By Peter A Byrne
In this thesis, the writer outlines the invention of an influence universal to consultant examples of all Li salt-free Wittig Reactions. the results of this type of universally acceptable impact are that every one such Wittig reactions ensue throughout the similar mechanism. even though the Wittig response used to be first found in 1953, its response mechanism hasn't ever been definitively settled with many various versions proposed and disproved. The paintings during this thesis exhibits conclusively that for [2+2] cycloadditions all Wittig reactions take place via a similar irreversible mechanism. moreover, the writer additionally describes a brand new chromatography-free technique for the elimination of phosphine oxide from the alkene crude fabricated from the Wittig response. The paintings during this thesis has ended in a number of guides in high-profile journals.
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Additional info for Investigation of Reactions Involving Pentacoordinate Intermediates: The Mechanism of the Wittig Reaction
Sample text
These experiments prove that there is no equilibration of either the betaine or of the OPA which are each necessarily formed in the path from b-HPS to alkene. The corresponding Wittig reaction must occur under kinetic control, since the formation of each of the possible intermediates is irreversible. e. 15 Stereospecific conversion of a cis-stilbene oxide to E-stilbene and b trans-stilbene oxide to Z-stilbene by nucleophilic ring cleavage and quaternisation of the resulting b-oxidophosphine the transient OPA intermediate by NMR is not possible in these cases.
This can either undergo ring closure to give cis or trans-OPA and hence deuterium-free Z or E-alkene respectively, or can revert to ylide and aldehyde and hence to crossover alkene. Dedeuteration is more likely at higher temperature (due to poorer discrimination of acidic sites by the base), so that more of each of the crossover product and the E-alkene are observed. There is also a lower proportion of deuterium in the Z-alkene that is produced, as some of it is derived from non-deuterated erythro-betaine, which is itself derived from b-hydroxy ylide.
2, but not structure 9. 19) [72], for which the presence of an apical oxygen was established in a similar manner to that described above in this paragraph. 2) as well as some examples of unconstrained OPAs are also likely to have oxygen in the apical position in their favoured solution pseudorotamer(s). Other spectra collected on unconstrained OPAs with the ylide a-carbon 13C labelled show 1JPC values of very similar magnitude for carbon-3 in the OPA ring [71]. Bangerter et al. 2) and in a separate experiment did the same for OPA pseudorotamers 15a and 15b by low temperature 31P NMR [88].
