Mechanism study on rhodium(III)-catalyzed CH functionalization of o-vinylphenols with alkynes: Regioselectivity and chemoselectivity

Computational and Theoretical Chemistry 1147 (2019) 40–50

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Mechanism study on rhodium(III)-catalyzed CeH functionalization of ovinylphenols with alkynes: Regioselectivity and chemoselectivity ⁎

Lusheng Chen, Xue Zhao, Fang Huang, Jianbiao Liu, Chuanzhi Sun , Dezhan Chen

T

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College of Chemistry, Chemical Engineering and Materials Science, Shandong Provincial Key Laboratory of Clean Production of Fine Chemicals, Institute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, PR China

A B S T R A C T

The mechanism of rhodium(III)-catalyzed CeH functionalization of o-vinylphenols (1a) with alkynes has been investigated with DFT calculations. The results suggest the whole reaction is comprised with four stages: (I) OeH deprotonation, (II) CeH activation induced by Rh(III) catalyst interacting with o-vinylphenols, (III) alkyne coordination and (IV) regeneration of Rh(III) catalyst. The CeH activation step proceeds under the concerted metalation deprotonation mechanism. The regioselectivity and chemoselectivity of the reaction also have been discussed, respectively. The regioselectivity for different CeH activation sites is depended on the coordination structure of the related Rh(III) complex. In addition, when the substrates are substituted with alkyl (or alkenes) at the terminal position of the alkene, such as (E)-2-(prop-1-en-1-yl)phenol 1b and 2-allylphenol 1c, the apparent activation energies (ΔG) are obviously higher than that of 1a, which indicates that they are unfavorable to occur. The effect of CO2Me substituent on the para position to the hydroxyl (1d) is also investigated.

1. Introduction Metal-catalyzed cycloadditions have attracted much attention in the past years and revolutionized the way of making cyclic compounds [1–5]. A great number of works on metal-catalyzed annulations involving the activation of CeH bonds as a key step have been reported in recent years [6–11].These reactions provided easy synthetic methods for a variety of five- and six-membered heterocycles through (3 + 2) or (4 + 2) cycloadditions [12–20]. However, the assembly of larger rings by means of related annulations is still challenging [21–23]. A related excellent work has been reported by Seoane and Gulías, and a new type of heteroannulation regarding to the Rh(III)-catalyzed CeH functionalization of o-vinylphenols with alkyne was provided [24]. This process allows the synthesis of benzoxepines from extremely simple precursors in a formal (5 + 2) cycloaddition reaction, and it exhibits very high regioselectivity and chemoselectivity. Under the catalysis of [Cp*RhCl2]2 and Cu(OAc)2 in toluene or acetonitrile, seven-membered benzoxepine 3a was selectively generated from 2 to vinylphenol 1a and alkyne 2a, without generation of the isomer benzofurane 3a' and the chromene 3a'', as shown in Scheme 1. When the terminal position of the alkene in 1a was substituted with alkyl substituents ((E)-2-(prop-1-en1-yl)phenol, 1b) or alkenes (2-allylphenol, 1c), the reaction does not proceed, as shown in Scheme 2. Although an increasing number of works about Rh catalyzed CeH activation have been reported [25–28], it is necessary to study the detailed mechanism of a new (5 + 2) cycloaddition reaction, especially

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for the good regioselectivity and chemoselectivity. In our previous work [29], Rh(III)-catalyzed CeH activations and annulations between benzamide derivatives and allenes have been studied, and the regioselectivity is dependent upon the carborhodation step of allenes. In this work, how is the regioselectivity controlled leading to a (5 + 2) cycloaddition reaction? In addition, why do 1b and 2a, 1c and 2a not react with each other under the same conditions and why is the reaction rate of 1d and 2a slower than that of the 1a and 2a? Computational results could provide useful information for chemists to gain a deeper understanding of the Rh(III)-catalyzed CeH functionalization reactions and to design new high selectivity catalysts. 2. Computational details All calculations were carried out using the Gaussian 09 suite of computational programs [30]. The hybrid density functional B3LYP [31–33] was employed. Geometries were optimized using 6-31G(d) basis sets [34,35] on nonmetal atoms and LANL2DZ [36–38] effective core potentials on Rh. Vibrational frequencies were computed at the same level to get the thermal corrections and to confirm whether the structures are minima or transition states. When necessary, IRC calculations were performed to verify the right connections among a transition state and its forward and reverse minima [39]. The M06 functional includes noncovalent interactions and can give accurate energies for transition metal systems [40–43], and single-point calculations with solvation effects modeled by SMD in solvent acetonitrile were applied

