Easy Preparation of [Bis(trifluoroacetoxy)iodo]arenes from Iodoarenes, with Sodium Percarbonate as the Oxidant

Abstract

Easy and effective preparations of the nearly pure [bis(trifluoroacetoxy)-iodo]arenes, ArI(OCOCF)₃, from some iodoarenes, ArI, are reported, using an anhydrous sodium percarbonate/(CF₃CO)₂O/CH₂Cl₂ system. The colorless, freshly prepared ArI(OCOCF₃)₂ thus obtained were 98-99% pure (by iodometry).

Introduction

[Bis(trifluoroacetoxy)iodo]arenes, ArI(OCOCF₃)₂, are potent and selective oxidants, widely used in modern organic synthesis. So far, they have been synthesized from either ArI, or ArIO, or ArICl₂, or directly from ArH, but the most common method depends on their preparation (in moderate yields) from the corresponding (diacetoxyiodo)arenes, ArI(OAc)₂, by their recrystallization from CF₃COOH [1,2].

Results and Discussion

Recently, in our laboratory we have devised a simple, easy, and efficient method for the direct preparation of ArI(OCOCF₃)₂ from the respective iodoarenes, ArI, in a binary anhydrous solvent system, (CF₃CO)₂O/CH₂Cl₂, using commercially available sodium percarbonate, 2Na₂CO₃^. 3H₂O₂, as the oxidant (Scheme 1 and Table 1):

Scheme 1.

Scheme 1.

In practice, only the fairly stable parent compound, PhI(OCOCF₃)₂ has found widespread use in organic synthesis [1,2,3], although it should be noted that p-Cl- and p-F-C₆H₄I(OCOCF₃)₂ are even more stable. A limited number of other ring-substituted ArI(OCOCF₃)₂ have been reported in the literature [4,5,6]. We have unexpectedly found that several other ArI(OCOCF₃)₂ compounds are less stable, particularly p-MeC₆H₄I(OCOCF₃)₂, and they usually deteriorate within several days to finally form oily residues with a strong acidic odor – even when stored in a dark cooler. As far as limitations of the reported method are concerned, we have found that it is not suitable for the preparation of the three O₂N-C₆H₄I(OCOCF₃)₂ isomers, as well as p-MeOC₆H₄I(OCOCF₃)₂. It is also seen from the data in Table 1 that 3-F₃CC₆H₄I(OCOCF₃)₂ was obtained in only 40% yield by our new method, cf. [6].

Experimental

General

Chemical structures of the compounds in Table 1 were confirmed by satisfactory microanalyses of the crude, freshly prepared products (C ± 0.2, H ± 0.1, I ± 0.2%) obtained at the Institute of Organic Chemistry, the Polish Academy of Sciences, Warsaw.

Optimized Procedure for Preparing [Bis(trifluoroacetoxy)iodo]arenes from Iodoarenes:

A solution of an appropriate iodoarene (5 mmol) in a mixture of (CF₃CO)₂O (12 mL) and CH₂Cl₂ (40 mL) was cooled with stirring to 0-2 ^oC. Next, sodium percarbonate (2.1 g; 13.4 mmol; 300% excess) was added portionwise, and the stirring was continued for 2 h at 0-2 ^oC, and further at room temperature for ca. 18 h. The precipitated CF₃COONa was collected by filtration under reduced pressure, washed with CH₂Cl₂ (2 x 20 mL), and discarded. The filtrates were evaporated under vacuum. The solid residues were triturated with hexane (10 mL), collected by filtration, washed with hexane on the filter, and quickly air-dried by the suction, leaving the colourless title products, ArI(OCOCF₃)₂ (40-87% yields). Their yields and melting points are given in Table 1 below. In practice these products need not to be recrystallized. They should be stored in the dark, preferably in a cooler, avoiding any moisture [3]. The freshly prepared products were 98-99% pure (by iodometry [7]).

Table 1. Yields and melting points (uncorrected) for the prepared [bis(trifluoroacetoxy)iodo]arenes, ArI(OCOCF₃)₂

Ar	Yield [%]	Mp [oC]	Lit. Mp [oC]
phenyl	87	122-124	120-122 [4]
2-methylphenyl	81	89-92	88-90 [4]
3-methylphenyl	86	93-96	87-90 [5]
4-methylphenyl	76	112-115	114-116 [4]
4-fluorophenyl	77	103-105	88-90 [4]
4-chlorophenyl	74	129-132	131-133 [5]
3-(trifluoromethyl)phenyl	40	97-100	99-100 [6]

Table 1. Yields and melting points (uncorrected) for the prepared [bis(trifluoroacetoxy)iodo]arenes, ArI(OCOCF₃)₂

Ar	Yield [%]	Mp [oC]	Lit. Mp [oC]
phenyl	87	122-124	120-122 [4]
2-methylphenyl	81	89-92	88-90 [4]
3-methylphenyl	86	93-96	87-90 [5]
4-methylphenyl	76	112-115	114-116 [4]
4-fluorophenyl	77	103-105	88-90 [4]
4-chlorophenyl	74	129-132	131-133 [5]
3-(trifluoromethyl)phenyl	40	97-100	99-100 [6]