Photogeneration of Benzhydryl Cations by Near-UV Laser Flash

Article pubs.acs.org/JPCA

Photogeneration of Benzhydryl Cations by Near-UV Laser Flash Photolysis of Pyridinium Salts Tobias A. Nigst, Johannes Ammer, and Herbert Mayr* Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377 München, Germany S Supporting Information *

ABSTRACT: Laser flash irradiation of substituted N-benzhydryl pyridinium salts yields benzhydryl cations (diarylcarbenium ions) and/or benzhydryl radicals (diarylmethyl radicals). The use of 3,4,5-triamino-substituted pyridines as photoleaving groups allowed us to employ the third harmonic of a Nd/YAG laser (355 nm) for the photogeneration of benzhydryl cations. In this way, benzhydryl cations can also be photogenerated in the presence of aromatic compounds and in solvents which are opaque at the wavelength of the quadrupled Nd/YAG laser (266 nm). To demonstrate the scope and limitations of this method, the rate constants for the bimolecular reactions of benzhydryl cations with several substituted pyridines were determined in acetonitrile and with water in acetone. The obtained data agree with results obtained by stopped-flow UV−vis spectroscopic measurements. The rate constants for the reaction of the 4,4′-bis[methyl(2,2,2-trifluoroethyl)amino]benzhydrylium ion with 4(dimethylamino)pyridine were also determined in dimethyl sulfoxide, N,N-dimethylformamide, and acetone. From the secondorder rate constants, we derived the nucleophilicity parameters N and sN for the substituted pyridines, as defined by the linear free energy relationship, log k2 = sN(N + E).

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INTRODUCTION Pyridinium salts have a rich photochemistry1 which includes their use as photoinitiators in cationic polymerizations.2−4 However, the heterolytic cleavage of the exocyclic C−N bond of N-alkylated pyridinium ions was only found in a few cases.4 The observation that photolyses of related onium salts, such as phosphonium5,6 or ammonium salts,7,8 gave excellent yields of carbocations thus prompted us to investigate substituted pyridinium salts as precursors for the carbocations. The use of highly substituted aminopyridines as photoleaving groups promised a good wavelength tunability, which would be interesting for applications as photoinitiators9,10 and for kinetic investigations of carbocation reactivities. Many bimolecular reactions of carbocations proceed on a timescale from nanoseconds to microseconds, which requires short pump pulses for time-resolved measurements. The most common and most affordable source of light pulses with pulse widths of a few nanoseconds is the Nd/YAG laser.11 Its fundamental emission in the infrared region (1064 nm) can be converted to the second, third, or fourth harmonic to generate laser pulses in the visible (532 nm) or UV range (355 or 266 nm). In the following discussion, we used the Nd/YAG laser as an example to address some problems related to the choice of excitation wavelength in laser flash photolysis experiments, but similar problems will also be encountered with other types of lasers (including tunable lasers). In previous investigations, we have generated benzhydryl cations 1 and other carbocations by heterolytic photocleavage of precursor molecules such as arylmethyl halides12 or arylmethyl triarylphosphonium salts6 which have significant UV absorbances at 266 nm, but not at 355 nm (Scheme 1). Typically, the carbocations were obtained by irradiating precursor solutions with A266 nm ≈ 0.1−1.0 with a © 2012 American Chemical Society

Scheme 1. Photoheterolysis of Substrates R-PLG (Photoleaving Group PLG− = Cl−, PR′3, etc.) and Subsequent Trapping of the Generated Carbocations (R+) by Nucleophiles (Nu)

266 nm laser pulse; subsequently, the UV−vis absorption decays of the carbocations were monitored in the presence of the nucleophilic reaction partners.8,13 This procedure is problematic when the reaction partners also have considerable absorbances at the excitation wavelength of 266 nm. In this case, the laser pulse may also generate reactive intermediates from the nucleophilic reaction partner, and we can no longer be sure which process is causing the decay of the carbocation. In the usual experimental setup11 featuring a 90° angle between pump and probe light, the opacity of the sample solutions for the 266 nm pump pulse also prevents sufficient excitation of the precursor molecules to generate the carbocations. The same problems not only occur when the nucleophilic reaction partner absorbs at 266 nm but also occur when experiments are carried out in solvents with a high UV cutoff, such as N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or acetone. In such cases, the third harmonic of the Nd/YAG laser (355 nm) would be suitable as a pump, but onium salts do not usually absorb at this wavelength. The use of photosensitizers, a common strategy for photoinitiator systems,9,10 cannot be Received: May 21, 2012 Revised: July 30, 2012 Published: July 31, 2012 8494

