Non-classical selectivities in the reduction of alkenes by cobalt-mediated hydrogen atom transfer

It is shown that the reduction of alkenes by hydrogen atom transfer provides selectivities that are distinct from classical hydrogenation catalysts. The first alkene hydrobromination, hydroiodination, and hydroselenylation reactions that proceed by hydrogen atom transfer processes are also reported.

Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S4 a Reactions employed 250 μmol each of 1a and 1b. All conversions and yields were calculated by 1 H NMR spectroscopy using mesitylene as an internal standard. Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S7 fluorine nuclear magnetic resonance spectra ( 19 F NMR) were recorded at 375 MHz or 470 MHz at 24 °C, unless otherwise noted. Chemical shifts are expressed in parts per million (ppm, δ scale) downfield from fluorotrichloromethane. Attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR) were obtained using a Thermo Electron Corporation Nicolet 6700 FTIR spectrometer referenced to a polystyrene standard. Data are represented as follows: frequency of absorption (cm -1 ), intensity of absorption (s = strong, m = medium, w = weak, br = broad). High-resolution mass spectrometry (HRMS) were obtained on a Waters UPLC/HRMS instrument equipped with a dual API/ESI high-resolution mass spectrometry detector and photodiode array detector. Unless otherwise noted, samples were eluted over a reverse-phase C18 column (1.7 μm particle size, 2.1 × 50 mm) with a linear gradient of 5% acetonitrile-water containing 0.1% formic acid95% acetonitrile-water containing 0.1% formic acid over 1.6 min, followed by 100% acetonitrile containing 0.1% formic acid for 1 min, at a flow rate of 600 μL/min. Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S8

Synthetic Procedures.
Preparation of 2-methylallyl 4-methoxybenzoate (1a): 4-Methoxybenzoyl chloride (1.50 g, 8.80 mmol, 1.10 equiv) was added dropwise via syringe to a solution of 2-methyl-2-propen-1-ol (576 mg, 8.00 mmol, 1 equiv) in pyridine (32 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (50 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (50 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  50 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford 2-methylallyl 4methoxybenzoate (1a) as a clear oil (1.60 g, 97%). Rf = 0.55 (20% ethyl acetate-hexanes; UV, KMnO4). 1 19.6 (CH3). 1 H and 13 C NMR data for 2-methylallyl 4-methoxybenzoate (1a) prepared in this way were in agreement with 46those previously described. [11] Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S9

Preparation of 1-((allyloxy)methyl)-4-methoxybenzene (1b):
A 250-mL round-bottomed flask that had been fused to a Teflon-coated valve was charged with sodium hydride (300 mg, 7.50 mmol, 1.50 equiv). The reaction vessel was evacuated and refilled using a balloon of argon. This process was repeated twice. Tetrahydrofuran (24 mL) was added to the reaction vessel via syringe and the resulting mixture was cooled to 0 C. A 250-mL round-bottomed flask was charged with 4-methoxybenzyl alcohol (691 mg, 5.00 mmol, 1 equiv). The vessel containing the starting material was evacuated and refilled using a balloon of argon. This process was repeated twice. Tetrahydrofuran (100 mL) was added to the vessel containing the starting material and the resulting solution was transferred via cannula to the sodium hydride suspension. The reaction mixture was stirred at 0 C for 45 min. Tetrabutylammonium iodide (92.3 mg, 250 μmol, 0.0500 equiv) and allyl bromide (786 mg, 6.50 mmol, 1.30 equiv) were then added in sequence. The reaction mixture was allowed to warm over 30 min to 24 C. The warmed reaction mixture was stirred for 12 h at 24 C. The product mixture was filtered through a pad of silica gel and the pad was rinsed with ethyl acetate (50 mL). The filtrates were collected and combined and the combined filtrates were concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ether-hexanes initially, grading to 10% ether-hexanes, linear