Eliminations giving alkynes

Rappoport has presented a detailed outline of the mechanisms of the reactions of vinyl halides with nucleophilic reagents. Modena et /. have provided further evidence in support of a spectrum of transition states for elimination from activated vinyl halides induced by alkoxide bases. Cristol and Whittemore have shown that the stereoselectivity of elimination from vinyl halides is largely determined by the choice of basic reagent alkoxide bases encourage am/-elimination, whereas syn-elimination and alpha-elimination become dominant with lithium alkyls. 

An (ElcB) ion-pair mechanism has been invoked for the elimination from 

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Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

Disubstituted silole derivatives are synthesized by the palladium-catalyzed reaction of (trialkylstannyl)di-methylsilane with terminal alkynes (Equation (107)).266 The mechanism is supposed to involve a palladium silylene complex, which is generated via /3-hydride elimination from LJ3d(SiMe2H)(SnBu3) (Scheme 62). Successive incorporation of two alkyne molecules into the complex followed by reductive elimination gives rise to the silole products. 

LDA is a strong base. Two E2 eliminations give an alkyne, which is deprotonated by the excess LDA to give an alkynyl anion. This species then reacts with Mel by an Sjsj-2 process. 

A proposed mechanism for the SiCaT reaction using the 1,6,11-triyne system as an example is illustrated in Scheme 7.21. The reaction proceeds through insertion of one of the terminal alkynes into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, generating an ethenyl-[Rh] intermediate, which undergoes addition to the second and third alkyne moieties to form intermediate III.2a. Subsequent carbocyclization followed by /9-hydride elimination gives the tricyclic silylbenzene derivative 70. Alternatively, ethenyl-[Rh] intermediate can be isomerized to the thermodynamically more... 

As a related reaction, the bicyclo[5.3.0]decane derivative 64 was obtained at 30 °C by the Rh-catalysed intramolecular [5+2] cycloaddition of the alkyne with the vinylcyclopropane moiety in 61. The latter behaves as a pseudo-1,3-diene in oxidative addition, and generates 62. This is followed by rearrangement to 63, whose reductive elimination gives 64 [20]. [Rh(CO)2Cl]2 is a better catalyst than RhCUPl+P. The reaction can be extended to alkenes [20a],... 

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. 

The Pd-catalyzed hydrocarbonations of methyleneaziridines 14 do not proceed through the formation of a Jt-allylpalladium intermediates, instead via a chelation effect. The hydropalladation of methyleneaziridines with the Pd(II) hydride species 16, followed by reductive elimination gives the non-ring-opened products 15. It is noteworthy to mention that the palladium-catalyzed intermolecular or intramolecular addition of nitriles to carbon-carbon multiple bonds can be explained by the hydropalladation mechanism, except for the intramolecular addition to the C=C triple bond of alkynes (vide infra). 

The Pauson-Khand reaction starts with the replacement of two CO molecules, one from each Co atom, with the alkyne to form a double a complex with two C-Co a bonds, again one to each Co atom. One CO molecule is then replaced by the alkene and this n complex in its turn gives a a complex with one C-Co a bond and one new C-C a bond, and a C-Co bond is sacrificed in a ligand coupling reaction. Then a carbonyl insertion follows and reductive elimination gives the product, initially as a cobalt complex. 

The electrophilic addition of bromine to aikenes is an oxidation. The starting alkene is at the alcohol oxidation level, but the product has two carbons at the alcohol oxidation level—the elimination reactions of dibromides to give alkynes that you met in the last chapter (p. 000) should convince you of this. There are a number of other oxidants containing electrophilic oxygen atoms that react with nucleophilic aikenes to produce epoxides (oxiranes). You can view epoxides as the oxygen analogues of bromonium ions, but unlike bromonium ions they are quite stable. 

