Preformed complex

The activity and enantioselectivity of the complex 6a-Cu were shghtly lower than those observed in dichloromethane, but the catalysts were stable after two reuses. After each reaction, the products were extracted with hexane and the catalyst remained in the ionic hquid phase, ready for reuse. In the case of the complex 6b-Cu, the enantioselectivities obtained in the former reactions (entry 3 in Table 5) were significantly lower than those observed in dichloromethane ( 90% ee) under the same conditions. When the complex was prepared in situ in the ionic hquid, as opposed to dissolving the preformed complex, the enantioselectivity results improved considerably (entry 5) but decreased again after catalyst recovery. 

Preformed complexes of type 52, 53 and 59, and in situ catalyst systems based on IMesX (X = CO, MeS03H, HCl, HBr, HI) and IPrHCl, have been tested for telomerisation of butediene with primary and secondary amines. Under optimised conditions, and low catalyst loadings, excellent activities and selectivities were observed [82]. 

In situ derived systems, in general, performed similarly to preformed complexes, in telomerisation of butadiene with MeOH, Tables 4.1 and 4.2 [68,70,71,77,78], In situ systans may be generated from free NHC or from imidazolium salt in combination with an appropriate Pd(0) or Pd(ll) source. Typically, 2-4 equivalents of imidazolium salt relative to Pd have been nsed [68,70,77], In situ catalysts derived from mono- and bis-Fc-snbstituted (Fc = ferrocenyl) imidazohnm and benzimidazolium salts (64-68) (Table 4.2) showed interesting telomerisation activities ascribed to the steric bulk of the Fc substituents [70]. Unsymmetrical salts 65 and 66 bearing A -Fc and A -Me... 

The standard method used to prepare these AHC s was by anchoring a preformed complex onto an alumina support which had been treated with a heteropoly acid such as phosphotungstic acid (PTA). Alternately, the AHC can be prepared by treating an anchored Rh(COD)2 precursor with an appropriate ligand (8). We report here the use of AHC s which have been prepared by this... 

Poor stirring during formylation of 2,5-dimethylpyrrole with the preformed complex of dimethylformamide with phosphoryl chloride caused eruption of the flask contents. Reaction of the complex with a local excess of the pyrrole may have been involved. 

Recently, very effective asymmetric conjugate addition of 1,3-dicarbonyl compounds to nitroalkenes has been reported, as shown in Scheme 4.10. The reaction of ethyl acetoacetate with nitrostyrene is carried out in the presence of 5 mol% of the preformed complex of magnesium triflate and chiral bis(oxazoline) ligands and a small amount ofW-methylmorpholine (NMM) to give the adduct with selectivity of 91%. The selectivity depends on ligands. The effect of ligands is presented in Scheme 4.10.63... 

The wide use of p-block and early transition metal chalcogenide materials for electronics applications (semiconductors, semi-metals, battery materials, etc.) has resulted in a large amount of work concerned with CVD using mixtures of metal halides and chalcogenoethers as dual source precursors and preformed complexes as single sources.166... 

Many of these problems have been removed through studies using exchange-inert complexes, particularly those incorporating Co(III) centers. In the 20-year period following 1967 studies with preformed complexes of the type cis-[Co(N)4(OH/H)(AAOR)]2+/3+ and [Co((N)4 (AAOR)]3+ (N)4 = (en)2, AA = Gly, 0-Ala, R = Me, Et, i-Pr) have allowed precise information to be acquired (43) and Table III lists rate data for the latter N carbonyl-O-chelated esters. What has been established for these systems may be summarized as follows. 

The stem-loop structure in the noncoding 3 region of selenoprotein mRNAs has also been termed a SECTS element in mammals although it has a different overall structure. ° In silica analysis of the human genome sequence, using this consensus SECTS element along with the presence of the characteristic UGA codon within an exon, has led to the discovery of several new selenoproteins, including a selenium-dependent methionine sulfoxide reductase. It has been shown that a specific complex exists for selenoprotein synthesis that shuttles between the nucleus and the cytosol. This possibly protects the preformed complex for nonsense-mediated decay to allow for more efficient selenoprotein synthesis. The specific tRNA needed for selenocysteine... 

Krenacs, T Uda, H., and Tanaka, S. (1991) One-step double immunolabeling of mouse interdigitating reticular cells simultaneous applications of preformed complexes of monoclonal rat antibody Ml 8 with horseradish peroxidase-linked antirat immunoglobulin and of monoclonal mouse antila antibody with alkaline phosphatase coupled antimouse immunoglobulins. J. Histochem. Cytochem. 39, 1719-1723. 

The thermal oxidation mechanism results [29,30] differ from the electrochemical results because different mechanisms apply. PVI-1 was found to be an excellent anti-oxidant in dry conditions, while it has no effect on the the anodic processes than thermal stability of the Cu(lnhlbitor) complex is probably important for the thermal oxidation whereas the preformed complex has little importance in the electrochemical processes. 

An alternative application of flash photolysis to study myoglobin electron transfer kinetics has been employed by Hofifinan and co-workers 156). In this approach, the photoactive zinc-substituted derivative of Mb is mixed with an equivalent amoimt of ferricytochrome bs to form an electrostatically stabilized binary complex. Upon transient irradiation, the strongly reducing Zn-Mb intermediate is formed, and the kinetics of ferricytochrome reduction within the preformed complex can be monitored spectrophotometrically. The resulting kinetics represents a mixed-order process consistent with electron transfer both within the electrostatically stabilized complex and between the dissociated components of the complex. 

A difference in reaction efficiency was observed depending on the catalyst used. Imidazolium salt 305 provides the highest yield of desired product. When preformed complex 307 is subjected to the reaction conditions, fran -2-ethylcyclohex-anol is detected by gas chromatography in 76% yield (Eq. 30). Alkylation starting with free carbene 306 results in only 28% yield of desired alkylated epoxide. 

Alternatively, the rhodium complexes of 4 exhibit enhanced performance relative to the ferrocene diphosphine analog 3 which cannot be explained simply as a switch from one binding mode to another. P NMR experiments on in situ formed rhodium complexes are useful for correlating rhodium binding of different ligands in solution when a single species dominates, but are inconclusive when multiple phosphorus resonances are observed. Preparation and isolation of preformed complexes may provide better systems for study by NMR spectroscopy. 

Polyelectrolyte complexes have also been studied as tablet matrices for controlled release applications. For example, the interpolymer complexes of chitosan with pectin and acacia were investigated as tablet matrices for release of chlorpromazine HCL [353]. The complex formed in situ by mixing chitosan with either pectin or acacia displayed the most efficient sustained release (compared to either pectin, acacia or a preformed complex). The results were attributed to the swelling and gel-forming capacity of the freshly formed complex in contrast to the preformed version. 

B) Displacement of a ligand from a preformed complex by dinitrogen, free or coordinated, e.g. reactions (5) and (6).6,8... 

Alkyl halides or nitriles Alcohols are converted conveniently into alkyl halides or nitriles by consecutive addition of a lithium halide or LiCN and the alcohol to a preformed complex of P(C6H5)3 and DEAD in anhydrous THF at 0° to 25°. Yields range from 60% to 97%. Reaction of secondary alcohols is accompanied by complete inversion. 

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