200 Acta Chim. Slov. 2005, 52, 200–206 Scientific Paper The Effect of Reaction Conditions on Activation Parameters for the Mild Introduction of Fluorine into Phenyl Substituted Alkenes with Selectfluor™ F-TEDA-BF/ Maja Papež-Iskra," Marko Zupan,"* and Stojan Stavber6* " Department of Chemistry of the Faculty for Chemistry and Chemical Technolog,; of the University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia, E-mail: stojan.stavber@ijs.si b “Jožef Štefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia, E-mail: stojan.stavber@ijs.si Received 23-12-2004 Dedicated to the memory ot Prof. Dr. Tatjana Malavašič Abstract l-Chloromethyl-4-fluoro-l,4-diazoniabicyclo[2.2.2]octane bis-(tetrafluoroborate) (Selectfluor™ F-TEDA-BF4) reacted with phenyl-substituted alkenes in acetonitrile and Ritter-type fluoroamidation was observed, while in the presence of a nucleophile (H20, MeOH), the formation of vicinal fluorohydroxy or fluoromethoxy alkanes was the predominant process. Mild fluorine introduction obeys a simple rate equation: v= d[F-TEDA-BFJldt= k2 [F-TEDA-BFJ [alkene] and the follovving second order rate constants were determined in acetonitrile at 22 °C: k2=9.0xlCT3]vrV1 for trans-stilbene (la), 2.0xlCT3]vrV1 for cis-stilbene (lb), 8.4xlO_3M_1s_1 for l,l-diphenylethene (le) and 0.29xlCT3]vrV1 for styrene (Id) (calculated from k2=3.9x KrtvrV1 at 50 °C), while the presence of a nucleophile had no a significant effect on the second order rate constants. Activation enthalpies were betvveen 72.7 kJmol-1 and 65.5 kJmol-1 in acetonitrile, while the presence of a nucleophile lovvered the activation enthalpies for la, lb, le, vvhereas higher values were observed for styrene. Activation entropies were betvveen -37.9 e.u. and -68.7 e.u. in acetonitrile, the presence of nucleophile lovvering the activation entropy for la, lb, le, while the opposite effect was observed for styrene. Solvent polarity variation (Grunwald-Winstein Ybemyl) had a very small effect on the rate of fluorination of styrene and l,l-diphenylethene, indicating a small change in the polarity of the rate determining transition state in comparison with the reactants. Key words: Selectfluor™ F-TEDA-BF4, alkenes, fluorination, kinetics, activation parameters Introduction Phenyl substituted alkenes are very convenient substrates for investigation of the role of the reagent on the funetionalization of alkenes. Rate constants, activation parameters and in some cases also the Hammett p values were often determined and used as convenient tools for mechanistic evaluation.14 Halogenation of phenvl substituted alkenes has been extensively studied. The kinetics of their bromination were most widely investigated,58 analogueous informations about chlorination reactions are less extensive,8 while recently some investigations dealing with the kinetics of fluorine introduction in organic compounds were reported.9 The lack of kinetic data on the mild introduction of fluorine into organic molecules using “electrophilic” fluorinating reagents like CF3OF, CF3COOF, CsS04F, XeF2, etc. could be aseribed in some cases to their high reactivity and high sensitivity to the reaction conditions.1015 On the other hand, one of the most important break-throughs in modern organofluorine chemistry has been accomplished in the nineties of the last century by the introduction and broad synthetic application of organic molecules ineorporating a reactive N-F bond as mild fluorinating reagents.16 Being easy handling beneh top materials, usually with optimal stability/reactivity