[7787-35-1] · BaMnO4 · Barium Manganate · (MW 256.28)
(selective oxidant; oxidizes allylic and benzylic alcohols to aldehydes; diols give dialdehydes, lactols or lactones)
Solubility: insol most organic solvents.
Form Supplied in: dark green crystals or blue-black powder with chunks; widely available.
Preparative Methods: from aqueous solution of KMnO4, BaCl2, NaOH (1 equiv each), and KI (0.12 equiv); dry crystals by azeotropic removal of H2O with C6H6; yield 60% on a 0.041 mol scale.2b
Purification: dry as above or at 100 °C for 24 h, or dry in vacuum oven (120 °C, 20 Torr); store in desiccator, and grind just before use.3
Handling, Storage, and Precautions: moisture sensitive; indefinitely stable and active; can be stored for months under dry conditions; use in a fume hood.
Barium manganate is a stable MnVI salt which can be regarded as an alternative to Manganese Dioxide. In general, the yields are equal to or better than those obtained using the latter reagent, and the reaction is more rapid. Barium manganate is claimed to be more attractive on a medium to large scale as its preparation is simple, it is easy to handle, no special activation is required for oxidations, product isolation is facile, and less oxidant is required (1:1 to 1:10 compared to 1:5 to 1:50 for MnO2).1,2b Barium manganate is unreactive towards styrene, toluene, 1-heptylamine, and a,b-unsaturated aldehydes and ketones.2b However, some aromatic aldehydes are converted to carboxylic acids and some aliphatic aldehydes polymerize on treatment with barium manganate.4
Simple allylic or benzylic alcohols are oxidized rapidly at room temperature (eqs 1 and 2); primary and secondary alcohols are also oxidized,2,5 but the reaction is much slower (eqs 3 and 4).2b The reaction also tolerates sensitive functionality such as C&tbond;C triple bonds (eq 5)2b and the furan nucleus (eq 6).6 For the latter reaction, MnO2 gives a much reduced yield (38%) and two-phase Jones oxidation results in a complex mixture.
The oxidation of benzylic and related diols with barium manganate often produces the corresponding dialdehyde (eqs 7-10),2 but lactones have been produced in some cases (see next section). As with the oxidation of monohydric alcohols, sensitive functionality is usually compatible with the oxidation conditions.
Barium manganate oxidation of the diol (1) produces the dialdehyde (2) (eq 11), which was used in a synthesis of the parent annulene derivative.7
In some cases the barium manganate oxidation of benzylic diols has been used to prepare the related lactones (eqs 12-14), although in unsymmetrical substrates the reaction often lacks selectivity.8 Substitution ortho to one of the hydroxymethyl groups appears to be important in determining the success of this reaction, as a related substrate which lacks such substitution gives a high yield of the dialdehyde (eq 15).7
The barium manganate oxidation of diol (3) has been used as a key step in the total synthesis of cinnamolide (4). This reaction can be controlled to give the lactol if 1-2 equiv of the oxidant is used, whereas with excess (10-20 equiv) cinnamolide (4) is obtained (eq 16).9
Barium manganate has found various other applications in synthesis, including the oxidation of thiols to disulfides (eqs 17 and 18).10 It is noteworthy that an aromatic amino group is unaffected in this oxidation, as aromatic amines have been reported to yield azo compounds with barium manganate (eq 19).2b
When hindered phenols are treated with barium manganate, oxidative coupling products are obtained (eq 20).11
Barium manganate is reported to be superior to manganese dioxide for the dehydrogenation of 2-imidazolines to imidazoles (eq 21).3
Longer reaction times also allow some aromatic aldehydes to be oxidized to carboxylic acids (eq 22).4 Under these conditions, heterocyclic aromatic aldehydes give no oxidation products and aliphatic aldehydes give only polymeric products, which are probably due to aldol reaction pathways.
Barium manganate is able to oxidatively cyclize benzylideneaminoanilines, -phenols, and -thiophenols to benzimidazoles, benzoxazoles, and benzothiazoles, respectively, at ambient temperature (eq 23).12
University of Salford, UK
Steven V. Ley & Grant H. Castle
University of Cambridge, UK