Oxidation of Aryl Alcohols by Morpholinium Fluorochromate(VI) on Silica Gel, a Selective and Efficient Heterogeneous Reagent

Morpholinium fluorochromate(VI), MFC, is easily synthesized by reacting morpholine to an aqueous solution of CrO3 and HF. This reagent selectively oxidizes aryl alcohols to their corresponding aldehydes and ketones under mild conditions. Moreover, it is inert towards aldehydes, ketons, oximes, thiols, sulfids, phenols, pyrans, trimethylsilanes, malonates and thioacetamides. The durability, reaction rate, ease of filtration and efficiency of MFC are considerably increased upon its absorption on silica gel.

Many workers have been carried out for preparation of oxidants with a number of specifications including: lower cost, higher yields, better selectivity, milder neutral conditions, easier preparations, less toxicity and short reaction times.Although, because of the growing demands is continued interest in the development of new chromium(VI) reagents for the effective and selective oxidation of alcohols under mild conditions.In this manuscript, we are reporting morpholinium fluorochromate(VI) (MFC) absorbed on silica gel as a new promising reagent with improved efficiency, selectivity and durability for the oxidation of aryl alcohols to their corresponding aldehydes and ketones under mild conditions.

Experimental
Morpholinie and chromium trioxide (CrO 3 ) were obtained from Fluka (Buchs, Switzerland).Melting points were measured on an Electrothermal 9100 apparatus.Elemental analyses were performed for C, H and N using a Heraeus CHN-O-Rapid analyzer.IR spectra were recorded on a Bomen FT-IR-MB100 Spectrometer. 1 H NMR was recorded using 90 MHz JEOL JNM-EX90A instrument. 13C and 19 F NMR spectra were determined on a BRUKER-DRX500 AVANCE instrument.

General procedure for oxidation of alcohols
A solution of the alcohol in the minimum amount of n-hexane was added drop wise at room temperature to a stirred suspension of MFC/wet silica gel (0.5 g MFC: 0.5 g silica gel/4 drops H 2 O) in n-hexane (5 mL).The molar ratio of alcohol to the oxidant was used 1:1 (Table 1).The progress of the reaction was monitored by TLC (solvent, 4:1, n-hexane/ethylacetate, v/v).After the completion of the reaction, the mixture was passed through a previously prepared short column of silica gel to give a clear solution.The solvent was evaporated and the products were purified by distillation, crystallization and/or column chromatography.Products were characterized by with authentic samples (NMR, IR, TLC and m.p./b.p. measurement).The % yields were found as isolated and/or determined by NMR analyses (Table 1).

Results and Discussion
In this paper we are reporting the synthesis of MFC.Different primary and secondary alcohols (1) were efficiently oxidized with MFC absorbed on wet SiO 2 (50% w/w) to their corresponding aldehydes and ketones (2), in n-hexane (Scheme 1).These oxidations take place under mild and completely heterogeneous conditions giving excellent yields (Table 1).Moreover, this oxidant is inert to aldehydes, ketons, oximes, thiols, sulfids, phenols, pyrans, trimethylsilanes, malonates and thioacetamides.PFC-Pyridinium fluorochromate used as oxidant 8 .IQFC-Isoquinolium fluorochromate used as oxidant 10 .c Products were characterized by comparison with authentic samples (NMR, IR, TLC and m.p./b.p. measurement) Oxidations may also occur using only MFC, in the absence of wet SiO 2 , but considerable improvements of both the yields and the corresponding reaction times were observed in the presence of the absorbent.The wet SiO 2 can act as a reactions medium, providing an effective heterogeneous surface area for the oxidation and at the same time making the simple work-up.
This oxidant proves to be many advantages as compared with similar oxidizing reagents including: easier preparations, application to pH sensitive molecules, good selectivity, high yields, short reaction times, lower oxidant/substrate ratio, lower solvent requirement and simple work-up.

Conclusion
A new reagent, morpholinium fluorochromate(VI) absorbed on silica gel, MFC/wet SiO 2 , selectively oxidizes aromatic alcohols.Many functional groups are inert towards this reagent, including aldehydes, ketons, oximes, thiols, sulfids, phenols, pyrans, trimethylsilanes, malonates and thioacetamides.Thus, this oxidant can not be used for the synthesis of highly functionalized molecules.Its advantages over similar oxidizing reagents include ease of synthesis and work-up, high yields, short reaction times, good selectivity, application to pH sensitive molecules, lower oxidant/substrate ratio and lower solvent requirement.

Table 1 .
Oxidations by MFC/silica gel, MFC, PFC a and IQFC b