Charge-Enhanced Acidity and Catalyst Activation Masoud Samet, Jordan Buhle, † Yunwen Zhou, and Steven R. Kass* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States * S Supporting Information ABSTRACT: Acidities are commonly measured in polar solvents but catalytic reactions are typically carried out in nonpolar media. IR spectra of a series of phenols in CCl 4 and 1% CD 3 CN/CCl 4 provide relative acidities. Non- protonated charged substituents with an appropriate counterion are found to enhance their Brønsted acidities and improve catalyst performance by orders of magnitude. A cid-base reactions are among the most common and fundamental transformations in all of chemistry. As a result, the development of new Brønsted acids and bases are of general interest and have been the subject of extensive research efforts. 1-5 The strengths of these reagents are most commonly measured in water and/or dimethyl sulfoxide (DMSO), both of which are very polar solvents with high dielectric constants. 6 Substituent effects are also routinely studied in polar media, whereas most organic transformations are carried out in less polar solvents. 7,8 Structure-reactivity insights from pK a and substituent effect data, consequently, can be misleading. In this work, IR spectroscopy is used to obtain relative acidities of a series of m- and p-substituted phenols in a nonpolar solvent, and these results are better fit by gas-phase acidities than the corresponding DMSO pK a values. This observation led us to examine charged substituents in nonpolar solvents, and enhanced acidity and catalytic performance is reported. In a clever study, Reed et al. showed that IR spectroscopy can be used to provide relative acidities of the strongest Brønsted acids known to date. 9 This was accomplished by comparing the N-H stretching frequencies of a series of trioctylammonium salts of deprotonated carboranes. It was found that the weaker the interaction of the base, the higher the frequency for this band. Inspired by this work and related studies, 10,11 we obtained the IR spectra of 20 m- and p-substituted phenols in carbon tetrachloride in the presence and absence of acetonitrile-d 3 (Table 1). 12-17 Dilute solutions (5 mM) of the phenols in the latter case gave rise to a sharp band for the “free” O-H stretch around 3600 cm -1 (Figure 1, solid line). Addition of a small amount of CD 3 CN (1% v/v) led to a large 150-220 cm -1 frequency reduction (i.e., a red shift) and a broadening of the band due to the formation of an ArOH··· NCCD 3 hydrogen bond (Figure 1, dotted line). 18,19 A plot of the experimental pK a values in DMSO versus the observed frequency shifts for both the meta and para isomers is reasonably well fit by a single line in which the p-nitro and p- acetyl derivatives are omitted from the least-squares analysis to improve the correlation coefficient from 0.89 to 0.94 (Figure 2). 20 A similar correlation between the gas-phase acidities (ΔG° acid ) of the phenols and the change in their O-H frequency shifts is obtained for all of the compounds including the p-NO 2 and p-COCH 3 derivatives, but in this case, the data are best fit by separate lines for the meta and para isomers (Figure 3). These results suggest that resonance delocalization is not as effective in carbon tetrachloride as it is in dimethyl sulfoxide because this is a completely stabilizing mechanism for solvent-separated ion pairs (DMSO) but not for tight ion pairs Received: February 17, 2015 Published: March 30, 2015 Table 1. Hydroxyl Stretching Frequencies for Substituted Phenols in CCl 4 along with Their DMSO pK a and Gas-Phase Acidity Values ν (cm -1 ) cmpd (XC 6 H 4 OH) X= CCl 4 1% ACN b Δν (cm -1 ) pK a (DMSO) ΔG° acid a (kcal mol -1 ) m-(CH 3 ) 2 N 3616 3464 152 19.1 343.5 ± 2.0 m-H 3611 3454 157 18.0 341.5 ± 1.0 c m-CH 3 3611 3448 163 18.2 341.3 ± 1.4 c m-CH 3 O 3611 3446 165 18.2 341.5 ± 2.0 m-F 3608 3423 185 15.8 337.2 ± 2.0 m-CF 3 3605 3415 190 15.6 332.4 ± 2.0 m-Cl 3606 3415 191 15.8 335.3 ± 2.0 m-NO 2 3599 3387 212 14.4 327.6 ± 2.0 m-CN 3606 3388 218 14.8 329.0 ± 2.0 p-(CH 3 ) 2 N 3616 3468 148 19.8 344.4 ± 2.0 p-CH 3 O 3616 3463 153 19.1 343.9 ± 2.0 p-CH 3 3613 3453 160 18.9 343.8 ± 2.0 p-F 3608 3442 166 18.0 340.4 ± 2.0 p-Cl 3607 3435 172 16.7 336.5 ± 2.0 p-Br 3607 3415 192 16.4 p-CH 3 CO 3599 3407 192 14.0 328.6 ± 2.0 p-CF 3 3602 3406 196 15.3 330.1 ± 2.0 p-CH 3 SO 2 3600 3390 210 13.6 324.2 ± 2.0 p-CN 3597 3380 217 13.2 325.5 ± 2.0 p-NO 2 3594 3373 221 10.8 320.9 ± 2.0 1 d 3041 3043 -2 12.5 ± 1.0 e 261.4 f 2 g 3576 3247 329 12.5 ± 1.0 e 261.4 f 3 h 3566 3196 370 12.4 ± 1.1 e 231.1 i a Equilibrium determinations from ref 16 unless otherwise noted; some values are the average of two similar results. b 1% ACN = 1% CD 3 CN/ 99% CCl 4 . c Measured by threshold collision-induced dissociation (ref 17). d 1 = p-HOC 6 H 4 N(n-C 8 H 17 ) 2 CH 3 + I - . e Measured by bracketing using two colored indicators. f B3LYP/6-31+G(d,p) computations on p-HOC 6 H 4 N(n-C 8 H 17 ) 2 CH 3 + . g 2 = p-HOC 6 H 4 N(n-C 8 H 17 ) 2 CH 3 + BAr F 4 - where Ar F stands for tetrakis(3,5-bis(trifluoromethyl)phenyl)- borate. h 3 = 3-hydroxy-N-octylpyridinium BAr F 4 - . i This calculated value is for 3-methylpyridinium phenol. Communication pubs.acs.org/JACS © 2015 American Chemical Society 4678 DOI: 10.1021/jacs.5b01805 J. Am. Chem. Soc. 2015, 137, 4678-4680