Phosphines ligation

Alkylnickel amido complexes ligated by bipyridine have been prepared that undergo reductive elimination of V-alkyl amines (Equation (54)).207,208 Unlike the phosphine-ligated palladium arylamides, these complexes underwent reductive elimination only after oxidation to nickel(III). Thermally induced reductive elimination of alkylamines from phosphine-ligated nickel complexes appears to occur after consumption of phosphine by arylazides 209... 

There are several polymer supported transition metal hydrofomylation catalysts (42 ). Most are attached by phosphine ligation and suffer fron catalyst leaching. There are no n5-cyclo-pentadienyl half sandwich systems despite the potentially, clearly advantageous presence of the relatively strong Cp-metal bond (43,MO. Resin 5 was used in the following brief study in which the potential of polystyrene-supported CpCo(C0)2 to function as a hydrofomylation catalyst was tested. 

In 1993 Burk, Brown, and coworkers confirmed that DuPHOS complexes exhibit the same anti-lock-and-key mechanistic motif as seen for aryl phosphine ligated catalysts [41], In 1998 by Burk and coworkers reported an unexpected and interesting result [34], With substrates having R<, = aryl, selectivity of 99% e.e. for the S product resulted from (S,S)-Me-DuPHOS-Rh hydrogenations, but the R product was obtained with similarly high enantioselectivity when Ra = t-Bu or adamantyl. In other words, the simple change of an aryl substituent to a bulky alkyl completely reverses the sense of enantioselection. 

B. D. Chandler, L. 1. Rubinstein, andL. H. Pignolet, Alkane dehydrogenation with silica supported Pt and Pt-Au catalysts derived from phosphine ligated precursors, J. Mol. Catal. A Chem. 133,... 

Phosphine—isocyanides, with technetium, 5, 843 Phosphine-ligated 15-electron complexes, with chromium,... 

Pd[P(o-tolyl)3]2] underwent oxidative addition of aryl halides to provide the unusual dimeric aryl halide complexes [Pd[P(o-tolyl)3](Ar)(Br) 2 (Eq. (44)) [77,102]. It is unusual for phosphine-ligated aryl halide complexes formed by oxidative addition to be dimeric. These oxidative addition products were isolated and structurally characterized. They remain dimeric in solution, as determined by solution molecular weight measurements, but react as the monomers, as described below. 

Scheme 9. Overall mechanism for aryl halide amination catalyzed by bis(phosphine)-ligated palladium complexes.

For the case of tri(o-tolyl)phosphine-ligated catalysts, the upper pathway appears to predominate. Oxidative addition occurs first via loss of a ligand from the bisphosphine precursor to form the oxidative adduct, which exists as a dimer bridged through the halogen atoms (equation 33). This dimer is broken up by amine, the coordination of which to palladium renders its proton acidic. Subsequent deprotonation by base leads to the amido complex, which can then reductively eliminate to form the product. When tert-butoxide is used as the base, the rate is limited by formation of and reductive elimination from the amido complex, while for the stronger hexamethyldisilazide, the rate-determining step appears to be oxidative addition. ... 

Coupling of allylic alcohols CH2=CHCH(OH)R with aryliodonium salts to give ArCH=CHCH(OH)R using polymeric phosphine ligated PdClj and NaHCOj enjoys many advantages. The catalyst can be easily removed from the reaction mixtures and reused more than 10 times. In the absence of base or with a catalyst prepared from Pd(OAc)j the yields are slightly lower. 

It was proposed that during the induction period phosphine is slowly dissociating from the complex to form Ij, and the concentration of Ij finally reaches a quasi-steady state when the rate of ADMET is observed to be constant In contrast, the more rapid phosphine dissociation of complex 6 results in the observation of a constant maximum ADMET rate from the first data point onward. Once the equilibrium state of phosphine dissociation is reached, complex 10 promotes ADMET twice as fast as 6 at 45 °C due to the higher reactivity of the NHC-derived Ij compared to the phosphine-ligated resulting from complex 6. 

A macrocycle with secondary phosphine ligating sites was prepared in an 80% yield by treating l,2-bis(phosphino)benzene first with n-butyllithium and then with bis(3-aminopropyl)methylamine under high-dilution conditions in THF (method AA-3) (Kyba and Heumuller, 1981). 

The 2-iodo derivative (21) is even less reactive than its 4-isomer. No alkenylation was observed with phosphine-ligated Pd (79CPB193 82CPB3647), but moderate yields of 2-alkenylated pyrimidines (22) are produced in the absence of triphenylphosphine (81H965). 

Aryl ketones. Benzyl alcohols and alkenes are united by a catalytic reaction using polymeric phosphine ligated RhClj hydrate. 

Ferrocene dendrimers are also of interest for reasons other than their redox activity. For example, metalloden-drimer 242 possesses planar chiral ferrocene units that make the bidentate phosphine ligation sites of potential interest for applications in asymmetric catalysis. Indeed, asymmetric hydrogenations of dimethyl itaconate catalyzed by Rh complexes of 242 showed impressive ee values of 98%. " ... 

While phosphine-ligated catalysts like 77 and 81 are effective in thermo-morphic systems, the oxidation of phosphine ligands in the presence of transition metal compounds and especially in the presence of Pd(0) poses experimental and practical problems in recycling catalysts in batch reactions. Such issues can be handled with more rigorous inert atmosphere equipment. Alternatively, more stable catalysts can be used. In the case of Heck and Suzuki chemistry, very stable pincer-type catalysts are available. These include phosphine-ligated species like 112 and thioether-ligated species like 113 [152-154]. 

Formation of Ni—CH3 species by the more likely biological route of Ni-based nucleophilic attack on a methyl electrophile has been demonstrated. The Ni complex, (40), supported by thioether and phosphine ligation, was reduced to the Ni adduct in situ. It was then oxidized to the Ni CH3 and Ni (C(0)CH3) species by reaction with CH3I and CH3C(0)C1, respectively. These low-spin, square-pyramidal complexes contain apical organic ligands and are related by CO insertion/extrusion, which was rapid even at -60 °C, precluding kinetic analysis. 

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