Phosphines, tertiary, oxidation

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). 

In the examples above, one or both of the reaction centers are already attached to the metal center. In many cases, the reactants are free before reaction occurs. If a metal ion or complex is to promote reaction between A and B, it is obvious that at least one species must coordinate to the metal for an effect. It is far from obvious whether both A and B enter the coordination sphere of the metal in a particular instance. A number of metal-oxygen complexes can oxygenate a variety of substrates (SOj, CO, NO, NO2, phosphines) in mild conditions. Probably the substrate and O2 are present in the coordination sphere of the metal during these so-called autoxidations. In the reaction of oxygen with transition metal phosphine complexes, oxidation of metal, of phosphine or of both, may result. The initial rate of reaction of O2 with Co(Et3P)2Cl2 in tertiary butylbenzene. 

Abou-Basha and Aboul-Enein [22] presented an isocratic and simple HPLC method for the direct resolution of the clenbuterol enantiomers. The method involved the use of a urea-type CSP made of hS )-indoline-2-carboxylic acid and (R)-1 -(naphthyl) ethylamine known as the Chirex 3022 column. The separation factor (a) obtained was 1.27 and the resolution factor (Rs) was 4.2 when using a mobile phase composed of hexane-1,2-dichloroethane-ethanol (80 10 10, v/v/v). The (+)-enantiomer eluted first with a capacity factor (k) of 2.67 followed by a (—)-enantiomer with a k of 3.38. Biesel et al. [23] resolved 1-benzylcyclohexane-1,2-diamine hydrochloride on a Chirex D-penicillamine column. Gasparrini et al. [24] synthesized a series of the chiral selectors based on /ra s -1,2 - d i a m i n o eye I o hexane. The developed CSPs were used for the chiral resolution of arylacetic acids, alcohols, sulfoxides, selenoxides, phosphinates, tertiary phosphine oxides, and benzodiazepines. In another study, the same authors [25] described the chiral resolution of /i-aminocstcrs enantiomers on synthetic CSPs based on a re-acidic derivatives of trans- 1,2-diaminocyclohexane... 

Analysis of the SnBr4—BU3P system shows that only AD2 complexes exist in solutions at —90°C independent of the D/A ratio. The values of 5(119Sn) and (31P) for the products isolated at D/A <0.5 at room temperature evidently show that formation of AD complexes takes place before the oxidation of phosphines occurs (oxidation of tertiary phosphines with excess of SnHaLt leads to an alternative mechanism for the formation of ICPHal+SnllalsM26. In the SnCLt—Me2PhP system, in which only weakly dissociated AD2 complex exists at room temperature independent of the D/A ratios, oxidation of the phosphine proceeds extremely slowly26. 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

Analogous to the oxidation of tertiary amines, tertiary phosphines are oxidized to phosphine oxides, (R3P=0). Triphenylphosphine is converted to ttiphenylphosphine... 

This is a very useful route for the preparation of phosphines, especially chiral phosphines. Tertiary phosphine oxides (and sulfides) and phosphonium salts are often precursors of choice in these reduction procedures. The following sections highlight reagents and reaction conditions in forthcoming sections further examples will be given. 

Various strategies exist for the preparation of alkyl- or aryl-substituted tertiary phosphines. Direct oxidation of alkyl-substituted phosphines by exposure to air is not practical since side products with P—O—C bonds might be formed.3 A few illustrative examples highlight the different routes that may be used. Arylphosphine oxides (e.g., (140)) can be made using aqueous H202 in diethylether as solvent (Equation (35)).302 The same procedure (aq. H202, THF, 0°C) can be used to prepare (141) (Scheme 10) in 49% yield the dioxide (142) can easily be separated from (141) by virtue of its poor solubility in THF.303... 

The Arbuzov rearrangement has been widely used for the synthesis of a variety of phosphorus related compounds, including phosphonates, phosphinates, tertiary phosphine oxides,phosphonyl, and phosphonyl halides. Some of these products can be used for the Wittig Reactions, such as in the synthesis of carotene. " ... 

Tertiary phosphines are oxidized via oxo-abstraction from dioxomolybdenum(VI) complexes of the type where L is S CNR, ... 

