Intermediates in Elimination Reactions

Carbanions play critical roles in a wide variety of reaction pathways. As stated in the Introduction, this chapter will not focus on the synthetic utility of carbanions, but will instead focus on their mechanistic significance. In this section, a sample of important reaction mechanisms that involve transient or relatively short-lived car-banion intermediates will be introduced. As you will see, the key element in these mechanisms is the ability to form a carbanion that is reasonably stable, and often the kinetics of the reactions are dominated by carbanion stability. The role of carbanion intermediates in elimination reactions will be presented in some detail as a way to illustrate some of the methods that have been developed to probe for carbanion intermediates in reaction mechanisms. Other processes including additions and rearrangement reactions will be presented in less detail, but the role of carbanion stability in these reactions will be outlined. 

It is well known that base-induced elimination reactions can proceed either by a single, concerted step (E2), or by two steps, proton transfer and leaving group expulsion, with a carbanion intermediate (ElcB) to yield an alkene. The 

The carbanion can be destroyed in two ways, k2 or k, and two limiting types of behavior are expected for ElcB mechanisms. If k2 3 fe-i[BH], then the carbanion always decomposes to the alkene product and the rate law simplifies to feobs = fei. In other words, the rate is only dependent on carbanion formation and the rate law has a form that is identical to what would be obtained in a concerted E2 reaction. This has been referred to as an EIcBirr mechanism, where IRR indicates that deprotonation is irreversible. On the other hand, if k2 fc i[BH], then carbanion formation rarely leads to the product and the rate law simplifies to the following  

These reactions often have been run in buffer solutions where [B ] and [BH] represent the concentrations of the buffer s components. In Eq. 15, it can be seen that the rate depends on the concentration of the proton donor in the buffer solution. The 

It has been argued that the occurrence of isotope exchange at the p-carbon does not prove an EIcBr mechanism, only that a carbanion is formed under the reaction conditions—it is possible that an E2 process is responsible for product formation (i.e., ki e2 k2hlk-i In other words, the carbanion is formed and 

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The isolation and characterization of the complexes 119,121 and 122 makes the [2.2]paracyclophane-l-yne a likely intermediate in elimination reactions, but direct spectroscopic evidence for the free cycloalkyne would certainly be even more convincing. 

Because of the instability of 1° carbocations and the conclusion that 1° alcohols undergo acid-catalyzed oxygen exchange by an Sn2 process, there may be some question whether 1° carbocations are true intermediates in elimination reactions. An alternative mechanistic possibility for dehydration of 1° alcohols is an acid-catalyzed E2 mechanism. Narayan and Antal proposed such a mechanism for the specific acid-catalyzed dehydration of 1-propanol with sulfuric acid in supercritical water (Figure 10.44). ... 

Intermediates in elimination reactions could also be detected using this three-phase test on the carbamylation reaction (R = NHCeH in resin 9). The reaction was catalyzed by triethylamine or a proton sponge [1,8-bis(dimethylamino)naphthalene]. The intermediate, phenyl isocyanate, was readily observed (infrared spectra) between the two resins and was trapped as the urea derivative of the resin (11). 

The TT-allylpalladiLim complexes formed as intermediates in the reaction of 1,3-dienes are trapped by soft carbon nucleophiles such as malonate, cyanoacctate, and malononitrile[ 177-179). The reaction of (o-iodophenyl-methyl) malonate (261) with 1,4-cyclohexadiene is terminated by the capture of malonate via Pd migration to form 262. The intramolecular reaction of 263 generates Tr-allylpalladium, which is trapped by malononitrile to give 264. o-[odophenylmalonate (265) adds to 1,4-cyciohexadiene to form a Tr-allylpalladium intermediate via elimination of H—Pd—X and its readdition, which is trapped intramolecularly with malonate to form 266)176]. 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

The reaction of allyl halides with terminal alkynes by use of PdClifFhCNji as a catalyst affords the l-halo-l,4-pentadienes 297. 7r-AlIylpalladium is not an intermediate in this reaction. The reaction proceeds by chloropalladation of the triple bond by PdCh, followed by the insertion of the double bond of the allyl halide to generate 296. The last step is the regeneration by elimination of PdCh, which recycles[148]. The cis addition of allyl chloride to alkynes is supported by formation of the cyclopentenone 299 from the addition product 298 by Ni(CO)4-catalyzed carbonylation[149]. 

If carbocations are intermediates, we should expect rearrangements with suitable substrates. These have often been found in elimination reactions performed under El conditions. 

The initially formed tetra-alkylferrate(II) represents the reactive intermediate in both reactions that undergoes a carboferration of the triple bond in eq. 2, Scheme 29. Transmetallation from Fe to Mg yields a vinyl-magnesium species, which liberates the desired olefin upon hydrolysis within the acidic work-up procedure. In the above two reactions, a competing p-hydride elimination from the ferrate yields the unreactive Fe-H species and hence is considered to be the deactivation step in the catalytic cycle. 

