Catalytic precursor

Homogeneous catalysis by transition metal clusters has been reviewed from the perspective of the specific transformations.Examples of very mixed-metal clusters catalyzing processes homogeneously are collected in Table IX. As is generally the case with homogeneous catalysis, the catalytic precursor is well defined, but the nature of the active catalyst is unclear. 

Chiral lactones were also obtained by cyclocarbonylation of chiral acetylenic alcohols with Pd and thiourea (H2NCSNH2) (Scheme 32). No loss in chirality was observed, but large amounts of Pd and thiourea were used (10 mol %) since the catalyst deactivates by forming metal particles. The catalytic precursor (Pdl2 > PdCl2) and the ratio of thiourea to Pd were very important, thiourea being necessary for this reaction. The active species was supposed to be [Pd(thiourea)3l]I, which forms in situ from [Pd(thiourea)4]l2 and [Pd(thiourea)2]l2. It had to be a partially dissociated species since [Pd(thiourea)4](Bp4)2 was inactive [121]. 

In the present chapter, we focus on the catalyst nature in solution using well-defined metal NPs as catal 4 ic precursors it means, soluble (or dispersible) heterogeneous pre-catalysts, as stated by Finke [6]. Some experiments described in the literature concerning the distinction between homogeneous and heterogeneous catalysts are discussed (see Section 3), followed by a particular case studied by us with regard to the catalyst nature in the allylic alkylation reaction, using preformed palladium NPs as catalytic precursors (see Section 4). 

TEM is a direct analysis tool for knowing if metal clusters are present in catalytic solutions. This technique permits to define the size, structure, morphology and state of aggregation of the metal particles [11]. Hence, this technique can be in particular useful to identify nanoclusters in metal-catalysed reactions, using molecular complexes as catalytic precursors. But the catalytic activity of these observed nanoclusters must be further checked. 

For example, when cyclohexene reduction using (NBu4)5 Na3[Ir(l,5-cod) P2Wi5Nb3062] as catalytic precursor was monitored, an induction period was detected corresponding to the formation of active species. Ir nanoclusters formed in situ were isolated from the reaction solution, characterised and reused as catalyst for the same organic process in the latter case, no induction period was observed [3]. 

For the Rh-catalysed amino-borane dehydrocoupling using [Rh(l,5-cod)(p-Cl)]2 as catal5dic precursor, the catalytic activity was completely suppressed by the addition of an excess of mercury. But for phosphino-borane, Hg(0) addition had no effect. Consequently, the authors proved that the catalyst nature depends on the substrate using the same catalytic precursor [15]. 

In particular when NPs are involved, either formed in situ from molecular complexes or preformed to be used as catalytic precursors, the catalyst identity remains difficult to prove. 

These metal vapor-derived nanostructured systems are valuable catalytic precursors for a wide range of reactions of great interest in fine chemistry. 

Solvated Metal Atoms as Homogeneous Catalytic Precursors... 

Mesitylene-solvated Rh atoms are valuable catalytic precursors for the selective silylformylation of a wide... 

The Cu vapor-derived nanopowders are efficient catalytic precursors for the oxidation of a wide range of organic substrates with molecular oxygen or air and are... 

In this chapter the potential of nanostructured metal systems in catalysis and the production of fine chemicals has been underlined. The crucial role of particle size in determining the activity and selectivity of the catalytic systems has been pointed out several examples of important reactions have been presented and the reaction conditions also described. Metal Vapor Synthesis has proved to be a powerful tool for the generation of catalytically active microclusters SMA and nanoparticles. SMA are unique homogeneous catalytic precursors and they can be very convenient starting materials for the gentle deposition of catalytically active metal nanoparticles of controlled size. 

Actually, the reactions led to the formation of the corresponding acyclic Mannich-type addition products, which were further transformed into their corresponding Diels-Alder adducts by treatment with TFA (Scheme 5.18). The 5, P-bidentate character of the ligands was proven by isolation of one complex and its X-ray analysis. Complexes prepared with CuCl afforded the expected product in up to 80% ee, whereas the use of CuBr as the catalytic precursor allowed an enantioselectivity of 97% ee to be obtained with the similar ligand. 

