJ/mol for the alkyl migration process (95). This value is well below the reported bond dissociation energy of 184 kJ/mol for the manganese-methyl carbon bond... 

In the late 70s and early 80s, tunneling theories indicated a preference for tunneling in symmetric double wells vs. asymmetric wells, supporting the concerted double proton transfer [63]. Thus, it seemed at that time that tunneling observed experimentally was only compatible with a concerted reaction pathway, supported... 

The ZnCh reaction seems to pass through a pericylic mechanistic pathway while the BF3-OEt2-cata-lyzed reaction resembles a Mukaiyama aldol reaction. Danishefsky described the process in which an activated diene is condensed with an aldehyde as a cyclocondensation reaction, thus avoiding the implication of a concerted reaction pathway in cases where aldol products are unambiguously isolated (Scheme 2). 

According to the concerted reaction pathway, in most cases the dipolarophile stereochemistry (( )/(Z)-configuration) is preserved during the cycloaddition. For cycloaddition reactions of nitrones to olefins both e/5 /-discrimination as well as endolexo selectivity have to be discussed." ... 

A final example of a reaction that may involve an azabicyclobutonium ion comes from studies of Deyrup and Clough on the interconversions of aziridine carboxylates and j -lactams. The stereoselectivity of the reaction of certain substrates, e.g., (27), suggests the intermediacy of cations such as (28). The NMR spectrum of the anhydride (29) in liquid SO 2 with added p-nitrobenzenesulfonyl chloride gave evidence of an intermediate. Based on the chemical shift data, the presence of (28) was claimed. The simplicity of the reported spectral data and the fact that (30) apparently did not ionize to (28) upon dissolution into a saturated solution of antimony pentafluoride in liquid sulfur dioxide suggests caution in accepting the conclusion that (28) was observed in the NMR experiments. It was pointed out that concerted reaction pathways are available that could account for the conversion of jS-lactams to aziridine carboxylates. Since a concerted mechanism can also be written to explain the stereospecific conversion of (27) to (30) [Eq. (19)], the intermediacy of (28) remains questionable. 

However, the dipole might be stabilised by the adjacent central hetero-atom X (nitrogen, oxygen, or sulfur) through resonance, and a non-concerted reaction pathway might also occur. Consequently, in some cases, the original stereochemistry of the alkene is not necessarily conserved, as depleted in Scheme 1.2. 

The reaction is diastereoselective a (Z)-olefin like 40 delivers the cis product 41, and the ( )-olefin 42 delivers the trans product 43 (Scheme 4.18). A concerted reaction pathway can be assumed. This is the reason why for the C,H-insertion also a direct concerted insertion rather than a stepwise process initiated by a [1,5]-H shift was assumed in Scheme 4.15 (species V directly to X and not V via W to X). 

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