INTRODUCTION:

Chromium(VI) based reagents are widely used in modern organic synthesis for the oxidation of organic substrates, in particular primary and secondary alcohols, under mild conditions. Even though, numerous oxidants for oxidation of alcohols are already reported, the growing demand for new oxidants of alcohols made us carry out the synthesis of Tetraphenylphosphonium Halochromates(VI), TPPXC. Many such reagents have been developed in recent years with some success (1), Some of the important entries in the list of reagents are quinolinium fluorochromate (QFC) (2), prolinium chlorochromate (3), caffeinilium chlorochromate (4), isoquinolinium chlorochromate(5),2,2'-bipyridinium chlorochromate (BiPCC) (6), 2,6-dicarboxypyridinium chlorochromate(7-8), pyridinium chlorochromate (PCC) (9), pyridinium dichromate (PDC) (10), pyridinium fluorochromate (PFC)(11), pyridinium bromochromate (PBC) (12), quinolinium chlorochromate (QCC) (13). and Tetramethlammonium fluorochromate (TMAFC) (14).

We have found that these reagents have certain advantages over similar oxidizing agents in terms of amounts of oxidants and solvents required, easier working up and high yields. In these respect, we wish to report that Tetraphenylphosphonium Halochromates, (TPPXC) able oxidizes alcohols to their corresponding aldehydes and ketones, under mild conditions.

EXPERIMENTAL:

Material and instruments

CrO₃ (Merck, P.A.) and Tetraphenylphosphonium Halochromates were obtained from Fluka (Buchs, Switzerland). Solvents were purified by standard methods. 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 and ¹³C NMR spectra were recorded using Bruker DRX-500 in CDCl₃ solutions. In the case of the reduced product of the oxidants, chromium was determined after oxidizing with acidic peroxodisulfate (K₂S₂O₈) solution. Department of Chemistry, OIRC, Tehran. Melting points were measured on Electrothermal 9100 melting point apparatus.

Synthesis of Tetraphenylphosphonium Fluorochromates (VI), [(C₆H₅)₄P][CrO₃F]

A 1g (10 mmol) sample of chromium (VI) oxide, CrO₃, and 9ml (20 mmol) of 40% hydrofluoric acid were added to 20 ml of water in a 100 ml polyethylene beaker with stirring. After 5-7 min the homogeneous solution was cooled to ca. 0 °C. To the resultant clear orange solution, Tetraphenylphosphonium Chloride (3.74g, 10 mmol) was added with stirring to this solution over a period of 0.5 h and stirring was continued for 0.5 h at 0 °C. The solid was washed with hexane and dried under vacuum for 1 h: C₂₄H₂₀CrFO₃P: Cacld. %C, 62.88; %H, 4.36. Found: %C, 64.62; %H, 4.51. IR.(KBr): 875 cm^-1ν₁(A₁) or ν(CrO₃), 640cm^-1ν₂(A₁) or ν(Cr-F), 947 cm^-1 ν₄(E) or ν(CrO₃) cm^-1. UV/Visible ¹³C-NMR and ¹H-NMR were all consistent with the TPPFC structure. Electronic absorption at 450 nm, corresponding to 1a₂→9e (ε = 276 M^-1 cm^-1); 363 nmto 8e→9e (ε = 462 M^-1 cm^-1); and 273 nm to 12a₁→9e (ε =986 M^-1 cm^-1).

Synthesis of Tetraphenylphosphonium Chlorochromates (VI), [(C₆H₅)₄P][CrO₃Cl]

Chromium(VI) oxide (1.0 g. 10 mmol) was dissolved in dry actonitrile (25 ml) in a beaker and was added under stirring at 0 °C. To the resultant clear orange solution, Tetraphenylphosphonium chloride (3.74 g, 10 mmol) was added under stirring over a period of 0.5 h and the stirring was continued for 0.5 h at 0 °C. The precipitated clear orange solid was isolated by filtration, washed with hexan and dried under vacuum for 1 h at room temperature. To this mixture, 30 ml CH₂Cl₂ is added. The orange organic phase is decanted and the solvent is distilled off. A reddish orange gel is separated and stored in the refrigerator. C₂₄H₂₀ClCrO₃P: Cacld. %C, 60.69; %H, 4.21. Found: %C, 63.02; %H, 4.40. IR (KBr): 905 cm^-1 v₁(A₁) or v(CrO₃), 429 cm^-1 v ₂ (A₁) or v(Cr-Cl), 945 cm^-1 v₄(E) or v(CrO₃) cm^-1. UV/Visible and ¹H-NMR were all consistent with the TPPCC structure. Electronic at 451 nm, corresponding to 1a₂→9e (ε = 173 M^-1 cm^-1); 364 nm to 8e→9e (ε = 532 M^-1 cm^-1); and 271 nmto 12a₁→9e (ε =1279 M^-1 cm^-1). UV/Visible, ¹³C NMR and ¹H NMR were all consistent with the TPPCC structure.

