Enolates vinyl halides

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Vinylic halides can react by a SrnI mechanism (p. 855) in some cases. An example is the FeCl2 catalyzed reaction of l-bromo-2-phenylethene and the enolate anion of pinacolone (t-BuCOCH2 ), which gave a low yield of substitution products along with alkynes. ... 

Vinylic lithium reagents (26) react with silyl peroxides to give high yields of silyl enol ethers with retention of configuration. Since the preparation of 26 from vinylic halides (12-37) also proceeds with retention, the overall procedure is a... 

Unfortunately, it quickly became apparent that a shortfall in this proposal was an inability to prepare the desired vinyl halide 25 in a straightforward and selective manner [19]. In contrast, we reasoned that the selective formation of an enol sulfonate, such as the enol triflate 26a, could be controlled by judicious tuning of enolization conditions starting from the corresponding ketone, and that such an enol sulfonate would possibly be a substrate for a palladium-mediated coupling (Scheme 9.17). In this way a common intermediate from the previously defined synthesis, that is, the racemic ketone rac-13 or its cyano equivalent rac-5 could be used to generate the required enamide. 

Alkenyl trifluoromethanesulphonates (enol triflates) undergo Heck coupling with alkenes efficiently (equation 123)209a 215. This reaction is a useful variation of the use of vinyl halides not only because they are easy to prepare from the corresponding carbonyl compounds, but also because yields are good, and the stereochemistry of the triflate is largely maintained. 

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

This preparation illustrates an efficient two-step process for the transformation of a cycloalkenone to the corresponding a-substituted derivative. The first step involves the installation of an a-iodo substituent by a process thought to involve nucleophilic addition of pyridine, iodine capture of the resulting enolate, and pyridine-promoted elimination of pyridine.5 The resulting vinyl iodides are superior to other vinyl halides as participants in a variety of transition-metal catalyzed coupling reactions, illustrated here by the Suzuki coupling with an arylboronic acid. Other coupling partners that... 

Photolysis of vinyl halides can induce both heterolysis of the C-X bond, thereby generating vinyl cations, and homolysis giving vinyl radicals. This competition between the two mechanisms was studied for 3-vinyl halides, 1,2,2-triphenylbromoethane (136) and 1-phenyl-2,2-bis(o-methoxyphenyl)-l-bromoethene and /3-styrene. Incursion of the photo-induced SrnI process, through the intermediate vinyl radical, is verified in the presence of reducing nucleophiles, such as the enolate ions of ketones and in part with (EtO)2PO . Incursion of the heterolytic pathway and the intermediacy of the radical cation, occurs in the presence of weak electron-donor anions, such as N02, Ns and Cl . The vinyl cation of /3-styrene gives phenylacetylene via an El-type elimination. 

The radicals generated from esters of halogenodifluoroacetic acid or halogenodi-fluorophosphonic acid add onto olefins and enolates. When these reactions are intramolecular, they afford tetrahydrofurans. In the presence of copper dust, ethyl bromodifluoroacetate can couple with aromatic and vinyl halides or can add onto Michael acceptors (Figure 2.13). ... 

Enol Ethers and Esters 0-15 O-Alkylation of carbonyl compounds with diazo alkanes 0-17 Transetherification 0-20 Reaction between acyl halides and active hydrogen compounds 0-23 Transesterification 0-24 Acylation of vinylic halides 0-94 Alkylation with ortho esters 0-107 O-Acylation of 1,3-dicarbonyl compounds... 

The most simple a-aryl substituted vinyl halide studied, a-bromostyrene, flash photolytically yields the enol of acetophenone in aqueous solution, which has been used to measure rates of ketonization302,303. With a,/2-diarylvinyl halides, loss of the /2 (vinylic) proton from the intermediate vinyl cation is competitive with nucleophilic trapping Irradiation of compounds such as 96 in methanol gives about equal amounts of tolan and... 

Examples are the reaction of the nitronate anion 4.14 with p-nitrobcnzyl chloride 4.15, and the reaction of the pinacolone enolate 4.16 with bromo-benzene 4.17. The former might have been a straightforward SN2 reaction, but actually takes the SrnI pathway because the nitro groups make the electron transfer exceptionally easy. The latter cannot take place by a conventional Sk2 reaction, because aryl (and vinyl) halides are not susceptible to direct displacement, and the SrnI pathway overcomes this difficulty. 

The reaction of less electrophilic halides, particularly aryl and vinyl halides, can be catalysed with a palladium compound (equations 14-63 and 14-64),115 and the tin enolate can be prepared in situ from the enol acetate and tributyltin methoxide, or lithium enolate and tributyltin trifluoroacetate, or silyl enolate and tributyltin fluoride. 

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