Sterically Modified Chiral (Salen)Cr(III) Complexes - Efficient Catalysts for the Oxo-Diels-Alder Reaction between Glyoxylates and Cyclohexa-1,3-diene

Abstract

A group of chiral [(salen)Cr(III)]⁺BF₄ ^- complexes, with enhanced steric hindrance in 3,3′-positions of salicylidene moiety, has been synthesized and applied for the oxo-Diels-Alder reaction of alkyl glyoxylates with cyclohexa-1,3-diene. A readily accessible complex that bears bulky adamanthyl substituents revealed its potential, leading to the cycloadducts with excellent selectivity (up to endo/exo 99:1, 98% ee), considerably better than the classic Jacobsen catalyst.

Key words

asymmetric catalysis - glyoxylates - hetero-Diels-Alder reaction - salen-chromium complexes

References and Notes

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• 18a

  Ref. 15.

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14

General Procedure for Cycloaddition of Cyclohexa-1,3-diene to Alkyl Glyoxylates.
To a solution of catalyst 5a-g (0.5-5 mol%) and alkyl glyoxylate (1 mmol, freshly distilled from P₂O₅) in toluene (1 mL) cyclohexa-1,3-diene (150 µL, 1.5 mmol) was added, and the solution was stirred for 24 h at r.t. Afterwards the reaction mixture was subjected to chromatography (hexanes-EtOAc, 9:1).

16

Analytical Data for Salen Ligand Precursor of 5g.
Mp 247-248 °C. ¹H NMR (500 MHz, CDCl₃): δ = 1.23 (s, 18 H), 1.15-1.35 (m, 2 H), 1.40-1.50 (m, 2 H), 1.65-2.00 (m, 10 H), 2.04-2.09 (m, 6 H), 2.13-2.18 (m, 12 H), 3.26-3,34 (m, 2 H), 6.96 (d, J = 2.2 Hz, 2 H), 7.24 (d, J = 2.2 Hz, 2 H), 8.29 (s, 2 H), 13.64 (s, 2 H). ¹³C NMR (125 MHz, CDCl₃): δ = 24.4, 29.2, 31.4, 33.3, 34.1, 37.2, 37.2, 40.4, 72.4, 117.9, 126.0, 126.7, 136.6, 139.9, 158.2, 166.0. [α]_D ²⁵ +325.7 (c 0.58, CHCl₃). IR (KBr): 2950, 2905, 2850, 1625, 1598 cm^-1. Anal. Calcd for C₄₈H₆₆N₂O₂: C, 82.00; H, 9.46; N 3.98. Found: C, 82.06; H, 9.39, N, 3.82. Catalyst 5g: HRMS: m/z calcd for C₄₈H₆₄N₂O₂Cr [M - BF₄]⁺: 752.4367; found: 752.4392.

19

Analytical Data for Cycloadducts 3.
Compound 3a (ref. 11): GC (column β-dex 120, i.d. 30 m × 0.25 mm, carrier gas - nitrogen 100 kPa, oven temp. 150 °C): t _{R1[ exo- 3a ]} = 29.9, t _{R2[ exo- 3a ]} = 30.7, t _{R[ ( 3 S )- endo- 3a ]} = 33.1, t _{R[ ( 3 R )- endo- 3a ]} = 34.1 min.
Compound 3b (ref. 7a,b and 8a): GC (column β-dex 120, i.d. 30 m × 0.25 mm, carrier gas - nitrogen 100 kPa, oven temp. 140 °C): t _{R1[ exo- 3b ]} = 18.8, t _{R2[ exo- 3b ]} = 19.5, t _{R[ ( 3 S )- endo- 3b ]} = 21.2, t _{R[ ( 3 R )- endo- 3b ]} = 21.8 min.
Compound 3c: GC (column β-dex 120, i.d. 30 m × 0.25 mm, carrier gas - nitrogen 100 kPa, oven temp. 140 °C):
t _{R1[ exo- 3c ]} = 19.8, t _{R2[ exo- 3c ]} = 20.5, t _{R[ ( 3 S )- endo- 3c ]} = 21.7,
t _{R[ ( 3 R )- endo- 3c ]} = 22.6 min. ¹H NMR for endo- 3c (500 MHz, CDCl₃): δ = 1.21 (d, J = 1.8 Hz, 3 H), 1.22 (d, J = 1.8 Hz, 3 H), 1.26-1.42 (m, 2 H), 1.71-1.77 (m, 1 H), 2.03-2.10 (m, 1 H), 3.06-3.10 (m, 1 H), 4.26 (d, J = 1.8 Hz, 1 H), 4.56-4.59 (m, 1 H), 5.01 (dq, J = 1.8, 1.8 Hz, 1 H), 6.23-6.28 (m, 1 H), 6.51-6.55 (m, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 20.9, 21.7, 25.7, 33.2, 66.4, 68.0, 74.2, 130.4, 134.7, 171.7.
Compound 3d: GC (column β-dex 120, i.d. 30 m × 0.25 mm, carrier gas - nitrogen 100 kPa, oven temp. 150 °C):
t _{R1[ exo- 3d ]} = 15.5, t _{R2[ exo- 3d ]} = 15.9, t _{R[ ( 3 S )- endo- 3d ]} = 16.2,
t _{R[ ( 3 R )- endo- 3d ]} = 16.7 min. ¹H NMR for endo- 3d (500 MHz, CDCl₃): δ = 1.18-1.28 (m, 1 H), 1.29 (s, 9 H), 1.81-1.87 (m, 1 H), 1.93-2.02 (m, 1 H), 2.16-2.24 (m, 1 H), 2.46-2.53 (m, 1 H), 4.51 (d, J = 7.3 Hz, 1 H), 4.59-4.63 (m, 1 H), 5.89-5.94 (m, 1 H), 6.18-6.23 (m, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 18.1, 23.4, 27.9, 39.7, 70.7, 71.5, 75.5, 122.2, 136.0, 175.4.