Pyridylstannanes, preparation

Several different approaches have been employed to prepare pyridylstannanes, one of which involves direct metalation followed by transmetalation with RjSnCl. A regioselective metalation of 2-methoxypyridine was achieved using a mixture of bases comprised of BuLi-LiO(CH2)2NMe2 [53], Subsequent transmetalation with BujSnCl led to 2-methoxy-6-(tributylstannyl)pyridine (68), which was then coupled with pyridyl-, pyrimidyl-, and quinolinyl bromides in 31-56% yield. 

The third methodology in pyridylstannane synthesis is unique to 2,6-dihalopyridines whose synthesis using the conventional lithiation method is low-yielding (17% yield) [57]. Schubert et al. prepared 2,6-bis(trimethyltin)pyridine (70) via an SnAt displacement of 2,6-dichloropyridine with sodium trimethylstannane, derived in situ from trimethyltin chloride [58-60]. 

Intriguingly, the Stille coupling of quaternary pyridylstannane 12 with 2-chloropyrazine (13) proceeded to afford adduct 14 [12]. A-Methylated 3-(tributylstannyl)pyridine 12 was easily prepared by refluxing 3-(tributylstannyl)pyridine (11) with methyl tosylate in EtOAc. By contrast, only 29% yield of the coupling adduct was isolated from the Stille reaction of 3-(tributylstannyl)pyridine A-oxide and 13. 

Pyridylstannanes have also been prepared by the reaction of halogeno pyridines with hexamethyldistannane in the presence of a palladium catalyst (Equation (62)).200... 

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