Sterically-hindered pyridine

Scheme 8.13. Disaccharide synthesis in the presence of a sterically hindered pyridine.

Studies of proton transfers involving small ions with localized charge have shown that these reactions may proceed indeed with rate constants close to or even slightly larger than the collision rate constants predicted by the ADO theory (Mackay et al., 1976). However, rate-constant measurements of proton-transfer reactions between delocalized anions (Farneth and Brau-man, 1976) and sterically hindered pyridine bases (Jasinski and Brauman, 1980) and of SN2 displacement reactions (Olmstead and Brauman, 1977 Pellerite and Brauman, 1980 Pellerite and Brauman, 1983 Caldwell et al., 1984 for a review see Riveros et al., 1985) have shown that the rate constants can span the range from almost collision controlled values down to ones too slow to be observed. For these reactions the wide variation in rate constants has been explained on the basis of a double potential-well model which for a hypothetical SN2 substitution is schematically shown in Fig. 4. 

Another type of steric effect is the result of an entropy effect. The compound 2,6-di-tcrt-butylpyridine is a weaker base than either pyridine or 2,6-dimethylpyridine. " The reason is that the conjugate acid (14) is less stable than the conjugate acids of non-sterically hindered pyridines. In aU cases, the conjugate acids are hydrogen bonded to a water molecule, but in the case of 14 the bulky tert-butyl groups restrict rotations in the water molecule, lowering the entropy. ... 

Little A-alkylation occurs during hydrogenation of pyridine in the presence of polyols (27, b). Substituted pyridines undergo little N-alkylation in the presence of a branched chain alcohol. An example of this is seen in the hydrogenation of 2-picoline in isopropyl alcohol at 230-240°. Only 7.3% of l-isopropyl-2-pipecoline was obtained (28, a). With more sterically hindered pyridines chain length of the alcohol is a factor (28, b). In the hydrogenation of 2,6-lutidine in various alcohols A -methylation occurred to the extent of 56.6% at 250°C. A dramatic drop occurred when ethyl alcohol was used giving only 19% of the 1-ethyl derivative. Less than 10% A-alkylation took place with n-propyl and w-butyl alcohols. 

Owing to their relatively low thermal stability, perfluoroacylperoxides [RpC(0)0]2 undergo thermal decomposition generating perfluoroalkyl radicals. Although [RpC(0)0]2 were successfully used for the perfluoroalkylation furane, thiophene, and pyrrole, the reaction of perfluoroacylperoxides with pyridine does not result in perfluoroalkylation, due to side reactions caused by interaction of the peroxides with a lone electron pair of nitrogen. Sterically hindered pyridines, however, were reported to give perfluoroalkylated products. For example, compound 1 reacts with peroxide 2 giving isomeric pyridines 3 and 4 in low yield (Fig. 7.1). ... 

Those in which a PFLA and adventitious moisture constitute the initiator system (Chart 5) [120-134]. For the majority of these systems, stopping experiments involving sterically hindered pyridines (SHPs) have demonstrated protonic initiation (Scheme 9). Of the PFLAs... 

---