Thallium triflate

Oxidation of easily enoUzable ketones such as alkyl aryl ketones affords either a-oxylated ketones or carboxyhc acid derivatives via oxythallation of the enol forms, the products depending on the kind of Tl(lll) salts and/or solvents employed (Scheme 9.24). Use of TTN usually induces aryl group migration leading to the formation of carboxyhc acids or their esters [26], making this oxidative transformation useful for organic synthesis (Schemes 9.25 [27], 9.26 [28], 9.27 [29] and 9.28 [30]). Use of TTA, thallium tosylate [ l l(OTs), ], thallium triflate [ l1(()Tf), ], and thallium mesylate [ l1(()Ms)j, on the other hand, produces -oxylated ketones which sometimes further react to give useful compounds (Schemes 9.29 [31], 9.30 [32], 9.31 [33], 9.32 [34] and 9.33 [35]). 

TABLE 1.15. 2-ALKYL-5-ARYLOXAZOLES FROM AROMATIC a-METHYL KETONES, NITRILES, AND THALLIUM TRIFLATE"... 

Reagents for oxidation 5,5 -dithio-hw-(2-nitrobenzoic acid) (DTNB, Ellman reagent)", silver triflate (AgOTf)", thallium trifluoroacetate (Tl(tfa)3), iodine, and guanidine HCT (Fluka or Sigma-Aldrich). 

Another approach to the synthesis of phosphirenes by formation of two P-C bonds involves the electrophilic addition of halo- and dihalophosphine-AlCls complexes with alkynes, yielding phosphirenium salts. In some cases, the use of thallium(l) triflate instead of AICI3 improves the method <1997JOM(529)189>. In the normal case, the chlorophosphirenium salts are not isolated but rather immediately reduced with tributylphosphine to furnish the phosphirenes (Scheme 12) <19960M1597>. 

Later, this cycloaddition reaction was improved by the pretreatment of the enamide ester with an equimoler amount of trialkylsilyl trifluoromethanesul-fonate and triethylamine at ambient temperature. The synthesis of tyro-phorine (119) was achieved by the above improved method (Scheme 55). The enamide ester 441 was subjected to annulation using t-butyldimethylsilyl triflate and triethylamine at 15°C to produce the bicyclic lactam 442 in 68% yield. Oxidation of 442 with thallium(III) trifluoroacetate and boron triflu-oroetherate in a mixture of dichloromethane and trifluoroacetic acid at 4°C produced (83%) the pentacyclic compound 443. Hydrolysis of 443, followed by decarboxylation of the resultant acid, gave the pentacyclic lactam 444. Reduction of 444 with sodium bis(2-methoxyethoxy)aluminum hydride in refluxing dioxane afforded tyrophorine (119) (85CC1159). 

Alkyl-5-aryloxazoles. Thallium(III) triflate formed in situ promotes the condensation of alkyl cyanides with aryl ketones (20 examples, 71-86%). 

Compared to the Heck arylation of electron-poor olefins, the analogous arylation of their electron-rich counterparts has been much less widely investigated. The problem of controlling regiochemistry has been solved by using aryl triflates instead of the more thermally stable halides, or by adding stoichiometric amounts of silver or thallium salts. However, low TON (<40) and TOF (<2 h" ) values have been documented. 

Silver and thallium salts have been widely employed with halide substrates, either to increase the rate of a Heck reaction [84], to minimize double-bond isomerization in the product [75], or to modify regioselectivity or enantioselectivity [lg]. These additives divert the Heck reaction to a pathway involving cationic palladium(II) intermediates. It is also possible to divert the Heck reaction of a triflate precursor from the cationic pathway to the neutral pathway (see Scheme 6-1) by the addition of halide salts. In one, possibly exceptional, case studied in our laboratory, the addition of halide salts dramatically improves enantioselectivity [65]. 

The reaction is not restricted to forming biaryls. Aryl boronic acids can also react with vinyl halides and vinyl triflates in the presence of palladium (0). Thallium (I) ethoxide is known to promote Suzuki cross coupling reactions for vinyl or arylboronic acids with vinyl and aryl halide partners. 3 synthetic example is... 

