Establishing the Trifluoromethylthio Radical Donating Abilities of

Article pubs.acs.org/joc

Establishing the Trifluoromethylthio Radical Donating Abilities of Electrophilic SCF3‑Transfer Reagents Man Li,†,§ Biying Zhou,†,§ Xiao-Song Xue,*,† and Jin-Pei Cheng†,‡ †

State Key Laboratory of Elemento-organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin 300071, China ‡ Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: The recent recognition of the novel application of a few traditional electrophilic trifluoromethylthiolating reagents as SCF3 radical sources offers a remarkable new opportunity for the development of radical trifluoromethylthiolation reactions. Herein, the first trifluoromethylthio radical donating ability (Tt•DA) scale of electrophilic SCF3-transfer reagents has been developed. This scale is based on Y-SCF3 bond dissociation energies, which were obtained by density functional calculations (M06-2X). Single electron transfer is revealed to exhibit a substantial Y-SCF3 bond-weakening effect, thus significantly facilitating the SCF3 radical (•SCF3) release. The results may aid in future novel radical SCF3-transfer reagent design and new reaction development.

1. INTRODUCTION The trifluoromethylthio (SCF3) group is an increasingly important functionality in pharmaceutical and agrochemical industries (Scheme 1).1 Incorporation of the SCF3 group into drug candidates will often bring beneficial effects on the drug’s metabolic stability and bioavailability1a,b,2 because of its strong electronegativity and high lipophilicity (Hansch lipophilicity parameter π = 1.44).3 Consequently, there has been considerable effort directed toward developing efficient strategies to incorporate the SCF3 group into organic molecules.4,5 In the past few years electrophilic and nucleophilic trifluoromethylthiolation methodologies have greatly advanced.4,5 In contrast, radical trifluoromethylthiolation,4e,j,6−13 although potentially attractive as a complementary strategy, has been much less studied due to the paucity of efficient and easy-tohandle radical SCF3 sources. Traditional radical trifluoromethylthiolating reagents, such as trifluoromethanesulythiol (CF3SH),6 bistrifluoromethyl disulfide (CF3SSCF3),7 and trifluoromethanesulyenyl chloride (CF3SCl;8 Figure 1a), are highly toxic gases and thus are not suitable for nonspecialized laboratories. Most recently, the groups of Shen,10 Glorius,11 Akita,12 Magnier,13a and Liu13b demonstrated that a few electrophilic trifluoromethythiolating reagents10b,14b−e (Figure 1b), such as Shen’s trifluoromethanesulfenate10b,14d and N-trifluoromethylthiosaccharin,14e Munavalli’s N-trifluoromethylthiophthalimide,14c and Haas’ N-trifluoromethylthiosuccinimide,14b could also serve as radical SCF3 sources under transition-metal or photoredox catalysis. Compared with traditional radical SCF3 sources,6−8 these newly developed electrophilic trifluoromethythiolating reagents10b,14b−e are generally safe and easier to handle, and some of them are now commercially available. © XXXX American Chemical Society

It is thus reasonable to expect that the discovery of the radical reactivity of electrophilic trifluoromethythiolating reagents10b,14 would offer a remarkable new opportunity for developing easy and mild radical trifluoromethylthiolation transformations. In this context, the knowledge of the tendency of an electrophilic trifluoromethylthiolating reagent to donate a •SCF3 and the factors that affect the •SCF3 donating ability is essential for the understanding of their relative reactivity as well as the mechanisms for SCF3 transfer. Herein, we reported the first trifluoromethylthio radical donating ability (Tt•DA) scale of electrophilic SCF3 transfer reagents. This scale is based on Y-SCF3 (where Y is the atom to which the SCF3 group is attached) bond dissociation energies (eq 1), which were obtained by

density functional calculations. The influence of structure variation and single electron transfer (SET) on the trifluoromethylthio radical donating ability will be disclosed.

