Methylimidazol units

In the complex with two methylimidazol units one of them is N-coordinated and the other a C-coordinating carbene ligand. It seems that in the synthesis of this complex a homoleptic rearrangement of the QFs precursor complex is accompanied by a rapid migration of the proton nitrogen to the initially coordinated carbon. 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with certain nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tin atoms.62 Such complexes are less stable for more-substituted stannylene acetals, such as those derived from carbohydrates.62 Unfortunately, the precise structures of these complexes have not yet been defined. Addition of nucleophiles to solutions of stannylene acetals in nonpolar solvents has been found to markedly increase the rates of reaction with electrophiles,63 and transient complexes of this type are likely intermediates. Similar rate enhancements were observed in reactions of tributylstannyl ethers.57 Tetrabu-tylammonium iodide was the nucleophile used first,57 but a wide variety of nucleophiles has been used subsequently tetraalkylammonium halides, jV-methylimidazole,18 and cesium fluoride64,65 have been used the most. Such nucleophilic solvents as N,N-dimethylformamide and ethers probably also act as added nucleophiles. As well as increasing the rates of reaction, in certain cases the added nucleophiles reverse the regioselectivity from that observed in nonpolar solvents.18,19... 

Letsinger and Klaus investigated the solvolysis of poly(acryIic acid-co-2,4-dinitro-plienyl p-vinylbenzoate) 9 catalyzed by partly protonated poly(N-vinyliniidazole) 4 (57). The poly(N-vinylinudazole) wed a sufficient activity at the imidazcde unit concentration mudi lower than that of N-methylimidazole II (Table 4—1), and the rate attained a limiting value at high catalyst concentrations. The rate saturation was... 

Kunitake and Okahata examined the hydrolysis of PNPA in the presence of vinyl polymers containing N-phenylhydroxamate (PHA) and methylimidazole(MIm) units (107). The reacticm of PHA-MIm-AAm tetpolymer 23 with large excesses of PNPA gives typical burst kinetics initial rapid liberation of p-nitrophenol (k ) followed by slower, steady release (ktt). 

Six-coordination is also found in [(dmp)YbCl2(N-Meim)2py].toluene (dmp = 2,6-dimesityl-phenyl N-Meim = N-methylimidazole) whilst distorted trigonal bipyramidal five-coordination exists in [Ln(Dnp)Cl2(THF)2] (Dnp = 2,6-di(l-naphthyl)phenyl) Ln = Y, Yb, Tm). Donor-functionalized terphenyl derivatives have also been made. [[(Danip)Yb(yU2-Cl)2(/x3-Cl)Li(thf)]2] and [[(Danip)Ln(/i2-Cl)2(/i2-Cl)Li(thf)]2] (Ln = Y, Sm) have structures based on LiCl-bridged (DanipLnCl2) units stabilized through additional coordination of two methoxy groups to lanthanum. (Danip = 2,6-di(o-anisol)phenyl.)... 

Fig. 16. Plot of nuoleophilio reactivity to various substrates vs. nucleophilic reactivity towards p-nitrophenyl acetate. Rate units are in M-l min-i. Symbols As , arsenate GEE, glycine ethyl ester gly, glycine glygly, glyoylglycine Iro, imidazole Melm, N-methylimidazole M or morph, morpholine Pi , phosphate, Piv, pivalate Pic, 4-picoline Py, pyridine pip, piperidine. Data taken from Bender and Glasson, (1959) Bruice and Willis (1965) Bruice and Benkovic (1964) Bruice and Mayahi (1960) Jencks and Carriuolo (1969) Jencks and Carriuolo (1960b) Di Sabato and Jeneks (1961a) Wolfenden and Jencks (1961) Johnson et al. (1966) Kirsch and Jencks (1964a) Butler and Gold (1961a), Reforonces for p-nitrophenyl acetate are in Table 8.

The standardization of ammonia caramek presents a special task. In the course of the manufacture of caramek in the presence of ammonia and ammonium compounds, 4(S)-methylimidazole is formed, and it is neurotoxic. Its content " in caramel is limited by the Food Laws of several countries (for instance, in Austria and the United Kingdom ) and by the WHO r jdations (see also articles by Thier and Wood ). (The specification and analysis of ammonia caramek manufactured in the United... 

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