Eliminations undesired side-reaction

Nitrile A-oxides, under reaction conditions used for the synthesis of isoxazoles, display four types of reactivity 1,3-cycloaddition 1,3-addition nucleophilic addition and dimerization. The first can give isoxazolines and isoxazoles directly. The second involves the nucleophilic addition of substrates to nitrile A-oxides and can give isoxazolines and isoxazoles indirectly. The third is the nucleophilic addition of undesirable nucleophiles to nitrile A-oxides and can be minimized or even eliminated by the proper selection of substrates and reaction conditions. The fourth is an undesirable side reaction which can often be avoided by generating the nitrile A-oxide in situ and by keeping its concentration low and by using a reactive acceptor (70E1169). 

Except for terpene chemistry, the Wagner-Meerwein rearrangement is of limited synthetic importance. It is rather found as an undesired side-reaction with other reactions, for example in the synthesis of alkenes by elimination reactions. 

The di- and monoalkyltin compounds are considered to be effective as stabilizers because they (i) inhibit the onset of the dehydrochlorination reaction by exchanging their anionic groups, X, with the reactive, allylic chlorine atoms in the polymer (ii) react with, and thereby scavenge, the hydrogen chloride that is produced and that would otherwise induce further elimination (jii) produce the compound HX, which may also help to inhibit other undesirable side reactions and iiv) prevent breakdown of the polymer initiated by atmospheric oxidation, i.e., by acting as antioxidants. 

The most comprehensive examination of the Rokita kinetic procedure from a synthetic standpoint was carried out by Barrero and coworkers.6 They examined the effects of various leaving groups, solvents, nucleophiles, and their equivalents on subsequent [4 + 2] cycloadditions. Avast excess of the intended nucleophile (50-100 equiv) must be employed, because the fluoride triggered (3-elimination proves nearly instantaneous at room temperature resulting in a high concentration of a species that is prone to undergo dimerization and other undesired side reactions that are irreversible at these low temperatures (Fig. 4.7). Use of fewer equivalents of the intended nucleophile led to a rapid drop off in yield. For example, 5-10 equivalents of ethoxy vinyl ether (EVE) affords only a 5-10% yield of the desired benzopyran... 

The complete elimination of functional groups is often an undesirable side reaction in organic synthesis, but on the other hand it is a possibility for the recycling of environmentally harmful compounds, for example phenols and haloarenes such as polychlorinated dibenzodioxins (PCDDs or dioxins ). For example, aryl chlorides can be effectively dechlorinated with Pd(0) NPs in tetra-butylammonium salts with almost quantitative conversions also after 19 runs (entry H, Table 1.4) [96]. On the other hand, a C-0 bond cleavage reaction also seems suitable for the fragmentation of sugar-based biomass such as cellulose or cello-biose in that way, sugar monomers and bioalcohol can be derived from renewable resources (entry F, Table 1.4) [164]. 

Excess benzene is always used in the reactors, also for two reasons. First, the benzene acts like a heat sponge, mitigating the rate at which the temperature increases due to the exothermic reaction. Second, excess benzene helps eliminate some of the undesirable side reactions that can take place, mainly the formation of di- or tri-isopropyl benzene (benzene hooking up with two or three propylenes) or other miscellaneous compounds. 

In the sulfoxidation, small to appreciable amounts of over oxidation with formation of undesired sulfone were observed, a result that implies that kinetic resolution may be involved in influencing the overall stereochemical result 105). This was shown to be the case. Indeed, some of the mutants are also excellent catalysts in the kinetic resolution of racemic sulfoxides such as 25 105). Directed evolution was then applied successfully to eliminate undesired sulfone formation, specifically, by going through a second cycle of epPCR 105). This is significant because it shows for the first time that an undesired side reaction can be eliminated by directed evolution. 

In comparison to carbanions, which maintain a full octet of valence electrons, carbenium ions are deficient by two electrons and are much less stable. Therefore, the controlled cationic polymerization requires specialized systems. The instability or high reactivity of the carbenium ions facilitates undesirable side reactions such as bimolecular chain transfer to monomer, /1-proton elimination, and carbenium ion rearrangement. All of that limits the control over the cationic polymerization. 

The small subunits of rubisco may help suppress undesirable side reactions.285 For example, the following deoxypentodiulose phosphate can be formed by (3 elimination from the second intermediate of Eq. 13-48. 

For the cobalt(III) complexes this method offers the additional advantage of rapid, controlled oxidation of the cobalt(II) intermediate complex to the cobalt(III) complex through the direct use of the elemental halogen corresponding to the anion of the desired salt, which eliminates the long air oxidation and the possible undesirable side reactions. 

Carbon monoxide may be eliminated from cyclopropanones either by thermal or photochemical processes (Table 12). In fact, decarbonylation is sometimes an undesirable side reaction in the synthesis of cyclopropanones. For example, the reaction of dimethylketene and ethyl diazoacetate affords carbon monoxide and ethyl acrylate 115 rather than the desired ketone. 2°)... 

The exceptional nucleofugality of the phenyliodonio group has been determined in an alkenyl salt and it is about 106 times greater than that of triflate [30]. This remarkable property makes alkenyl iodonium salts excellent vinyl cation equivalents in nucleophilic substitutions. The chemistry of alkenyl iodonium salts is dominated by the transfer of their aliphatic moiety to a variety of nucleophiles other important reactions involve Michael-type addition and alkylidenecarbene generation, along with elimination to alkynes which is actually an undesirable side-reaction. 

The reactions used in the preparation of intermediates are, for the most part, simple operations. Frequently, they proceed quantitatively according to the rules of stoichiometry. In other cases, side reactions are encountered which complicate the reaction and greatly reduce the yield. It is one of the important tasks of the dye chemist to study these undesirable side reactions suflSciently to understand their nature and then, if possible, to select the reaction conditions which will favor only the main reaction leading to the desired intermediate. This end is not always attained, because often the set of conditions which will eliminate the side reactions is not known, but the chemist must always bear in mind the possibihty of achieving diese conditions by further study. The preparation of l,8-aminonaphthol-3,6-disulfonic acid (H acid) illustrates this point. This compound has been known for nearly fifty years and is still being studied extensively in many laboratories, yet to this day has not been prepared in satisfactory yield. 

One of the crucial side reactions in the displacement reactions of alkyl halides is elimination, which can generally be neglected in the reaction of primary alkyl halides. On the other hand, a considerable amount of alkene invariably forms in the reaction of secondary alkyl halides. This undesirable side reaction can be minimized by the use of cesium carboxylates. ... 

The reactions of alkyl sulfonates with sodium, potassium or tetraalkylammonium carboxylates generally proceed with inversion of configuration. In the case of f-alkyl sulfonates, however, a varied amount of elimination products is invariably formed. Of all metal carboxylates, cesium salts (Scheme 43) have proved vastly superior with respect to reactivity and the suppression of undesirable side reactions. ... 

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