Corresponding authors. E-mail addresses: [email protected] (C. Sun), [email protected] (D. Chen).

https://doi.org/10.1016/j.comptc.2018.12.003 Received 15 September 2018; Received in revised form 4 December 2018; Accepted 4 December 2018 Available online 06 December 2018 2210-271X/ © 2018 Elsevier B.V. All rights reserved.

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Scheme 1. Different annulation options for o-vinylphenols using a Rh(III) catalyst.

Scheme 2. The (5 + 2) annulation reactions of 1b, 1c with 2a.

3.1. OeH deprotonation and CeH activation

for all gas-phase optimized structures at the M06/[6-311++G(d,p) +LANL2DZ] level at 298.15 K. B3LYP for geometry optimizations and M06 for single-point energy calculations have been proved to be effective methods by numerous studies, and it can successfully produce energy profiles including transition metals of reactions [44–51]. The free energies are used in the following discussion. All optimized structures, as well as their cartesian coordinates, are given in the Supporting Information.

Complex IM1a is first formed via the coordination of lone pair electrons of O3 atom in 1a to Rh atom and the O1…H hydrogen bond interaction, as shown in Fig. 1. Then, IM2a and HOAc are generated by the hydrogen transfer from O3 atom of 1a to O1 of OAc- via the transition state TS1a, as shown in Fig. 2. The distances of O1-H, O3-H and Rh-O3 are changed from 1.448, 1.051 and 2.221 Å to 1.238, 1.169 and 2.174 Å, respectively. The electronic energy of TS1a is 0.43 kcal/mol higher than that of IM1a in gas phase. However, in the acetonitrile solvent, the free energy of TS1a is 2.1 kcal/mol lower than that of IM1a, which suggests that the process of IM1a to IM2a occurs easily. IM3a is obtained by the C2-C4 bond rotation via TS2a, in which the C1 atom is nearer to the Rh atom than in IM2a. The free energy barrier of this step is only 5.4 kcal/mol. In the following reaction process, two different mechanisms, i.e., concerted metalation deprotonation mechanism (CMD) and an intramolecular electrophilic attack of the rhodium to conjugated alkene, are considered for the CeH activation. In the CMD mechanism, IM4a is formed by the isomerization of IM3a, in which the distances of Rh-C1 and Rh-O are shortened to 2.216 and 2.123 Å. A hydrogen bond between O2 and H1 (1.948 Å) is formed in IM4a. Next, IM5a, which includes the HOAc part is provided by the hydrogen transfer from C1 to the O2 center via a six-membered (RheOeCeO2eHeC1) transition state TS4a with the free energy of

3. Results and discussion The whole reaction can be characterized by four steps, including (stage I) OeH deprotonation, (stage II) CeH activation, (stage III) alkyne coordination and (stage IV) regeneration of Rh(III)-catalyst. Cp*Rh(OAc)2 is chosen as the active catalyst in this work based on the experimental and theoretical literature [11,52–58], which is generated by the reaction between [Cp*RhCl2]2 and Cu(OAc)2. As shown in Scheme 3, in stage I, a molecular HOAc is released by the reaction of 1a and 2a, and intermediate IM2a forms in this stage. In stage II, the second molecular HOAc is released and a terminal CeH bond of a vinyl group is activated, and IM6a is generated. Then, in stage III, the alkyne coordinates to the Rh atom forming IM7a. A (5 + 2) annulation reaction occurs and IM9a forms through intermediate IM8a. The stage IV is mainly related with the regeneration of Rh(III)-catalyst. 41

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Scheme 3. The mechanism of Rh(III) catalyzed reaction between 1a and 2a. R1 = R2 = Ph.

