Phosphane oxides

Phosphinous amides bearing protons at the nitrogen atom, that here we will call NH phosphinous amides, such as 6, may be involved in prototropic equilibria with their PH phosphazene forms 7 [18,68-70] (Scheme 10). This kind of prototropic equilibria, paralleling that between phosphinous acids R2P-OH and phosphane oxides R2P(0)H [4], have been evidenced in some particular... 

Aqueous HCI solutions hydrolyze the P-N bond to give the amine hydrochloride and R2P-OH, which then disproportionates and is oxidized to diphenylphosphinic acid. A free phosphinous amide anion, with the countercation complexed by a crown ether, has been shown to be hydrolyzed and oxidized to the corresponding phosphinite with unusual ease [119]. Formic acid in toluene can be utilized for converting P,P-disubstituted phosphinous amides into their respective phosphane oxides [30]. 

There do not appear to be any simple phosphines that bear a CH2F group. However, fluorine NMR spectra of phosphonates, phosphane oxides, and phosphonium compounds with CH2F and —CHF- bound to phosphorous have been reported. Examples are given in Scheme 3.26, including spectral data for the useful Horner-Wadsworth-Emmons reagent, triethyl 2-fluoro-2-phosphonoacetate. 

The first spirophosphorane with an azaphosphetidine ring 63 was prepared from the intramolecular cyclization and dehydration of phosphane oxides with DEAD/PPh3 <1996AGE1096, 1996PS489>. Dissolution of 63 in [r/8]-toluene led to some isomerization to the pseudorotamer 64, where the electronegative nitrogen occupies the equatorial position (Scheme 32). Such rotamers were previously unknown for the analogous oxaphosphetanes. 

Treatment of (5,5)-/>/.v(2-hydroxypropyl)phcnylphosphine oxide with initially base followed by Cl(CH2)20(CH2)20Tf gave the dichloride intermediate, which with base and either catechol or tosyl amine in DMF and elevated (150 °C) temperatures generated the macrocyclic phosphane oxide 35 in 15% yield or the l-phospha-10-aza-18-crown-6-ether in 44% yield <06EJOC154>. 

Whereas the reactions of allenephosphonates 171 (R2 = OEt) with primary aliphatic and aromatic amines 172 and the reactions of the phosphane oxides 171 (R2 = Ph) with aliphatic amines 172 afford the conjugated addition products 173 always in good yields, the addition of anilines to 171 (R2 = Ph) leads to an equilibrium of the products 173 and 174 [231]. However, treatment of both phosphane oxides and phos-phonates of type 171 with hydroxylamines 172 (R3 = OR4) yields only the oximes 174 [232, 233]. The analogous reaction of the allenes 171 with diphenylphosphinoylhy-drazine furnishes hydrazones of type 174 [R3 = NHP(0)Ph2] [234],... 

In fact, tris(pyrrolidino)phosphane oxide has a donor number of 1.22. 

The stereochemical trends discussed above are not limited to a, yS-unsaturated carbonyl compounds other Michael acceptors such as nitroalkenes and unsaturated phosphane oxides display similar behavior. A representative example for the nitroalkene class of Michael acceptors is shown with substrate 70 in Scheme 6.13 [28]. The best results were thus obtained for arylcuprates. Other organocuprates were much less selective, which severely restricts their application in organic synthesis. 

Similar observations were made in a related series of unsaturated phosphane oxides (such as 73, Scheme 6.14) [29]. Whereas dialkylcuprates mostly reacted non-selectively, the best diastereoselectivities were observed for disilylcuprates (74). 

Scheme 6.14. Diastereoselective cuprate addition to ,/ -unsaturated phosphane oxide 73 (TBS = t-butyidimethylsilyl).

The first publications to describe the phosphane oxide-catalyzed carbodi-imide synthesis from isocyanate appeared in 1962. In this case iminophos-phoranes were recognized as important intermediates. The first mechanistic studies also appeared at this time. Scheme 24 depicts the proposed two-step mechanism (62JA3673, 62JA4288 66CJC2793). 

The rate-determining first step is the reaction of the isocyanate with phosphane oxide, resulting in the formation of isocyanate and CO2. Once the isocyanate is formed, it reacts with additional isocyanate to give carbodi-imide and regenerate the catalyst. A more detailed investigation of the reaction mechanism (Scheme 25) followed years later (69ACSA2697 72ACSA1777). 

