3 years ago

Improved Synthetic Route to Heteroleptic Alkylphosphine Oxides

Improved Synthetic Route to Heteroleptic Alkylphosphine Oxides
David R. Tyler, Daniel T. Seidenkranz, Alexander J. Kendall
A new method for the synthesis of heteroleptic alkylphosphine oxides (R2R1P═O, where R ≠ R1) from secondary phosphine oxides (or SPOs, R2HP═O) is presented. These reactions were fast at room temperature, sterically selective, high yielding, and >95% pure after an aqueous wash. Deprotonation of an SPO generates a phosphinite anion ([R2P–O]) that was found to be highly selective for nucleophilic P–C bond formation (as opposed to O–C bond formation) with alkyl halides. Surprisingly, most strong organometallic bases failed to deprotonate SPOs to their respective phosphinite anions (pKas for most SPOs are <27). Only sodium bis(trimethylsilyl)amide (NaHMDS) cleanly formed the phosphinite anion, which was stable in solution (0.1 M, 23 °C in THF) for over 24 h. The need for a very specific base to deprotonate suggests that both ion pairing and the conjugate acid play a role in stabilizing the phosphinite anion. Phosphinite anion reactivity followed the expected trend for an SN2 mechanism on reaction with alkyl halides; elimination products were never observed. A wide variety of heteroleptic alkylphosphine oxides were isolated in near-quantitative yield with only an aqueous wash as purification. This methodology was then used to make new bis(phosphine oxide)alkanes and unsymmetrical α,ω-bis(phosphine oxide)alkanes (R2P(O)(CH2)3P(O)R12) on the benchtop with unprecedented ease.

Publisher URL: http://dx.doi.org/10.1021/acs.organomet.7b00304

DOI: 10.1021/acs.organomet.7b00304

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