Alkyl phosphines and phosphites

If molecules in which the donor atom is in the third, fourth or fifth row of the periodic table are added to Figure 2, then similar results to those of Figure 1 are found. The heavier donor atoms lie below the line, corresponding to weak homolytic bond energies. Exceptions occur for alkyl phosphines and phosphites, which lie above the line. These exceptions are related to the fact that the ionization potentials of these molecules are anomalously large (23). 

The tetrakis-phosphino and -arsino complexes reported in Table 3 have very different thermal and air stabilities. In general, complexes with alkyl-phosphines and -arsines are air-unstable or pyrophoric. The tetrakis(arylphosphino) complexes (but not the bis or tris derivatives) are moderately air- and heat-stable, and the tetrakis phosphites may be handled in the atmosphere. Ni(PF3)4 is a stable liquid compound, whereas Ni(PCl3)4 and Ni(PBr3)4 are solids and stable only in dry air. 

In addition to a- and p-C-H activation, the possibility occasionally arises for y- or even 8-functionalization. This is particularly common for aryl phosphine and phosphite ligands that may undergo metallation of the ortho-C-H bond of an aryl substituent. This process may be reversible however, if a suitable co-ligand is present which can undergo subsequent reductive elimination of the hydride, stable metallacyclic organyls are obtained (Figure 4.31). The formation of metallacyclic alkyls may confer some stability, as does the possibility of increased hapticity, e.g. in the case of xylyene ligands (see also Chapter 6). 

As noted above, <7-carbon complexes derived from HCo(CO)4 are of low thermal stability and most of the isolated examples contain phosphine and phosphite ligands. Thus Co(PMe3)4 is readily alkylated by Mel to MeCo(PMc3)4. With excess Mel, oxidative addition with loss of one phosphine to Me2CoI(PMe3)3 is found. Higher alkyls are subject to /3-elimination (see -Elimination). 

Alkylation at Phosphorus. Phosphines and phosphites undergo easy quaternization. Thus methylation of tris(2,6-dimethyl-phenoxy)phosphine with MeOTf, followed by treatment of the product with sodium 2,6-dimethylphenoxide, gave methyltetrakis (2,6-dimethylphenoxy)phosphane. Methoxyphosphonium tri-flates are relatively stable intermediates in Arbuzov reactions Phosphine oxides and sulfides are alkylated. S-Methylation of chiral phosphine sulfides, followed by treatment with hexam-ethylphosphorous triamide, has been advocated as a general synthesis of optically active phosphines. ... 

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. ... 

Similar reactions between diketopiperazine and either trialkyl phosphites or alkyl phosphinates produced the related cyclic analogs 17 and 18 (24). 

Low oxidation states are generally stabilized by ligands which have both a donor (lone pairs) and n acceptor (either empty d orbitals or empty antibonding Jt orbitals) capability. Examples of common ligands with these characteristics are carbon monoxide, cyanide ion, alkyl and aryl isocyanides, tertiary phosphines and arsines, and alkyl or aryl phosphites. 

The nature of alkyl halide and the environment of the phosphorus atom make a substantial contribution to the direction of the reaction. Thus, cyclic phosphites [108], like trialkyl phosphites [110], react with preservation of the coordination of the P(III) phosphorus atom (126-132 ppm). If the alkoxyl group is substituted by amide, alkyl, or aryl, the nucleophilicity of the phosphorus atom in the corresponding amidophosphite (phosphonite) increases in comparison with the trialkyl phosphite. This probably promotes attack by 1 -haloalkyl-2-pyrrolidone at the phosphorus atom and not the oxygen, and this was confirmed experimentally. As a result of the investigated reactions amidophosphonates or ethyl phenyl phosphinates llOa-c were isolated compounds with P(III) were not detected in these cases. 

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