Oxygen activation phosphine oxidation

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. 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

Some oxygen-containing heteroelement compounds, in which other elements are not coordinatively active, could be attributed to O-ligands. This group includes in particular nitrogen, phosphorus, arsenic, and antimony organo-oxides [1,3,11,112], The most studies amongst them are phosphine oxides, on whose basis complexes, with structure 295, of practically all metals have been obtained [1, vol.2 3] ... 

Similarly, a recent study141 of the homogeneous oxidation of cyclohexene by various low-valent phosphine complexes of Group VIII transition metals yielded no definite proof for initiation by oxygen activation. Results were consistent with reactions involving chain initiation via the usual redox reactions of the metal complexes with traces of hydroperoxides. Long induction periods were observed with peroxide-free hydrocarbons. 

We believe that the activity of the phosphine oxides, the phosphonic and the phosphinic acids is related to the acidity of the compounds, as well as the thermal stability of the carbon-phosphorus bond in the compounds and of the phosphorus-oxygen bond of the derived acids. 

Aqua(phosphine)ruthenium(II) complexes [121] are useful for activation of molecular oxygen, and catalytic oxidation of cyclohexene can be carried out with 1 atm of O2 [121a,bj. The ruthenium catalyst bearing perfluorinated 1,3-diketone ligands catalyzes the aerobic epoxidation of alkenes in a perfluorinated solvent in the presence of i-PrCHO [122]. Asymmetric epoxidations of styrene and stilbene proceed with 56-80% e.e. with ruthenium complexes 38-40 (Figure 3.2) and oxidants such as PhI(OAc)2, PhIO, 2,6-dichloropyridine N-oxide, and molecular oxygen [123-125]. 

The origin of the highly enantioselective catalysis by 55 is the simultaneous activation of aldehydes and TMSCN by the Lewis acid (Al) and the Lewis base (the oxygen atom of the phosphine oxide), respectively. The enantioselectivity of the reaction may be explained by the working model depicted in Fig. 1, with the... 

The activation of oxygen in systems containing MTO has been assumed to arise from its reaction with a dioxorhenium(V) complex, giving the peroxo complex that rapidly converts phosphine to phosphine oxide [84]. 

Oxidation of thiophosphoryl and thiocarbonyl compounds (4, 194). Compounds containing group (1) or (2) are oxidized by DMSO containing a catalytic amount of iodine to the oxygen analogs. Oxidation of optically active phosphine... 

The noticeable differences in chemical properties between the phosphines and amines, are due to certain marked differences between nitrogen and phosphorus. The action of oxygen on the two elements furnishes a striking case nitrogen and many of its compounds are inert, while phosphorus and certain of its derivatives are readily oxidized. This marked difference in activity between the two elements is met with in certain of their organic derivatives. Triethylamine, for example, is stable in the air, whereas triethylphosphine rapidly absorbs oxygen and forms an oxide, (C2H6)sPO. Some phosphines oxidize so rapidly that they are spontaneously inflammable. 

The catalyst is the phosphonium salt of the li salt of TPPMS (cf Section 2.2.3.2). This catalyst shows, as desired, sufficient activity at a high P/Pd ratio and no appredable time-dependent deterioration of the activity after repeated catalyst cycles. The formation of phosphine oxides by traces of oxygen in the feed is also minimized. It has been demonstrated that the catalyst is stabilized at high levels if carbonate anions are present along with monoamines or tertiary ammonium carbonates. 

The primary focus of research using transition metal phosphine complexes for oxidations is in the complexation and activation of molecular oxygen. These oxygen complexes have been variously regarded as complexes of coordinated peroxide, superoxide, or singlet oxygen, and their reactivity with reduced substrate has been interpreted on such a basis. In this chapter, we will focus on the chemical reactivity of these compounds for oxygen atom transfer oxidation reactions, with a particular emphasis on the mechanistic features of these processes. 

Stackman [29] carried out a study to find systems suitable for reducing the flammability of polyethylene terephthalate (PET) and poly-1,4-butylene terephthalate (PBT) while retaining the chemical and physical properties of the original polymers. The additives used were phosphine oxides, phosphonates and phosphates and their activity was assessed by means of an oxygen index test. Most of the phosphorus esters were found to be volatile under the blending conditions and both the halogenated phosphorus esters and halogenated derivatives of phosphorus oxide proved to be ineffective as flame retardants. 

---