Synthesis and Reactions of N-methylbenzylammonium Fluorochromate(vi) on Silica Gel, a Selective and Efficient Heterogeneous Oxidant

N-Methylbenzylammonium fluorochromate(VI) (MBAFC) is easily synthesized by addition of N-methylbenzylamine to an aqueous solution of CrO 3 and HF. MBAFC shows selectivity in the oxidation of aryl alcohols to their corresponding aldehydes and ketones under mild conditions. The durability, ease of work up and efficiency of MBAFC are considerably increased upon its absorption on silica gel.


Results and Discussion
Different primary and secondary alcohols 1 were subjected to oxidations with N-methylbenzylammonium fluorochromate(VI) (MBAFC) absorbed on wet SiO 2 (50% w/w), in dichloromethane (Scheme 1).These oxidations take place under mild and completely heterogeneous conditions giving excellent yields (Table 1).

Scheme 1
The heterogeneous reaction mixtures are thoroughly stirred, at room temperature, for 10-33 minutes.The corresponding aldehyde and ketone products 2 can then be easily isolated by simple filtration and evaporation of the solvent.Yields obtained using MBAFC/silica gel are mostly higher than those reported in the literature, while both reagent ratios as well as reaction times are lower (Table 1).
Oxidations may also occur using only MBAFC, in the absence of wet SiO 2 , but considerable improvements of both the yields and the corresponding reaction times are observed in the presence of the absorbent.This implies that the wet SiO 2 may act as a reaction medium, providing an effective heterogeneous surface area for the oxidation and at the same time making the work-up much more convenient.
The selectivity of MBAFC is well demonstrated through its oxidations of 4-chlorobenzyl alcohol (yield of product: 91 %), and/or benzyl alcohol (yield of product: 93%), in the presence of equimolar amounts of 2-phenylethyl alcohol (no product detected).Neither carboxylic acids overoxidation products nor other by-products are formed upon oxidation of alcohols via MBAFC (Table 1).Functional groups such as methoxy and methyl attached on the phenyl ring are inert to this reagent.In addition, we have shown that this reagent does not oxidize a variety of other substrates, including diphenyl sulfide, thiophenol, 3,5-dimethoxyphenol, 2-benzyl-3,4-dihydro-2Hpyran, benzyloxytrimethylsilane, N-methyl-2-phenylthioacetamide and sodium 2-hydroxyiminomalonate.

Conclusions
A new reagent, N-methylbenzylammonium fluorochromate(VI) (MBAFC) absorbed on silica gel, is easily synthesized.It proves to be a low cost, readily available and highly selective oxidizing reagent for a variety of aromatic alcohols.Its advantages include higher yields, shorter reaction times, lower alcohol/oxidant molar ratios, application to pH sensitive molecules and ease of separation of products.Moreover, during the reaction, the color of the oxidant changes from orange to brown, thus providing a visual means for ascertaining the progress of the oxidation.Many functional groups are inert towards this oxidizing agent, including thiols, sulfides and phenols, enhancing the usefulness of the oxidant and the oxidation conditions for the synthesis of highly functionalized molecules.