Synthesis of Nitrosobenzene Derivatives via Nitrosodesilylation

Article Cite This: J. Org. Chem. 2018, 83, 3915−3920

pubs.acs.org/joc

Synthesis of Nitrosobenzene Derivatives via Nitrosodesilylation Reaction Corinna Kohlmeyer,†,‡ Maike Klüppel,‡ and Gerhard Hilt*,† †

Institut für Chemie, Universität Oldenburg, Carl-von-Ossietzky-Straße 9-11, 26111 Oldenburg, Germany Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, 35043 Marburg, Germany

‡

S Supporting Information *

ABSTRACT: The electrophilic ipso-substitution of trimethylsilylsubstituted benzene derivatives into nitrosobenzene derivatives is reported. The optimization of the reaction conditions was performed for moderately electron-deficient, electron-rich, and sterically hindered starting materials by varying reaction time, temperature, and equivalents of NOBF4. Also, a stable intermediate of the nitrosation reaction could be characterized by 19F NMR which can be assigned to a NO+ adduct with the nitrosobenzene derivative. This complex decomposes upon aqueous workup and liberates the desired nitrosobenzene derivative.

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Scheme 1. Nitrosoarene Synthesis via ipso-Substitution Reaction

INTRODUCTION Aromatic nitroso compounds are a class of compounds of high importance in organic synthesis as they are applied in various transformations, such as Diels−Alder reactions,1 nitroso-ene reactions,2 nitroso-aldol reactions,3 allylborations,4 or the addition of Grignard reagents.5 Recently, several asymmetric6 and catalytic7 methods have been examined. Therefore, the development of novel synthetic approaches for nitrosoarenes is still an important field in organic chemistry.8 Their synthesis is most often accomplished by oxidation of the corresponding aniline derivative using peracids,9 oxone,10 or H2O2 as oxidant and various metal peroxo complexes as catalyst.11 Alternatively, nitrosoarenes can be synthesized by direct nitrosation using suitable nitrosating agents for in situ formation12 or a nitrosonium salt,13 such as NOBF4, as electrophile for electrophilic substitution. However, this method is limited to electron-rich arenes such as anisole derivatives. In a seminal work, Molander extended this synthetic strategy to a broad range of arenes by introducing potassium trifluoroborate as the leaving group, which allowed the electrophile to attack in the ipso-position, forming a nitrosoarene with a defined regioselectivity (Scheme 1).14 Using a slight excess of NOBF4, the reaction conditions are very mild and compatible with various functional groups. In addition to potassium trifluoroborate, a limited number of transformations with a trimethylsilyl group is known to act as the directing group to introduce a nitroso group.15 Based on this, we were interested in reacting trimethylsilyl-substituted arenes with NOBF4 to synthesize nitrosoarenes. Similar to the aryl trifluoroborates, trimethylsilylsubstituted arenes are cost-efficient and convenient to synthesize, as they are readily accessible from the corresponding aryl bromide.16 An alternative synthetic approach to trimethylsilyl-substituted arenes is the cobalt-catalyzed Diels− Alder reaction that has been on our scientific agenda for a long time.17 Recently, we reported a related reaction to the proposed synthesis of nitrosoarenesa trimethylsilyl iodine exchange reaction that demonstrated the suitability of the trimethylsilyl © 2018 American Chemical Society

group as a directing group in electrophilic substitution reactions. We were able to develop a protocol for the chemical18 as well as for the electrochemical19 version of these iodination reactions. This prompted us to examine a nitrosodesilylation reaction using NOBF4 as an electrophile.

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RESULTS AND DISCUSSION For the investigation of suitable conditions for a nitrosodesilylation reaction, we chose 4-methoxyphenyltrimethylsilane (1a) as the screening substrate as it is readily available and the corresponding product is well described in the literature.20 As the nitrosonium source, we chose NOBF4 because it is commercially available and easy to handle.21 Promisingly, in a first test reaction (Scheme 2), quantitative conversion of the Received: January 29, 2018 Published: February 27, 2018 3915

