Quantitative Scale for the Trifluoromethylthio Cation-Donating Ability of

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Quantitative Scale for the Trifluoromethylthio Cation-Donating Ability of Electrophilic Trifluoromethylthiolating Reagents Man Li,† Jinping Guo,† Xiao-Song Xue,*,†,‡ and Jin-Pei Cheng*,†,‡,§ †

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

ABSTRACT: A new parameter, trifluoromethylthio cationdonating ability (Tt+DA), is introduced as a quantitative descriptor for the propensity of electrophilic trifluoromethylthiolating reagents to transfer a CF3S moiety in organic synthesis. The first Tt+DA scale of popular reagents has been established through DFT calculations. Excellent correlation has been identified between the Tt+DAs of NSCF3-type reagents and the pKa of the corresponding acids, offering a powerful avenue for the rational design of novel reagents.

he trifluoromethylthio moiety (CF3S) is one of the most lipophilic substituents.1 Introduction of a CF3S group into drug candidates could significantly enhance their cell membrane permeabilities.2 The high electronegativity of the CF3S group could substantially improve the metabolic stability of drug candidates.3 Hence, this unique group has attracted substantial interest from pharmaceutical and agrochemical industries as well as the academic community for its use in isostere-based drug design.2,4 In response to a growing demand for CF3S-containing compounds in medicinal chemistry, chemists are continuously seeking efficient and practical strategies for selective installation of the CF3S group onto the desired scaffold.2,4,5 Among the powerful methods developed,2,4,5 electrophilic trifluoromethylthiolation has attracted considerable interest in recent years.4e,i The rapid progress of this direction is due to the invention of a series of power-variable electrophilic CF3S transfer reagents6−15 (Figure 1) by several groups, such as Munavalli,8a

T

Billard and Langlois,9a,10a Lu and Shen,11a,13a,14 and Shibata.15 The availability of these reagents has greatly accelerated the discovery of new trifluoromethylthiolation reactions16 and thus contributed significantly to an efficient synthesis of compounds possessing fabulous biological properties. Surprisingly, despite significant achievements in developing various reagents and great synthetic utility, scant attention has been devoted to detailed structure−reactivity relationship studies. Consequently, the rational design of novel reagents has been seriously hampered. Very recently, Shen and Lu presented an excellent structure− reactivity study on their trifluoromethanesulfenate reagent (Figure 1, Lu and Shen 2013). On this basis, they were able to find simple and cheaper second-generation reagents (Figure 1, Lu and Shen 2015).14 Additionally, since the available reagents showed very rich and diverse reactivity,4e,i this field would be well served by a scale to quantitatively rank the trifluoromethylthiolating abilities of various reagents. However, such a scale was not available. As a sequence, the current development of new reactions and optimization of reagents still rely largely on the traditional trial-and-error approach. Therefore, establishment of such a scale is urgently needed. Herein, we introduce a previously unexplored parameter, the trifluoromethylthio cation-donating ability (Tt+DA), as a quantitative descriptor of the tendency of a given reagent to release the trifluoromethylthio cation. The Tt+DA parameter of a reagent is defined as the Gibbs free energy change of the reaction (ΔGrxn) at 298.15 K in eq 1, which was obtained by DFT calculations. On the basis of DFT calculations, the first Tt+DA scale of popular reagents is established, and a good correspondence has been found between the computed Tt+DA

Figure 1. Typical electrophilic CF3S transfer reagents.

Received: December 1, 2015

© XXXX American Chemical Society

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DOI: 10.1021/acs.orglett.5b03433 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

Table 1. Calculated Relative Tt+DA Values of Electrophilic CF3S Transfer Reagents in DCM values of the reagents and the experimentally observed relative trifluoromethylthiolating capabilities. Furthermore, the effect of structure variations on the trifluoromethylthio cation-donating abilities is disclosed. Truhlar et al.’s M06-2X hybrid functional has been shown to provide accurate predictions for main group thermochemistry.17 Accordingly, the M06-2X functional was employed for calculating the Tt+DA values. Geometry optimizations and frequency computations were performed using Gaussian 0918 at the M06-2X/[6-31+G(d)+LANL2DZ(I)] level of theory,17 in conjunction with the SMD model19 to account for the solvation effects of dichloromethane (DCM), a commonly used solvent in electrophilic trifluoromethylthiolations. To obtain more accurate electronic energies, single-point energy calculations were performed at the SMD-M06-2X/[6-311++G(2df, 2p)+Def2QZVPPD(I)] level with the SMD-M06-2X/[6-31+G(d)+LANL2DZ(I)] structures. Notably, this level of theory provides reliable predictions of the known experimental C/N/ O−S bond dissociation enthalpies with a mean unsigned error of 1.1 kcal mol−1 (for details, see Table S1). The free “CF3S+” intermediate has been proposed to be involved in trifluoromethylthiolations.20 However, it is found in this calculation that the geometry optimization of the CF3S+ species always led to a +CF2SF species (Scheme 1), suggesting Scheme 1. Sulfur Cation versus Carbocation

