Stannanes aryl, palladium-catalyzed coupling

The same methodology has also been applied to the synthesis of stannylated furans. Heating bis(tributylstannyl)acetylene and 55 in a sealed mbe at 185°C for 10 days provided 3,4-bis(tributylstannyl)furan 139 and 3-(tributylstannyl)furan 140 in 22 and 10% isolated yield, respectively (Fig. 3.41). These stannanes underwent palladium-catalyzed couplings with aryl, vinyl, and benzylic halides as well as acid chlorides. 

The added advantage of the C (1 )-stannylated glycals is their abUity to participate in palladium-catalyzed coupling reactions with organic halides, a process independently reported by Beau [75] and Friesen [81]. Vinyl stannane 237 can be benzylated, allylated or acylated provided that appropriate catalysts are used [75,77] and representative examples are given in Scheme 59. The C-arylation of... 

Pyrrolyl- and indolyl-stannanes and -boronic acids, which can be prepared from the corresponding organolithium derivatives, have received increasing use in palladium-catalyzed coupling reactions with aryl halides (Scheme 82) (91S613,92JOC1653). 

The alkaloid dubamine contains a single bond between the two heteroaiene units. This iond was formed in 79% yield by the generally valuable palladium-catalyzed coupling of an ryltrimethylstannane with an aryl triflate (see section 1.6). The requisite stannane was prepared from l,3-benzodioxol-5-yl triflate and hexamethykfistannane with the same palladium atalyst, the triflate ester was obtained from 2(lff)-quinolinone and trifluoromethanesulfonic. nhydride (A.M. Echavarren, 1987). An earlier attempt to perform this aryl coupling by dassical means gave a yield of only 1 %. 

Aldous and co-workers also reported extensive results on palladium-catalyzed coupling reactions. They introduced the utility of electron-deficient pyridazinyl triflate 136 in Stille and Suzuki biaryl cross-couplings with electron rich aryl stannanes and aryl boronales 137 <01SL150>. Electron deficient aryl stannanes resulted in more sluggish reactions and poor yields. In general, Suzuki reactions were found to be better than the corresponding Stille couplings. 

In contrast, the Stille reaction was one of the first cross-coupling reactions performed on solid support. For the palladium-catalyzed coupling of various aryl, vinyl, or alkynyl stannanes with aryl or alkenyl bromides, iodides, or triflates, the reaction conditions employed were chosen in analogy to the liquid phase procedures and often feature an arsine or trifurylphosphine as an added ligand. Due to the tedious separation of tin and organo tin reagents or by-products in solution phase, this carbon-carbon bondforming reaction is particularly suitable for solid-phase synthesis. ... 

Li and Yue also reported the intermolecular palladium catalyzed cross coupling reactions of bromo quinoxalines 214 and 216 with aryl boronic acids and heterocyclic stannanes, respectively <99TL4507>. The Suzuki couplings (i.e., 214 215) required the use of a... 

In 2001, Nolan described the palladium/imidazoilium salt-catalyzed coupling of aryl halides with hypervalent organostannanes.The imidazolium salt 36 in combination with Pd(OAc)2 and TBAF was found to be most effective for the cross-coupling of aryl bromides and electron-deficient aryl chlorides with aryl and vinyl stannanes. 

Scheme8.5. Palladium-catalyzed cross-coupling reactions of stannanes and other carbon nucleophiles with aryl, allyl, and vinyl bromides [56, 69-72],...

The palladium-catalyzed cross-coupling reaction of a vinyl or aryl stannane with an arylhalogenide or -triflate is known as a Stille reaction. The mechanism of this Stille reaction is outlined below The palladium precatalyst loses two ligands and forms the catalytic species 36. The catalytic cycle starts with the oxidative addition of the catalytic species 36 into the carbon-triflate bond of 23 forming complex 41, which, however, does not undergo the required transmetallation step with stannane 22. Therefore, the triflate ion is... 

Tin, zinc, boron, and mercury metallopyrimidines in alkylation and arylation reactions Acylation by electrophilic substitution Acylation by palladium-catalyzed reactions with stannanes Tin metallopyrimidines in acylation reactions Coupling reactions in quinazolines and perimidines Alkylation and arylation by organometallic adduct formation... 

Boronic acids derived from 3- and 5-substituted indoles are useful intermediates for palladium-catalyzed vinylation, arylation, and heteroarylation (Equation (47)) <92H(34)1395, 93TL2235>. As with the stannanes, a reverse process can be utilized. 6- and 7-Bromoindole can be coupled directly with arylboronic acids to give the 6- and 7-arylindoles, respectively a feature of the reaction is that protection of the indole nitrogen is unnecessary (Equation (48)) <94SL93>. A new route to 2-substituted indoles has been developed from trialkyl-(l-methylindol-2-yl)borates. The borate (153),... 

The most important palladium-catalyzed processes include Heck vinylation of halides and sulfonates and various cross-coupling reactions in which a nucleophilic intermediate (stannane, organo-zinc halide or boronic acid) is coupled with an electrophile (halide or sulfonate). These coupling reactions are usually restricted to arylation and vinylation because of the tendency of alkylpalladium species to undergo elimination. The pyrrole and indole rings can participate in cross-coupling as either the nucleophilic or electrophilic component. 

Mann et al. showed that various acetophenones can be synthesized by carbonylative coupling of aryl triflates with SnMe4 [65]. Under mild conditions using the catalyst system Pd(OAc)2/dppp, good yields (70-94 %) were obtained (Scheme 4.32). In 2002 Mido and colleagues reported the palladium-catalyzed carbonylative coupling of tributyl(l-fluorovinyl)stannane with aryl halides and aryl triflates [66]. In the presence of a catalytic amount of palladium catalyst and CsF in DMF, aryl 1-fluorovinyl ketones were formed in good yields. 

Several groups have reported procedures for palladium-mediated cross-coupling of indoles. This can involve the indole reacting as the nucleophilic (eg. stannane, zinc or boronic acid derivatives) or electrophilic (halide or triflate) component. The variety of such procedures that are now available indicates that Pd-catalyzed cross-coupling is the most versatile method for synthesis of many aryl and vinyl indoles. 

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