Article pubs.acs.org/joc
Cite This: J. Org. Chem. 2017, 82, 11052-11055
Aryltrimethylstannane Cation Radical Fragmentation Selectivities That Depend on Codonor: Evidence for Reactions from Heterodimer Cation Radicals Pu Luo† and Joseph P. Dinnocenzo* Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, United States S Supporting Information *
ABSTRACT: The aryl/methyl fragmentation selectivities for the photooxidations of phenyltrimethylstannane and (4-methylphenyl)trimethylstannane by 1,2,4,5-tetracyanobenzene in acetonitrile were found to depend on the codonor used to generate the stannane cation radical intermediates. The aryl/ methyl fragmentation selectivities for phenyltrimethylstannane and (4methylphenyl)trimethylstannane varied by factors of 26 and 5.6, respectively, depending on the structures of the codonors. The fragmentation selectivities could be correlated with the oxidation potentials of the codonors and their steric bulk. The results can be interpreted by the intermediacy of heterodimer cation radicals formed between the stannane cation radicals and the neutral codonors, which thereby affect the fragmentation selectivities.
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INTRODUCTION We recently described the generation and spectral characterization of several aryltrialkylstannane cation radicals, including those of 1 and 2 (see structures below).1 1+• and 2+• were both found to undergo fragmentation with loss of aryl or methyl radicals by a nucleophile-assisted mechanism. An unusual feature of these fragmentation reactions was the selective loss of aryl radical over methyl radical, an otherwise unprecedented result in cation radical chemistry. Steady-state photooxidation experiments to generate 1+• and 2+• were performed using 1,2,4,5-tetracyanobenzene (TCB) as the photosensitizer in CH3CN in the presence of a high concentration of a variety of aromatic codonors. The function of the codonor is to intercept the singlet excited state of TCB (1TCB*) to initially generate a TCB−•/codonor+• geminate pair. After separation of the ion radical pair, the free codonor+• oxidizes the stannanes by single electron transfer to generate the stannane cation radicals, which subsequently react with CH3CN as the nucleophile. Use of a codonor in the TCB photooxidations is helpful because direct interception of 1TCB* by the stannanes results in rapid intersystem crossing to form the triplet excited state of TCB (3TCB*), which is a much poorer photooxidant. One naturally expects the follow-up chemistry of stannane cation radicals generated through oxidation by the codonor cation radical to be independent of the structure of the codonor. The aryl/ methyl loss selectivities for the photooxidations of 1 and 2 were indeed independent of the codonors used (o-difluorobenzene, PhCl, PhBut, or PhMe).
We describe herein the surprising discovery that the aryl/ methyl loss selectivities for the TCB-photosensitized oxidations of phenyltrimethylstannane (3) and (4-methylphenyl)trimethylstannane (4) in CH3CN do depend on the structure of the codonor used. As described below, the results suggest that, depending on the codonor, the fragmentations of 3+• and 4+• can occur via complexes between the stannane cation radicals and the codonors, and the structure of the codonor can thereby influence the fragmentation selectivity for aryl vs methyl radical loss.
