Secondary phosphine oxide

Stmcture is isomeric with secondary phosphine oxides. 

In general, if the desired carbon—phosphoms skeleton is available in an oxidi2ed form, reduction with lithium aluminum hydride is a powerful technique for the production of primary and secondary phosphines. The method is appHcable to halophosphines, phosphonic and phosphinic acids as well as thein esters, and acid chlorides. Tertiary and secondary phosphine oxides can be reduced to the phosphines. 

The secondary phosphine oxides oi sulfides can be oxidized to phosphinic acids oi thiophosphinic acids. 

Phosphine Oxides. Controlled oxidation of secondary or tertiary phosphines using H2O2 yields the corresponding phosphine oxides. Control... 

Because of their relative instabiUty, primary phosphine oxides caimot be isolated and must be converted direcdy to derivatives. Primary and secondary phosphine oxides undergo reactions characteristic of the presence of P—H bonds, eg, the base-cataly2ed nucleophilic addition to unsaturated compounds such as olefins, ketones, and isocyanates (95). 

Two contrasting conclusions have been reported in the reactions of lithium aluminium hydride in THF with phosphine oxides and phosphine sulphides respectively. The secondary oxide, phenyl-a-phenylethylphos-phine oxide (42), has been found to be racemized very rapidly by lithium aluminium hydride, and this observation casts some doubt on earlier reports of the preparation of optically active secondary oxides by reduction of menthyl phosphinates with this reagent. A similar study of the treatment of (/ )-(+ )-methyl-n-propylphenylphosphine sulphide (43) with lithium aluminium hydride has revealed no racemization. These results have been rationalized on the basis of the preferred site of attack of hydride on the complexed intermediate (44), which, in the case of phosphine oxides (X = O), is at phosphorus, and in the case of the sulphides (X = S), is at sulphur. Such behaviour is comparable to that observed during the reduction of phosphine oxides and sulphides with hexachlorodisilane. ... 

Isopropyl (/ )-( —)-methylphosphinate (134) has been prepared" in > 90% optical purity by Raney nickel desulphurization of optically pure O-isopropyl (5)-(-f-)-methyIphosphonothioate (135). The phosphonate (134) is rapidly racemized by base, but not by acid, unlike secondary phosphine oxides"" [although whether these have been prepared optically active now seems doubtful (see Chapter 4)]. The phosphinate (134) can be reconverted into 89% optically pure (5)-( + )-(135) by addition of sulphur in dioxan. As shown in the Scheme, a series of interconversions has been used to establish the configurations. 

The deoxygenation of peroxycarbonates (53) with phosphines and phosphites has been examined. Reaction with phosphites favours pyrocarbonate formation (Path A) whilst phosphines favour carbonate formation (Path B). Secondary phosphine oxides are oxidized to phosphinic acids by perbenzoic acid. The kinetics of the deoxygenation of hydroperoxides by triphenylphosphine have been examined and the reaction shown to be catalysed by strong acids. ... 

The cyclic phosphonium salts 140,141,143,145, and 146 so obtained are evidence for the mechanism of the oxaphospholic cyclization and especially for the main role of the tertiary carbocation formation during the process. The additional data which support this assumption, come from the investigation of the same reaction, but with different substrate, i.e., dimethyl(l,2-hexadienyl)phosphine oxide 147. In this case, the reaction mechanism involved formation of secondary carbocation that gives oxaphosphole product 148 only in 10% yield (Scheme 60) [124],... 

Reports on the sensitivity of many neutral and cationic NHP derivatives towards air and moisture reveal in most cases pronounced reactivity even with traces of H20 causing P-X bond hydrolysis whereas genuine oxidation processes appear to play a role only for P-H and P-alkyl NHPs [71]. Controlled hydrolysis proceeds at low temperature as depicted in Scheme 9 to give secondary phosphine oxides 17 as initial product which may react further with excess NHP to phosphinous acid anhydrides 18.3 Both products may be obtained as isolable products starting from P-chloro NHPs [48], Hydrolysis at ambient temperature may be unselective and... 

