Induction catalyst precursor

In addition, careful kinetic measurements and product analysis has revealed that the activation of the catalyst precursor 26b during the induction period occurs by hydrogenation of the coordinated maleic anhydride to succinic anhy-... 

During the late 1960s, Homer et al. [13] and Knowles and Sabacky [14] independently found that a chiral monodentate tertiary phosphine, in the presence of a rhodium complex, could provide enantioselective induction for a hydrogenation, although the amount of induction was small [15-20]. The chiral phosphine ligand replaced the triphenylphosphine in a Wilkinson-type catalyst [10, 21, 22]. At about this time, it was also found that [Rh(COD)2]+ or [Rh(NBD)2]+ could be used as catalyst precursors, without the need to perform ligand exchange reactions [23]. 

The ketone hydrosilylation shown in Fig. 7 was used as a test reaction. This can be catalyzed by the fluorous rhodium complexes 16-Rf6 and 16-Rfs under fluorous/organic liquid/liquid biphase conditions [55,56]. These red-orange compounds have very httle or no solubihty in organic solvents at room temperature [57]. However, their solubilities increase markedly with temperature. Several features render this catalyst system a particularly challenging test for recovery via precipitation. First, a variety of rest states are possible (e.g., various Rh(H)(SiR3) or Rh(OR )(SiR3) species), each with unique solubility properties. Second, the first cycle exhibits an induction period, indicating some fundamental alteration of the catalyst precursor. 

The reaction exhibits an induction period. Comparison of different catalyst precursors shows that the behaviour of Pd/C is similar to that of PdCl2 (Table 2). Examination of the medium at the first stages of the reaction indicates in both cases the formation of dimethyl carbonate. This product is known to be formed by the reaction of CO and MeOH in the presence of 02 and PdCl2 (ref. 13). 

Complex (3) may be regarded as an immediate precursor of the species taking part in the catalytic cycle. If (2) is used as catalyst precursor, induction periods are observed during which (3) is formed by hydrogenolysis. The induction period may be prolonged if no base is present to neutralize the HC1 formed. The use of a polar solvent instead of a base also promotes the hydrogenolysis.4... 

When (PPh3)3PdCl2 is used as the catalyst precursor and optically active alcohols are used as the hydrogen donors, a small asymmetric induction occurs (see Table II) which could be due to a solvent effect (25). However, the large influence of the alcohol structure on re-gioselectivity suggests that the alcohol residue is present in (at least one of) the catalytic complexes. 

S)-3-sec-Butylpyridine, which interacts with the cobalt catalytic system (as shown by the increase of the reaction rate), does not cause asymmetric induction when styrene is used as the substrate17). Alkoxyalkylidenetricobaltmonocarbonyl cluster complexes bearing chiral alkoxy groups used as catalyst precursors do not give optically active aldehydes either18). 

In general, in rhodium-catalysed hydroformylation reactions conducted in ionic liquids the catalyst is generated in situ, typically from catalyst precursors such as Rh(OAc)4, or more commonly Rh(acac)(CO)2, together with an excess of an appropriate phosphine ligand. An induction period is often observed with the full activity only being reached after several catalytic runs. This has been attributed to sluggish conversion of the catalyst precursor to the active hydride species,140 421 and to impurities present in the added ligand that are only removed after several product-extraction steps.1401... 

Polyether melt TPPTS 1-Tetradec. 16-95 (0.4-3.9) RI1CI3 as catalyst precursor leaching 0.5 %, recycled 6 times TOF decreases only slowly after induction period. [48]... 

When HRli(CO)(PPh3)3 as a catalyst precursor, there is no induction... 

C2-chiral bidentate fluoroarylphosphinite ligands, efficiently catalyze the asymmetric Diels-Alder reaction between enals and 1,3-dienes [36], Electronic factors apart, the catalyst creates a chiral contour that favors enal coordination, and subsequently this was extended to Ru Lewis acids [37]. These are stable at room temperature, and can be recycled almost quantitatively after the reaction. The immediate catalyst precursor, Ru(Cp)(BIPHOP-F)I is readily available via a one-pot synthesis from Ru3(CO)i2. Although the Ru-catalysts were at first not quite as active as the Fe analogues and produced lower asymmetric induction than the Fe analogues, structural data showed the way to improve the situation (Scheme 10.21). 

Zeise s salt, modified with chiral aminophosphane-phosphinites (AMPP), can also be used as a catalyst precursor in asymmetric styrene hydroformylation113. Other catalytic platinum systems for the hydroformylation of styrene are platinum(O)- alkene complexes of the type [Pt(C2H4)(L2)] L2 = l,2-bis[(diphenylphosphino)methyl]benzene and ( + )-Diop 24. When activated with methanesulfonic acid, catalysts for styrene hydroformylation are formed, which give total yields of aldehydes ranging from 44 to 67% and selectivities towards linear 3-phenyl-propanal ranging from 80 to 88%. Smaller amounts of the corresponding alcohols (3-18%) are also obtained with a pronounced selectivity towards 3-phenylpropanol of 94-96%. However, when Diop complexes of this type are used, no asymmetric induction in hydroformylation can be detected24. 

The cluster [PPN][Ru6G(GO)i6(GH3)], which contains an interstitial carbide within an octahedral metal core, was found to be a catalyst precursor for hydrogenation of olefins at 333 K, but after an induction period. Reaction of the... 

The sterically hindered hydride [(lndenyl)2YH]2 [8] is an effective catalyst to accomplish homo- and co-dimerisation of a wide range of terminal olefins CH2=CHR, R=Ph, n-Bu, /-Pr, t-Bu, SiMcj, CH2Ph. The presence of substituents and functionalities in the monomer are usually tolerated. An induction period of ca. 30 min may be necessary to cleave the dimeric structure of the catalyst precursor the yields are nearly quantitative within two days. Co-dimerisation of a-olefins with styrene proceeds via initial insertion of the a-olefin into the Y-H bond, followed by a 2,1-insertion of styrene into the Y-C bond of the alkyl intermediate. Subsequent P-H abstraction leads to the releasing of the dimer (Scheme 16). 

Precatalytic Reactions and Xpre. The catalyst precursor must transform under reaction conditions into intermediates to obtain an active system. This transformation may involve, in a small number of cases, only a single elementary step, for example, the dissociation of a ligand from a transition-metal complex. However, a series of elementary reaction steps are usually required to convert the catalyst precursor. Useful examples include (1) the degradation of a polynuclear precursor to mononuclear intermediates, (2) the modification of a precursor with a ligand L which is used to control selectivity, and (3) the transformation of finely divided metal. The characteristic time scale for the precatalytic reaction will be denoted tpre, and the instantaneous reaction rate will be denoted Ppre- Precatalytic phenomena and the associated induction periods have been directly monitored in a number of in situ spectroscopic studies using a variety of mononuclear, dinuclear, polynuclear, and metallic precursors (11). 

Addition of compounds containing divalent sulfur, selenium or tellurium also shortens the induction period and increases the rate of reaction. These compounds are highly active if nickel or cobalt halides are used as catalyst precursors. The increase in the rate of reaction may well be explained by the faster regeneration of the active intermediate nickel carbonyl complexes [370]. 

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