DOI: 10.1021/acs.joc.8b00262 J. Org. Chem. 2018, 83, 3915−3920

Article

The Journal of Organic Chemistry Scheme 2. Nitrosation of 1 by ipso-Substitution with Nitrosonium Tetrafluoroborate

significantly improved by lowering the temperature and simultaneously decreasing the amount of the electrophile. The best yield was obtained at −40 °C using 1.3 equiv of NOBF4 (entry 11). With those optimized conditions for the nitrosodesilylation reaction in hand, we investigated a small number of other trimethylsilyl-substituted benzene derivatives of type 1 to evaluate the scope of the reaction (Scheme 3).

starting material could be achieved, resulting in the formation of a mixture of 4-methoxynitrosobenzene (2a, 68%) and 4methoxynitrobenzene (3a, 32%), verified by 1H NMR and GCMS analysis of the reaction mixture. The occurrence of the nitrobenzene derivative can be explained by traces of oxygen as was described by Kochi.22 In order to improve the yields and decrease the nitroarene byproduct, we examined the nitrosodesilylation reaction in more detail by modifying the reaction temperature, the reaction time, and the amount of NOBF4. The results of these optimizations are summarized in Table 1.

Scheme 3. Application of the Reaction Conditions to Other Starting Materials

a

10% starting material 1d remaining. The product also contains 5% 1fluoro-4-nitrobenzene 3d. The ratio was determined by 1H NMR analysis.

Table 1. Optimization of Nitrosodesilylation Reaction of 1a to 2aa entry 1 2 3 4 5 6 7 8 9 10 11 12b 13c 14 15

solvent CH2Cl2 (CH2Cl)2 EtOAc THF CH3CN CH3CN/CH3OH (5:1) CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN

NOBF4 (equiv)

temp (°C)

2a/3a/1a ratio (byproduct)

2.2 2.2 2.2 2.2 2.2 2.2

20 20 20 20 20 20

42:7:0 (51) 41:8:36 (15) 0:0:3 (97) 66:10:10 (14) 87:13:0 (0) 10:0:90 (0)

2.2 2.2 2.0 1.5 1.3 1.3 1.3 1.1 1.0

20 −40 −40 −40 −40 −40 −40 −40 −40

Using the optimized conditions, product 2a could be isolated in a very good yield of 91%. Whereas the yield for the derivative 2b was still acceptable (59%), rather disappointing results were obtained for the electron-rich 2-methoxy-substituted derivative 2c. Accordingly, the reaction conditions proved to be unsuitable for ortho-substituted arenes. This finding was supported by 1H NMR analysis of the crude product for the synthesis of 2c, which indicated that the reaction generated a number of undesired byproducts. Also, this substrate gave no complete conversion, likely because of its steric hindrance. Therefore, we conducted a second optimization using (2methoxyphenyl)trimethylsilane 1c as starting material to identify the critical aspects for the nitrosodesilylation reaction with this substrate. The results for the nitrosodesilylation reaction of 1c to 2c are summarized in Table 2.

93:7:0 (0) 93:2:0 (5) 95:5:0 (0) 95:2:0 (3) 98:2:0 (0) 87:4:2 (7) 97:1:2 (0) 97:1:2 (0) 92:1:7 (0)

Table 2. Optimization of the Nitrosodesilylation Reaction of 1c to 2ca,b

a

Reaction conditions: all reactions were performed on a 0.50 mmol scale, and the reaction time was 30 min. The ratio of the products was determined by 1H NMR analysis. b0.05 M instead of 0.03 M. c0.01 M instead of 0.03 M.

The use of CH2Cl2, which is the preferred solvent for the cobalt-catalyzed Diels−Alder reaction,17 led to a mixture of the nitrosoarene 2, the nitroarene 3, and another uncharacterized byproduct (entry 1). When switched to a coordinating solvent, such as EtOAc or THF, the yield of the nitrosoarene 2 was improved, but the conversion of the starting material 1 was incomplete (entries 3 and 4). Complete conversion could be achieved by using acetonitrile; the product could be isolated in a good yield (87%), and only a minor quantity of the nitroarene (13%) was formed. Furthermore, formation of the unknown byproduct could be suppressed completely (entry 5). Next, we attempted to increase the selective nitrosation by adding methanol in order to improve the ability of the trimethylsilyl group acting as a leaving group.23 However, the reaction was strongly inhibited by this additive (entry 6). Thus, acetonitrile proved to be the solvent of choice for this transformation. In a second step, we optimized the temperature and the applied NOBF4 equivalents. The outcome of the reaction was quite sensitive toward those parameters, and the yield could be

entry

NOBF4 (equiv)

temp (°C)

time

1c/2c/byproduct ratio (yield in %)

1 2 3 4 5

1.3 2.0 2.0 2.0 2.0

20 −40 30 20 20

2h 10 min 10 min 10 min 5 min

50:26:24 (nd) 0:85:15 (52%) 0:71:29 (nd) 0:90:10 (58%) 0:96:4 (50%)

a

Reaction conditions: all reactions were performed on a 0.50 mmol scale. The ratio of the products was determined by 1H NMR analysis. b Acetonitrile/methanol = 10:1.

