Insights into the mechanism of silver-catalyzed decarboxylative fluorination

Computational and Theoretical Chemistry 1082 (2016) 11–20

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Insights into the mechanism of silver-catalyzed decarboxylative fluorination Xiang Zhang School of Food Science and Biotechnology, Zhejiang Gongshang University, Hangzhou 310035, China

a r t i c l e

i n f o

Article history: Received 6 February 2016 Received in revised form 21 February 2016 Accepted 21 February 2016 Available online 2 March 2016 Keywords: Ag(I)-catalyzed Decarboxylative fluorination Selectfluor Radical Theoretical calculation

a b s t r a c t A systematic study has been given to the Ag(I)-catalyzed decarboxylative fluorination of the carboxylic acid with Selectfluor using density functional theory (DFT). It is proposed that the catalytic cycle consists of five steps, namely the formation of the H2O-ligated Ag-carboxylate, oxidation, homolytic cleavage of the O–Ag(II) bond, decarboxylation (loss of CO2) and fluorine abstraction. It is the formation of the H2O-ligated Ag-carboxylate rather than that of Ag(II)–F or Ag(III)–F intermediate that initiates the fluorodecarboxylation. As for the rate-determining step, there exist two possibilities: (i) the formation of the H2O-ligated Ag-carboxylate and its oxidation by Selectfluor; (ii) decarboxylation (loss of CO2), and which one is dominant depends on the structure of the substrate. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Carbon–fluorine bond formation is playing an ever-growing role in the synthesis of chemicals and pharmaceuticals, for the introduction of fluorine atom alters markedly the physical, chemical, and biological properties of the organic molecules [1–3]. All the time people are committed to developing new methods to construct C–F bond [4]. Especially major progress has been achieved in the transition metal catalysis for the C–F bond formation. For example, Pd-catalyzed nucleophilic fluorination of aryl bromides, iodides and triflates has been achieved by Buchwald’s group with the famous phosphorus ligand (e.g. AdBrettPhos) [5–9]. Ritter’s group has reported Ag-catalyzed electrophilic fluorination of arylstannanes [10,11] and Pd-mediated fluorination of arylboronic acids [12], which exhibit excellent functional group tolerance and substrate scope. Besides, Cu [13], Au [14], Ru [15] and Mn [16] etc have also been applied to the construction of the C–F bond. Through the literature, most of work focuses on the formation of aromatic C(sp2)–F bonds. Methods for the general and sitespecific formation of C(sp3)–F bonds are limited [16,17]. Recently, Li’s group has reported a novel decarboxylative fluorination of aliphatic carboxylic acids catalyzed by AgNO3 without organic ligands [18], shown in Eq. (1). This kind of reaction has its benefits – including remarkable chemo-selectivity and wide functional group compatibility. In the meantime, preliminary

E-mail address: [email protected] http://dx.doi.org/10.1016/j.comptc.2016.02.016 2210-271X/Ó 2016 Elsevier B.V. All rights reserved.

mechanistic studies have been conducted by Li’s group and they suggested an Ag(III)-mediated fluorine-transfer pathway, shown in Scheme 1a. Specifically, the oxidation of Ag(I) by the electrophilic fluorinating reagent 1-chloromethyl-4-fluoro-1,4-diazo niabicyclo [2.2.2] octane bis(tetrafluoroborate) (F-TEDA–BF4, Selectfluor) leads to an Ag(III)–F intermediate, which subsequently undergoes single electron transfer (SET) with a carboxylate anion to form the Ag(II)–F intermediate and carboxyl radical. The alkyl radical results from the decarboxylation of the corresponding carboxyl radical then abstracts the fluorine atom of the Ag(II)–F intermediate to give the alkyl fluoride product and regenerate the Ag(I) catalyst.

