Pyridine, 2-ethyl-, catalytic synthesis

Borabenzene complexes of cobalt such as Co(C5H5BPh)(COD) (51) and its 5-ethyl analog show the same type of catalysis but improved activity and chemoselectivity (77). Thus, 51 as the catalyst precursor gave the hitherto best results in the catalytic synthesis of the valuable 2-vinylpyridine from C2H2 and CH2=CHCN (120°C, 51 bar, 2 hours, turnover number 2164) (77,101). Furthermore, this catalyst for the first time allowed the synthesis of pyridine from C2H2 and HCN under mild conditions (110°C, 23 bar, 60 minutes, turnover number 103) (77). 

The joint action of pyridine and thionyl chloride on aliphatic or aromatic aldehydes RCHO results in 1-chloromethylpyridinium chlorides 272, which react with ethyl jS-aminocrotonate to give 1,4-dihydropyridines 273 (equation 113). The reaction constitutes a modified Hantzsch synthesis it has the advantage over the latter that it proceeds under milder conditions and gives better yields138. The dihydropyridine 273 (R = H) is also readily obtained from ethyl / -aminocrotonate and formaldehyde in the presence of catalytic amounts of 4-(dimethylamino)pyridine139. 

Using functionalized 3,4-dialkyl pyridines, Shiao and co-workers have described an alternative synthesis of actinidine (183) based on mixed metal reactions with l-(alkoxycarbonyl)pyridinium salts. Thus, ethyl 3-iodopropi-onate was successively treated with zinc and cuprous cyanide, and after being cooled to —78°C the resulting solution was reacted with 5-methyl-methylnicotinoate (158) and the mixture warmed to room temperature to afford 159. Cyclization and decarboxylation was effected with sodium hydride followed by heating in aqueous solution to afford 160. A Wittig reaction on 160 gave the olefin 161, and catalytic hydrogenation (Pd-C) afforded ( )-actinidine (134) (Scheme 5) (183). 

A selectivity of 100 % was reported in the carbonylation of nitrobenzene to ethyl phenylcarbamate, but the actual number is probably lower, since some of the added aniline was probably also carbonylated. It should be noted that a comparison of the examples in [89] and [90] indicates that, although the seleetivity is surely higher in the second case, the rate of the reaction may not be so. Curiously, the addition of an aniline to the system is not mentioned in later patents even from the same company. A similar catalytic system composed of [Bu4N][Pd(CO)Cl3] and VCl3 3 ITiF (but no pyridine) has also been used in the synthesis of acylureas by carbonylation of nitro compounds in the presence of an amide [91]. 

The use of copper compounds as catalysts for the reaction here described is even more limited than the one of iron compounds. We are aware of only one case in which a copper catalyst was used for the synthesis of a carbamate [194]. Thus a mixture of PhN02 (5.0 mmol), CuCl2 2H20 (0.5 mmol), EtOH (86 mmol) and pyridine (0.1 mmol) was reacted at 180 and 68 atm CO for 4 h, to yield a 76 % conversion, with a 90 % selectivity in ethyl phenylcarbamate. The selectivity achieved is good, but the catalytic ratio is very low (10), indicating a very low activity of copper. 

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