To avoid incomplete conversion, the temperature was increased again to ambient temperature. Nevertheless, conversion of the starting material utilizing 1.3 equiv of NOBF4 was still incomplete after 2 h reaction time (entry 1). Increasing the amount of nitrosonium ions and increasing the reaction temperature to 30 °C gave complete conversion, but at the same time, the amount of the undesired byproducts also 3916


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good yields with the exception of the 4-cyano-substituted derivative 1l. Mechanistically, the described nitrosodesilylation reaction proceeds via an electrophilic aromatic substitution with NO+ acting as the electrophile. 19F NMR spectroscopy was used to follow the course of the reaction subjecting (4-fluorophenyl)trimethylsilane (1d) to the reaction conditions shown in Scheme 3. Monitoring the chemical shift of the aromatic fluorine atom, it becomes apparent that the nitrosoarene 2d was not formed until the aqueous workup was performed (see Figures S1−S8 in the Supporting Information). This could be explained by the formation of nitroso-substituted arenes which form a complex with additional NOBF4 as described by Kochi.13a,24 Also, the additional high field signal at δ(19F) = −158.0 ppm reveals characteristic 29Si satellites, which indicates the formation of Me3SiF. This assumption was confirmed by a 29 Si{H} INEPT NMR spectrum showing the chemical shift of δ(29Si) = 33.6 ppm and a coupling constant of 1J(Si−F) = 272.6 Hz (see Figures S1−S6 in the Supporting Information).25 These results suggest that the trimethylsilyl group abstracted a fluoride anion from BF4− under formation of Me3SiF and BF3. The proposed mechanism of the nitrosodesilylation is shown in Scheme 5.

increased (entries 2−4). The best conditions were found to be 5 min reaction time and 2.0 equiv of NOBF4 at ambient temperature, giving the highest product/byproduct ratio of 96:4 and full conversion of the starting material (entry 5). On the other hand, the moderate yield of 52% can be attributed to purification issues as the product is not trivial to isolate by column chromatography. Nevertheless, a number of substrates were tested again in the nitrosodesilylation reaction with the optimized reaction conditions for less reactive substrates, and the results are summarized in Scheme 4. Scheme 4. Scope of the Nitrosodesilylation Reaction

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CONCLUSION In summary, we realized a new approach for the synthesis of nitrosoarenes by nitrosodesilylation reaction for a number of different starting materials with good success. The reaction conditions had to be adjusted to the electronic and steric conditions present in the starting materials. Accordingly, the trimethylsilyl-substituted alkynes that we initially used in our cobalt-catalyzed Diels−Alder reactions can be seen as a synthon for a nitrosyl-substituted alkyne. Finally, we were able to monitor the ipso-nitrosation reaction by 19F NMR analysis and characterized the proposed intermediate 4 that is formed with additional NO+ ions. The intermediate 4 was converted into the desired nitrosoarene derivatives during the aqueous workup.

a

The yield was determined by 1H NMR analysis using CH2Br2 as the internal standard. bThe ratio of nitrosoarene/protodesilylation byproduct is 81:19 and was determined by 1H NMR analysis.

Various electron-rich trimethylsilyl-substituted substrates were suitable for the explored reaction. The best yield could be achieved with methyl- and methoxy-substituted derivatives (52−68% yield). Benzo[d][1,3]dioxol-5-yltrimethylsilane 1k afforded the corresponding nitrosoarene 2k in 73% yield. In contrast, we discovered that the reaction of [1,1′-biphenyl]-4-yltrimethylsilane 1i to afford product 2i led to the formation of significant quantities of the protodesilylated byproduct. Applying electrondeficient derivatives to the nitrosodesilylation reaction led to