ð1Þ Later on, Flowers II and co-workers suggested a different mechanism for the Ag(I)-mediated decarboxylative fluorination on the basis of the spectroscopic and kinetic studies [19], as shown in Scheme 1b. The reaction starts with the formation of the silvercarboxylate, which is then oxidized by Selectfluor to afford Ag(II) intermediate and TEDA–BF4 radical cation in the rate-limiting step. The resulting Ag(II) intermediate oxidizes the carboxylate ligand to form CO2 and an alkyl radical, which abstracts fluorine from

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X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

(a)

Cl N+

(b)

N+

R-F

Ag(I)

N+

F

F-Ag(III)

F

Ag+

R-CO2H

RCOO-Ag

+ F-

N+

N+

rate-limiting step F-Ag(II) R

Cl

Cl N+

TEDA-BF4 radical cation

TEDA-BF4 radical cation

Ag2+-OOCR

R-CO2H Cl

CO2

R-CO2H

N+

(RCOO)2-Ag

N+

CO2 + R

F

Cl N+ N+

+ R-F

TEDA-BF4 radical cation Scheme 1. (a) Ag(III)-mediated fluorine-transfer mechanism proposed by Li’s group; (b) mechanism for the decarboxylative fluorination of aliphatic carboxylic acids proposed by Flowers II et al.

Selectfluor to give product and the TEDA–BF4 radical cation. It is to be observed that the TEDA–BF4 radical cation can oxidize Ag(I) to continue the catalytic cycle. By implication, the dispute about the mechanism for the decarboxylative fluorination includes: (i) whether the intermediates (Ag (III)–F, Ag(II)–F) exist in the fluorination is uncertain; (ii) the role of Ag(I) in the oxidative decarboxylation remains unclear. Owing to the difficulty in trapping the unstable intermediates in the reaction solution, we cannot fully grasp every detail of the reaction. Fortunately, quantum chemical calculation, especially the density functional theory (DFT) [20,21], provides us a tool to investigate theoretically the reaction mechanism at closer range [22–24]. In the present work, as the continuation of our study on the decarboxylation [25–27], we shall conduct a systematic study on the mechanism for the Ag(I)-catalyzed decarboxylative fluorination with density functional theory (DFT), including the ratedetermining step, characteristics of the key intermediates. It is expected that the present work will improve the understanding of the Ag(I)-catalyzed decarboxylative fluorination.

(BS2: SDD [37,38] basis set was used for Ag, and the 6-311++G (2df, 2p) basis set was used for other atoms). In addition, singlepoint solvation energies were calculated by using CPCM solvation model (radii = uaks) [39,40]. As the fluorodecarboxylation runs in a 50:50 ratio of acetone/water media, the dielectric constant e of the solvent was estimated to be 49.4. In the present calculations, DMSO (e = 46.8) was chosen as an alternative solvent. The Gibbs free energies presented in this work are M06 (B3LYP energies were listed in parentheses) electronic energies modified with zeropoint-energy, thermal, and entropy corrections and solvationenergy corrections. Computed molecular structures were drawn with the CYLview program [41]. As for the single-electron transfer (SET) between the metal complex MLnx+ and organic molecule A–B (Eq. (2)), the activation barriers for SET were estimated by using the outer-sphere Marcus–Hush model [42–44] and Savéant’s model [45,46], according to corresponding situations.

ð2Þ

2. Computational methods All calculations were carried out with the Gaussian 09 package [28]. Geometry optimizations were performed with B3LYP [29–32]/BS1 method (BS1: LANL2DZ + f (1.611) basis set with ECP was used for Ag [33,34], and the 6-31G (d) basis set was used for other atoms). All the gas phase minima and transition structures (here also referred to as transition states) were characterized by frequency analysis. Frequency calculations identified minimum structures with all real frequencies, while transition states with only one imaginary frequency. To confirm the pivotal transition states connecting the designated intermediates, intrinsic reaction coordinate (IRC) calculation was carried out. Zero point energy (ZPE) corrections were applied at the same level of theory [35]. Single-point energy calculations of all of these stationary points were carried out with M06 [36] (or B3LYP)/BS2 method