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azaprefulvenes or Dewar-pyridines upon ∼254 nm irradiation.16 Therefore, their reactivities cannot be characterized readily with a method that uses 266 nm pump pulses. Rate constants of the reactions of photolytically generated carbocations with pyridines could previously only be determined when the employed precursors incorporated a highly conjugated π system which absorbed at λ > 300 nm.17 The reactivity data acquired in this study will subsequently be employed to determine the nucleophilicity parameters, N and sN, for the pyridines 4−8 according to the linear free energy relationship eq 1, which allows us to predict second-order rate constants, k2, for polar organic reactions with one electrophile-specific parameter, E, and two solvent-dependent nucleophile-specific parameters, N and sN.18

employed for kinetic studies because the diffusion-limited bimolecular excitation transfer is too slow. It was, therefore, desirable to develop a carbocation precursor that can be irradiated at 355 nm. For this purpose, we took advantage of the fact that amino substitution of pyridinium salts causes red shifts of the UV absorptions.1a In this work, we report the generation of benzhydryl cations 1f−n by 355 nm irradiation of benzhydryl pyridinium salts 2f−n-R derived from 3,4,5triamino-substituted pyridines 3-R, the so-called superDMAPs14,15 (Scheme 2). Scheme 2. Generation of Benzhydrylium Ions 1f−n by 355 nm Laser Flash Photolysis of Pyridinium Ions 2f−n-R with Different Substituents (R) on the Pyridine Moietya

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log k 2 = s N(N + E)

(1)

For testing our approach, we measured the rate constants of the reactions of benzhydrylium ions 1 (Table 1) with pyridines

RESULTS AND DISCUSSION Photogeneration of Benzhydrylium Ions. Since the photoelectrofuges (i.e., carbocations-to-be) do not absorb at 355 nm, the excitation must occur at the photonucleofuge (i.e., PLG moiety in Scheme 1). Our recent investigation of 3,4,5triamino-substituted pyridines 3-R (Chart 2) showed that these compounds are strong Lewis bases and that the pyridinium ions 2-R (i.e., their adducts with benzhydrylium ions 1) show UV absorptions around 355 nm.15

Table 1. List of the Reference Electrophiles 1 Used in This Study

Chart 2. Pyridines Employed as Photoleaving Groups in this Studya

a

For substitution patterns f−n of the benzhydryl moiety, see Table 1. Counteranion X− = Cl− or BF4−.

a

dmaBz = 4-(dimethylamino)benzoyl.

The pyridines 3-R were obtained in four to six steps from 4pyridone as described by David et al.15 In order to use the costly materials efficiently, we did not isolate the benzhydryl pyridinium tetrafluoroborates 2f−j-R (X− = BF4−) but generated them in solution by combining the pyridines 3-R with solutions of the isolated18a benzhydrylium tetrafluoroborates 1f−j BF4− (i.e., the reverse of the reaction depicted in Scheme 2). The formation of the pyridinium salts 2f−j-R was indicated by the immediate disappearance of the color of 1f−j and the appearance of UV−vis absorption bands at 340−380 nm. The benzhydryl pyridinium chlorides 2j−o-R (X− = Cl−) were obtained from reactions of 3-R with the corresponding benzhydryl chlorides, Ar2CH-Cl. The syntheses of 2n-R and 2o-R by this method required longer reaction times of 2 h or overnight, as revealed by the full development of the pyridinium bands in the UV−vis spectrum. Figure 1 shows the UV−vis absorption spectra of several 1benzhydryl pyridinium salts 2l-R derived from the pyridines 3R. The parent compound 2l-H (not shown) and the N-alkylsubstituted derivatives, 2l-Bn, 2l-Me (Figure 1), and 2l-Et (not shown), have absorption maxima near 355 nm. The absorption maxima of the N-acyl-substituted compounds 2l-Ac and 2l-Bz

a

Electrophilicity parameters, E, were taken from ref 18a, unless noted otherwise. bFrom ref 6b.

4−8 (Chart 1). Pyridines have strong UV absorptions below 300 nm and are known to undergo photoisomerizations via Chart 1. Pyridines Employed as Nucleophiles in This Study

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Figure 1. UV−vis absorption spectra of pyridinium salts 2l-R (X− = Cl−) with different substituents.

