[15306-22-6]  · C20H23CoN3O2  · Bis(3-salicylideneaminopropyl)aminecobalt(II)  · (MW 396.36)

(catalyst for the homogeneous oxygenation of phenols with dioxygen in organic media)

Alternate Name: Co(Salpr).

Solubility: sol MeOH, CH2Cl2, CHCl3, C6H6, THF, C5H5N and DMF.

Form Supplied in: green crystals.

Preparative Method: prepared under nitrogen from bis(salicylaldehydehydato)cobalt(II) dihydrate and bis(3-aminopropyl)amine.2,3 The long green prisms are collected by suction, washed with cold methanol, and dried in a vacuum desiccator.

Purification: may be recrystallized from methanol if necessary; normally used without further purification.

Handling, Storage, and Precautions: store in a sealed container under an inert atmosphere.

Oxidation of 4-Substituted 2,6-Di-t-butylphenols to Quinols.

Co(Salpr), which binds dioxygen reversibly (1 O2:2 Co), catalyzes the regioselective oxidation of 4-substituted 2,6-di-t-butylphenols in methanol to p-quinols, via alkylperoxycobalt(III) complexes and hydroperoxy intermediates, in high yields (eq 1).2,4-10 The peroxy complexes give the quinols quantitatively upon treatment with silica gel.2 Either the complex or intermediate may be isolated. The regioselectivity of dioxygen incorporation depends on the nature of the substituent in the 4-position of the phenols.9 In methanol, with 4-alkyl substituents, the oxygenation takes place in the 4-position (eq 1), and with 4-aryl substitution, oxygenation occurs exclusively at the 2-position (eq 2).2,9 Complex product mixtures are obtained in benzene or chloroform.

4-Alkyl-2,6-di-t-butylphenols give p-quinols in alkaline ethanol (cf. eq 1) and epoxy-p-quinols in DMF containing t-BuOK (eq 3).10 The 4-aryl-substituted phenols afford o-quinols at low temperature (cf. eq 2), epoxy-o-quinols at 40 °C, and cyclopentadienones at 70 °C (eq 4).10

2,6-Di-t-butylphenols bearing electron-withdrawing groups at the 4-position are oxygenated exclusively at the 2 position with Co(Salpr) in CH2Cl2, to give the corresponding 3,5-di-t-butyl-6-hydroperoxy-2,4-cyclohexadienone derivatives (cf. eq 2; Ar = COR; CN; C=NR; C(R)=NOMe).11,12 The 4-cyanophenol ultimately affords 3-cyano-2,5-di-t-butylcyclopentadienone (eq 5), and the imines also yield products resulting from intramolecular decomposition of the peroxy-o-quinolato CoIII(Salpr) intermediates (eq 6).12

Oxidation of Alkyl Substituted Phenols to p-Benzoquinones.

Oxygenation of alkyl substituted phenols without a substituent para to the oxygen functionality in the presence of Co(Salpr) affords p-benzoquinone derivatives.13-15

Oxygenation of t-Butylphenols with an Unsaturated Side Chain.

The oxygenation of 4-alkenyl-2,6-di-t-butylphenols with Co(Salpr) in dichloromethane followed by filtration through a short column of silica gel affords the intermediate hydroperoxyquinone methides and p-acylphenols (eq 7).9 The yields of the products depend on the nature of the alkenyl substituent. With 2-alkenyl-4,6-di-t-butylphenols, cleavage of the double bond to form only 2-acyl-4,6-di-t-butylphenols is observed.9 These results indicate that dioxygen is incorporated exclusively into the alkenyl side chain. Each sp hybridized carbon atom is oxidized to a carbonyl group (1,2-dione) during the oxygenation of 4-alkynyl-2,6-di-t-butylphenols.9,16

Oxidative Phenolic Coupling.

Carpanone, a lignan with five contiguous asymmetric centers, is synthesized in excellent yield by the Co(Salpr) catalyzed oxidation of trans-2-(1-propenyl)-4,5-methylenedioxyphenol (eq 8).17 Co(Salpr) and Co(Salen) (Salcomine) are more effective catalysts than either Fe(Salen) or Mn(Salen).

Oxidative Cyclization to Flavanones and 1,3-Benzodioxoles.

