Stille cross coupling Mechanism

Scheme 1.6 (a) General conditions for the Stille cross-coupling reaction, (b) The proposed mechanism for the Stille cross-coupling arylation procedure [12a]. 

In the Stille cross-coupling, an organotin compound, such as a trimethylstannyl or tributylstannyl species, is coupled with an sp -hybridized halide in the presence of a catalytically active complex of Pd(0). The mechanism has been well... 

Even with the limitation on yield implied by the statistical process, cross-dimerization is still useful when one of the reactants is an alkane, because the products are easy to separate, and because of the few other ways to functionalize an alkane. The cross-coupling of an alkane with trioxane is especially valuable, because hydrolysis of the product (10-6) gives an aldehyde, thus achieving the conversion RH RCHO. The mechanism probably involves abstraction of H by the excited Hg atom, and coupling of the resulting radicals. 

A new C-C bond is formed between a nucleophilic C-Sn and an electrophilic C-Br. This Stille coupling proceeds through the standard oxidative addition, transmetallation, reductive elimination process characteristic of Pd-catalyzed cross-couplings. The mechanism was discussed in the text (Section 6.3.4). 

Dichloropyridine reacts in the dark (88% of disubstitution), whereas 2,6- and 3,5-dichloropyridines require photostimulation to afford ca. 80% yield of disubstitution products [26]. Aryltrialkylstannanes are valuable intermediates in organic synthesis, and the fact that they can be easily synthesized through the SRN1 mechanism, opens up important synthetic routes to different reaction schemes. For over a decade, the Pd(0)-catalyzed cross-coupling of organotin compounds with electrophiles, known as the Stille reaction, has been a very important tool in product design [104]. 

Mechanism The reaction proceeds first by the oxidative addition of organohalide to the Pd(0) complex to give a palladium(II) intermediate as in the case of Stille coupling. The Pd(II) complex then undergoes transmetallation with the base-activated boronic acid to give complex B. This is followed by reductive elimination to form the active Pd(0) species, HX and the cross-coupled product (Scheme 5.17). 

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... 

The general mechanism for the cross coupling of organostannanes and aryl halides (the Stille reaction) is given in Scheme The transmetalation step takes place... 

Several new alternatives to the classical methods of synthesis of biheterocycles have been developed. Most notable among these are low-valent transition metal-catalyzed homo-coupling reactions, which are particularly applicable to the synthesis of biheterocycles, and which give much better yields than the earlier Ullmann and Busch procedures. Furthermore, un-symmetrical biheterocycles are now more readily available by way of palladium-catalyzed cross-coupling reactions, such as the Stille and the Suzuki procedures. Thus, many new members of the biheterocyclic series are now available for study. As a consequence of the increased sophistication of molecular structure determination techniques, aromatic biheterocycles have been the subject of many recent spectroscopic and X-ray crystallographic studies, as well as numerous molecular orbital and molecular mechanics calculations. 

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