15.6 kcal/mol. In TS4a, the distances of Rh-C1and HeO2 bonds are shortened from 2.216 and 1.948 Å in IM4a to 2.085 and 1.319 Å, respectively. Ultimately, IM6a is formed by the dissociation of HOAc from IM5a. In the electrophilic reaction mechanism, IM3a transforms to IM4a' (shown in Fig. 1) by the direct dissociation of an OAc- group with the free energy barrier of 15.8 kcal/mol. Then, the six-membered intermediate IM5a' is given by the Rh-C1 bond formation via TS4a' with the free energy barrier of 2.1 kcal/mol. Subsequently, IM6a' is formed by the recoordination of the released OAc- group on the rhodium atom. Finally, the C1eH bond breaks and the OeH bond forms generating IM6a and HOAc via TS6a'. The free energy barrier of this step is 9.7 kcal/mol. Comparing the two mechanisms, it can be seen that the rate-limiting step of the CMD pathway is the C1-H bond cleavage (TS4a, 15.6 kcal/ mol). While in the electrophilic reaction mechanism, the Rh-C1 bond formation step (TS4a', 20.1 kcal/mol) is the highest point in the free energy profile. Therefore, the CMD mechanism should be the preferred pathway in the CeH activation stage. Seoane et al also suggest that CeH bond cleavage is involved in the rate-limiting step in their experimental work [24], which is consistent with our computational results.

The free energy profiles of the full catalytic cycle for Rh(III)-catalyzed reaction of 1a with 2a is shown in Fig. 5. The efficiency of the catalyst in this work is evaluated according to the energetic span model proposed by Kozuch and Shaik [60]. In this model, they suggest that neither one transition state nor one reaction step possesses all the kinetic information that determines the efficiency of a catalyst. The apparent activation energy of catalytic cycle depends on the largest energy differences between any given transition state and any given intermediate, regardless of if they are adjoined as one single step or which occurs first. The apparent activation free energy (ΔG) is based on the turnover-frequency(TOF)-determining transition state (TDTS) and the TOF-determining intermediate (TDI), and it can be calculated as follows:

TDTS − TDI if TDTS appears after TDI Δ G= ⎧ TDTS TDI ΔG if TDTS appears before TDI − + ⎨ r ⎩ ΔGr represents the free energy of the whole reaction.According to the free energy profile (Fig. 5), the apparent activation energy (ΔG) for 1a + 2a reaction is 11.0 kcal/mol (TS7a – IM8a – 22.7), which will be used to explain the chemoselectivity in the following section. 3.3. Regioselectivity for the reaction of 1a and 2a In IM2a, three different sites may be activated in the reactive process (as shown in Scheme 1), which are named as C-H1, C-H2 and C-H3 activation, respectively. For C-H1 activation, the olefinic CeH functionation might lead to the seven-membered ring (3a, benzoxepine). For C-H2 and C-H3 activation, the reactions might provide the six (chromene 3a', through (4 + 2) cycloadditions) and the five (benzofurane 3a'', through (3 + 2) cycloadditions) membered rings, respectively. However, only the benzoxepine 3a is detected in the experiment. In order to get the reasonable explanations, the activation of three different CeH sites has been studied in detail. The related structures of all stationary points and their relative free energies are shown in Figs. 6 and 7, respectively. It can be seen that the rate-determining step in this process is the CeH activation step (as shown in Fig. 7). Comparing three different CeH activation pathways, the free energy barrier of TS4a is the lowest. In addition, the intermediate IM5a is more stable than IM5a'' and IM5a''' in energy. Thus, according to kinetics and thermodynamics, the above results suggest that the pathway pointed to product 3a is the most favorable, which is consistent with the experimental observations, and C-H1 is the easiest activation site among the three kinds of sites. The frontier molecular orbital [FMO] analysis was performed for IM2a to further understand the mechanism of the regioselectivity, as shown in Fig. 8. The HOMO distribution, the natural atomic charges,

