Activations phosphites

In the proposed mechanism (Scheme 5-33), both the phosphite and the aldehyde are coordinated to the catalyst before nucleophilic attack forms the P-C bond. Slow addition of the aldehyde was found to improve ees it was suggested that this minimizes unselective attack of the activated phosphite on free aldehyde instead of the desired selective attack on complexed aldehyde [26]. In related chemistry with para-anisaldehyde, Shibuya found that ees also depended on the rare earth (La, Eu, Sm) in the heterobimetallic catalyst [24]. 

LLB, a so-called heterobimetallic catalyst, is believed to activate both nucleophiles and electrophiles.162 For the hydrophosphonylation of comparatively unreactive aldehydes, the activated phosphite can react with only the molecules precoordinated to lanthanum (route A). The less favored route (B) is a competing reaction between Li-activated phosphite and unactivated aldehyde, and this unfavored reaction can be minimized if aldehydes are introduced slowly to the reaction mixture, thus maximizing the ratio of activated to inactivated aldehyde present in solution. Route A regenerates the catalyst and completes the catalysis cycle (Fig. 2-9). 

Due to their peroxidolytic activity, phosphites are normally used in combination with hindered phenols in order to deactivate hydroperoxides formed as a consequence of the antioxidant function of hindered phenols, see reaction 1. The enhanced melt stability of tocopherol-containing PP extruded in the presence of a phosphite is clearly illustrated in Fig. 5a with an optimum melt stabilising performance at a 1 2 w/w ratio in favour of the phosphite. 

The effects of the slow addition of the aldehydes on enantioselection can be best explained as follows. Heterobimetallic catalysts such as LLB are believed to activate both nucleophiles and electrophiles. For the hydrophosphonylation of comparatively unreactive aldehydes, the activated phosphite can react only with aldehydes that are precoordinated to lanthanum. However, in the case of reactive aldehydes such as 54 and 124, the Li-activated phosphite may be able to undergo a competing reaction with the unactivated aldehyde. If such aldehydes are added in one portion, the ee of the product will thus be reduced. Slow addition of the... 

Following a comparison of the behaviours of trialkyl phosphites, mixed alkyl phenyl phosphites and triphenyl phosphite towards iodomethane and, in the last case, the breakdown of the phosphonium salt when treated with an alcohol, Landauer and Rydon considered that all the reactions involve a stage identical with that of the normal Michaelis-Arbuzov reaction. The absence of any rearrangement during the decomposition of complexes from neopentyl phosphites, and the configurational inversion which occurs when optically active 2-halooctanes are produced from optically active phosphite triesters (themselves obtained from optically active octan-2-ol), suggest that the mode of breakdown of the intermediate complexes is of S 2 character. 

Miscellaneous.- New optically active phosphites (103) and (104) derived from R- or 5-2,2 -binaphthol have been prepared by standard methods and used as ligands for asymmetric hydroformylation or hydrocyanation reactions. Low to moderate asymmetric induction was achieved in the synthesis of S,6-disubstituted cyclohexadienes from benzene chromium complexes when the optically active phosphites (105) were ligands. ... 

The highly functionalized imidazole 3 can be polymerized by active phosphite ester methods [4] to afford the A-B polyamide. The monomer is dissolved in a mixed solvent system of N-methylp3n rolidinone and pyridine containing dissolved lithium chloride. Upon addition of triphenyl phosphite, the reaction immediately takes on a yellow color. During the course of the polymerization, the color increases in intensity until the pol5rmerization mixture appears black. The products are isolated as fine yellow powders by precipitation in methanol followed by a methanol wash. 

They carried out some experiments in order to clarify the mechanism of the reaction that allowed them to propose the transition state depicted in the Figure 12.7, where the in situ generated imine coordinates to the catalyst in the equatorial position, avoiding the steric repulsion of the ligand. In addition, after coordination of the oxygen atom of the P-component to the metallic center, the active phosphite is formed, which then attacks to the azomethine carbon of the imine by its si face, thus providing the experimentally observed (5)-enantiomer. 

