Stereochemistry of Nucleophilic Substitutions on Benzyl-silane and

Nov 2, 2017 - ... on Benzyl-silane and -germane Cation Radicals: Application of the ... University of Rochester, Rochester, New York 14627, United Sta...
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Stereochemistry of Nucleophilic Substitutions on Benzyl-silane and -germane Cation Radicals: Application of the Endocyclic Restriction Test Mary S. Lenczewski, H. J. Peter de Lijser, David M. Turner, and Joseph P. Dinnocenzo* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *

ABSTRACT: Benzyltrialkylgermane cation radicals were generated and spectroscopically characterized by nanosecond transient absorption spectroscopy. The germane cation radicals were found to rapidly react with nucleophiles (e.g., alcohols) in reactions that were first-order in cation radical and first-order in nucleophile. The geometries of the transition states for nucleophilic substitutions on benzyl-silane and -germane cation radicals were investigated by using the endocyclic restriction test. Cation radicals containing tethered nucleophiles that required endocyclic transition states with small angles between the bond being formed to the nucleophilic atom and the bond to the leaving group reacted ∼250 times more slowly than cation radicals with tethered nucleophiles where a large bond angle can be accommodated. The results are consistent with the nucleophile-assisted fragmentations proceeding through an inversion transition state.

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INTRODUCTION A variety of organosilane cation radicals, including benzylsilanes,1 α-aminosilanes,2 silylenol ethers,3 disilanes,4 and oligosilanes,5 have been shown to undergo nucleophile-assisted fragmentation reactions. Analogous fragmentations of organogermane and -stannane cation radicals have also been reported in the literature,6,7 although relatively little mechanistic evidence has been provided that these reactions are also nucleophile-assisted.8 Experimental data that support a nucleophile-assisted mechanism for organosilane cation radicals are largely from transient kinetics experiments, which show the reactions are first-order in cation radical and first-order in nucleophile. Mechanistic experiments for analogous SN2 reactions of closed shell molecules have typically relied on both kinetics and a determination of the stereochemical outcome of the reactions. To date, only one experiment has been performed to elucidate the stereochemical outcome of an organosilane cation radical nucleophilic substitution. Silacyclobutanes 1 and 2 were subjected to one-electron oxidation by photoinduced electron transfer and found to react with methanol as the nucleophile with inversion of configuration at silicon (i.e., 1+• → 3 and 2+• → 4).1f The use of analogous larger ring silacycloalkanes or acyclic silanes is not possible because the methoxysilane products are not configurationally stable in the presence of excess nucleophile. Thus, an alternative approach is needed to more generally investigate the stereochemistry of nucleophilic substitutions on silane cation radicals and for other cation radicals where the substitution products are not configurationally stable. We describe herein use of the endocyclic restriction test to probe the stereochemical outcome of nucleophilic substitutions on benzyl-silane and -germane cation radicals. © 2017 American Chemical Society

The endocyclic restriction test (ERT) has been most extensively used to probe the stereochemistry of substitutions at a variety of nonstereogenic atoms, typically heteroatoms (e.g., N, O, S, P, Br, etc.).9 In its most common embodiment, the ERT is applied to substitution reactions where a substrate containing an atom whose substitution stereochemistry is to be probed additionally contains a tethered atom or group that will perform the substitution. When the appropriately attached tether is sufficiently short, intramolecular substitution can only realistically proceed by a retention mechanism. To distinguish between intramolecular (retention) and intermolecular (inversion) substitution pathways, double-labeling/crossover experiments are typically the method of choice. When probing the stereochemical outcome of substitution reactions with the ERT using a double-labeling/crossover experiment, it is imperative that any reactive intermediates formed from unlabeled and doubly labeled substrates do not rapidly react with residual reactants to give products, which would compromise the crossover experiment. As explained below, Received: July 28, 2017 Published: November 2, 2017 12112

