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Synlett 2006(20): 3389-3394  
DOI: 10.1055/s-2006-956465
LETTER
© Georg Thieme Verlag Stuttgart · New York

P-Chirogenic α-Carboxyphosphine Boranes as Effective Pre-Ligands in Palladium-Catalyzed Asymmetric Reactions

Franck Dolhema, Magnus J. Johanssona, Thomas Antonssonb, Nina Kann*a
a Organic Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, 41296 Göteborg, Sweden
Fax: +46(31)7723657; e-Mail: kann@chalmers.se;
b Medicinal Chemistry, AstraZeneca R&D Mölndal, 43183 Mölndal, Sweden
Further Information

Publication History

Received 6 July 2006
Publication Date:
08 December 2006 (online)

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Abstract

P-Chirogenic α-carboxyphosphine boranes have been applied directly as pre-ligands in palladium-catalyzed asymmetric allylic alkylation and amination reactions, with in situ deprotection of the borane moiety. Using dimethylmalonate as the nucleophile afforded the alkylated product in up to 91% ee, while reaction with benzylamine gave amination in up to 94% ee. In both cases, the opposite enantiomer could be accessed in high ee by using the antipode of the ligand, easily accessible by exchanging (-)-sparteine for a (+)-sparteine mimic in the desymmetrization process used for preparing the ligands.

Key words

asymmetric catalysis - allylic alkylation - allylic amination - phosphine ligands - P-chiral