Because dialkyl phosphonates undergo phosphite-phosphonate tautomerism, a simple strategy to accelerate the phosphite-phosphonate tautomerization is the deprotonation of phosphonates using a base. Inorganic bases, such as potassium carbonate, which have relatively weak basicity and low solubility in organic solvents, could made the dialkyl phosphonate to generate an active phosphite anion at... 

Phosphites, such as triisopropyl and triphenyl phosphite, are weaker electron donors than the corresponding phosphines, but they are used in some reactions because of their greater rr-accepting ability. The cyclic phosphite trimethylol-propane phosphite (TMPP) or 4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2]oc-tane (8), which has a small cone angle and small steric hindrance, shows high catalytic activity in some reactions It is not commercially available, but can be prepared easily[27]. 

The roles of phosphines are not clearly understood and are unpredictable. Therefore, in surveying optimum conditions of catalytic reactions, it is advisable to test the activity of all these important types of phosphines and phosphites. which have different steric effects and electron-donating properties. 

Recent patent activity suggests that DuPont is developing a new generation of chelating diphosphite—nickel catalysts for this technology which are significantly more active than the monodentate phosphite based catalyst system used for the last two decades (61—64). 

Phosphites. Tertiary phosphites are also commonly used and are particularly effective ia most mixed metal stabilizers at a use level of 0.25—1.0 phr. They can take part ia a number of different reactions duting PVC processing they can react with HCl, displace activated chlorine atoms on the polymer, provide antioxidant functionaHty, and coordinate with the metals to alter the Lewis acidity of the chloride salts. Typical examples of phosphites are triphenyl phosphite [101 -02-0], diphenyl decyl phosphite [3287-06-7], tridecyl phosphite [2929-86-4], and polyphosphites made by reaction of PCl with polyols and capping alcohols. The phosphites are often included in commercial stabilizer packages. 

Another impo2rtant P—C-hond-forming reaction is the base-cataly2ed Michael addition to activated double bonds. For example, dimethyl phosphite can be added to dimethyl maleate to yield tetramethylphosphonosucciaate [2788-26-3] (TMPS), an iatermediate ia the synthesis of 2-phosphonobutane-l,2,4-tricarboxyhc acid [37971-36-1] (PBTC) with 98% yield (20). 

In general, the amines are more active than the phenoHcs which are in turn more active than the phosphites. Amine antioxidants, however, often cause staining problems and are therefore used mainly in black stocks. The phenoHcs and phosphites are relatively nonstaining and are normally used ia light-colored mbbers. 

Manufacture. Trichloromethanesulfenyl chloride is made commercially by chlorination of carbon disulfide with the careful exclusion of iron or other metals, which cataly2e the chlorinolysis of the C—S bond to produce carbon tetrachloride. Various catalysts, notably iodine and activated carbon, are effective. The product is purified by fractional distillation to a minimum purity of 95%. Continuous processes have been described wherein carbon disulfide chlorination takes place on a granular charcoal column (59,60). A series of patents describes means for yield improvement by chlorination in the presence of dihinctional carbonyl compounds, phosphonates, phosphonites, phosphites, phosphates, or lead acetate (61). 

When two antioxidants are used together, a synergistic improvement in activity usually results. Synergism can arise from three combinations (1) homosynergism — two chemically similar antioxidants (for instance, two hindered phenols) (2) autosynergism — two different antioxidants functions that are present in the same molecule (3) heterosynergism — the cooperative effect between mechanistically different classes of antioxidants, such as the combined effect of primary and secondary antioxidants. Thus, combinations of phenols and phosphites are widely used to stabilize synthetic rubbers. 

The SATE ester is formed from a phosphite using PvCl activation followed by oxidation to the phosphate with l2/H20. ... 

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