SILVER IODATE 1 Silver Iodate a,c-Biladienes with exocyclic rings have been utilized in silver iodate–zinc acetate mediated cyclization.7,8 The reaction of a,c-biladienes bearing six-membered carbocyclic rings with silver AgIO 3 iodate in dimethylformamide followed by demetalation with 5% sulfuric acid in trifluoroacetic acid affords the isolated porphyrin in [7783-97-3] IO3Ag (MW 282.77) 12% yield (eq 3). The syntheses of petroporphyrin bearing a seven- 9 InChI = 1/Ag.HIO3/c;2-1(3)4/h;(H,2,3,4)/q+1;/p-1/fAg.IO3/ membered exocyclic ring, such as C32 15,17-butanoporphyrin qm;-1 and its 3-methyl homolog,10 have been reported by Lash and John- InChIKey = YSVXTGDPTJIEIX-YIVJLXCRCQ son (eq 4).8 Treatment of a,c-biladiene salts with silver iodate and zinc acetate affords desired petroporphyrins via oxidative cycliza- (reagent used as a versatile oxidative amidation and cyclization tion in good yields under mild conditions.5 However, attempts to component) cyclize a,c-biladienes bearing exocyclic rings under other condi- Physical Data: mp >200 ◦C; d 5.53 g cm−3. tions such as copper(II) chloride in dimethylformamide or cop- Solubility: soluble in aqueous ammonia; practically insoluble in per(II) acetate in pyridine result in only trace amounts of the water (0.3 g L−1 at 10 ◦C). cyclized petroporphyrins due to the geometry enforced on the Form Supplied in: white crystalline powder; commercially avail- tetrapyrrolic intermediate by the carbocyclic ring conformation.11 able. It has been shown that the silver iodate–zinc acetate mediated Handling, Storage, and Precautions: irritant; light sensitive; conditions can increase the stability of the cyclizing tetrapyrroles, causes ignition with reducing agents or combustibles; store in resulting in improvement of the cyclization yield.12 cool and dry conditions in well-sealed containers; handle in fume hood. Et 1 Et Oxidative Cyclizations. Like copper(II) acetate and other NH HN metal salts,2 silver iodate has been utilized as an oxidant to AgIO3, Zn(OAc)2 3,4 5 2Br transform a,c-biladienes into porphyrins via cyclization. For CuCl , DMF, rt NH HN 2 example, treatment of an a,c-biladiene containing a free β-position 12% adjacent to the terminal methyl group with zinc acetate and silver iodate delivers the corresponding porphyrin in 31% yield Et Et in dimethylformamide at high temperature (160 ◦C) after the removal of zinc (eq 1). It is necessary to use anhydrous zinc acetate Me Et as a chelating agent to bring the methyl groups in close proxim- N N 1,6 ity and promote the cyclization. In the absence of chelating Cu (2) agents or in the presence of other agents such as magnesium or N N mercuric salts, the cyclization gives low yields under the same conditions. In the presence of copper(II) chloride, silver iodate can also promote the cyclization at room temperature rather than Et the typical harsh conditions involving boiling dimethylformamide (eq 2).3,5 It has been shown that the formation of a stable square Silver iodate has been applied toward the multistep synthesis planar chelate between copper and the a,c-biladiene results in 13,14 the straightforward oxidation to the planar porphyrin.1 of meso- and β-substituted chlorin building blocks. Chlorins provide the basis for plant photosynthesis,15,16 but the unavailability of suitable chlorin building blocks has led to the inevitable use of synthetic model systems that employ sur- Et rogate porphyrins.17,18 A bromodipyrromethane carbinol is prepared by acylation and bromination of a 5-substituted dipyrro- Et methane followed by reduction. Chlorin formation is achieved NH HN 1. AgIO3, Zn(OAc)2 DMF, 160 °C by acid-catalyzed condensation and metal-mediated oxidative cy- 2Br 2. TFA clization. The applied copper-free conditions use zinc acetate, ◦ NH HN 31% silver iodate, and piperidine in toluene at 80 Cfor2htoob- tain the zinc chlorins in around 10% yield (eq 5). These can be easily demetalated to give the corresponding base chlorins. This Et Et general synthetic process is applicable to a range of meso- and Me Et β-substituents (eq 6). The metal-mediated oxidative cyclization NH N of a dihydrobilene results in the final formation of the chlorins. (1) This process proceeds in five steps: oxidation of the dihydrobi- N HN lene intermediate to afford a dihydrobilatriene, imine–enamine tautomerization, metal complexation with a divalent metal ion, carbon–carbon bond formation through an 18π-electrocycli- Et zation,19 and hydrogen bromide (HBr) elimination to yield