attached: (1) N-SCF3 reagents (1A−1H); (2) O-SCF3 reagents (2A−2K); and (3) F-SCF3, Cl-SCF3, and CF3S-SCF3 reagents (3A−3C). Among these reagents, the N-SCF3 family is arguably the most popular CF3S+ source and has been utilized in trifluoromethylthiolation of a wide range of nucleophiles. The calculation predicted that Shen’s N-trifluoromethylthiosaccharin 1A113a has the lowest Tt+DA value of 17.9 kcal mol−1 among these reported N-SCF3 reagents, meaning that it should be the most powerful N-SCF3 reagent reported so far. Replacing the sulfonyl unit of Shen’s reagent 1A1 with a carbonyl group leads to a significant decrease in the ability to transfer a trifluoromethylthio cation, as seen for Munavalli’s reagent 1B18a (33.0 kcal mol−1). The decreased ability to release a trifluoromethylthio cation from Munavalli’s reagent 1B1 than from Shen’s reagent 1A1 could be because the resulting nitrogen anion stabilized by the carbonyl group is not as effective as by the sulfonyl group.1b Removing the π-system of Munavalli’s reagent 1B1 yields Haas’s reagent 1C,7a which is 1.9 kcal mol−1 less favorable to transfer a trifluoromethylthio cation compared to reagent 1B1. Replacement of two carbonyl groups of Haas’s reagent 1C with a phenyl group and a hydrogen leads to a dramatic increase in the Tt+DA value, as seen for Billard and Langlois’s reagent 1D19a (59.7 kcal mol−1). Substitution of the phenyl of reagent 1D1 with a benzyl further increases the Tt+DA value by ca. 22 kcal mol−1 (1D1 59.7 vs 1E9a 81.4 kcal mol−1), while an electron-withdrawing carbobenzyloxy substitution (1F9a) decreases the Tt+DA value by ca. 12 kcal mol−1. The Tt+DA values of Billard and Langlois’s reagents 1D1 and 1G are very close to each other (1D1 59.7 vs 1G 59.1 kcal mol−1). Replacing the phenyl group with an electronwithdrawing p-toluenesulfonyl group yields Billard’s secondgeneration reagent 1H,9a which is ca. 16 kcal mol−1 more favorable to transfer a trifluoromethylthio cation than reagent

that the putative CF3S+ intermediate should not be an energy minimum on the potential energy surface and, hence, should not be recommended for trifluoromethylthiolation as a valid intermediate. Though at first glance, this may not be so conceivable, by closer inspection, one could find that this is indeed reasonable. As a result of the relatively high electronegativity of the sulfur atom compared to carbon, the electrondeficient sulfur cation (if generated) should be significantly less stable than the carbocation, thus resulting in its rapid isomerization to the +CF2SF species. It is worth emphasizing that this has an important implication for the mechanism of electrophilic trifluoromethylthiolations. That is, electrophilic transfer of a CF3S group from reagent to substrate should not proceed via an SN1-type of process involving a free CF3S+ intermediate but rather via an SN2-type mechanism. The unexpected finding mentioned above led us to evaluate the Tt+DA value of a given reagent in a relative manner by using the CF3S exchange reaction in eq 2. One merit that eq 2 can offer

is the cancellation of potential error. In this study, the relative Tt+DA values anchored to the CF3CO2SCF3 with an arbitrarily assigned value of 0.0 kcal mol−1. The calculated relative Tt+DA values for 46 electrophilic CF3S transfer reagents in DCM are presented in Table 1. To elucidate the effects of structural variations on their Tt+DAs, these reagents are grouped into three categories according to the heteroatom to which SCF3 is B

