Hydrogen secondary phosphine oxides

Secondary phosphine oxides are known to be excellent ligands in palladium-catalyzed coupling reactions and platinum-catalyzed nitrile hydrolysis. A series of chiral enantiopure secondary phosphine oxides 49 and 50 has been prepared and studied in the iridium-catalyzed enantioselective hydrogenation of imines [48] and in the rhodium- and iridium-catalyzed hydrogenation functionalized olefins [86]. Especially in benzyl substituted imine-hydrogenation, 49a ranks among the best ligands available in terms of ex. 

Hydrogen phosphonates [(R0)2P(0)H] and secondary phosphine oxides R2P(0)H exist in equilibrium with their P(III) tautomers, (RO)2P(OH) and R2P(0H), respectively, the P(V) tautomers being more favored under ambient conditions. As ligands, they coordinate, like tertiary phosphines, to transition metals to form complexes, which have been used as catalysts for organic reactions. However, catalytic addition reactions of P(V)-H bonds have not been scrutinized until recently. 

Addition of the H-P bond in secondary phosphine oxides (hydrophosphiny-lation) also proceeds in the presence of palladium complexes (Scheme 36) [32]. Secondary phosphine oxides appear more reactive than hydrogen... 

As expected, hydrogen phosphinate, which is a hybrid structure of hydrogen phosphonate and secondary phosphine oxide, adds to alkynes in the presence of the Pd-diphenylphosphinic acid catalyst system (Scheme 44) [36]. Normally, branched isomers are the major products, while trimethylsily-lacetylene exceptionally affords the corresponding terminally phosphinylat-ed product. Diphenylacetylene also reacts to afford the corresponding adduct in 99% yield. 

Methyl triflate is a powerful alkylating agent, which methylates tetracovalent P=0, P=S, and P=Se systems at the oxygen, sulfur, and selenium atoms, respectively (1,2, 3). Its reaction with analogs containing a hydrogen substituent (e .j>., phosphinate, phosphonate or secondary phosphine oxide species), however, appears not to have been reported. 

The simplest supramolecular bidentate ligand derives from secondary phosphine oxides (SPO). Complexes of transition metals with SPOs have been known for 45 years, and they were introduced as catalysts by van Leeuwen and Roobeek in the early 1980s (66). The complex used first was a platinum hydride containing two SPOs, cormected to one another by a strong hydrogen bond, and a triphenylphosphine to complete the coordination sphere. SPOs have a very strong tendency to occur in pairs connected by hydrogen bonds in many metal complexes they act as bidentate monoanions. 

The secondary phosphine oxides were conveniently oxidized to phos-phinic acids with hydrogen peroxide (122,195). Proof of structure... 

The hydrolysis of tervalent phosphorus acid derivatives with two P—C bonds leads to secondary phosphine oxides (50) and with one P—C bond to phosphonus acid derivatives (51). Chlorophosphines react rapidly with water, but aminophosphines, phosphinites and phosphonites often survive a short wash with aqueous NaHC03, an effective way to remove contaminating ammonium salts in the crude products. However, aminophosphines with small substituents, e.g. dimethylaminodimethylphosphine, aryl phosphinites and phosphonites and trimethylsilyl phosphinites and phosphonites are hydrolysed too quickly for such a treatment. The hydrolyses are catalysed by acids (the hydrolyses of phosphinites and phosphonites are also catalysed by OH ) and are much faster than hydrolyses of the corresponding phosphoryl compounds [up to a factor of 10 for acid-catalysed hydrolysis of (MeO)3P compared with (MeO)3P=0 ]. Dialkyl phosphonites are rapidly hydrolysed to the monoalkyl esters (51, X = OR) in weakly acidic water, whereas hydrolyses to phosphonous acids require reflux with strong acid or base, e.g. equation 131 Bis-(dialkylamino) phosphines may also be partially hydrolysed to phosphonous acid amides (51, X = NR2). Tervalent phosphorus acid derivatives with hydrogen sulphide give secondary phosphine sulphides or phosphonodithious acids, e.g. equation 156 . ... 

All other potential phosphinous acids exist as secondary phosphine oxides (6.207). These may be made by oxidation of primary phosphines with hydrogen peroxide at 0°C (6.26), or by the acid-catalysed addition of phosphines to ketones (6.129). 

Secondary phosphinic acids (6.254) may be prepared by the oxidation of secondary phosphine oxides (6.26) or by thermal decomposition of the latter (6.133). Air, oxygen, hydrogen peroxide, dilute nitric acid or bromine water may be used for such oxidations. Many phosphinic acids can be prepared by refluxing their alkyl esters with 20% aqueous HCl (6.255). Sulphur dioxide and phosphines may also be used (6.268). 

Rh- and Ru-based systems are the catalysts of choice to hydrogenate dehydroamino acids. When those systems fail, switching to iridium can lead to improved results. An example is the hydrogenation of the sterically hindered dehydroamino ester 33 with a secondary phosphine oxide as ligand, depicted in Scheme 7.14. 

The undesired hydrolysis of phosphoramidites can lead to an unexpected alteration of catalytic results. Thus, during Rh-catalyzed asymmetric hydrogenation of a-substituted ethenylphosphonic acid, by chance Ding and coworkers [40] observed the hydrolysis of the P-N bond with adventitious water (Scheme 2.134). The newly formed secondary phosphine oxides were more efficient than the parent phosphoramidite ligands in the respective catalytic reaction. When an equimolar amount of NHj was added to the reaction mixture before hydrogenation was started, the reaction did not occur. This clearly indicates that the phosphine oxide is formed from the phosphoramidite only in acidic media. 

Phosphine and secondary phosphine oxide-stabilized Ru NPs for hydrogenation reactions Carbene-stabilized nanoparticles for hydrogenation reactions 57... 

Organophosphinates (90) may also be prepared by the oxidation of secondary phosphines or halophosphines with hydrogen peroxide or sulfur ... 

---