(i) If the cleavage of the A–B bond doesn’t occur during SET (or if SET and the cleavage of the A–B bond occur in different steps and involve the formation of the A–B radical anion), the activation barrier may be estimated from the outersphere Marcus–Hush model. Specifically: According to the Marcus equation, the solvent reorganization energy k0 could be calculated from Eq. (3):



 1 1 1 1 1 k0 ¼ ð332 kcal=molÞ þ   2a1 2a2 R eop e

ð3Þ

where a1 and a2 are the radii of the molecules involved in the electron transfer, R = a1 + a2, eop is the optical dielectric constant, and e is the static dielectric constant for the solvent. As there are not sufficient data to calculate the inner reorganization energy for the reactants, ki, we estimated ki  0. Thus, the total reorganization energy k = k0 + ki could be obtained.

X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

According to the Marcus theory, the activation barrier DGzET could be estimated. – DG– 1þ ET ¼ DG0

DGr 4DG– 0

!2 ð4Þ

where DGr is the reaction energy, and DGz0 is the intrinsic barrier (the activation energy at zero driving force) for the outer-sphere electron transfer. DGz0 is related to the reorganization energy (k) by:

DG– 0 ¼

k 4

(ii) If SET and the cleavage of the A–B bond are concerted (namely the concerted dissociative ET), Savéant’s model was used. Specifically: The intrinsic barrier DGz0 includes contribution from the bond dissociation free energy (BDFE) of the A–B bond:

DG– 0 ¼

k ki þ k0 þ BDFE ¼ 4 4

k0 and ki are the same as those in the Marcus–Hush theory. Then, according to the Marcus theory (Formula (4)), the activation barrier DGzET for the concerted dissociative ET could be estimated. 3. Results and discussion According to the work reported by Li [18] and Flowers II et al. [19], some information about the mechanism for the decarboxylative fluorination can be obtained, including: (i) the radical ‘‘clock” experiment reveals that the radical mechanism is involved in the silver-catalyzed process; (ii) 1H NMR of the AgNO3 solution suggests that H2O is ligated to the Ag(I) ion; (iii) an induction period is observed in this reaction, in which there is no loss of Selectfluor

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and instead an Ag-carboxylate complex is formed; (iv) the reactivity of the carboxylic acids decreases in the order: Multistage > primary  aromatic. With the vital clue in hand, we then set out to explore the mechanism of the decarboxylative fluorination using 2,2-dimethylpentanedioic acid (M1) as the model substrate, shown in Scheme 2a. The coordination of Ag+ is the first problem to be solved. Although no organic ligand (e.g. Bpy or Phen) was used, the solvent H2O was suggested to be ligated to the Ag(I) ion on the basis of 1H NMR. DFT calculations also show that the coordination of H2O to the naked Ag(I) ion is thermodynamically favorable by 7.4 kcal/mol, while subsequent coordination of H2O to Ag (H2O)+ is thermodynamically unfavorable by 6.0 kcal/mol (Scheme 2b). In addition, the coordination of acetone to Ag(H2O)+ is also thermodynamically unfavorable by 3.0 kcal/mol (Scheme 2b). Thus, water acts as a solvating ligand and it is Ag (H2O)+ that is involved in the reaction with carboxylic acid. We reasoned that Ag(H2O)+ reacts with 2,2-dimethylpentanedioic acid to give the corresponding H2O-ligated Ag-carboxylate and H+, leading to the decrease of pH value of the reaction solution, as is shown in Scheme 2c. Calculations show that the formation of the H2O-ligated Ag-carboxylate (IM-1) is endergonic by about 11.4 kcal/mol [47]. Compared with tertiary carboxylic acid, deprotonation of the primary one by Ag(H2O)+ leading to the corresponding Ag-carboxylate (IM-2) is harder by about 0.8 kcal/mol (Scheme 2c). On the other hand, as for the oxidation of the Ag (H2O)+ by Selectfluor to Ag(II)–F and Ag(III)–F [48], the reaction free energies were calculated to be about 45.8 and 108.2 kca/mol, respectively, indicating that the conversions are unlikely at room temperature (Scheme 2d). According to the calculations, the formation of the H2O-ligated Ag-carboxylate (IM-1 or IM-2) is prior to that of the Ag(II)–F or Ag(III)–F, which is consistent with the experiment reported by Flowers II et al. [19]. It deserves noting that the reaction (or activation) free energies mentioned in the

