Cobalt bis(acetylacetonate)–tert-butyl hydroperoxide–triethylsilane: a general reagent combination for the Markovnikov-selective hydrofunctionalization of alkenes by hydrogen atom transfer

We show that cobalt bis(acetylacetonate) [Co(acac)2], tert-butyl hydroperoxide (TBHP), and triethylsilane (Et3SiH) constitute an inexpensive, general, and practical reagent combination to initiate a broad range of Markovnikov-selective alkene hydrofunctionalization reactions. These transformations are believed to proceed by cobalt-mediated hydrogen atom transfer (HAT) to the alkene substrate, followed by interception of the resulting alkyl radical intermediate with a SOMOphile. In addition, we report the first reductive couplings of unactivated alkenes and aryldiazonium salts by an HAT pathway. The simplicity and generality of the Co(acac)2–TBHP–Et3SiH reagent combination suggests it as a useful starting point to develop HAT reactions in complex settings.

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.
Preparation of 3-methylbut-2-en-1-yl 4-methoxybenzoate (3c): 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-

Preparation of p-toluenesulfonyl bromide (S1):
A solution of sodium bromide (815 mg, 7.92 mmol, 0.333 equiv) and sodium bromate (2.43 g, 16.1 mmol, 0.667 equiv) in water (14 mL) was added dropwise to a suspension of p-toluenesulfonyl hydrazide (4.47 g, 24.0 mmol, 1 equiv) in an aqueous solution of hydrochloric acid (10% w/w, 80 mL) at 24 C. The product mixture was stirred for 10 min and filtered immediately. The residue obtained was recrystallized from petroleum ether to afford p-toluenesulfonyl bromide (S1) as a white crystalline solid (3.99 g, 71%).  H and 13 C NMR data for p-toluenesulfonyl bromide (S1) prepared in this way were in agreement with those previously described [8] .

Preparation of Se-phenyl seleno-p-toluenesulfonate (S2):
A suspension of p-toluenesulfonyl hydrazide (1.86 g, 10.0 mmol, 1 equiv) in methanol (8.0 mL) was added dropwise over 15 min to a suspension of benzeneseleninic acid (1.89 g, 10.0 mmol, 1.00 equiv) in methanol (8.0 mL) at 0 C. Vigorous evolution of nitrogen gas was observed and a yellow precipitate was formed. The reaction mixture was cooled to -5 C overnight. The suspension was filtered to afford Sephenyl seleno-p-toluenesulfonate (S2) as a light yellow solid (2.83 g, 91%).  H and 13 C NMR data for Se-phenyl seleno-p-toluenesulfonate (S2) prepared in this way were in agreement with those previously described [9] .

Preparation of (phenylsulfonyl)methanal O-benzyl oxime (6b):
Six equal portions of m-chloroperbenzoic acid (4.40 g, 25.5 mmol, 2.20 equiv) were added over 1 h to a suspension of phenyl N-(benzyloxy)methanimidothioate (2.82 g, 11.6 mmol, 1 equiv) and sodium bicarbonate (2.00 g, 23.8 mmol, 2.05 equiv) in dichloromethane (50 mL) at 0 C. The resulting mixture was stirred at 0 C for 15 min. The reaction vessel was then placed in an oil bath that had been previously heated to 40 C. The reaction mixture was stirred and heated at 40 C for 1 h. The product mixture was allowed to cool down to 24 C over 30 min and the cooled product mixture was transferred to a separatory funnel that had been charged with dichloromethane (100 mL) and a saturated aqueous sodium bicarbonate solution (50 mL). The layers that formed was separated and the organic layer was washed with aqueous saturated sodium thiosulfate solution (3 × 50 mL). The organic layer was dried and the dried solution was filtered. The filtrate was concentrated to dryness and the residue obtained was purified by flash-column chromatography (eluting with 10% ethyl acetate-hexanes initially, grading to 20% ethyl acetate-hexanes, linear gradient) to afford (phenylsulfonyl)methanal O-benzyl oxime (6b) as a white solid (3.03 g, 95% H and 13 C NMR data for (phenylsulfonyl)methanal O-benzyl oxime (6b) prepared in this way were in agreement with those previously described [4] .

