Trialkylphosphines

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. 

Ligand-Modified Cobalt Process. The ligand-modified cobalt process, commercialized in the early 1960s by Shell, may employ a trialkylphosphine-substituted cobalt carbonyl catalyst, HCo(CO)2P( -C4H2)3 [20161 -43-7] to give a significantly improved selectivity to straight-chain... 

Complexes of triisopropylphosphine (cone angle 160°) [69, 117]. In contrast to less bulky trialkylphosphines, a complex RhCl3(PPr3)3 has only been made by reaction at 0°C at higher temperatures, hydrides like RhHCl2(PPr 3)2 result (the less bulky tri(n-propyl)phosphine gives the 3 1 complex at reflux). 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Ylids are usually prepared from triphenylphosphine, but other triarylpho-sphines, trialkylphosphines, and triphenylarsine " have also been used. The Wittig reaction has also been carried out with other types of ylids, the most important being prepared from phosphonates " ... 

Thereafter, however, P-chirogenic phosphine ligands were the subject of less investigation since the synthesis of highly enantiomerically enriched P-stereo-genic phosphines often proves difficult. Another reluctance Hes in the fact that this class of phosphines, especially diaryl- and triarylphosphines, is conforma-tionally unstable and gradually racemize at high temperature [57,58]. In contrast, optically active trialkylphosphines are known to be optically stable even at considerably elevated temperature. 

B. By Hydrolysis Reactions.—Details have appeared of the synthesis of dibenzophosphorin oxides (15) from 5-alkyldibenzophospholes, by reaction with methyl propiolate in the presence of water, and of confirmatory syntheses from phosphinic acid chlorides, as shown below. Evidence for the suggested mechanism of the ring-expansion reaction is presented. The hydrolysis of enamine phosphine oxides is an efficient, although somewhat indirect, method for the preparation of j8-ketoalkylphosphine oxides (16) [see Section 3(iii), for the preparation of enamine oxides]. Reasonable yields (48—66%) of trialkylphosphine oxides (17) have been obtained by the alkaline hydrolysis of the products from the pyrolysis at 220 °C of red phosphorus with alkyl halides, in the presence of iodine. 

The chromatographic behaviour of condensed phosphates was found to be strongly influenced by water content and pH. The Ry of tris(dimethyl-amino)phosphine chalcogenides is generally lower than those of the trialkyl-phosphine chalcogenides but the difference is greatest for the P " compounds, and whereas the trialkylphosphines have the highest Ry values in the whole series, tris(dimethylamino)phosphine has the lowest. ... 

Trialkylphosphine oxides and tris(dimethylamino)phosphine oxide are undergoing this reaction as well (62). 

Acyclic phosphoranes. - A number of difluorotris(perfluoro-alkyl)phosphoranes (7) have been prepared by electrochemical fluor-ination of trialkylphosphine oxides (6) in anhydrous HF17. The phosphoranes are conveniently converted into the perfluoroalkyl-phosphine oxides (8) by reaction with hexamethyldisiloxane and the phosphoranes are regenerated by treatment of (8) with HF. 

At least partial substitution of other inherently strong ligands by phosphines can be achieved. Trialkylphosphines substitute partially into arylisocyanide complexes [Co(CNR)5)]+, with monosubstitution and at best disubstitution observed, influenced by the steric bulk of the phosphine.152... 

Bis(trialkylphosphine oxide) complexes [NiBr2(0=PR3)2] reversibly bind S02 to give species formulated as (259).734... 

In the absence of a solvent, quatemation of trialkylphosphines with methyl iodide may proceed explosively. 

The reduction of a solution of a trialkylphosphine copper(I) iodide complex (CuIPR ) with preformed lithium naphthalide (LiNp) in THF or DME under argon was found to give a more reactive copper species, which will undergo oxidative addition with a variety of organic substrates at room... 

The cross-coupling of aryl or alkenylsilanes with aryl chlorides bearing electron-withdrawing substituents has been reported to require trialkylphosphine (P Pr3) ligands.407... 

The other use of the bulky trialkylphosphines is in the cases where PPh3 or similar ligands can interfere with the cross-coupling reaction by forming by-products (115).412... 

A major advance in homogeneous catalysis was the introduction of a trialkyl-phosphine to supplement the role of carbon monoxide in catalyst stabilization. [3] A ligand modifier such as trialkylphosphine serves three principal roles in a homogeneous catalytic process. It stabilizes the metal, it influences the reaction rate, and it influences process selectivity. 

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