DOI: 10.1021/acs.joc.7b01892 J. Org. Chem. 2017, 82, 12112−12118

The Journal of Organic Chemistry

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this requirement cannot be practically met for nucleophilic substitutions on benzyl-silane and -germane cation radicals. We therefore opted to use a variant of the ERT in which substitution rate constants for two cation radicals are compared: one in which a tethered nucleophile is capable of intramolecular substitution only by a retention mechanism and a second in which a comparable tethered nucleophile is capable of intramolecular substitution by inversion or retention. Our experiments to probe the stereochemistry of nucleophilic substitutions on benzyl-silane and -germane cation radicals utilized silanes 5 and 6 and germanes 7−9, which contain comparably tethered alcohols as nucleophiles. These substrates were chosen for several reasons: prior work had shown that their cation radicals undergo clean fragmentation of the benzylic-Si or -Ge bonds upon photooxidation, similarly substituted silane cation radicals could be readily observed by nanosecond transient absorption spectroscopy (NTAS), and the cation radicals rapidly react with alcohol nucleophiles.1,6d Additionally, we previously showed that the cation radical of 6 undergoes rapid first-order decay by intramolecular nucleophilic substitution to displace the 4-methoxybenzyl radical.1d Although analogous experiments with benzylgermane cation radicals have not been previously reported, we show herein that their cation radicals, while significantly more reactive with nucleophiles than comparable silane cation radicals, can nonetheless also be generated and observed by NTAS.

Article

RESULTS AND DISCUSSION

We begin by discussing the reactions of benzylgermane cation radicals with nucleophiles because the kinetics for these reactions have not been previously described in the literature. Reaction of Benzylgermane Cation Radicals with Nucleophiles. Benzylgermanes 10−12 were prepared by standard methods (see Experimental Section) to initially determine if their cation radicals reacted with nucleophiles in a manner analogous to that of comparably substituted benzylsilane cation radicals.1

The cation radicals of 10−12 were generated by the general method previously described.1e Briefly, O2-saturated 1,2dichloroethane (DCE) solutions containing ∼1 mM Nmethylquinolinium hexafluorophosphate (NMQ+), toluene (0.4 M), and ∼10 mM of a benzylgermane were excited with a nanosecond laser (∼10 ns, 343 nm). Excitation of NMQ+ produces its singlet-excited state (1NMQ+*), which is rapidly intercepted by toluene to efficiently produce toluene+• and the N-methylquinolinyl radical (NMQ•) by photoinduced electron transfer. NMQ• is scavenged by dioxygen in 70 times shorter in CH2Cl2.1d The somewhat smaller ratio for the lifetimes of 8+• vs 9+• (∼14) may be reflective of differences in the solvent used for the experiments.

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CONCLUSIONS Benzyltrialkylgermane cation radicals have been shown to react with nucleophiles by an SN2 mechanism, similar to analogous silane cation radicals previously studied, although the germane cation radicals are considerably more reactive. Endocyclic restrictions tests demonstrate that the reactions of both the benzyl-silane and the -germane cation radicals with nucleophiles prefer a substitution mechanism that proceeds via an inversion pathway. The kinetic ERT method used here should be generally applicable for studying the stereochemical pathway of other reactions involving highly reactive intermediates where conventional ERT methods (e.g., double-labeling/crossover) cannot be readily applied.

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EXPERIMENTAL SECTION

General Experimental Procedures and Techniques. Unless otherwise noted, the following conditions were used for all nonaqueous reactions. Reactions were conducted at room temperature in oven-dried glassware (125 °C) under a nitrogen atmosphere, and solutions were stirred magnetically using Teflon-coated magnetic stirbars. Air- and moisture-sensitive reagents and solutions were transferred via syringe or cannula and were introduced to the apparatus through rubber septa or three-way stopcocks under a vigorous nitrogen purge. Routine 1H NMR spectra were recorded at 400 MHz. Chemical shifts (δ) in ppm relative to tetramethylsilane using the residual proton in the solvent as an internal standard. Proton−proton coupling constants are measured line spacings. The following abbreviations were used: s (singlet), d (doublet), t (triplet), q (quartet), pent (pentet), dd (doublet of doublets), dt (doublet of triplets), m (multiplet), and br (broad). 13C NMR spectra were recorded at 100 MHz; chemical shifts were referenced to internal chloroform-d. Purifications by column chromatography were performed using 230− 400 mesh silica gel. Materials. Diethyl ether, tetrahydrofuran (THF), and acetonitrile were purified by passing through a column of activated alumina from a solvent purification system.11 Dichloromethane and 1,2-dichloroethane were distilled from phosphorus pentoxide before use. 1,1,1,3,3,3-Hexafluoroisopropanol was distilled from sodium bicarbonate and stored over 3 Å sieves before use. Toluene was distilled from calcium hydride. N-Methylquinolinium hexafluorophosphate,1e 10,12 13,13 16,14 and 1,2-dichlorotetramethyldigermane15 were prepared by literature procedures. Other materials were commercially available and used as received. Preparation of Germanes 11 and 12. A 100 mL three-necked flask was charged with 0.612 g (25.2 mmol) of magnesium powder, 0.091 (0.5 mmol) anthracene, and 20 mL of THF. The mixture was sonicated for 4 h and then cooled to −30°. A solution containing 1.85 12116