characteristics and attractive costs, N-F reagents have revolutionarily changed the perception of synthesis of fluorinated organic compounds. Three main type of N-F reagents are used for this purpose: neutral N-fluoro amines (RJR2NF), N-fluoropyridinium and related salts, and quaternary N-F salts (F-N+R1R2R3Y). The most widely used member of the last group is 1-chloromethyl-4-fluoro-l,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) known under the trade name of Selectfluor™ F-TEDA-BF4.16e f In order to obtain some further information about the mechanism of seleetive mild introduction of a fluorine atom into organic compounds with the electrophilic-type of fluorinating Papež-Iskra et al. Introduction of Fluorine into Phenyl Substituted Alkenes Acta Chim. Slov. 2005, 52, 200–206 201 reagents, we decided, encouraged by the fact that various reactions of F-TEDA-BF4 could be easily followed by iodometric titration,17 to study their reactions with phenyl substituted olefins with special attention to the effect of the structure of the target compounds and solvent polarity on the rate of fluorination. Results and discussion It has already been demonstrated that phenyl substituted olefins18 reacted with F-TEDA-BF4 at room temperature or at slightly evaluated temperatures, while the structure of the product depended on the reaction conditions and the structure of the alkene (1). Vicinal fluorohydroxy (3) or fluoromethoxy derivatives (4) were formed in the presence of water or methanol,19'20 while Ritter-type21 of functionalization giving vicinal fluoroacetamides(2) was observed in acetonitrile (Scheme 1). First we investigated the kinetics of the reaction of trans-stilbene (la) with F-TEDA in an acetonitrile-water mixture, following the progress of reaction by iodometric titration. We found that the rates of fluorination obey a simple second order rate equation: v= d[F-TEDA]/dt= k2 [F-TEDA] [alkene]. The effect of variation of trans-stilbene concentration on the pseudo first order rate constants is presented in Figure 1, while the effect of alkene structure on its fluorination with F-TEDA-BF4 in acetonitrile is shown in Figure 2. Similar second order rate behaviour was also observed for the reactions of F-TEDA-BF4 with c/s-stilbene, l,l-diphenylethene and styrene in acetonitrile, acetonitrile-water and acetonitrile-methanol mktures (molar ratio 1: F-TEDA-BF4: ROH= 2: 1: 7.5). As evident from Table 1, the presence of a nucleophile did not have an important effect on the second order rate constants, in spite of the f act that different products were actually formed (2, 3, 4). Ph- Ph. tf c=c/ R1/ V 1 0/—\0 cih2c-n^Vn-f (BF4~)2 R-OH/ 'CH3CN 1a: R1=R3=H, R2=Ph 1b: R1=R2=H, R3=Ph 1c: R1=Ph, R2=R3=H 1d: R1=R2=R3=H R1 R2 -C-C-R3 NH F COCH3 R1 R2 Ph-C-C-R3 OR F 3: R=H 4: R=CH3 Scheme 1 Further, we investigated activation parameters for the introduction of fluorine into phenyl substituted alkenes (1) in acetonitrile, acetonitrile-water and acetonitrile-methanol mixtures, and from their dependence on temperature AH* and AS* were determined. As evident from Table 1, the values of the activation enthalpies in acetonitrile are almost the same for stvrene, trans- and c«-stilbene, but lower for l,l-diphenylethene. On the other hand, larger differences were observed for the entropy factors, being the largest for stvrene and the lowest for trans-stiVoene for the reactions in acetonitrile. The presence of ROH lowered AH* of fluorination for l,l-diphenylethene, cis and trans-stilbene, while an increase in activation entalpy was observed in reactions with stvrene, where the enthalpy was found to be higher in methanol than in water. A similar effect of the variation of solvent was also evident from the entropy factor, where values were lower in the presence of a nucleophile in the čase of l,l-diphenylethene, cis- and trans-stilbene, while in the čase of stvrene a shift from -68.7 e.u. in acetonitrile to -26.3 e.u. in the presence of methanol was observed. 