2. COMPUTATIONAL METHODS We have found that the M06-2X/[6-311++G(2df, 2p)+Def2QZVPPD(I)]//M06-2X/[6-31+G(d)+LANL2DZ(I)] method is reliable to predict known experimental N/O/C−S bond dissociation enthalpies.15 This method was thus applied to the evaluation of Tt•DAs of SCF3-transfer reagents. The Tt•DA of a given reagent is assessed by the Y-SCF3 bond dissociation enthalpy, which is equal to the enthalpy change at 298.15 K for the reaction shown in eq 1. Received: July 15, 2017 Published: July 24, 2017 A

DOI: 10.1021/acs.joc.7b01771 J. Org. Chem. XXXX, XXX, XXX−XXX

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The Journal of Organic Chemistry Scheme 1. Examples of SCF3-Containing Biologically Active Compounds

be better SCF3 radical donors than CF3SH under thermal activation. Among the reagents shown in Table 1, the N-SCF3 family is well-known as shelf-stable electrophilic trifluoromethylthiolating reagents, and some of them have already been employed as radical SCF3 sources.11−13 The calculation predicted that Billard and Langlois’s reagent 1A14i has the lowest Tt•DA value of 57.0 kcal mol−1, meaning that this reagent should potentially be a good •SCF3 donor. Replacing the methyl with a hydrogen leads to an increase in the Tt•DA value by 3.3 kcal mol−1 as seen for reagent 1B114i (60.3 kcal mol−1). This is likely due to the fact that the resulting electron deficient N radical could be more effectively stabilized by methyl (through interaction of the unpaired spin with adjacent C−H bonds19a) than by the hydrogen. Examination of the para-substituent effect on the Tt•DA of reagent 1B114i obtained an excellent linear correlation between the Tt•DA values and the Brown constants (σp+; Figure 2). This can be understood largely in terms of Walter’s criteria for radical behavior: the resulting phenylaminyl radicals belong to the “Class O” radicals,20 which can be stabilized by electron-donating substituents and destabilized by electronwithdrawing substituents. Replacement of the phenyl group of 1A (1B1) with the electron-withdrawing p-toluenesulfonyl group (p-nitrophenylsulfonyl) leads to an increase in Tt•DA value by 10 (12.1) kcal mol−1 as seen for Billard’s second-generation SCF3 reagent 1C14k of 67.0 kcal mol−1 (1D14o of 72.4 kcal mol−1). This is a result of less efficient stabilization of the resulting electron deficient N radical by p-toluenesulfonyl than by phenyl: the radical stabilization energies (RSE)19 of the former and the latter are −6.1 and −14.2 kcal mol−1, respectively. The calculated Tt•DA values for Shen’s chiral N-trifluoromethylthiocamphorsultams 1E1−1E 314p are from 62.3 to 62.7 kcal mol−1, which are at least 27 kcal mol−1 more facile to deliver a •SCF3 than radical reagent CF3SH. This means that Shen’s N-trifluoromethylthio-camphorsultams14p should be potential chiral radical trifluoromethylthiolating reagents. Computational analysis showed that Shen’s N-trifluoromethylthiosaccharin 1F114e is 13.5 kcal mol−1 less favorable to transfer a SCF3 radical than reagent 1E1 (Table 1). Incorporation of substituents on the aromatic ring of 1F1 does not much affect the trfluoromethylthio radical donating ability. The calculated Tt•DA value for N-trifluoromethylthio-dibenzenesulfonimide 1G14m,n is lower than that of N-trifluoromethylthiosaccharin 1F1, possibly as a result of better stabilization of

Figure 1. Typical radical and electrophilic SCF3 sources. Geometry optimizations and frequency computations were carried out at the M06-2X/[6-31+G(d)+LANL2DZ(I)] level in conjunction with the SMD solvation model16 to account for the effects of dichloromethane (CH2Cl2), one of the most commonly used solvents in trifluoromethylthiolations. Single-point energy calculations were conducted at the SMD-M06-2X/[6-311++G(2df, 2p)+Def2-QZVPPD(I)] level with the SMD-M06-2X/[6-31+G(d)+LANL2DZ(I)] optimized structures. All of the calculations were conducted using Gaussian 09.17

3. RESULTS AND DISCUSSION 3.1. Tt•DAs of Electrophilic Trifluoromethylthiolating Reagents. The calculated Tt•DA values for 48 electrophilic trifluoromethylthiolating reagents in CH2Cl2 are listed in Table 1.18 For the convenience of discussion, these reagents are classified into three families: (a) N-SCF3 reagents (1A−1I), (b) O-SCF3 reagents (2A1−2F), and (c) CF3S-SCF3, Cl-SCF3, and F-SCF3 reagents (3A−3C). Examination of Table 1 reveals that the Tt•DA values of 48 reagents cover a range from 57.0 kcal mol−1 for Billard and Langlois’s reagent (1A)14i to 88.1 kcal mol−1 for Haas’ reagent (1I),14b which are all lower than that of the well-known radical trifluoromethylthiolating reagent CF3SH (90.5 kcal mol−1).6 This has important implications for the development of radical trifluoromethylthiolation reactions: all of these electrophilic trifluoromethylthiolating reagents should B

