Alkyl model system

Furthermore, gallium compounds can serve as model systems for aluminum congeners. Cationic gallium alkyls are of interest in synthesis and catalytic applications involving polar substituents because of the relative stability of the Ga—R bond toward hydrolysis and electrophilic cleavage compared to the otherwise superior Al-R species [11]. 

Experiments have been carried out to mimic the reactions of model systems for coenzyme F430 that is involved in the terminal step in the biosynthesis of methane, and that is able to dechlorinate CCI4 successively to CHCI3 and CH2CI2 (Krone et al. 1989). Nickel(I) isobacteriochlorin anion was generated electrolytically and used to examine the reactions with alkyl halides in dimethylformamide (Helvenston and Castro 1992). The three classes of reaction were the same as those observed with Fe(II) deuteroporphyrin IX that have already been noted. 

The reversibility of QM adducts also creates numerous challenges. For example, measuring the full burden of DNA alkylation by a QM can be obscured by the loss of its labile products during or before chemical identification can be completed. Results from a deoxynucleotide model system indicated that only a small fraction of the possible adducts could be measured after the interval required for analysis of DNA. Perhaps the kinetic products of QMs also contribute to the cellular activity of these intermediates although this has yet to be explored. QM equivalents can be envisioned to migrate from one reversible nucleophile such as the N1 of adenine in such cofactors as ATP to another until quenched by a compound such as glutathione that is present in cells as a defense against undesirable electrophiles. 

In addition to providing many new precursors to functionalized poly(alkyl/arylphosphazenes), the deprotonation/substitution reactions of these N-silylphosphoranimines serve as useful models for similar chemistry that can be carried out on the preformed polymers. New reactions and experimentation with reaction conditions can first be tried with monomers before being applied to the more difficult to prepare polymeric substrates. A considerable amount of preliminary work [e.g., with the silylated monomers (z z) and polymers (2 o) has demonstrated the feasibility of this model system approach. 

The above considerations stimulated investigations of the polymerization of model systems, namely ethylene oxide in the presence of dialkyldichlorostannanes30). R2SnCl2 has been found to be a very active catalyst for the polymerization of ethylene oxide, the polymerization rate increasing considerably with the length of the alkyl substituent at the tin atom. 

The feasibility of identifying these edges of water base pairs has been supported by our studies of mitomycin C interacting with the model system for AT base pairs 29). Interactions of either component with mitomycin C are not observed but a complex is formed when all three components are present. Chemical shift changes observed in the NMR spectra support the structure 47 for the termolecular complex. The broader implication is that mitomycin C will likewise recognize the minor groove side of a G-C pair (it is known to alkylate the guanidine on this side)31 ... 

Many model systems which mimic both the redox behaviour [for example, ready reduction to Co(i) species] and the alkyl binding ability of vitamin Bu derivatives have been investigated. The most studied of these has involved bis(dimethylgloximato)cobalt systems of type (307), known as the cobaloximes (Bresciani-Pahor et al., 1985). Other closely related... 

Fig. 8 Cationic alkyl generic model system 22 and real system derived from 5a...

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

Asymmetric reduction of ketones or aldehydes to chiral alcohols has received considerable attention. Methods to accomplish this include catalytic asymmetric hydrogenation, hydrosilylation, enzymatic reduction, reductions with biomimetic model systems, and chirally modified metal hydride and alkyl metal reagents. This chapter will be concerned with chiral aluminum-containing reducing re-... 

There are several potential sources of error. Both methods of analysis use a binary model mixture, composed of sulfidic and thiophenic components. Thickness effects in the XANES of these model systems would alter the calibrations. There may be contributions from species not adequately represented by a simple dibenzothiophene-dibenzylsulfide model. While the XPS data are represented by 163.3 eV and 164.1 eV components, the model compound data base is as yet limited and not sufficient for a definitive interpretation in terms of alkyl sulfide and thiophenic forms. Examination by both XPS and XANES of a wider variety of model compounds and multiple component model compound mixtures will better define the sulfur species represented by these quantification methods. 

Once this process is explored with the model system to assess the level of enantioselectivity, we will then prepare alkyl zinc reagent 48 from 44 using standard methods - - and cross couple 48 to aryl bromide 18 using the appropriate chiral catalysts (Scheme 7). Although the acetonide stereocenter in 48 is somewhat remote from the coupling site, the stereocenter may serve to enhance the stereoselectivity of the cross-coupling process because the two possible products are diastereomers, not simply enantiomers. This reaction will produce 49 from (S)-48 and 30 from (R)-48 that can then be converted to epoxides 31 and 32 using standard methods. Epoxide 31 leads to heliannuol D 4 after base-promoted epoxide cyclization and deprotonation. Similarly, epoxide 32 leads to heliannuol A 1 after acid-promoted cyclization. 

Although it is clear in model systems that artemisinin can efficiently alkylate heme-based models, the role of this event in the mechanism of action of artemisinin has... 

Discussions here have focused on the more well-characterized salts based on the [CiCilm]+ cation, but this is a somewhat model system, and may accentuate the interactions observed in many of these examples. Where longer alkyl chains are present on the cation liquid and solid-state packing are disrupted and can lead to the formation of ionic/nonionic microdomains in the liquid and which may have a considerable influence on the dissolution of solutes [49]. 

Interposing an additional carbon atom between the indazole nucleus and the acetic acid side chain provides another compound that shows anti-inflammatory activity in model systems. Reduction of the carboxylic acid in the indazole (38-1) by means of hthium aluminum hydride leads to the carbinol (38-2). Alkylation of the enolate from the alcohol with methyl 2-bromo-2-methylpropionate leads to the corresponding ether. Saponification gives the free acid and thus bindarit (38-3) [40]. 

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