3 2.5 2 1.5 1 0.5 f 0 0 c [10-2M] Figure 1. The effect of trans-stilbene (la) concentration on the pseudo first order rate constant of fluorination with F-TEDA (c0=lxlO~2M) at 22 °C in an acetonitrile-water mixture (C„ate, =7-5X1°~2M)- la 97--92- 87 82-77 72-b 67 2000 4000 6000 8000 10000 12000 t [s] Figure 2. The effect of alkene structure on fluorination with F-TEDA in acetonitrile at 22 °C for o trans-stilbene (la), A cis-stilbene (lb), () l,l-diphenylethene (le) and at 30 °C for n styrene (Id). Important information about the nature of the intermediates involved in transformation of alkenes l 2 3 2 0 Papež-Iskra et al. Introduction of Fluorine into Phenyl Substituted Alkenes 202 Acta Chim. Slov. 2005, 52, 200–206 could be obtained by using solvents with various dielectric constants, or by using a mixtures of solvents where Grunvald-Winstein Y values were already determined22 as in the čase of acetonitrile-water mixtures.23 As evident from Table 2, variation of Y in the range of 3.19 units had a very small effect on second order rate constants for l,l-diphenylethene and styrene. The effect of solvent polarity on various functionalizations of the sp2 carbon atom is presented in Figure 3; in bromination of methylideneadamantane24 and solvolysis of p-methoxybenzoyl chloride25 very large effects were observed (ionic reactions) in comparison with the values determined for styrene fluorination with F-TEDA-BF4, indicating small changes in the polarity of the rate determining transition state in comparision with the reactants. Table 2. The effect of solvent polaritv on the second order rate constants for fluorination of l,l-diphenylethene and stvrene in acetonitrile-water solution." k2 [lO^M-1«-1] l,l-Diphenyletene Stvrene 90An -1.45 4.9 2.6 80An -0.35 3.2 2.1 70An 2.8 60An 0.81 2.8 2.3 50An 3.4 40An 1.74 3.6 " Reactions at 22 °C for l,l-diphenylethene and at 50 °C for stvrene.h % v/v of acetonitrile in water solution. c Values from ref. 23. 10 T -2 -4 -2 Yi benzyl Figure 3. The effect of solvent polarity (Ybenzyl) on fluorination of styrene with F-TEDA-BF4 in an acetonitrile-water mixture at 50 °C (n) in comparison with bromine addition to methylidene-adamantane24 and solvolysis of/>-methoxybezoyl chloride.25 Table 1. The effect of alkene structure (1), nucleophile (Nu)a and reaction temperature on the second order rate constants for fluorination with F-TEDA. Alkene Nu T [°C] K2 [lO^M-V1] AH" [kJmor1] AS" [Jmor1^1] 17.0 5.3 ±0.4 - 22.0 9.0±0.3 72.7 -37.9 27.0 14.8±0.4 PK || 17.1 5.5±0.1 Ph H20 22.0 9.5±0.1 71.1 -43.0 26.9 14.9±0.5 17.0 5.7±0.1 MeOH 22.0 9.1 ±0.4 68.9 -50.6 27.0 15.1±0.5 16.9 1.2 ±0.1 - 22.0 2.0±0.1 72.2 -51.7 27.0 3.5±0.1 PhvN || 16.9 1.4±0.0 Pri H20 21.9 2.3 ±0.06 68.4 -63.7 27.0 3.8±0.15 17.1 1.5±0.1 MeOH 22.0 2.3 ±0.2 64.0 -78.7 27.1 3.7±0.1 17.0 5.3 ±0.1 - 21.9 8.4±0.2 65.5 -62.6 27.0 13.5±0.5 PlK/Ph 17.2 5.3 ±0.1 H20 22.0 8.2±0.2 64.7 -65.6 le 27.1 13.3 ±0.2 17.1 5.5±0.3 MeOH 22.0 8.8±0.5 60.9 -78.0 27.1 13.0±0.5 50.0 3.9±0.0 - 54.9 6.4±0.3 72.0 -68.7 50.0 9.0±0.2 Id 30.0 0.57±0.1 H20 34.9 0.94 ±0.1 76.1 -56.0 40.0 1.57±0.01 30.1 0.51 ±0.04 MeOH 35.0 0.88±0.01 85.5 -26.3 39.9 1.52±0.02 " Concentration of alkene (2xlO~2M), F-TEDA-BF4 (lxlO~2M) and R-OH (7.5xlO~2M); molar ratio of alkene: F-TEDA-BF4 : R-OH= 2: 1: 7.5. In order to compare the reaction parameters for fluorination reactions of phenyl-substituted alkenes by F-TEDA-BF4 with parameters for funetionalization by other reagents, second order rate constants for various transformations of styrene and relative rates for the same transformations for 1,1-diphenylethene and trans-stilbene are presented and compared in Table 3. 