Olefin-forming elimination reactions

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). 

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. 

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). 

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. 

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. 

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. 

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 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. 

When the carbanion is formed in a rapid pre-equilibrium, isotopic exchange between the substrate and the solvent should be a faster process than formation of the olefin. The elimination reaction of 2-(p-nitrophenyl)ethyltrimethyl-ammonium ion in aqueous solution is accelerated by hydroxide ion but retarded by acid. This observation led to the postulation of the carbanion mechanism acid depressing the rate by reversing the equilibrium step (22). 

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)... 

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—O" is then eliminated and a new C—C bond is formed. Such reactions take place because the formation of a Y—0 bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

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 ... 

In 1970, it was disclosed that it is possible to achieve the conversion of dimethylformamide cyclic acetals, prepared in one step from vicinal diols, into alkenes through thermolysis in the presence of acetic anhydride." In the context of 31, this two-step process performs admirably and furnishes the desired trans alkene 33 in an overall yield of 40 % from 29. In the event, when diol 31 is heated in the presence of V, V-dimethylforrnamide dimethyl acetal, cyclic dimethylformamide acetal 32 forms. When this substance is heated further in the presence of acetic anhydride, an elimination reaction takes place to give trans olefin 33. Although the mechanism for the elimination step was not established, it was demonstrated in the original report that acetic acid, yV, V-dimethylacetamide, and carbon dioxide are produced in addition to the alkene product."... 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

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