(not shown) are too far in the UV but can be red shifted by the introduction of the NMe2 substituent on the benzoyl group (→ 2l-dmaBz, Figure 1). In agreement with previous reports,1a the absorption maximum of the pyridinium salts does not depend on the substituent at the pyridine nitrogen atom (see Figure S1 of the Supporting Information). When we irradiated ∼1.2 × 10−4 M solutions of the parent pyridinium salt 2i−l-H or the N-alkyl derivatives 2i−l-Bn, 2i−lMe, or 2i−l-Et in CH3CN with a 7 ns laser pulse from the frequency-tripled Nd/YAG laser (λexc = 355 nm, ∼50 mJ/ pulse), we detected the previously described12 UV−vis absorption bands of the moderately stabilized benzhydrylium ions 1i−l at λmax = 450−600 nm in the transient spectra and observed the disappearance (bleaching) of the pyridinium salts 2i−l-R at ∼350−370 nm. Figure 2 shows the transient spectra obtained by irradiation of the N-benzyl-substituted pyridinium salts 2f−n-Bn with different substituents on the benzhydryl moiety. The different absorbances of the benzhydrylium ions 1f−n in Figure 2(panels a−d) are mostly due to the different absorption coefficients and correspond to concentrations of the benzhydrylium ions of 0.6−2.3 × 10−6 M. The spectrum obtained by irradiation of 2n-Bn (X− = Cl−) additionally shows a small band at ∼338 nm, which we assign to the phenyl(ptolyl)methyl radical by comparison with its previously published spectrum.12 More electrophilic carbocations such as the parent benzhydrylium ion 1o could not be obtained in concentrations which were sufficient for kinetic investigations; in these cases we mainly observed the benzhydryl radicals. The yields and lifetimes of the carbocations 1i−l obtained by photolysis of the parent pyridinium salts 2i−l-H and the Nalkylated derivatives 2i−l-Bn, 2i−l-Me, and 2i−l-Et were almost independent of the nature of the photonucleofuge 3R. Irradiation of the N-acyl derivative 2l-dmaBz, however, did not yield any carbocations 1l. For our kinetic investigations, we selected the N-benzyl derivatives 2-Bn because the pyridine 3Bn has the highest Lewis basicity in the series15 and, therefore, forms the most stable pyridinium ions (see below). In the photolyses of benzhydryl triphenylphosphonium salts, the yields and lifetimes of the benzhydryl cations greatly depend on the counteranion of the precursor salt.6a The generation of benzhydryl radicals by electron transfer in electronically excited phosphonium chloride and bromide ion pairs was described, but this process is unimportant at low concentrations of the precursor salts in CH3CN, as the phosphonium halides are mostly unpaired under these conditions.6a However, the lifetimes of the photogenerated benzhydryl cations are reduced significantly by the diffusioncontrolled reactions with the halide counteranions.6a Therefore, we also investigated the influence of the counteranion in the photolysis of 2j-Bn on the lifetime of the photogenerated carbocation 1j. When we irradiated a 1.2 × 10−4 M solution of

Figure 2. Transient UV−vis spectra obtained by irradiation of 1.2 × 10−4 M solutions of (a) 2f-Bn (X− = BF4−), (b) 2i-Bn (X− = BF4−), (c) 2k-Bn (X− = Cl−), or (d) 2n-Bn (X− = Cl−) in acetonitrile with a 7 ns laser pulse (λexc = 355 nm).

the pyridinium halide 2j-Bn (X− = Cl−), we observed a monoexponential decay of the benzhydryl cation 1j with a rate constant of kobs = 1.9 × 106 s−1, which corresponds to a secondorder rate constant of 1.6 × 1010 M−1 s−1 for the reaction of 1j with 2j-Bn Cl−. As this value is slightly larger than the previously reported value of 9.39 × 109 M−1 s−1 for the reaction of 1j with Cl− in acetonitrile,19 one might assign this decay to the combination of 1j with chloride ions. On the other hand, when we employed the pyridinium tetrafluoroborate 2j-Bn (X− = BF4−) as a precursor, the decay of the benzhydryl cation 1j was not monoexponential, and we found an initial fast decay with a rate constant comparable to that obtained with 2j-Bn Cl−, which was followed by a slower decay on the microsecond timescale. We explain this behavior by a fast reversible reaction of 1j with the tertiary amine functions of the precursor salt 2jBn, which is followed by a slower unidentified subsequent reaction. This behavior is analogous to that of other tertiary amines, where we also observed fast reversible reactions followed by slower decays due to unknown subsequent reactions.8 Due to the high nucleophilic reactivity of the tertiary amine centers of the precursors 2-Bn, the choice of the counteranion X− is thus not so important, and we did not 8496