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EXPERIMENTAL SECTION

General Information. All reactions with air- or water-sensitive starting materials were carried out under argon atmosphere in heatgun-dried glassware using Schlenk techniques. THF was refluxed under nitrogen atmosphere and freshly distilled from sodium/ benzophenone and stored under nitrogen atmosphere. Dichloromethane and acetonitrile were stored over molecular sieves under a nitrogen atmosphere. Co(dppp)Br2 was prepared following literature

Scheme 5. Proposed Mechanism of the Nitrosodesilylation

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The Journal of Organic Chemistry procedures.26 ZnI2 was dried in vacuo at 220 °C and stored in a Schlenk tube under an argon atmosphere. Commercially available materials were used without further purification. The trimethylsilylsubstituted arenes 1b, 1c, 1e, 1g, 1k, and 1l were prepared with n-BuLi and 1a and 1d by using Grignard reagents according to a known procedure.16 For thin layer chromatography, Merck TLC plates (Silica 60, F254 with fluorescence indicator) were used. Column chromatography was carried out on Macherey-Nagel silica gel 60 (230−400 mesh). 1H and 13 C{1H} NMR spectra were recorded on either a Bruker AV II 300 or AV III HD 300 (1H NMR at 300 MHz, 13C{1H} NMR at 75 MHz) or AV III 500 (1H NMR at 500 MHz, 13C{1H} NMR at 125 MHz, 19F NMR at 470 MHz, 29Si{1H} INEPT NMR at 99 MHz). The 29Si{H} INEPT NMR spectra were recorded with delay times D3 = 0.0068 s and D4 = 0.0313 s. Chemical shifts are reported in parts per million (ppm) and are referenced to the solvent peak of CDCl3 (δ = 7.26 ppm for 1H NMR and δ = 77.16 ppm for 13C{1H} NMR) or CD3CN (δ = 1.94 ppm for 1H NMR) as internal standard. C6F6 (δ(19F) = −164.9 ppm) was used as internal standard for 19F NMR analysis. GC-MS spectra were measured on an Agilent 6890 GC system including a Hewlett-Packard 5973 mass selective detector. A JEOL AccuTof GCv spectrometer with an energy of 70 eV was used for for EI-HRMS, and for ESI-HRMS, a LTQ-FT Ultra mass spectrometer (Thermo Fischer Scientific) was used. IR spectra were recorded on a Bruker IFS 88 (FTIR) spectrometer. General Procedure for the Synthesis of TrimethylsilylSubstituted Arenes by Using Cobalt-Catalyzed Diels−Alder Reaction.17b Under argon atmosphere, Co(dppp)Br2 (0.20 or 0.50 mmol, 10 mol %), zinc powder (0.40 or 1.00 mmol, 20 mol %), and anhydrous zinc iodide (0.40 or 1.00 mmol, 20 mol %) were suspended in anhydrous dichloromethane (2.0 or 5.0 mL, 1.0 M), and the mixture was