Oxidative addition, Stille coupling, mechanism

The group R1 can be allyl, acyl, or alkynyl, and arynes can also act as the acceptors. The catalysts are usually Ni(cod)2, or ligated palladium. The mechanisms are not understood in detail, but a catalytic cycle involving the product of oxidative addition, Sn-M-R1, is thought to be involved. The stannylalkenes that are formed can then be subjected to reaction with electrophiles (e.g., AczO or RCH=0), or to coupling reactions in the presence of transition metals (e.g., the Stille reaction). 

The mechanism is the same as a regular Stille coupling, except that coordination of CO and insertion into the Pd-C bond intervenes between the oxidative addition and transmetallation steps At some point the TfO- group on Pd is exchanged for a Cl- group. 

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). 

Fig. 16.18. Representative mechanism of the Pd-catalyzed C,C coupling of an organoboron compound. The elementary steps, discussed in the text, are (1) complexation, (2) oxidative addition, (3) transmetalation of the alkenylboron compound to afford an alkenylpalladium compound, (4) reductive elimination, and (5) dissociation of the coupled product from the metal. - Note Regarding the arrangement of the ligands around the metal center of the individual intermediates and the details of the transmetalation the present mechanistic analysis is less complete than the mechanistic analysis of other Pd-catalyzed C,C couplings, namely the Stille coupling (Figure 16.27) or Heck reaction (Figure 16.35, part II), which have been investigated in great detail.

The mechanism is very similar to that of the Stille coupling. Oxidative addition of the vinylic or aromatic halide to the palladium(O) complex generates a palladium(II) intermediate. This then undergoes a transmetallation with the alkenyl boronate, from which the product is expelled by reductive elimination, regenerating the palladium(O) catalyst. The important difference is the transmetallation step, which explains the need for an additional base, usually sodium or potassium ethoxide or hydroxide, in the Suzuki coupling. The base accelerates the transmetallation step leading to the borate directly presumably via a more nucleophilic ate complex,... 

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... 

In his review of 1986, Stille proposed a mechanism based primarily on data obtained from the coupling of benzoyl chloride with tri-n-butyl(phenyl)tin. This proposal already clearly stated four main steps of the catalytic cycle oxidative addition, transmetalation, isomerization, and reductive elimination. 

Another important o-bond activation/formation process discussed in this article is vinyl-vinyl coupling, shown in Scheme 7. Vinyl-vinyl coupling opens a convenient route to conjugated 1,3-dienes and is widely employed in many catalytic coupling reactions. The great potential of the field is still under continuous development [26,27] and, therefore, elucidation of the C-C bond formation mechanism and the factors controlling it are very crucial. In literature, numerous mechanistic studies on C-C reductive elimination and reverse process, oxidative addition (C-C bond activation), have been reported for di-... 

Mechanistically, while the reaction pathways of Pd-catalyzed systems have been properly investigated, the reaction pathway of Ni-catalyzed cross-couphng reactions remains relatively unexplored. On the basis of Stille s proposal [145] and the cross-coupling reactions developed by the Fu group, oxidative addition by a radical mechanism seems reasonable, as the stereogenic center is racemized... 

The Stille Coupling (Section 24.5C) The Stille coupling is the palladium-catalyzed reaction of a vinyl tin reagent with an organic halide or triflate.The mechanism involves oxidative addition of the organic halide/triflate, transmetallation of the vinyl group on Sn to Pd, and reductive elimination to form the new C—C bond. 

The possibilities for the formation of carbon-carbon bonds involving arenes have been dramatically increased in recent years by the use of transition metal catalysis. Copper-mediated reactions to couple aryl halides in Ulknann-type reactions [12, 13] have been known for many years, and copper still remains an important catalyst [14, 15]. However, the use of metals such as palladium [16,17] to effect substitution has led to such an explosion of research that in 2011 transition metal-catalyzed processes comprised more than half of the reactions classified as aromatic substitutions in Organic Reaction Mechanisms [18]. The reactions often involve a sequence outlined in Scheme 6.6 where Ln represents ligand(s) for the palladium. Oxidative addition of the aryl halide to the paiiadium catalyst is followed by transmetalation with an aryl or alkyl derivative and by reductive elimination to give the coupled product and legeuCTate the catalyst. Part 6 of this book elaborates these and related processes. 

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