Radical mechanisms phosphite radicals

Fig. 4.11 The assumed mechanism of the synthesis of vinylphosphonic acid via addition of the phosphite radical to acetylene (de Graaf et al., 1997)...

The mechanism of formation of PCBs from the trichlorobenzene solvent involves hydrogen abstraction by the urea present, leading to the formation of trichlorophenyl radicals. Two such radicals can then combine (Scheme 2.2) to form a PCB molecule (2.36). This reaction can be suppressed by adding antioxidants such as hydroquinone or sodium phosphite to yield hydrogen (H ) radicals, which convert (Scheme 2.3) the trichlorophenyl radicals back to the parent substance (2.37). 

Allyl)CpFe(CO)(PR3) complexes (14) have been prepared from the dicarbonyls, by photolysis in the presence of phosphine or phosphite. The substitution is often aided by a trace of (CpFe(CO)2)2, which is indicative of a radical (see Radicals) chain substitution mechanism. Phosphite ligand (as in 15) has been reported as being a particularly good replacement ligand from the standpoint of thermal stability.Nevertheless, neither C-3-substituted ()] -allyl)Fp complexes (14b) nor cyclic allyl complexes (see Allyl Complexes) may be made directly by this method the carbocyclic cases have been prepared by methoxide-induced proton abstraction of aUcene cation (16) (Section 4.3.2). " ... 

Interesting possibilities for the synthesis of new types of polysaccharide derivatives are offered by the reaction of addition to double bonds, which proceeds by a free-radical mechanism. The presence of initiators of free-radical polymerization (benzoyl peroxide, tert-butyl peroxide, dicumyl peroxide, dinitrile of azodiisobutyric acid), also irradiation with ultraviolet light, has effected the addition to 5,6-cellulosene of chloroform, carbon tetrachloride, methylmonochloroacetate, dimethyl-phosphite and other compounds that decompc under the conditions of a reaction with the formation of free radicals (45,46). The reaction proceeds as follows ... 

A peculiai- case of phosphoryl radical addition to difluoroalkenes involves the reaction of trialkyl phosphites with l-bromo-2-iodo-l,l,2,2-tetfafluoroethane under ultraviolet irradiation (254 nm). Surprisingly, the corresponding 2-iodo-l,l,2,2-tetrafluoroethylphosphonates are formed in 42 8% yields with no detectable amount of the bromo derivative (Scheme 3.39). The proposed mechanism involves a halide-induced dealkylation of the trialkyl phosphite radical cation followed by addition of the product phosphoryl radical to tetrafluoroethene (generated by halide anion elimination) and iodide radical abstraction from the starting haloalkane. s... 

The reaction can be conducted under thermal conditions (reflux), radical conditions (the presence of traces of benzoyl peroxide induces a fourfold increase in the thermal reaction rate and a slightly better yield), or photochemical conditions (where the reaction proceeds under UV irradiation at room temperature to give the same yield as above no reaction is observed in the dark at room temperature).° ° The mechanism of the reaction has been studied extensively, °° ° °°° °° and it has been concluded that the thermal reaction of triethyl phosphite with CCI4 involves an SnCP substitution. In the presence of UV light or free-radical chain initiators, the radical mechanism generally dominates. The ability of the trichloromethyl radical to initiate a radical chain reaction depends on the relative concentrations of the reagents. The final product mixture is the same as in the ionic casc.°°°... 

Several mechanisms are possible for these cyclizations. Formation of the dioxide (4) from the unknown 2,2 -dinitrosobiphenyl would parallel the dimerization observed with acyclic nitrosoarenes, and all the other cycliza-tion steps shown in Scheme 1 could be simple condensations, or involve radical anions, or radicals. Where deoxygenating reagents are used, there is also the possibility of cyclization via nitrene intermediates. It has been reported that low yields of benzo[c]cinnolines result from reduction of 2,2 -dinitrobiphenyls with carbon monoxide in the presence of iron penta-carbonyF or by heating in triethyl phosphite. In polarographic reduction studies, Ross et al - claimed to have demonstrated the sequence ... 

Further investigation of the disulfide reaction, first reported from our own laboratory (143), has revealed that the ionic mechanism appears to be the favored pathway. Thus, the reaction of triethyl phosphite with n-propyl disulfide (125) proceeds with comparable facility in the presence or absence of hydroquinone as a radical inhibitor to yield in either case the mixed thioether which is predicted by the ionic but not the radical mechanism. Symmetrical aryl disulfides are more reactive than alkyl disulfides, and with the unsymmetrical disulfides studied, the transformation takes place exothermically at room temperature. 

It is known that triethyl phosphite reacts with carbon tetrachloride to give ethyl chloride and diethyl (trichloroethyl)phosphonate. Ionic and radical mechanisms have been suggested to account for this remarkable reaction. According to the most reasonable, fundamental one (61), the reaction is initiated by a nucleophilic attack on chlorine (not an Arbusov reaction) followed by formation of the phosphonium salt [71] (Atkinson et al., 1969). 

In the presence of a radical initiator, disulphides react with phosphites to give an alkoxyphos-phine sulphide and a thioether (13.202)-(13.204). The oxidation of triethyl phosphite by nitric oxide proceeds by a free radical mechanism (13.205). 

