Iron complexes quinones

The Hill Reaction. The photochemical reaction has been studied with isolated chloroplasts and chloroplast fragments (grana). These evolve oxygen in the presence of suitable oxidants, such as various iron complexes, quinones, and dyes. These reactions are known as Hill reactions, in recognition of the pioneer studies of Hill in demonstrating and defining the properties of isolated chloroplasts (X). ... 

For the quinone imine cyclization of iron complexes to carbazoles the arylamine is chemoselectively oxidized to a quinone imine before the cyclodehydrogenation [99]. The basic strategy of this approach is demonstrated for the total synthesis of the 3-oxygenated tricyclic carbazole alkaloids 4-deoxycarbazomycin B, hyellazole, carazostatin, and 0-methylcarazostatin (Scheme 17). 

Over the past 15 years, we developed three procedures for the iron-mediated carbazole synthesis, which differ in the mode of oxidative cyclization arylamine cyclization, quinone imine cyclization, and oxidative cyclization by air (8,10,557,558). The one-pot transformation of the arylamine-substituted tricarbonyl(ri -cyclohexadiene) iron complexes 571 to the 9H-carbazoles 573 proceeds via a sequence of cyclization, aromatization, and demetalation. This iron-mediated arylamine cyclization has been widely applied to the total synthesis of a broad range of 1-oxygenated, 3-oxygenated, and 3,4-dioxygenated carbazole alkaloids (Scheme 5.24). 

Electrophilic aromatic substitution of 708 with the iron-coordinated cation 602 afforded the iron-complex 714 quantitatively. The iron-mediated quinone imine cyclization of complex 714, by sequential application of two, differently activated, manganese dioxide reagents, provided the iron-coordinated 4b,8a-dihydrocarbazole-3-one 716. Demetalation of the iron complex 716 with concomitant... 

Electrophilic substitution at the arylamine 709 using the complex salt 602, provided the iron complex 725 quantitatively. Sequential, highly chemoselective oxidation of the iron complex 725 with two, differently activated, manganese dioxide reagents provided the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727) via the non-cyclized quinone imine 726. Demetalation of the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727), followed by selective O-methylation, provided hyellazole (245) (599,600) (Scheme 5.70). 

Electrophilic aromatic substitution of the arylamine 780a using the iron-complex salt 602 afforded the iron-complex 785. Oxidative cyclization of complex 785 in toluene at room temperature with very active manganese dioxide afforded carbazomycin A (260) in 25% yield, along with the tricarbonyliron-complexed 4b,8a-dihydro-3H-carbazol-3-one (786) (17% yield). The quinone imine 786 was also converted to carbazomycin A (260) by a sequence of demetalation and O-methylation (Scheme 5.86). The synthesis via the iron-mediated arylamine cyclization provides carbazomycin A (260) in two steps and 21% overall yield based on 602 (607-609) (Scheme 5.86). 

The total synthesis of carbazomycin D (263) was completed using the quinone imine cyclization route as described for the total synthesis of carbazomycin A (261) (see Scheme 5.86). Electrophilic substitution of the arylamine 780a by reaction with the complex salt 779 provided the iron complex 800. Using different grades of manganese dioxide, the oxidative cyclization of complex 800 was achieved in a two-step sequence to afford the tricarbonyliron complexes 801 (38%) and 802 (4%). By a subsequent proton-catalyzed isomerization, the 8-methoxy isomer 802 could be quantitatively transformed to the 6-methoxy isomer 801 due to the regio-directing effect of the 2-methoxy substituent of the intermediate cyclohexadienyl cation. Demetalation of complex 801 with trimethylamine N-oxide, followed by O-methylation of the intermediate 3-hydroxycarbazole derivative, provided carbazomycin D (263) (five steps and 23% overall yield based on 779) (611) (Scheme 5.91). 