3.2. Alkyne coordination on Rh atom and regeneration of Rh(III)-catalyst The key optimized geometries and the free energy profiles of alkyne coordination on Rh atom are illustrated in Figs. 3 and 4, respectively. Firstly, IM7a is generated by the coordination of the C5^C6 bond of 2a to the Rh atom, in which the distances of the RheC5 and RheC6 are 2.404 and 2.306 Å, respectively. Then, an eight-membered intermediate IM8a is formed with the Rh-C1 bond broken, and RheC6 and C1eC5 bond formations via TS7a with the free energy of 17.1 kcal/mol. In TS7a, the distances of RheC6 and C1eC5 are shortened from 2.306 and 2.404 Å in IM7a to 2.089 and 2.023 Å, respectively. Next, IM8a transforms to the seven-membered intermediate IM9a through TS8a with the free energy of 14.3 kcal/mol. In addition, the energy barrier of this single step is relative high (30.9 kcal/mol). However, the experimental results reported that the reaction temperature is 85 °C [24]. Thus, the relative high barrier seems reasonable here. In this step, O3eC6 bond is formed accompanied by the elongation of RheO3 and RheC6 bonds. The detailed mechanism for the regeneration of Rh(III) catalyst has been systematically studied by Maseras et al. According to their study, the reoxidation of the Rh(I) complex with Cu(OAc)2 can regenerate the Rh(III) catalyst in the reaction [59]. The calculated ΔG for the regeneration of the active catalyst is −12.5 kcal/mol, which is favorable thermodynamically. 42

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Fig. 1. Optimized structures of the stationary points of OeH deprotonation and CeH activation step, along with the key bond lengths (in Å).

influence of the CeH bond strength on the regioselectivity has also been explored. The wiberg index of the CeH bonds in TS4a, TS4a'' and TS4a''' are obtained using the NBO analysis, which are 0.459, 0.448 and 0.385, respectively. It can be found that the C1-H bond is the most hard to be broken. However, the energy barrier of TS4a is the lowest in the three transition states, which indicates that the strength of CeH bond is not the factor determining the regioselectivity. Subsequently, the coordination structures of C and O atoms on the Rh center were studied to verify whether there is a relation between the structures and the regioselectivity, as shown in Fig. 9. The angles of CeRheO in TS4a/TS4a''/TS4a''' (indicated by the doted oval in Fig. 9) are different. In TS4a, there is a hexatomic ring by the construction of

and the free energy of corresponding transition states are listed in Table 1. As can be seen, the active C1 atom contributes the largest atomic orbitals to the HOMO (11.18%) and possesses the largest natural atomic charge (−0.441). Correspondingly, the attack of the C1 site of IM2a to the Rh center enjoys the lowest free energy (15.6 kcal/mol). The calculated free energies for the CeH activation at different sites obey the order of C1 < C2 < C3, which is inversely proportional to the contribution of atom orbital to the HOMO. Thus, the contribution of atom orbital to HOMO and the atomic charge of C may be one reason of the regioselectivity. In TS4a/TS4a''/TS4a''', the CeH activation is the process of CeH bond cleavage and C-Rh bond formation simultaneously. Therefore, the 43

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Fig. 2. Free energy profile of OeH deprotonation and CeH activation step (values are given in kcal/mol).

Fig. 3. Optimized structures of the stationary points of alkyne coordination step along with the key bond lengths (in Å).

Fig. 4. Free energy profile of alkyne coordination step (values are given in kcal/mol).

