Conjugate eliminations

In order to study the reactivity of the nitrogen atom in saccharidic OZT moieties Rollin and co-workers13,55 explored some standard reactions IV-acylation, IV-sulfonylation, IV-vinylsulfonylation by Michael addition, reductive elimination, conjugated addition and cycloaddition. 

Mossoba, M.M., Kramer, J.K.G., Fritsche, J., Yurawecz, M.P., Eulitz, K., Ku, Y., and Rader, J.I. 2001 a. Application of standard addition to eliminate conjugated linoleic acid and other interferences in the determination of total trans fatty acids in selected food products by infrared spectroscopy.. /. Am. Oil Chem. Soc. 78 631-634. 

Unimolecular Elimination, Conjugate Base (Elcb) Focus On Box, pages 333—334... 

To limit the size of this report, a restriction was placed on the compounds that were eligible for inclusion. Firstly, to be included, the compound had to contain at least one five-membered heterocycle in its structure. Secondly, the number of atoms in the internal ring pathway of the macrocycle was required to contain at least 17 atoms (one more than the number of atoms present in the inner core of porphine Fig. 1). Thus this definition, which is in two parts, eliminates conjugated macrocycles containing other small heterocycles such as pyridines. It also rules... 

Convergent plans for synthesis Thermodynamic control Selection of reagents for enol(ate) conjugate addition Tandem reactions and Robinson annelation Substitution may be elimination-conjugate addition In disguise... 

This reaction type is called the ElcB mechanism, which stands for unimolecular elimination conjugate base reaction, because the conjugate base of the starting material is being formed as the reactive intermediate. It is sometimes called the carbanion mechanism. As this mechanism results from the removal of a proton, it is not surprising that it is favoured by those substrates that possess an acidic hydrogen atom. Thus, would you expect the ElcB mechanism to be more prevalent in reactions that result in a carbon/carbon double bond or in reactions that result in a carbon/carbon triple bond ... 

Most chemists would consider that there are four separate steps in this reaction elimination, conjugate addition, aldol reaction and dehydration. Mechanistically we should double this number as each step requires preliminary enolate ion formation. 

Elimination. Conjugated enynes and styrenes are formed from p-hydroxy sulfides that are derived from 2-alkynyl- and l-aryl-l,3-dithiolanes. The reaction is considered as... 

Humans eliminated conjugates of 3-phenoxybenzoic acid (CAS no. 3739-38-6), 3-(4 -hydroxyphenoxy) benzoic acid (CAS no. 35065-12-4), and cyclopropane-carboxylic acid (DCCA CAS no. 55701-05-8) in urine according to Woollen et al. (1992). In the Woollen et al. study, cypermethrin was administered orally to six male volunteers as a single dose (3.3 mg cis trans 1 1) and dermally to six volunteers at a dose of 31 mg/800 cm (cis trans 56 44) of skin. Cypermethrin was orally absorbed between 27 and 57% (mean of 36%) based on the elimination of DCCA and four times greater based on the recovery of benzoic acid conjugates. In the case of the dermal studies, 1.2% of the appUed dose was recovered in urine as benzoic acid conjugates. 

Where still more heat resistance is needed, we now have the polyimides, polybenzimidazole polyquinoxalines and other linear polymers in which the methylene content is reduced or eliminated. Conjugated unsaturation increases the thermal stability of these materials. Unfortunately, as the extent of condensation approaches that of the ladder polymer pyr-rones which might provide optimum heat resistance, the polymers become more intractable. 

For chemical processes, some examples are the elimination of aromatics by sulfonation, the elimination of olefins by bromine addition on the double bond (bromine number), the elimination of conjugated diolefins as in the case of the maleic anhydride value (MAV), and the extraction of bases or acids by contact with aqueous acidic or basic solutions. 

Here, A is an undeteiinined matrix of the coordinates (A is its Hermitian conjugate). Our next step is to obtain an A matrix, which will eventually simplify Eq. (16) by eliminating the Xm matiix. For this purpose, we consider the following expression ... 

Tertiary amines capable of eliminating a secondary amine to form a conjugated system can react with hydrogen cyanide to form y-keto nitriles by amine replacement. Thus (I) yields p-benzoylpropionitrile (IV) ... 

Treatment of conjugated enyne systems, H C-C=C-CeC-CH-OR, with a suitable base in principle may give rise to 1,6-elimination of HOR with primary formation of the system C=C-C=C=C=C, which subsequently may undergo isomerization to a dienyne. ... 

Fully conjugated cyclopolyynes, so-called cyciocarbons, constitute another class of carbon modifications besides diamond, graphite, and the recently discovered fullerenes (see section 5.6). Syntheses of these unstable rings might be possible by mild elimination, extrusion, or... 

The reaction of alkenyl mercurials with alkenes forms 7r-allylpalladium intermediates by the rearrangement of Pd via the elimination of H—Pd—Cl and its reverse readdition. Further transformations such as trapping with nucleophiles or elimination form conjugated dienes[379]. The 7r-allylpalladium intermediate 418 formed from 3-butenoic acid reacts intramolecularly with carboxylic acid to yield the 7-vinyl-7-laCtone 4I9[380], The /i,7-titisaturated amide 421 is obtained by the reaction of 4-vinyl-2-azetidinone (420) with an organomercur-ial. Similarly homoallylic alcohols are obtained from vinylic oxetanes[381]. 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

When allylic compounds are treated with Pd(0) catalyst in the absence of any nucleophile, 1,4-elimination is a sole reaction path, as shown by 492, and conjugated dienes are formed as a mixture of E and Z isomers[329]. From terminal allylic compounds, terminal conjugated dienes are formed. The reaction has been applied to the syntheses of a pheromone, 12-acetoxy-1,3-dode-cadiene (493)[330], ambergris fragrance[331], and aklavinone[332]. Selective elimination of the acetate of the cyanohydrin 494 derived from 2-nonenal is a key reaction for the formation of the 1,3-diene unit in pellitorine (495)[333], Facile aromatization occurs by bis-elimination of the l,4-diacetoxy-2-cyclohex-ene 496[334],... 

In steroid systems, the homoannular diene in ring A and the heteroannular diene in AB rings are generated. The allylic 3a-carbonate 514 affords the homoannular conjugated diene 515 as a main product and a small amount of the heteroannular diene 516. On the other hand, the heteroannular conjugated diene 516 is obtained exclusively from 33-carbonates 517. The elimination reaction proceeds smoothly at room temperature. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

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