This section describes selected arylations of phosphorus, sulfur, and halide nucleophiles under metal-free and metal-catalyzed conditions. Arylations of other nucleophiles, e.g., selenium and tellurium, have been reviewed previously [4]. Aryl phosphonates [ArPO(OR)2] can be synthesized by arylation of phosphite anions with diaryliodonium salts and NaH in DMF at 70-80 °C [158]. A copper-catalyzed arylation of various phosphorous nucleophiles, e.g., diarylphosphine oxides and //-phosphonates, was recently reported to proceed at room temperature. The observed chemoselectivity with unsymmetric salts was opposite to the general trend in metal-catalyzed reactions (see Sect. 2.1), which was explained by a radical mechanism [159]. 

Extrusion of sulphur from disulphides " and trisulphides gives sulphides and disulphides, respectively, by treatment with phosphines (a polymeric aminophosphine has been advocated ), phosphites, and MeHgOAc. Ionic pathways continue to be preferred for these reactions, and concurrent electrophilic and nucleophilic mechanisms are in operation, according to interpretations of kinetic data for the reaction with dimethyl disulphide. A new view on nucleophilic attack is advanced in this work, where the encounter involves the o orbital of the S—S bond. Further support is given by c.i.d.n.p. to a radical mechanism that involves PhCHjSe for the reaction of dibenzyl selenide with PhjP. The process PhTeTePh + RN2 Cl - RTeCljPh in the presence of lCu(OAc)2l may be more easily understood on this basis. 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

Examples of the intermolecular C-P bond formation by means of radical phosphonation and phosphination have been achieved by reaction of aryl halides with trialkyl phosphites and chlorodiphenylphosphine, respectively, in the presence of (TMSlsSiH under standard radical conditions. The phosphonation reaction (Reaction 71) worked well either under UV irradiation at room temperature or in refluxing toluene. The radical phosphina-tion (Reaction 72) required pyridine in boiling benzene for 20 h. Phosphinated products were handled as phosphine sulfides. Scheme 15 shows the reaction mechanism for the phosphination procedure that involves in situ formation of tetraphenylbiphosphine. This approach has also been extended to the phosphination of alkyl halides and sequential radical cyclization/phosphination reaction. ... 

The mechanism of secondary stabilization by antioxidants is demonstrated in Figure 15.5. TnT-nonylphenyl phosphites, derived from PCI3 and various alcohols, and thio-compounds are active as a secondary stabilizer [21], They are used to decompose peroxides into non-free-radical products, presumably by a polar mechanism. The secondary antioxidant is reacting with the hydroperoxide resulting in an oxidized antioxidant and an alcohol. The thio-compounds can react with two hydroperoxide molecules. 

A useful new method of preparing arylphosphonates (123) involves the reaction of trialkyl phosphites with aryl halides in the presence of a nickel catalyst.The suggested mechanism is via the nickel complex (124), and is non-radical. 

Reduction by hydrogen atom donors involves free radical intermediates and usually proceeds by chain mechanisms. Tri-n-butylstannane is the most prominent example of this type of reducing agent. Other synthetically useful hydrogen atom donors include hypophosphorous acid, dialkyl phosphites, and tris-(trimethylsilyl)silane. The processes that have found most synthetic application are reductive replacement of halogen and various types of thiono esters. 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

The mechanism of inhibitory action of aryl phosphites seems to be relatively complex. Phosphites reduce hydroperoxide and thus decrease chain autoinitiation. The formed peroxyl and alkoxyl radicals react with phosphites to form aroxyl radicals. The latter terminates the chains by reaction with peroxyl radicals. On the other hand, phosphites are hydrolyzed with... 

The electrochemistry of RH-Nu systems is well established (Eberson and Nyberg, 1976 Eberson et al., 1991 Childs et al., 1991). The radical cation mechanism has been shown to prevail for most situations where Nu = F , Cl-, RCOCT, OCN", CN", NO-r, Py and triethyl phosphite, all of them nucleophiles that are difficult to oxidize (Table 5). The initial formation of Nu" is indicated for the redox-reactive SCN", NJ, I- and N02, with Br and (N02)3C occupying a somewhat indeterminate position. 

Both mechanisms explain the complete retention of configuration at silicon because free silyl radicals are not involved. The 16-electron complex may add both one phosphite molecule (when it is present in excess) or one carbonyl (formed in the first steps and remaining in solution before being slowly evolved). 

The reactions of sodium dimethyl and diisopropyl phosphite with 4-nitrobenzyl chloride, 9-chlorofluorene, and diphenylchloromethane provided information that supported the proposed reaction mechanism. The RaPO anion acts towards an arylmethyl chloride as a base and abstracts a proton to form a carbanion, which can then participate in single-electron transfer processes to produce carbon-centred radicals. ... 

While investigating the mechanism, Pobedimskii and Buchachenko (1968a, 1968b) concluded that these reactions have an ion-radical nature and consist of electron transfer from phosphites or sulfides (denoted further as D) to hydroperoxides as follows ... 

The light-induced addition elimination reaction of diethyl phosphite to both 1,3,3,4,4-penta-fluorocyclobutene and l-chloro-3,3,4,4-tetrafluorocyclobutene afforded a mixture of diethyl 3,3,4,4-letrafluorocyclobutene-l-phosphonate (11) and tetraethyl 3,3,4,4-tetrafluorocyclobutane-1,2-diphosphonate (12). The postulated mechanism of addition involves phosphonyl radical attack at the cyclobutene moiety.17... 

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