Enantioselective Synthesis of C ₂-Symmetric Dimethyl Cyclohexadiene­dicarboxylates through Cationic Rhodium(I)/Modified-BINAP-Catalyzed [2+2+2] Cycloadditions

Abstract

A cationic rhodium(I)/(S)-Tol-BINAP complex was employed to catalyze an enantioselective intramolecular [2+2+2] cycloaddition of a trans enediyne leading to a C ₂-symmetric tricyclic dimethyl cyclohexadienedicarboxylate in 95% yield with 59% ee. A cationic rhodium(I)/(R)-H₈-BINAP complex was employed to catalyze an intermolecular [2+2+2] cycloaddition of 1,6-diynes with dimethyl fumarate leading to C ₂-symmetric bicyclic dimethyl ­cyclohexadienedicarboxylates in 35-96% yields with ee values of 82-98%.

asymmetric catalysis - BINAP - cycloadditions - cyclohexadienes - rhodium

References and Notes

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<A NAME="RU02407ST-16">16</A>

General Procedure for Intramolecular [2+2+2] Cycloadditions (Table 1): Under an Ar atmosphere, BINAP-type ligand (0.0125 mmol) and [Rh(cod)₂]BF₄ (cod: 1,5-cyclooctadiene; 0.0125 mmol) were dissolved in CH₂Cl₂ (2.0 mL) and the mixture was stirred at r.t. for 5 min. H₂ (1 atm) was introduced to the resulting solution in a Schlenk tube. After stirring at r.t. for 0.5 h, the resulting solution was concentrated and redissolved in CH₂Cl₂ (8.0 mL). To this solution was added dropwise over 10 min a solution of 1 (0.125 mmol) in CH₂Cl₂ (2.0 mL), and the remaining substrate was washed away by using CH₂Cl₂ (2.5 mL). After stirring at r.t. for 16 h, the resulting solution was concentrated and product 2 was isolated by preparative TLC. Product (-)-2a: [α]²⁵ _D -31.0° (acetone, c = 0.010; 48% ee). IR (neat): 3390, 2930, 2860, 2360, 1730, 1680, 1040, 900 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 5.86 (s, 2 H), 4.55 (d, J = 13.8 Hz, 2 H), 4.35 (d, J = 13.8 Hz, 2 H), 4.23-4.31 (m, 2 H), 3.41-3.53 (m, 2 H), 2.63-2.81 (m, 2 H). HRMS (ESI): m/z [M]⁺ calcd for C₁₀H₁₂O₂: 164.0837; found: 164.0841. HPLC: CHIRALPAK AD-H; hexane-i-PrOH = 99:1; 1.0 mL/min, t _R = 12.6 min (major isomer), t _R = 13.5 min (minor isomer). Product (+)-2b: [α]²⁵ _D +64.9° (acetone, c = 0.665, 59% ee). IR (neat): 3390, 2950, 2850, 1720, 1440, 1300, 1210, 1150, 1020, 920 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 4.98 (d, J = 17.4 Hz, 2 H), 4.52 (d, J = 17.4 Hz, 2 H), 4.25-4.36 (m, 2 H), 3.76 (s, 6 H), 3.41-3.50 (m, 2 H), 2.84-3.02 (m, 2 H). ¹³C NMR (CDCl₃, 75 MHz): δ = 165.4, 155.8, 120.5, 72.8, 70.3, 51.9, 45.4. HRMS (ESI): m/z [M]⁺ calcd for C₁₄H₁₆O₆: 280.0947; found: 280.0988. HPLC: CHIRALPAK AS; hexane-2-PrOH = 95:5; 1.0 mL/min;
t _R = 29.3 min (major isomer), t _R = 39.0 min (minor isomer).