Mixed phosphines,113 phosphates,114,115 phosphinites (diphenylphosphine oxide) and related diphenylphosphine acids,116-119 phospholes,120 and other phosphorus ligands121-123 have been used in Pt-catalyzed hydroformylation. [PtCl2(COD)] has been used as starting material for the preparation of catalytic precursors.114,124-126... 

The catalytic cycle for hydroboration is now widely accepted and direct examples of several intermediate species have been isolated and well characterized (Scheme 3).5-7 These now include (j-borane complexes, which have in some instances been found to be catalytic precursors for hydroboration.8-10 Oxidative addition of an H—B bond to a coordinatively unsaturated metal fragment... 

The supported aqueous phase methodology was applied to the system Pd(OAc)2/5 TPPTS, a catalytic precursor for the Trost-Tsuji reaction. The characterization of the solid by 31P MAS NMR confirms the presence of Pd°(TPPTS)3 as the main surface species. The catalytic properties of the solid were tested for the allylic substitution of E-cinnamylethylcarbonate by different nucleophiles such as ethyl acetoacetate, dimethyl malonate, morpholine, phenol, and 2-mercapto-pyridine. The absence of palladium leaching was demonstrated, and having solved the problem of water leaching from the solid to the organic phase, the SAP-Pd catalyst was successfully recycled several times without loss in its activity. It was used in a continuous flow experiment which... 

Following these early investigations, the major breakthrough in this field has been the change in both the structure of the ligand frame and that of the anion in the starting rhodium catalytic precursor. The [Rh(TFA) (S)-(Cp,Cp)oxoPro-... 

In order to obtain lactones from natural alkenols, we investigated the cyclocarbonylation of monoterpenic alcohols. The catalytic precursor is [PdCl2L2] in the presence of a slight excess of tin chloride and phosphine ligands. Dihydromyrcenol, a representative acyclic terpene containing a termi-... 

In summary, it must be concluded that amides as a category do not seem to have a substantial advantage over LiAlH system doped with catalytic precursors. There is no evidence that the amides could reversibly desorb/absorb at temperatures lower than 200°C. At the temperatures higher than 200°C amides and related materials barely desorb 5 Their kinetics of isothermal desorption at the atmospheric pressure... 

Yamaguchi et al. also reported the use of another new Cp Ir NHC complex 9 bearing basic 2-(dimethylamino)ethyl group in a Cp ring as the catalytic precursor in Oppenauer-type oxidation (Scheme 5.7) [34]. Owing to the basic amino... 

Results of catalytic reactions in which allenylidene complexes work as catalytic precursors [51] are not included in this chapter. 

Trust s group has shown that another selective reaction involving C—O bond formation followed by rearrangement and C—C bond formation occurred when Cp-containing ruthenium complexes were used as catalytic precursors. With RuCl(Cp)(PPh3)2 in the presence of NH4PF6, an additive known to facilitate chloride abstraction from the metal center, the addition of allylic alcohols to terminal alkynes afforded unsaturated ketones [46, 47]. It has been shown that the key steps are the... 

Another type of neutral gold( I) complex is [ AuX(P R3)[ where X is an anionic oxygen or nitrogen donor ligand such as [Au N(S02CF3)2 (PR3)] or [Au(OR)(PR3)] or even [Au(OS02CF3)(PR3)[, which are relevant as catalytically-active species or catalytic precursors [26, 108]. 

The assays are reported in Table I. The selectivity of the reaction is lower when rhodium rather than cobalt is used, and it is lower still when rhodium is used in the presence of triphenylphosphine. Thus, the catalytic precursor changes the selectivity to the possible isomeric aldehydes markedly. In each case it is possible to obtain one of the three aldehydes in good yields and in a high state of purity. 

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