DOI: 10.1021/acs.joc.7b01771 J. Org. Chem. XXXX, XXX, XXX−XXX

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The Journal of Organic Chemistry Table 1. Calculated Tt•DA Values of SCF3-Transfer Reagents in CH2Cl2

the resulting electron deficient nitrogen radical by the sulfonyl than by the carbonyl (RSE for methylsulfonylamidyl and methylacetylamidyl are −7.7 and −0.1 kcal mol−1, respectively).19b Indeed, replacing the sulfonyl unit of N-trifluoromethylthiosaccharin 1F1 by the carbonyl group further increases the Tt•DA value by 8.8 kcal mol−1 as seen for Munavalli’s reagent 1H1.14c Introduction of substituents on the aromatic ring of 1H1 has only a marginal effect on the Tt•DA value, while removing its aromatic ring increases the Tt•DA value by 3.3 kcal mol−1 as seen for Haas’s reagent 1I.14b The calculated Tt•DA values of the O-SCF3 reagents10b,14d,g,h are in a narrow range from 61.3 to 68.2 kcal mol−1 (Table 1), suggesting that they are at least 22 kcal mol−1 more favorable to deliver a •SCF3 in comparison to CF3SH. Thus, these O-SCF3 reagents should potentially be better SCF3 radical donors than CF3SH under thermal activation. Additionally, calculations predicted that the Tt•DA values of CF3S-SCF3 (3A),21a Cl-SCF3 (3B),21b and F-SCF3 (3C)14f are 66.1, 65.2, and 83.0 kcal mol−1,

Figure 2. Plot of Tt•DAs of para-substituted reagents 1B1 against the Brown substituent parameters. C

DOI: 10.1021/acs.joc.7b01771 J. Org. Chem. XXXX, XXX, XXX−XXX

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The Journal of Organic Chemistry

Figure 3. Effect of single electron reduction on Tt•DAs of representative SCF3 transfer reagents in CH2Cl2 (kcal mol−1).

electrophilic trifluoromethythiolating reagents.11b,12,13a Thus, it is highly valuable to evaluate the effect of single electron reduction on the trifluoromethylthio radical donating abilities of electrophilic SCF3 transfer reagents. We have evaluated the effect of single electron reduction on the trifluoromethylthio radical donating abilities of representative reagents, and the results are shown in Figure 3. Clearly, single electron reduction exhibits a substantial effect on activation of the Y-SCF3 bonds in these trifluoromethylthiolating reagents,24 thus remarkablely facilitating the SCF3 radical release. This may explain why the generation of the SCF3 radical by photoredox catalysis could take place under exceptionally mild conditions. For a better comparison, we compiled a Tt•DA scale (Figure 4) for representative electrophilic CF3S-transfer reagents. To further demonstrate the value of the Tt•DA scale, we applied it to Shen’s silver-catalyzed decarboxylative trifluoromethylthiolation (Figure 5).10b Indeed, the experiment observation that reagent

Figure 4. Tt•DA order of representative reagents in CH2Cl2.

respectively. Indeed, reagents CF3S-SCF3 (3A)6 and Cl-SCF3 (3B)7 have long been utilized as radical SCF3 sources. 3.2. Effect of Single Electron Reduction on Tt•DAs. Recently, photoredox catalysis has emerged as a powerful tool to generate reactive radical species under extraordinarily mild conditions.22 Indeed, the development of radical trifluoromethylthiolation by using electrophilic trifluoromethythiolating reagents as radical SCF3 sources under photoredox catalysis is receving increasing attention.11−13 The formation of the SCF3 radical is supposed to involve single electron reduction23 of

Figure 5. Silver-catalyzed decarboxylative trifluoromethylthiolation.10b

2A2 showed much higher radical reactivity than reagent 1F1 is consistent with their Tt•DAs: the former is 11.5 kcal mol−1 more facile to transfer a SCF3 radical than the latter. It should be noted that, like the commonly used pKa scale, the Tt•DA is a thermodynamic quantity15,25 and thus may not work to predict kinetically determined reaction outcomes.26 For such cases, a D