the Avoid Skin Contact with All Reagents 2 SILVER IODATE final chlorin.13,20,21 Although very little is known of the reactiv- vides the dinaphthoporphyrin in 6.8% yield (eq 7). This low yield ity of the intermediates or the efficacy of the individual reactions, is thought to be due to deleterious steric interactions between the factors that can affect the outcome of this process include choice two naphthalene units. In addition, the purity of the a,c-biladiene of solvent, metal complex, oxidant, temperature, and base. As may also be an important factor.22 described above, silver iodate is more commonly used in the cy- clization of a,c-biladienes and it is believed that silver cations Br facilitate the elimination of bromides during chlorin formation. HN In studies of the cyclization reaction, it has been found that vari- N H 1. TFA, CH3CN, 0 °C ous quinines and oxidants in place of silver iodate do not produce + Ms 2. AgIO3, Zn(OAc)2 the desired product. Similarly, combinations of the oxidants man- N NH piperidine, toluene ganese dioxide and silver iodate in toluene or 1,4-dioxane show 80 °C, 2 h 13,14 no improvement in the yield of the chlorin product. Ts 10% Et N N Zn Ms (5) NH HN N N 1. AgIO3, Zn(OAc)2 2Br 2. H SO , TFA NH HN 2 4 12% Ts Et Et Et NH N HO (3) 1. TFA, CH3CN N HN NH HN 2. AgIO3, Zn(OAc)2 + piperidine, toluene Et Et N HN 80 °C, 2.5 h I 18% Br NH HN 1. AgIO3, Zn(OAc)2 2Br NH HN 2. H2SO4, TFA Et 26% N N (6) Zn Et N N I N HN (4) Oxidative Amidations. The use of silver iodate has been NH N Et reported in the copper-catalyzed oxidative amidation reactions of simple aldehydes and amines.23 After screening a variety of Et copper salts, copper(I) iodide has been determined to be the most effective catalyst to generate the desired amide. Reducing cata- lyst loading results in a significant increase in amide formation. The synthesis of Type E dinaphthoporphyrin systems has been For example, the amidation of benzaldehyde with ethylamine hy- achieved using cyclization of a,c-biladiene intermediates with sil- drochloride gives a quantitative yield under silver iodate mediated ver iodate and zinc acetate.22 While the conventional cyclization conditions (eq 8). A proposed mechanism of the oxidative amida- procedure using copper(II) chloride in dimethylformamide pro- tion with silver iodate and copper(I) iodide includes several steps duces virtually none of the desired porphyrin products, treatment such as deprotonation of amine hydrochloride salts to obtain free with silver iodate and zinc acetate in dimethylformamide results amines, nucleophilic addition to generate carbinolamine interme- in the zinc complex.5,8 Demetalation with trifluoroacetic acid pro- diates, and final oxidation to deliver desired amide products.24,25 A list of General Abbreviations appears on the front Endpapers SILVER IODATE 3 O The addition of a base improves the amidation reaction signifi- CuI, AgIO3 cantly. Although the base has been shown to be critical for the + NH2·HCl Ph H CaCO3, t-BuOOH success of the amidation reaction, it is also the source of unde- MeCN, 40 °C, 6 h sirable side reactions. An insoluble base such as CaCO3 allows 73% clean and successful reactions. O (9) Ph N H O O H CuI, AgIO3 1. AgIO3, Zn(OAc)2 NH HN + EtOCCH2NH2·HCl CaCO , t-BuOOH DMF, 160 °C 3 2Br MeO MeCN, 40 °C, 6 h 2. TFA 78% NH HN 6.8% O O Bu Bu CH COEt (10) N 2 H MeO O CuI, AgIO NH N + 3 (7) Ph H CaCO3, t-BuOOH MeOOC NH2·HCl N HN MeCN, 40 °C, 6 h 91% Bu Bu O (11) Ph N COOMe H O O CuI, AgIO3 1. Johnson, A. W.; Kay, I. T., J. Chem. Soc. 1961, 2418. + EtNH2·HC1 Et (8) Ph H NaHCO3, t-BuOOH Ph N 2. (a) Grigg, R.; Johnson, A. W.; Kenyon, R.; Math, V. B.; Richardson, K., 80 °C, overnight H J. Chem. Soc. 1969, 176. (b) Clarke, D. A.; Grigg, R.; Harris, R. L. N.; 99% Johnson, A. W.; Kay, I. T.; Shelton, K. W., J. Chem. Soc. 1967, 1648. (c) Dolphin, D.; Harris, R. L. N.; Huppatz, J.; Johnson, A. W.; Kay, I. T., J. Chem. Soc. 1966, 30. 3. Smith, K. M.; Kehres, L. A., J. Chem. Soc., Perkin Trans. 1 1983, 2329. The scope of the oxidative amidation reaction includes a 4. Smith, K. M.; Langry, K. C.; Minnetian, O. M., J. Org. Chem. 1985, 49, variety of aldehydes and amine hydrochloride salts. The optimized 4602. reaction conditions entail the use of 1.0 equiv of aldehyde, 1.5 5. Smith, K. M.; Minnetian, O. M., J. Chem. Soc., Perkin Trans. 1 1986, equiv of amine hydrochloride salt, 1.1 equiv of calcium carbonate, 277. 1.1 equiv of tert-butyl hydroperoxide,26 1.0 mol % of copper(I) 6. Cavaleiro, J.