Dimeric olefin-forming eliminations

If the initial proton removal occurs in a rapid reversible equilibrium, then the determination of the reaction order in the substrate should suffice to distinguish between the two mechanisms. 

Hauser et were able to isolate the intermediate dimeric halide when 9-chlorofluorene was allowed to react with only one equivalent of sodium amide in liquid ammonia. In t-butyl alcohol containing its potassium salt or a dilute aqueous solution of benzyltrimethylammonium hydroxide, the rate of formation of bifluorenylidene is second-order in 9-bromofluorene and first-order in the basicity of the reaction medium as measured by the ionisation of nitroaniline indicators . Under the same reaction conditions, protium exchange of 9-deutero-9-bromofluorene and elimination from 9-bromo-9,9 -bifluorenyl are much faster reactions than the conversion of 9-bromofluorene into bifluorenylidene. These facts are consistent with the displacement mechanism. 

With sodium or potassium r-butoxides as the base, the rate of elimination of 9-halogenofluorenes sometimes shows a first-order dependence on both the medium basicity and the substrate, even though exchange of 9-deuterium is still a rapid process . Factors which promote this change in kinetic order are the presence of powerful electron-withdrawing substituents in the fluorene 

In view of the above findings, rather than attribute a first-order dependence of rate on the substrate concentration to a rate-determining carbene formation, a modification of the displacement mechanism was suggested in which ion-pair dissociation to give more reactive solvent separated ions or free ions becomes the slow step under certain conditions, viz. 

Despite earlier claims of a carbene mechanism , more recent kinetic studies indicate that the conversion of diphenylmethyl chloride into tetraphenylethyl-ene by r-butoxide in dimethyl sulphoxide follows the displacement mechanism. The reaction is third-order, second in substrate and first in the medium basicity and a-hydrogen exchange in the substrate is a rapid process . 

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The rate law does, of course, not decide on the exact pathway by which the olefin dimer is formed from the excited 1 2 complex. One possibility is a radical-radical-dimerization with intermediate formation of a five membered metallacycle that could form the product by reductive elimination. Such a sequence is not subject to the restrictions of a concerted electrocyclic mechanism and the final stereochemistry of the cycloaddition product would be largely determined by the favored stereochemical arrangement in the 1 2 complex. An isolated and structurally characterized intermediate which is cited for support is the Ir complex 5, shown in Scheme 4, formed from [(COD)IrCl]2 and NBD followed by metathesis with 2,5-pentane-dionate [17]. 

Additions of silyl-substituted synthons 965 to nitrones such as 962 a in the presence of LDA result in the products 966 which eliminate the lithium salt of O-tri-methylsilyl-N-phenylhydroxylamine 968 to give the olefins 967a or 967b in 72 and 39% yield, respectively [68, 69] (Scheme 7.20). The intermediate lithium salt 968 dimerizes with elimination of LiOSiMe3 98 to form azobenzene and azoxybenzene 961 [68, 69]. 

The complex, XLYI, may add to another molecule of isobutylene to yield a higher polymer complex or eliminate aluminum chloride to yield the dimer in the latter case intramolecular migration (a 1,5-shift 1) of hydrogen must be postulated in order to form an olefin. On the other hand, cyclization may readily occur (particularly after a 1,2-shift of a proton from a methyl group) with the resultant formation of a naphthene. 

During the early attempts to synthesize free NHC, Wanzlick and colleagues tried to prepare l,3-diphenylimidazolidin-2-ylidene (2) by thermal elimination of chloroform from 1, but they rather obtained the dimeric electron-rich olefin 3 (Scheme 1) [15-17]. Wanzlick postulated that the carbene 2 could be formed as an intermediate during the formation of 3, and proposed the existence of an equilibrium between 2 and 3. Evidence supporting this equilibrium came later with the works performed by the research groups of Denk, Hahn, Lemal, and Cavell [18-21]. 

The evidence is in accord with an addition-elimination mechanism (addition of ArPdX followed by elimination of HPdX) in most cases." In the conventionally accepted reaction mechanism," a four-coordinate aryl-Pd(II) intermediate is formed by oxidative addition of the aryl halide to a Pd(0) complex prior to olefin addition. This suggests that cleavage of the dimeric precursor complex, reduction of Pd , and ligand dissociation combine to give a viable catalytic species." If these processes occur on a time scale comparable to that of the catalytic reaction, nonsteady-state catalysis could occur while the active catalyst is forming, and an... 

Hence, in one instance, the glucal (1) affords the nitroso nitrate (4) (which subsequently dimerizes to 5). In the other, the glucal might be expected to yield a nitro nitrite (8), but the latter, if indeed it is formed as a primary adduct, would certainly be prone to loss of nitrous acid by elimination and thereby to produce the nitro-olefin (6) that... 

Ti o.9i jji most cases the organometallic undergoing insertion is formed in situ by a metal hydride addition (insertion) with an olefin rather than by the exchange reaction. The olefin reacting with the hydride to form the alkyl may be the same one that undergoes the insertion with the metal alkyl, or a different one. This sequence with a single olefin produces olefin dimers after the final / -hydride elimination. A mechanism for the RhClj-catalyzed dimerization of ethylene is ... 

The rearranged olefin 50 may be used in a variety of synthetic applications. For instance it can be converted into the novel di-bis-tetrahydrofuran-acetal 95 in a one-pot operation using trimethylsilyl iodide in dichloromethane at 22 °C (Scheme 16) (12). The mechanism involves the formation of an oxonium intermediate 96 which undergoes a Prins cyclization to form the cation 97. Subsequent pinacol rearrangement generates 98 which cyclizes to 99. This acetal dimerizes under elimination of trimethylsilylbenzyl ether and benzyliodide. The structure of 95 has been elucidated by X-ray analysis (Fig. 3) which nicely shows the C2-symmetry of the dimeric structure. 

The polymerization and oligomerization of alkenes has been one of the most successful applications of organometallic chemistry to the synthesis of organic products on a large scale. As noted in the introduction to this chapter, organometallic complexes are involved in the synthesis of close to, or in excess of, fifty to one hundred million metric tons of polyolefins and a-olefins per year. In most cases, these products are formed by a series of alkene insertions into metal alkyl complexes in competition with p-hydrogen elimination processes. In other cases, selective dimerization or trimerization of alkenes occurs by the intermediacy of metallacyclic intermediates. 

DFT calculations have shown that the experimentally observed decomposition pathway likely occurs through insertion of the benzylidene into the chelating Ru-C bond. The computed free-energy profile for the decomposition of complex 22 is shown in Figure 7.21. Insertion of the alkylidene into the chelating ruthenium-carbon(adamantyl) bond required 29.7 kcalmoC and formed alkyl ruthenium complex 28. Complex 28 then underwent facile fi-hydride elimination to form ruthenium hydride 30, which then converted to the q -bound olefin complex 32 and eventually the dimer complex 26. The a-hydride ehmination pathway from intermediate 28 via 33-ts required much higher activation energy than the fi-hydride elimination. 

There are two potential side reactions that can derail the catalytic cycle shown in Scheme 10.6, namely, hydroboration of the olefin monomer by borane and ligand exchange reactions between the borane and cocatalysts containing aluminum alkyl groups. Fortunately, borane compounds containing B-H groups usually form stable dimers (Figure 10.1) that are unreactive toward olefins in typical olefin polymerization solvents (e.g., hexane, toluene). To eliminate the second process, however. 

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