Preparation of N-methoxypyridinium methyl sulfate (6c):
A 25-mL round-bottomed flask fitted with a rubber septum was charged with pyridine N-oxide (2.17 g, 22.8 mmol, 1 equiv). The reaction vessel was evacuated and refilled using a balloon of argon. This process was repeated twice. The reaction vessel was then cooled to 0 C. Dimethyl sulfate (2.16 mL, 22.8 mmol, 1.00 equiv) was added to the reaction vessel dropwise via syringe over 5 min. The reaction vessel was placed in an oil bath that had been preheated to 100 C. The reaction mixture was stirred and heated for 5 h at 100 C. The product mixture was concentrated in vacuo (0.1 Torr) overnight to afford N-methoxy pyridinium methyl sulfate (6c) as a colorless low-melting solid (4.99 g, 99%).

Preparation of 4-methoxybenzenediazonium tetrafluoroborate (5a):
A solution of tetrafluoroboric acid in water (48% w/w, 6.15 mL, 33.6 mmol, 2.00 equiv) was added to a solution of p-anisidine (2.07 g, 16.8 mmol, 1 equiv) in water (8.0 mL) at 0 C. The resulting suspension was stirred for 10 min at 0 C. A solution of sodium nitrite (1.16 g, 16.8 mmol, 1.00 equiv) in water (2.0 mL) was added dropwise to the reaction vessel over 2 min at 0 C. The reaction mixture was stirred for 15 min at 0 C and the resulting mixture was filtered immediately. The residue obtained was recrystallized with acetone-ether to afford 4-methoxybenzenediazonium tetrafluoroborate (5a) as white needles (2.95 g, 79%). H and 13 C NMR data for 4-methoxybenzenediazonium tetrafluoroborate (5a) prepared in this way were in agreement with those previously described [10] .

Preparation of 4-fluorobenzenediazonium tetrafluoroborate (5b):
A solution of tetrafluoroboric acid in water (48% w/w, 3.66 mL, 17.6 mmol, 2.00 equiv) was added to a solution of 4-fluoroaniline (947 μL, 10.0 mmol, 1 equiv) in water (2.0 mL) at 0 C. The resulting suspension was stirred for 10 min at 0 C. A solution of sodium nitrite (690 mg, 1.0 mmol, 1.00 equiv) in water (1.0 mL) was added dropwise to the reaction vessel over 2 min at 0 C. The reaction mixture was stirred for 15 min at 0 C and the resulting mixture was filtered immediately. The residue obtained was recrystallized with acetone-ether to afford 4-fluorobenzenediazonium tetrafluoroborate (5b) as an off-white solid (1.76 g, 84%). H and 13 C NMR data for 4-fluorobenzenediazonium tetrafluoroborate (5b) prepared in this way were in agreement with those previously described [11] .

Preparation of 4-bromobenzenediazonium tetrafluoroborate (5d):
A solution of tetrafluoroboric acid in water (48% w/w, 3.66 mL, 20.0 mmol, 2.00 equiv) was added to a solution of 4-(trifluoromethyl)aniline (1.72 g, 10.0 mmol, 1 equiv) in water (2.0 mL) at 0 C. The resulting suspension was stirred for 10 min at 0 C. A solution of sodium nitrite (690 mg, 10.0 mmol, 1.00 equiv) in water (1.0 mL) was added dropwise to the reaction vessel over 2 min at 0 C. The reaction mixture was stirred for 15 min at 0 C and the resulting mixture was filtered immediately. The residue obtained was recrystallized with acetone-ether to afford 4-bromobenzenediazonium tetrafluoroborate (5d) as a white solid (2.41 g, 89%). H and 13 C NMR data for 4-bromobenzenediazonium tetrafluoroborate (5d) prepared in this way were in agreement with those previously described [12] .

