Decarboxylation fluoroformates

Therefore, much effort has been devoted to the research of new industrializable routes to fluorinated aromatics derivatives which are as far as possible versatile and without the drawbacks of classical processes. Among the reported reactions (refs. 4-7) aryl fluoroformate decarboxylation seems to be very attractive. Aryl fluoroformates are easily prepared from the corresponding phenol, phosgene and anhydrous hydrogen fluoride, which are widely available and cheap raw materials. 

Also unsuccessful have been all attempts to decarboxylate fluoroformates of various carbohydrates, but it has been shown that an excess of reagent, solvent, temperature and time of reaction are all of great importance in obtaining high yields for the fluorination of carbohydrate chloroformates (Table l).5... 

Attempts to apply the thermal decarboxylation reaction in the liquid phase to aromatic halofor-mic acid esters have shown that their reaction is different from that ol the aliphatic haloformates. It was found that evolution of carbon dioxide occurs, but only high boiling products could be isolated.136 On heating in the presence of aromatics and Lewis acids, aryl chloroformates do not react to give chlorinated aromatics with concomitant decarboxylation, but undergo a Friedel-Crafts reaction to give phenyl benzoates.137 Under similar conditions phenyl fluoroformate undergoes only polymerization and carbonate formation.137... 

Alkyl fluoroformates can be decarboxylated, for example by heating in the presence of pyridine or BF, to give the corresponding fluoride [380,382], or they can be further treated with SF4 to produce ROCF, [1856b,1856c] ... 

Aryl fluoroformates can be decarboxylated into aromatic fluorides by gas-phase decomposition at 600-800 C [380,382], or at around 300 C by passing the vaporized aromatic fluoroformate over an alumina catalyst [86]. 

The reaction of carbonyl difluoride with carbonyl compounds follows the general pattern of addition of the F and C(0)F units across the unsaturated C=0 entity. However, the resulting fluoroformate can be frequently readily decarboxylated to the corresponding... 

As with phosgene, the lactone 3-propanolide is ring-opened upon treatment with COF j (100 C over 8 h in the presence of pyridine) to give the fluoroformate derivative (as opposed to the decarboxylated product), according to [632] ... 

Phosgene is reported to combine with a wide range of oxygenated materials, including alcohols, ethers, ketones, carboxylic acids, anhydrides, lactones, esters, carbonic acid derivatives, etc. Only the reactions of COCIF with alcohols, phenols and cyclic ethers have been reported, resulting usually in the generation of fluoroformates. Such materials can often be usefully converted into the corresponding fluoro compound by means of decarboxylation in the presence of BF3, EtjO, pyridine, or other materials. 

The thermal decomposition of aryl fluoroformates was examined by Christe and Pavlath (using a platinum gauze catalyst) [380-383], who found that temperatures typically in the range of 700-800 "C were required in order to produce the most favourable yields of the corresponding fluorides. These temperatures, however, are considered to be beyond the scope of common industrial practice. More recently, the decarboxylation of phenyl fluoroformate, for example, has been achieved in a flow system, employing alumina catalysts, at temperatures of around 300 "C [86,87]. 

By using a three molar equivalent of COCIF at 50 "C and 1.3 Mpa, over 5 h in the presence of tributylamine, a conversion of 97.2% of phenol with 99.9% yield of phenylfluoroformate was obtained [380], Phenyl fluoroformate may be decarboxylated to give fluorobenzene ... 

Thermal decarboxylation of the resulting fluoroformate derivatives at around 700-800 C gave the corresponding substituted fluorobenzene, in yields of up to about 50% [381]. 

The reaction of methanal with COCIF has been examined in both the gas and liquid phase. In the liquid phase, in the presence of a catalytic amount of a tertiary amine in toluene, the novel fluoroformate CH2CI0C(0)F is produced and undergoes subsequent rapid decarboxylation over an alumina catalyst at >100 "C to give a high yield (ca. 95%) of CHjClF [ICI118]. 

Depending upon the reaction conditions, ethanal can combine with phosgene to produce 1,1-dichloroethane, chloroethene, vinyl chloroformate or 1-chloroethyl chloroformate (see Section 10.3.3.1). However, imder conditions which produce 1,1-dichloroethane from phosgene and ethanal (charcoal catalyst, 150 C), carbonyl difluoride reacts (using a charcoal catalyst impregnated with caesium fluoride) to produce a mixture of the saturated 1 -fluoroethyl fluoroformate and the unsaturated vinyl fluoroformate (see Section 13.14.6.3.1). Thus, the COFj/CHjCHO/charcoal system is unstable with respect to dehydrofluorination, whilst the COClj/CHjCHO/charcoal system is unstable with respect to decarboxylation ... 

Fluoroformic and chloroformic esters of secondary alcohols may be decarboxylated, thermally or catalytically, to the corresponding fluoro-i or chloro-i i compound. Although 6-0-(chloroformyl)-1,2 3,4-di-O-iso-propylidene-a-D-galactopyranose is converted into 6-chloro-6-deoxy-l, 2 ... 

The decarboxylation of arylchloroformate (ref. 8) or thiochloroformate (ref. 9) into the corresponding chloroaromatic and the decarboxylation of aliphatic fluoroformate occurs by an ionic mechanism (ref. 10). But, the arylfluoroformate decomposition by this mechanism does not lead to the formation of fluorinated aromatics but instead to phenols and tars (ref. 4a). 

The thermal, selective, decarboxylation of arylfluoroformate was reported for the first time by Christe and Pavlath (ref. 4). The use of vapor phase conditions and of temperatures as high as 700 - 800 °C permits the selective transformation even without catalyst. Under the conditions, the use of Pt gauze as the catalyst improves the selectivity of the reaction. Fluorobenzene can then be obtained with high yield (up to 90 %), but this methodology is of little versatility. Yields are much lower with substituted aryl fluoroformates (ref. 4b). Moreover, these temperatures are considered to be beyond the scope of common industrial practice and particularly for the preparation of fine chemicals. 

Sodium trifluoroacetate, in the presence of copper(l) iodide, was also used as trifluoromethyl source to replace halogen by trifluoromelhyl group in the thiophene system. Sodium trifluoroacetate was decarboxylated, forming fluoroform, when heated alone in aqueous N-methylpyrrolidin-2-one. The addition of copper(l) iodide increased the rate of decarboxylation dramatically. The mechanism of this process was explored and an intermediate [CFsCul] was proposed. Introduction of higher perfluoroalkyl groups from their corresponding sodium perfluoroalkane carboxyl-ates was also shown to be possible [58],... 

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