We have already seen examples of carbanions involved as intermediates, e.g. (40), in elimination reactions, i.e. those that proceed by the ElcB pathway (p. 251), for example ... 

There had been some evidence that alkoxide ligands slow down reactions which involve elimination of a p-hydride from an alkyl ligand. a-01efins are dimerized to a mixture of head-to-tail and tail-to-tail dimers by olefin complexes of the type Tafr -CsMes)-(CH2=CHR)Cl2 (10). The p,p1- and a,p -disubstituted tantalacyclo-pentane complexes are intermediates in this reaction. Their decomposition involves the sequence shown in equation 5. When one... 

In previous experiments, Bald et al. [2] had tried to liberate 131 from 3-bromo-6,7-benzobicyclo[3.2.1]octa-2,6-diene (134) by /3-elimination of hydrogen bromide with KOtBu. Both the trapping products 140 of the intermediate with DPIBF and the enol ether 138 appeared to provide evidence in favor of 131, but did not exclude the cycloalkyne 136 [94]. The generation of 136 in the presence of DPIBF by two routes that cannot lead to 131 gave rise to cycloadducts different from 140, which were converted into 140 on treatment with KOtBu, however [95, 96], On the basis of these results and further investigations [97, 98], the formation of 131 from 134 is considered unlikely. Instead of 131, 136 seems to arise and to be the source of the products observed. In contrast, the phenyl derivative 137 of 131 is the obvious intermediate in the reaction of 135 with KOtBu, which furnished the enol ether 139 [2]. 

Considering the success of the condensation route to carboranes in the hydride bath (vide supra), other alkynylboranes than diethyl(propyn-l-yl)borane might be equally suitable. By heating a mixture of bis(diethylboryl) ethyne (65) and excess of (Et2BH)2 ( hydride bath ) at 110-120 °C, l,2,3,4-tetraethyl-5,6,7,8-tetracarba-mdo-octaborane(8) 64c was obtained by distillation in ca. 20% yield as a colorless liquid, stable to air and H2O (Scheme 3.2-35) [87]. Possible intermediates in this reaction can be proposed as 67 and 68, where 67 results from double hydroboration of bis(diethylboryl)ethyne (65), which dimerizes to 68 and finally yields the carbo-rane 64c by elimination of Et3B. 

Propoxur (313) is another herbicide which can be eliminated from the environment by means of photochemical treatments. In fact, it has been shown that direct irradiation of aerated aqueous solutions of propoxur leads to formation of PFR products, disappearing almost completely the starting material. By laser flash photolysis, it has been demonstrated that the key intermediate in this reaction is the 2-isopropoxyphenoxy radical [297]. 

In the case of unsaturated chloramines 209, the DTBB-catalyzed lithiation had to be carried out in the presence of the electrophile in THF at —78 °C in order to avoid decomposition of the corresponding functionalized organolithium intermediate through elimination reactions. Final hydrolysis yielded a mixture (variable ratios 11.5/1 to 1/19 depending on the electrophile) of the a- and y-products 210 (Scheme 72) . ... 

Apart from the azirine pathway, a vinylnitrene 526 was postulated as a possible intermediate in the reaction (Scheme 14). The nitrene may be formed by a base-promoted elimination of the leaving group on the nitrogen and gives rise to the 277-azirine by electrocyclization (nitrene insertion) and, in view of current data, this pathway cannot be excluded. 

Another clear example of an acetylene insertion reaction was reported by Chiusoli (15). He observed that allylic halides react catalytically with nickel carbonyl in alcoholic solution, in the presence of CO and acetylene, to form esters of cis-2,5-hexadienoic acid. The intermediate in this reaction is very probably a 7r-allylnickel carbonyl halide, X, which then undergoes acetylene insertion followed by CO insertion and alcoholysis or acyl halide elimination (35). Acetylene is obviously a considerably better inserting group than CO in this reaction since with acetylene and CO, the hexadienoate is the only product, whereas, with only CO, the 3-butenoate ester is formed (15). [See Reaction 59]. 

A similar palladium-catalyzed cyclization procedure has recently been developed which involves enol ethers capable of (3-H elimination.373 Significant evidence has been accumulated suggesting that an oxa-ir-allyl complex is not an intermediate in these reactions, but that it is better characterized as an enolate addition to a Pdn-alkene complex.376 377 Synthetic applications of this reaction have also appeared.376-379... 

The decomposition of mandelic acid (70),62 the elimination of CO2 from the intermediate in the reaction of phosgene with carboxylic acids,64 the thermal decomposition of haloacetic acids (74),65 and the hydrolysis of aryl carbazates (140)119 were discussed earlier. 

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