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Vallejo, S.; Bonesi, S.; Postigo, A. Org. Biomol. Chem. 2016, 14, 7150. (m) Guo, Y.; Huang, M.-W.; Fu, X.-L.; Liu, C.; Chen, Q.-Y.; Zhao, Z.G.; Zeng, B.-Z.; Chen, J. Chin. Chem. Lett. 2017, 28, 719. (n) Zhao, X.; Luo, J.; Liu, X. Synlett 2017, 28, 397. (5) For selected examples of trifluoromethylthiolations, see: (a) Zhang, C.-P.; Vicic, D. A. J. Am. Chem. Soc. 2012, 134, 183. (b) Chen, C.; Chu, L.; Qing, F.-L. J. Am. Chem. Soc. 2012, 134, 12454. (c) Weng, Z.; He, W.; Chen, C.; Lee, R.; Tan, D.; Lai, Z.; Kong, D.; Yuan, Y.; Huang, K.-W. Angew. Chem., Int. Ed. 2013, 52, 1548. (d) Alazet, S.; Zimmer, L.; Billard, T. Angew. Chem., Int. Ed. 2013, 52, 10814. (e) Pluta, R.; Nikolaienko, P.; Rueping, M. Angew. Chem., Int. Ed. 2014, 53, 1650. (f) Arimori, S.; Takada, M.; Shibata, N. Org. Lett. 2015, 17, 1063. (g) Huang, Z.; Yang, Y. D.; Tokunaga, E.; Shibata, N. Org. Lett. 2015, 17, 1094. (h) Glenadel, Q.; Alazet, S.; Tlili, A.; Billard, T. Chem. - Eur. J. 2015, 21, 1. (i) Chachignon, H.; Maeno, M.; Kondo, H.; Shibata, N.; Cahard, D. Org. Lett. 2016, 18, 2467. (j) Yang, Y.; Xu, L.; Yu, S.; Liu, X.; Zhang, Y.; Vicic, D. A. Chem. - Eur. J. 2016, 22, 858. (k) Jiang, L.; Qian, J.; Yi, W.; Lu, G.; Cai, C.; Zhang, W. Angew. Chem., Int. Ed. 2015, 54, 14965. (l) Liu, J.-B.; Xu, X.-H.; Chen, Z.-H.; Qing, F.-L. Angew. Chem., Int. Ed. 2015, 54, 897. (m) Maeno, M.; Shibata, N.; Cahard, D. Org. Lett. 2015, 17, 1990. (n) Xiong, H. Y.; Besset, T.; Cahard, D.; Pannecoucke, X. J. Org. Chem. 2015, 80, 4204. (o) Yin, G.; Kalvet, I.; Englert, U.; Schoenebeck, F. J. Am. Chem. Soc. 2015, 137, 4164. (p) Luo, J.; Liu, Y.; Zhao, X. Org. Lett. 2017, 19, 3434. (q) Wei, F.; Zhou, T.; Ma, Y.; Tung, C.-H.; Xu, Z. Org. Lett. 2017, 19, 2098. (r) Kovács, S.; Bayarmagnai, B.; Goossen, L. J. Adv. Synth. Catal. 2017, 359, 250. (s) Zeng, Y.; Hu, J. Org. Lett. 2016, 18, 856. (t) Kalvet, I.; Guo, Q. Q.; Tizzard, G. J.; Schoenebeck, F. ACS Catal. 2017, 7, 2126. (u) Zeng, J. L.; Chachignon, H.; Ma, J. A.; Cahard, D. Org. Lett. 2017, 19, 1974. (v) Zhang, Z.; Sheng, Z.; Yu, W.; Wu, G.; Zhang, R.; Chu, W.-D.; Zhang, Y.; Wang, J. Nat. Chem. 2017, DOI: 10.1038/ nchem.2789. (w) Zhao, B.-L.; Du, D.-M. Org. Lett. 2017, 19, 1036. (x) Li, H.; Shan, C.; Tung, C. H.; Xu, Z. Chem. Sci. 2017, 8, 2610. (y) Zheng, J.; Cheng, R.; Lin, J. H.; Yu, D. H.; Ma, L.; Jia, L.; Zhang, L.; Wang, L.; Xiao, J. C.; Liang, S. H. Angew. Chem., Int. Ed. 2017, 56, 3196. (6) For the seminal report using CF3SH as radical SCF3 source, see: Harris, J. F.; Stacey, F. W. J. Am. Chem. Soc. 1961, 83, 840. (7) For the seminal report using CF3S-SCF3 as radical SCF3 source, see: Haran, G.; Sharp, D. W. A. J. Chem. Soc., Perkin Trans. 1 1972, 34. (8) For examples using CF3SCl as radical SCF3 source, see: (a) Harris, J. F. J. Am. Chem. Soc. 1962, 84, 3148. (b) Harris, J. F. J. Org. Chem. 1966, 31, 931. (c) Harris, J. F. J. Org. Chem. 1972, 37, 1340. (d) Munavalli, S.; Rossman, D. I.; Rohrbaugh, D. K.; Durst, H. D. J. Fluorine Chem. 1998, 89, 189. (e) Munavalli, S.; Rohrbaugh, D. K.; Berg, F. J.; McMahon, L. R.; Longo, F. R.; Durst, H. D. Phosphorus, Sulfur Silicon Relat. Elem. 2002, 177, 1117. (9) For examples using AgSCF3 as radical SCF3 source, see: (a) Yin, F.; Wang, X.