Addition of Electrochemically Prepared Arylzinc Species onto Activated Olefins via a Cobalt Catalysis

Abstract

The consumable anode process allows the electrochemical preparation of arylzinc compounds in the presence of cobalt chloride as catalyst in a mixed solvent acetonitrile/pyridine (9:1). Conjugate addition of these organometallic reagents to olefins is also obtained in good yields via a new method using CoBr₂(2,2′-bipyridine)₂ as catalyst.

catalysis - organometallic - Michael addition - zinc - transition metal

References

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<A NAME="RG18202ST-10">10</A>

Experimental procedure: The electrochemical cell fitted with a zinc rod as the anode and a stainless steel grid as cathode was flushed with argon. Acetonitrile (45 mL) and pyridine (5 mL) were added using a syringe. To the solution were added anhyd zinc chloride (0.7 g, 0.3 equiv), cobalt chloride (0.2 g, 0.13 equiv) and tetrabutylammonium tetra-fluoroborate in order to increase the conductivity of the medium. The functionalised phenyl halide (10 mmol, 1 equiv) and dodecane (0.2 mL) as internal standard were added, and a constant current intensity of 0.2 A was applied until complete disappearance of the starting compound (ideally 2000 C). The reaction was monitored using gas chromatography after iodation of the intermediate organo-zinc compound. The solution was then heated to 50 °C, activated olefin (1.5 equiv/ArZnX) and the complex of CoBr₂(bpy)₂ (0.5 equiv/ArZnX) were added under vigorous stirring. The reaction mixture was allowed to warm over
15 h. The mixture was then poured into 50 mL of 2 M HCl solution. The organic materials were then extracted with diethyl ether and the organic layer washed with brine, dried with magnesium sulfate and evaporated under reduced pressure. The resulting crude oil was purified over silica gel using pentane/diethyl ether mixture as an eluent. Some characteristic data of new compounds (Table [1] , entry 11) and (Table [1] , entry 17) are as follows. (Table [1] , entry 11): The product was purified on silica gel using a mixture of pentane and diethyl ether (80:20) as eluent to give 4-(4-acetylphenyl)-2-pentanone. Yield: 44% ¹H NMR (200 MHz) δ (ppm): 7.82 (2 H, d, J = 8.3 Hz), 7.23 (2 H, d, J = 8.3 Hz), 3.36-3.26 (1 H, m), 2.75-2.55 (2 H, m), 2.51 (3 H, s), 2.01 (3 H, s), 1.20 (3 H, d, J = 7.0 Hz) ¹³C NMR (50 MHz) δ (ppm): 207.2, 197.2, 152.0, 128.8-127.2-126.8, 51.2, 35.1, 30.6, 26.3, 22.0 MS, m/z (%): 205 (M + 1, 14), 204 (M, base), 189 (M - CH₃, 69), 161 (M - COCH₃, 26), 147 (19), 143 (19), 132 (12), 129 (16) IR (cm^-1): 2980, 2940, 1680, 1620. Anal. Calcd for C₁₃H₁₆O₂: C, 76.44; H, 7.89; O, 15.67. Found: C, 76.51; H, 7.98; O, 15.51 (Table [1] , entry 17): the product was purified on silica gel using a mixture of pentane and ether (90:10) as eluent to give methyl 3-(4-butoxy-carbonylphenyl)propionate. Yield: 48%. ¹H NMR (200 MHz) δ (ppm): 8.06 (2 H, d, J = 8.2 Hz), 7.38 (2 H, d, J = 8.2 Hz), 4.17 (2 H, t, J = 6.6 Hz), 4.00 (3 H, s), 3.11 (2 H, t, J = 7.6 Hz), 2.74 (2 H, t, J = 7.6 Hz), 1.87-1.61 (2 H, m), 1.56-1.34 (2 H, m), 1.00 (3 H, t, J = 7.2 Hz). ¹³C NMR (50 MHz) δ (ppm): 172.0, 166.5, 145.8, 129.9-128.4-127.0, 64.5, 51.6, 35.4, 31.0-30.6, 19.1, 13.7. MS, m/z (%): 264 (M, 31), 233 (M - OCH₃, 25), 209 (24), 208 (base), 177 (24), 163 (37), 162 (58).
IR (cm^-1): 2960, 1720, 1640. Anal. Calcd for C₁₅H₂₀O₄: C, 68.16; H, 7.23; O, 24.21. Found: C, 68.20; H, 7.53; O, 23.77.

<A NAME="RG18202ST-11">11</A> Buriez O. Nédélec J.-Y. Périchon J. J. Electroanal. Chem.  2001,  506:  162