Bis(trimethylsilyl)chromate Catalyzed Oxidations of Alcohols to Aldehydes and Ketones with Periodic Acid

A facile, selective and high yielding bis(trimethylsilyl) chromate (BTSC) catalyzed selective oxidation of alcohols to aldehydes and ketones with periodic acid is reported.


Introduction
The selective oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones is a fundamental transformation in synthetic organic chemistry [1].A plethora of reagents are available for this interconversion, but most of these reagents, which are often expensive and toxic, must be used in stoichiometric quantities.Some of the most applicable, efficient and on the other hand most notorious and hazardous reagents used in oxidations are chromium based chemicals.The utility of chromium (VI) reagents in the oxidative transformation is compromised due to their inherent toxicity (they are cancer-causing suspects) and other problems such as difficulties in preparation and handling of its complexes, susceptibility to ignition and explosivity, difficulties in terms of product isolation and waste disposal, etc. [2].Consequently, from both the environmental and economical points of view catalytic oxidative transformations are thus promising and valuable and those employing less hazardous chromium reagent in catalytic amounts are particularly attractive.Accordingly, several methods have been recently reported for the catalytic oxidation of primary and secondary alcohols to carbonyl compounds using a variety of catalysts [3].

Results and Discussion
Our laboratory has been engaged in the exploration of practical green chemistry procedures that eliminate or minimize the use of hazardous substances which are dangerous to human life and environments [4] but necessary for chemical reactions.We are also interested in oxidative transformations, especially those which involve user friendly oxidative reagents and catalysts [5].Although we have previously reported bistrimethylsilyl chromate (BTSC) as a versatile reagent for various oxidative transformations [6] and it is being used routinely in organic synthesis, the need to use an excess or at least a stochiometric amount of BTSC to perform the oxidations is a drawback, due to all the known disadvantages of chromium based compounds.Therefore, new oxidation protocols that only require catalytic amounts of chromium based reagents and generate less chromium waste are still in much demand.In this communication, we wish to report a BTSC catalyzed oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones with H 5 IO 6 in high yields using only 3.8% mol of the catalyst (Scheme 1).
Bis(trimethylsilyl)chromate was prepared following the reported procedure [6a] from hexamethyldisiloxane and CrO 3 in dichloromethane.The oxidation reaction was conducted by refluxing benzyl alcohol (1 equiv.),BTSC (3.8 mol %), periodic acid (1 equiv.).The progress of the reaction was monitored by TLC.Upon the completion of the reaction the solvent was evaporated to dryness and the crude product was directly subjected to column chromatography using a small pad of silicagel and a 9:1 mixture of pet.ether-ethyl acetate as eluent to afford benzaldehyde in almost quantitative yield.To establish the generality of method various alcohols including aromatic, aliphatic, benzylic, primary and secondary were oxidized to the corresponding carbonyl compounds in high yields.The results are summarized in Table 1.No overoxidation products (to carboxylic acid) were observed.It is worthwhile to mention that in the presence of alkyne (propargyl alcohol) and alkene (cinnamyl alcohol) groups this protocol for oxidation of hydroxyl groups to carbonyl groups did not work.Oxidation of hexanol to hexanal (entry 10) took 4 h to complete.Furthermore the oxidation of benzopinacol (entry 9) gave benzophenone and the oxidation of pinacol gave a complicated mixture.Benzoin (entry 11) afforded a mixture of benzoic acid (45%) and benzaldehyde (37%).It is noteworthy to mention that in this reaction no addition of water is required, in contrast to the CrO 3 /H 5 IO 6 oxidation [7].The yield of the oxidation of benzyl alcohol to benzaldehyde (96%), relative to the amount of BTSC used, clearly shows the catalytic nature of this reaction.A plausible mechanism is illustrated in Scheme 2. The only shortcoming of this method is the lack of reactivity of aliphatic alcohols.However, we took advantage of this apparent disadvantage to show the selectivity of the protocol for the selective oxidation of benzylic alcohol in the presence of hexanol.Benzyl alcohol was selectivity oxidized in 1 h to benzaldehyde in 90% yield in the presence of hexanol without detection of hexanal.In continuation of our ongoing program to develop the use of solventless systems for environmentally benign synthetic protocols utilizing microwave irradiation [8], we examined the use of this procedure for oxidation of benzyl alcohols in a solventless system.No conversion was observed.

Conclusions
In summary, we have developed an effective and versatile catalytic system for the selective oxidation of various types of alcohols to carbonyl compounds.Primary alcohols can be oxidized to the corresponding aldehydes without any appreciable overoxidation to the corresponding carboxylic acids.This method for oxidation of alcohols offers a simple protocol, and is much less hazardous when compared to other conventional chromium oxidations currently used.Due to the simplicity and environmentally friendly conditions, this methodology should find utility in organic synthesis.

General
All products were known and their physical and spectroscopic data were compared with those of authentic samples.

Oxidation of benzyl alcohol: Typical procedure
Benzyl alcohol (1 mmol), periodic acid (1.05 mmol) and BTSC (3.8 mol%) were refluxed in acetonitrile (5 mL).The progress of reaction was monitored by TLC, using a 9:1 mixture of pet.etherethyl acetate as eluent.Upon the completion of the reaction, the solvent was evaporated to dryness.To the crude product ethyl acetate (10 mL) was added and the solution was washed with 10 mL of 1:1 brine-water.The organic layer was separated, dried over Na 2 SO 4 and evaporated to dryness.The residue was directly passed through a small silica gel column to afford benzaldehyde in 96% yield.
reactions were conducted in CH 3 CN except for entries 3, 5 and 9 which were done in THF.b) Yields are based on GC analysis.The products were characterized by comparison of their IR with those of authentic samples and their boiling points and melting points with those reported in CRC Handbook of Chemistry.c) Boiling point, CRC Handbook of Chemistry.

Table 1 .
Oxidations of alcohols to aldehydes and ketones with BTSC/H 5 IO 6 .