-S. Org. Lett. 2014, 16, 1128. (b) Zhang, K.; Liu, J.-B.; Qing, F.-L. Chem. Commun. 2014, 50, 14157. (c) Zhu, L.; Wang, G.; Guo, Q.; Xu, Z.; Zhang, D.; Wang, R. Org. Lett. 2014, 16, 5390. (d) Fuentes, N.; Kong, W.; Fernandez-Sanchez, L.; Merino, E.; Nevado, C. J. Am. Chem. Soc. 2015, 137, 964. (e) Guo, S.; Zhang, X.; Tang, P. Angew. Chem., Int. Ed. 2015, 54, 4065. (f) Li, C.; Zhang, K.; Xu, X.-H.; Qing, F.-L. Tetrahedron Lett. 2015, 56, 6273. (g) Qiu, Y.-F.; Zhu, X.-Y.; Li, Y.-X.; He, Y.-T.; Yang, F.; Wang, J.; Hua, H.-L.; Zheng, L.; Wang, L.-C.; Liu, X.-Y.; Liang, Y.-M. Org. Lett. 2015, 17, 3694. (h) Wu, H.; Xiao, Z.; Wu, J.; Guo, Y.; Xiao, J.-C.; Liu, C.; Chen, Q.-Y. Angew. Chem., Int. Ed. 2015, 54, 4070. (i) Zeng, Y.-F.; Tan, D.-H.; Chen, Y.; Lv, W.-X.; Liu, X.-G.; Li, Q.; Wang, H. Org. Chem. Front. 2015, 2, 1511. (j) Huang, F.-Q.; Wang, Y.-W.; Sun, J.-G.; Xie, J.; Qi, L.-W.; Zhang, B. RSC Adv. 2016, 6, 52710. (k) Jin, D.-P.; Gao, P.; Chen, D.-Q.; Chen, S.; Wang, J.; Liu, X.-Y.; Liang, Y.-M. Org. Lett. 2016, 18, 3486. (l) Pan, S.; Huang, Y.; Qing, F. L. Chem. - Asian J. 2016, 11, 2854. (m) Song, Y.-K.; Qian, P.-C.; Chen, F.; Deng, C.-L.; Zhang, X.-G. Tetrahedron 2016, 72, 7589. (n) Wu, W.; Dai, W.; Ji, X.; Cao, S. Org. Lett. 2016, 18, 2918. (o) Chen, M.-T.; Tang, X.-Y.; Shi, M. Org. Chem. Front. 2017, 4, 86. (p) He, B.; Xiao, Z.; Wu, H.; Guo, Y.; Chen, Q.-Y.; Liu, C. RSC Adv. 2017, 7, 880. (q) Ji, M. S.; Wu, Z.;

detailed study of transition states and reaction paths would have to be conducted.

4. CONCLUSION To sum up, in this work we have estalished the first trifluoromethylthio radical donor ability scale of electrophilic SCF3 transfer reagents. The large variety of SCF3 transfer reagents and wide span of the scale (from 9.4 to 90.5 kcal mol−1) make the scale a useful tool not only for the future rational design of novel reagents but also for the judicious choice of appropriate ones to explore new radical trifluoromethylthiolations. We believed that the results from this study would contribute to future rapid developemnt of the area of radical trifluoromethylthiolation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b01771. Table S1, Figure S1, and optimized geometries of all computed species. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiao-Song Xue: 0000-0003-4541-8702 Jin-Pei Cheng: 0000-0001-8822-1577 Author Contributions §

M.L. and B.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support from Natural Science Foundation of China (NSFC, Grant Nos. 21390400 and 21402099), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), the State Key Laboratory on Elemento-organic Chemistry, and the Fundamental Research Funds for the Central Universities for financial support.

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REFERENCES

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DOI: 10.1021/acs.joc.7b01771 J. Org. Chem. XXXX, XXX, XXX−XXX