Phosphine oxide, formation

In media of low proton availability (MeCN, dmf, hmpa)762, ylide formation was found and products derived from the radical cleavage of the phosphonium ion have been observed. The initial cation would interfere in the reaction process as an acid. A competition can exist between the one-electron pathway (dimerization, disproportionation of R ) and the two-electron pathway (ylide formation, Hofmann degradation, phosphine oxide formation) (Table 24). 

H-atom transfer disproportionation Second wave electrode electron transfer Second wave solution electron transfer Hofmann degradation Ylide formation Phosphine oxide formation... 

This chapter covers the main synthetically useful phosphonylation reactions, the corresponding processes of phosphinylation and tertiary phosphine oxide formation along with some related reactions. In all these reactions the phosphorus reactant (a phosphite, phosphonite, phosphinite, or derivative or tautomer thereof) is the nucleophilic component, herein these reactants are referred to collectively as phosphorus(III) reactants/acids, as appropriate in general these reagents are best used freshly distilled. Syntheses of phosphonates, phosphinates and tertiary phosphine oxides by nucleophilic substitution at phosphorus is not covered (for reviews of this area see Refs 6 and 16). 

Carbodiimide formation is markedly accelerated when phosphine oxides (R PO) or phosphates are used as catalysts. Intermediates with P—NR bonds have been postulated as iatermediates ia these reactioas (59,60). 

Garbodiimide Formation. Carbodiimide formation has commercial significance in the manufacture of Hquid MDI. Heating of MDI in the presence of catalytic amounts of phosphine oxides or alkyl phosphates leads to partial conversion of isocyanate into carbodiimide (95). The carbodiimide (39) species reacts with excess isocyanate to form a 2 + 2cycloaddition product. The presence of this product in MDI leads to a melting point depression and thus a mixture which is Hquid at room temperature. 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

Another class of hydrocarbon binders used in propints are the carboxy-terminated polybutadiene polymers which are cross-linked with either tris[l-(2-methyl)aziridinyl] phosphine oxide (MAPO) or combinations with phenyl bis [l -(2-methyl)aziridinyl] phosphine oxide (Phenyl MAPO). Phenyl MAPO is a difunctional counterpart of MAPO which makes possible chain extension of polymers with two carboxylic acid groups. A typical propint formulation with ballistic properties is in Table 11 (Ref 83) Another class of composites includes those using hydroxy-terminated polybutadienes cross-linked with toluene diisocyanate as binders. The following simplified equations illustrate typical reactions involved in binder formation... 

P-coupling occurs in the formation of azophosphonic esters [ArN2PO(OCH3)2] from diazonium salts and dimethyl phosphite [HPO(OCH3)2] (Suckfull and Hau-brich, 1958). P-coupled intermediates are formed in the reaction between diazonium salts and tertiary phosphines, studied by Horner and Stohr (1953), and by Horner and Hoffmann (1956). The P-azo compound is hydrolyzed to triphenylphosphine oxide, but if a second equivalent of the tertiary phosphine is available, phenyl-hydrazine is finally obtained along with the phosphine oxide (Scheme 6-26 Horner and Hoffmann, 1958). It is likely that an aryldiazene (ArN = NH) is an intermediate in the hydrolysis step of the P-azo compounds. 

In the course of this study, the authors determined /Lvalues for dibenzyl, methyl phenyl, methyl p-nitrophenyl, di-p-tolyl, di-isopropyl and tetramethylene sulphoxides and for diethyl, dipropyl and dibutyl sulphites. The /Lscales are applied to the various reactions or the spectral measurements. The /Lscales have been divided into either family-dependent (FD) types, which means two or more compounds can share the same /Lscale, family-independent (FI) types. Consequently, a variety of /Lscales are now available for various families of the bases, including 29 aldehydes and ketones, 17 carboxylic amides and ureas, 14 carboxylic acids esters, 4 acyl halides, 5 nitriles, 10 ethers, 16 phosphine oxides, 12 sulphinyl compounds, 15 pyridines and pyrimidines, 16 sp3 hybridized amines and 10 alcohols. The enthalpies of formation of the hydrogen bond of 4-fluorophenol with both sulphoxides and phosphine oxides and related derivatives fit the empirical equation 18, where the standard deviation is y = 0.983. Several averaged scales are shown in Table 1588. 

