Xantphos complex

Torisawa [17] developed an alternative oxidative amidation of aldehydes using palladium chloride (PdCl2)-xantphos complex as a catalyst. The use of hydrogen peroxide (H2O2)-urea complex as oxidant prevents the formation of imine from the carbinolamine intermediate and minimizes the level of benzoic acid side... 

The two-layered ONIOM(B3LYP HF) method describing the whole system at quantum mechanical level, without the support of a molecular mechanics method, was employed by Landis and Uddin to explore the hydroformylation of 1-alkenes by a Rh-xantphos complex [119]. The authors state that their results dispel the assumption that only phosphine diequatorial isomers... 

Arylative cyclization of propargyl-substituted malonate esters RC=CCH2CH-(C02Me)2 with aryl halides, catalysed by the Pd-xantphos complex, has been reported to afford alkylidenecyclopropanes. The reaction proceeds via an initial flnft-carbopalladation of the C C bond, which controls the configuration of the resulting C=C in the product. ° ... 

Numerous attempts have been made to immobilize rhodium or its complexes on polymeric or on inorganic supports. Immobilization without ligands leads to systems that mainly hydrogenate the alkene [13]. Recently van Leeuwen and coworkers described the immobilization ofRh - Xantphos complexes in silicagel [14]. The system can be recycled 8 times without significant loss ofRh or activity, but the reaction rate is very low. 

These conclusions confirm the results with Ir(Xantphos) complexes by Eisenberg s group from 2006 [11]. For some of these H2lr complexes, a trans coordination of the diphosphine was found. The hydrido complexes HIr(CO)2(Xantphos) and H3lr(CO)(Xantphos) exhibited only modest hydroformylation activity for the transformation of 1-hexene and styrene (Hj/CO = 2 1, 3 atm 75 °C). The aldehydes were produced in a yield of 10%. More than 50% 1-hexene isomerization was observed. Complete inhibition of the reaction took place in the presence of a twofold excess of the bidentate ligand. It was speculated that in some cases dissociation of Xantphos could be a precondition for catalysis to occur. 

The authors optimised conditions for the general reaction of 1,1-diphenylethylene and piperidine (Scheme 9.5). They obtained the highest TOF (288 h" ) and all linear product for this specific reaction when using complex 31 and 5 1 H iCO at 125°C for 24 h. An important note here is that the sterics of the substrate, 1,1-diarylethyl-enes, are responsible for generation of only linear products instead of the catalyst. With a one-pot method, the authors procured the active pharmaceuticals prozapine, fendilline, milverine, and diisopromine in 85%, 91%, 35% and 88% yield, respectively. The catalyst activities compare well to the established Rh-Xantphos system [33]. 

Phosphacyclic diphosphines (73a) and (73b) with wide natural bite angles were synthesized and the effect of the phosphacyclic moieties on the coordination chemistry in the [(diphosphine) Rh(CO)2H] complexes was studied. Both NMR and IR spectroscopy showed that the phosphacyclic xantphos ligands exhibit an enhanced preference for diequatorial chelation compared to the diphenylphosphino-substituted parent compound. In the hydroformylation of 1-octene the introduction of the phosphacyclic moieties leads to higher reaction rates. The dibenzophospholyl- and phenoxaphosphino-substituted xantphos ligands exhibit a high activity and selectivity in the hydroformylation of trans-2- and 4-octene to linear nonanal. CO dissociation rates from the... 

The synthesis, aggregation behavior, and catalytic activity of Rh complexes of Xantphos derivatives (129) with surface-active pendant groups have been described.416 The complex [HRh(CO)(TPPTS)3] was used as a catalyst precursor in the hydroformylation of 1-butene, 1-octene, and styrene under biphasic reaction conditions 417 The two-phase hydroformylation of buta-1,3-diene with [HRh(CO)(TPPTS)3], with excess TPPPS, gives high yields of C5-monoaldehydes.418 The coordination behavior of the catalytic species HRh(130)(CO)2] was studied by HP NMR spectroscopy which showed the desired bis-equatorial coordination of the ligand to the rhodium center.419... 