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opposite order. Thus, the coordination structure of C and O on Rh is another influential factor for the regioselectivity. As shown in Fig. 7, the preferred pathway leading to (5 + 2) annulation involves the lowest energy of transition state TS4a, and the regioselectivity of this pathway is favorable both in kinetically and thermodynamically. 3.4. Chemoselectivity for 1b(1c) and 2a In the experiment, as shown in Scheme 2, the reaction does not proceed if the substrates are substituted with alkyl (or alkenes) at the terminal position of the alkene, such as (E)-2-(prop-1-en-1-yl)phenol 1b and 2-allylphenol 1c. The reaction mechanisms of 2a with 1b(1c) was also investigated to compare with that of the reaction between 2a and 1a. The free energy profile for the reaction of 1b with 2a is shown in Fig. 10, and the optimized structures of the stationary points are given in Fig. S1. The mechanism of the reaction between 1b and 2a is similar to that of 1a and 2a. In the CeH activation process, the highest point in the energy profile of 1b + 2a is TS4b with the free energy of 19.2 kcal/ mol, which is higher than that of TS4a (15.6 kcal/mol). And the highest point in the whole profiles is TS7b with the free energy of 22.8 kcal/ mol (17.1 kcal/mol for TS7a). Therefore, when the terminal position of the alkene is substituted by alkyl, the reaction barriers of the key steps are obviously increased. The free energy profile for the reaction of 1c with 2a is shown in Fig. 11, and the optimized structures of all stationary points are given in Fig. S2. It can be seen that the highest energy point in the CeH activation process is TS4c with the free energy of 22.2 kcal/mol, which is higher than those of TS4a (15.6 kcal/mol) and TS4b (19.2 kcal/mol).

Fig. 5. The free energy profiles of the full catalytic cycle for 1a + 2a reaction.

six atoms and the degree of C-Rh-O is 89.2°. However, there are fiveand four-membered rings in TS4a'' and TS4a''', and the CeRheO degrees are 79.6° and 64.0°, respectively. In order to find the standard coordination structure for the Rh(III) complex, a similar model (TS4) in which the C and O atom is relax rather than binding in a ring is optimized. In TS4, the CeRheO angle degree is 87.7°, which means that without any binding for C and O atoms, the CeRheO angle is inclined to be about 87.7°. By comparison, TS4a possesses the lowest energy and its CeRheO angle is most close to that in TS4. Furthermore, it can be found that the order of degrees for CeRheO angle is TS4a > TS4a'' > TS4a''', and the energies of three transition states obey the

Fig. 6. Optimized structures of the stationary points of the reaction between 1a and 2a along with the key bond lengths (in Å). 45

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Fig. 7. Free energy profiles of different options of CeH activation (values are given in kcal/mol).

Fig. 8. The distribution of highest occupied molecular orbital (HOMO) and Natural atomic charges of IM2a.

TS7c were investigated by NBO analysis, as shown in Table 2. It can be seen from the NBO results that the charge of C1 in TS7b and TS7c becomes more positive relative to that in TS7a, which may make the C1eRh bond unstable and further pull up the energy of the transition states. Thus, the energies of the three transition states obey the order of TS7c > TS7b > TS7a, and the Wiberg Index of C1eRh bond obey the opposite order.

Table 1 Relationship between the HOMO distribution of IM2a, the Natural Atomic Charges and the Regioselectivity. Regioselectivity

HOMO distribution (%)

Natural atomic charges

Gibbs free energy (kcal/mol)

C1 C2 C3

11.18 2.24 0.05

−0.441 −0.232 −0.206

15.6 22.1 25.7

3.5. The effect of CO2Me substituent on the para position to the hydroxyl group

Additionally, the free energy of TS7c is 27.4 kcal/mol, which is 10.3 kcal/mol higher than that of TS7a. According to the energetic span model, the apparent activation energies (ΔG) of catalytic reactions of 1b + 2a and 1c + 2a are 15.6 kcal/mol (TS7b – IM8b – 22.0) and 27.3 kcal/mol (TS7c – IM8c –17.9), respectively, which are higher than that of 1a + 2a (11.0 kcal/mol). Thus, the reaction of 1b(1c) with 2a is much more difficult than that of 1a + 2a. The calculated results are well in agreement with the experimental results. For rate determining steps, the order of free energies is TS7c > TS7b > TS7a, and the reasons for it were also investigated here. As shown in Fig. 12, the degrees of C-Rh-O angles of TS7a, TS7b and TS7c are 90.0°, 90.1° and 88.9°, respectively, which are little difference between each other. Therefore, the different energies cannot be explained from the viewpoint of coordination structures. The Natural Charge of C1, Rh and Wiberg Index for C1eRh bond of TS7a, TS7b and