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RESULTS AND DISCUSSION
The fragmentation selectivities for aryl vs methyl radical loss from 3+• and 4+• in TCB-sensitized photooxidation experiments in the presence of a codonor were determined as previously described.1 Argon-purged acetonitrile solutions containing 3 or 4 (∼5 mM), TCB (∼5 mM), and a codonor (∼0.4 M) were irradiated at 313 nm. Following irradiation, the solutions were treated with LiCl to convert the stannylium cation−acetonitrile adducts to Me3SnCl and ArSnMe2Cl (Scheme 1). The stannyl chloride product ratios were determined by 1H NMR spectroscopy using peak assignments from authentic materials. The product ratios were independent Scheme 1. Conditions for Photooxidation of Aryltrimethylstannanes
Received: August 7, 2017 Published: October 10, 2017 © 2017 American Chemical Society
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DOI: 10.1021/acs.joc.7b01989 J. Org. Chem. 2017, 82, 11052−11055
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The Journal of Organic Chemistry
heterodimer cation radicals formed between 3+• or 4+• and the codonors, we would expect that, barring significant steric or electronic (e.g., MO symmetry) factors, the stannane cation radicals would tend to interact most strongly with codonors of similar oxidation potential. In principle, it should be possible to detect the presence of the proposed heterodimer cation radicals formed with 3+• and 4+• by transient absorption spectroscopy. However, as described previously,1 no cation radical was detected by nanosecond transient absorption spectroscopy upon photooxidation of 3, presumably because 3+• and/or the proposed heterodimer cation radicals are too short-lived. The same was found to be true for photooxidation of 4. Stannane cation radicals 3+• and 4+• are highly reactive and, not surprisingly, their cyclic voltammograms do not show reversible behavior. The ionization potentials of 3 and toluene are identical5 within experimental error; thus, it is reasonable to assume that their oxidation potentials will also be similar. As the data in Table 1 show, the Ph/Me fragmentation ratio for 3 is close to a maximum with toluene as the codonor. Thus, if the photooxidation of 3 proceeds via [3/codonor]+• complexes, the experimental results are consistent with the most selective fragmentation reactions occurring for codonors where the degree of mixing between the aromatic donors is maximal. The lower selectivity observed for 3 with mesitylene as codonor is consistent with this hypothesis. We estimate that the oxidation potential of 4 will be lower than that of 3 by an increment similar to the difference in the oxidations of benzene and toluene (0.22 V; see Table 1), i.e., ∼2.04 V vs SCE. For the codonors studied, the fragmentation ratios for photooxidation of 4 are indeed the largest for codonors whose oxidation potentials are similar to that estimated for 4 (m-xylene and mesitylene). The Ar/Me fragmentation selectivities decrease significantly for the photooxidations of both 3 and 4 with the two sterically hindered codonors 1,4-di-tert-butylbenzene and 1,3,5-tri-tertbutylbenzene when compared to codonors of comparable oxidation potential. Presumably the hindered codonors form less stable heterodimer cation radicals due to the larger interring separation enforced by the bulky tert-butyl groups. That the fragmentation ratios for these two sterically hindered codonors remain different for the photooxidations of 3 and 4 provides some indication of the severe steric requirements needed to fully block heterodimer cation radical formation. The present data do not allow a determination of the fraction of the stannane cation radical fragmentations for 3+• and 4+• that proceed via heterodimer cation radical intermediates vs uncomplexed cation radicals. In the future we plan to study the fragmentation selectivities vs codonor concentration to address this question. On the basis of the range of selectivities in Table 1, however, one can at least conclude that the differences in selectivity for the free vs complexed stannane cation radicals must be at ≳25 for 3 and ≳6 for 4. As discussed above, the fragmentation selectivities for the cation radicals of (4-biphenyl)trimethylstannane (1+•) and (4methoxyphenyl)trimethylstannane (2+•) do not show a dependence on the codonor used to generate them.1 This is likely due, in part, to the greater differences in the oxidation potentials of the codonors relative to 1 and 2. In addition, for 1+•, prior work has shown that biphenyl cation radicals are less susceptible to dimer cation radical formation than benzene cation radicals with donors of comparable oxidation potential.2 Given the renewed interest in photoredox processes6 (a.k.a. photoinduced electron transfer reactions), the results presented
of conversion percentages (∼10−50%) within experimental error. Mass balances determined using an internal standard were uniformly high (>90%). The statistically corrected Ar/Me fragmentation ratios from photooxidation of 3 and 4 with eight different codonors are summarized in Table 1. We note that Table 1. Statistically Corrected Aryl/Methyl Loss Ratios from Photooxidation of Stannanes 1−4 with Various Codonors Ar/Me loss ratioc,d codonora
Eoxb
o-F2B PhH PhBut PhMe m-xylene mesitylene 1,4-But2B 1,3,5-But3B
∼2.53 2.48 2.28 2.26 2.10 2.05 2.07 2.05
e
1f
2f
3
4
11.5(9) 12.2(2)g 12(1) 11.7(4)
1.1(1) 1.1(1)g 1.1(1) 1.0(1)
4.2(3) 8.1(4) 9.8(8) 12.3(5) 12.8(9) 4.0(4) 2.1(1) 0.5(1)
5.5(3) 11.0(3) 11.0(6) 14.2(4) 23(2) 26(2) 13.0(6) 4.6(2)
a
o-F2B, o-difluorobenzene; 1,4-But2B, 1,4-di-tert-butylbenzene; 1,3,5But3B, 1,3,5-tri-tert-butylbenzene. bOxidation potentials of codonors (V vs SCE) in CH3CN (ref 2). cStatistically corrected. dStandard deviation in the last significant digit given in parentheses. eEstimated from a correlation of Eox and ionization potential. fData from ref 1. g With chlorobenzene as codonor, which has Eox of 2.46 V vs SCE (ref 2).
