Highly Chemoselective Crossed Imino Pinacol Coupling Reaction Using the Synergetic Effect of Boron Trifluoride Etherate and Trichloromethylsilane

Abstract

Use of boron trifluoride etherate and trichloromethyl­silane in the presence of zinc-copper couple effects a crossed imino pinacol coupling reaction to give 1,2-diamines in good yields with high diastereoselectivities.

crossed imino pinacol coupling - boron trifluoride etherate - trichloromethylsilane - 1,2-diamines - zinc-copper couple

References

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6

A typical experimental procedure is as follows: To a suspension of Zn-Cu (32.7 mg, 0.50 mmol) in MeCN (0.25 mL) was added a mixture of BF₃·OEt₂ (78.1 mg, 0.55 mmol) and MeSiCl₃ (0.147 mL, 1.25 mmol) in MeCN (1.0 mL) at 0 °C under an argon atmosphere. To the resulting mixture was added a solution of N-benzylidene-p-anisidine (52.8 mg, 0.25 mmol) and N-benzylidene-p-chlorophenyl aniline (53.9 mg, 0.25 mmol) in acetonitrile (1.25 mL) at 0 °C. After being stirred at room temperature for 1 h, the reaction was quenched with sat. aqueous NaHCO₃. Usual work-up followed by purification on preparative silica gel TLC⁹ deactivated with phosphate buffer gave 1-(4-chloro-phenylamino)-2-(4-methoxyphenylamino)-1,2-diphenyl-ethane (76.2 mg, 71%) as a yellow amorphous.

7

The relative stereochemistry of the product was determined using ¹H NMR (NOE) after transforming into the corresponding imidazolidinone as in the following examples (Scheme [6] ).

Scheme 6

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9

The buffered silica gel TLC was used for the purification. The buffered silica gel was prepared by suspending 93 g of silica gel (Merck 60F₂₅₄) in 230 mL of phosphate buffer solution (pH 7.0) for 2 h and dried.