Vinylic triflate

Vinylation can also be done by Pd-catalysed cross-coupling in which one component is used as a halide or triflate and the other as a stannane (Stille reaction) or boronic acid (Suzuki reaction). Entry 9, Table 11.3, is an example of the use of a vinylstannane with a haloindole. lndole-3-boronic acids, which can be prepared by mcrcuration/boration, undergo coupling with vinyl triflates (Entry 10). 

Highly unstable vinyl cations, generated in situ from vinyl triflates have also been arylated (the triflate group is replaced by the aromatics) to give vinyl aromatics under Friedel-Crafts conditions (28). 

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. 

An important feature of the Stille reaction is that it is not particularly susceptible to steric effects. Indeed, vinyl triflate 120, despite its somewhat hindered nature, couples smoothly with the indicated vinylstannane in the presence of a catalytic amount of Pd(PPh3)4 and LiCl to give 1,3-diene 122 in 90% yield (see Scheme 32).49a As expected, vinyl triflate 119 is converted to the regioisomeric 1,3-diene 121 under identical conditions. 

The key step in a short and efficient synthesis of pleraplysillin-1 (127) is the palladium-catalyzed cross-coupling of vinylstannane 125 with vinyl triflate 126 (see Scheme 33). This synthesis is noteworthy in two respects. First, vinyl triflate 126 is generated regio-specifically from the kinetic enolate arising from a conjugate reduction of enone 124 the conjugate reduction of an enone is, in fact, a... 

Vinyl triflates derived from lactone enolates are also viable coupling partners In Ni(ll)/Cr(ll)-mediated carbon-carbon bond forming reactions... 

For a review of vinyl triflate chemistry, see Ritter, K. Synthesis 1993, 735. 

More recently, Dodd and co-workers reported that treatment of triflate 24 (Scheme 3.9) with benzylamine in DMF afforded aziridine lactone 25 in 43% yield [32], Treatment of vinyl triflate 26 with (J )-phenylethylamine 27 gave a mixture of diastereoisomers 28 and 29 in 71% yield and a ratio of 1 1 [33]. 

S)-N-methyl-2-vinyloxymethylpyrrolidine have also been used as an alkene partner in Heck reactions on vinyl triflates under microwave irradiation [91]. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

For conversion of vinylic triflates to carboxylic esters and amides, see Cacchi, S. Morera, E. Ortar, G. Tetrahedron Lett., 1985, 26, 1109. 

Vinylic copper reagents react with CICN to give vinyl cyanides, though BrCN and ICN give the vinylic halide instead." Vinylic cyanides have also been prepared by the reaction between vinylic lithium compounds and phenyl cyanate PhOCN." Alkyl cyanides (RCN) have been prepared, in varying yields, by treatment of sodium trialkylcyanoborates with NaCN and lead tetraacetate." Vinyl bromides reacted with KCN, in the presence of a nickel complex and zinc metal to give the vinyl nitrile. Vinyl triflates react with LiCN, in the presence of a palladium catalyst, to give the vinyl nitrile." ... 

Coupling of vinylic triflates with organometallic reagents... 

Hanack, Schleyer, Stang and co-workers (167) recently have independently achieved the synthesis of vinyl triflates from ketones. This procedure. 

Solvolyses of these cyclic vinyl triflates at 100 in 50% aqueous ethanol, buffered with triethylamine, lead exclusively to the corresponding cyclo-alkanones. Treatment of 176 with buffered CH3COOD gave a mixture of cyclohexanone (85%) and 1-cyclohexenyl acetate (15%). Mass spectral analysis of this cyclohexanone product showed that the amount of deuterium incorporation was identical to that amount observed when cyclohexanone was treated with CH3COOD under the same conditions. This result rules out an addition-elimination mechanism, at least in the case of 174, and since concerted elimination is highly unlikely in small ring systems, it suggests a unimolecular ionization and formation of a vinyl cation intermediate in the solvolysis of cyclic triflates (170). The observed solvent m values, 174 m =. 64 175 m =. 66 and 16 m =. 16, are in accord with a unimolecular solvolysis. 

The mechanism of reaction of primary vinyl triflates and the possibility of solvolytic generation of primary vinyl cations needs further exploration. Along these lines, an examination of the behavior of the simplest vinyl system CH2=CHOTf and the possibility of generating the parent vinyl cation needs to be done. 

Tetrahydropyridine-4-carboxylic acid 113 and related 2- and/or 6-methylated analogs were synthesized using the methoxycarbonylation of their corresponding vinyl triflates <96H(43)2131>. 

Pretreatment with nickel bromide causes normally unreactive aryl chlorides to undergo Pd-catalyzed substitution,139 and aryl and vinyl triflates have been found to be excellent substrates for Pd-catalyzed alkenylations.140... 

The 6 -phosphines dppe, dppp, and dppb were also effective for this coupling. Entry 3 is an example of use of a vinyl triflate. Entries 4 and 5 illustrate the use of perfluorobu-tanesulfonate (nonaflate) as an alternative leaving group to triflate. The organozinc... 

Similarly, nickel catalysis permits the extension of cross coupling to vinyl phosphates, which are in some cases more readily obtained and handled than vinyl triflates.273... 

Vinyl ethers and vinyl esters are not employed extensively in organic transformations. During the synthesis of phlegmarine, however, the stereogenic center at C-10 was to be introduced stereoselectively by catalytic hydrogenation-hydrogenolysis of the vinyl triflate (Scheme 4.7). 

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