Elimination reactions stereoselectivity

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... 

Compounds containing carboxyl groups on adjacent carbons (succinic acid derivatives) can be bisdecarboxylated with lead tetraacetate in the presence of O2 263 jjjg reaction is of wide scope. The elimination is stereoselective, but not stereospecific (both meso- and dl- 2,3-diphenylsuccinic acid gave trans- stilbene) a concerted mechanism is thus unlikely. The following mechanism is not inconsistent with the data ... 

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 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). 

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. 

Stereochemical investigation on the vinylogous 1,5-elimination reactions of 16 to give cyclopropylalkenes has revealed highswi stereoselectively of the intramolecular SN process (equation 18)30. The stereoselectivity of the cyclization reaction of 17 to give the ( )-isomer is improved by the increase in steric bulkiness of the sulfonyl substituent (equation 19)31. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

A reaction of dibromoacetic acid with different aldehydes promoted by Sml2, followed by an elimination reaction also promoted by samarium diiodide, affords ( > ,/l-unsaturalcd carboxylic acids with total stereoselectivity (Scheme 8).42 A mecha- (g) nism that involves chelation of the Sm(III) centre with the oxygen atom of the alcohol group through a six-membered chair-like transition state has been described. 

As a further stereoselective organic synthesis [40-47] using reactive sp2 carbon-centered radicals, eq. 10.23 shows the preparation of chiral 4-te/7-butylcyclohexene (49) from the optically pure o-bromophenyl sulfoxide (48) through 1,5-H shift by sp2 carbon-centered radical, followed by (3-elimination. This reaction looks like a thermal concerted intramolecular elimination reaction (Ei). 

Anionic cyclisation can be used to form cyclopropanes when the product cyclopropylmethyllithium can undergo an elimination reaction.140111 The simple primary organolithium 270 requires the presence of TMEDA to cyclise, but gives 88% of vinylcyclopropane. 271 cyclises with moderate stereoselectivity. 

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