Olefin forming /3-eliminations

Chapter 4 is limited to a discussion of the most important eliminations, which are the olefin-forming /3-eliminations. 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Yields of chlorides are good to excellent for primary and secondary alcohols, but a competing olefin-forming elimination process renders the method of limited value for preparing tertiary chlorides.12 An adaptation of the procedure using carbon tetrabromide allows the synthesis of alkyl bromides. Some examples are the preparation of rt-C5H11Br (97%) and C H6CH2Br (96%).12 Farncsyl bromide has been prepared in 90% yield from fame sol.23... 

On solid acid—base catalysts, beside elimination, addition and substitution, some other reactions also proceed. Of these, especially skeletal isomerisation of hydrocarbons and double bond shift should be mentioned. The latter can influence the product composition in olefin-forming eliminations and thus distort the information on orientation being sought. 

In organic chemistry, elimination processes are those decompositions of molecules whereby two fragments are split off and the multiplicity of the bonds between two carbon atoms or a carbon atom and a hetero atom is increased. Such a broad definition also embraces the dehydrogenation of hydrocarbons and alcohols which is dealt with in Chap. 2. Here we shall restrict our review to the olefin-forming eliminations of the type... 

Homogeneous olefin-forming eliminations have been studied extensively, especially in the liquid phase and comprehensive treatments of the subject are available [64,65]. The rules governing the course of homogeneous eliminations and their mechanisms are well established and the interpretation of the results obtained with heterogeneous catalytic sys-... 

The equilibrium of reaction (B) is less favourable for the formation of an alkyne and, in order to achieve equal conversion, much higher temperatures would be required than are necessary for the olefin-forming elimination (A). Therefore little attention has been paid to this type of reaction and this section will be devoted solely to type (A) dehydrohalogenation. 

The reader may notice a terminological inconsistency with respect to other olefin-forming eliminations. The reaction is called dealkylation in spite of the fact that the second product is an alkane or arene. The reasons are historical because the aromatic hydrocarbons (and similarly the alkanes) were held to be the more important components of the products. Dealkanation and dearenation are the proper names for the reactions. 

J. Sicher, The syn and anti Course in Bimolecular Olefin-Forming Eliminations, Angew. Chem., Inti. Ed., 11, 201 (1972). 

For similar reactions, comparison of die p values can be used to determine which reaction has a greater charge development. Comparison of the olefin-forming eliminations below reveals which reaction has greater charge development at die benzylic position and thus which has a greater degree of proton removal in die activated complex. 

In these alkylation reactions primary alkyl halides (the bromide for preference) should be used as the alkylating agents, since secondary and tertiary halides undergo extensive olefin-forming elimination reactions in the presence of the strongly basic acetylide ion. A typical synthesis is that of hex-l-yne (Expt 5.26). 

Bartsch, R. A. Bunnett, J. F. Orientation of olefin-forming elimination in reactions of 2-substituted hexanes with KOtBu—tBuOH and NaOMe-MeOH./. Am. Chem. Soc. 1969, 91, 1376-1382. 

The Steric Course of Bimolecular Olefin-forming Eliminations Future Developments. ... 

The use of DMSO as a mechanistic tool is not restricted to rate variation effects (Section 4). Advantage can also be taken of its unique molecular properties which enable it to stabilize certain types of structures, such as the anionic intermediates in SnAt reactions. Moreover, as a consequence of its influence on ion association constants, it is found to affect the product distribution and the stereochemical course of bimolecular olefin-forming eliminations. These two illustrative systems which have been chosen for discussion are intended to demonstrate the versality of this solvent in mechanistic studies and may suggest other avenues of investigation. 

Olefin-forming elimination reactions permit a similar division into cis and trans processes, and will be discussed in these two groups. The related reactions of epoxide ring closure and opening have sufficient similarity to the formation and reactions of olefins to warrant their inclusion in the same chapter. 

Yields (isolated) are about 80 % in the case of primary aliphatic halides and tosylates. Yields are less satisfactory in the case of secondary substrates owing to olefin-forming elimination reactions, which can be minimized by use of TIIF as solvent. Primary... 

The relative instability of the c/s-2-oxopyrido[I,2-a]pyrimidines (158 R = Ph) was explained by the steric conditions of the molecule the anti-periplanar arrangement of bonds a and b facilitates a concerted olefin-forming elimination reaction leading to the amide (159 R = Ph). The reaction is also aided by the unfavorable interactions arising among the cis-4-phenyl groups and between the 4-phenyl group and the 6-H atom. 

Olefin-Forming Eliminations Correlated by the Taft EqtieUion RX —r olefin - EX... 

Nitrate and sulphonate esters can undergo carbonyl-forming eliminations along with substitution and olefin-forming elimination in basic solution, e.g. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

Brown, H. C. Wheeler, O. H. Steric Effects in Elimination Reactions. IX. The Effect of the Steric Requirements of the Leaving Group on the Direction of Bimolecular Elimination in 2-Pentyl Derivatives J. Am. Chem. Soc. 1956, 78, 2199-2202. Also see Bartsch, R. A. Bunnett, J. F. Orientation of Olefin-Forming Elimination in Reactions of 2-Substituted Hexanes with Potassium frrf-Butoxide-fiprf-Butyl Alcohol and Sodium Methoxide-Methanol /. Aw. Chem. Soc. 1969, 91, 1376-1382. Provide the products expected from the following olefin-forming reactions. (CJH-7)... 

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