DOI: 10.1021/acs.orglett.5b03433 Org. Lett. XXXX, XXX, XXX−XXX

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been verified. For instance, Lu and Shen found that the reagents in Figure 4 exhibited dramatically different reactivity in the

1G. Finally, examination of a remote substituent effect on the Tt+DAs of the three most popular N-SCF3 reagents 1A1, 1B1, and 1D1 obtained a good linear correlation between the Tt+DA values and the Hammett σp constants (see Figure S1), allowing their trifluoromethylthio cation-donating abilities to be finetuned by substituents. For the O-SCF3 reagents, the Tt+DA values of Lu and Shen’s first- (2A) and second- (2C-2I) generation reagents11a,14 display a narrow distribution around 50.5−57.4 kcal mol−1, indicating that the change of structures does not affect their trifluoromethylthio cation transfer abilities much. This is mainly because the substituents are located far apart from the reaction center. In contrast, direct substitution at the reaction center with the electron-withdrawing acetyl and trifluoroacetyl groups can enhance the ability to transfer a trifluoromethylthio cation by around 32 and 54 kcal mol−1 for Haas’s reagents 2J21 and 2K,21b respectively. Finally, calculation predicted that the Tt+DA values of reagents Cl-SCF3 (3A),22 F-SCF3 (3B),23 and CF3S-SCF3 (3C)6 are 11.1, 18.3, and 25.1 kcal mol−1, respectively, suggesting that their electrophilic trifluoromethylthiolating capability should be stronger than that of Munavalli’s reagent. Interestingly, plotting the Tt+DAs of N-SCF3 reagents against the pKa values24 of the corresponding acids yields an excellent linear relationship with a regression coefficient of 0.998 (Figure 2), indicating that the linear free energy relationship holds in

Figure 4. Electrophilic trifluoromethylthiolation of 2-(naphthalen-2yl)ethanol.

trifluoromethylthiolation of 2-(naphthalen-2-yl)ethanol.4i,13a Clearly, one can see that there is good correspondence between the computed Tt+DAs and the experimentally observed reactivities of these reagents. Moreover, Lu and Shen’s first(2A) and second- (2C-2I) generation reagents showed very similar efficiency in the trifluoromethylthiolation of βketoester,12,14 which, again, can be explained by a similarity of their Tt+DA values. Furthermore, this calculation predicted that Billard’s trifluoromethanesulfanylamides9a (1D1 and 1G) should have a relatively weak electrophilic trifluoromethylthiolating ability. This corresponds well with the experimental observations that a strong Lewis/Brønsted acid was required as an activator in the related transformations.4e,j Note that the Tt+DA scale, like other thermodynamic measures, such as the recently developed methyl cation affinity scale27 and halenium affinity (HalA) scale,28 may be applied under thermodynamically controlled conditions. For cases where kinetic factors play a role, energetic analyses of transition states and reaction paths would have to be considered. In summary, we have developed a quantitative scale for the trifluoromethylthio cation-donating ability of electrophilic CF3S transfer reagents. Excellent linear correlation has been found between the Tt+DAs of N-SCF3-type electrophilic trifluoromethylthiolating reagents and the pKa values of the corresponding acids, which would offer a desired quantitative guide for future rational design of novel reagents and reactions.

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Figure 2. Plot of calculated Tt+DAs of CF3S transfer reagents against experimental pKa values (DMSO) of the corresponding acids.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.5b03433. Table S1, Figure S1, complete ref 18, and optimized geometries of all computed species (PDF)

these two systems.26 This offers a good opportunity for future design of new reagents, considering that substantial pKa data have already been accumulated in the past.24,25 Finally, for easy comparison, we compiled a Tt+DA scale for representative electrophilic trifluoromethylating reagents (Figure 3). With the Tt+DA scale in hand, we naturally wanted to know how well these theoretically obtained results can explain experiments. Indeed, the practical value of the Tt+DA scale has

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Basic Research Program of China (973 Program, 2012CB821600), Natural Science Foundation (NSFC, Grant Nos. 21390400 and 21402099), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), the State Key Laboratory on Elemento-organic Chemistry, the Fundamental Research

Figure 3. Relative trifluoromethylthio cation-donating ability scale of representative reagents in dichloromethane. C

DOI: 10.1021/acs.orglett.5b03433 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.5b03433 Org. Lett. XXXX, XXX, XXX−XXX