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Bromofluorination of alkenes: 1‐bromo‐2‐fluoro‐2‐phenylpropane product: 1‐Bromo‐2‐fluoro‐2‐phenylpropane

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Triethylamine tris(hydroflu0ride)2-Fluoro-2-phenylalkanoic acids 2 are obtained by oxidation of the corresponding P-fluoro-p-phenyl alcohols 6. These compounds are shown to be accessible by two alternative pathways, either by bromo fluorination of a-alkyl styrenes 3, subsequent bromine-by-acetate replacement in the bromo fluorides 4 and hydrolysis of the acetates 5 or by a BF3 . OEt2 catalyzed ring opening of corresponding styrene oxides 8 with triethylamine tris(hydrof1uoride).Fluorine-containing compounds are widely used in scientific research and industry. Among the carbonyl compounds fluorinated carboxylic acids are used as substrates for biochemical studies and as building blocks for the synthesis of large, selectively fluorinated molecules ['].Utilization of fluorinated acids for analytical purposes became particularly common[2] since Mosher et aI.L3] described the efficiency of 2-methoxy-2-phenyl-2-(trifluoromethy1)acetic acid (MTPA) (1) as a chiral dcrivatizing agent to determine the enantiomeric excess of optically active alcohols and amines by 19F-NMR spectroscopy after esterification or amidation, respectively.However, the applicability of compound 1 is sometimes limited. Shift differences in I9F-NMR spectra in some cases are very due to the distance of the fluorine atoms to the stereogenic centers originating from the alcohol or amine part of the formed esters or amides, respectively. In these cases optically pure carboxylic acids like perfluoro-2-propoxypropionic acidL4] or compounds of general structure 2 bearing the fluorine atom at the stereogenic center in aposition to the carboxylic function were shown to be a significant impro~ement [~-~].The use of 2-fluoro-2-phenylacetic acid (2e) as the simplest representative of this class of compounds increases the shift differences of the diastereomeric esters in 19F-NMR spectra up to ten times in comparison with MTPA l[3b].[*I Present address: Department of Chemistry, State University of New York at Albany, Albany, New York 12222, USA.Replacement of the a-hydrogen in 2e by a methyl group often causes a further enhancement of the shift differences and also prevents the possibility of racemization. Best results with respect to large shift differences were obtained in the case of diastereomeric esters of a-cyano-a-fluorophenylacetic acid (2f) [7]. In spite of these interesting results only two methods for the synthesis of the optically active compound 2a are known. In both cases fluorine is introduced into methyl 2-phenylpropionates either by use of N-fluorocamphorsultame after metalation[*] giving a low enantiomeric excess of the product or by replacement of the amino group in racemic 2-amino-2-phenyl-propionate by fluoride[@. The separation into the enantiomerically pure compounds was accomplished by classical fractional crystallization of phenylethylamino salts. Higher yields (65 or 83%, respectively) of racemic methyl 2-fluoro-2-phenylpropionate were obtained by fluorination of the parent ester using Nfluoro-bissulfonyl imide~ [~]. Results and Discussio...