Scheme 2. (a) Decarboxylative fluorination of 2,2-dimethylpentanedioic acid (M1) catalyzed by AgNO3; (b) coordination of H2O (or acetone) to the Ag(I) ion; (c) formation of the H2O-ligated Ag-carboxylate (IM-1 or IM-2); (d) formation of the Ag(II)–F and Ag(III)–F intermediates (the free energies were calculated with M06 functional) (the free energies in parenthesis were calculated with B3LYP functional) (kcal/mol).

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X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

present work were calculated with M06 functional unless otherwise specified. In addition, similar conclusion can also be obtained when the corresponding free energies were calculated with B3LYP functional (Scheme 2). Next, we will focus our attention on the oxidation of the H2O-ligated Ag-carboxylate (IM-1 and IM-2 in Scheme 2c) by Selectfluor. The reduced product of Selectfluor is TEDA–BF4 radical cation, which can continuously oxidize the H2O-ligated Ag-carboxylate, shown in Fig. 1. Taking the case of IM-1 (Fig. 1a), the oxidation of IM-1 by Selectfluor to IM-3 is exergonic by 3.6 kcal/mol, and the corresponding activation free energy was calculated to be about 14.9 kcal/mol with Savéant’s model, which has been applicated to the copper-catalyzed Ullmann-type reactions (Specifically, Savéant’s model was used to calculate the activation free energy for the concerted dissociated electron transfer between Cu(I) and PhI) [49]. On the other hand, the oxidation of IM-1 by TEDA–BF4 radical cation is endergonic by 4.3 kcal/mol. The

corresponding activation free energy was calculated to be about 6.3 kcal/mol with frequently-used Marcus–Hush model [50]. Obviously, oxidation of IM-1 by TEDA–BF4 radical cation is kinetically favorable, while Selectfluor has evident advantage in thermodynamics. For the oxidation product IM-3, generally Ag(I) was proposed to be oxidized to Ag(II), which then oxidized –COO to –COO. Alternatively, the homolytic cleavage of the O–Ag(II) bond in IM-3 can also give –COO radical and Ag(I). Although the latter is rarely reported, similar situation, such as the homolytic cleavage of the N–Ag(II) bond to give nitrogen radical, is often seen in the literature [51]. To our knowledge, quantum chemistry is not good at dealing with the intramolecular electron transfer. Herein, we reasoned that the carboxyl radical was derived from the homolytic cleavage of the O–Ag(II) bond and this process is exergonic by 2.4 kcal/mol. Then, the loss of CO2 from the carboxyl radical leads to the corresponding alkyl radical, which subsequently abstracts fluorine from Selectfluor to yield the final product and TEDA–BF4

Fig. 1. Detailed pathway for the decarboxylative fluorination of (a) IM-1; (b) IM-2 (the energies in parenthesis were calculated with B3LYP functional) (kcal/mol) (Å).

