Secocorrin nickel complex

The secocorrin nickel complex (101) can also be induced to undergo an electrochemical cyclization to corrin (100) but only in low yield. The experimental conditions involve a one-electron oxidation, followed by a one-electron reduction.269 The same secocorrin complex (101) is the starting material for two cyclization sequences leading to a didehydrocorrin complex (107) with a chromophore of seven double bonds (Scheme 67).269,270 The most interesting feature of these sequences is the remarkably easy acid-catalyzed ring closure of the secocorrinoid complex (106) to corrin (107). 

For the ring closure of macrocycles, the use of templates is a common tool to enhance the yield and selectivity. Substrates using metal ions as templates were successfully converted to the macrocyclic product. The templating metal center remains in the product. The nickel complex 20 is oxidized on platinum, undergoes hydrogen shift, and is trapped by water providing nickel secocorrinate 21 [36] (Scheme 8). 

Photoinduced cycloisomerisation can also be carried out in the presence of other metal ions serving as templates. For example, secocorrin coordinated to metal ions such as lithiiun(I), magnesirun(II), zinc(II) or cadmium(II) readily cyclises in the absence of oxygen in almost quantitative yield at room temperature [441, 444]. Platinum(II) and palladium(II) complexes (quantum )deld 0.008 in chloroform at 20°C) also cyclise, but more slowly. However, nickel(II), cobalt(III) and copper(II) secocorrinates do not undergo such a transformation [441, 444]. If, however, the nickel(II) l-methylidene-l,19-secocorrinate is subjected to one-electron electrochemical oxidation, followed by direct reduction of the isomerised cation-radical, then such a transformation is realisable [450]. 

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