Naturally occurring bipyridines

Although discussion of the chemistry and biochemistry of pyridylpiperi-dines, bipiperidines, and related reduced bipyridines is outside the scope of a review on bipyridines, it is necessary for the understanding of subsequent sections of this review to record the structures of reduced 2,3 -bipyridines that occur naturally. Several compounds of this type are important alkaloids isolated from several plant species, notably. Anabasis, Duboisia, Mackinlaya, Marsdenia, Nicotiana, and Priesteya. Anabasine (also known as neonicotinel has structure 12 (R = H), and is insecticidal. The closely related structures 1-methylanabasine (12 R = anatabine (13 R = its... 

Ru(ll) and Os(ll) tris-bipyridine chromophores have been widely used in the design and synthesis of artificial antenna systems. Mixed metal triads of type 2 and 3 have been recently studied. With 2, very efficient energy transfer from the Ru(II) chromophores to the Os(ll) unit was observed. With 3, where the spacer unit between the three chromophores is larger, a lower efficiency of energy transfer was obtained. The results are complicated by the flexible nature of the connector. In 3, conformers in which energy transfer takes place very rapidly and conformers in which it does not occur at all seem to be simultaneously present. 

One of the problems of radical polymerization is high-termination-rate constants by combination ( ) or by disproportionation ( ). In view of this, polymer chains of controlled chain length cannot be formed and this technique is ill-suited for precise control of molecular structure (e.g., in star, comb, dendri-mers, etc.) required for newer apphcations like microelectronics. The major breakthrough occurred when nonterminatmg initiators (which are also stable radicals) were used. Because of its nonterminating nature, this is sometimes called living radical polymerization and the first initiator that was utilized for this purpose was TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxo) [36,37]. A variation of this is atom-transfer radical polymerization (ATRP) in which, say for styrene, a mixture of 1 mol% of 1-phenyl ether chloride (R—X) and 1 mol% CuCl with two equivalents of bipyridine (bpy) is used for initiation of polymerization. Upon heating at 130°C in a sealed tube, bpy forms a complex with CuCl (bpy/CuCl),... 

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