Eliminative processes

Double dehydrohalogenation of vicinal dihalides (Section 9 7) Dihalides in which the halogens are on adjacent carbons undergo two elimination processes analogous to those of gemmal dihalides... 

Hydrocarbyl Complexes. Stable homoleptic and heteroleptic uranium hydrocarbyl complexes have been synthesized. Unlike the thorium analogues, uranium alkyl complexes are generally thermally unstable due to P-hydride elimination or reductive elimination processes. A rare example of a homoleptic uranium complex is U(CH(Si(CH2)3)2)3, the first stable U(I11) homoleptic complex to have been isolated. A stmctural study indicated a triganol... 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

Addition and elimination processes are the reverse of one another in a formal sense. There is also a close mechanistic relationship between the two reactions, and in many systems reaction can occur in either direction. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

The discussion of /3-elimination processes thus far has focused on reactions that involve removal of a proton bound to carbon. It is the electrons in the C—H a bond, however, that are essential to the elimination process. Compounds bearing other substituents that can release electrons can undergo similar /3 eliminations. Many such processes are known, and frequently the reactions are stereospecific. 

There are a number of synthetically important /3-elimination processes involving... 

The rate of elimination of a chemical compound from the body is usually proportional to the amount of the chemical in the body. Elimination processes include biotransformation, exhalation, and excretion in the urine, bile, saliva, and sweat, and even in the hair and nails. The first-order... 

Systematic error, as stated above, can be eliminated— not totally, but usually to a sufficient degree. This elimination process is called calibration. Calibration is simply a procedure where the result of measurement recorded by an instrument is compared with the measurement result of a standard. A standard is a measuring device intended to define, to represent physically, to conserve, or to reproduce the unit of measurement in order to transmit it to other measuring instruments by comparison. There are several categories of standards, but, simplifying a little, a standard is an instrument with a very high accuracy and can for that reason be... 

Dichlorotetrafluoro-2,5-dihydrothiophene is converted to the perfluoro-p-dithiin, probably by an addition-elimination process [i] (equation 3). 

An interesting class ot covalent Inflates are vin l and ar>/ or heteroaryl Inflates Vinyl inflates are used for the direct solvolytic generation of vinyl cations and for the generation of unsaturated carbenes via the a-elimination process [66] A triflate ester of 2-hydroxypyridine can be used as a catalyst for the acylation of aromatic compounds with carboxylic acids [109] (equation 55)... 

Synthesis of heterocycles using the intramolecular Heck reaction involving a formal nnd-elimination process 99H(51)1957. 

An instructive example on how stereochemical features influence the stereochemical outcome of the elimination is the pyrolysis of xanthates from erythro-and t/zrco-l,2-diphenyl-l-propanol. The erythro-dlcohoX 8 is converted into fi-methylstilbene 9 only, and the threo-dlcohoX 10 is converted into the corresponding Z-isomer 11 only. These results support the assumption of a syn-elimination process through a cyclic transition state ... 

In a somewhat similar vein, alkylation of the urea derivative 109 with methyl iodide affords the S-methyl ether 110. Condensation of that with taurine (111), leads to the guanidine 112, again by an addition elimination process. The product is the anthelmintic agent netobimin (112) [271. 

Drug elimination may not be first order at high doses due to saturation of the capacity of the elimination processes. When this occurs, a reduction in the slope of the elimination curve is observed since elimination is governed by the relationship Vmax/(Km- -[conc]), where Vmax is the maximal rate of elimination, Km is the concentration at which the process runs at half maximal speed, and [cone] is the concentration of the drug. However, once the concentration falls below saturating levels first-order kinetics prevail. Once the saturating levels of drugs fall to ones eliminated via first-order kinetics, the half time can be measured from the linear portion of the In pt versus time relationship. Most elimination processes can be estimated by a one compartment model. This compartment can... 

Thiepins stabilized by two annulated benzene rings can generally be prepared by conventional elimination processes. 

Isoquinoline 2-oxide was converted by bromine in acetic anhydride in the presence of sodium acetate into the 4-bromo 2-oxide, presumably via an addition-elimination process (84MI2). Metallic derivatives have been used occasionally to prepare bromoisoquinolines, as in the formaton of 79, a process accompanied by ring-opening [87JCS(PI)1865]. 

There are two general methods within this subcategory, involving one- or two-step mechanisms. Nitrenes and metalonitrenes thus add to alkenes by a direct azir-idination reaction, whereas nonmetallic nitrenoids usually react through an addition-elimination process (Scheme 4.6). 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

The lithium enolate of the oc-silyl-substituted iron-acyl complex 19 reacts with aldehydes, however, products of the Peterson elimination process (E)- and (Z)-22 are usually isolat-ed22- 23,36.37 for t[1js anc other preparations of a,/t-unsaturated iron-acyl complexes see Section I.3.4.2.5.I.3.). 

Zero-order kinetics describe the time course of disappearance of drugs from the plasma, which do not follow an exponential pattern, but are initially linear (i.e. the drug is removed at a constant rate that is independent of its concentration in the plasma). This rare time course of elimination is most often caused by saturation of the elimination processes (e.g. a metabolizing enzyme), which occurs even at low drug concentrations. Ethanol or phenytoin are examples of drugs, which are eliminated in a time-dependent manner which follows a zero-order kinetic. 

In the case of electron-deficient monomers (e.g, acrylics) it is accepted that reaction occurs by initial addition of the sulfate radical anion to the monomer. Reactions of sulfate radical anion with acrylic acid derivatives have been shown to give rise to the sulfate adduct under neutral or basic conditions but under acidic conditions give the radical cation probahly by an addition-elimination process. 

Starnes et al.hl have also suggested that the head adduct may undergo p-scission to eliminate a chlorine atom which in turn adds VC to initiate a new polymer chain. Kinetic data suggest that the chlorine atom does not have discrete existence. This addition-elimination process is proposed to he the principal mechanism for transfer to monomer during VC polymerization and it accounts for the reaction being much more important than in other polymerizations. The reaction gives rise to terminal chloroallyl and 1,2-dichlorocthyl groups as shown in Scheme 4.8. 

An extension of this general elimination process can be seen in the use of 2-trimethylsilylethanol and its derivatives as protecting/masking groups for molecules with labile hydrogens ... 

An alternative to the above mechanism is that acetoxylation is an addition-elimination process involving N02 and OAc , leading to nitro and acetoxy products77, and it follows that this process would be less likely to occur, for example, with mesitylene and significantly perhaps, experiments seeking acetoxylation in mesitylene have failed78 on the other hand, mesitylene is a very reactive substrate so it could be that an alternative nitrating species is involved here. 

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