Synthetic processes transmetallation

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

Low temperatures are necessary with orr/io-halolithiopyridines in order to prevent pyridyne formation, and an added complication with some bromopyridines is that rearrangement of the initial lithio derivative can occur via an intermolecular transmetalation process (Scheme 108)(79T1625 85T3433). This result has been used synthetically to give 2-bromo-3-substituted derivatives from 2,6-dibromopyridine [90JOM-... 

In the original process using tin amides, transmetallation formed the amido intermediate. However, this synthetic method is outdated and the transfer of amides from tin to palladium will not be discussed. In the tin-free processes, reaction of palladium aryl halide complexes with amine and base generates palladium amide intermediates. One pathway for generation of the amido complex from amine and base would be reaction of the metal complex with the small concentration of amide that is present in the reaction mixtures. This pathway seems unlikely considering the two directly observed alternative pathways discussed below and the absence of benzyne and radical nucleophilic aromatic substitution products that would be generated from the reaction of alkali amide with aryl halides. 

The SN2(Sn)(coord) mechanism, however, is familiar in the synthetically important Sn/Li transmetallation reaction that occurs between tetraalkyltin compounds and alkyl lithiums. This process is discussed in Section 19.1.1, and its applications in synthesis in Section 22.1. 

When the C-H activation process is inefficient or difficult to carry out, one can often resort to alternative methods such as transmetallation reactions, as in the synthesis of the interesting cycloruthenated complex 13. The corresponding C-H activation process for 2 gives only a 38% yield." A similar method was employed for the synthetically useful cyclopalladated ketone derivative 15 and for the organomolybdenum complex 17.Oxidative additions of suitably substituted iodo derivatives on Pd(0) were employed to synthesize the primary 8-napthylamine complex 19 and the complexes 21 with a variety of substitution patterns on the thioether group. 

---