For the reaction of phosphane oxide with isocyanate, the rate-determining step is the formation of the oxazaphosphetane 45 via P—O—bond formation of the intermediate betaine (44), since the stable and energetically favorable P=0 double bond is broken here. Subsequent rapid decomposition of the oxazaphosphetane 45 into iminophosphorane and carbon dioxide occurs. Within the actual aza-Wittig step, the intermediate betaine (46) is generated in a rate-determining step by nucleophilic attack of the iminophosphorane nitrogen on the carbonyl C. By P —O-bond formation, betaine (46) is then converted into an oxazaphosphetane (47), which decomposes... 

Unlike phosphane oxide, the reaction rates of the irainophosphoranes exhibit no dependence on the P substituents. The initial step is a factor of 1(P-10 slower than the second step. However, the nucleophilicity of the iminophosphoranes can influence the reaction rate thus, electron-donating... 

The reaction of V-aminotriphenyliminophosphoranes with aldehydes and ketones is influenced by the presence or absence of water under anhydrous conditions phosphazines (65TL1447) are formed, while trace amounts of water lead to the formation of hydrazones (64AG991) and phosphane oxide. 

In another approach, 2-(alkylamino)alcohol is employed as starting material for aziridine syntheses with the aid of dihalogenophosphoranes (70BCJ1185). Intramolecular transformation of 3-azidopropyloxirane 73 results in a simultaneous formation of a condensed aziridino[l,2-a]pyrrol-idine system (Scheme 39). The azide group is first transformed into imino-phosphorane 74, the nucleophilic N atom cleaves the oxirane to form betaine 75 [as in the Mitsunobu reaction (81S1)], and the phosphorus is shifted from N to O and then eliminated as phosphane oxide under simultaneous cyclization to bicyclic 76 (89JA7500). 

Starting from acylated 2-azidomethylbenzimidazoles (145), an additional imidazole ring can be condensed by transformation of the azido group with tri-n-butylphosphane into the appropriate iminophosphorane intermediate 146. After extrusion of phosphane oxide, cyclization occurs to the 1-substituted 4//-imidazo[l,5-a]benzimidazole 147 (Scheme 58) (89T1823 94S1197). 

An easy one-pot procedure (Scheme 80) gives pyrimido[4,5-h]quinolines by reacting the iminophosphorane 215 prepared from o-azidobenzaldehyde with A(N-dialkylbarbituric acid (216). Refluxing in pyridine leads to the elimination of phosphane oxide and to quinoline 217 (92S827). 

In the group of Izod, the tris(phosphane oxide) 19 was 1,2-dilithiated by the reaction with two equivalents of w-butyllithium in THF at room temperature (Scheme 7). The similarity of the structural formula of compound 20 (Lewis formula) to 1,2-dilithium compounds found by Sekiguchi and coworkers (see Section n. E), where two lithium centres are bridging a C2 unit, is not maintained in the solid state. The X-ray structural analysis reveals a centrosymmetric dimer containing no carbon-lithium contacts (Figure 8). 

Das Filtrat enthalt das parallel gebildete Triphenyl-phosphan-oxid. 

It is important to use dry air and dry equipment because difluorotris(perfluoroalkyl)-25-phos-phanes react very rapidly with water to form tris(perfluoroalkyl)phosphane oxides. Under these conditions, the electrochemical perfiuorination of triethyl-, tripropyl-, and tributylphosphanc oxides yields 15-50% of the corresponding difluorotris(perfluoroalkyl)-A5-phosphanes. A decrease in yield is observed with a longer carbon chain (chain with 2C atoms 42%, with 6C atoms 24 %).63 Byproducts are perfluorinated alkanes. Addition of bromine makes it possible to electrochemically fluorinate trialkylphosphane oxides with a carbon chain length of C5 to... 

The direct sulfonation with sulfuric acid discovered by Hanson et al. (18) is more effective. Another method by which the formation of phosphane oxide is strongly inhibited was published in 1995 the use of B(OH)3 in combination with sulfur trioxide hinders the oxidation and offers the possibility to control the number and the position of sulfonate groups (19). 

---