Co(Salpr) in methanol under nitrogen promotes the conversion of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanediones to retro-Claisen reaction products and flavanones.18 Similarly, 2-hydroxychalcones are also converted to flavanones (eq 9).19 Substituted 2-hydroxyacetophenone 4-bromophenylhydrazones are oxygenated readily in the presence of Co(Salpr) in ethanol to give 2-(4-bromophenylazo)-1,3-benzodioxoles in good yield (eq 10).20

Other Applications.

Co(Salpr) and other cobalt-Schiff base complexes, in primary or secondary alcohols under dioxygen, catalyze the oxygenation of alkenes substituted with an aromatic or an electron-withdrawing group to ketones without double bond cleavage (eq 11).21

1. (a) Freeman, F. In Organic Synthesis By Oxidation With Metal Compounds; Mijs, W. J.; de Jonge, C. R. H. I., Eds.; Plenum: New York, 1986; Chapter 5. (b) de Jonge, C. R. H. I. In Organic Synthesis By Oxidation With Metal Compounds; Mijs, W. J.; de Jonge, C. R. H. I., Eds.; Plenum: New York, 1986; Chapter 7. (c) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic: New York, 1981. (d) Nishinaga, A.; Tomita, H.; Shimizu, T.; Matsuura, T. In Fundamental Research in Homogeneous Catalysis; Ishii, Y.; Tsutsui, M., Eds.; Plenum: New York, 1978, Vol. 2, p 241.
2. Nishinaga, A.; Tomita, H.; Nishizawa, K.; Matsuura, T.; Ooi, S.; Hirotsu, K. JCS(D) 1981, 1504.
3. Nishikawa, H.; Yamada, S. BCJ 1964, 37, 8.
4. Nishinaga, A.; Tomita, H.; Matsuura, T. TL 1979, 2893.
5. Matsuura, T.; Watanabe, K.; Nishinaga, A. CC 1970, 163.
6. Nishinaga, A.; Watanabe, K.; Matsuura, T. TL 1974, 1291.
7. Nishinaga, A.; Nishizawa, K.; Tomita, H.; Matsuura, T. JACS 1977, 99, 1287.
8. Nishinaga, A.; Itahara, T.; Shimizu, T.; Matsuura, T. JACS 1978, 100, 1820.
9. Nishinaga, A.; Iwasaki, H.; Shimizu, T.; Toyoda, Y.; Matsuura, T. JOC 1986, 51, 2257.
10. Nishinaga, A.; Shimizu, T.; Fujii, T.; Matsuura, T. JOC 1980, 45, 4997.
11. Nishinaga, A.; Shimizu, T.; Matsuura, T. TL 1981, 22, 5293.
12. Nishinaga, A.; Shimizu, T.; Toyoda, Y.; Matsuura, T.; Hirotsu, K. JOC 1982, 47, 2278.
13. Ichikawa, Y.; Yamanaka, Y.; Tsuruta, H. Jpn. Patent 7 399 130, 1973 (CA 1974, 80, 95 500).
14. Kato, T.; Yamanaka, I.; Komtsu, A. Jpn. Patent 7 424 464, 1974 (CA 1975, 82, 97 840).
15. Jouffret, M. Ger. Patent 2 518 028, 1974 (CA 1976, 84, 73 905).
16. Nishinga, A.; Iwasaki, H.; Kondo, T.; Matsuura, T. CL 1985, 5.
17. Matsumoto, M.; Kuroda, K. TL 1981, 22, 4437.
18. Nishinaga, A.; Maruyama, K.; Ando, H.; Sato, R.; Mashino, T.; Inada, A.; Nakanishi, T. TL 1990, 31, 3171.
19. Maruyama, K.; Tamanaka, K.; Nishinaga, A.; Inada, A.; Nakanishi, T. TL 1989, 30, 4145.
20. Nishinaga, A.; Yamazaki, S.; Matsuura, T. TL 1984, 25, 5805.
21. Nishinaga, A.; Yamada, T.; Fujisawa, H.; Ishizaki, K.; Ihara, H.; Matsuura, T. J. Mol. Catal. 1988, 48, 249.

Fillmore Freeman

University of California, Irvine, CA, USA

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