Synthesis of 3-Phenoxy-4-Fluorobenzaldehyde

One fluorine atom in position 4 of 3-phenoxy benzaldehyde improves the insecticidal activity of its ensuing pyrethroid esters (cyfluthrin and flumethrin) and nonesters by a factor of at least two (see p. 42, Vol. 4 of this series). Production of this aldehyde 288 certainly is more expensive. An essential step is also the Ullmann-reaction with a fluorine atom in the nucleus of one component 289 (Reaction scheme 206) [599-604]. 

Synthesis of this fluorine-containing intermediate poses a special synthetic problem. Several proposals for making 3-bromo-4-fluorotoluene 291, -benzaldehyde 290 or -benzoic acid are matter of patent applications. The next Reaction schemes 207/208 outline the reactions starting with fluoroaromatics. 

Introduction of the fluorine can be done by chloride-fluoride exchange (Reaction scheme 210) or according to the Balz-Schiemann decomposition of diazonium tetrafluoroborate 292 or triazene 293 in liquid hydrogen fluoride (Reaction scheme 210). 

Reduction of the acid stage to the aldehyde 2S8 or 290 is possible by catalytic hydrogenation of the corresponding a-ketophosphonic esters [615] and subsequent termal [617] or alkaline [618] decomposition of the a-hydroxyphosphonic esters. Another possibility is the selective reduction of the ester with deactivated aluminum-hydrides [619] or catalytic reduction of the acid chloride [620]. On a bench scale, reduction of the bromofluoro benzonitrile with Raney nickel in formic acid [621] is particularly advisable. 

Functionalization of the 3-phenoxy-4-fluorotoluene follows the lines as mentioned earlier. 

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