stirred for 10 min at ambient temperature. Then 2,3-dimethyl-1,3butadiene (2.20 or 5.50 mmol, 1.1 equiv) and the trimethylsilylsubstituted alkyne (2.00 or 5.00 mmol, 1.0 equiv) were added, and the reaction mixture was stirred at ambient temperature. After complete conversion of the alkyne (monitored by GC-MS analysis), the reaction mixture was filtered through a small plug of silica gel, and the solvent was removed under reduced pressure. Without further purification, the obtained product was dissolved in dichloromethane (0.2 M) and cooled to 0 °C. In three portions, 2,3-dichloro-5,6-dicyano-1,4benzoquinone (2.60 or 6.50 mmol, 1.3 equiv) was added, and the mixture was stirred at ambient temperature for 30 min. Then the reaction mixture was filtered through a small plug of deactivated silica gel (n-pentane), and the solvent was removed under reduced pressure, yielding the pure trimethylsilyl-substituted arene. Trimethyl(2,4,5-trimethylphenyl)silane (1e): colorless oil; 82% (331 mg, 1.75 mmol); eluent, n-pentane; 1H NMR (300 MHz, CDCl3, ppm) δ = 7.22 (s, 1H), 6.97 (s, 1H), 2.41 (s, 3H), 2.24 (s, 6H), 0.32 (s, 9H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 141.1, 137.7, 135.9, 135.4, 132.9, 131.6, 22.5, 19.6, 19.3, 0.1; MS (EI) m/z (%) 192 (19, [M+]), 177 (100), 161 (8), 149 (9), 133 (8), 119 (5), 105 (8), 91 (8), 73 (8); HRMS (EI, m/z) calcd for C12H20Si 192.1334, found 192.1316; IR (film, cm−1) 2997, 2956, 1604, 1449, 1248, 1164, 1015, 978, 832, 754, 687, 639, 470, 442. (3,4-Dimethylphenyl)trimethylsilane (1i): pale yellow oil; 84% (750 mg, 4.22 mmol); eluent, n-pentane; 1H NMR (300 MHz, CDCl3, ppm) δ = 7.27 (d, J = 8.5 Hz, 2H), 7.14 (d, J = 7.3 Hz, 1H), 2.28 (s, 3H), 2.27 (s, 3H), 0.25 (s, 9H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 137.6, 137.5, 136.0, 134.8, 131.1, 129.3, 19.9 (2C), −0.9. The analytic data are in accordance with the literature.27 Preparation of (1,1′-Biphenyl)-4-yltrimethylsilane (1h). According to a procedure of Hilt,28 under argon atmosphere, phenylboronic acid (730 mg, 6.00 mmol, 1.2 equiv), (4-bromophenyl)trimethylsilane 1f (1.14 g, 5.00 mmol, 1.0 equiv), K3PO4·6H2O (6.91 g, 30.0 mmol, 6.0 equiv), and Pd(PPh3)4 (170 mg, 0.15 mmol, 3 mol %) were dissolved in a mixture of degassed THF and water (10:1, 55 mL, 0.09 M). The reaction mixture was heated to 60 °C and stirred overnight. After complete conversion of the aryl bromide (monitored by GC-MS analysis), the mixture was filtered through a small plug of silica gel (npentane). The solvent was removed under reduced pressure. The

