In situ complexation

Another successful approach to catalyst immobilisation involves attachment of the carbene precursor to a peptide on solid support. Treatment with base generates the corresponding carbenes that undergo in situ complexation to Pd(ll) centres (Scheme 6.33). Again, the main drawback of this approach was the low reactivity of the catalytic system that only allowed the coupling of aryl iodides and bromides [116], The reasons for this outcome are in need of further studies. 

The research group of Van Leeuwen reported the use of carbosilane de-ndrimers appended with peripherial diphenylphosphino end groups (i.e. 25, Scheme 26) [37]. After in situ complexation with allylpalladium chloride, the resultant metallodendrimer 25 was used as catalyst in the allylic alkylation of sodium diethyl malonate with allyl trifluoroacetate in a continuous flow reactor. Unlike in the batch reaction, in which a very high activity of the dendrimer catalyst and quantitative conversion of the substrate was observed, a rapid decrease in space time yield of the product was noted inside the membrane reactor. The authors concluded that this can most probably be ascribed to catalyst decomposition. The product flow (i.e. outside the membrane reactor)... 

The in situ complexation of the ligand has the advantage of not having to prepare and isolate the free NHC. In cases where the carbene is hardly stable, not yet accessible at all, or difficult to handle, this approach offers the only chance to prepare the desired complex. 

In the in situ complex formation method, the desired complex of Co(III) ions has been allowed to form in presenee of CMS-CH2CH2CO2H. For this purpose Co(OAe)2 4H2O and 4-eyanopyridine are stirred in water having suspended particles of CMS-CH2CH2CO2H. Complexation is indieated by the change in the colour of the solid partieles to purple. The... 

A general NMR spectroscopic method has been developed by Kyba19 using in situ complexes of diphosphines with (—)-bis(/r-chloro)bis[(7 )-dimethyl(a-methylbenzyl)aminato-C2,A]-dipalladium(II). 

Nevertheless, solubilization still could be required for very insoluble molecules. Several approaches have been reported. Use of cyclodextrin as a complexation agent was shown to be effective and yielded both accelerated and pH-independent release.48 Further, in situ complexation between drug molecules and cyclodextrin in the matrix was encouraging because it would not require organic processing to render such complexation prior to matrix preparation. Recently, other approaches, such as incorporating... 

Different solutions can be applied to avoid the use of polar solvents. The most usual is to make the sugar more lipophilic by introducing hydrophobic groups by simple chemical transformations [56]. In addition, sugars can be solubilized in hydrophobic solvents by in situ complexation with phenylboronic acid [57]. 

A different stereochemical behavior has, however, been observed in methanol . In this solvent XeF, reacts with the solvent to form an unstable reactive species (CHsOXeF), which gives quantitatively formaldehyde by disproportionation in the absence of unsaturated hydrocarbons or with unreactive alkenes. Hydrogen fluoride generated in situ complexes the electrophilic CHsOXeF species to form a protonated derivative a which reacts with activated dienes such as 2,3-dimethylbutadiene, as an apparent fluorine electrophile to give 1,4- and 1,2-fluoromethoxy products, together with 1,2- and 1,4-difluoro derivatives (equation 25). 

Leonard GD, Swain SM. Ductal carcinoma in situ, complexities and challenges. J Natl Cancer Inst. 2004 96 906-920. 

In the presence of a polyolic compound used as starter, the Lewis acid generates in situ complex superacids ... 

In a different reaction scheme, one can take advantage of the fimctional porphyrin macrocycle to create metalloporphyrin compounds and nanoarchitectures in 2D. Upon exposure of regular TPyP arrays self-assembled on Ag(lll) to iron monomers supplied by an atomic beam, selective com-plexation occurs whereby the template structure is strictly preserved [156]. This expands the diversity of metalloporphyrin layers conventionally realized by evaporation of integral species, because in-situ metalation provides a route towards novel metalloporphyrin nano architectures and patterned surfaces [156-158]. In a related reaction pathway, evidence could be obtained for in-situ complexation and metal center-induced switching of phenanthroline-based catenane units deposited the Ag(lll) surface [182]. 