gradient) to afford 1-((allyloxy)methyl)-4-methoxybenzene (1b) as a light yellow oil (890 mg, 99%). Rf 1 H and 13 C NMR data for 1-((allyloxy)methyl)-4-methoxybenzene (1b) prepared in this way were in agreement with those previously described. [12] Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S10 Preparation of allyl 4-methoxybenzoate (1c): 4-Methoxybenzoyl chloride (1.50 g, 8.80 mmol, 1.10 equiv) was added dropwise via syringe to a solution of allyl alcohol (464 mg, 8.00 mmol, 1 equiv) in pyridine (32 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (50 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (50 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  50 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford allyl 4-methoxybenzoate (1c) as a clear oil (1.54 g, 99%).  13 C NMR (150 MHz, CDCl3) δ 166.0 (C), 163.4 (C), 132.5 (CH), 131.6 (CH), 122.5 (C), 117.9 (CH2), 113.6 (CH), 65.2 (CH2), 55.4 (CH3). 1 H and 13 C NMR data for 2-methylallyl allyl 4-methoxybenzoate (1c) prepared in this way were in agreement with those previously described. [13] Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S11 Preparation of 2-chloroallyl 4-methoxybenzoate (1d): 4-Methoxybenzoyl chloride (1.02 g, 5.96 mmol, 1.10 equiv) was added dropwise via syringe to a solution of 2-chloro-2-propen-1-ol (500 mg, 5.41 mmol, 1 equiv) in pyridine (22 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (20 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (20 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  20 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford 2-chloroallyl 4-methoxybenzoate (1d) as a clear oil (1.20 g, 98%).  H and 13 C NMR data for 2-chloroallyl 4-methoxybenzoate (1d) prepared in this way were in agreement with those previously described. [14] Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S12 Preparation of 3-bromobut-3-en-1-yl (4-methoxyphenyl)carbamate (1e): 1-Methylimidazole (71.4 μL, 900 μmol, 0.300 equiv) and 4-methoxyphenyl isocyanate (386 μL, 3.00 mmol, 1.00 equiv) were added in sequence to a solution of 3-bromo-3-buten-1-ol (298 μL, 3.00 mmol, 1 equiv) in acetonitrile (7.5 mL) at 24 C. The reaction mixture was stirred for 12 h at 24 C. The product mixture was concentrated and the residue obtained was purified by automated flash-column chromatography (eluting with 10% ethyl acetate-hexanes initially, grading to 33% ethyl acetate-hexanes, linear gradient) to afford 3-bromobut-3-en-1-yl (4-methoxyphenyl)carbamate (1e) as a white solid (809 mg, 90%). Rf  H and 13 C NMR data for 3-bromobut-3-en-1-yl (4-methoxyphenyl)carbamate (1e) prepared in this way were in agreement with those previously described. [14] Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S13

Preparation of 1-fluoro-4-(hex-1-yn-1-yl)benzene (4f):
A 25-mL round-bottomed flask that had been fused to a Teflon-coated valve was charged sequentially with 1-fluoro-4-iodobenzene (1.11 g, 5.00 mmol, 1 equiv), bis(triphenylphosphine)palladium dichloride (175 mg, 250 μmol, 0.0500 equiv), and copper iodide (95.2 mg, 500 μmol, 0.100 equiv). The reaction vessel was evacuated and refilled using a balloon of argon. This process was repeated twice. Triethylamine (10.0 mL) and 1-hexyne (860 μL, 7.50 mmol, 1.50 equiv) were added sequentially to the reaction vessel. The reaction vessel was sealed and the sealed vessel was placed in an oil bath that had been preheated to 50 C. The reaction mixture was stirred and heated for 4 h at 50 C. The product mixture was allowed to cool over 20 min to 24 C. The cooled product mixture was filtered through a pad of silica gel and the pad was rinsed with ethyl acetate (100 mL). The filtrates were combined and the combined filtrates were concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with hexanes, isocratic gradient) to afford 1-fluoro-4-(hex-1-yn-1-yl)benzene (4f) as a light yellow oil (723 mg, 82%). H and 19 F NMR data for 1-fluoro-4-(hex-1-yn-1-yl)benzene (4f) prepared in this way were in agreement with those previously described. [18] Ma and Herzon "Non-Classical Selectivities in the Reduction of Alkenes by Cobalt-Mediated Hydrogen Atom Transfer" Chem. Sci. S23 Preparation of (E)-pent-2-en-1-yl 4-methoxybenzoate (S1): 4-Methoxybenzoyl chloride (563 mg, 3.30 mmol, 1.10 equiv) was added dropwise via syringe to a solution of (E)-pent-2-en-1-ol (258 mg, 3.00 mmol, 1 equiv) in pyridine (12 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (20 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (20 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  20 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford (E)-pent-2-en-1-yl 4methoxybenzoate (S1) as a clear oil (621 mg, 94%). Preparation of (Z)-pent-2-en-1-yl 4-methoxybenzoate (S2): 4-Methoxybenzoyl chloride (563 mg, 3.30 mmol, 1.10 equiv) was added dropwise via syringe to a solution of (Z)-pent-2-en-1-ol (258 mg, 3.00 mmol, 1 equiv) in pyridine (12 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (20 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (20 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  20 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford (Z)-pent-2-en-1-yl 4methoxybenzoate (S2) as a clear oil (661 mg, 99%). 4-Methoxybenzoyl chloride (563 mg, 3.30 mmol, 1.10 equiv) was added dropwise via syringe to a solution of 3-methylbut-2-en-1-ol (258 mg, 3.00 mmol, 1 equiv) in pyridine (12 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (20 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (20 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  20 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford 3-methylbut-2-en-1-yl 4methoxybenzoate (S3) as a clear oil (661 mg, 99%). Preparation of but-2-yn-1-yl 4-methoxybenzoate (S4): 4-Methoxybenzoyl chloride (563 mg, 3.30 mmol, 1.10 equiv) was added dropwise via syringe to a solution of but-2-yn-1-ol (210 mg, 3.00 mmol, 1 equiv) in pyridine (12 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (20 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (20 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  20 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford but-2-yn-1-yl 4-methoxybenzoate (S4) as a clear oil (612 mg, 99%).

Preparation of 2-bromoallyl 4-methoxybenzoate (1k):
4-Methoxybenzoyl chloride (938 mg, 5.50 mmol, 1.10 equiv) was added dropwise via syringe to a solution of 2-bromoallyl alcohol (685 mg, 5.00 mmol, 1 equiv) in pyridine (20 mL) at 0 C. The reaction mixture was stirred for 30 min at 0 C, and then the ice bath was removed. The reaction mixture was stirred for 24 h at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (20 mL). The diluted product mixture was washed with saturated aqueous sodium bicarbonate solution (20 mL). The aqueous layer was isolated and the isolated aqueous layer was extracted with ethyl acetate (3  20 mL). The organic layers were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The residue obtained was purified by automated flash-column chromatography (eluting with 5% ethyl acetate-hexanes initially, grading to 10% ethyl acetate-hexanes, linear gradient) to afford 2-bromoallyl 4-methoxybenzoate (1k) as a clear oil (1.26g, 93%). Preparation of (E)-2-methylallyl pent-2-en-1-yl terephthalate (9): N,N'-Diisopropylcarbodiimide (1.55 mL, 10.0 mmol, 2.00 equiv) and 4-dimethylaminopyridine (122 mg, 1.00 mmol, 0.200 equiv) were added sequentially to a suspension of 2-methylallyl alcohol (371 mg, 5.00 mmol, 1 equiv) and 4-formylbenzoic acid (750 mg, 5.00 mmol, 1 equiv) in dichloromethane (25 mL) at 24 C. The reaction mixture was stirred for 2 h at 24 C. The product mixture was concentrated and the residue obtained was diluted with a mixture of ethyl acetate and hexanes (1:4, v/v, 100 mL). The diluted product mixture was filtered through a pad of celite and the pad was rinsed with a mixture of ethyl acetate and hexanes (1:4, v/v, 100 mL). The filtrates were collected and combined. The combined filtrate was concentrated. The 2-methylallyl 4-formylbenzoate prepared in this way was immediately used in the following step without further purification.
