Enol triflates palladium complexes

Initially the Pd(0) complex oxidatively adds to enol triflate 6 to form a vinyl-Pd(II) species. Carbon monoxide then inserts into the new Pd—C o-bond to yield a palladium(ll)-acyl complex which captures methanol. The methanolysis step is formally a reductive elimination reaction in which the Pd(0) catalyst is regenerated to propagate the catalytic cycle (Scheme 6.8).7... 

Three new chirality centers are formed with high enantio- and complete diastereoselectivity in the course of the reaction of the enol triflate 37 to the bicyclo [3.3.0]octane derivative 38 (Scheme 11) [15]. In this transformation, the intermediate 39, formed by oxidative addition, leads to the cationic palladium-7r-allyl complex 40, which is finally converted to the isolated product 38 by regio- and diastereoselective nucleophilic addition of an acetate anion. The bicyclic product 38 is of interest as a building block for the synthesis of capnellene sesquiterpenes. 

The neutral palladium(II) compound 43 is transformed by addition of AgOTf into the cationic complex 44. In the presence of water an exchange of the triflate anion to hydroxide occurs (44 — 45). Finally, the palladium enolate 46 is formed from the palladium complex and the silyl enol ether. 

If the electrophile is a vinyl triflate, it is essential to add LiCl to the reaction so that the chloride may displace triflate from the palladium o-complex. Transmetallation takes place with chloride on palladium but not with triflate. This famous example illustrates the similar regioselectivity of enol triflate formation from ketones to that of silyl enol ether formation discussed in chapter 3. Kinetic conditions give the less 198 and thermodynamic conditions the more highly substituted 195 triflate. 

A new catalytic elimination/isomerization pathway of stereodefined enol triflates capable of providing the corresponding functionalized, highly substituted, 1,3-dienes in synthetically useful yields has been discovered. Preliminary mechanistic studies support a distinct catalytic pathway that rationalizes the stark reactivity differences between E and Z enol triflates through stereoisomeric cationic vinyl palladium(II) complexes. 

Freskos reported the synthesis of chiral succinate using Pd-catalyzed carbonylation of enol triflate 26 (Scheme 18) followed by asymmetric hydrogenation of resulting a,/3-unsaturated ester 27 using a ruthenium complex. - " In the absence of HCOOH, the yield was low (15-20%). Similarly, when water was substituted for formic acid, low product yields (15-20%) were observed. As one possible pathway to produce 27, reductive elimination from the palladium complex to yield a mixed anhydride derived from the product and triflic acid would be considered. 

StiiieP l and Ortar independently reported that enol triflate could be oxidatively added to a low-valent palladium complex to give vinylpalladium complex. Since enol triflate is easily prepared from a keto-carbonyl group by treatment with Tf20 or PhNTf2 in the presence of a base, this means that a new carbon-carbon bond formation can be achieved on the carbonyl carbon using a palladium catalyst. 

Up to now, palladium complexes do not play a significant role in the hydroformylation of olefins [1]. However, because of their widespread use in the related hydrocarboxylation, hydroesterification, and olefin copolymerization with CO [2], occasionally their utility for hydroformylation was elucidated [3]. Moreover, palladium catalysts have been used for the hydroformylation of aryl and enol triflates to produce the corresponding unsaturated aldehydes [4]. 

In the total synthesis of the marine terpenoid (-)-frondosin B, Trauner and Hughes described an intramolecular palladium-catalyzed alkenylation reaction between a benzofuran and an enol triflate (124 to 125, Scheme 10.41). Although the mechanism to form the key seven-membered ring is still unclear, a reasonable hypothesis would involve oxidative addition of palladium(0) to the C—OTf bond, C3-palladation of the benzofuran and reductive elimination to form the new C—C bond. This work is notable as it was the first example of heteroaromatic C—H activation in a complex molecule setting. 

Process research efforts relative to the design of a practical synthesis of a structurally complex MRSA carbapenem (1) culminated in the establishment of two distinct synAeses. The first is based on a unique Stille coupling between a carbapenem enol triflate and a stannatrane appended via a methylene spacer to a complex and extended heteroaromatic side chain. The second is founded upon a novel n-allyl palladium reaction to incorporate die entire complex side chain. Details on die development of both methods are described. 

With this end in view, phenyldimcthylsilyl tri-n-butylstannane was added under the influence of zero-valent palladium compound with high regioselectivity and in excellent yield to the acetylene 386 to give the metallated olefin 387 (Scheme 56). The vinyl lithium carbanion 388 generated therefrom, was then converted by reaction with cerium(lll) chloride into an equilibrium mixture (1 1) of the cerium salts 389 and 390 respectively. However, the 1,2-addition of 389 to the caibonyl of 391, which in principle would have eventually led to ( )-pretazettine, did not occur due to steric reasons — instead, only deprotonation of 391 was observed. On the other hand, 390 did function as a suitable nucleophile to provide the olefinic product 392. Exposure of 392 to copper(II) triflate induced its transformation via the nine membered enol (Scheme 55) to the requisite C-silyl hydroindole 393. On treatment with tetrafluoroboric acid diethyl ether complex in dichloromethane, compound 393 suffered... 

Heck-type reactions with enol carboxylates (e.g., vinyl acetate) are generally complex. Most common are reactions in which vinyl acetate is employed as an ethylene equivalent (see Scheme 24). However, an example of a preparatively useful reaction with an intact acetate function is given in entry 44.The reaction of vinyl triflates with vinyl phosphonates affords the corresponding conjugate dienylphosphonates (entry 45).f A new access to reactive nonaflates via a one-pot nonaflation-Heck reaction was recently reported (entry 46). " This reaction sequence starts from silyl enol ethers and provides functionalized 1,3-dienes in a simple manner, lodonium salts can be used as RPd precursors (entry 47). It is notable that the palladium(O) insertion preferentially occurs inbetween the iodonium ion and the vinylic, rather than the arylic sp -hybridized carbon (entry 47). Some years ago, Jeffery used acetylenic halides to achieve (JiJ-enynoates and (Ji)-enynones in fair yields at room temperature (entry 48). ... 

Scheme 5.49 Palladium- and nickel-catalyzed enantioselective reaction of aryl triflates with enolates derived from racemic ketones 149, catalyzed by palladium or nickel complexes with ligand 150.

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