DOI: 10.1021/acs.joc.7b01892 J. Org. Chem. 2017, 82, 12112−12118

Article

The Journal of Organic Chemistry

Preparation of Germane 19. A solution of 18 (2.7 g, 10.4 mmol) in 25 mL of THF was added dropwise over 30 min to a solution of lithium aluminum hydride (0.398 g, 10.5 mmol) in 25 mL of THF. After 18 h, ethyl acetate (20 mL) was added dropwise over 30 min, followed by 20 mL of water. The layers were separated, and the organic layer was dried over anhydrous MgSO4, filtered, and concentrated to give a pale yellow oil that was distilled (47−48 °C, 0.05 mmHg) to give 1.71 g (73%) of a clear, colorless oil. 1H NMR (CDCl3): δ 6.96 (d, 2 H, J = 8.8 Hz), 6.79 (d, 2 H, J = 8.8 Hz), 3.95 (sept, 1 H, J = 3.2 Hz), 3.78 (s, 3 H), 2.24 (d, 2 H, J = 2.8 Hz), 0.18 (d, 6 H, J = 3.2 Hz). 13C NMR (CDCl3): δ 156.6, 133.0, 128.5, 113.8, 55.2, 22.3, −5.3. HRMS (EI-TOF) m/z: [M]+ calcd for C10H16OGe 226.0407; found 226.0413. Preparation of Germane 20. 19 (0.50 g, 2.2 mmol) was added dropwise over 5 min to a solution of di-μ-chloro-dichlorobis(ethylene)diplatinum (0.010 g, 0.017 mmol) in but-3-en-1-yl acetate (0.39 mL, 3.14 mmol) contained in an ice-water bath. After warming to room temperature, the reaction mixture was stirred for 10 h. The resulting black solution was diluted with 10 mL of 10:1 hexanes:ethyl acetate and filtered through a pad of silica gel. The filtrate was concentrated to give a dark oil that was purified by column chromatography eluting with 5:1 hexanes:ethyl acetate followed by distillation (113−114 °C, 0.05 mmHg) to give 0.247 g (33%) of a clear, colorless oil. 1H NMR (CDCl3): δ 6.91 (d, 2 H, J = 8.4 Hz), 6.78 (d, 2 H, J = 8.4 Hz), 4.05 (t, 2 H, J = 6.8 Hz), 3.78 (s, 3 H), 2.15 (s, 2 H), 2.06 (s, 3 H), 1.62 (quint, 2 H, J = 6.8 Hz), 1.39 (m, 2 H), 0.71 (m, 2 H), 0.07 (s, 6 H). 13C NMR (CDCl3): δ 171.2, 156.4, 133.0, 128.4, 113.6, 64.2, 55.2, 32.0, 23.6, 21.3, 21.0, 14.6, −4.4. HRMS (CITOF) m/z: [M + Na]+ calcd for C16H26O3GeNa 363.0986; found 363.0994. Preparation of Germane 21. 19 (0.50 g, 2.2 mmol) was added dropwise over 5 min to a solution di-μ-chloro-dichlorobis(ethylene)diplatinum (0.016 g, 0.018 mmol) in pent-4-en-1-yl acetate (0.45 mL, 3.16 mmol) contained in an ice-water bath. After warming to room temperature, the reaction mixture was stirred for 10 h. The resulting black solution was diluted with 10 mL of 10:1 hexanes:ethyl acetate and filtered through a pad of silica gel. The filtrate was concentrated to give a colorless oil that was purified by column chromatography eluting with 10:1 hexanes:ethyl acetate followed by distillation (126− 127 °C, 0.03 mmHg) to give 0.281 g (36%) of a clear, colorless oil. 1H NMR (CDCl3): δ 6.91 (d, 2 H, J = 8.4 Hz), 6.78 (d, 2 H, J = 8.4 Hz), 4.05 (t, 2 H, J = 6.8 Hz), 3.78 (s, 3 H), 2.14 (s, 2 H), 2.06 (s, 3 H), 1.62 (quint, 2 H, J = 7.2 Hz), 1.36 (m, 4 H), 0.70 (m, 2 H), 0.06 (s, 6 H). 13C NMR (CDCl3): δ 171.2, 156.4, 133.1, 128.4, 113.6, 64.5, 55.2, 29.4, 28.2, 24.6, 23.6, 21.0, 14.9, −4.4. HRMS (CI-TOF) m/z: [M + Na]+ calcd for C17H28O3GeNa 377.1142; found 377.1143. Preparation of Germane 8. A solution of 20 (0.362 g, 1.07 mmol) in 2 mL of THF was added dropwise over 3 min to a solution of lithium aluminum hydride (0.0482 g, 1.27 mmol) in 2 mL of THF. The reaction was refluxed for 2 h before being cooled in an ice-water bath and diluted with 10 mL of ether. Sequentially, 50 μL of water, 50 μL of 15 wt % sodium hydroxide, and 150 μL of water were added dropwise. After 15 min, 0.1 g of anhydrous MgSO4 was added. After 15 min, the mixture was filtered, and filtrate was concentrated to give a colorless oil that was purified by column chromatography eluting with 2:1 hexanes:ethyl acetate to give 0.192 g (60%) of a clear, colorless oil. 1 H NMR (CDCl3): δ 6.91 (d, 2 H, J = 8.4 Hz), 6.78 (d, 2 H, J = 8.4 Hz), 3.78 (s, 3 H), 3.64 (t, 2 H, J = 6.4 Hz), 2.15 (s, 2 H), 1.57 (quint, 2 H, J = 7.2 Hz), 1.41 (m, 2 H), 1.19 (s, 1 H), 0.72 (m, 2 H), 0.07 (s, 6 H). 13C NMR (CDCl3): δ 156.4, 133.1, 128.4, 113.6, 62.6, 55.2, 36.2, 23.6, 21.1, 14.8, −4.6. HRMS (CI-TOF) m/z: [M + Na]+ calcd for C14H24O2GeNa 321.0880; found 321.0883. Preparation of Germane 9. A solution of 21 (0.245 g, 0.69 mmol) in 2.5 mL of THF was added over 3 min to a solution of lithium aluminum hydride (0.0548 g (1.44 mmol) in 1 mL of THF. The reaction mixture was refluxed for 2 h and then stirred at room temperature for an additional 5 h. The reaction mixture was diluted with 10 mL of ether and then sequentially 60 μL of water, 60 μL of 15 wt % sodium hydroxide, and 180 μL of water were added dropwise. After 15 min, 0.1 g of anhydrous MgSO4 was added. After 15 min, the