X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

radical cation. The resultant TEDA–BF4 radical cation re-enters the catalytic cycle. Although we cannot find the transition states for the loss of CO2 and fluorine abstraction, the restrictive structural optimization indicates that these two processes can proceed with no barrier, as shown in Fig. 1a. Analogously, as for IM-2 (Fig. 1b), calculations show that the activation free energies for each step are 15.4 (oxidation by Selectfluor), 6.8 (oxidation by TEDA–BF4 radical cation), 2.4 (cleavage of the O–Ag(II) bond), 0 (loss of CO2) and 0 (fluorine abstraction) kcal/mol. Fig. 2 shows the entire potential energy surface for the Ag(H2O)+-mediated decarboxylative fluorination of 2,2-dimethylpentanedioic acid (M1) with Selectfluor as oxidant and fluorine source. Accordingly, the following observations are drawn: (i) the decarboxylative fluorination includes five steps: formation of H2O-ligated Ag-carboxylate, oxidation, homolytic cleavage of the O–Ag(II) bond, loss of CO2 and fluorine abstraction; (ii) The rate-determining step includes the formation of the Ag-carboxylate (IM-1 or IM-2) and its oxidation by Selectfluor. It should be noted that the activation free energies in the oxidation step (Selectfluor) calculated by the M06 functional are higher than those calculated by the B3LYP functional by about 5.0 kcal/mol. The total activation free energy for the decarboxylative fluorination of M1 calculated by B3LYP functional (22.3 kcal/mol in Fig. 2) is very close to the experimental result (21 ± 2 kcal/mol) [19]. (iii) The tertiary carboxyl in 2,2-dimethylpentanedioic acid has a stronger reactivity than the primary one, for the formation of P1 is prior to P2 by 1.3 kcal/mol in kinetics and 11.2 kcal/mol in thermodynamics, which is consistent with the experiment [18,19]. Besides M1, we have also studied the decarboxylative fluorination of 2,2-dimethylbutyric acid M2 and isobutyric acid M3. These

15

substrates showed excellent reactivities at room temperature according to the work reported by Flowers II. The corresponding potential energy surfaces for these conversions are displayed in Figs. 3 and 4. Similar to that of 2,2-dimethylpentanedioic acid M1, the decarboxylative fluorinations of M2 and M3 contain five steps and the rate-determining steps include the formation of the Ag-carboxylates (IM-9 and IM-14) and their oxidations by Selectfluor. The total activation free energies calculated by M06 functional are 26.1 and 25.4 kcal/mol, respectively. The corresponding values calculated by B3LYP functional are 22.2 and 21.4 kcal/mol, respectively. According to the calculations (Especially the B3LYPenergies), the decarboxylative fluorinations of M2 and M3 take place easily at room temperature, in agreement with the experiments. On the other hand, according to the Li’s work, other substrate, such as 2-(4-chlorophenoxy) acetic acid M4, requires higher reaction temperature (about 55 °C). The aromatic acid, such as 4-chlorobenzoic acid M5, fails to give any desired product even at refluxing temperature. The following content will focus on the reaction pathways of M4 and M5. The corresponding potential energy surfaces are displayed in Figs. 5 and 6. Analogously, for either M4 or M5, the whole decarboxylative fluorination includes five steps. However, the rate-determining step has changed into the decarboxylation [52]. Specifically, for 2-(4-chlorophenoxy) acetic acid M4 (Fig. 5), the activation free energy was calculated to be about 47.0 kcal/mol with M06 functional (B3LYP: 36.5 kcal/mol). Especially the calculation with B3LYP functional is more consistent with the experiment (the decarboxylative fluorination of M4 occurs at 55 °C). For 4-chlorobenzoic acid M5 (Fig. 6), the activation free energy was calculated to be about

Fig. 2. The entire potential energy surface for the Ag(H2O)+-mediated decarboxylative fluorination of 2,2-dimethylpentanedioic acid (M1) with Selectfluor (the energies in parenthesis were calculated with B3LYP functional) (kcal/mol).

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X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

Fig. 3. The entire potential energy surface for the Ag(H2O)+-mediated decarboxylative fluorination of 2,2-dimethylbutyric acid (M2) with Selectfluor (the energies in parenthesis were calculated with B3LYP functional) (kcal/mol).