UV−vis spectra do not show any spectroscopic evidence for the formation of charge transfer complexes between TCB and 3 or 4 at 0.01 M in the presence or absence of the codonors. We begin by discussing the results from the photooxidation of phenyltrimethylstannane (3). For the codonors studied, the range of the observed fragmentation Ph/Me ratios varies by a factor of ∼25! It is of interest to note that for the relatively unhindered codonors (the first six entries in Table 1), the fragmentation ratios increase with decreasing oxidation potential of the codonor, reaching a maximum of ∼12−13. A plausible reason for the lower selectivity observed for mesitylene will be discussed below. A similar trend is observed in the photooxidation of (4-methylphenyl)trimethylstannane (4), except that the maximum for the Ar/Me loss ratio is observed for codonors with lower oxidation potentials. While the range of fragmentation ratios for 4 (∼6) is less than for 3 (∼25), it is still significant. Interestingly, for the two most sterically hindered donors, 1,4-di-tert-butylbenzene and 1,3,5tri-tert-butylbenzene, the fragmentation ratios decrease relative to the other codonors for the photooxidations of both 3 and 4. The Ar/Me loss ratios in Table 1 can plausibly be explained if the fragmentations of the intermediate stannane cation radicals produced by photoinduced electron transfer proceed via complexes between 3+• or 4+• and the neutral codonors, i.e., heterodimer cation radicals. Homodimer cation radicals of aromatic compounds have long been known.3 There are fewer examples of analogous heterodimer cation radicals, but a number of well documented cases have been reported.4 As clearly shown by Meot-Ner (Mautner),4b the interaction energy for heterodimer cation radicals increases as the difference in ionization energy between the partners decreases. Simple orbital interaction diagrams4c also predict that the extent of charge delocalization (resonance mixing) is expected to be maximal when the oxidation potentials of the monomer units are similar. Applying these principles to the potential 11053
DOI: 10.1021/acs.joc.7b01989 J. Org. Chem. 2017, 82, 11052−11055
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The Journal of Organic Chemistry
authentic samples, and the mass balance was determined by integration against the internal standard. Sufficient 1H NMR delay times were used to ensure accurate integrations.
here provide both a caution and an opportunity. For photoredox reactions involving organic cation radicals where selectivity (e.g., regio-, chemo-, stereo-, etc.) is a relevant factor, one should be aware that moieties that can form heterodimer cation radicals with the intended cation radicals of interest might be noninnocent. On the other hand, the deliberate use of addends to promote heterodimer cation radical formation might, in some circumstances, be productively used to optimize cation radical reaction selectivities.