0.25 mL of 15 wt % sodium hydroxide, and 0.75 mL of water were added dropwise. Anhydrous MgSO4 was added, and the reaction mixture was warmed to room temperature, filtered, and concentrated under reduced pressure to give a colorless oil. Purification by distillation (94−95 °C, 0.05 mmHg) gave 0.858 g (71%) of a clear, colorless oil. 1H NMR (CDCl3): δ 6.96 (d, 1 H, J = 2.8 Hz), 6.93 (d, 1 H, J = 8.4 Hz), 6.75 (dd, 1 H, J = 8.4, 2.8 Hz), 4.60 (d, 2 H, J = 5.6 Hz), 3.80 (s, 3 H), 2.09 (s, 2 H), 1.47 (t, 1 H, J = 5.6 Hz), 0.001 (s, 9 H). 13C NMR (CDCl3): δ 156.8, 138.2, 130.2, 130.1, 113.2, 113.1, 63.5, 55.3, 21.7, −1.5. HRMS (EI-OF) m/z: [M]+ calcd for C12H20O2Si 224.1227; found 224.1235. Preparation of Germane 7. A solution of 0.590 g (1.99 mmol) of 15 in 5 mL of THF was added over 3 min to a solution lithium aluminum hydride of (0.0792 g, 2.09 mmol) in 3 mL of THF. The resulting mixture was refluxed for 3 h, diluted with 10 mL of ether, and cooled in an ice-water bath. Sequentially, 80 μL of water, 80 μL of 15 wt % sodium hydroxide, and 240 μL of water were added dropwise. After warming to room temperature, 0.10 g of anhydrous MgSO4 was added, and the mixture was stirred for 15 min. The mixture was filtered through a pad of Celite, and the filtrate was concentrated under reduced pressure to give colorless oil that was distilled (79−81°, 0.03 mmHg) to yield 0.305 g (57%) of a pale yellow oil. 1H NMR (CDCl3): δ 6.94 (d, 1 H, J = 2.8 Hz), 6.92 (d, 1 H, J = 8.4 Hz), 6.75 (dd, 1 H, J = 8.4, 2.8 Hz), 4.61 (d, 2 H, J = 5.6 Hz), 3.80 (s, 3 H), 2.23 (s, 2 H), 1.49 (t, 1 H, J = 5.6 Hz), 0.11 (s, 9 H). 13C NMR (CDCl3): δ 156.7, 137.8, 131.1, 129.7, 113.20, 113.04, 63.5, 55.2, 21.3, −2.1. HRMS (CI-TOF) m/z: [M + Na]+ calcd for C12H20O2GeNa 293.0567; found 293.0569. Preparation of Silane 17. A solution of di-μ-chloro-dichlorobis(ethylene)diplatinum (0.010 g, 0.017 mmol) in 1.37 g (14.9 mmol) of but-3-en-1-yl acetate was stirred for 5 min. Then, 2 mL (10.4 mmol) of 16 was added over 7 min. The reaction mixture was stirred for 3 h and then purified by column chromatography eluting with 10:1 hexanes:ethyl acetate to give 2.42 g (79%) of a clear, colorless oil. 1 H NMR (CDCl3): δ 6.91 (d, 2 H, J = 8.8 Hz), 6.78 (d, 2 H, J = 8.8 Hz), 4.05 (t, 2 H, J = 6.4 Hz), 3.78 (s, 3 H), 2.06 (s, 3 H), 2.01 (s, 2 H), 1.63 (quint, 2 H, J = 6.8 Hz), 1.35 (m, 2 H), 0.51 (m, 2 H), −0.04 (s, 6 H). 