Transmetallation pathway

However, when the Pd-TBAF system is used in the above reaction, the transmetallation from silicon to palladium can be accelerated by the fluoride ion and the reaction pathway is changed from carbometallation to transmetallation pathway (Scheme 5.19). 

The mechanism for this transformation is not clear, but one could speculate that the reaction proceeds via a transmetallation pathway similar to fluoride-induced silicon to palladium transmetallation mechanism. 

SnCU is effective in promoting the addition of nucleophiles to simple aldehydes. Among the most synthetically useful additions are allylstannane and -silane additions. The product distribution in the stannane reactions can be influenced by the order of addition, stoichiometry, and reaction temperature. The anti geometry of the tin-aldehyde complex is favored because of steric interactions. Furthermore, the six-coordinate 2 1 complex is most probably the reactive intermediate in these systems. The use of crotylstaimanes provides evidence for competing transmetalation pathways (Eq. 35) [60]. TiCU results in superior selectivity. 

In a second stereochemical investigation of the mechanism of oxidative addition, tosylate 3 was treated with Pd/P(t-Bu)2Me in the absence of a coupling partner (Scheme 3). With no transmetalation pathway available to the alkyl-... 

This work demonstrated the existence of some competitive transmetallation pathways in the Negishi coupling reaction that had not been invoked before [286b]. 

As a result, in order to distinguish the two potential transmetallation pathways for the formation of the key complex 6, recently the Amatore [13] and Hartwig [14] groups have independently conducted systematic experimental studies to understand this key step. Using a common hydroxide base (NBU4OH) and palladium... 

In this case, most/ if not all, of the reaction proceeds by a transmetalation pathway producing allyltrichlorostannane, which is then responsible for the production of addition... 

Importantly, the results derived from the kinetic model b provided negligible values for kf, thus indicating that the competitive transmetalation pathway via 5+ can be mled out. Besides, both kinetic models consistently provided almost identical values for ks and for 4. ... 

Various allylic compounds react with hard carbon nucleophiles > 25) including organometallic compounds of Zn, B, Al, Sn and Si via a transmetallation pathway (see 1.1.8.3). For example, the reaction of allylic lactone 46 with phenylzinc chloride gave the product 48 in excellent yield. Note the inversion of the stereochemistry at the allylic carbon (see 1.1.8.3). 

Through the use of Inis, Baba et al. had managed to dispense with TMSCI to regenerate the catalyst [30]. Only 10 to 20 mol% of Inis was required for the allylation of aldehydes and aromatic ketones with allyltributyltin, and stoichiometric amounts of Inis was found to inhibit the reaction. Reaction was expected to proceed via an entirely transmetallation pathway owing to the lower Lewis acidity of Inis. Thus, for y-substituted allylic tin compounds, typical anti preference for the addition product was observed. A chelation-controlled addition was used to explain the rate acceleration and higher yields of carbonyl compounds with vicinal alkoxy group, and the predominance of the syn addition product for such compounds (Figure 8.12). 

However, replacement of R=OEt, OMe with R = OSi(Pr-i)j on steric or stereo-electronic grounds completely restores the transmetallation pathway (Scheme 12.4). In support of these arguments, all attempts to effect the above-described sequence with amides 3e,f produces the alkenes 5e and 5f, respectively, but they do not undergo transmetallation reactions [5]. 

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