8 6 4 2 0 -1 0 2 3 Papež-Iskra et al. Introduction of Fluorine into Phenyl Substituted Alkenes Acta Chim. Slov. 2005, 52, 200–206 203 Table 3. The effect of reagent on second order rate constants for functionalization of styrene (1d) and the relative reactivities for 1,1-dipheylethene (1c) and trans-stilbene (1a). Reagent React. cond. Ref. k2 [M-V1] Id Relative rates lc/ld la/ld Br2 MeOH; 25 °C 26 1.53X103 28 0.0071 Cr02Cl2 10 °C, CC14 27 2.9X10' 12.3 0.16 Cr03 0.002MH2SO4m 95% MeC02H; 25 °C 28 2.78X10-2 2.2 1.32 MeC03H MeCOOH; 26 °C 29 1.12X10-2 4.29 0.6 F-TEDA 7.5xlO-2M H20 in MeCN; 22 °C 2.5X10-* 32.8 38 * calculated from k2 of 0.57x10 3 M xs 1 at 30 °C. It is evident that the introduction of a second phenyl group on the same carbon atom increased the rate of the transformation and the effect strongly depended on the reagent used. In ali demonstrated examples l,l-diphenylethene was more reactive than styrene, and increased reactivity was especialh/ evident for the reaction with F-TEDA-BF4, while only in the čase of the reaction with ArSCl was a decrease in reactivity observed, where a relative rate factor of 0.4 was found.30 Introduction of a second phenyl group on the second carbon atom was reflected in lower reactivity compared to stvrene in the čase of bromination with Br2, chlorination with Cr02Cl2 and epoxidation with MeCOOOH, a small increase of reactivity was observed in the čase of reaction with Cr03, while interestingly, fluorination of trans-stilbene with F-TEDA was found to be significantly faster than reaction with styrene. The effect of the reagent on the activation parameters for various transformations of styrene, />-nitrostyrene and l,l-diphenylethene are presented in Table 4. It is evident that fluorine introduction with F-TEDA-BF4 into these alkenes required a higher energy of activation than chlorine and Table 4. The effect of reagent styrene (ld),/>-nitrostyrene and on activation parameters for l,l-diphenylethene (le). Alkene Reagent Ref. AH* [kJmor1] AS* [Jmort1] Stvrene (Id) Br2 31 19.7 -157.4 ISCN 30 32.2 -184.2 Cr02Cl2 27 33.9 -99.6 NOC1 30 38.1 -154.9 ArSCl 30 54.0 -129.8 MeC03H 32 54.4 -131.8 PI1CO3H 30 59.4 -104.6 F-TEDA 76.1 -56.0 p-N02-Styrene Br2 Cl2 34 34 37.7 22.2 -159.0 -123.5 l,l-Diphenyl- Cr02Cl2 27 13.8 -147.8 ethene (le) F-TEDA 64.7 -65.6 bromine addition reactions; however the similarity in energy profile with peroxyacids is evident. The effect of alkene geometry and reagent on relative reactivities and activation parameters for transformations oicis- and trans-stilbene are presented in Table 5. It is known that the rate of bromine addition was strongly influenced by solvent polarity and activation parameters for bromine addition in acetic acid33 were closer to F-TEDA-BF4 fluorination of stilbene than those observed for stvrene derivatives31 (Table 4). However, relative reactivities were quite different and bromination of the cis-isomer is faster than for trans-analogue, while for fluorination with F-TEDA-BF4 the opposite situation was