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Compounds 2a,c,d,22 2b,23 2e 24 and 2f 3 were prepared as described in the literature. Compounds 2g and 2h were prepared via the corresponding phosphine sulfides using the same procedure as for 2f.3 Compound 2i was prepared from PCl3 and 2,4,6-tri(isopropyl)-1-phenylmagnesium bromide, following a literature procedure.25 P , P -Dimethyl(2-biphenyl)phosphine Sulfide (Precursor to 2g)
Waxy solid; 89% yield; mp 99-101 °C. IR (KBr): ν = 2986, 2873, 1770, 1458, 1383, 1140 cm-1. 1H NMR (CDCl3): δ = 1.58 (d, J = 13.2 Hz, 6 H), 7.18-7.47 (m, 8 H), 8.20-8.27 (m, 1 H). 13C NMR (CDCl3): δ = 23.56 (d, J = 56.9 Hz), 127.39 (d, J = 12.2 Hz), 127.87, 128.05, 129.54, 130.80 (d, J = 3.1 Hz), 131.23 (d, J = 9.1 Hz), 131.96 (d, J = 75.8 Hz), 132.23 (d, J = 12.1 Hz), 140.88 (d, J = 3.1 Hz), 144.07 (d, J = 8.3 Hz). Anal. Calcd for C14H15PS: C, 68.27; H, 6.14. Found: C, 68.21; H, 6.10.
P , P -Dimethyl(2-biphenyl)phosphineborane (2g) White powder; 90% yield; mp 91-93 °C. IR (KBr): ν = 2918, 2383, 1466, 1433, 1300, 1064, 952, 921 cm-1. 1H NMR (CDCl3): δ = 1.22 (d, J = 10.0 Hz, 6 H), 7.24-7.52 (m, 8 H), 7.97 (dd, J = 13.6, 8.8 Hz, 1 H). 13C NMR (CDCl3): δ = 13.76 (d, J = 39.4 Hz), 127.48 (d, J = 11.4 Hz), 127.97 (s), 128.10 (s), 128.79 (d, J = 50.0 Hz), 129.53 (s), 130.56 (d, J = 2.2 Hz), 131.24 (d, J = 6.1 Hz), 133.61 (d, J = 15.9 Hz), 141.09 (d, J = 2.3 Hz), 146.29 (d, J = 2.3 Hz). Anal. Calcd for C14H18BP: C, 73.72; H, 7.95. Found: C, 73.64; H, 8.06.
P , P -Dimethyl[3,5-di( tert -butyl)-6-methoxyphenyl]phos-phine Sulfide (Precursor to 2h) White powder; 25% yield; mp 150-155 °C. IR (KBr): ν = 2970, 2840, 1590, 1576, 1475, 1270, 1240, 1000, 920 cm-1. 1H NMR (CDCl3): δ = 1.33 (s, 18 H), 1.86 (d, J = 13.2 Hz, 6 H), 3.60 (s, 3 H), 7.63 (d, J = 14.0 Hz, 2 H). 13C NMR (CDCl3): δ = 23.21 (d, J = 56.9 Hz), 32.00 (s), 36.20 (s), 64.54 (s), 126.90 (d, J = 83.5 Hz), 128.58 (d, J = 12.9 Hz), 144.36 (d, J = 12.1 Hz), 162.55 (d, J = 3.8 Hz). Anal. Calcd for C17H29OPS: C, 65.35; H, 9.36. Found: C, 65.12; H, 9.27.
P , P -Dimethyl[3,5-di( tert -butyl)-6-methoxyphenyl]phos-phineborane (2h) White powder; 79% yield; mp 120-126 °C. IR (KBr): ν = 2960, 2367, 1500, 1400, 1230, 1150, 1067, 1006, 930 cm-1. 1H NMR (CDCl3): δ = 1.45 (s, 18 H), 1.55 (d, J = 10.0 Hz, 6 H), 3.72 (s, 3 H), 7.59 (d, J = 11.6 Hz, 2 H). 13C NMR (CDCl3): δ = 13.14 (d, J = 38.7 Hz), 31.68 (s), 35.83 (s), 64.15 (s), 123.66 (d, J = 58.5 Hz), 129.14 (d, J = 11.4 Hz), 144.22 (d, J = 9.9 Hz), 162.0 (br s). Anal. Calcd for C17H32BOP: C, 69.40; H, 10.96. Found: C, 69.26; H, 11.07.
P , P -Dimethyl[2,4,6-tri(isopropyl)phenyl]phosphine-borane (2i). White powder; 60% yield; mp 158-162 °C. IR (KBr): ν = 2961, 2370, 1601, 1555, 1458, 1150, 1074, 939 cm-1. 1H NMR (CDCl3): δ = 1.10 (br m, 3 H), 1.26 (d, J = 7.2 Hz, 6 H), 1.27 (d, J = 6.8 Hz, 12 H), 1.75 (d, J = 9.6 Hz, 6 H), 2.89 (q, J = 6.8 Hz, 1 H), 3.88 (q, J = 6.8 Hz, 2 H), 7.09 (d, J = 3.2 Hz, 2 H). 13C NMR (CDCl3): δ = 16.94 (d, J = 41.0 Hz), 23.58 (s), 25.07 (s), 30.56 (d, J = 6.1 Hz), 34.03 (s), 121.0 (d, J = 49.8 Hz), 123.12 (d, J = 8.3 Hz), 151.75 (d, J = 2.3 Hz), 154.33 (d, J = 9.1 Hz). Anal. Calcd for C17H32BP: C, 73.39; H, 11.59. Found: C, 73.31; H, 11.68.

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Compounds 3a-d were prepared according the literature,22,23 affording ligands of the following enantiopurity as determined by reduction to the corresponding alcohol: 3a (96%), 3b (85%), 3c (90%), 3d (92%).
General Procedure for 3e-i
To a solution of (-)-sparteine or (+)-1 (1.12 equiv) in anhyd Et2O (2.5 mL/mmol) under argon at -78 °C was added a 1.3 M solution (1.1 equiv) of sec-BuLi in hexane. The mixture was stirred for 30 min at -78 °C before the slow addition of a Et2O solution (3 mL/mmol) of the prochiral phosphine borane. The mixture was stirred at -78 °C for 3 h before CO2 was bubbled through the solution. The flask was slowly warmed up to r.t. over a period of 1 h and then acidified with 1 M HCl. The aqueous layer was extracted with EtOAc (3 ×). The pH of the combined organic layers was adjusted to pH 12 using sat. Na2CO3. After phase separation, the aqueous layer was acidified with HCl and extracted with EtOAc. The organic layer was washed with brine, dried over Na2SO4, filtered and concentrated to afford the P-chirogenic α-carboxyphosphine boranes. To measure the enantiomeric excess, the ligands were first reduced to their corresponding alcohol using BH3·DMS, and then subjected to analysis by HPLC on a Chiralcel OD-H column, eluting with n-hexane-i-PrOH.