Synthesis of Tetraphenylphosphonium Bromochromates (VI), [(C₆H₅)₄P][CrO₃Br]

Tetraphenylphosphonium Bromochromate can be easily prepared in excellent yield from the reaction of CrO₃ with Tetraphenylphosphonium Bromide in water in a molar ratio of 1:1. To a solution of chromium trioxide (1 g, 10 mmol) dry actonitrile (25 ml) was cooled to 0°C and a stoichiometric amoute of Tetraphenylphosphonium Bromide (4.19 g, 10 mmol) was added under stirring at room temperature. Within 1h, a clear orange solution fored which upon refrigerating gave solid TPPBC, which was isolated by filtration. The solid was washed with hexan and dried under vacuum for 1 h. C₂₄H₂₀BrCrO₃P: Cacld. %C, 55.49; %H, 3.85. Found: %C, 57.62; %H, 3.99. IR. (KBr): 905cm^-1 υ₁(A₁) or υ(Cro₃), 945 cm^-1 υ₄(E) or υ(Cro₃). UV/Visible was all consistent with the TPPBC structure. Electronic at 452 nm, Corresponding to 1a₂→9e (ε = 174 M^-1 cm^-1); 365 nmto 8e→9e (ε = 670 M^-1 cm^-1); and 270 nm to 12a₁→9e (ε = 1786 M^-1 cm^-1).

Oxidation of alcohols: General Method

To TPPXC (0.001 mol) in CH₂Cl₂ (25 ml) was added the alcohol (0.001 mol) dissolved in a small amount of the solvent at room temperature. The mixture was stirred and refluxed for the time indicated in the Table 1 at room temperature, diluted with CH₂Cl₂ and filtered. Evaporation of solvent furnished the product. The molar ratio of substrate to oxidant was 1:1. The solution became homogeneous briefly before the black-brown reduced reagent precipitated. The progress of the reaction was monitored by TLC and UV/Vis spectrophotometry.

RESULTS AND DISCUSSION:

These reagents works as efficiently as activated manganese dioxide or Collins reagents. (15). These new oxidants efficiently oxidize number of organic substrates including primary, secondary alcohols. The results obtained with Tetraphenylphosphonium Halochromates are very satisfactory and show the new reagents to be a valuable addition to the existing oxidizing agents. (Table 1) summarizes the products, yields and reaction times of TPPXC with various substrates.

They concluded that the inequality between the Cr-O and the Cr-X bonds are responsible for the higher reactivity. TPPXC appears to be a stronger reagent than the others; these could be due to their lower symmetry. They have also been found that these reagents have certain advantages over similar oxidizing agents in terms of the amounts of oxidants and solvent required, and especially in the short reaction times required and in the higher yields of the product (Table I) (11,15,16).

Because of the stability and solubility of Tetraphenylphosphonium Halochromates, reactions could be performed at room temperature and the separation of the products is facile. During the reactions, the color of the oxidants changes from orange to brown, providing visual means for ascertaining the progress of the oxidation. The mechanism for the presents oxidation are still unclear. However we assume that the mechanism of oxidation are similar to that of other Halochromates. In addition these oxidants and the oxidation conditions can be used for the synthesis of highly functionalized molecules.

Oxidations may also occur using only TPPXC, in the absence of wet SiO₂, but considerable improvements the yields and the corresponding reaction times are observed in the presence of the absorbent. These imply that the wet SiO₂ 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.

Table1: Oxidations via TPPFC, TPPCC and TPPBC.

TPPBC		TPPCC		TPPFC		Product	Substrate	Sr. No.
Yield (%)	Time (min)	Yield (%)	Time (min)	Yield (%)	Time (min)

90	150	90	160	92	170	n-C₂H₅-CHO	n-C₃H₇-OH	1
90	110	90	120	90	130	2-C₂H₅-CHO	2-C₃H₇-OH	2
91	110	93	110	94	120	n-C₃H₇-CHO	n-C₄H₉-OH	3
92	90	92	95	94	100	n-C₃H₇-CHO	2-C₄H₉-OH	4
87	80	88	85	90	90	n-C₄H₉-CHO	n-C₅H₁₁-OH	5
85	70	86	70	88	70	n-C₇H₁₅-CHO	n-C₈H₁₇-OH	6
87	30	88	30	90	35			7
94	40	95	40	96	45			8
90	5	92	5	94	5			9

CONCLUSION:

The synthesized oxidants efficiently oxidize number of organic substrates including primary, secondary alcohols. TPPXC appears to be a stronger reagent than the others; these could be due to their lower symmetry. Oxidations may also occur using only TPPXC, in the absence of wet SiO₂, but considerable improvements the yields and the corresponding reaction times are detected in the presence of the absorbent. These suggest that the wet SiO₂ might act by way of a reaction medium, providing an effective heterogeneous surface area for the oxidation and at the same time making the work-up much more convenient.

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Received on 26.12.2015 Modified on 25.01.2016

Asian J. Research Chem. 2016; 9(5): 193-196

DOI: 10.5958/0974-4150.2016.00032.8