Cobalt Salophen Complexes1

(R, X absent)

[17457-14-6]  · C20H14CoN2O2  · Cobalt Salophen Complexes  · (MW 373.29)

(reagents for the generation of carbon-centered radicals; vitamin B12 mimics2)

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

Solubility: sol most organic solvents, e.g. benzene, methanol, THF, dichloromethane.

Preparative Methods: RCoIII(salophen) complexes have been prepared in the same manner as for the corresponding CoIII(salen) and CoIII(dmgH)2 complexes, and they undergo similar reactions (see Bis(dimethylglyoximato)(methyl)(pyridine)cobalt(III) and Cobalt Salen Complexes). The formation of organoCoIII(salophen) complexes can be accomplished by reacting CoI(salophen) or CoII(salophen) with electrophiles or radicals, respectively.1 The supernucleophilic NaCoI(salophen) complex reacts with a variety of alkyl halides2 and acyl halides3 to form RCoIII(salophen) species.

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

Reactions of CoI(salophen) Complexes with Aryl Halides.

NaCoI(salophen) has the advantage that it is generally more nucleophilic than CoI(dmgH)2 and CoI(salen) complexes, particularly in reactions with aryl iodides. Reactions of CoI(salophen) complexes with aryl halides do not lead to isolable ArCoIII(salophen) complexes. Instead, NaCoI(salophen) initially gives the corresponding aryl radical via a single-electron transfer process. For example, (1) reacts with CoI(salophen) to give the corresponding aryl radical which then undergoes a smooth 5-exo-trig radical cyclization, followed by trapping with CoII(salophen), to give the RCoIII(salophen) complex (2); photolysis of (2) in C6H6 leads to b-elimination of HCoIII(salophen) (eq 1).4 Photolytic homolysis of (2) in the presence of styrene leads to a new alkene product resulting from radical addition to the C-C double bond in styrene, followed by b-elimination of HCoIII(salophen) (eq 2).5 Radicals produced from photolysis of (2) have also been trapped with O2, TEMPO, NO, SO2, (PhS)2, (PhSe)2, MeSO2Cl, BrCl3C, and I2, to give oxygen, nitrogen, sulfur, selenium, and halogen functionalized products, respectively (eq 2).6

Reactions of AcylCoIII(salophen) Complexes.

AcylCoIII(salophen) species are conveniently prepared from acid chlorides, or the mixed acid anhydrides with 2,6-dichlorobenzoic acid, following treatment with NaCoI(salophen).3 In this manner a range of primary, secondary, tertiary, vinyl, aryl, arylmethyl, oxy, and aminylacyl salophens, all of which are brightly colored, stable crystalline solids, have been prepared. Irradiation of deaerated, refluxing solutions of acylCoIII(salophen) complexes in methylene dichloride results in acyl carbon-cobalt bond homolysis to produce acyl radicals. These acyl radicals may add to activated alkenes intermolecularly in a Michael fashion (eq 3), or in an intramolecular manner leading to ring synthesis (eq 4).7 The resulting product radicals are trapped by CoII(salophen) to give RCoIII(salophen) complexes which undergo dehydrocobaltation leading to conjugated enones.8 This method of generating acyl radicals has the advantage over conventional reductive methods for their preparation in that functionality is retained in the products (see Tri-n-butylstannane). ArylmethylCoIII(salophen) and allylacylCoIII(salophen) complexes undergo carbon-cobalt bond homolysis and in situ decarbonylation, producing new alkyl radical centers which can be intercepted with oxygen, nitrogen, halogen, sulfur, and selenium containing radical trapping agents, leading to functionalized nor-alkanes (eq 5).3 The sequence constitutes a useful, and in some cases more flexible, variant of the classical Hundsdiecker reaction, and amounts to the cobalt equivalent of the Barton radical decarboxylation reaction of carboxylic acids via their corresponding thiohydroxamic acids.9 N-AllylcarbamoylCoIII(salophen) complexes undergo efficient 4-exo-trig oxidative cyclization giving rise to b-lactams; this procedure has provided a new approach to the antibiotic substance thienamycin.10 A range of g- and d-lactams is available by this chemistry (eq 6).11

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. Meth. 1983, 13. (e) Gupta, B. D.; Roy, S. ICA 1988, 146, 209. (f) Pattenden, G. CSR 1988, 17, 361.
2. Bigotto, A.; Costa, G.; Mestroni, G.; Peliizer, G.; Puxedda, A.; Reisenhofer, E.; Stefani, L.; Tauzher, G. ICA 1970, 4, 41.
3. Patel, V. F.; Pattenden, G. JCS(P1) 1990, 2729.
4. Bhandal, H.; Patel, V. F.; Pattenden, G. JCS(P1) 1990, 2691.
5. (a) Bhandal, H.; Patel, V. F.; Pattenden, G. JCS(P1) 1990, 2709. (b) Patel, V. F.; Pattenden, G. CC 1987, 871.
6. Bhandal, H.; Patel, V. F.; Pattenden, G. JCS(P1) 1990, 2703.
7. Coveney, D. J.; Patel, V. F.; Pattenden, G. TL 1987, 28, 5949.
8. Coveney, D. J.; Patel, V. F.; Pattenden, G. JCS(P1) 1990, 2721.
9. Barton, D. H. R.; Zard, S. Z. PAC 1986, 32, 259, and references therein.
10. Pattenden, G.; Reynolds, S. J. TL 1991, 32, 259.
11. Gill, G. B.; Pattenden, G.; Reynolds, S. J. TL 1989, 30, 3229.

Gerald Pattenden

University of Nottingham, UK

Andrew J. Clark

University of Warwick, Coventry, UK

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