Preparation of 2-methylbenzenediazonium tetrafluoroborate (5f):
A solution of tetrafluoroboric acid in water (48% w/w, 3.66 mL, 20.0 mmol, 2.00 equiv) was added to a solution of 2-methylaniline (1.06 mL, 10.0 mmol, 1 equiv) in water (2.0 mL) at 0 C. The resulting suspension was stirred for 10 min at 0 C. A solution of sodium nitrite (690 mg, 10.0 mmol, 1.00 equiv) in water (1.0 mL) was added dropwise to the reaction vessel over 2 min at 0 C. The reaction mixture was stirred for 15 min at 0 C and the resulting mixture was filtered immediately. The residue obtained was recrystallized with acetone-ether to afford 2-methylbenzenediazonium tetrafluoroborate (5f) as a light yellow solid (1.76 g, 85%). H and 13 C NMR data for 2-methylbenzenediazonium tetrafluoroborate (5f) prepared in this way were in agreement with those previously described [12] .

Preparation of 3-phenoxybenzenediazonium tetrafluoroborate (5g):
A solution of tetrafluoroboric acid in water (48% w/w, 3.66 mL, 20.0 mmol, 2.00 equiv) was added to a solution of 3-phenoxyaniline (1.85 g, 10.0 mmol, 1 equiv) in water (2.0 mL) at 0 C. The resulting suspension was stirred for 10 min at 0 C. A solution of sodium nitrite (690 mg, 10.0 mmol, 1.00 equiv) in water (1.0 mL) was added dropwise to the reaction vessel over 2 min at 0 C. The reaction mixture was stirred for 15 min at 0 C and the resulting mixture was filtered immediately. The residue obtained was recrystallized with acetone-ether to afford 3-phenoxybenzenediazonium tetrafluoroborate (5g) as a beige solid (2.58 g, 91%).

Preparation of 3,4-methylenedioxybenzenediazonium tetrafluoroborate (5h):
A solution of tetrafluoroboric acid in water (48% w/w, 3.66 mL, 20.0 mmol, 2.00 equiv) was added to a solution of 3,4-methylenedioxyaniline (1.37 g, 10.0 mmol, 1 equiv) in water (2.0 mL) at 0 C. The resulting suspension was stirred for 10 min at 0 C. A solution of sodium nitrite (690 mg, 10.0 mmol, 1.00 equiv) in water (1.0 mL) was added dropwise to the reaction vessel over 2 min at 0 C. The reaction mixture was stirred for 15 min at 0 C and the resulting mixture was filtered immediately. The residue obtained was recrystallized with acetone-ether to afford 3,4-methylenedioxybenzenediazonium tetrafluoroborate (5h) as a black solid (1.90 g, 81%). H and 13 C NMR data for 3,4-methylenedioxybenzenediazonium tetrafluoroborate (5h) prepared in this way were in agreement with those previously described [12] .

Preparation of 1-naphthlenediazonium tetrafluoroborate (5i):
A solution of tetrafluoroboric acid in water (48% w/w, 3.66 mL, 20.0 mmol, 2.00 equiv) was added to a solution of 1-naphthylamine (1.43 g, 10.0 mmol, 1 equiv) in water (2.0 mL) at 0 C. The resulting suspension was stirred for 10 min at 0 C. A solution of sodium nitrite (690 mg, 10.0 mmol, 1.00 equiv) in water (1.0 mL) was added dropwise to the reaction vessel over 2 min at 0 C. The reaction mixture was stirred for 15 min at 0 C and the resulting mixture was filtered immediately. The residue obtained was recrystallized with acetone-ether to afford 1-naphthlenediazonium tetrafluoroborate (5i) as a purple solid (1.31 g, 54%). H and 13 C NMR data for 1-naphthlenediazonium tetrafluoroborate (5i) prepared in this way were in agreement with those previously described [14] .