Stabilizing phosphines

Palladium is generally introduced in catalytic amounts as the stable complexes Pd(PPh3)4 and Pd(dba)2 (dba = dibenzylideneacetone), frequently accompanied by a stabilizing phosphine [PPh3, Ph2P — (CH2) — PPh2, or others], or as a form of Pd(II) such as acetate, chloride, or acetylacetonate plus a phosphine. In the last case, the Pd(II) is reduced in situ to the catalytically active Pd(0) species. 

Cationic P compounds exhibit several modes of reactivity, including coordination to Lewis acids oxidation by acids, water, and alkyl chlorides and substitution of the stabilizing phosphine ligands by stronger donors (Scheme 17). Some of these P and As cations have also been shown to be useful sources of P and As ions that provide zirconium complexes (12) containing unique square-planar Pn environments (equation 22). ... 

Herrmann emphasized that aryl chlorides are unsuitable as arylating agents in Heck reactions primarily because P—C cleavage and loss of Pd(0)-stabilizing phosphines ultimately occur, leading to catalyst deactivation and palladium black formation, rather than resistance of aryl chlorides to oxidative addition.f " ... 

In terms of complex stability, phosphines with strong a-donor properties are advantageous. Noteworthy, the p/Cj value of tertiary phosphines correlates indirectly with the rate of the hydroformylation [18]. Thus, the coordination of... 

Organophosphorus compounds. Phosphorus-carbon bond fonnation takes place by the reaction of various phosphorus compounds containing a P—H bond with halides or tritlates. Alkylaryl- or alkenylalkylphosphinates are prepared from alkylphosphinate[638]. The optically active isopropyl alkenyl-methylphosphinate 778 is prepared from isopropyl methylphosphinate with retention[639]. The monoaryl and symmetrical and asymmetric diarylphosphi-nates 780, 781, and 782 are prepared by the reaction of the unstable methyl phosphinate 779 with different amounts of aryl iodides. Tnmethyl orthoformate is added to stabilize the methyl phosphinate[640]. 

This dicarboxyhc ester is then copolycondensed with the other reactants in PET manufacture to produce a flame-retardant polyester [63745-01-7]. The advantage of this rather unusual phosphinate stmcture is its high thermal and hydrolytic stability. The fabric is probably used mainly for flirnishings in pubhc buildings in Japan. 

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. 

However, when either P(CgH )(CH2)2 or P(CgH )2(CH2) is used to form cis- or /n j -M(N2)2(PR3)4j M = Mo or W, respectively, followed by treatment with acid, ammonia yields of about 2 mol or 0.7 mol pet mole of complex for M = W and Mo, respectively, are produced (193,194). These and related data have been used to suggest a possible stepwise sequence for the reduction and protonation of N2 on a single molybdenum atom ia nitrogeaase (194). However, acidificatioa leads to complete destmctioa of the complex. Using both the stabilizing effect of the chelating phosphine triphos,... 

Heteroleptic complexes of uranium can be stabilized by the presence of the ancillary ligands however, the chemistry is dominated by methyl and benzyl ligands. Examples of these materials include UR4(dmpe) (R = alkyl, benzyl) and U(benzyl)4MgCl2. The former compounds coordinate "soft" chelating phosphine ligands, a rarity for the hard U(IV) atom. 

Chemical Reactivity - Reactivity with Water Reacts vigorously with water, generating phosphine, which is a poisonous and spontaneously flammable gas Reactivity with Common Materials Can react with surface moisture to generate phosphine, which is toxic and spontaneously flammable Stability During Transport Stable if kept dry Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

The stability of the phosphine adducts is notable as is the fact that thioethers readily form such adducts whereas ethers do not. Bis-ligand adducts of moderate stability play an important role in activating decaborane for several types of reaction to be considered in moie detail in subsequent paragraphs, e.g. ... 

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

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