As mentioned before, allenes can be formed by prototropic rearrangement of alkynes or, if an appropriate hydrogen is present in the allylic position, by direct elimination. The bromovinyl ether yields, in a trans elimination, the alkyne ether (Scheme 39), but the other isomer, where trans elimination is not possible, gives both the alkynyl ether and the allenyl ether (Scheme 40). This corresponds to the problem of Hofmann and Zaitsev orientation in alkene synthesis. 

Eliminations. Homopropargylic alcohols are formed from 3,3-disubstituted 2-methyleneoxetanes. Alkenyl triflates give alkynes. ... 

Because of P-elimination of palladium, this reaction cannot be used with most alkyl halides. However, vinyl halides are fine as their palladium o-complexes do not undergo P-elimination to give alkynes. Here is a simple example. 

Shi et al. [161-164] studied a different reaction of an array of triphenylmethane and triarylmethane derivatives such as alkanes, alkenes, alkynes, phosphonates, phosphonic acids and esters, dialkylamines, triaryl acetic acid, triaryl acetonitriles, triaryl acetates, and tetraarylmethanes and published a review of their work [65]. Mainly from product studies they proposed the special case of di-Jt-methane and oxa-di-7u-methane reactions [165], viz. a,a-elimination gives a biaryl and the corresponding carbenes and operates in polar and nonpolar sol-... 

Allenes. Substituted allenes are prepared from ketones via elimination of the enol phosphates. Interestingly, enol Inflates tend to give alkynes. The method can be applied to protected aminoalkenyl phosphates. Silyl enol ethers" also undergo elimination. 

The double elimination of HHal from 1,1- and 1,2-dihalogeno-alkanes to give alkynes (terminal and internal) under very mild conditions is preparatively very simple in petroleum ether, using solid KOBu and catalytic amounts of 18-crown-6 polyether.Different transition-state structures within the E2 mechanism, as well as different initial-state solvation conditions, have been proposed to rationalize the effects of equimolar amounts of crown ether and base on the dehydrochlorination of (p-ClC6H4)2CH2CH(3 x)Ch (x = 1, 2, or 3). ... 

Numerous carbo- and heterocyclic compounds of various sizes have been prepared by HR-type monocyclization. Cyclopropanes are formed only when neopentylpalladium intermediates are formed in the absence of anions. Intramolecular carbopalladation of the alkenyl triflate 201 generates the neopentylpalladium 202, and its 3-exo cyclization and jd-H elimination afford the cyclopropane 203 [95]. Intermolecular carbopalladation of the alkyne 204 generates 205. Its 5-exo cyclization provides the neopentylpalladium 206, which undergoes 3-exo cyclization to construct the three-membered ring 207. Finally y3-H elimination gives rise to the cyclopropane 208 [96]. 

The indenone 70 is obtained by the reaction of o-iodobenzaldehyde (66) with alkyne [22,23], Two mechanisms are suggested. One of them involves the formation of Pd(IV) species 68 from 67 by oxidative addition of aldehyde, and its reductive elimination affords the indenone 70. Another possibility is the insertion of carbonyl group (or nucleophilic attack) to form the indenyloxypalladium 69, and /i-H elimination gives the indenone 70. 

The second type of reactions proceed by transmetallation of the complexes 1. MR (M = main group metals) and metal hydrides MH undergo the transmetallation with 1 to generate 6. Subsequent reductive elimination gives the allene derivative 7. Also reactions of 1 with 1-alkynes in the presence of Cul to afford allenylalkynes belong to this type. 

Cyclization of the 1,6-enyne 99 with HSnBu3 provided the cyclopentane 102 at room temperature. Ligandless Pd(OAc)2 was used. In this reaction, regioselective hydropalladation of the alkyne generates 100 at first, and subsequent alkene insertion yields 101. Finally reductive elimination gives rise to the cyclized product 102 [30]. 

An alkyne is hydrolyzed to an enol, which tautomerizes to a carbonyl, and halo-genation of a carbonyl followed by elimination gives an alkyne. 

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