The arylation of secondary phosphines 201 with ortho-aiy iodides, catalyzed by generated in situ complex Pda (dba>3 x CHQ3, containing chiral ligand Et,Et-FerroTANE 207 and LiBr, led to the formatiOTi of corresponding tertiary phosphines with enantioselectivity of 90% cc [ 132,137]. The palladium complex 209 also showed high enantioselectivity in arylation of secondary phosphines [131,132]. Some examples of arylation reaction of secondary phosphines with low ee were described. The asymmetric arylation of phosphine boranes with anisyl iodide, catalyzed by chiral complex of oxazoline phosphine 208, led to the formation of enantiomerically enriched tertiary phosphines 206 with 45% ee [134]. The Pd complex 210 of (R )-t-Bu-JOSlPHOS ligand catalyzed arylation of PH(Me)(Ph)(BH3) by o-anisyl iodide with the formation of PAMP-BH3 with 10% ee (Table 3) [112]. 

The advantage of the triple systems consists in the fact that the formed in situ complexes Ni(II)(acac)2 L PhOH are active for a long time, and are not transformed in the course of the process into inactive particles. Thus, the application of triple systems, including Ni(II)(acac)2, electron donor ligand I and PhOH, as homogeneous catalysts is one of the most effective methods of control of selective ethylbenzene oxidation by dioxygen into PEH. 

Liquori et al. [23] first discovered that isotactic and syndiotactic PMMA chains form a crystalline stereocomplex. A number of authors have since studied this phenomenon [24]. Buter et al. [25,26] reported the formation of an in situ complex during stereospecific replica polymerization of methyl methacrylate in the presence of preformed isotactic or syndiotactic PMMA. Hatada et al. [24] reported a detailed study of the complex formation, using highly stereoregular PMMA polymers with narrow molecular weight distribution. The effect of tacticity on the characteristics of Langmuir-Blodgett films of PMMA and the stereocomplex between isotactic and syndiotactic PMMA in such monolayers at the air-water interface have been reported in a series of papers by Brinkhuis and Schouten [27,27a]. Similar to this system, Hatada et al. [28] reported stereocomplex formation in solution and in the bulk between isotactic polymers of / -(+)- and S-(—)-a-methylbenzyl methacrylates. 

In 1953, in the cell-free extract of a o-arabinose-adapted strain of Escherichia coli, enzymatic activity was found which converted o-arabinose (13) into d-eryt/zro-pentulose (o-ribulose, 14) [8]. Moreover, this enzyme was able to convert L-fucose (6-deoxy-L-galactose, 15) into the corresponding ketose 16 189]. In-depth investigations by the same authors confirmed this finding and revealed that the equilibrium could be shifted from originally 11% of L-fuculose (16) to over 80% by in situ complexation of the ketose with excess borate [90]. 

Of particular importance is the in situ complexation of the strong and highly oxophihc dialkyl aluminum hydrides, for example, DIBAL [6, 11]. 

Garcia R, Seco JM, Vazquez SA, Quinoa E, Riguera R. Absolute configuration of secondary alcohols by H NMR in situ complexation of a-methoxyphenylacetic acid esters with barium(n). J. Org. Chem. 2002 67 4579-4589. 

Exchangeable cationic complexes are usually generated by in-situ complexation of transition metal ions. This process is obviously influenced by a variety of steric constraints. Occluded neutral species, either captured during synthesis or sorbed by the finished zeolite, may be subject to ligand substitution or react with structural (anionic or cationic) species. Complexes incorporated in framework sites have not been described so far their formation will obviously necessitate in most cases the breakage of T-O-M bonds, T and M being metal lattice sites. 

The authors indicate that the elution orders for samples of metal ions can be accurately predicted using complexometric calculations. The algorithm used requires inputs of mobile phase ligand concentrations, mobile phase pH, and the formation constants for reactions between metal ions and the mobile phase ligands. In addition to their own experimental results, the authors have successfully calculated elution orders for many of the literature reported in situ complexation-based separations which have been achieved on either cation-exchange or d3mamically modified reverse-phase columns [31]. 

The first METSCAN instrument developed for automated monitoring of metals in industrial effluents was based on in situ complex formation of rapidly formed metal dithiocarbamate complexes. Complex formation was achieved by including dtc in the mobile phase and a schematic diagram of the on-line instrument is presented in Fig. 7.9. Automatic injection and extensive use of microprocessors for experiment control distinguish the on-line system relative to the off-line method described... 

Fig. 7.9. Schematic diagram of the initial version of the automated METSCAN system used for use with in situ complex formation. Reproduced by courtesy Anal. Chem. 55 (1983) 718.

On line determination of copper and nickel with in situ complex formation... 

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