Electrophilic aromatic substitution of 5-hydroxy-2,4-dimethoxy-3-methylaniline by reaction with the iron complex salts affords the corresponding aryl-substituted tricarbonyliron-cyclohexadiene complexes. O-Acetylation followed by iron-mediated arylamine cydization with concomitant aromatization provides the substituted carbazole derivatives. Oxidation using cerium(IV) ammonium nitrate (CAN) leads to the carbazole-l,4-quinones. Addition of methyllithium at low temperature occurs preferentially at C-1, representing the more reactive carbonyl group, and thus provides in only five steps carbazomycin G (46 % overall yield) and carbazomycin H (7 % overall yield). 

Figure 6 Model of the X-ray crystal structure of the photosynthetic reaction center from Rb. sphaeroides. The bacteriochlorophyU special pair ((Bchl)2), accessory bacteriochlorophyUs (Bchl), bacteropheophytins (Bphe), quinones (Q), and iron complex (Fe) are shown in black. Electron transfer proceeds primarily along the A branch...

Qq - 2,3-dimethoxy-5-methyl-l,4-benzoquinone - primary quinone acceptor Qg - secondary quinone acceptor Q Fe - quinone-iron complex QHj - dihydroquinone or quinol Q - oxidizing quinone binding site also called Q, - reducing quinone binding site also called Q, Q - electronic transition moments... 

Figure 4. Proposed plastoquinine (QB) and herbicide binding site on the 32 kDalton D-1 polypeptide of photosystem II. The quinone is bound through an iron-complexed histidine residue (his 215) and hydrogen bonding to ser 264. Further interactions occur with arg 269 and phe 255 lying above and below the binding site. Amino acid substitutions in herbicide-tolerant mutants have been identified at the residues numbered 219. 255, 264 and 275. Reproduced with permission from Ref. 57. Copyright 1986 Verlag der Zeitschrift fur Naturforschung.

A third pathway leads via the quinone imine intermediates 38 to 3-hydro-xycarbazoles 41 (mode C in Scheme 12) [97, 98, 108, 109]. Oxidation of the complexes 36 with manganese dioxide afforded the quinone imines 38, which on treatment with very active manganese dioxide undergo oxidative cyclization to the tricarbonyl(ri" -4b,8a-dihydrocarbazol-3-one)iron complexes 39. Demetalation of 39 with trimethylamine iV-oxide and subsequent aromatization lead to the 3-hydro-xycarbazoles 41. The isomerization providing the aromatic carbazole system is a... 

An intense new absorption band occurs in a number of nickel-quinone complexes such as pOCVn). This has been characterized as a charge transfer band. It might be noted that an iron complex of coenzyme Q (XXVni) has been proposed " as an intermediate in oxidative phosphoryl-... 

The relation of the structure and organization of the Photosystem II reaction centers to those from Photosystem I or from the green or purple bacteria presents an interesting example of comparative biochemistry. Similarities between PS II and purple bacterial reaction centers include aspects of the reaction center proteins, the stoichiometry of chlorophyll and pheophytin in the reaction center and the complex of iron with quinones as the primary electron acceptor. In each of these respects the reaction centers of PS I or green bacteria, however, have no obvious similarity. 

Use of excess oxidant leads to decomplexation and aromatization of the product. In the case of electron-rich aromatic products, oxidation can go further to quinone-like compounds. This has been used in the synthesis of carbazole natural products (Scheme 10.33)." " The substituted aniline 10.130 underwent electrophilic substitution by the iron complex 10.95. Regioselective oxidation to give a new i/-complex 10J12 allowed a second nucleophilic attack to generate the carbazole skeleton 10.133 in situ. Further oxidation resulted in decomplexation, aromatization of the ring to give carbazole 10.134 and some formation of iminoquinone 10.135. 

Different from the enzymatic system, this simple ligand system forms quinone, which is not reactive with iron complexes. The quinone 6 thus formed is catalytically oxygenated in the presence of 2,6-di-r rr-butylhydroquinone, which converts 6 to 1, as shown in Fig. 1. 

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