13C NMR (CDCl3): δ 171.3, 156.5, 132.1, 128.8, 113.7, 64.3, 55.2, 32.3, 24.2, 21.0, 20.2, 14.3, −3.7. HRMS (CI-TOF) m/z: [M + Na]+ calcd for C16H26O3SiNa 317.1543; found 317.1539. Preparation of Silane 6. A solution of 17 (1.5 g, 5.1 mmol) in 5 mL of THF was added over 5 min to a solution of lithium aluminum hydride (0.210 g, 5.5 mmol) in 5 mL of THF. After 4 h, the resulting solution was added slowly to 60 mL of a saturated potassium sodium tartrate solution. After being stirred for 10 h, the solution was extracted with 3 × 25 mL ether, washed with brine, dried over anhydrous MgSO4, filtered, and concentrated to give a pale yellow oil. Purification by column chromatography eluting with 2:1 hexanes:ethyl acetate containing 0.5% triethylamine followed by distillation at reduced pressure (103−104 °C at 0.03 mmHg) gave 1.07 g (83%) of a clear, colorless oil. 1H NMR (CDCl3): δ 6.92 (d, 2 H, J = 8.8 Hz), 6.79 (d, 2 H, J = 8.8 Hz), 3.78 (s, 3 H), 3.65 (q, 2 H, J = 6.4 Hz), 2.02 (s, 2 H), 1.59 (quint, 2 H, J = 6.8 Hz), 1.37 (m, 2 H), 1.19 (t, 1 H, J = 5.6 Hz), 0.52 (m, 2 H), −0.34 (s, 6 H). 13C NMR (CDCl3): δ 156.5, 132.2, 128.8, 113.7, 62.7, 55.2, 36.6, 24.2, 20.0, 14.5, −3.6. HRMS (CI-TOF) m/z: [M + Na]+ calcd for C14H24O2SiNa 275.1438; found 275.1442. Preparation of Germane 18. A solution of 1,2-dichlorotetramethyldigermane (3.32 g, 12.02 mmol) and 4-methoxybenzyl chloride (1.81 g, 11.56 mmol) in 60 mL of benzene was added dropwise over 10 min to a solution of tetrakis(triphenylphosphine)palladium (0.0565 g, 0.049 mmol) in 10 mL of benzene. After refluxing for 12 h, the reaction mixture was diluted with 10 mL of ethyl acetate and filtered through a pad of silica gel. The filtrate was concentrated to yield a dark yellow oil that was purified by distillation (79−80 °C, 0.03 mmHg) to yield 2.47 g (82%) of a colorless oil that solidified upon standing to a white solid. 1H NMR (CDCl3): δ 7.02 (d, 2 H, J = 8.8 Hz), 6.82 (d, 2 H, J = 8.8 Hz), 3.79 (s, 3 H), 2.62 (s, 2 H), 0.63 (s, 6 H). 13C NMR (CDCl3): δ 157.4, 129.0, 128.8, 114.1, 55.2, 29.2, 2.5. Anal. Calcd for C10H15ClGeO: C, 46.32; H, 5.83. Found: C, 46.25; H, 5.83. 12117

