Elimination stereospecificity

In contrast to 2-pyridyl sulfoxide, both 3- and 4-pyridyl p-tolyl (or phenyl) sulfoxides were found exclusively to undergo the ligand-exchange reactions to eliminate stereospecifically 3- and 4-pyridyl Grignard reagents and phenyl p-tolyl sulfoxide. The optical activity of phenyl p-tolyl sulfoxide indicated that the stereochemistry had been completely inverted. The resulting 3- and 4-PyMgBr... 

To establish the double bond of the respective allylstannane, the aldol products were submitted to very mild deprotection conditions for the allylester (Scheme 3.35). The respective hydroxy acids 165 and 167 were eliminated stereospecifically with N,N-dimethyl-ortho-formamide. The syn-aldol product 165 gave the E double bond 166, and the anti-aldol product 167 gave the Z double bond 168. 

The stereospecific synthesis of an A ring synthon of la-hydroxy vitamin D has been carried out. The ( )-allcene is cyclized to give the (E -c.xo-diene 155, and the (Z)-allcene affords the (Z)-e,xo-diene 156 stereospecifically[125,126]. These results can be understood by the cis addition and syn elimination mechanism. 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

Montedison and Mitsui Petrochemical iatroduced MgCl2-supported high yield catalysts ia 1975 (7). These third-generation catalyst systems reduced the level of corrosive catalyst residues to the extent that neutralization or removal from the polymer was not required. Stereospecificity, however, was iasufficient to eliminate the requirement for removal of the atactic polymer fraction. These catalysts are used ia the Montedison high yield slurry process (Fig. 9), which demonstrates the process simplification achieved when the sections for polymer de-ashing and separation and purification of the hydrocarbon diluent and alcohol are eliminated (121). These catalysts have also been used ia retrofitted RexaH (El Paso) Hquid monomer processes, eliminating the de-ashing sections of the plant (Fig. 10) (129). 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

Alkylaziridines can be stereospecifically deaminated to alkenes by reaction with m-chioroperbenzoic acid (70AG(E)374). The reaction and work-up are carried out in the dark to avoid isomerization of the cw-alkene, and the mechanism is thought to involve an initial oxidation to an amine oxide followed by a concerted elimination. Aziridine oxides have been generated by treating aziridines with ozone at low temperatures (71JA4082). Two... 

Synthesis of olefins by stereospecific rerbctfve elimination of S-hydroxyaikyl selenides (a variant at Peterson olefinatlon) by means of MeSOjCI. HCIO4 or P2I4. 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

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. 

Although there is usually a preference for anti elimination in acyclic systems, syn elimination is competitive in some cases. In acyclic systems, the extent of anti versus syn elimination can be determined by use of stereospecifically deuterated substrates or by use of diastereomeric reactants which will give different products by syn and anti elimination. The latter approach showed that elimination from 3-phenyl-2-butyl tosylate is a stereospecific anti process. ... 

The occurrence of syn elimination in 5-decyl systems has been demonstrated with the use of diastereomeric deuterium-labeled substrates. Stereospecifically labeled 5-substituted decane derivatives were prepared and subjected to appropriate elimination conditions. By comparison of the amount of deuterium in the E and Z isomers of the product, it was... 

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. 

These reactions proceed by alkoxide or fluoride attack at silicon which results in C—Si bond cleavage and elimination of the leaving group from the fi carbon. These reactions are stereospecific anti eliminations. 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Difluorododecane is unaffected by sodium methoxide in methanol, but its treatment with potassium tert-butoxide in tetrahydrofuran eliminates hydrogen fluonde stereospecifically rneso and dl compounds give, respectively, ( )- and (2) 6 fluoro 6 dodecene [12] (equation 11)... 

Stereospecific reaction (Section 7.13) Reaction in which stereoisomeric starting materials give stereoisomeric products. Terms such as syn addition, anti elimination, and inversion of eonfiguration describe stereospecific reactions. 

The stereospecific generation of enamines by -elimination reactions (187) and a vinylogous elimination, which leads to a dienamine (188), have been reported. The loss of an a substituent from a tertiary amine is seen in the generation of enamines by elimination of hydrogen cyanide from benzylic a-aminonitriles (189,190). 

The stereospecificity of the methylene transfer provides compelling support for a concerted mechanism and this conclusion has rarely been disputed. It is instructive, however, to review the experimental evidence that allowed for the elimination of the alternative mechanistic proposals, namely, a radical addition and a carbome-tallation (Scheme 3.4). 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

Eor the elimination of trimethylamine and water from the erythro- and threo-isomer of trimethyl-1,2-diphenylpropylammonium iodide 6 and 7 respectively, by treatment with sodium ethoxide, a stereospecific -elimination has been found to take place thus supporting a E2-mechanism. Erom the -isomer... 

Compounds 1, which bear in X an electrofugical leaving group, such as trialkylsilyl or diphenylphosphinyl, are of high value for the synthesis of stereochemically homogeneous 1,3-dienes via stereospecific anti or urn elimination. 

It has been generally assumed that thermal decomposition of thiirane oxides proceeds stereospecifically to the corresponding olefins by elimination of sulfur monoxide, possibly through a concerted nonlinear chelatropic reaction96 with retention of configuration of the liberated olefin. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

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