Metal-Free Phosphine Oxide Reductions Catalyzed by B(C₆F₅)₃ and Electrophilic Fluorophosphonium Cations

Abstract

The hydrosilylation/reduction of tertiary and secondary phosphine oxides to phosphines is catalyzed by B­(C6F5)3 or electrophilic fluorophosphonium cations (EPCs). B­(C6F5)3 is an effective catalyst for phosphine oxide reduction using (EtO)3SiH, PhSiH3, and Ph2SiH2 at elevated temperature (105 °C), while EPCs effect the same reduction at significantly lower temperature with PhSiH3 as reducing agent, allowing for good functional-group tolerance.

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…The reduction of phosphine oxides using silane reagents is also catalyzed by phosphonium compounds 8 and 9 (Scheme 15). [47] Catalysts 8 and 9 are more active phosphine deoxygenation catalysts than B(C6F5)3, which is attributed to their greater Lewis acidity. The proposed mechanism is analogous to that suggested for hydrosilations catalyzed by B(C6F5)3, with the phosphonium-silane interaction promoting nucleophilic attack at silicon by the phosphine oxide.…”

B(C6F5)3‐Catalyzed Reduction of Aromatic and Aliphatic Nitro Groups with Hydrosilanes

“…We began with testing various B(C 6 F 5 ) 3 /hydrosilane combinations in the reduction of nitrobenzene to aniline ( 1 → 2 ; Table ). The use of B(C 6 F 5 ) 3 /PhSiH 3 in toluene at 100 °C that had yielded full conversion in the aforementioned phosphine oxide reduction showed hardly any consumption of the nitroarene, but conversion rose to 75 % without solvent (Entries 1 and 2). No reaction was seen with (EtO) 3 SiH (Entries 3 and 4), another result that is not in line with those of the phosphine oxide deoxygenation Decomposition and little conversion, respectively, were obtained with tetramethyldisiloxane (TMDS; Entries 5 and 6).…”

“…The use of B(C 6 F 5 ) 3 /PhSiH 3 in toluene at 100 °C that had yielded full conversion in the aforementioned phosphine oxide reduction showed hardly any consumption of the nitroarene, but conversion rose to 75 % without solvent (Entries 1 and 2). No reaction was seen with (EtO) 3 SiH (Entries 3 and 4), another result that is not in line with those of the phosphine oxide deoxygenation Decomposition and little conversion, respectively, were obtained with tetramethyldisiloxane (TMDS; Entries 5 and 6). Conversely, polymethylhydrosiloxane (PMHS) was moderately effective in the reduction of the nitro group (Entries 7 and 8); the use of PMHS had been a complete failure with phosphine oxides .…”

Metal‐Free Reduction of Phosphine Oxides, Sulfoxides, and N‐Oxides with Hydrosilanes using a Borinic Acid Precatalyst

“…Later, Werner reported the use of triflic acid ( 2 ) (Scheme ) . In 2016, Oestreich and Stephan jointly disclosed the use of B(C 6 F 5 ) 3 and fluorophosphonium cations as efficient catalysts for the reduction of a range of phosphine oxides in the presence of phenylsilane (Scheme ) . Interestingly, the electrophilic cations allowed the use of a lower temperature than that required for B(C 6 F 5 ) 3 ; and furthermore, a higher functional‐group tolerance was achieved…”

“…Electron‐rich tricyclohexylphosphine ( 10 h ) was also isolated successfully in 78 % yield. Our methodology was next applied to the reduction of diphosphine oxides, which are known to be more difficult substrates owing to the possible formation of complex mixtures of di‐ and monophosphines . Nevertheless, under our conditions Xantphos ( 10 i ) and ferrocene bis(diphenylphosphine) ( 10 j ) were efficiently delivered in 71–89 % yield, and the formation of the monophosphine was not detected (Table ).…”