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convert the chlorides 2j−n-Bn (X− = Cl−) to the tetrafluoroborates. Because of the proximity of the diffusion limit, the reactions of 1j−n with chloride19 or tertiary amines8 proceed with almost the same rate constants, and we observed similar decay rates for all investigated benzhydrylium ions obtained from different precursors 2j−n-Bn (X− = Cl− or BF4−). Kinetic Investigations. With a method at hand to generate the benzydrylium ions 1 using 355 nm laser pulses, we determined rate constants of reactions of 1 with nucleophiles. After irradiation of the pyridinium salts 2f−l-Bn (∼1.2 × 10−4 M) in the presence of a large excess of added nucleophiles (7 × 10−4 to 1 × 10−1 M), we observed monoexponential decays of the absorbances, A, of the photogenerated benzhydrylium ions 1f−l, as illustrated in Figure 3 for the reaction of 1h with

Table 2. Second-Order Rate Constants, k2, for the Reactions of the Reference Electrophiles 1 with the Pyridines 4−8 at 20 °C and Resulting N and sN Parameters

Figure 3. Plot of the absorbance decay of 1h at λ = 612 nm observed after irradiation of 2h-Bn (1.2 × 10−4 M, X− = BF4−) with a 7 ns laser pulse (λexc = 355 nm, ∼50 mJ/pulse) in the presence of 4(dimethylamino)pyridine ([4] = 2.12 × 10−3 M). Inset: plot of kobs vs [4]; kobs = 1.37 × 106 [4] + 857 (R2 = 0.9996).

DMAP (4). The decays of the absorbances were fitted with the exponential function A(t) = A0e−kobst + C to obtain the firstorder rate constants kobs (s−1). Plots of kobs versus the nucleophile concentrations were linear in all cases, and the slopes of these plots provided the second-order rate constants k2 (M−1 s−1) for the reactions of 1f−l with the nucleophiles. In order to study some systems of known reactivity, we measured the second-order rate constants, k2, for the reactions of some benzhydrylium ions 1 with 4-(dimethylamino)pyridine (DMAP, 4) in acetonitrile (Table 2). The second-order rate constant for the reaction of 4 with 1f was previously determined with the stopped-flow method,20 and we wanted to reproduce this measurement with our new method. However, due to the similar electrofugalities of 1f (Ef = 4.84) and 1c (Ef = 4.83),21 the formation of 1f by thermal dissociation of 2f-Bn in acetonitrile solutions is expected to occur with a rate constant similar to that reported for the dissociation of 2c-Bn (9.6 × 10−2 s−1).15 In the presence of high concentrations of DMAP (4), the benzhydrylium ions 1f will then be trapped by 4 to give a pyridinium salt with no absorption at 355 nm. As a consequence, we had to work quickly and keep the nucleophile concentration below ca. 5 × 10−3 M in order to maintain a sufficient concentration of 2f-Bn, which is needed to generate the carbocation 1f with the 355 nm laser pulse. The rate constant for the reaction of 4 with 1f (2.97 × 105 M−1 s−1) determined in this way is in fair agreement with the previously reported value from stopped-flow experiments (2.31 × 105 M−1 s−1).20 The rate constants for the reactions of 4 with 1h and 1i are too fast for the stopped-flow method and could not be measured with the established laser flash photolytic method using 266 nm laser pulses because 4 absorbs at the excitation wavelength. These rate constants could now be determined by laser flash photolysis of 2-Bn at 355 nm (Table 2), and Figure 4 shows that these rate constants extend the correlation line for

