Reduction of Functionalized Tertiary Phosphine Oxides with BH₃

Abstract

A direct stereoselective conversion of tertiary hydroxyalkylphosphine oxides to the corresponding tertiary hydroxyalkylphosphine-boranes involving facile reduction of the P═O bond by BH3 under mild conditions has been developed. The unprecedented facility of reduction of the strong P═O bond by BH3, a mild reducing agent, has been achieved through an intramolecular P═O···B complexation directed by proximal α- or β-hydroxy groups present in the phosphine oxide structures. As established by two chemical correlations, the developed transformation of hydroxyalkylphosphine oxides into hydroxyalkylphosphine-boranes takes place with complete inversion of configuration at P.

Asymmetric Electrophilic Reactions in Phosphorus Chemistry

“…Phosphine boranes were obtained from the tertbutyl(hydroxyalkyl)phenylphosphine oxide by treatment with an excess borane complex (BH3 . Me2S or BH3 THF) (Equation (115)) [165,166]. (114) Aromatic and enolizable aliphatic aldehydes turned out to be the most suitable reagents in this case, leading to the formation of one diastereomer at room temperature.…”

“…Quantitative addition of formaldehyde to secondary phosphine oxides gave enantiomerically pure (hydroxyalkyl)phosphine oxides in quantitative yield. Phosphine boranes were obtained from the tert-butyl(hydroxyalkyl)phenylphosphine oxide by treatment with an excess borane complex (BH 3 •Me 2 S or BH 3 THF) (Equation (115)) [164,165].…”

“…Phosphine boranes were obtained from the tertbutyl(hydroxyalkyl)phenylphosphine oxide by treatment with an excess borane complex (BH3 . Me2S or BH3 THF) (Equation (115)) [165,166]. (115) A number of optically pure α-and β-hydroxyalkylphosphine boranes were prepared using a three-step method from readily available adamantyl H-phosphinates.…”

“…The reduction of the P=O bond with BH 3 is possible due to the intramolecular complexation of P=O•B coordinated by αor β-hydroxy groups present in the phosphine oxide structure. The conversion of hydroxyalkylphosphine oxides to hydroxyalkylphosphine boranes were obtained with a full inversion of the configuration at phosphorus [163,[165][166][167][168]. It was found that the P-alkylation reaction proceeds with good yield and high enantiospecificity (ee > 95%) with various alkyl halides.…”

Which future for stereogenic phosphorus? Lessons from P* pincer complexes of iron(ii)

“…91 Therefore, we developed an alternative approach inspired by Buono's masked secondary phosphine boranes (4); 92 we treated diastereomerically pure (R P )-(À)-menthyl-H-phenylphosphinate (prepared on a multi-gram scale from (À)-menthol and PhPCl 2 ) 93 with MeLi, quenched the lithium salt of the methylphenylphosphine oxide with acetaldehyde, and reduced (S P )-(1-hydroxyethyl)methylphenylphosphine oxide 8 with BH 3 ÁSMe 2 to (S P )-(1-hydroxyethyl)methylphenylphosphine borane (4) with formal retention (Scheme 23). 94 This route is easily scalable, and (S P )-4 was routinely prepared on a 45 g scale. 89 Pyridine based pincer complexes…”