Fig. 4. The entire potential energy surface for the Ag(H2O)+-mediated decarboxylative fluorination of isobutyric acid (M3) with Selectfluor (the energies in parenthesis were calculated with B3LYP functional) (kcal/mol).

X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

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Fig. 5. The entire potential energy surface for the Ag(H2O)+-mediated decarboxylative fluorination of 2-(4-chlorophenoxy) acetic acid (M4) with Selectfluor (the energies in parenthesis were calculated with B3LYP functional) (kcal/mol).

57.2 kcal/mol with M06 functional (B3LYP: 46.3 kcal/mol), indicating that the decarboxylative fluorination of M5 is unlikely to take place. According to the above analysis, the silver-catalyzed decarboxylative fluorination of the carboxylic acid contains five steps: formation of H2O-ligated Ag-carboxylate, oxidation, homolytic cleavage of the O–Ag(II) bond, decarboxylation (loss of CO2) and fluorine abstraction, as shown in Scheme 3. It is the formation of the H2O-ligated Ag-carboxylate rather than that of Ag(II)–F or Ag (III)–F intermediate that initiates the fluorodecarboxylation. Obviously the present calculations well support the mechanism proposed by Flowers II et al. (Scheme 1b). However, as for the rate-determining step, besides the formation of the H2O-ligated Ag-carboxylate and its oxidation by Selectfluor (similar to that proposed by Flowers II), there exists another possibility: namely decarboxylation (loss of CO2 from the carboxyl radical). The latter was not taken in account by Flowers II et al. might be due to the fact that the type of the substrates in the experiment is incomplete. For the two probable rate-determining steps, which one is dominant depends on the structure of the substrate. For example, for the multistage carboxylic acid, such as M1, M2 and M3, loss of CO2 from the corresponding carboxyl radical occurs easily (nearly with no barrier) and the rate-determining step is the formation of the H2O-ligated Ag-carboxylate and its oxidation by Selectfluor. In this respect, the present calculations are well consistent with the experiments reported by Flowers II. On the other hand, for the primary carboxylic acid and aromatic acid, such as M4 and M5, loss of CO2 is hard (the activation free energy is greater than 30 kcal/mol, Figs. 5 and 6) and this step determines the speed of

the whole reaction. Here, it is important to point out that we cannot make a simple judgment just on the basis of the type of carboxylic acid, for there are many exceptions. For instance, although the intermediate IM-6 (Fig. 2) belongs to the primary carboxyl radical, loss of CO2 is easy and this step is not the ratedetermining step. On the contrary, the adamantane-1-carboxylic acid belongs to the tertiary carboxylic acid, however loss of CO2 from the corresponding carboxyl radical is hard and this step is rate-determining step (see Fig. S2 in the supporting information). Although the substrates selected in the work reported by Flowers II are limited in the multistage carboxylic acids (such as M1, M2 and M3), detailed experimental data provide us very useful information for the study of the mechanism. Taking 2,2-dimethylbutyric acid M2 for example, we shall carry out a further mechanical analysis to verify the theoretical calculation. Calculations show that the reaction rate depends on the formation of the H2O-ligated Ag-carboxylate (IM-9 in Fig. 3) and its oxidation by Selectfluor and this accords with the fact that a first-order rate dependence was found for AgNO3 or Selectfluor by kinetic order studies. In contrast, a rate order of 1.5 for the carboxylic acid M2 was observed experimentally, indicating that the carboxylic acid was inhibiting reaction progress through formation of an Ag-(carboxylate)2 intermediate. Calculations also show that the formation of the Ag-(carboxylate)2 intermediate (namely IM-13 in Fig. 3) between M2 and IM-9 (Fig. 3) is exergonic by 3.0 kcal/mol (B3LYP: 1.7 kcal/mol), revealing this process is roughly in equilibrium. Increasing the amount of M2 obviously facilitates consuming IM-9, leading to the decrease in the fluorination rate. It is evident that the theoretical calculation is well consistent with the kinetic

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X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

Fig. 6. The entire potential energy surface for the Ag(H2O)+-mediated decarboxylative fluorination of 4-chlorobenzoic acid (M5) with Selectfluor (the energies in parenthesis were calculated with B3LYP functional) (kcal/mol).

