Elimi nation

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

FIGURE 8 11 When a Lewis base reacts with an alkyl halide either substitution or elimination can occur Sub stitution (Sn2) occurs when the Lewis base acts as a nu cleophile and attacks carbon to displace bromide Elimi nation (E2) occurs when the Lewis base abstracts a pro ton from the p carbon The alkyl halide shown is iso propyl bromide and elimi nation (E2) predominates over substitution with alkox ide bases... 

A reaction useful only with sub strates that do not undergo E2 elimi nation readily It is rarely used for the synthesis of alcohols since alkyl halides are normally prepared from alcohols... 

Alkenylcarbene complexes react with in situ-generated iodomethyllithium or dibromomethyllithium, at low temperature, to produce cydopropylcarbene complexes in a formal [2C+1S] cycloaddition reaction. This reaction is highly diastereoselective and the use of chiral alkenylcarbene complexes derived from (-)-8-phenylmenthol has allowed the enantioselective synthesis of highly interesting 1,2-disubstituted and 1,2,3-trisubstituted cyclopropane derivatives [31] (Scheme 9). As in the precedent example, this reaction is supposed to proceed through an initial 1,4-addition of the corresponding halomethyllithium derivative to the alkenylcarbene complex, followed by a spontaneous y-elimi-nation of lithium halide to produce the final cydopropylcarbene complexes. 

The most problematic cases for the Stille reaction involve coupling saturated systems. The tendency for p-elimi nation of alkylpalladium compounds requires special conditions. Zh. v-(dialkylamino)cyclohexylphosphincs have shown considerable success... 

An alternative means of preparing alkylidene complexes of titanium is by the [Pg.479]

Initial theoretical studies focused on steps (1) and (2). Several model systems were examined with ab initio calculations.1191 For the reaction of methyl amine with methyl acetate, it was shown that the addition/elimi-nation (through a neutral tetrahedral intermediate) and the direct displacement (through a transition state similar to that shown in Figure 5a) mechanisms for aminolysis had comparable activation barriers. However, in the case of methyl amine addition to phenyl acetate, it was shown that the direct displacement pathway is favored by approximately 5 kcal/mol.1201 Noncovalent stabilization of the direct displacement transition state was therefore the focus of the subsequent catalyst design process. 

FIGURE 5.13 Protecting group combinations employed in solid-phase synthesis. The protector written in italics is removed after each residue is incorporated into the chain. (A) Boc/Bzl3 is not an orthogonal system — all substituents are removed by acidolysis. (B) Fmoc/tBu27 [Atherton et al., 1978] is an orthogonal system - Fmoc is removed by P-elimi-nation, other substituents by acidolysis. More suitable variants of the side-chain protectors are also used. 

Carbamic acid, hexyl-, methyl ester, 56, 40 CARBENF GENERATION BY a-ELIMI NATION, 58, 37... 

The frequently observed preference for anti-elimination over syn-elimi-nation on alumina (for a summary of earlier results see ref. 7, later especially ref. 96) has been a cause of much controversy. However, as has been explained in Sect. 2.1.2, it is a natural reaction course for concerted elimination, provided that suitably spaced acidic and basic sites are available on the surface. Catalysts which operate by means of the El-like mechanism... 

The observation of this y-elimination reaction has raised the question, whether in DNA the related C(l ) radical can also lead to strand breakage. However due to the negative charge at the phosphate group in 3 -position, the rate of the y-elimi-nation reaction (54) should be considerably slower than that of reaction (53). 

Fig. 4.5. Mechanism-independent occurrence of some stereocontrol in /3-elimi-nations of Het and H from a substrate Het—C(Ph)Me— CH2—R. Only eliminations in which the removed H atom does not come from the methyl group are considered.

Intramolecular nucleophilic addition of alkoxides to vinyl sulfoxides (90) provided a route to (3-alkoxysuIfoxides (91) (Scheme 24). The cij-product was formed with up to 18 1 selectivity.58 Alkoxide nucleophiles undergo an addition/elimi-nation protocol with P-iodo vinyl sulfoxides to yield tetrahydrofurans and tetrahy-dropyrans, which contain either an endo- or an exo-cyclic double bond.59,60... 

Jin and Fuchs reported that vinyl sulfones, using basic phase-transfer catalyst conditions, were regiospecifically alkylated at the a-position.78 No P-elimination products were observed in systems capable of undergoing anion-promoted P-elimi-nation. He also reported that y-methoxy vinyl sulfones 116 can be converted to the corresponding substituted enones 119 using this protocol (Scheme 32).79 On reaction with tert-butyllithium, 116 is converted to the y-methoxy allylsulfonyl anion 117, which was regiospecifically trapped by a variety of electrophiles to provide the enol ether 118. On hydrolysis the -substituted enone 119 was obtained. 

Michaet-reiro-M ichael (M rM) addthon/elimi nation... 

The first indication, reported in 1971, for a nickel-catalyzed intramolecular allylation of an alkenic bond was the dimerization of butadiene to 2-methylvinylcyclopentane (263). This efficient transformation (90%) proceeded with 1.2 mol % of a Ni catalyst (prepared in situ from (Bu3P)2NiBr2 and BuLi) in the presence of methanol and presumably involves a nickel-ene reaction of (262) followed by a -elimi-nation of Ha (Scheme 55). 

Ally halides (equation 37), vinyl halides (equation 38) and vinyl ethers are good substrates for the preparation of organosilanes having functional groups at the 3- or 7-position. Reduction of these substrates sometimes takes place to give the parent alkenes (equation 37), which are derived from 3-elimi-nation of the regioisomeric hydrosilylation intermediates. ... 

The key step in the preparation of 5,6-dihydro-5-pyrones with an incorporated 1,3-polyol system (an example, compound 105 possessing inhibitory activity against C. cucumerinum is shown in Scheme 45) consisted of formation of a 2,3-unsaturated furanose derivative 104, which was prepared from 103. Abstraction of an acidic proton from the C-2 induced 8-elimi-nation, providing the desired target 104 [94]. 

Selenoxide elimination occurs under relatively mild conditions in comparison to the elimination reactions described above. Selenoxides undergo spontaneous yn-elimi-nation at room temperature or below and thus have been used for the preparation of a variety of unsaturated compounds. The selenide precursors can be obtained by displacement of halides or sulfonate esters with PhSeNa. Oxidation of the selenides with hydrogen peroxide or tert-huiyX hydroperoxide, sodium periodate, or peroxycar-boxylic acids furnishes the corresponding selenoxides. Their eliminations usually favor formation of the less substituted olefin in the absence of heteroatom substituents or delocalizing groups. Since selenium compounds are toxic, they should be handled with care. 

Firstly, the stabilization by chelate formation should increase the exothermicity of olefin insertion. Since part of the stabilization will already be felt in the transition state, it will reduce the barrier for olefin insertion in a palladium-acyl bond. Secondly, chelate formation should oppose or slow down termination by ff-elimi-nation from palladium-alkyl. For -elimination to occur the P-R atom has to approach the palladium ion, but that is opposed by coordination of the carbonyl group (eq. (13)). Inhibition ofy9-H elimination in metallacycles is well known [27]. 

In the previous sub-section, we saw that c/////-elimi nation requires the two groups that are to be eliminated to be anti-periplanar to each other. If there is a deviation from this angle then a different mechanism is observed. 

If the groups are lost from a single center (a-elimi-nation, 1/1/elimination), the resulting product is a car-bene or a carbene analog. 

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