Catalytic Asymmetric Cyano-Phosphorylation of Aldehydes Promoted by Heterobimetallic YLi₃tris(binaphthoxide) (YLB) Complex

Abstract

A highly enantioselective cyano-phosphorylation of aldehydes catalyzed by YLi₃tris(binaphthoxide) complex (YLB, 1) is described. Slow addition of diethyl cyanophosphonate (4) to aldehydes 5 in the presence of 1 (10 mol%), H₂O (30 mol%), tris(2,6-dimethoxyphenyl)phosphine oxide (10 mol%, 3a), and BuLi (10 mol%) afforded cyanohydrin O-phosphates 6 in up to 98% yield and 97% ee.

Key words

asymmetric catalysis - cyanohydrins - Lewis acid - phosphorylation - yttrium

References

• For recent reviews for catalytic asymmetric cyanation reaction of carbonyl compounds, see:
• 1a Brunel JM. Holmes IP. Angew. Chem. Int. Ed.  2004,  43:  2752 
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General Procedure for the Cyano-Phosphorylation Reaction. To 3a (164.6 mg, 0.3 mmol) in a test tube were added the (S)-YLB·H₂O solution (5.00 mL, 0.3 mmol, 0.06 M, THF) [2] and BuLi (0.3 mmol) in hexane at r.t. After dissolving 3a completely, the mixture was cooled to -78 °C, and 5a (3.0 mmol) in THF (4.50 mL) was added to the catalyst mixture. After stirring for 10 min at -78 °C, 4 (0.55 mL, 3.6 mmol) in THF (0.50 mL) was slowly added to the reaction mixture over 1 h, and the reaction mixture was stirred at -78 °C for additional 1 h. Then, HOAc in THF cooled to -78 °C was added to the solution, and the mixture was diluted with H₂O. The organic component was extracted with EtOAc. The organic layer was washed with sat. aq NaHCO₃, H₂O, brine, and dried over Na₂SO₄. After evaporating solvent, the residue was purified by silica gel flash column chromatography (hexane-EtOAc = 7:1) to give 6a (97% yield, 92% ee, R); colorless oil. IR (neat): ν = 1269, 1024 cm^-1. ¹H NMR (CDCl₃): δ = 1.16 (dt, J _{( H,P)} = 0.6 Hz, J = 7.0 Hz, 3 H), 1.32 (dt, J _{( H,P)} = 0.6 Hz, J = 7.0 Hz, 3 H), 3.92-3.99 (m, 2 H), 4.12-4.17 (m, 2 H), 6.02 (d, J _{( H,P)} = 8.6 Hz, 1 H), 7.39-7.40 (m, 3 H), 7.49-7.50 (m, 2 H). ¹³C NMR (CDCl₃): δ = 15.8 (d, J _{( C,P)} = 7.3 Hz), 16.0 (d, J _{( C,P)} = 7.1 Hz), 64.6 (d, J _{( C,P)} = 6.3 Hz), 64.8 (d, J _{( C,P)} = 6.1 Hz), 66.5 (d, J _{( C,P)} = 4.1 Hz), 116.1 (d, J _{( C,P)} = 6.3 Hz), 127.5, 129.2, 130.6, 132.4 (d, J _{( C,P)} = 4.1 Hz). ³¹P NMR (CDCl₃): δ =
-1.93. LRMS (ESI, MeOH): m/z = 292 [M + Na⁺]. HRMS (FAB): m/z [N + H⁺] calcd for C₁₂H₁₇NO₄P: 270.0890; found: 270.0894. [α]_D ^21.7 +18.9 (c 1.2, CHCl₃; 92% ee, R); HPLC (DAICEL CHIRALPAK^®) AD-H, hexane-2-PrOH = 9:1, flow rate = 1.0 mL/min, retention time 14.5 min (S)/17.1 min (R). The absolute configuration of 6a was determined comparing retention time in HPLC analysis with that of the authentic product synthesized from commercially available (R)-mandelonitrile (Aldrich Co. Ltd.) with 4.

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Detailed mechanistic studies of asymmetric cyanation reaction using YLB(1) and the cyanide source 2 and 4 will be reported elsewhere in due course.