product was obtained in 85% yield (970 mg, 4.30 mmol) as a red oil: 1 H NMR (300 MHz, CDCl3, ppm) δ = 7.62−7.60 (m, 6H), 7.47−7.42 (m, 2H), 7.38−7.33 (m, 1H), 0.31 (s, 9H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 141.8, 141.3, 139.4, 133.9, 128.9, 127.5, 127.3, 126.6, −0.9. The analytic data are in accordance with the literature.29 General Procedure for Nitrosodesilylation Reaction. Procedure A. A round-bottom flask was charged with acetonitrile (8.3 or 16.7 mL, 0.03 M), and trimethylsilyl-substituted arene (0.25 or 0.50 mmol, 1.0 equiv) and cooled to −40 °C. After NOBF4 (0.33 or 0.65 mmol, 1.3 equiv) was added, the reaction mixture was stirred for 30 min. Then, the mixture was poured into a 1:1 mixture of water/ dichloromethane. The phases were separated; the aqueous layer was extracted with dichloromethane, and the combined organic layers were dried (MgSO4), filtered, and concentrated under reduced pressure. If necessary, the crude product was purified by column chromatography or by filtering through a small plug of silica gel. The yield was either determined gravimetrically or by adding CH2Br2 as an internal standard for 1H NMR analysis. Procedure B. A round-bottom flask was charged with acetonitrile (8.3 or 16.7 mL, 0.03 M), and trimethylsilyl-substituted arene (0.25 or 0.50 mmol, 1.0 equiv) and NOBF4 (0.50 or 1.00 mmol, 2.0 equiv) were added. After being stirred at ambient temperature for 30 min, the reaction mixture was poured into a 1:1 mixture of water/dichloromethane. The phases were separated; the aqueous layer was extracted with dichloromethane, and the combined organic layers were dried (MgSO4), filtered, and concentrated under reduced pressure. If necessary, the crude product was purified by column chromatography or by filtering through a small plug of silica gel. The yield was determined either gravimetrically or by adding CH2Br2 as an internal standard for 1H NMR analysis. 1-Methoxy-4-nitrosobenzene (2a). (4-Methoxyphenyl)trimethylsilane (90.2 mg, 0.50 mmol, 1.0 equiv) and NOBF4 (75.9 mg, 0.65 mmol, 1.3 equiv) were reacted according to procedure A. The crude product was purified by column chromatography on silica gel (npentane/diethyl ether = 25:1) to afford 91% yield (62.3 mg, 0.45 mmol) of 2a as a green solid: 1H NMR (300 MHz, CDCl3, ppm) δ = 7.91 (d, J = 8.5 Hz, 2H), 7.07−6.97 (m, 2H), 3.94 (s, 3H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 165.8, 164.2, 124.6, 114.0, 56.1; HRMS (EI) m/z calcd for C7H7NO2 ([M+]) 137.0477, found 137.0487. The analytic data are in accordance with the literature.14,30 1,2-Dimethoxy-4-nitrosobenzene (2b). (3,4-Dimethoxyphenyl)trimethylsilane (105 mg, 0.50 mmol, 1.0 equiv) and NOBF4 (75.9 mg, 0.65 mmol, 1.3 equiv) were reacted according to procedure A. The crude product was purified by column chromatography on silica gel (npentane/diethyl ether = 25:1 → 5:1) to afford 59% yield (49.5 mg, 0.27 mmol) of 2b as a green oil: 1H NMR (300 MHz, CDCl3, ppm) δ = 8.49 (dd, J = 8.5, 2.2 Hz, 1H), 7.14 (d, J = 8.5 Hz, 1H), 6.54 (d, J = 2.2 Hz, 1H), 4.02 (s, 3H), 3.86 (s, 3H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 163.6, 156.1, 149.8, 128.7, 110.1, 94.5, 56.6, 56.6. The analytic data are in accordance with the literature.11b 1-Methoxy-2-nitrosobenzene (2c). (2-Methoxyphenyl)trimethylsilane 1c (45.1 mg, 0.25 mmol, 1.0 equiv) and NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. The crude product was purified by column chromatography on silica gel (n-pentane/ethyl acetate = 10:1) to afford 52% yield (49.5 mg, 0.27 mmol) of 2c as a brown oil: 1H NMR (300 MHz, CDCl3, ppm) δ = 7.67 (ddd, J = 8.7, 7.1, 1.7 Hz, 1H), 7.35 (d, J = 8.5 Hz, 1H), 6.84 (ddd, J = 8.1, 7.1, 1.1 Hz, 1H), 6.29 (dd, J = 8.1, 1.7 Hz, 1H), 4.27 (s, 3H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 161.9, 158.2, 138.5, 119.5, 114.7, 108.6, 56.9. The analytic data are in accordance with the literature.11b 1-Fluoro-4-nitrosobenzene (2d). (4-Fluorophenyl)trimethylsilane 1d (126.2.1 mg, 0.75 mmol, 1.0 equiv) and NOBF4 (113.3 mg, 0.98 mmol, 1.3 equiv) were reacted according to procedure A. Without further purification, product 2d was obtained in 79% yield (74.0 mg, 0.59 mmol) as a brown oil: 1H NMR (300 MHz, CDCl3, ppm) δ = 8.00−7.88 (m, 2H), 7.32−7.23 (m, 2H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 163.8 (d, J = , 124.1 (d, J = 9.6 Hz), 116.5 (d, J = 23.1 Hz). The fluoro-substituted carbon signal could not be resolved. The analytic data are in accordance with the literature.31 3918