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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.7b01989. NMR spectra (PDF)
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EXPERIMENTAL SECTION
General Methods. 1H NMR spectra were recorded at 400 MHz. Chemical shifts (δ) are in ppm relative to tetramethylsilane using the residual proton in the solvent as an internal standard (CHD2CN, δ = 1.95; CHCl3, δ = 7.27). Proton−proton coupling constants reflect assumed first-order behavior. Peak integrations were normalized by multiplying the fractional peak area (area peak/sum of all peak areas) by the total number of protons in the proposed structure. 13C NMR spectra were recorded at 125 MHz. Chemical shifts (δ) were referenced to internal chloroform-d (CDCl3, δ = 78.0 ppm). 119SnNMR spectra were recorded at 186.5 MHz. Chemical shifts (δ) in ppm are relative to tetramethyltin. Materials. Unless otherwise stated, materials were from commercial sources and used as received. 1,2,4,5-Tetracyanobenzene was purified by recrystallization from chloroform. Diethyl ether was purified by passage over a bed of activated alumina.7 Benzene, toluene, o-difluorobenzene, and tert-butylbenzene were distilled before use. 1,4Di-tert-butylbenzene and 1,3,5-tri-tert-butylbenzene were generously provided by Eastman Kodak. Phenyltrimethylstannane, 8 (4methylphenyl)trimethylstannane,9 dimethylphenylstannyl chloride,10 and bis(4-methylphenyl)dimethylstannane11 were prepared by literature procedures. Techniques. Steady-state photolysis reactions were performed using a 200 W or a 500 W mercury arc lamp equipped with a liquid filter filled with deionized water to absorb IR light. The light was successively filtered by a 309 nm cutoff filter (Oreil no. 59450) and a 313 nm interference filter (Oriel no. 56511). Preparation of Dimethyl(4-methylphenyl)stannyl Chloride. To a 50 mL round-bottomed flask containing 3.31 g (10 mmol) of bis(4-methylphenyl)dimethylstannane in 20 mL of Et2O was added dropwise 5.3 mL (10 mmol) of a 1.9 M HCl solution in Et2O under a nitrogen atmosphere. The volatiles were subsequently removed under reduced pressure, resulting in a colorless oil that was taken up in hexanes and filtered through a plug of basic alumina. Vacuum distillation (bp 69 °C, 0.03 Torr) gave a colorless oil (1.82 g, 66%). 1H NMR (CDCl3 400 MHz): δ 7.48 (d + d + d, J = 7.6 Hz, JSn−H = 57.2 Hz, 2.0 H), 7.27 (d, J = 7.6 Hz, 2.4 H (integral includes residual CHCl3), 2.38 (s, 3.0 H), 0.84 (s, JSn−H = 58.4, 6.1 H). 1H NMR (CD3CN 400 MHz): δ 7.56 (d + d + d, J = 7.6 Hz, JSn−H = 57.2 Hz, 1.9 H), 7.27 (d + d + d, J = 7.6 Hz, JSn−H = 15.2 Hz, 2.0 H), 2.36 (s, 3.0 H), 0.83 (s + d, J119Sn−H = 64.2 Hz, 6.2 H). 13C NMR (CDCl3 125 MHz): 141.0 (JSn−C = 12.3 Hz), 137.6 (J117Sn−C = 543.6, J119Sn−C = 568.8 Hz), 135.9 (JSn−C = 50.2 Hz), 130.5 (JSn−C = 61.0 Hz), 22.4, −1.23 (J117Sn−C = 375.9, J119Sn−C = 393.5 Hz). 119Sn NMR (CDCl3 186.5 MHz): δ 99.00. HRMS (EI-TOF) m/z: [M]+ calcd for C9H13ClSn 275.9728; found 275.9716. Representative Procedure for Photooxidations in Acetonitrile. A stopcocked, quartz cuvette containing a small Teflon-coated magnetic stir bar was charged with 0.5 mL of a 0.010 M 1,2,4,5tetracyanobenzene solution in CD3CN, 40 μL of a 0.13 M solution of 3 (or 4) in CD3CN, 0.4 mL CD3CN, and an aromatic codonor (0.4 M). The resulting solution was purged with argon for ∼15 min and photolyzed with stirring at 313 nm. To the resulting solution was added 30 μL of a 0.17 M dioxane solution (internal standard) in CD3CN and 20 μL of a 4 M aqueous LiCl solution. After stirring for 10 min, the solution was quickly dried over MgSO4, filtered into an NMR tube, and analyzed by 1H NMR spectroscopy. The tin-methyl group hydrogens of the tin chloride products were integrated to calculate the aryl/Me fragmentation ratio by comparison with
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ORCID
Joseph P. Dinnocenzo: 0000-0003-0206-3497 Present Address †
Dow Chemical, 400 Arcola Rd., N1320, Collegeville, PA 19426. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The NSF (CHE-1464629) is gratefully acknowledged for financial support. REFERENCES
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