Dienes from vinyl triflates

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

In reactions of alkenyl triflates with stereogenic C=C double bonds, coupling reactions of these kinds convert the Csp2—X bond of the alkenyl triflate into the Csp2—C bond of the substitution product with complete retention of configuration. The stereoselective synthesis of a 1,3-diene from an alkenyl triflate and (vinyl)2CuLi provides an example (Figure 13.1). 

Later, the dimethylphenylsilyl adduct 75 d was not only converted to the vinyl-ketene acetal 81 by a Wittig-Horner reaction via the ester 80, but also by reduction of 80 to the aldehyde 84 followed by silylation of the corresponding anion to the silylenol ether 85. Vinylcyclohexenes without terminal substitution (e. g., 83) was prepared via an alcohol derived from 80 or 84 [73,74]. The least substituted diene 83 was alternatively prepared from the triflate 82 in a Stille coupling with trialkylvinyl stannane [75] (Scheme 24), a reaction also used by Toshima etal.[76]. 

Vinyl triflates (C=C-0S02CF3) react with vinyl tin derivatives in the presence of palladium catalysts to form dienes, in what is known as the Stille coupling. Vinyl triflates can be prepared from the enolate by reaction with Al-phenyl trifli-... 

Three cyclopropyl-substituted vinyl triflates have yielded mixtures of cyclopropylpropa-1,2-diene, 1-cyclopropylpropyne and other products when subjected to solvolysis conditions in aqueous 2,2,2-trifluoroethanol buffered with pyridine. Table 14 shows the product distributions obtained from the three substrates after three days at 80 "C for different concentrations of 2,2,2-trifluoroethanol. 

Now let us look at a complete synthesis.28 The natural product pleraplysillin-1 199 is found in a marine sponge and in the nudibranch (a kind of apparently defenceless shell-less mollusc) that eats it. It has a defensive role - the nudibranch is rapidly rejected as a food source by carnivorous fish .29 Pleraplysilin-1 has a diene joined to a furan at the difficult 3-position. Disconnection between the two double bonds suggests a vinyl triflate 201 from a cyclic alkene, as we can make that regioselectively from the corresponding ketone, and a vinyl stannane from the furan half 200. 

Hydrogenation with Raney nickel removes the chiral auxiliary and, as we had hoped, hydrogenates the alkene from the less hindered exo-face, pushing the side chain inside the folded molecule 83. The ketone is converted into the only possible vinyl triflate for Stille coupling with a vinyl stannane to give the diene 84. 

When the Uthium enolate intermediate 9 was trapped by addition of a solution of Af-phenyltriflimide in hexamethylphosphoramide (HMPA) or fVA/ -dimethylpropylene urea (DMPU), the reaction smoothly delivered the Z-vinyl triflate 11 in 74% yield (Scheme 3). With the pure Z-vinyl triflate in hand, we turned our attention to its palladium-catalyzed cross-coupling with tin species. Under optimized crmditions, Pd2(dba)3 and triphenylarsine in fV-methyl-2-pyrrohd(Mie (NMP) at 40 °C, the cross-coupled products were obtained in good yields with retention of the Z-stereochemistry of the starting Z-vinyl triflate. These optimized conditions appUed to tributyl(vinyl)stannane and the Z-vinyl triflate from 11 dehvered the Z-diene compoimd 12,... 

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). ... 

Catalytic hydrogenation of both double bonds yielded the quinolizidine-1,4-dione ( )-2234, the ketone of which was converted into a vinyl triflate before Stille coupling with aUyltributylstannane afforded the conjugated diene ( )-2235. Hydrogenation then produced a 5 2 mixture of racemic di-astereomers 2236 and 2237, which were separated by HPLC. Finally, addition of allylmagnesium chloride to the separated lactams followed by reduction of the adducts with sodium cyanoborohydride gave ( )-quinolizidine 2071 (rac-1728) and ( )-l-ept-quinoHzidine 2071 (rac-2238) as the sole isomers from 2236 and 2237, respectively. 

Kobayashi et al. found that lanthanide triflates were excellent catalysts for activation of C-N double bonds —activation by other Lewis acids required more than stoichiometric amounts of the acids. Examples were aza Diels-Alder reactions, the Man-nich-type reaction of A-(a-aminoalkyl)benzotriazoles with silyl enol ethers, the 1,3-dipolar cycloaddition of nitrones to alkenes, the 1,2-cycloaddition of diazoesters to imines, and the nucleophilic addition reactions to imines [24], These reactions are efficiently catalyzed by Yb(OTf)3. The arylimines reacted with Danishefsky s diene to give the dihydropyridones (Eq. 14) [25,26], The arylimines acted as the azadienes when reacted with cyclopentadiene, vinyl ethers or vinyl thioethers, providing the tet-rahydroquinolines (Eq. 15). Silyl enol ethers derived from esters, ketones, and thio-esters reacted with N-(a-aminoalkyl)benzotriazoles to give the /5-amino carbonyl compounds (Eq. 16) [27]. The diastereoselectivity was independent of the geometry of the silyl enol ethers, and favored the anti products. Nitrones, prepared in situ from aldehydes and N-substituted hydroxylamines, added to alkenes to afford isoxazoli-dines (Eq. 17) [28]. Addition of diazoesters to imines afforded CK-aziridines as the major products (Eq. 18) [29]. In all the reactions the imines could be generated in situ and the three-component coupling reactions proceeded smoothly in one pot. 

Reaction of l-chloro-l,3,3-triphenylallene (19) with p-toluidine in the presence of silver triflate gave 2,3,4-triphenylbuta- l-aza-1,3-diene derivative 22a via a novel l, 2-phenyl shift not reported earlier in the solvolysis of allenyl chlorides. The reaction takes place via the formation of the allenyl cation, which is captured as its canonical propargyl cation, first affording the protonated amine 20 (R = Me). Proton transfer from the nitrogen to the acetylenic carbon is followed by migration of a phenyl group in the intermediate vinyl cation 21 (R = Me) to afford the iminium triflate 22a. The latter is hydrolyzed by aqueous sodium hydroxide to the azabutadiene 23 (equation 5). Similar reaction of 19 with aniline and silver triflate afforded the corresponding iminium triflate 22b. ... 

The kinetics of the Cope rearrangement have been measured using 1,1-dideuteriohexa-1,5-diene and the possibility of a boat transition state in the Cope rearrangement of hexa-1,5-diene has bwn examined. Trifluoromethanesulphonates (triflates) have been used to study the stereochemistry of solvolytic displacement at unsaturated (vinyl sp ) carbon. Solvolysis of the (Z)- and ( -triflates (611) and (612) in trifluorethanol gives rise to dissimilar ratios of products (613) and (614) a greater proportion of (613) is formed from (611) than from (612). The resultsarebestaccommodated... 

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