Radicals phosphinyl

The acyl phosphonates, acyl phosphine oxides and related compounds (e.g. 81. 82) absorb strongly in the near UV (350-400 nm) and generally decompose by rescission in a manner analogous to the benzoin derivatives.381"285 Quantum yields vary from 0.3 to 1.0 depending on structure. The phosphinyl radicals are highly reactive towards unsaturated substrates and appear to have a high specificity for addition v.v abstraction (see 3.4.3.2). 

Phosphinyl radicals (e.g. 103-107) arc generated by photodecomposition of acyl phosphinates or acyl phosphine oxides (see 3.3.4.LI)282,466 474,473 or by hydrogen abstraction from the appropriate phosphine oxide.467... 

The reactivities of the various phosphinyl radicals with monomers have been examined (Table 3. lO).283-465,467-475 Absolute rate constants are high, lying in the range 106-I08 M 1 s 1 and show some solvent dependence. The rate constants are higher in aqueous acetonitrile solvent than in methanol. The high magnitude of the rate constants has been linked to the pyramidal structure of the phosphinyl radicals.46- ... 

The phosphinyl radicals (103-107) all show nucleophilic character (eg. VAc is substantially less reactive than the acrylic monomers). However, the... 

Although their chemistry is less developed than that of phosphonyl, phospho-niumyl or phosphoranyl radicals, many structural studies have been devoted to phosphinyl radicals [1]. Like their nitrogen analogs,phosphinyl radicals are 7i-type radicals (Fig. 1) and because of the very small s character of their SOMO, the magnitude of their phosphorus hyperfine coupling constants flp is below 15 mT [1]. 

Due to their weak P-H bonds (-370 kj mol"0 [2] and the high rate constants for the transfer of the P-H hydrogen [3] (/c=1.5 10 L rnoL s" for Ph2PH and k=5,0 10 L mol s for (c-hexyl)2PH), diaryl and dialkyl phosphines present a high interest as H-donors. Since the corresponding phosphinyl radicals are good chain carriers [4,5], diaryl and dialkyl phosphines can be added to olefinic or acetylenic compounds through radical chain reactions. Simpkins et al. [6] used... 

Scheme 1 5-exo Cyclization initiated by phosphinyl radical addition onto an alkyne... 

Phosphinyl radicals, obtained by hydrogen abstraction from dialkyl phosphites, have been trapped with t-nitrosobutane and the resultant nitroxyl radicals examined by e.s.r, The reaction of phosphinyl radicals, e.g. (5) and (6), with olefins has been shown to occur with retention of configuration at phosphorus. " These radicals have also been postulated as intermediates in the reactions of dialkyl disulphides and diaryl disulphides with phosphinates. " From the reaction of diphenyl disulphide... 

Phosphide anions, phosphinyl radicals, and phospheniutn cations are two-coordinate phosphorus species which are related by gain or loss of one electron ... 

Phosphide anions are well documented and are used widely as reagents. However, much less is known concerning phosphinyl radicals and phosphenium cations. 

Interesting work by Lappert, Goldwhite, and co-workers (1,2) has established that phosphinyl radicals can be stabilized by means of bulky substituents such as (Me3Si)2CH. We have found that these radicals constitute a new class of ligand intermediate in character between phosphenium cations and phosphide anions. 

Fig. 10-7. Magnetic field dependence of the absorbance (A(t)) observed at r = 100 or 200 ns after laser excitation for some phosphinyl radicals in cyclohexane at room temperature. This figure is reproduced from Ref [8].

Isomerization of cis- to traos lefins. Sgoutas and ICummerow reported in 1969 that c i-unsaluraled fatly acid esters are isomerized to tro/is-eslers when heated at 65° in a sealed vial with thiols or diphenylphosphine in the presence of azobisisobutyronitrile. The equilibrium mixture contains 75-80% lran.i double bonds. There is no migration of the double bonds. Presumably addition of thiyi or phosphinyl radicals to the double bond is involved. 

Sulfophosphamides, ArS02PPh2, are reduced more easily than the corresponding sulfonamides [269] the radical anion is cleaved to sulfinic acid and phosphinyl radical, which is further reduced to the phosphine anion, Ph2P . This anion, however, reduces the arylsulfinate to arylmercaptane according to ... 

The group VB hydrides show trends in reactivity similar to those of group IVB. The N-H bond can be reacted with alkenes only under the influence of catalysts or under forcing conditions. The P-H bond can be added to alkenes (hydrophosphination) in a free radical chain process, or under photolytic conditions. Such reactions proceed in good yield and in an anti-Markovnikov manner. Some typical free radical P-H additions are listed in Table 1 . The addition of phosphinyl radicals is reversible and can lead to... 

For several years, our laboratory has been interested in the chemistry of main group elements in low coordination number environments (7). Some low coordinate species from Group 15 are shown below. They include two-coordinate cations (phosphenium ions), two-coordinate radicals (phosphinyl radicals), as well as stable double-bonded and even triple-bonded neutral molecules. For many years it was thought that... 

A review has recently been published on the subject Phosphorus Radicals . Once again e.s.r. spectroscopy has proved to be an invaluable tool for investigating the kinetics of reactions of phosphorus radicals. Thus the rate constants for the bimolecular reactions of several types of phosphorus radicals, e.g. phosphinyl, phosphonyl, and phosphoranyl radicals, with like species have been determined and shown to be of the same order as, or very close to, the diffusion-controlled limit.These annihilation reactions presumably involve formation of a P—P-bonded dimer. An interesting observation recorded in the paper reporting this work was that t-butoxyl radicals react with (25) to give phosphinyl radicals. 

Other reported reactions involving phosphinyl radicals include the reaction of dimethylphosphine and tetramethylbiphosphine with tetrafluoro-ethylene. The gas-phase reaction of dimethylphosphine with the olefin occurs quantitatively, and a free-radical chain mechanism was postulated. Detailed kinetic analysis of the reaction indicated that a rather unusual type of initiation reaction occurs, in which, it is thought, the olefin (26) reacts as a biradical. The... 

The reactivities of the various phosphinyl radicals with monomers have been... 

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