Elimination to Give Alkynes

The El process is very unlikely vinyl cations are quite unstable. Both E2 and ElcB are viable options vinyl anions are reasonably stable (remember the orbital effect in considering anion stability). 

FIGURE 10.34 Possible mechanisms of elimination to give alkynes. 

FIGURE 10.36 Synthesis of alkynes as intermediates in totai synthesis. 

FIGURE 10.37 Elimination of HBr from bromobenzene to give benzyne. 

The two groups that have been eliminated here are carboxylate and chloride ion. The first thing that happens when we add triethylamine to the starting material is the deprotonation of the carboxylic acid to give a carboxylate anion. Loss of CO2 is always a good reaction, because of the entropy gain from a mole of escaping gas. The elimination has no choice but to be sy -coplanar. 

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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. 

The 2-substituted 3-acylindoles 579 are prepared by carbonylative cycliza-tion of the 2-alkynyltrifluoroacetanilides 576 with aryl halides or alkenyl tri-flates. The reaction can be understood by the aminopalladation of the alkyne with the acylpalladium intermediate as shown by 577 to generate 578, followed by reductive elimination to give 579[425]. 

Starting from 63, the carbonylation may proceed via coordination and insertion of CO into the vinyl-C-Pd bond to provide an a,P-unsaturated acyl complex. This complex reacts with (ArY) 2, and subsequently the C-Y bond is formed by reductive elimination to give 64 (Scheme 7-14). Because the compound 64 could be directly converted into the corresponding enal 65 by the Pd-catalyzed reduction with BujSnH, this sequence is synthetically equivalent to the regio- and stereoselective thioformy-lation and selenoformylation of alkynes (Eq. 7.49) [53, 54]. 

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

Based on his previous work on the catalytic double addition of diazo compounds to alkynes173 using Cp RuCl(COD),174 Dixneuf has developed an efficient one-step synthesis of alkenyl bicyclo[3.1.0]-hexane derivatives of type 163 from enyne precursors 162 (Scheme 43). The catalytic cycle starts with the formation of an Ru=CHR species. It then adds to an alkyne to form ruthenacyclobutene 166, which evolves into vinylcarbene 167. [2 + 2]-Cycloaddition of 167 gives ruthenacyclobutane 168. The novelty in this transformation is the subsequent reductive elimination to give 170 without leading to the formation of diene 169. This can be attributed to the steric hindrance of the CsMes-Ru group. 

Coordination of Ni(0) to the alkyne gives a n complex, which can be written in its Ni(II) resonance form. Coordination and insertion of another alkyne forms the new C6-C7 bond and gives a nickelacyclopenta-diene. Maleimide may react with the metallacycle by coordination, insertion, and reductive elimination to give a cyclohexadiene. Alternatively, [4+2] cycloaddition to the metallacycle followed by retro [4+1] cycloaddtion to expel Ni(0) gives the same cyclohexadiene. The cyclohexadiene can undergo Diels-Alder reaction with another equivalent of maleimide to give the observed product. 

Zhang has proposed a mechanism for the rhodium-catalyzed Alder-ene reaction based on rhodium-catalyzed [4-1-2], [5-i-2], and Pauson-Khand reactions, which invoke the initial formation of a metallacyclopentene as the key intermediate (Scheme 8.1) [21]. Initially, the rhodium(I) species coordinates to the alkyne and olefin moieties forming intermediate I. This intermediate then undergoes an oxidative cycHzation forming the metallacyclopentene II, followed by a y9-hydride elimination to give the appending olefin shown in intermediate III. Finally, intermediate III undergoes reductive elimination to afford the 1,4-diene IV. 

Alkenes substituted with potential leaving groups are masked alkynes and are thus useful alternative dipolarophiles. They react with diazo compounds, producing pyrazolines, which can undergo elimination to give 3//-pyrazoIes. 

After addition of the alkyne anion to the alkylborane, iodination facilitates alkyl group migration from boron to carbon in a transfer that resembles the one seen in the synthesis of (Z)-alkenes described in Section B4.1. Elimination to give the product alkyne occurs under the iodination reaction conditions (Figure B4.4). 

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. 

Enol phosphates treated with NaNH2 in liquid ammonia or with lithium dialkylamides give the alkynes in good yields (Scheme 49). Vinyl selenoxides can also be used as a starting material for the preparation of alkynes. Thermolysis at 85-95 C takes place in the presence of DABCO if a syn elimination to the alkyne is possible (Scheme 50). If a syn elimination to an alkyne is blocked by a substituent, allene formation can occur. ... 

Not only can 1° and 2° C(sp3)-X undergo /3-elimination by the E2 mechanism under basic conditions, but so can 3° C(sp3)-X systems. Alkenyl halides also undergo /5-el i mi nation readily. When there are H atoms on either side of an alkenyl halide, either an alkyne or an allene may be obtained. Even alkenyl ethers (enol ethers) can undergo /3-elimination to give an alkyne. 

The problem with this scenario is that alkynes are more acidic than most hydrocarbons (pK 25), but they are not sufficiently acidic to be deprotonated by amines (p/temperature required for the Sonogashira coupling from >100 °C to room temperature. The Cul may convert the alkyne (RC=CH) to a copper(I) acetylide (RC=C-Cu), a species that can undergo transmetallation with Pd(II). Of course, now the question is, How is RC=C-H converted to RC=C-Cu The alkyne may form a tt complex with Cu, and this complex may be deprotonated (E2-like elimination) to give the Cu acetylide, which can transmetallate with Pd. 

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). ... 

The reaction is explained by the following mechanism. At first, Cul activates 1-alkynes 1 by forming the Cu acetylides 6, which undergo transmetallation with arylpalladium halides to form the alkynylarylpalladium species 7, and reductive elimination to give 2 is the final step. However, the coupling proceeds even in the absence of Cul under certain conditions, and it may be possible to form the alkynylarylpalladium species 7 directly from 1-alkynes. As another less likely possibility, carbopalladation of a triple bond with Ar-Pd-X (or insertion of the triple bond to Ar-Pd-X) generates the alkenylpalladium 8 which undergoes dehydropal-ladation to afford disubstituted alkynes 2. In this mechanism, Cul plays no role. The mechanism of -H elimination of alkenylpalladium to form alkynes is not clearly known. 

Two reactions via the propargylpalladiums 2 (12), namely, hydrogenolysis to form alkynes 13 and j6-H elimination to give enynes 14, are known. 

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