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1-Methoxy-3-nitrosobenzene (2e). (3-Methoxyphenyl)trimethylsilane 1e (45.1 mg, 0.25 mmol, 1.0 equiv) and NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. The crude product was purified by filtration through a small plug of silica gel (n-pentane/ethyl acetate = 10:1) to afford 57% yield (25.7 mg, 0.14 mmol) of 2e as a brown oil: 1H NMR (300 MHz, CDCl3, ppm) δ = 8.03 (dt, J = 7.5, 1.3 Hz, 1H), 7.61 (dd, J = 8.5, 7.4 Hz, 1H), 7.29 (ddd, J = 8.4, 2.7, 1.0 Hz, 1H), 6.91 (dd, J = 2.7, 1.7 Hz, 1H), 3.87 (s, 3H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 167.0, 160.6, 130.5, 122.9, 119.6, 100.0, 55.8. The analytic data are in accordance with the literature.30 1,2,4-Trimethyl-5-nitrosobenzene (2f). Trimethyl(2,4,5trimethylphenyl)silane 1f (37.3 mg, 0.25 mmol, 1.0 equiv) and NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. Then CH2Br2 was added to the crude product to determine the yield of 56%: 1H NMR (300 MHz, CDCl3, ppm) δ = 7.27 (s, 1H), 6.15 (s, 1H), 3.23 (s, 3H), 2.28 (s, 3H), 2.20 (s, 3H); 13 C{1H} NMR (75 MHz, CDCl3, ppm) δ = 165.2, 147.0, 140.3, 134.3, 133.7, 108.8, 20.5, 19.0, 16.8. The analytic data are in accordance with the literature.13a 1-Bromo-4-nitrosobenzene (2g). (4-Bromophenyl)trimethylsilane 1g (57.3 mg, 0.25 mmol, 1.0 equiv) and NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. The crude product was purified by filtration through a small plug of silica gel (n-pentane/ ethyl acetate = 10:1) to afford 71% yield (32.8 mg, 0.18 mmol) of 2g as a green oil: 1H NMR (300 MHz, CDCl3, ppm) δ = 7.78 (s, 4H); 13 C{1H} NMR (75 MHz, CDCl3, ppm) δ = 164.0, 132.9, 131.8, 122.3. The analytic data are in accordance with the literature.11b 1-Methyl-4-nitrosobenzene (2h). (4-Methylphenyl)trimethylsilane 1h (41.1 mg, 0.25 mmol, 1.0 equiv) and NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. Then CH2Br2 was added to the crude product to determine the yield of 68%: 1H NMR (300 MHz, CDCl3, ppm) δ = 7.80 (d, J = 8.0 Hz, 2H), 7.38 (d, J = 8.0 Hz, 2H), 2.44 (s, 3H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 165.7, 147.3, 129.9, 121.4, 22.1. The analytic data are in accordance with the literature.11b,33 4-Nitroso-1,1′-biphenyl (2i). (1,1′-Biphenyl)-4-yltrimethylsilane 1i (56.6 mg, 0.25 mmol, 1.0 equiv) and NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. Then CH2Br2 was added to the crude product to determine the yield of 81%. The mixture contained 15% of protodesilylation byproduct.32 The product was obtained as an orange solid: 1H NMR (300 MHz, CDCl3, ppm) δ = 7.98 (d, J = 8.7 Hz, 2H), 7.83 (d, J = 8.7, 2H), 7.70−7.64 (m, 2H), 7.55−7.42 (m, 3H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 165.1, 148.2, 139.3, 129.3, 129.1, 128.0, 127.6, 121.8. The analytic data are in accordance with the literature.14 1,2-Dimethyl-4-nitrosobenzene (2j). (3,4-Dimethylphenyl)trimethylsilane 1j (44.6 mg, 0.25 mmol, 1.0 equiv), NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. Then CH2Br2 was added to the crude product to determine the yield of 59%: 1H NMR (300 MHz, CDCl3, ppm) δ = 7.71 (dd, J = 8.0, 2.0 Hz, 1H), 7.60 (s, 1H), 7.35 (d, J = 8.0 Hz, 1H), 2.39 (s, 3H), 2.34 (s, 3H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 166.3, 146.1, 137.8, 130.4, 121.8, 119.7, 20.5, 18.9. The analytic data are in accordance with the literature.33 5-Nitrosobenzo[d][1,3]dioxole (2k). Benzo[d][1,3]dioxol-5-yltrimethylsilane 1k (48.6 mg, 0.25 mmol, 1.0 equiv) and NOBF4 (58.4 mg, 0.50 mmol, 2.0 equiv) were reacted according to procedure B. The crude product was purified by column chromatography on silica gel (npentane/ethyl acetate = 10:1) to afford 73% yield (27.5 mg, 0.18 mmol) of 2k as a green solid: 1H NMR (300 MHz, CDCl3, ppm) δ = 8.59 (dd, J = 8.1, 1.9 Hz, 1H), 7.14 (d, J = 8.2 Hz, 1H), 6.42 (d, J = 1.9 Hz, 1H), 6.10 (s, 2H); 13C{1H} NMR (75 MHz, CDCl3, ppm) δ = 165.0, 154.2, 149.4, 133.0, 108.3, 102.9, 91.8. The analytic data are in accordance with the literature.34

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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.8b00262. Analytical data and NMR spectra of isolated compounds; 1 H and 13C{1H} NMR spectra for selected compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gerhard Hilt: 0000-0002-5279-3378 Notes

The authors declare no competing financial interest.

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REFERENCES

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Synthesis of Nitrosobenzene Derivatives via Nitrosodesilylation

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