Tertiary oxides containing a hydroxy groups are less stable than simple alkyl derivatives, and undergo thermal decomposition at about 100°C to form secondary phosphine oxides (6.120). Tertiary oxides can be reduced to tertiary phosphines with lithium aluminium hydride (6.58). Alkali hydrides form phosphinite derivatives (6.121). 

Primary and secondary phosphines can oxidatively add to low-valent transition metal complexes to form phosphido complexes. In contrast to phosphines, metal phosphido complexes are known to undergo fast pyramidal inversion, often on the NMR timescale. Inversion barriers for some platinum complexes, determined by NMR spectroscopy, range from 42 to 67kJmol. As a consequence, phosphido complexes containing other chiral ligands are mixtures of interconverting diastereomers. Reaction of these complexes with electrophiles yields tertiary phosphines and derivatives (Scheme 6.1). 

Another very important reaction initially involving nucleophilic attack on an aldehyde carbonyl is the Wittig reaction. An yUd adds to the carbonyl forming a betaine intermediate which then decomposes to produce an olefin and a tertiary phosphine oxide. 

Much effort has been placed in the synthesis of compounds possessing a chiral center at the phosphoms atom, particularly three- and four-coordinate compounds such as tertiary phosphines, phosphine oxides, phosphonates, phosphinates, and phosphate esters (11). Some enantiomers are known to exhibit a variety of biological activities and are therefore of interest Oas agricultural chemicals, pharmaceuticals (qv), etc. Homochiral bisphosphines are commonly used in catalytic asymmetric syntheses providing good enantioselectivities (see also Nucleic acids). Excellent reviews of low coordinate (coordination numbers 1 and 2) phosphoms compounds are available (12). 

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. 

If aromatic aldehydes or ketones are used, the tertiary phosphine product sometimes rearranges to a mixed phosphine oxide. 

Tertiary phosphines and primary and secondary phosphines can be oxidized by elemental halogen to halophosphine halides. 

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

Tertiary phosphine oxides are stable. The temperatures required for thermal decomposition are approximately 300°C higher than the corresponding amine oxides (96). Trimethyl phosphine oxide is stable to 700°C. 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

Like Ag, Au also readily forms linear 2-coordinate complexes such as [AuX2] (X = Cl, Br, I) " and also the technologically important [Au(CN)2] . But it is much more susceptible to oxidation and to disproportionation into Au and Au which renders all its binary compounds, except AuCN, unstable to water. It is also more clearly a class b or soft metal with a preference for the heavier donor atoms P, As and S. Stable, linear complexes are obtained when tertiary phosphines reduce Au in ethanol. 

Pyridlne-A -oxide may be formed in a manner analogous to the reaction of dibenzoyl peroxide with tertiary phosphines, i.e., Eq, (18)... 

Within the osmium complexes in oxidation states (II-IV) [11,12] the stability of the +4 oxidation state becomes more important. Ammine and tertiary phosphine complexes have been selected for detailed examination. 

Syntheses of some of these important tertiary phosphine complexes are summarized in Figure 1.60, which represent reactions typical of a tertiary phosphine (e.g. PMe2Ph), showing complexes in the oxidation states +6, +4, +3 and +2 [78a]. 

It is a volatile orange-red crystalline solid (m.p. 30°C), stable to over 100°C. On reduction with tertiary phosphines or sodium amalgam, Os(NBu )3 is formed, which is dimeric (ButN)2Os(/x-NBut)2Os(NBu )2. This can be oxidized to the osmium(VII) dication with concomitant shortening in the Os-Os distance from 3.1 to 2.68 A. 

Rhodium(III) forms a wide range of complexes with tertiary phosphines and arsines [108, 109], though in some cases other oxidation states are possible. Table 2.5 summarizes the complexes produced from reaction of RhCl3 with stoichiometric quantities of the phosphine. 

The most important of the tertiary phosphine complexes of platinum(IV) are Pt(QR3)2X4, generally prepared by halogen oxidation [174] of cis- or trans-Pt(QR3)2X2 (Q = P, As, R = alkyl Q = Sb, R = Me), since direct reaction of the platinum(IV) halides with the ligands leads to reduction. Once made, the platinum(IV) compounds are stable to reduction ... 

Methylplatinum phosphine complexes have been synthesized in both the +2 and +4 oxidation states (see also section 3.8.4). Syntheses for a number of these with the tertiary phosphine PMe2Ph, which appear to be typical, are shown in Figure 3.115. 

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