Detailed investigations of the chemical reactivity of the diketiminato-stabilized phosphenium cations like 28 (Scheme 17) are to date rare and include only two reports dealing with the substitution and reduction of P-halogen-derivatives. Thus, reaction of 28 (X=Br) with sodium hydroxide in toluene was reported to proceed with displacement of the halide substituent at phosphorus and conservation of the heterocyclic ring to give a mixture of bromide and triflate salts containing a P-hydroxy-substituted cation, both of which were isolated in small yields [89], The products are remarkable as they represent one of very few examples of a stable phosphinous acid which does not rearrange to the tautomeric secondary phosphine oxide. Potassium reduction of the P-chloro-substituted derivative 34 produced the... 

Preparation of (SP)-f-butyl(phenyl)(a-hydroxybenzyl) phosphine oxide — Reaction of a chiral secondary phosphine with an aldehyde under basic conditions... 

Similarly, monobasic forms of other trivalent phosphorus species have been used successfully in such conjugate addition processes, including monoesters of phosphonous acids375 425 426 and secondary phosphine oxides.427-429 The notable exception to the last of these species is the addition of the anion from diphenyl phosphine oxide to unsaturated aldehydes, which appears always to proceed by addition to the carbonyl carbon.427... 

Emmick, T.L. and Letsinger, R.I., Unsymmetrical secondary phosphine oxides. Synthetic, isotopic, and stereochemical studies, J. Am. Chem. Soc., 90, 3459, 1968. 

The resultant material (isolated in low yield) is an intriguing tricyclic system that could be viewed as an overall antiaromatic system or as two aromatic rings with two phosphorus sites connecting them. In any event, this tricyclic product readily undergoes addition of water in even trace amounts to generate a bis-secondary phosphine oxide (Equation 4.26). 

Monodentate Phosphinites, Aminophosphinites, Diazaphospholidines and Secondary Phosphine Oxides... 

Scheme 28.13 Monodentate phosphinites, aminophosphinites, diazaphospholidines and 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. 

Addition reactions of secondary phosphine oxides to acetyl cyanide (35),30 and to azo-esters (36),31 have been described. Complexes of dimethylphosphine sulphide... 

The secondary phosphine oxide (104) has been reported as an intermediate in the reaction of benzylbis(a-hydroxybenzyl)phosphine oxide (103) with amines to give (105).79 The same authors now report the results of direct reaction of (103) with imines80 to give (105) and the reaction of (103) with aldehydes81 to give acetals (106). 

A synthesis of A-substituted a-aminobenzylphosphinic acids starts from ammonium hypophosphite this is allowed to react with primary amines together with aldehydes or ketones in the presence of HC1.60 The nature of the products and general success of the Atherton-Todd reaction for the preparation of dialkyl- and diaryl-phosphinic amides from secondary phosphine oxides depends on the order in which reactants are mixed and on the choice of polyhalogen reactant.61... 

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. 

The proportions of primary and secondary phosphine oxide formed are dependent mainly on steric effects. 

One of the most reactive, with respect to phosphine, ketones, hexafluoro-cyclobutanone, produces with phosphine primary and secondary 1 hydroxy-fluorocyclobutylphosphines instead of phosphine oxides ... 

The addition of phosphine to 5,6-dideoxy-l,2-0-isopropylidene-D-xylo-hex-5-enfuranose 14) takes place when the reaction mixture is irradiated with UV-light. A mixture of 5,6-dideoxy-l, 2-0-isopropylidene-o -D-xylo-hexa-furanose-phosphine 15) and bis-6-(5,6-dideoxy-l, 2-0-isopropylidene-a-D-xylo hexanose)phosphine (f 6) is probably formed but the components could not be separated. In the presence of atmospheric oxygen these are converted to the corresponding phosphonous acid 17) and the secondary phosphine oxide (25), respectively... 

The sole report of homogeneous palladium catalysis invoking vinylidene intermediates comes from the laboratory of Buono and coworkers [38]. They discovered a reaction unique to air-stable palladium catalysts 122, which form from the self-assembly of secondary phosphine oxides with Pd(II) (Equation 9.12). 

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