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Fosforowe odczynniki nukleofilowe typu >P-O w reakcjach halofilowej substytucji

Rachoń J.

Wiadomości Chemiczne

2000

[Z] 54, 1-2

105-126

One of the oldest pathways for the formation of carbon - phosphorus bond is the Michaelis - Becker reaction. The mechanism of this reaction is assumed to be SN2 process involving nucleophiles of the >P-O- type and C-halogen electrophiles. On the other hand dialkyl phosphite anions are known to participate in aromatic as well as aliphatic SRN1 processes. In the literature one can find some examples of an unusual course of the Michaelis-Becker reaction. The reaction between bromotriphenylmethane and sodium diethyl phosphite was claimed to be a free radical process. It was shown recently that the anions of the >P-O- type as well as >P-S- undergo reaction with a-bromocarboxylates and - phosphonates yielding debrominated products. The anions of the >P-O- type are of special interest; they are nucleophilic ambident reagents, strong bases, and are suggested to be single electron donors. On the other hand the compounds of the >P(O)H structure can act as proton or hydrogen ources; depending on the structure and reaction conditions. The results of current investigations show that in the case of the carbon bromine bond, also bromine can be a target for nucleophilic attack by the phosphorus reagent of the >P-O- as well as >P-S- types with the release of the carbanion as nucleofuge. It was demonstrated that according to the reduction potential of p-substituted benzyl bromides and the solvent used in the reaction of these starting materials and the nucleophilic reagent of the >P-O- type the formation of the P-C bond, debromination and/or dimerization occur. The influence of light on the products distribution as well as the results of the free radical trap experiments have resulted in a reasonable picture of the mechanism of the reaction between p-substituted benzyl bromides and >P-O- nucleophiles outlined in Scheme 16. The results of current research strongly speak for the X-philic substitution as the principal process in the reaction between bromoalkanes possessing the electron - withdrawing groups and >P-O- nucleophiles. This nucleophilic attack of the >P-O- anion on the bromine atom results in formation of carbanion , which, depending on the redox potentials, can participate in the proton and / or electron transfer) processes producing debrominated products and / or dimers . The anions of the >P-O- type can act rather as a carbon radical trap and not as a single electron donor.