Stereoselective, elimination reactions

The stereoselective elimination reaction of suitably substituted allylic compounds is a reasonable approach to the construction of the propadiene framework. Central chirality at the allylic position is transferred to axial chirality of the allene by stereoselective /3-elimination (Scheme 4.53). 

Predominantly -alkenes can be formed by stereoselective elimination reactions... 

PREDOMINANTLY E ALKENES CAN BE FORMED BY STEREOSELECTIVE ELIMINATION REACTIONS... 

Stereoselectivity was defined and introduced in connec tion with the formation of stereoisomeric alkenes in elimination reactions (Sec tion 5 11)... 

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. 

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

S-N bond cleavage 159 S-O bond lengths 543 Solvated electrons 897, 905 Solvent effects 672 in elimination reactions 772 S-O stretching frequencies 543, 545, 546, 552-555, 560-562 Spiroconjugation 390 Stereoselectivity 779, 789 of cylcoaddition reactions 799 of sulphones 761 Steroids... 

In general, the elimination reactions are anti under acidic conditions and syn under basic conditions. This stereoselectivity is the result of a cyclic mechanism under basic conditions, whereas under acidic conditions an acyclic (3-elimination occurs. 

Chapter 6 looks at concerted pericyclic reactions, including the Diels-Alder reaction, 1,3-dipolar cycloaddition, [3,3]- and [2,3]-sigmatropic rearrangements, and thermal elimination reactions. The carbon-carbon bond-forming reactions are emphasized and the stereoselectivity of the reactions is discussed in detail. 

The cyclopropylphosphonate anion undergoes the stereoselective addition reaction with aldehydes [106]. Although the Wittig-Horner elimination is not observed at this stage, the olefinic compounds are produced by treatment of the adduct alcohols 201 with NaH in the presence of 18-crown-6 as illustrated in Scheme 76. 

Compound 874, as a representative of derivatives with an electron-withdrawing substituent at C-[1 of the vinyl group, is easily prepared by elimination of one benzotriazole from 2,2-/fo(benzotriazol-l-yl)ethyl methyl ketone 873. The stereoselective elimination catalyzed by NaOH gives exclusively the (E) isomer of derivative 874. Addition of nucleophiles to the double bond of vinyl ketone 874 followed by elimination of benzotriazole leads to a,P unsaturated ketones 875. Amines used as nucleophiles do not need any catalysis, but reactions with carbon and sulfur nucleophiles require addition of a base. The total effect is nucleophilic substitution of the benzotriazolyl group at the i-carbon of orji-iinsaturatcd ketone (Scheme 142) <1996SC3773>. 

Reactions of propynyl alcohols and their derivatives with metal hydrides, such as lithium aluminum hydride, constitute an important regio- and stereoselective approach to chiral allenes of high enantiomeric purity63-69. Formally, a hydride is introduced by net 1,3-substitution, however, when leaving groups such as amines, sulfonates and tetrahydropyranyloxy are involved, it has been established that the reaction proceeds by successive trans-1,2-addition and preferred anti-1,2-elimination reactions. The conformational mobility of the intermediate results in both syn- and ami- 1,2-elimination, which leads to competition between overall syn- and anti-1,3-substitution and hence lower optical yields and/or a reversal of the stereochemistry. 

Fluorines present in the /i-position (/f-fluorination) can also influence the outcome of elimination reactions of fluorocarbons as earbanion intermediates are involved in the ElcB elimination mechanism.83 The regio- and stereoselectivity of the elimination step is governed by CH acidities,84 which, as has been discussed, arc influenced by /f-fluorination. 

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