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( S )-(-)-Mesityl(ethanoic acid)methylphosphine-borane (3e) Transparent gel; 90% yield. IR (KBr): ν = 3100, 2960, 2720, 2520, 2170, 1705, 1640, 1340, 1180, 995 cm-1. 1H NMR (CDCl3): δ = 1.1 (m, 3 H), 1.88 (d, J = 9.2 Hz, 3 H), 2.24 (s, 3 H), 2.55 (s, 6 H), 2.98 (dd, J = 14.0, 9.6 Hz, 1 H), 3.18 (dd, J = 14.0, 9.6 Hz, 1 H), 6.85 (d, J = 3.2 Hz, 2 H), 10.96 (br s, 1 H). 13C NMR (CDCl3): δ = 15.9 (d, J = 39.5 Hz), 21.04 (s), 24.10 (d, J = 4.6 Hz), 35.61 (d, J = 27.3 Hz), 121.14 (d, J = 47.0 Hz), 131.38 (d, J = 9.1 Hz), 141.16 (d, J = 3.1 Hz), 143.26 (d, J = 9.9 Hz), 173.69 (d, J = 4.5 Hz). [α]D 21 -15.0 (c 1, CHCl3, 80% ee). Anal. Calcd for C12H20BO2P: C, 60.54; H, 8.47. Found: C, 60.28; H, 8.43. Retention times for alcohol (90% n-hexane, 10% i-PrOH): 0.5 mL/min, 228 nm, t R (-) = 19.4 min, t R (+) = 20.4 min.
( S )-(+)-1-Naphtyl(ethanoic acid)methylphosphine-borane (3f) Recrystallized from n-hexane-Et2O; white solid, 86% yield; mp 92-98 °C. IR (KBr): ν = 3000, 2414, 1834, 1713, 1500, 1424, 1260, 1130, 920, 840, 800, 770 cm-1. 1H NMR (CDCl3): δ = 1.1 (br m, 3 H), 1.95 (d, J = 9.6 Hz, 3 H), 3.09 (dd, J = 13.6, 9.2 Hz, 1 H), 3.31 (dd, J = 13.6, 10.8 Hz, 1 H), 7.49 (dt, J = 8.8, 1.6 Hz, 1 H), 7.55 (dt, J = 8.8, 0.8 Hz, 1 H), 7.63 (dt, J = 8.8, 1.2 Hz, 1 H), 7.87 (dd, J = 7.2, 0.8 Hz, 1 H), 7.91 (d, J = 7.6 Hz, 1 H), 7.99 (d, J = 8.4 Hz, 1 H), 8.40 (d, J = 8.4 Hz, 1 H), 10.00 (br s, 1 H). 13C NMR (CDCl3): δ = 11.14 (d, J = 39.5 Hz), 33.86 (d, J = 25.8 Hz), 123.79 (d, J = 50.1 Hz), 124.8 (d, J = 12.2 Hz), 125.18 (d, J = 6.8 Hz), 126.46 (s), 127.36 (s), 129.64 (s), 132.75 (d, J = 10.6 Hz), 132.85 (d, J = 9.1 Hz), 133.20 (d, J = 2.3 Hz), 133.71 (d, J = 6.9 Hz), 173.01 (d, J = 5.3 Hz). [α]D 21 +24.0 (c 1, CHCl3, 94% ee). Anal. Calcd for C13H16BO2P: C, 63.46; H, 6.55. Found: C, 63.34; H, 6.43. Retention times for alcohol (90% n-hexane, 10% i-PrOH): 1 mL/min, 224 nm, t R (+) = 16.99 min, t R (-) = 20.38 min.
( S )-(+)-[(2-Phenyl)phenyl](ethanoic acid)methylphosphine-borane (3g) Recrystallized from n-hexane-Et2O; white solid; 93% yield; mp 98-106 °C. IR (KBr): ν = 3000, 2360, 1695, 1440, 1300, 1130, 1064, 920, 840, 800, 770 cm-1. 1H NMR (CDCl3): δ = 0.90 (br m, 3 H), 1.35 (d, J = 10.0 Hz, 3 H), 2.68 (dd, J = 10.0, J = 2.0 Hz, 2 H), 7.28 (ddd, J = 7.6, J = 3.2, J = 1.2 Hz, 1 H), 7.36 (m, 2 H), 7.48 (m, 5 H), 7.94 (dd, J = 13.6, J = 7.6 Hz, 1 H), 10.00 (br s, 1 H). 