The role of geometry on regioselectivity and rate of fluorination of fluorene and diphenylmethane with Selectfluor™ F-TEDA-BF4

Abstract

Diphenylmethane and fluorene were used as target molecules in an investigation of the effect of the geometry of aromatic molecules on the regioselectivity and rate of fluorination with 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor F-TEDA-BF4). In acetonitrile at 80 degrees C ring fluorination of diphenylmethane was accompanied by oxidation of the saturated carbon atom, while in trifluoroacetic acid only ring fluorination with an ortho-para regioselectivity of 1.8:1 was observed. Fluorene was converted in acetonitrile as well as in trifluoroacetic acid into 2- and 4-fluoro substituted products in the relative ratio of 2:1 and 1.2:1, respectively. The reactions in acetonitrile obey a simple rate equation: v = d[F-TEDA]/dt = k2 x [F-TEDA] x [Substrate] and the second order rate constants for the reactions in acetonitrile at 65 degrees C were determined; values of 0.6 x 10(-4) M-1 s-1 for diphenylmethane and 35.5 x 10(-4) M-1 s-1 for fluorene were obtained. The reaction rates for the various functionalisations of fluorene relative to those for diphenylmethane were found to be considerably influenced by the type of functionalisation. Relative rate factors (k(rel) = k2(fluorene)/k2(diphenylmethane)) with values between 59 for fluorination and 712 for chlorination were determined, while the corresponding data for the biphenyl/diphenylmethane pair were only slightly dependent on the type of functionalisation. A reaction pathway involving electron transfer, thus forming cation radical intermediates, was proposed as the main process in the case of fluorination of fluorene with F-TEDA-BF4.

Reactivities of electrophilic N–F fluorinating reagents

“…The liberated iodine was then titrated with Na2S2O3, to afford second-order rate constants. The rates of fluorination of anisole 74, 96 fluorene 76, 97 diphenylether 78, 96 dibenzofuran 80, 96 biphenyl 82 96 and…”

“…Please do not adjust margins diphenylmethane 84 97 using Selectfluor™ 25 were determined in MeCN (Table 3). The synthetic reactions are summarised in Scheme 5 to show regioselectivities and isolated yields.…”

“…Scheme 5: Fluorination of anisole 74, 96 fluorene 76 97 diphenylether 78, 96 dibenzofuran 80, 96 biphenyl 82 96 and diphenylmethane 84 97 using Selectfluor™ 25.…”

“…Summary of second-order rate constants, k2, for the fluorination of aromatic systems by Selectfluor™ 25 in MeCN at 65 °C 96,97. …”

Electrophilic fluorination of 6-methyl- and 1,3,6-trimethyluracils in water

“…Preliminarily prepared solutions of compound 1 (c = 0.079 M) and F-TEDA-BF 4 (c = 0.0159 M) were kept for 60 min at 40°C and were then mixed together. The progress of the reaction was monitored by the concentration of F-TEDA-BF 4 (samples were withdrawn at definite time intervals), which was determined by iodometric titration [24]. The kinetic study of the fluorination of compounds 2 and 3 was performed in a similar way.…”