Stille coupling mechanisms

The mechanism " of the Suzuki reaction is closely related to that of the Stille coupling reaction, and is also best described by a catalytic cycle ... 

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). 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

The self-coupling mechanism still remains unclear, although its synthetic options appear to be rather encouraging. Mechanistically, it is likely that Pd(IV) intermediates were involved. ... 

One difference between the Suzuki mechanism and that of the Stille Coupling is that the boronic acid must be activated, for example with base. This activation of the boron atom enhances the polarisation of the organic ligand, and facilitates transmetallation. If starting materials are substituted with base labile groups (for example esters), powdered KF effects this activation while leaving base labile groups unaffected. 

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). 

This reaction is highly stereoselective. Cine substitution is known with this reaction, and its mechanism has been studied. Using ArSnCl3 derivatives, Stille coupling can be done in aq. KOH. A related reaction couples reagents with C=C-I Ph reagents, in the presence of a palladium catalyst. Aryl halides and het-... 

Problem 6.21. Draw a mechanism for the following Stille coupling. 

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