Seoane's experiment reported that when the para position to the hydroxyl group is substituted by CO2Me (1d), the reaction rates of 1d + 2a is slower than that of 1a + 2a [24]. The energy profile of 1d + 2a reaction has been investigated, as shown in Fig. 13. The optimized structures of the stationary points are given in Fig. S3. The reaction pathway between 1d and 2a is similar to that of 1a and 2a. Both pathways experience four steps, i.e., OeH deprotonation, CeH activation, alkyne coordination and regeneration of Rh(III) catalyst. In the CeH activation process, the free energy of the rate determining step (TS4d) is a little lower than that of TS4a by 1.7 kcal/mol, which indicated that the CeH activation step should be a little easier in 1d + 2a reaction. It can be seen that the highest energy in the whole energy profiles of 1d + 2a is TS8d, and the TDTS and TDI are TS8d and IM8d, respectively. The apparent activation energy of 1d + 2a reaction is 46

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Fig. 9. The coordination structures of transition states TS4a, TS4a'' and TS4a'''.

Fig. 10. Free energy profiles of 1b and 2a reaction (values are given in kcal/mol).

36.2 kcal/mol (TS8d – IM8d), which is 25.2 kcal/mol higher than that of 1a + 2a reaction (11.0 kcal/mol). Therefore, the rate of 1d + 2a reaction is slower than that of 1a + 2a reaction. The calculated results are in agreement with that in the experiment.

functionalization of o-vinylphenols with alkynes was investigated using DFT calculations. The whole reaction is comprised of four stages: (I) OeH deprotonation, (II) CeH activation induced by Rh-catalyst interacting with o-vinylphenols, (III) alkyne coordination and (IV) regeneration of Rh(III) catalyst. The regioselectivity and chemoselectivity of the reaction also have been studied, respectively. The distribution of HOMO and natural atomic charges of IM2a reveal that the contribution of atom orbital of C1 to the HOMO is the

4. Conclusion A

detailed

mechanism

of

rhodium(III)-catalyzed

CeH 47

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Fig. 11. Free energy profiles of 1c and 2a reaction (values are given in kcal/mol).

Fig. 12. The optimized coordination structures of TS7a, TS7b and TS7c.

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Table 2 The Natural Charge of C1, Rh and Wiberg Index for C1eRh bond and corresponding Gibbs free energies of TS7a, TS7b and TS7c. Transition state

Charge of C1

Charge of Rh

Wiberg Index of C1eRh bond

Gibbs free energy (kcal/mol)

TS7a TS7b TS7c

−0.353 −0.129 −0.105

0.645 0.651 0.636

0.503 0.467 0.400

17.1 22.8 27.4

Fig. 13. Free energy profiles of 1d and 2a reaction (values are given in kcal/mol).

highest (11.18%), and the natural atomic charge of C1 is most negative. This may be one of the reasons for the activation of the C1eH1 sites. In addition, the regioselectivity also depends upon the coordination structure of the related transition states. For example, the degree of CeRheO angle in TS4a is close to that observed in typical structures of Rh(III) complexes. Therefore, the energy of TS4a is the lowest among the three transition states, and C1eH1 activation is preferred. The reaction pathways of 1b(1c) and 2a are similar to that of 1a and 2a. However, the free energies of the rate determining steps in the 1b(1c) + 2a reactions are obviously higher than that of 1a + 2a, which indicates that they are unfavorable to occur. The effect of a CO2Me substituent on the para position to the hydroxyl (1d) was also investigated, which has the same catalytic cycle as that of 1a. However, the apparent activation energy of 1d + 2a reaction is 25.2 kcal/mol higher than that of 1a + 2a reaction. Thus, the rate of 1d + 2a reaction is slower than that of 1a + 2a reaction. The calculated results agree with the experimental results well.

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Acknowledgments This work was supported by the National Natural Science Foundations of China (No. 21403134). Shandong Province Development Program of Science and Technology (2014GGX102019), 49

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