a Laser flash photolysis of 2-Bn, unless noted otherwise. bStopped-flow UV−vis measurement. cFrom ref 20. dThe average of both rate constants from laser flash photolysis and stopped-flow measurements was used for the correlation analysis. eWith 105 M−1 s−1 for the reactions of 1f−k with the substituted pyridines 5−8 (Table 2) in acetonitrile with the 355 nm laser flash photolytic method.22 These data are supplemented with rate constants for slower reactions (k2 < 106 M−1 s−1), which were determined with the stopped-flow method under first-order conditions by using the nucleophiles in a large excess (>10 equiv over the benzhydrylium tetrafluoroborates 1a−i) as described previously18a (Table 2). Again we observed good linear correlations of log k2 versus E (Figure 4) for the data from both methods. As previously observed, 1g8,23a,b (not shown) and 1j23c always react somewhat faster in acetonitrile solution than expected from their E parameters which have been determined in CH2Cl2 solutions. In the case of 7, only the reactions with 1h and 1i can be followed with the stopped-flow method, as the better stabilized carbocations show no conversion in reactions with 7 due to its low Lewis basicity. We have now been able to characterize the nucleophilicity of 7 over a wider reactivity range by including the data determined by the laser flash photolysis method. From the correlations in Figure 4, we derived the nucleophilicity parameters of 4−8 listed in Table 2. As expected, the nucleophilicities of the 4-amino-substituted pyridines 4−7 decrease with the electron-withdrawing character of the substitutents (Table 2 and Figure 4). The N-acetyl-4aminopyridine (8) is of similar reactivity as the 3-bromosubstituted DMAP 6. To demonstrate the applicability of the method in solvents with high UV cutoff, we also measured the second-order rate constants for the reactions of 1i with 4 in DMSO, DMF, and acetone (Table 2). The order of reactivity (acetone > acetonitrile > DMF > DMSO) agrees with that previously reported for the reactions of 4 with 1f.20 Furthermore, we determined first-order rate constants for the reactions of 1l with 80% and 90% aqueous acetone (Table 3) and found good

such as 1o cannot be generated efficiently, and the precursors 2-Bn of carbocations, which are less Lewis-acidic than 1f, are not stable in the presence of nucleophiles that trap the small concentrations of benzhydrylium ions, which exist in solutions of 2a−f-Bn in the dark. Nevertheless, we could generate benzhydrylium ions 1f−n covering more than 10 orders of reactivity and investigate the rates of their reactions with nucleophiles. We employed this method to characterize the nucleophilic reactivities of the electron-rich pyridines 5−8, which will be used for further studies in our group. Moreover, we have demonstrated that the third harmonic of the Nd/YAG laser (355 nm) can be employed to generate benzhydrylium ions 1 in solvents with a high UV cutoff such as acetone (cutoff = 330 nm)25 which is opaque to the quadrupled Nd/YAG laser (266 nm) and the XeCl excimer laser (308 nm). Laser flash photolysis of substituted pyridinium salts at 355 nm thus supplements the established kinetic methods and is particularly useful for characterizing nucleophiles which do not react with stabilized carbocations for thermodynamic reasons and which cannot be studied with 266 nm laser flash photolytically generated carbocations due to the absorbance of the sample solutions.

Table 3. Comparison of Experimental and Calculated FirstOrder Rate Constants (s−1) for Reactions of 1l with Acetone/Water Mixtures

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solventa

k1b (s−1)

90A10W 80A20W

6.23 × 10 6.61 × 106 6

kcalcc (s−1)

k2/kcalc

4.35 × 10 7.17 × 106

1.43 0.92

6

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ASSOCIATED CONTENT

S Supporting Information *

Additional UV−vis spectra and details of the kinetic studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+49) 892180-77719. Fax: (+49) 89-2180-77717. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (SFB 749) and the Fonds der Chemischen Industrie for financial support and Raman Tandon for samples of 3-Me and 3-Et. ABBREVIATIONS Bn, benzyl; Bz, benzoyl; dmaBz, 4-(dimethylamino)benzoyl; PLG, photoleaving group; DMAP, 4-(dimethylamino)pyridine; DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; YAG, yttrium aluminum garnet

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a Mixtures given in v/v (A = acetone, W = water). bLaser flash photolysis of 2l-Bn. cCalculated from eq 1 using the previously published N1 and sN parameters of acetone/water mixtures (90A10W: N1 = 5.70, sN = 0.85; 80A20W: N1 = 5.77, sN = 0.87).24

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CONCLUSION We have demonstrated that the 355 nm laser flash photolysis of pyridinium salts 2-Bn is a suitable method for the generation of benzhydrylium ions 1 in the presence of reactants or solvents that absorb at 266 nm. The scope of carbocations which can be generated is more restricted than in the 266 nm photolyses of quaternary phosphonium salts.6,8 Highly reactive carbocations 8498

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dx.doi.org/10.1021/jp3049247 | J. Phys. Chem. A 2012, 116, 8494−8499