Vinylpyridines hydroboration

Vinylpyridine, on the other hand, is strongly complexed with 9-BBN and no hydroboration has been observed. Moreover, the unreacted starting material cannot be recovered following usual alkaline hydrogen peroxide oxidation. 4-Vinylpyridine is known to undergo polymerization [40] readily. 

The complex formed by treatment of 2-methyl-5-vinylpyridine with 9-BBN (1 1 molar ratio) gets dissociated readily at 25 or 65 °C due to 2-methylsubstitu-ent, and hydroboration proceeds smoothly to place the boron atom predominantly at the (5-carbon [38]. The electron-withdrawing property of pyridine nitrogen here is transmitted weakly as compared with vinylpyridine. The comparable hydroboration with BMS, 9-BBN, Clix BH, and SiajBH for their steric and electronic consideration along with their molar ratio studies are presented in Table 5.10 [38]. 

The complexation adducts of vinylpyridines and BFj can be hydroborated with a 1 1 molar ratio of 9-BBN, but without much success (Table 5.11) [38]. 

Table 5.11 (continued) Hydroboration of vinylpyridine-borontrifluoride complexes [38]... 

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