RING EXPANSION Allyl triflate. Chromium carbonyl. Ethyl diazOacetate. Thallium(III) nitrate. 

Lee and Hong" described the first direct synthesis of 2-alkyl-5-aryloxazoles 206 from aromatic a-methyl ketones (Scheme 1.57). They used thallium(III)triflate generated in sim to convert a ketone to the a-ketotriflate 204. Reaction of 204 with a nitrile most likely produces a nitrilium salt 205, as proposed by Meyers and Sicar, which subsequently cyclizes to 206. The reaction works well for aliphatic nitriles regardless of steric hindrance. Aromatic nitriles are not as useful,... 

The selective formation of the arylated or vinylated allylalcohol was rationalized by assuming a cationic four-manbered intermediate, in which the hydrogen atom (H ) on the hydroxyl-bearing carbon is located unfavorably for a syn palladium hydride elimination. The cationic intermediates were generated from organic triflates, iodonium salts,or organic iodide/thallium or silver salt combinations (Scheme 28). 

When the substrate is a triflate, or when a halide is subjected to appropriate reaction conditions in the presence of a halide scavenger (such as silver or thallium salts), the reaction proceeds via the cationic manifold. After oxidative addition, dissociation of X yields cationic intermediate 67. Alkene coordination provides 68 and migratory insertion delivers 69. /S-Hydride elimination then yields the desired product 66. Of importance in the pathway 62- 7— 68— 69— 66 is that both phosphines maintain contact with the metal throughout the process. This is ostensibly the factor responsible for the high enantioselectivities observed for reactions that are thought to proceed along this pathway. This contrasts with the course of events in the neutral pathway, where phosphine dissociation is thought to be responsible for low enantioselectivities. 

The use of phenyl triflate (2) was favoured over phenyl iodide, which gave very poor enantioselection, and it was postulated that employing the triflate allows the reaction to proceed by the cationic as opposed to the neutral pathway (Scheme 11.5). The cationic pathway was proposed as early as 1990 and begins with the dissociation of X (X = triflate or a halide in the presence of a halide scavenger such as silver or thallium cations) from the cationic complex with the accompanying counterion X (X = halide or triflate) [9]. Complexation of the alkene... 

The coupling of electron-rich olefins in conventional solvents often requires additives such as silver triflate or thallium acetate to obtain high a/p selectivity. 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4) is an excellent catalytic system for highly regioselective a-arylation of several electron-rich olefins. Butyl vinyl ether 9 reacted with p-bromobenzaldehyde 10 selectively afp ratio >99 1) to 11 (Scheme 15.3 [42]). Comparative experiments in conventional solvents (e.g. DMF) resulted in product mixtures with aip ratios of 46 54 to 69 31. It has been suggested that the ionic pathway is responsible for the unique regiocontrol in the ionic liquid [48-50]. 

The cationic variant of the asymmetric intramolecular Mizoroki-Heck reaction is illustrated in Scheme 16.1 [8, 9]. This pathway accounts for the outcome of asymmetric Mizoroki-Heck reactions of unsaturated triflates or unsaturated haUdes in the presence of silver(I) or thallium(l) additives. Following oxidative addition of the palladium(O) catalyst, either triflate dissociation occurs or halide abstraction is promoted by silver(l) or thallium(I) salts. This event vacates a coordination site, permitting coordination of the... 

Aside from thallation, trivalent thallium is best known as a versatile oxidant in organic chemistry many of the reactions proceed with unique rearrangements. Let us first consider one that does not involve rearrangement. Thus, thallium(III) triflate (see Section 1.14 for the structure of the triflate anion), which may be obtained from the more common thallium(III) nitrate by treatment with trifluoromethanesulfonic (triflic) acid in DMF, oxidizes acetophenone to its a-trifluoromethanesulfonyloxy derivative ... 

We won t write a mechanism for the formation of the organothallium intermediate it forms via thallation of the enol form of acetophenone, and we urge you to write out the details. A triflate anion, which got kicked out in the first step, then displaces the thallium, which simultaneously undergoes reductive elimination to produce Tl OTf ... 

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