13C NMR (CDCl3): δ = 11.97 (d, J = 38.7 Hz), 34.28 (d, J = 28.1 Hz), 125.56 (d, J = 50.1 Hz), 127.46 (d, J = 12.2 Hz), 127.96 (s), 128.25 (s), 129.36 (s), 131.01 (s), 131.32 (d, J = 6.8 Hz), 133.68 (d, J = 16.7 Hz), 140.39 (s), 146.36 (s), 172.6 (d, J = 5.3 Hz). [α]D 21 +13.0 (c 1, CHCl3, 95% ee). Anal. Calcd for C15H18BO2P: C, 66.21; H, 6.67. Found: C, 66.03; H, 6.58. Retention times for alcohol (95% n-hexane, 5% i-PrOH): 0.4 mL/min, 220 nm, t R (-) = 59.80 min, t R (+) = 62.30 min.
( S )-(-)-[3,5-Di( tert -butyl)-6-methoxyphenyl](ethanoic acid)methylphosphine-borane (3h) Precipitated in n-hexane; white solid; 89% yield; mp 118-121 °C. IR (KBr): ν = 2960, 2360, 1725, 1410, 1225, 1130, 1064, 1000, 920, 840, 800, 770 cm-1. 1H NMR (CDCl3): δ = 0.88 (br m, 3 H), 1.42 (s, 18 H), 1.77 (d, J = 10.0 Hz, 3 H), 2.94 (dd, J = 10.4, 6.4 Hz, 2 H), 3.71 (s, 3 H), 7.63 (d, J = 12.0 Hz, 2 H), 9.05 (br s, 1 H). 13C NMR (CDCl3): δ = 10.41 (d, J = 37.9 Hz), 31.78 (s), 35.13 (d, J = 26.5 Hz), 36.04 (s), 64.39 (s), 120.0 (d, J = 50.0 Hz), 130.03 (d, J = 11.4 Hz), 144.65 (d, J = 11.4 Hz), 162.8 (s)173.7 (d, J = 5.3 Hz). [α]D 21 -21.5 (c 1, CHCl3, 81% ee). Anal. Calcd for C18H32BO3P: C, 63.92; H, 9.57. Found, C: 64.04; H, 9.49. Retention times for alcohol (99% n-hexane, 1% i-PrOH): 0.75 mL/min, 224 nm, t R (-) = 30.7 min, t R (+) = 32.9 min.
( S )-(-)-[2,3,6-Tri(isopropyl)phenyl](ethanoic acid)methylphosphine-borane (3i) Recrystallized from n-hexane-Et2O; white solid; 72% yield; mp 116-123 °C. IR (KBr): ν = 3000, 2414, 1834, 1713, 1500, 1424, 1260, 1130, 920, 840, 800, 770 cm-1. 1H NMR (CDCl3): δ = 1.20 (br m, 3 H), 1.26 (d, J = 6.8 Hz, 6 H), 1.26 (dd, J = 6.4, 4.8 Hz, 12 H), 1.90 (d, J = 8.8 Hz, 3 H), 2.89 (q, J = 6.8 Hz, 1 H), 3.13 (d, J = 9.6 Hz, 2 H), 3.85 (dq, J = 6.8, 2.0 Hz, 2 H), 7.10 (d, J = 3.2 Hz, 2 H), 8.90 (br s, 1 H). 13C NMR (100 MHz, CDCl3): δ = 16.15 (d, J = 40.2 Hz), 23.79 (s), 25.26 (d, J = 11.4 Hz), 30.84 (d, J = 6.0 Hz), 34.30 (s), 36.53 (d, J = 28.1 Hz), 120.14 (d, J = 47.8 Hz), 123.76 (d, J = 9.1 Hz), 152.76 (s), 155.04 (d, J = 9.9 Hz), 173.4 (d, J = 3.0 Hz). [α]D 21 -3.6 (c 1, CHCl3, 71% ee). Anal. Calcd for C18H32BO2P: C, 67.09; H, 10.01. Found: C, 66.87; H, 9.89. Retention times for alcohol (99% n-hexane, 1% i-PrOH): 0.75 mL/min, 227 nm, t R (-) = 24.25 min, t R (+) = 25.91 min.

