Enantioselective Catalysis of Intramolecular Morita-Baylis-Hillman and Related Reactions by Chiral Rhenium-Containing Phosphines of the Formula (η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂PAr₂)

Abstract

The racemic rhenium-containing phosphine (η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂PPh₂) catalyzes intramolecular Morita-Baylis-Hillman reactions of four R(CO)CH=CH(CH₂) _n CH₂CHO species (n = 1 or 2; R = Ph, S-i-Pr, p-Tol, Me) in benzene or chlorobenzene at 20 °C. The products R(CO)CH=CH(CH₂) _n CH₂CHOH are isolated in 88-99% yields and 38-74% ee when enantiopure S-configured catalyst is used. Similar reactions of R(CO)CH=CHCH₂CH₂CH=CH(CO)R (R = Ph, S-i-Pr) give R(CO)C=CHCH₂CH₂CHCH₂(CO)R in 63-87% yields and 42-56% ee.

Morita-Baylis-Hillman - Rauhut-Currier - phosphine - enantioselective catalysis - organocatalysis - cyclization

References and Notes

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General Procedures

<A NAME="RG34706ST-13A">13a</A>

Racemic catalysts: A Schlenk flask was charged with the educt (typically 0.060-0.080 g). Then C₆H₅Cl or C₆H₆ solutions that were 0.0125 M in ClCH₂CH₂Cl (reference for ¹H NMR integration) were added to give 0.100 M educt solutions. These were equilibrated to 20 °C using a cryostat. Solutions of C₆H₅Cl or C₆H₆ that were 0.0100 M in catalyst and 0.0125 M in ClCH₂CH₂Cl were cooled to 0 °C. Equal volumes, corresponding to 10 mol% loading, were added dropwise over ca. 5 min to the educt solutions. An aliquot (0.6 mL) was transferred to an NMR tube, and ¹H NMR spectra were periodically recorded. When the reaction was complete (or no further reaction took place), 5 volumes of hexane were added with stirring. The mixture was filtered through a short plug of silica gel (removing catalyst), and the plug was washed with hexane-EtOAc (9:1 v/v). The solvent was removed from the filtrates by rotary evaporation. Reactions conducted in C₆H₅Cl were further purified by silica gel column chromatography, except in the case of 5a.

<A NAME="RG34706ST-13B">13b</A>

Enantiopure catalysts: The preceding reactions were repeated on 0.0010-0.0020 g scales. The products were analyzed by HPLC using Chiralcel OD, Chiralpak AD-H or Chiralpak AS-H columns.

<A NAME="RG34706ST-14">14</A>

All products were characterized by NMR (¹H, ¹³C) and IR spectroscopy, and these data are available from the authors upon request; 3a,d and 5a have been reported previously.^11a,15,16
Typical data (3b): ¹H NMR (400 MHz, CDCl₃): δ = 1.39 [d, (CH ₃)₂CH, ³ J(H,H) = 7.2 Hz, 6 H], 1.82-1.91 (m, C=CHCHH′, 1 H), 2.27-2.46 (2 m, C=CHCHH′, CHH′CHOH, 2 H) 2.62-2.73 (m, CHH′CHOH, 1 H), 2.78 (br s, CHOH, 1 H), 3.74 [sep, ³ J(H,H) = 7.2 Hz, (CH₃)₂CH, 1 H], 5.13-5.16 (m, CHOH, 1 H), 6.89 [dd, ³ J(H,H) = 2.8, 2.8 Hz, C=CHCHH′, 1 H] ppm. ¹³C{¹H} NMR (101 MHz, CDCl₃): δ = 23.0 [s, ( C H₃)₂CH], 30.9 (s, C=CHCH₂), 31.8 (s, CH₂CHOH), 34.3 [s, (CH₃)₂ CH], 75.7 (s, CHOH), 144.4 [s, (CO)C=CH], 145.2 [s, (CO)C=CH], 190.1 [s, (CO)C=CH] ppm. IR (thin film): 1613 (s, ν_C=C), 1648 (s, ν_CO), 3450 (br, ν_OH) cm^-1.

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