DOI: 10.1021/acs.joc.7b01892 J. Org. Chem. 2017, 82, 12112−12118

Article

The Journal of Organic Chemistry mixture was filtered, and filtrate was concentrated to give a colorless oil that was purified by column chromatography eluting with 2:1 hexanes:ethyl acetate to give 0.162 g (75%) of a clear, colorless oil. 1 H NMR (CDCl3): δ 6.91 (d, 2 H, J = 8.8 Hz), 6.78 (d, 2 H, J = 8.8 Hz), 3.78 (s, 3 H), 3.63 (t, 2 H, J = 6.4 Hz), 2.15 (s, 2 H), 1.56 (m, 2 H), 1.36 (m, 4 H), 1.27 (s, 1 H), 0.71 (m, 2 H), 0.07 (s, 6 H). 13C NMR (CDCl3): δ 156.4, 133.4, 128.4, 113.6, 62.9, 55.2, 32.4, 29.3, 24.7, 23.7, 15.0, −4.3. HRMS (CI-TOF) m/z: [M + Na]+ calcd for C15H26O2GeNa 355.1037; found 335.1046. Nanosecond Transient Absorption Spectroscopy. A XeCl excimer laser (308 nm) was used to pump a dye laser containing pterphenyl laser dye for 343 nm excitation. Transient spectral absorptions were monitored at a right angle to the laser excitation by using a home-built, xenon flashlamp system equipped with a PerkinElmer FX-193 flashlamp to generate the analyzing light. The analyzing light was focused into the end of a fiber optic cable and onto the entrance slit of a monochromator equipped with an intensified CCD. A pulsed xenon arc lamp was used as the monitoring light source for kinetics experiments. The monitoring light was passed through a monochromator and detected using a photomultiplier tube. The signal from the PMT was directed into a digitizing oscilloscope and then to a computer for viewing, storage, and data analysis. General Procedure for the Generation and Observation of Cation Radicals by Nanosecond Transient Absorption Spectroscopy. Transient absorption experiments were typically performed in quartz, stopcocked cuvettes containing solutions with ∼1 mM NMQ+ PF6− (OD ∼ 1 at 343 nm), 0.4 M toluene, and ∼0.01 M silane or germane in dioxygen-saturated 1,2-dichloroethane, dichloromethane, 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), or HFIP:dichloromethane. Transient spectra were generally recorded at least 100 ns after the laser pulse to avoid interference from the N-methylquinolyl radical.17 For this reason, fitting of transient decays or growths for kinetic experiments were done only after ∼100 ns.