Amides and sulfonamides undergo intramolecular chemistry to form aryl amides and aryl sulfonamides (Equations (17)—(19)) in the presence of palladium catalysts ligated by arylphos-phines.35,89 Initially, complexes of P(furyl)3 and P(o-tol)3 were most effective catalysts, but complexes of Hayashi s MOP and van Leeuwen s DPEphos and xantphos have lately been shown to be more active.90 In the presence of catalysts containing one of these ligand systems, five-, six-, and seven-membered rings were formed from halogenated benzamides or from substrates containing an acetamide, an A-carbobenzyloxy, or a t-butylcarbamate substituent tethered to the aryl halide (Equations (18) and (19)) ... 

A set of core-functionalized dendrimers was synthesized by Van Leeuwen et al. and one compound was applied in continuous catalysis. [45] The dendritic dppf, Xantphos and triphenylphosphine derivatives (Figures 4.22, 4.30 and 4.31) were active in rhodium-catalyzed hydroformylation and hydrogenation reactions (performed batch-wise). Dendritic effects were observed which are discussed in paragraph 4.5. The dendritic rhodium-dppf complex was applied in a continuous hydrogenation reaction of dimethyl itaconate. 

The last example of a dendritic effect discussed in this chapter is the use of core-functionalized dendritic mono- and diphosphine rhodium complexes by Van Leeuwen el al. [45] Carbosilane dendrimers were functionalized in the core with Xantphos, bis(diphenylphosphino)ferrocene (dppf) and triphenylphosphine (Figures 4.22, 4.32 and 4.33). 

More recently, during research aimed at supporting the highly linear selective hydroformylation catalyst [Rh(H)(Xantphos)(CO)2] onto a silica support, the presence of a cationic rhodium precursor in equilibrium with the desired rhodium hydride hydroformylation catalyst was observed. The presence of this complex gave the resulting catalyst considerable hydrogenation activity such that high yields of linear nonanol could be obtained from oct-1-ene by domino hy-droformylation-reduction reaction [75]. 

In line with the above mechanism, catalyst deactivation by formation of palladium black can be retarded by increasing the [P]/[Pd] ratio, however, only on the expense of the reaction rate. Bidentate phosphines form stronger chelate complexes than TPPMS which may allow at working with lower phosphine to palladium ratios. Indeed, the palladium complex of sulfonated XANTPHOS (51) proved to be an effective and selective catalyst for hydroxycarbonylation of propene, although at [51]/[Pd] < 2 formation of palladium black was still observed. The catalyst was selective towards the formation of butyric acid, with 1/b = 65/35 [41]. 

Table 6.1 shows data for rhodium complexes containing xantphos ligands in which the electronic properties were systematically varied [13]. With few exceptions, we see that the rate of the hydroformylation reaction increases when the rhodium center is more electron deficient. This is as expected, since dissociation of... 

The decay of the carbonyl bands of the HRh(diphosphine)("CO)2 complexes with time follows simple first-order kinetics in all experiments. Plots of ln[HRh (diphosphine)("CO)2] vs. time are linear for at least two half-lives. Comparison of the rate constants, kj, obtained for ligands 32 and 33 [54] with those obtained for other xantphos ligands [52] shows that the CO dissociation rate for ligand 32 is in the same range as other ligands. The CO dissociation rate for ligand 33, however, proves to be four to six times higher. 

The luminescent properties of complexes [Au2C12(P P)] [P P = bis(2-diphenyl-phosphino)phenylether (dpephos), 9,9-dimethyl-4,5-bis(diphenylphosphino)xan-thene (xantphos), and 4,6-bis(diphenylphosphino)dibenzofuran (dbfphos)] have been studied. Aurophilic intramolecular interactions are present in [Au2Cl2(dephos)] and [Au2Cl2(xantphos)] which are probably responsible for the presence of a low energy band [209]. 

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