Double Elimination reaction

Several of the isomeric 5,10-pentadecadienols, compounds which are of interest because of their use as insect pheromones as well as flavoring and perfume chemicals, have been prepared from 2-chloromethyltetrahydrofuran (163) and 2-chloromethyltetrahydropyran (167) through double elimination reactions (77HCA1161). On treating (163) with three... 

A more recent variation of the general procedure described in the previous section involves the use of adducts of l-bromo-2-chlorocyclopropene as substrates for the double elimination reaction.Thus, l/f-cyclopropa[6]phenanthrene (4) was obtained in 89% yield in the final step of a sequence that included cycloaddition to diene 1 followed by aromatization. ... 

Dienes from epoxides. In refluxing dichloroethane in the presence of triethyl-amine, epoxides undergo a double-elimination reaction, giving conjugated dienes in good yields. Crowded epoxides remain unchanged thus, the mixture of cis- and frani-cyclododecene oxides affords a clean mixture of (IE, 3Z)-cyclododecadiene and rran -cyclododecene oxide (50% yield). 

The following reaction, in which two molecules of HBr are eliminated to yield an alkyne, is an example of a double elimination reaction. 

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

Zaitsevs rule (Section 5 10) When two or more alkenes are capable of being formed by an elimination reaction the one with the more highly substituted double bond (the more sta ble alkene) is the major product Zwitterion (Section 27 3) The form in which neutral amino acids actually exist The ammo group is in its protonated form and the carboxyl group is present as a carboxylate... 

The stereochemistry of the most fundamental reaction types such as addition, substitution, and elimination are described by terms which specify the stereochemical relationship between the reactants and products. Addition and elimination reactions are classified as syn or anti, depending on whether the covalent bonds which are made or broken are on the same face or opposite faces of the plane of the double bond. 

The nature of the transition state in elimination reactions is of great importance, since it controls the regiochemistry of p elimination in compounds in which the double bond can be introduced in one of several positions. These effects are discussed in the next section. 

According to Bredt s rule such olefins of small ring size are unstable ordinary elimination reactions usually yield an isomeric olefin where a bridgehead carbon does not participate in the double bond. 

According to Zaitsev s rule, formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons. In the following two cases, for example, the more highly substituted alkene product predominates. 

I Ignoring double-bond stereochemistry, what products would you expect from elimination reactions of the following alkyl halides Which will be the major product in each case ... 

Anti stereochemistry (Section 7.2) The opposite of syn. An anti addition reaction is one in which the two ends of the double bond are attacked from different sides. An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

The water elimination reactions of Co3(P04)2 8 H20 [838], zirconium phosphate [839] and both acid and basic gallium phosphates [840] are too complicated to make kinetic studies of more than empirical value. The decomposition of the double salt, Na3NiP3O10 12 H20 has been shown [593] to obey a composite rate equation comprised of two processes, one purely chemical and the other involving diffusion control, for which E = 38 and 49 kJ mole-1, respectively. There has been a thermodynamic study of CeP04 vaporization [841]. Decomposition of metal phosphites [842] involves oxidation and anion reorganization. 

Early investigations have indicated that sulfinyl radicals apparently do not add, at least in the usual way, to olefmic double bonds24. However, some recent results by lino and Matsuda25 obtained by studying the thermal decomposition of benzhydryl p-tolyl and benzhydryl methyl sulfoxides in the presence of cis-/2-deuteriostyrene lead one to believe that sulfinyl radicals add reversibly to CH2 =CHPh. The molar ratio of trans to cis /3-deuteriostyrene that they observed at nearly 50% conversion was explained by addition-elimination reaction of sulfinyl radicals. 

FIGURE 18.8 In an elimination reaction, two atoms or groups (the orange and yellow spheres) attached to neighboring carbon atoms are eliminated from the molecule, leaving a double bond between the two carbon atoms. 

The double bonds in alkenes can be generated by elimination reactions. 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

Notice that there is one long flow of electron density, illustrated with three arrows. We begin at the tail of the arrow on the base, because that is where the flow starts. This arrow is going from a lone pair to form a bond. The second arrow goes from a bond to form a bond, and the third arrow goes from a bond to form a lone pair on X. This type of reaction is called an elimination reaction, because we are eliminating H+ and X to form a double bond ... 

Consider the elimination reaction below, which uses a strong base. The product will be a double bond. This reaction will produce two Zaitsev products. One will be cis and one will be trans. Draw these products, and identify which is cis and which is trans. 

In a substitution reaction, the leaving group is replaced with a nucleophile. In an elimination reaction, a beta ((3) proton is removed together with the leaving group, forming a double bond. In the previous chapter, we saw two mechanisms for substitution reactions (SnI and Sn2). In a similar way, we will now explore two mechanisms for elimination reactions, called El and E2. Let s begin with the E2 mechanism. 

Answer In this problem, we are starting with an alkane. There are no leaving groups, so we cannot do a substitution or an elimination reaction. There are also no double bonds, so we cannot do an addition. It seems that we are stuck, with nothing to do. Clearly, our only way out of this situation is to introduce a functional group into the compound, via radical bromination. Radical bromination will place a Br at the most substituted position (the tertiary position), and then we can eliminate ... 

Now we are one step closer to solving this problem. The next step is to ask if there is a way to turn the starting material into this double bond. And there is. We just do an elimination reaction to get the double bond. So now we have solved our synthesis by working backward ... 

This section describes reactions in which elimination to form a double bond or a new ring occurs as a result of thermal activation. There are several such thermal elimination reactions that are used syntheses, some of which are concerted processes. The... 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

A chelation-assisted Pd-catalyzed Cope rearrangement was proposed in the reaction of phenanthroline to generate isoquinolinone derivatives (Eq. 12.78).177 The use of aqueous media and ligands enables a double-Heck reaction on a substrate favoring alkene insertion over (J>-hydride elimination. 

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