established. Table 5. The effect of reagent strueture and geometry of 1,2-diphenylethene on relative rates and activation parameters. Alkene Ph Ph Ph lb Ph la Reagent Br233 Cr02Cl227 F-TEDA Br233 Cr02Cl227 F-TEDA AH* [kJmor1] 53.5 37.5 68.4 52.3 34.2 71.1 AS* [Jmol^K-1] -90.0 -158.7 -63.7 -100.9 -110.9 -43.0 k(lb)/k(la) 2.96 1.79 0.24 It has been suggested that the electron transfer process is the main reaction path for mild introduction of fluorine into phenyl substituted olefins with the N-F type of reagents.16'35'36 Laser flash photolysis generation of ion-radicals from styrene and its substituted derivatives clearly indicated that the primary process is attack of a nucleophile at position C-2, and formation of a radical intermediate has also been proven.37'38 On the other hand, it has been demonstrated that attack of the nucleophile on ion radicals at position C-2 is also very dependent on the nucleophile strueture and steric arrangement at C-2; the second order rate constants for methanol ineorporation are as follows:37 1.8xl08 M_1s_1 for stvrene, 1.9x 108 M-1 s-1 for 2-phenyl-propene, 9.7x 106 M-1 s-1 for E-l-phenyl-propene and 2xl05 M_1s_1 for 1-phenyl-2-methyl-propene. Scheme 2 presents possible reaction pathways for mild introduction of fluorine into alkenes with F-TEDA-BF4. In the first step the formation of a n complex is suggested, followed by an electron transfer from the alkene to F-TEDA-BF4 thus inducing a formation of cation radical with a positive charge on C-2 (similar to laser flash photolysis studies37) and of F-L" which could decompose to a pair consisting of the fluoride anion and L", or a fluorine radical and L: (intermediate A). However, attack of acetonitrile, water or methanol on C-2 was not observed, but in spite of this f act the situation in the proposed intimated pair A is not clear since attack by fluoride anion on the ion radical Papež-Iskra et al. Introduction of Fluorine into Phenyl Substituted Alkenes 204 Acta Chim. Slov. 2005, 52, 200–206 R1x R2 ©e >=< + F-L BF4 p/ R3 ©e F-L BF4 R1.,,,, 1 „iR2 Ph^ NR3 7t-COMPLEX D C Ph' © 8- BF4 F-L „iR2 R3 BF4 rV® k„r2 Ph' Ph R1 R2 C CR3 NH F COCH3 A R3 R1 R2 Ph-C-C-R3 OR F R1'.,,,. ©,„iR2 Pt/ N3 F-L* • © L F © . BF4 L RV . f „R2 Ph' *R3 >' ©e F-L BF4 f:l i BF4 B j\ ROH \ _R1...... ^R2 Pt/^R3 OR ©/----\® CIH2C-N''^N-F (BF4~)2 Scheme 2 could not be excluded. Solvent polarity studies and structural variation of the alkene indicated only a small difference in the polarity of the reactants and the rate determining transition state, supporting the formation of a fluoro-substituted radical E. The nonpolar nature of the rate determining step is also supported by the low Hammet value p + of -1.42 for fluorination of substituted l,l-diphenylethenes20 (the following p+ values for styrene derivatives were determined: -1.3 for C6H5C03H,40'41 -2.03 for ArSCl,30 -2.08 for NOC1,30 -2.48 for CF3OF,42 -2.59 for JSCN,30 -3.18 for methoxymercuration,43 -3.22 for chlorination,34 -4.5 for bromination44). On the other hand, the recent observation that with F-TEDA-BF4 norbornene gave rearranged 2-exo-acetamido-7-syn-fluoronorbornane and 2-exo-acetamido-7-anti-fluoronorbornane suggested the carbonium ion nature of the intermediate,39 and for this reason we believe that the rate determining transition state in fluorination of alkenes has a nonpolar nature C, while further processes are much faster. The fluoro carbonium ion D reacted either with acetonitrile giving Ritter-type45 of products or with methanol or water, where the nucleophile entered according to