Phosphine oxides arylphosphine

Extensive 13C and 81P n.m.r. studies have been reported for phosphine oxides and selenides, and the inversion-recovery technique has been used to establish 2J and ZJ values for 18C-31P coupling.52 Shift reagents have been used to establish alkene geometry in the oxides (61).53 Coupling and shift data have been published for the arylphosphine derivatives (62).54... 

Triarylphosphines were prepared by the reaction between lithium diphenylphosphide in THF and m-and p-iodotoluene (or the corresponding bromo compounds), 4-bromobiphenyl and p-dibromobenzene in yields of 70-80% (isolated after oxidation, as the phosphine oxides).143 The absence of cine substitution products is a synthetic advantage and would have been taken as a prima facie indication that the displacements are examples of the 5rn1 reaction, had the mechanism been recognized at the time. Operation of the radical ion mechanism in DMSO, or liquid ammonia, in which marginally improved yields are obtained, was confirmed by Swartz and Bunnett,48 but no extension to the scope of the reaction was made. Rossi and coworkers have developed a procedure for one-pot preparation of triarylphosphines starting from elemental phosphorus (Scheme 6).146 As an example of the synthesis of a symmetrical tri-arylphosphine, triphenylphosphine (isolated as its oxide) was obtained in 75% yield, with iodobenzene as the aryl halide (ArX in Scheme 6, steps i-iii only). Unsymmetrical phosphines result from the full sequence of reactions in Scheme 6, and p-anisyldiphenylphosphine (isolated as its oxide) was produced in 55% yield, based on the phosphorus used, when chlorobenzene (ArX) and p-methoxyanisole (AiOC) were used. 

Rates of oxidation of para-substituted arylphosphines with singlet oxygen show good correlation with the Hammett a parameter (p = —1.53) and with the Tolman electronic parameter. The only products are the corresponding phosphine oxides. However, for ortho-substituted phosphines with electron-donating substituents, there are two products, namely a phosphinate formed by intramolecular insertion and phosphine oxide. Kinetic analyses demonstrated that both products are formed from the same intermediate, a phosphadioxirane. VT NMR experiments showed that perox-idic intermediates can only be detected for highly hindered and very electron-rich arylphosphines 243... 

The palladium-catalysed cross-coupling of aryl halides or vinyl halides with dialkyl phosphonates (31) to yield dialkyl arylphosphonates and dialkyl vinylphosphonates, respectively, was first reported by Hirao and co-workers 69 the halides used most frequently are bromides and the reaction is stereospecific with haloalkenes. Subsequently, analogous reactions of alkyl alkylphosphinates (32), alkyl arylphosphinates (32), alkyl phosphinates (33), and secondary phosphine oxides (34), replacing [P—H] bonds with [P—C] bonds to yield various phosphinates and tertiary phosphine oxides, have been developed (Figure 7.1). Alkyl phosphinates (33) may be mono- or diarylated as desired by the selection of appropriate conditions. Aiyl and vinyl triflates have also found limited... 

Alkylation of diastereomerically enriched menthyl arylphosphinates leads to menthyl alkylarylphosphinates, the alkylation occurring stereospecifically. Of only marginal interest in monoterpenoid chemistry, the method is valuable for the preparation of mixed alkyldiaryl- and aryldialkyl-phosphine oxides of known absolute configuration. " ... 

Methods for the synthesis of C-functionalised arylphosphines based on the direet introduetion of phosphino groups into aryl halides or tosylates, eatalysed by a variety of metals, have eontinued to develop. The reaetions of seeondary phosphines (and seeondary phosphine oxides) with bromo- or iodo-arenes, eatalysed by palladium aeetate or other palladium complexes, have been used... 

Organophosphorus compounds find wide use in the chemical industry as catalysts, intermediates, complexes, and end-use products. Arylphosphines and phosphine oxides are often produced by the reaction of a preformed Grignard reagent with a halophosphine or phosphine oxide. Yields are reduced by the production of unwanted side-reaction products such as biaryls. These unwanted products are reduced when the reaction is conducted under Barbier conditions. When alkyl and aryl halides are reacted with magnesium metal, a trihalophosphine or phosphine oxide, a metal halide or amine catalyst, in THE benzene mixtures, at reflux, good yields of phosphines or phosphine oxides are obtained [74]. For example, triphenylphosphine can be prepared in a 97.2% yield from the reaction of bromobenzene, trichlorophosphine, magnesium metal, aluminum chloride, and sodium chloride in THF-benzene at 70 80 C. 

Catalysts (25) are the Lewis acid-Lewis base bifunctional catalysts in which Lewis acid-Al(III) moiety activates acyl iminium ion and the Lewis base (oxygen of phosphine oxide) does TMSCN, simultaneously (Scheme 5.7). Halogen atoms at the 6-position enhanced both yields and enantioselectivity in Reissert-type cyanation of the imino part of 26. However, the order for the activation is not parallel to the electronegativity of the halogen atoms and, moreover, the strong electron-withdrawing trifluoromethyl group provided unexpectedly the worst result for the activation [13]. It is not simple to explain this phenomenon only in terms of the increased Lewis acidity of the metal center. Trifluoromethylated BINOL-zirconium catalysts (28) for asymmetric hetero Diels-Alder reaction (Scheme 5.8) [14], trifluoromethylated arylphosphine-palladium catalyst (32) for asymmetric hydrosilylation (Scheme 5.9) [15], and fluorinated BINOL-zinc catalyst (35) for asymmetric phenylation (Scheme 5.10) [16] are known. 

A commercially available and inexpensive proline- or pipecoline acid-promoted copper-catalyst system has been developed for the preparation of arylphosphonates, arylphosphinates and aryl phosphine oxides (136) through P-arylation of //-phos-phonates (137) (Scheme 48). ... 

The palladium-catalyzed cross-coupling of arenediazonium tetrafluoroborates and secondary phosphine oxides generates arylphosphine oxides in moderate to excellent yields (Scheme 4.196) [292]. Palladium acetate was the most effective palladium precursor, and no additional ligand was needed for solubilization and stabilization of the palladium center. The chemistry was successful with arenes bearing esters, ethers, and halogens. 

Secondary phosphine oxides have been used in nickel-catalyzed cross-coupling reactions with aryl tosylates and mesylates for the preparation of arylphosphine oxides (Scheme 4.205) [341], These substrates are typically more stable than aryl triflates and can be readily prepared from a wide range of phenols. In terms of the metal catalyst, the authors used a discrete species ((dppONiCl ) and added extra supporting ligand (2 equiv per metal center) to prevent catalyst decomposition. The addition of zinc dust was essential to the success of the reaction, and no arylphosphine oxide was observed without it. 

The nickel-catalyzed cross-coupling of boronic acids with secondary phosphine oxides is an attractive approach for the preparation of arylphosphine oxides (Scheme 4.208) [346], After some experimentation, the authors found that nickel(II) bromide was the most effective nickel source. A mineral base was needed and potassium carbonate was effective. The substrate scope of this reaction was exceptionally high, and a range of functionalized boronic acids as well as secondary phosphine oxides were successfully cross-coupled. One drawback to this system was the use of 1,2-dichloroethane as the solvent for this reaction. It was noteworthy that the chemistry could be carried out under an atmosphere of air for some examples with only a minor reduction in the yields of the arylphosphine oxides. Thus, this reaction is very attractive as no glovebox or vacuum manifold was needed. This chemistry has the potential to generate a large number of different arylphosphine oxides due to the vast array of boronic acids that are readily available. 

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