Experimental Tests of the Stereoelectronic Effect at Phosphorus: Nucleophilic Reactivity of Phosphite Esters.

摘要

Stereoelectronic effects have been suggested to significantly influence the rates, products,and stereochemistry of rections of organophosphorus compounds. In contrast to the large body of experimental and theoretical work supporting the role of orbital orientation(the stereoelectronic effect) in carbon chemistry, no direct experimental evidence has previously existed to support this hypothesis in the reactions of organophosphorus compounds. Triethyl phosphite rapidly reacts with ethyl benzenesulfenate or diethyl peroxide to yield pentaethoxyphosphorane. In contrast, 1-methyl-4-phosha-3,5,8-trioxabicyclo(2.2.2)octane (1) fails to react with either electrophile to yield the expected bicyclic phosphorane 5. The poor reactivity of the bicyclic phosphite 1 is due to a kinetic rather than a thermodynamic barrier because 5 is formed smoothly from an equimolar mixture of P(OEt)5 and the triol 1,1,1-tris(hydroxymethyl)ethane. This result is interpreted in terms of the stereoelectronic effect. The order of nucleophilic reactivity of trialkyl phosphites with 3-benzylidene-2,4-pentanedione is also shown to be consistent with the stereoelectronic effect. The bicyclic phosphite 1 reacted 750 times slower than the pseudoequatorial 2-methoxy ester of hexahydrobenzo-1,3,2-dioxaphosphorinane in a Michael addition reaction with 3-benzylidene-2,4-pentanedione.