Markovnikov type of regioselectivity. In addition, in the present study we observed some similarity in the kinetic behaviour of fluorination of phenyl substituted olefins with F-TEDA-BF4 and their epoxydation with peracids what indicates possible analogy in geometry of the structure of rate determining transition state.32 Experimental l-Chloromethyl-4-fluoro-l,4-diazoniabicyclo[ 2.2.2]octane bis(tetrafluoroborate) was crystallised from an acetonitrile-methanol mixture and dried in a vacuum at 20 °C for 5 hours. Styrene (Merck), l,l-diphenylethene (Merck, Aldrich), c/s-stilbene (Aldrich) were distilled and trans-stiVoene was crystallised before use. Acetonitrile (Merck) and methanol (Merck) were purified by distillation and stored over molecular sieves. KI (Merck) and standard solution of Na2S203 (Riedel-deHaën)) were used as received. Characterisation data for the products 2, 3, and 4 were already reported.1921 Determination of pseudo first order rate con stan ts for the reaction of trans-stilbene with F-TEDA-BF4: To 35 mL of a thermostatted acetonitrile-water solution (4.125 mmol of water) of various amount of trans-stiVoene (0.275, 0.55, 1.1, 1.65 mmol), 20 mL of a thermostatted acetonitrile solution of F-TEDA-BF4 (0.55 mmol) was added and stirred at 22 °C. After various times 10 mL aliquots were mixed with 20 mL of ice cold 0.02M KI and the liberated iodine titrated with 0.05M Na2S203. A linear correlation was found for lnc= f(t). Pseudo first order * E Papež-Iskra et al. Introduction of Fluorine into Phenyl Substituted Alkenes Acta Chim. Slov. 2005, 52, 200–206 205 rate constants were calculated and the effect of trans- stilbene concentration on kt is presented in Figure 1. Determination of rate order and activation parameters for fluorination with F-TEDA-BF4: To 35 mL of a thermostatted acetonitrile (or mixtures containing 4.125 mmol of water or methanol) solution of 1.1 mmol of substrate (la, lb, le, Id), 20 mL of a thermostatted solution of F-TEDA-BF4 (0.55 mmol) was added and stirred at various temperatures. The progress of F-TEDA-BF4 consumption was monitored by iodometric titration. Second order rate constants were calculated from the equation: 1/(A-B) ln(Ba/Ab) = k2t and a linear relationship was found. The effect of alkene strueture on second order rate constants is presented in Figure 2 and Table 1. Further, we investigated the effect of the temperature on k2. A linear correlation was found and activation parameters were calculated by linear regression from the equation: In (kJT) = In (kjh) +AS*/R - AH*/RT. Results are presented in Table 1. 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Povzetek Reakcije l-klorometil-4-fluoro-l,4-diazoniabiciklo[2.2.2]oktan bis(tetrafluoroborata) (Selectfluor™F-TEDA-BF4) s fenil substituiranimi alkeni vodijo v acetonitrilu ob prisotnosti nukleofilov (voda, metanol) do nastanka vicinalnih fluorohidroksi oz fluorometoksi alkanov, medtem ko v samem acetonitrilu prevladuje Ritterjev tip transformacije in nastanek vicinalnih fluoro acetamidov. Hitrost reakcije sledi enačbi: v = d[F-TEDA-BF4] / dt = k2 [F-TEDA-BFJ [alken] in za reakcije v acetonitrilu pri 22 °C so bile določene sledeče konstante reakcijskih hitrosti: k2=9.0xlO~3M~V1 zafram-stilben (la), 2.0xlO_3M"V1 za cw-stilben (lb), 8.4xlO"3M_1s_1 za 1,1-dipfenileten (le) and 0.29xlO~3M~V1 za stiren (Id) (izračunano iz k2=3.9x lO^M^s-1 pri 50 °C). Ugotovljeno je bilo, da prisotnost nukleofila ne vpliva bistveno na vrednosti konstant reakcijske hitrosti. 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