Nickel complexes corrins

The chemistry outlined in Scheme 24 was then put into effect catalytic hydrogenation of the tris-isoxazole (302) and recyclization with triethylamine gave a tricyclic ligand which was chelated with nickel ions to give (303). Introduction of the fourth nitrogen atom was accomplished by treatment of (303) with ammonium acetate, giving (304). Treatment with cyanide removed the nickel ion which was then replaced with zinc(II) to give (305). The reasons for this transmetallation step were two-fold firstly, zinc(II) corrins, as shown by Eschenmoser, can be readily demetallated, and this fact opens up many options later in the synthesis, but secondly, and more importantly, Eschenmoser s photochemical cyclization of seco-corrins (see Section 3.07.3.4.2.3) does not proceed with nickel complexes of seco-corrins, whereas zinc(II) seco-corrins can be cyclized in almost quantitative yield... 

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). 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

The two halves (266) and (267) were connected as shown in Scheme 20 by condensation of the sodium salt of (266) with the iminoether function in (267) to give the tetracyclic substance (280). Chelation with nickel(II) ions, followed by treatment with butoxide, gave the corrin perchlorate (265 M = Ni), and the cobalt complex was similarly prepared. 

Both of the above approaches employed a metal ion as a template about which the corrin cyclization was performed, but the nickel or cobalt ions could not subsequently be removed. In order to obtain metal-free corrins, a new route was therefore devised (67AG865) which employed the novel principle of sulfide contraction (Scheme 22). Thus the sodium salt of the precorrin (284) (Scheme 23) was transformed into the thiolactam (285), and loose complexation with zinc(II) ions caused cyclization to give (286), which was treated with benzoyl peroxide and acid to give the ring-expanded compound (287). Contraction with TFA/DMF gave the corrins (288) and (289), and the major of these (289) was desulfurized with triphenylphosphine and acid to give (288). Finally, demetallation with TFA gave the required metal-free corrin (290), a source for a whole variety of metal derivatives. 

Nickel(II) complexes have also been reported with reduced porphyrins, usually referred to as chlorins and corrins. Some nickel(II) complexes with chlorins (406)2883 have been obtained as by-products in the template synthesis of tetraalkylporphyrins. The main difference between [Ni(tmc)] and [Ni(tmp)] (tmc = deprotonated tetramethylchlorin, tmp = deprotonated tetra-methylporphyrin Table 110) is the lack of symmetry in the former complex with respect to the latter. The synthesis and reactivity properties of a number of corrin-nickel(II) complexes have been reported, mostly by Johnson and co-workers.2910-2915 Scheme 62 is a typical example of oxidative cyclization in the presence of a nickel salt.2914... 

The cadmium secocorrinoid carboxylic acid (102 M = Cd) also undergoes photocyclization to the acid (103 M = Cd), which on transmetallation to the nickel(II) complex (103 M = Ni) and treatment with triethylamine and acetic acid yields the parent corrin complex (100 M = Ni).268 The decarboxylation process is extremely facile. A related base-catalyzed cyclization of the secocorrinoid aldehyde (104) gives the corrin complex (105), which can be decarbonylated to the parent complex (100 M = Ni) by treatment with potassium hydroxide (Scheme 66).268... 

The first example reported in the literature is the cyclization of dihydrobilin to octadehydrocorrin [51-54]. The reaction is catalyzed by the presence of nickel or cobalt salts. As in the case of corrole and its metal complexes such ring closure reaction has been carried out in alcoholic solution, it is oxidative and base catalyzed. It has been demonstrated that the formation of the corrin ring is part of an equilibrium where the oxidative ring closure is coupled with a reductive ring opening reaction [55]. 

The use of metal ions as templates for macrocycle synthesis has an obvious relevance to the understanding of how biological molecules are formed in vivo. The early synthesis of phthalocyanins from phthalonitrile in the presence of metal salts (89) has been followed by the use of Cu(II) salts as templates in the synthesis of copper complexes of etioporphyrin-I (32), tetraethoxycarbonylporphyrin (26), etioporphyrin-II (78), and coproporphyrin-II (81). Metal ions have also been used as templates in the synthesis of corrins, e.g., nickel and cobalt ions in the synthesis of tetradehydrocorrin complexes (64) and nickel ions to hold the two halves of a corrin ring system while cycliza-tion was effected (51), and other biological molecules (67, 76, 77). 

Subsequent to the original quest for vitamin B12 (1), driven by medicinal purposes mainly, further investigations on the natural corrinoids laid bare the central roles of the Bi2-coenzymes in the metabolism of microorganisms, in particular. These primitive organisms uniquely possess the capacity to build up the complex B12 structure in nature, in which they may vary the constitution of the nucleohde ftmchon in a species-specihc way (Figure 2). The cobalt-corrins, in turn, have been proposed to be structural and functional renmants of early (primihve) forms of life, where presumably, central metabolic processes could rely considerably on organometalhc chemistry at cobalt and nickel centers. ... 

The polyene derivative XIX can be cyclized to form 18% of the nickel corrin complex XX (R = H), if it is first oxidized at -1-1.22 V to the radical cation, which undergoes a 1,16-hydrogen shift and then is coupled intramolecularly through cathodic reduction (—0.3 V) [131a]. 

Figure 3.4 nickel ethylenediamine complex reacts with acetone to form a nirrin, the four planar positions of the JVz " " bonds templating the reaction. In evolutionary terms, such reactions permitted great strides forward, as simple molecules such as the two above immediately became corrins surrounding the cobalt of vitamin B12 chlorophyll if magnesium and haem if ferrous was at the centre (from R.W. Hay, in An Introduction to Bio-inorganic Chemistry , ed. D.R. Williams, by permission of the Publishers,... 

Essentially the same reaction was then used to combine the east and west halves of corrin chromophores regioselectively. Donor (enamine anion) and acceptor (imidic ester) were both located on the western half. The eastern half carried an imidic ester as acceptor. Upon deprotonation of the western half, it was deactivated toward self-dimerization, because two anions would not react with each other. It rather reacted with the eastern half s imidic ester to form an open-chain tetrapyrroline. Complexation with nickel (template effect) and treatment with base (removal of an allylic proton) then led to the desired nickel corrinate in 80% yield (Scheme 7.3.19) (Eschenmoser, 1974 Pfaltz et al. 1997). 

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