[14167-18-1] · C16H14CoN2O2 · Salcomine · (MW 325.25)
(oxidizing agent for conversion of phenols to quinones;3-8 N-dealkylation of anilines;10 1,4-acetoxylation of dienes;14 nonoxidative hydration of alkenes and alkynes;16-18 oxidative carbonylation of amines;19,20 oxidative cleavage of fused heterocycles;21,22 oxidation of hydrazones to diazo compounds;23 catalyst for rearrangements25,26)
Alternate Names: bis(salicylidene)ethylenediiminocobalt(II); CoII(salen); Co(salen).
Solubility: sol nonpolar solvents including DMF, methanol, chloroform.
Form Supplied in: brown or black-grey powder.
Preparative Method: see Diehl and Hack.1
Handling, Storage, and Precautions: may be stored at room temperature in air for long periods of time.
Salcomine is usually the reagent of choice for the oxidation of para-unsubstituted phenols to 1,4-quinones. It serves as a catalyst in this transformation in which molecular Oxygen is the stoichiometric oxidant. Salcomine compares favorably in ease of preparation, handling, and convenience of workup with the well-known Fremy's salt, previously the standard reagent for this conversion. Reactions take place at room temperature and are generally conducted in DMF.
Originally studied as a synthetic oxygen carrier,2 salcomine attracted attention in the 1980s when its catalysis of the phenol-to-quinone oxidation3 was discovered and exploited by synthetic chemists.4 Its use appears to be restricted to the oxidation of phenols which do not bear electron-withdrawing groups. For example, Parker and Petraitis were unable to oxidize 2-hydroxy-3-methoxypropiophenone to the corresponding p-quinone,5 but found that the acetalized substrate gave a good yield of the acetal quinone (eq 1).6
A particularly useful example of the phenol-to-quinone conversion is the oxidation of inexpensive 1,5-naphthalenediol to a 5:1 mixture of juglone and its o-quinone regioisomer (eq 2). The salcomine oxidation of 5-methoxy-1-naphthol produces juglone methyl ether and its regioisomer as a 4:1 mixture.7
Thymoquinone, viewed as a convenient starting material for the synthesis of sesquiterpenes, was conveniently obtained from thymol in 71-84% yield and from carvacrol in 79-93% yield (eq 3).8
The oxidation of estrone to the corresponding quinol and an interesting, highly oxygenated steroid was effected by salcomine catalysis (eq 4).9
Salcomine-catalyzed O2 oxidation of N-alkyl aromatic amines (substituted at the meta or para position) has been examined. Optimized conditions convert N-n-butyl-4-phenylaniline to 4-phenylaniline in 69% yield.10 For some examples the imine intermediate has been isolated.11,12 A 4-methoxy-2,6-disubstituted aniline was converted to the corresponding quinone imine. However, 2,4,6-trisubstituted anilines (where the substituents are t-butyl and aryl) are converted to 4-peroxy-2,5-cyclohexadien-1-imines and nitrobenzenes.13
A triple catalytic system involving PdII, 1,4-Benzoquinone, salcomine, and air was used to effect the 1,4-acetoxylation of conjugated dienes (eq 5).14 Co(salophen) (1) and Co(TPP) (TPP = tetraphenylporphyrin) were more efficient reagents for this transformation, however. Co(salophen) and Co(TPP) also served to reoxidize hydroquinone to quinone in an allylic oxidation system (eq 6). Iron(II) Phthalocyanine (Fe(Pc)) was more effective in this system and Co(salen) was not active.
An oxidation-reduction mechanism is postulated for the salcomine-catalyzed hydration of styrenes. Further oxidation of the product affords aryl ketones.15 The Co(TPP) oxidation can be carried out in the presence of borohydride, and under these conditions only the benzylic alcohol product is obtained.16 When chiral Co(salen) derivatives were used in the styrene hydration, modest yields (18-31%) of 1-phenylethanol with modest enantiomeric excesses (0-38%) were reported.17
The analogous hydration of terminal alkynes is also proposed to proceed through an oxidation-reduction mechanism.18 Mandelic and glyoxylic ester byproducts are also produced.
Salcomine catalyzes the oxidative carbonylation of primary aromatic amines in methanol. The primary products in most cases are the symmetrical urea and the methyl carbamate. Under optimized conditions, o-aminobenzamide affords the quinazolinedione in 64% yield (eq 7).19 Iodide was found to promote the oxidative carbonylation in some systems.20
Several other oxidations have been the subject of limited studies. Salcomine and its derivatives catalyze the oxidative cleavage of fused heterocycles, including the conversion of 3-hydroxyflavones to o-aryloxybenzoates21 and 3-substituted indoles to o-formylaminoacetophenone derivatives.22 Hydrazones are oxidized to diazo compounds.23 The reduction of NO to N2 is effected by Sodium Borohydride in the presence of CoII(salen).24
Co(TPP) and CoII(salen) catalyze the rearrangement of monocyclic endoperoxides to furans.25 Salcomine and several analogs are reported to catalyze the cycloreversion of TMDC-Q to TMDC-NBD (eq 8). Carboxy-N,N-disalicylidenephenylenediiminatocobalt was immobilized on alumina to afford a packing material for a fixed bed reactor which was effective in catalyzing this reaction.26
Cobalt Salen Complexes;
Kathlyn A. Parker & David Taveras
Brown University, Providence, RI, USA