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( S )-(-)- tert -Butyl(ethanoic acid)methylphosphine-borane (4) Recrystallized from n-hexane-Et2O; white solid; 86% yield; mp 107-112 °C. IR (KBr): ν = 2970, 2910, 2370, 1705, 1295, 925 cm-1. 1H NMR (CDCl3): δ = 0.42 (br q, J = 88.0 Hz, 3 H), 1.19 (dd, J = 14.4, 4.0 Hz, 9 H), 1.41 (dd, J = 10.0, 3.6 Hz, 3 H), 2.72 (m, 2 H), 9.93 (br s, 1 H). 13C NMR (CDCl3): δ = 5.67 (d, J = 32.4 Hz), 24.96 (s), 28.15 (d, J = 31.9 Hz), 29.26 (d, J = 21.3 Hz), 174.17 (d, J = 4.5 Hz). [α]D 21 -9.2 (c 1, CHCl3, 90% ee). Anal. Calcd for C7H18BO2P: C, 47.77; H, 6.14. Found: C, 47.86; H, 10.30.

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General Procedure for the Palladium-Catalyzed Allylic Asymmetric Alkylation Reactions. To the phosphine-borane (0.019 mmol, 9.5% mol) under an argon atmosphere was added an anhyd degassed solution of Pd(AcO)2 (2.3 mg, 5 mol%) in 0.8 mL THF. This mixture was stirred at r.t. for 15 min whereupon 1,3-diphenyl-propenyl acetate (5, 50 µL, 1 equiv) or 1,3-diphenylpropenyl ethylcarbonate (6, 56 µL, 1 equiv) was added. After 15 min at r.t., dimethylmalonate (70 µL, 3 equiv) and BSA (150 µL, 3 equiv) were successively added. The mixture was stirred at r.t. and the reaction was monitored by HPLC and TLC. The mixture was purified by flash chromatography on silica gel, eluting with PE-EtOAc 95:5. The enantiomeric excess was determined by chiral HPLC, Chiralpak AD-H, n-hexane-i-PrOH (9:1) 1 mL/min, 254 nm, retention time: t R (R) = 9.60 min, t R (S) = 13.22 min.

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General Procedure for the Palladium-Catalyzed Asymmetric Allylic Amination Reactions. To the phosphine-borane (0.019 mmol, 9.5 mol%), evacuated and flushed with argon, was added an anhyd, degassed THF solution (0.8 mL) of Pd(OAc)2 (2.3 mg, 5 mol%). This mixture was stirred at r.t for 15 min, followed by the addition of 1,3-diphenylpropenyl acetate (5, 50 µL, 1 equiv) or 1,3-diphenylpropenyl ethylcarbonate (6, 56 µL, 1 equiv). After 15 min the reaction was cooled to 5 °C and benzylamine (66 µL) was added. The mixture was stirred at 5 °C and monitored by HPLC and TLC. The mixture was purified by flash chromatography on silica gel, eluting with PE-EtOAc 95:5. The enantiomeric excess was determined by chiral HPLC, Chiralpak AD-H, n-hexane-i-PrOH (9:1) 0.35 mL/min, 254 nm, retention time : t R (S) = 28.64 min, t R (R) = 33.92 min.