<A NAME="RU02407ST-17">17</A>

General Procedure for Intermolecular [2+2+2] Cycloadditions (Table 2): Under an Ar atmosphere, BINAP-type ligand (0.010 mmol) and [Rh(cod)₂]BF₄ (0.010 mmol) were dissolved in CH₂Cl₂ (2.0 mL) and the mixture was stirred at r.t. for 5 min. H₂ (1 atm) was introduced to the resulting solution in a Schlenk tube. After stirring at r.t. for 0.5 h, the resulting solution was concentrated and redissolved in CH₂Cl₂ (0.5 mL). To this solution was added a CH₂Cl₂ (0.5 mL) solution of 13 (1.00 mmol) and then a CH₂Cl₂ (1.0 mL) solution of 12 (0.20 mmol) was added dropwise over 20 min at r.t. After stirring at r.t. for 1 h, the resulting solution was concentrated and product 14 was isolated by preparative TLC. Product (-)-14a: mp 41.0-41.9 °C; [α]²⁵ _D -334.7° (c = 1.53, CHCl₃; 90% ee). IR (neat): 2953, 1731, 1434, 1253, 1206, 1169, 1007 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 4.50 (d, J = 12.9 Hz, 2 H), 4.43 (d, J = 12.9 Hz, 2 H), 3.68 (s, 6 H), 3.55 (s, 2 H), 1.82 (s, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 172.7, 132.4, 117.3, 70.0, 52.2, 47.8, 19.2. HPLC: CHIRALCEL OD-H, hexane-i-PrOH = 97:3; 1.0 mL/min; t _R = 10.2 min (minor isomer),
t _R = 11.8 min (major isomer). Product (-)-14b: mp 128.9-130.5 °C; [α]²⁵ _D -175.2° (c = 4.78, CHCl₃; 82% ee). IR (neat): 2952, 1732, 1430, 1346, 1305, 1259, 1160 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 7.73 (d, J = 7.8 Hz, 2 H), 7.35 (d, J = 7.8 Hz, 2 H), 3.98 (d, J = 12.6 Hz, 2 H), 3.91 (d, J = 12.6 Hz, 2 H), 3.91 (s, 6 H), 3.52 (s, 2 H), 2.44 (s, 3 H), 1.81 (s, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 172.1, 143.6, 132.9, 129.7, 129.0, 127.6, 120.0, 52.2, 50.1, 47.2, 21.4, 19.1. HPLC: CHIRALPAK AD-H, hexane-i-PrOH = 90:10; 1.0 mL/min; t _R = 21.0 min (major isomer), t _R = 23.5 min (minor isomer). Product (-)-14c: [α]²⁵ _D -229.0° (c = 0.59, CHCl₃; 98% ee). IR (neat): 2954, 1733, 1435, 1253, 1201, 1165 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 3.72 (s, 6 H), 3.66 (s, 6 H), 3.50 (s, 2 H), 3.01 (d, J = 16.5 Hz, 2 H), 2.94 (d, J = 16.5 Hz, 2 H), 1.85 (s, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 172.9, 171.8, 132.4, 119.8, 58.7, 52.8, 52.1, 47.9, 37.1, 19.2. HPLC: CHIRALPAK AD-H, hexane-i-PrOH = 97:3; 1.0 mL/min; t _R = 12.5 min (major isomer),
t _R = 15.9 min (minor isomer).

<A NAME="RU02407ST-18">18</A>

Products 16 and (-)-17: [α]²⁵ _D -25.3° {CHCl₃, c = 0.160 [based on the calculated content of (-)-17], >99% ee}. ¹H NMR (300 MHz, CDCl₃): δ = 3.81 (d, J = 12.6 Hz, 2 H, 17), 3.80 (d, J = 12.6 Hz, 2 H, 16), 3.58 (d, J = 12.6 Hz, 2 H, 17), 3.57 (d, J = 12.6 Hz, 2 H, 16), 3.43 (d, J = 9.0 Hz, 2 H, 17), 3.27 (d, J = 9.0 Hz, 2 H, 16), 2.74 (d, J = 9.0 Hz, 2 H, 17), 2.59 (d, J = 9.0 Hz, 2 H, 16), 2.44 (s, 6 H, 16), 2.43 (s, 6 H, 17), 1.48-1.63 (m, 2 H + 2 H, 16 + 17), 1.19-1.33 (m, 2 H + 2 H, 16 + 17), 0.90 (s, 6 H + 6 H, 16 + 17). HPLC: CHIRALPAK AD-H, hexane-i-PrOH = 90:10; 1.0 mL/min; t _R = 81.8 min [minor dl-isomer (+)-17], t _R = 98.5 min (meso-isomer 16), t _R = 109.8 min [major dl-isomer (-)-17].