2-Methyl-2-butene (6.36mL, 60.0 mmol, 12.0 equiv) and a solution of sodium chlorite (3.00 g, 33.3 mmol, 6.65 equiv) and sodium phosphate monobasic (3.68 g, 26.7 mmol, 5.34 equiv) in water (22.7 mL) were added sequentially to a suspension of 2-methylallyl 4-formylbenzoate (nominally, 5.00 mmol) in t-butanol (71.4 mL) at 24 C. The reaction mixture was stirred for 30 min at 24 C. The product mixture was transferred to a separatory funnel that had been charged with ethyl acetate (250 mL). The diluted product mixture was washed with saturated ammonium chloride (3  50 mL). The organic layer were combined and the combined organic layers were dried over sodium sulfate. The dried solution was filtered and the filtrate was concentrated. The 4-(((2-methylallyl)oxy)carbonyl)benzoic acid prepared in this way was immediately used in the following step without further purification.
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Representative Procedure for Competition Experiments (Table S2, Condition A).
A 25-mL round-bottomed flask fitted with a rubber septum was charged sequentially with the target substrate (250 μmol, 1 equiv), the competition substrate (250 μmol, 1 equiv), and cobalt bis(acetylacetonate) (64.3 mg, 250 μmol, 1.00 equiv). A 16-gauge needle was penetrated through the septum. n-Propanol (830 μL), 1,4-dihydrobenzene (238 μL, 2.50 mmol, 10.0 equiv), a solution of tertbutyl hydroperoxide in nonane (~5.5 M, 45.5 μL, 250 μmol, 1.00 equiv), and triethylsilane (400 μL, 2.50 mmol, 10.0 equiv) were added sequentially to the reaction vessel via syringe. The reaction mixture was stirred at 24 C and a solution of tert-butyl hydroperoxide in nonane (~5.5 M, 22.8 μL, 125 μmol, 0.50 equiv) was added every hour until the consumption of the target substrate was complete (as determined by TLC analysis). The product mixture was filtered through a short column of silica gel and the short column was rinsed with 20% ethyl acetate-hexanes (100 mL). The filtrates were combined and the combined filtrates were concentrated. The conversion of the competition substrate and the yield of the competition product were determined by 1 H NMR analysis of the unpurified product mixture using mesitylene as an internal standard. The NMR sample was concentrated to dryness and the residue obtained was purified by automated flash-column chromatography to afford the target hydrogenation product.
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Representative Procedure for Competition Experiments (Table S2, Condition B).
A 10-mL round-bottomed flask fitted with a reflux condenser was charged sequentially with the target substrate (250 μmol, 1 equiv), the competition substrate (250 μmol, 1 equiv), and cobalt bis(acetylacetonate) (64.3 mg, 250 μmol, 1.00 equiv). The reaction vessel was evacuated and refilled using a balloon of argon. This process was repeated twice. n-Propanol (830 μL), 1,4-dihydrobenzene (238 μL, 2.5 mmol, 10.0 equiv), and triethylsilane (400 μL, 2.50 mmol, 10.0 equiv) were added sequentially to the reaction vessel via syringe. The reaction vessel was placed in an oil bath that had been preheated to 40 C. A solution of tert-butyl hydroperoxide in nonane (~5.5 M, 45.5 μL, 250 μmol, 1.00 equiv) was added dropwise via syringe pump to the reaction vessel. The reaction mixture was stirred and heated at 40 C until the consumption of the target substrate was complete (as determined by TLC analysis). The product mixture was filtered through a short column of silica gel and the short column was rinsed with 20% ethyl acetate-hexanes (100 mL). The filtrates were combined and the combined filtrates were concentrated. The conversion of the competition substrate and the yield of the competition product were determined by 1 H NMR analysis of the unpurified product mixture using mesitylene as an internal standard. The NMR sample was concentrated to dryness and the residue obtained was purified by automated flash-column chromatography to afford the target hydrogenation product.
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