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Lijser, H. J. P.; Snelgrove, D. W.; Dinnocenzo, J. P. J. Am. Chem. Soc. 2001, 123, 9698. (2) (a) Zhang, X.; Yeh, S.-R.; Hong, S.; Freccero, M.; Albini, A.; Falvey, D. F.; Mariano, P. S. J. Am. Chem. Soc. 1994, 116, 4211. (b) Gould, I. R.; Godleski, S. A.; Zielinski, P. A.; Farid, S. Can. J. Chem. 2003, 81, 777. (3) Schmittel, M.; Kelley, M.; Burghart, A. J. Chem. Soc., Perkin Trans. 2 1995, 2327. (4) Guirado, G.; Haze, O.; Dinnocenzo, J. P. J. Org. Chem. 2010, 75, 3326. (5) Rijksen, B.; van Lagen, B.; Zuilhof, H. J. Am. Chem. Soc. 2011, 133, 4998. (6) (a) Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 5593. (b) Kyushin, S.; Masuda, Y.; Matsushita, K.; Nakadaira, Y.; Ohashi, M. Tetrahedron Lett. 1990, 31, 6395. (c) Nakanishi, K.; Mizuno, K.; Otsuji, Y. Bull. Chem. Soc. Jpn. 1993, 66, 2371. (d) Lenczewski, M. S. Mechanistic Studies on Benzylsilane and Benzylgermane Cation Radicals. Ph.D. Thesis, University of Rochester, Rochester, NY, 2012. (7) (a) Gardner, H. C.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 2460. (b) Eaton, D. F. J. Am. Chem. Soc. 1981, 103, 7235. (c) Fukuzumi, S.; Kuroda, S.; Tanaka, T. J. Chem. Soc., Chem. Commun. 1986, 1553. (d) Takuwa, A.; Tagawa, H.; Iwamoto, H.; Soga, O.; Maruyama, K. Chem. Lett. 1987, 16, 1091. (e) Kyushin, S.; Nakadaira, Y.; Ohashi, M. Chem. Lett. 1990, 19, 2191. (f) Lochynski, S.; Boduszek, B.; Shine, H. J. J. Org. Chem. 1991, 56, 914. (g) Yoshida, J.; Ishichi, Y.; Isoe, S. J. Am. Chem. Soc. 1992, 114, 7594. (h) Butcher, E.; Rhodes, C. J.; Standing, M.; Davidson, R. S.; Bowser, R. J. Chem. Soc., Perkin Trans. 2 1992, 1469. (i) Kyushin, S.; Otani, S.; Nakadaira, Y.; Ohashi, M. Chem. Lett. 1995, 24, 29. (j) Yoshida, J.; Izawa, M. J. Am. Chem. Soc. 1997, 119, 9361. (k) Fukuzumi, S.; Yasui, K.; Itoh, S. Chem. Lett. 1997, 26, 161. (8) Luo, P.; Dinnocenzo, J. P. J. Org. Chem. 2015, 80, 9240. (9) For reviews, see: (a) Beak, P. Acc. Chem. Res. 1992, 25, 215. (b) Beak, P. Pure Appl. Chem. 1993, 65, 611. (10) Eberson, L.; Hartshorn, M. P.; Persson, O. J. Chem. Soc., Perkin Trans. 2 1995, 1735. (11) (a) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. (b) Alaimo, P. J.; Peters, D. W.; Arnold, J.; Bergman, R. G. J. Chem. Educ. 2001, 78, 64. (12) Lambert, J. B.; Singer, R. A. J. Am. Chem. Soc. 1992, 114, 10246. (13) Stará, I. G.; Stary, I.; Kollárovic, A.; Teply, F.; Fiedler, P. Tetrahedron 1998, 54, 11209. (14) Boebel, T. A.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 7534. (15) Barrau, J.; Rima, G.; El Amine, M.; Satge, J. Synth. React. Inorg. Met.-Org. Chem. 1988, 18, 21. (16) Love, B. E.; Jones, E. G. J. Org. Chem. 1999, 64, 3755. (17) Guirado, G.; Fleming, C. N.; Lingenfelter, T. G.; Williams, M. L.; Zuilfhof, H.; Dinnocenzo, J. P. J. Am. Chem. Soc. 2004, 126, 14086.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b01892. NMR spectra (1H and 13C), representative kinetic plots, and tables of rate constants for bimolecular rate constants (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mary S. Lenczewski: 0000-0001-6220-1068 Joseph P. Dinnocenzo: 0000-0003-0206-3497 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The NSF (Grant CHE-1464629) is gratefully acknowledged for financial support. REFERENCES

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DOI: 10.1021/acs.joc.7b01892 J. Org. Chem. 2017, 82, 12112−12118