Pyridynes formation from

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-... 

There is a complication if the nucleophile used in reactions with halopyridines is also a strong base for now the formation of a pyridyne is possible, and with sodamide in liquid ammonia (providing the NH2 ion , B), for example, both 3-aminopyridine and 4-aminopyridine are formed from 4-bromopyridine (Scheme 2.16). 

In 1972 Berry and co-worker detected 3,4-pyridyne (101) by MS. Trapping experiments also provided evidence for the existence of this intermediate, although the chemistry of 101 differs considerably from that of o-benzyne. Thus, neither anthracene nor dimethylfulvene form Diels-Alder adducts with 101. Nam and Leroi were able to generate 101 in nitrogen matrices at 13 K and characterized it by IR spectroscopy. Irradiation of 3,4-pyridinedicarboxylic anhydride (103) with 1 > 340 nm results in formation of 101, which upon short wavelength photolysis (k > 210 nm) fragments to buta-l,3-diyne (104) and HCN, and to acetylene (105) and cyanoacetylene (106, Scheme 16.24). The assignment of an intense... 

Similarly, 4-lithiated 3-bromo and 3-chloro pyridines generated from substrates 40, are stable between -60 and -40°C, and lithium halide elimination to 2-fluoro-3,4-pyridyne occurs only upon warming to room temperature, as evidenced by the formation of adduct 41 (Scheme 13) [72CR(C)(275)1439, 72CR(C)(275)1535]. 

The scope of this methodology is restricted to the generation of 3,4-pyridyne. To date, analogous formation of 2,3-pyridyne from 2-halopyridines has not been reported. 

Recently H. L. Jones and D. L. Beveridge have presented molecular orbital calculations on the electronic structure of 2,3-pyridyne explaining the exclusive formation of 2-aminop3rridine from this intermediate [Tetrahedron Letters No. 24, 1677 (1964)]. 

As mentioned already (p. 204), the formation of 4-phenacylpyridine and 4-aminopyridine from S-bromopyridine, acetophenone and sodamide in liquid ammonia led Levine and Leake to postulate the occurrence of 3,4-pyridyne as an intermediate in the reaction. The results of other reactions in which a 3,4-pyridyne plays a part are collected in Table 5,23. 3-Fluoro-... 

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