Synthesis, reactivity and decomposition of aryl phospha-enolates Stephanie Urwin, Jose M. Goicoechea University of Oxford, UK Lithium enolates are integral synthetic intermediates with wide ranging applications. A key characteristic which drives the diverse reactivity of the enolate functionality is the 1,3-delocalisation of the negative charge upon deprotonation of the β-carbon atom. Replacing this carbon with phosphorus to create a phospha-enolate, unlocks a higher potential for delocalisation through the accessible lone pair on the heteroatom. Here, we present the preparation of unsupported lithium phospha-enolates, and explore their diverse reactivity and thermal decomposition (Scheme 1). On reaction with aryl lithium reagents, the C≡P bond of tri(isopropyl)silyl phosphaethynolate is reduced, and is accompanied by a 1,2-silyl migration to form [RP=C(Si i Pr 3 )OLi] 2. Initially dimeric in both the solid and solution state, phospha-enolate aggregation can be modulated by the addition of coordinating bases such as THF or crown ethers, and in the monomeric form delocalisation of negative charge across the PCO unit increases. Silylation with Me 3 SiCl affords the corresponding silyl enol ether, which reduces the extent of delocalisation, and provides a synthetic route to the heavier potassium phospha-enolate. Salt metathesis reactivity is also seen with (IDipp)AuCl, initially forming a gold phosphide with the less bulky phospha- enolate, followed by an unexpected entropically-driven rearrangement to an unusual η 1 -phosphaalkene structure.
Scheme 1. Summary of phospha-enolate chemistry. On addition of water, the lithium phospha-enolate is protonated to RP=C(Si i Pr 3 )(OH). For the sterically smaller example (R = Mes), this phospha-enol rapidly tautomerises to the corresponding acyl phosphine MesP(H)C(Si i Pr 3 ) (O), which on heating extrudes CO. In contrast, bulkier phospha-enol (R = Mes*) is stable to rearrangement at room temperature and thermally decomposes to RH and i Pr 3 SiPCO. Evidently, the chemistry of these lithium phospha-enolates is dependent on the steric environment and directed by a freely migrating silyl group.
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