[139033-95-7] · C20H22CoCr2N4O8 · Tetrakis(pyridine)cobalt(II) Dichromate · (MW 609.39)
Alternate Names: TPCD; tetrakis(pyridine)cobalt (II) bis(chromate).
Physical Data: dec. 307 °C
Solubility: sol DMF, DMSO, AcOH, hot H2O; insol acetone, Et2O, dichloromethane.
Form Supplied in: deep brown or black crystals; commercially available.
Preparative Methods: to a solution of Chromium(VI) Oxide (2 equiv) and Cobalt(II) Acetate (1 equiv) in H2O is added Pyridine (4 equiv) at 60 °C. TPCD is collected as brown-colored crystals and washed with acetone. After drying under vacuum, it is available for use.
Handling, Storage, and Precautions: the dry crystals are nonhygroscopic and air stable. The sample stored in a brown-colored bottle under nitrogen over one year will keep its original activity. The reagent should not be heated above its decomposition point without solvent.
As a neutral CrVI complex, TPCD oxidizes primary and secondary alcohols, benzylic halides, and benzylamines to their corresponding aldehydes and ketones under essentially neutral conditions. It is very suitable for oxidation of some alcohols with acid-sensitivity and limited solubility (eq 1), for benzylic halides bearing electron-withdrawing substituents (eq 2), and for non-tertiary benzylic amines (eq 3); tertiary amines and quaternary ammonium salts are unaffected by TPCD.
The behavior of oxime compounds is different and depends on the solvent used. Hence, aromatic aldoximes are converted smoothly into nitriles in DMF or carboxylic acids in aq HOAc (eq 4),2 in which the dehydro dimer of aldoxime (i.e. a bis-nitrone) is identified as an intermediate. Benzaldoxime dehydro dimer can be trapped by Methyl Acrylate to yield a 1,3-cycloaddition product.3 When chalcone oxime is warmed with TPCD at 60 °C, within 1 min an intramolecular 1,3-cycloaddition occurs to give 3,5-diphenylisoxazole (eq 5).4
Under the influence of CoII, the behavior of TPCD, as expected, is different from that of other CrVI-based oxidants. Thus, it can effect the cleavage of hydrazides selectively to generate their acids in the presence of ester or amide groups (eq 6). It also converts benzil dihydrazones into diphenylacetylene derivatives in excellent yields, whereas 1,2-cyclodecanedione dihydrazone affords an 18% yield of cyclodecyne (eq 7).5
In the presence of TPCD, derivatives of N-acylpyridinium bromides are converted into 1,2,3-triacylindolizines according to a 1,5-dipolar cycloaddition mechanism (eq 8).6 1,3-Dipolar cycloaddition of N-phenacylpyridinium bromide with alkenes can be accomplished to give the aromatic indolizines in one step (eq 9).7 Both of these reactions appear to proceed by a dipolar cycloaddition first, followed by a dehydrogenative aromatization. CoIII may be regarded as a
real oxidant and CrVI as a reoxidant in these cases.
Yuefei Hu, Xudong Wei & Hongwen Hu
Nanjing University, P. R. China