Cobalt Salen Complexes1

(X = Br, R = PPh3)

[15603-80-2]  · C34H29BrCoN2O2P  · Cobalt Salen Complexes  · (MW 587.53)

(reagents for the generation of carbon centered radicals and vitamin B12 mimics)

Physical Data: dark brown-black solids; usually stable up to 200 °C.

Solubility: sol most organic solvents, e.g. benzene, THF, dichloromethane; also partially sol H2O.

Preparative Methods: RCoIII(salen) complexes have been prepared in the same manner as the corresponding RCoIII(salophen) and RCoIII(dmgH)2 complexes, and they undergo similar reactions (see also Bis(dimethylglyoximato)(methyl)(pyridine)cobalt(III) and Cobalt Salophen Complexes). The formation of RCoIII(salen) complexes can be accomplished by reacting CoI(salen) complexes, CoII(salen) complexes, or CoIII(salen) complexes with electrophiles, radicals, and nucleophiles, respectively.1 NaCoI(salen) can be prepared by reduction of the BrCoIII(salen)PPh3 complex2 using sodium, metal amalgams,3 NaBH4/cat PdIICl2,4 or electrochemically.5 The supernucleophilic NaCoI(salen) complex reacts with a variety of alkyl halides, vinyl halides, acyl halides,3,5 and alkyl tosylates to form RCoIII(salen) species. Alkyl and vinyl RCoIII(salen) complexes can be prepared by the addition of NaCoI(salen) to alkenes and alkynes, respectively.1,3

Handling, Storage, and Precautions: often light- and air-sensitive and should be stored in tinted bottles.

Reactions of NaCoI(salen) with Aryl Halides.

NaCoI(salen) complex has the advantage that it is more nucleophilic than NaCoI(dmgH)2, particularly in reactions with aryl iodides. Reactions of NaCoI(salen) with aryl halides initially give the corresponding aryl radical via a single-electron transfer process (see Cobalt Salophen Complexes). These radicals can add either intramolecularly or intermolecularly to alkenes and alkynes (eq 1).5b,6 The new carbon radical formed from these additions is then trapped by the CoII(salen) formed in situ to form either alkyl or vinyl RCoIII(salen) complexes, respectively.

Reactions of RCoIII(salen) Complexes.

Owing to the weak nature of the CoIII-carbon bond (18-25 kcal mol-1),7 photolysis or thermolysis leads to homolytic cleavage and the formation of carbon-centered radicals. In practice, RCoIII(salen) complexes often undergo b-elimination to give alkenes unless the initially formed radical is allowed to react with an added radical trap (eq 1) (see also Cobalt Salophen Complexes).5b,6 In the presence of oxygen, alkylperoxy CoIII(salen) complexes (ROOCoIII(salen)) result, which can then be reduced with Sodium Borohydride to give alcohols (eq 2),5b while thermolysis in the presence of (PhS)2, SO2, (PhSe)2, NO, MeSO2Cl, BrCCl3, or I2 gives the corresponding sulfur, selenium, nitrogen, or halogen functionalized products, respectively (eq 2).1f The combination of the reaction of NaCoI(salen) with an aryl halide followed by radical cyclization and b-elimination of the resulting intermediate CoIII(salen) complex has been used in an approach to physovenine (eq 3).8

The oxidative radical cyclization process has the advantage over conventional reductive cyclization processes (see also Tri-n-butylstannane and Tris(trimethylsilyl)silane) in that functionality is retained in the products.

Reactions of CoII(salen).

CoII(salen) catalyzes the oxidation of phenols to the corresponding p-benzoquinones or p-diphenoquinones in high yield (eq 4).9 Oxidation in the presence of t-Butyl Hydroperoxide leads to t-butylhydroperoxyquinol ethers, and the reagent catalyzes the oxidation of 2,4,6-trisubstituted anilines to give either 4-t-butylperoxy-2,5-cyclohexadiene-1-imine or nitrobenzene derivatives, depending upon the nature of the substituents in the substrate.10 CoII(salen) also catalyzes the oxidation of alcohols and amines.11

Related Reagents.

Salcomine.


1. (a) Dodd, D.; Johnson, M. D. Organomet. Chem. Rev. 1973, 52, 1. (b) Pratt, J. M.; Craig, P. J. Adv. Organomet. Chem. 1973, 11, 331. (c) Toscano, P. J.; Marzilli, L. G. Prog. Inorg. Chem. 1984, 31, 105. (d) Scheffold, R.; Rytz, G.; Walder, L. Mod. Synth. Methods 1983, 13. (e) Gupta, B. D.; Roy, S. ICA 1988, 146, 209. (f) Pattenden, G. CSR 1988, 17, 361. (g) Schrauzer, G. N. ACR 1968, 1, 97.
2. Costa, G.; Mestroni, G.; Stefani, L.; JOM 1967, 7, 493.
3. Costa, G.; Mestroni, G.; Pellizer, G. JOM 1968, 11, 333.
4. Schrauzer, G. N.; Sibert, J. W.; Windgassen, R. J. W. JACS 1968, 90, 6681.
5. (a) Puxeddu, A.; Costa, G.; Marsich, N. JCS(D) 1980, 1489. (b) Bhandal, H.; Patel, V. F.; Pattenden, G.; Russel, J. J. JCS(P1) 1990, 2691.
6. (a) Patel, V. F.; Pattenden, G.; Russell, J. J. TL 1986, 27, 2703. (b) Clark, A.; Jones, K. TL 1989, 30, 5485.
7. Halpern, J. ACR 1982, 15, 238.
8. Clark, A.; Jones, K. TL 1992, 48, 6875.
9. Vogt, L. H., Jr.; Wirth, J. G.; Finkbeiner, H. L. JOC 1969, 34, 273.
10. Nishinaga, A.; Furster, S.; Eihhorn, E.; Speiser, B.; Rieker, A. TL 1992, 33, 4425.
11. Nishigai, K.; Amano, N.; Takasawa, T. CL 1991, 1093.

Gerald Pattenden

University of Nottingham, UK

Andrew J. Clark

University of Warwick, Coventry, UK



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