Synthesis and characterization of two new triphenylphosphonium halochromates

Two new mild oxidizing agents, triphenylphosphonium fluorochromate (TriPPFC), triphenylphosphonium bromochromate (TriPPBC), have been easily prepared. These reagents are suitable for oxidizing various primary and secondary alcohols to their corresponding carbonyl compounds. TriPPFC and TriPPBC have many advantages over similar oxidizing agents including: ease of preparation, involving, simple cationic exchange; high yields of the oxidized species; short reaction times; good selectivity; lower oxidant/substrate ratio and lower solvent requirement.


Results and Discussion
TriPPFC and TriPPBC are non-hygroscopic, air-stable but moderately light-sensitive solids which should be protected from light during preparation and storage.The structures of the reagents were confirmed by elemental analysis, infrared spectra and UV-Visible spectroscopy.The infrared absorption frequencies for fluorochromate group in TriPPFC found and assigned as: 877 cm -1 ν 1 (A 1 ) or ν(CrO 3 ), 640 cm -1 ν 2 (A 1 ) or ν(Cr-F), 947 cm -1 ν 4 (E) or ν(CrO 3 ) cm -1 .
The infrared absorption frequencies for bromochromate group at 945 and 913 cm -1 in triphenylphosphonium bromochromate are attributable to ν asym (Cr-O), ν sym (Cr-O) and ν(Cr=O), respectively.][15] These reagents are soluble in dimethylformamide and dimethyl sulfoxide, sparingly soluble in dichloromethane, acetonitrile and chloroform and insoluble in benzene, toluene, ether, nitrobenzene and ethyl acetate.These results are indicative of the ionic nature of these compounds.The compounds are diamagnetic.
Different primary and secondary alcohols were subjected to oxidations with TriPPFC and TriPPBC in dichloromethane.These oxidations take place under mild and quantitative conditions giving excellent yields (Table 1).

Scheme 1
The corresponding aldehyde and ketone products can then be easily isolated by simple filtration and evaporation of the solvent.Yields obtained using TriPPFC and TriPPBC are mostly higher than other similar reported reagents in the literature, while both reagents ratios as well as reaction times are lower (Table 1).

Experimental Section
General Procedures.Triphenylphosphonium Bromide and chromium trioxide were obtained from Fluka (Buchs, Switzerland).Melting points are measured on an Electrothermal 9100 apparatus.Elemental analyses (C, H, and N) were performed using a Heraeus CHN-O-Rapid analyzer.
Infrared spectra were recorded as KBr disks on a Shimadzu model 420 spectrophotometer.The UV/Visible measurements were made on an Uvicon model 922 spectrometer. 1H-NMR and 13 C-NMR spectra were measured on a BRUKER-DRX500 AVANCE instrument.

General procedure for oxidation of alcohols with TriPPFC and TriPPBC
To a stirred suspension of TriPPFC or TriPPBC (1 mmol) in dichloromethane (25 ml), a solution of the substrate (1 mmol) in the minimum amount of dichloromethane was added drop wise (Table I).The progress of the reaction is monitored by TLC and UV/Vis spectrophotometry.Filtration and evaporation of solvent furnished the product.The molar ratios of substrate to oxidants were 1:1.The solution became homogeneous briefly before the black-brown reduced reagent precipitated.The products are purified by distillation, crystallization or columns.