Scheme 3. Proposed mechanism for the Ag(I)-catalyzed decarboxylative fluorination of carboxylic acid on the basis of the present calculations.

X. Zhang / Computational and Theoretical Chemistry 1082 (2016) 11–20

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Scheme 4. (a) Hydrogen abstraction between TEDA radical cation and M2 and electron transfer between TEDA radical cation and IM-9; (b) electron transfer between NFSI and IM-9; (c) formation of the Ag-carboxylate IM-26 between Ag(Phen)+2 and M2 and dissociative electron transfer between Selectfluor and Ag(Phen)+2 (the free energies in parenthesis were calculated with B3LYP functional) (kcal/mol).

experiments. In addition, we also need to clarify several issues, shown in Scheme 4. First, according to the mechanism proposed by Flowers II, the key intermediate TEDA–BF4 radical cation, which is derived from the single electron reduction of Selectfluor and the fluorine abstraction between alkyl radical and Selectfluor, can go on oxidizing the Ag-carboxylate to complete the catalytic cycle. An alternative pathway that the TEDA–BF4 radical cation can abstract hydrogen from the carboxylic acid (such as M2) has been excluded. The present calculations support this opinion, as shown in Scheme 4a. Specifically, the activation free energies for the oxidation of the Ag-carboxylate by the TEDA–BF4 radical cation was calculated to be about 6.3 kcal/mol (M06), far less than the reaction free energy for the hydrogen abstraction between the TEDA–BF4 radical cation and the carboxylic acid M2 by 7.3 kcal/mol (the corresponding value calculated with B3LYP functional is 4.7 kcal/mol). Secondly, Li’s group has pointed out that upon replacement of Selectfluor with N-fluorobenzenesulfonimide (NFSI) no reaction occurred. The calculations show that the reaction free energy for the oxidation of the Ag-carboxylate (IM-9) by NFSI is as high as 60.4 kcal/mol (M06) (Scheme 4b), indicating that this conversion is unlikely to take place. Finally, according to Li’s report, bidentate silver complexes such as Ag(Phen)2OTf (Phen = 1, 10-phenanthroline) failed to initiate the fluorodecarboxylation. The reaction free energy for the formation of the Ag-carboxylate (IM-26 in Scheme 4c) was calculated to be 22.2 kcal/mol (M06), while the activation free energies for the oxidation of Ag(Phen)+2 by Selectfluor was calculated to be about 8.1 kcal/mol (M06). Obviously it is unlikely to form the Ag-carboxylate to initiate the fluorodecarboxylation. Similar conclusion can be drawn on the basis of the B3LYP calculations.

4. Conclusion In this work, we have conducted a systematic theoretical study on the Ag(I)-catalyzed decarboxylative fluorination of the carboxylic acid. The following conclusions can be drawn from our results: 1. This kind of reaction is proposed to include five steps: formation of H2O-ligated Ag-carboxylate, oxidation, homolytic cleavage of the O–Ag(II) bond, decarboxylation (loss of CO2) and fluorine abstraction. 2. The calculations indicate that the formation of Ag(II)–F or Ag (III)–F intermediate is unlikely. The fluorodecarboxylation is activated by the formation of the H2O-ligated Ag-carboxylate. Herein, water acts as a solvating ligand and the ligation of H2O to the Ag(I) ion is thermodynamically favorable. The present calculations are well in conformity with the experiments reported by Flowers II et al. 3. As for the rate-determining step, there exist two possibilities: (i) the formation of the H2O-ligated Ag-carboxylate and its oxidation by Selectfluor (reported by Flowers II); (ii) decarboxylation (loss of CO2). Which one is dominant depends on the structure of the substrate.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2016.02. 016.

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