A similar quantitative treatment of sulphoxides as hydrogen bonding acceptors has been obtained by comparing the IR frequency shift AvOH of the C—I bond in an acetylenic iodide such as IC=CI (Avc j) due to formation of a C—T complex with phenol in various bases. This investigation suggests that sulphoxides belong to the same family as carbonyls, phosphine oxides, arsine oxides and their derivatives90. 

With a-dfsubstituted yUdes the reaction results in the formation of a carbonyl compound and a tertiary phosphine oxide (Scheme 6). 

Wittig reactions are versatile and useful for preparing alkenes, under mild conditions, where the position of the double bond is known unambiguously. The reaction involves the facile formation of a phosphonium salt from an alkyl halide and a phosphine. In the presence of base this loses HX to form an ylide (Scheme 1.15). This highly polar ylide reacts with a carbonyl compound to give an alkene and a stoichiometric amount of a phosphine oxide, usually triphenylphosphine oxide. 

C. Reactions not involving P=0 or P=S Groups.—Enamine phosphine oxides (45) have been prepared by the addition of amines to 1-alkynyl-phosphine oxides, and the reactions of their anions with various electrophiles have been reported. - With ketones a Wittig-type reaction leads to the formation of a/3-unsaturated ketones, in 53—70% yield, while with epoxides cyclopropyl ketimines are formed. A Diels-Alder reaction of l-phenyl-A -phospholen-l-oxide (46) with 1,4-diacetoxybutadiene has been used in the preparation of l-phenyl-benzo[/>]phosphole (47), as... 

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. ... 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

An unusual two-component domino Michael/aldol process was described by Tomioka and coworkers in which the initiating step is the formation of an a-lithiated vinyl-phosphine oxide [28] or vinyl phosphate [29]. 

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],... 

The P-nitroso phosphine oxide 406 behaves as an N-O heterodienophile and reacts with the 1,3-diene part of the molecule in a cycloaddition reaction to form the 2,4 ,5,6,7,8-hexahydro-8-phenyl-[l,2]azaphosphorino[l,6-3][l,2]oxa-zine 8-oxide 407 containing an stereogenic cyclic phosphorus atom (Scheme 64). 31P NMR spectroscopy shows one single peak indicating the formation of only one diastereomer <2002JOG6174>. 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

Nickel and palladium complexes also catalyze the formation of the carbon-phosphorus bonds in phosphorus(V) and phosphorus(III) compounds. Indeed, this chemistry has become a common way to prepare phosphine ligands by the catalytic formation of phosphine oxides and subsequent reduction, by the formation of phosphine boranes and subsequent decomplexation, or by the formation of phosphines directly. The catalytic formation of both aryl and vinyl carbon phosphorus bonds has been accomplished. 

Tertiary phosphines are oxidized catalytically by nickel(O) complexes with formation of phosphine oxides. Also, complexed tert-butylisonitriles can be oxidized to the corresponding isocyanates (examples 1 and 2, Table IX) (225-226). 

An interesting approach to rhenium bioconjugate formation that has been developed but apparently has yet to be fully evaluated from the standpoint of applicability and stability is the use of chelating and nonchelating phosphine imine and phosphine oxide ligands [110]. For... 

Formation of the conjugate base of phosphine under the conditions of aqueous dioxane and KOH with red phosphorus allows also for the formation of C-P bonds by attack of oxiranes.33 Under these conditions, mixtures of phosphines and phosphine oxides are formed. Using red phosphorus in liquid ammonia with sodium metal and f-butyl alcohol, good yields of primary (2-hydroxyalkyl)phos-phines are obtained.34... 

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