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DOI: 10.1055/s-2007-973861
A New Versatile Synthesis of Esters from Grignard Reagents and Chloroformates
Publikationsverlauf
Publikationsdatum:
26. März 2007 (online)
Abstract
Cross-coupling reactions of chloroformates with organocopper reagents, derived from Grignard reagents, cuprous bromide and lithium bromide, provide a rapid and straightforward method for the synthesis of esters.
Key words
organometallic reagents - cross-coupling - chemoselectivity - esters
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References and Notes
Typical Experimental Procedure
A THF solution (10 mL) of anhyd LiBr (0.665 g, 7.66 mmol) was added at r.t., under nitrogen, to CuBr (0.549 g, 3.83 mmol). A freshly prepared THF solution of 2,6-dimethyl-phenylmagnesium bromide (4.7 mL, 3.83 mmol) and soon afterwards phenyl chloroformate (0.4 mL, 3.19 mmol) in THF (5 mL) were quickly added to the stirred solution of salts. The mixture was stirred at r.t. for 15 h, quenched with sat. aq NH4Cl and extracted with EtOAc. The organic extracts were dried over Na2SO4 and concentrated under vacuum. The residue was purified by column chromatography (silica gel, PE-EtOAc, 9.8:0.2) leading to phenyl 2,6-dimethylbenzoate 4c (0.707 g, 98% yield). The residual solid was crystallized from hexane giving white crystals of 4c, mp 50-51 °C. IR (KBr): νmax = 1744, 1600, 1587, 1491, 1466, 1261, 1244, 1182, 1105, 1053, 916, 764, 731, 690 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.48-7.41 (m, 2 H), 7.32-7.22 (m, 4 H), 7.10 (d, J = 7.2 Hz, 2 H), 2.48 (s, 6 H). 13C NMR (100.6 MHz, CDCl3): δ = 168.3, 150.6, 135.2, 132.9, 129.8, 129.6, 127.7, 126.1, 121.5, 19.9. MS: m/z (%) = 226 (<1) [M+], 133 (100), 105 (27), 77 (13), 65 (6), 51 (5). The side product phenyl 2,6-dimethylphenyl carbonate (<2%) was identified by the comparison with mass spectral data of an authentic sample independently synthesized: MS: m/z (%) = 242 (99) [M+], 198 (62), 197 (25), 183 (97), 165 (49), 155 (18), 148 (44), 121 (27), 120 (100), 105 (32), 103 (20), 94 (20), 92 (48), 91 (72), 79 (40), 78 (22), 77 (73), 51 (25).
At present, the formation of this side product is not yet clear and is subject to further investigations.