Stereoselective Cross-Coupling Reaction of 2,4-Diaryl-1,1-diboryl-1,3-butadienes: Stereocontrolled Approach to 1,3,4,6-Tetraarylated 1,3,5-Hexatrienes

Abstract

The boryl group cis to the C(3)=C(4) group in 2,4-aryl-1,1-bis(pinacolatoboryl)-1,3-butadienes undergoes Pd-catalyzed cross-coupling reaction with aryl iodides stereoselectively at room temperature, giving rise to the corresponding boronates as a single diastereomer. Subsequent coupling of the boronates with alkenyl ­iodides allows us to synthesize 1,3,4,6-tetraarylated 1,3,5-hexa­trienes stereoselectively.

Key words

alkenes - boron - cross-coupling - palladium - stereoselectivity

References and Notes

• 1a Electronic Materials: The Oligomer Approach   Müllen K. Wegner G. Wiley-VCH; Weinheim: 1998. 
• 1b Tour JM. Chem. Rev.  1996,  96:  537 
• 1c Martin RE. Diederich F. Angew. Chem. Int. Ed.  1999,  38:  1350 
• 1d Bendikov M. Wudl F. Perepichka DF. Chem. Rev.  2004,  104:  4891 
• 1e Nielsen MB. Diederich F. Chem. Rev.  2005,  105:  1837 
• 2 Shimizu M. Nakamaki C. Shimono K. Schelper M. Kurahashi T. Hiyama T. J. Am. Chem. Soc.  2005,  127:  12506 
• Diborylalkenes 1 were prepared by 1,1-diborylation of the corresponding 1,1-dibromo-1-alkenes with bis(pinacolato)diboron. See:
• 3a Hata T. Kitagawa H. Masai H. Kurahashi T. Shimizu M. Hiyama T. Angew. Chem. Int. Ed.  2001,  40:  790 
• 3b Kurahashi T. Hata T. Masai H. Kitagawa H. Shimizu M. Hiyama T. Tetrahedron  2002,  58:  6381  Compound 1a: mp 119.9 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.00 (s, 12 H), 1.34 (s, 12 H), 6.20 (d, J = 15.6 Hz, 1 H), 7.20-7.36 (m, 10 H), 7.87 (d, J = 15.6 Hz, 1 H). ¹³C NMR (67.8 MHz, CDCl₃): δ = 24.3, 25.0, 83.1, 83.2, 127.0, 127.6, 127.8, 128.5, 129.1, 132.4, 135.0, 137.5, 142.5, 163.6. IR (KBr): 2976, 2930, 2357, 2340, 1556, 1489, 1447, 1379, 1360, 1296, 1257, 1142, 984, 854, 756, 698 cm^-1. MS (EI, 70 eV): m/z (%) = 459 (8) [M⁺ + 1], 458 (28) [M⁺], 457 (13) [M⁺ - 1], 330 (100). HRMS: m/z calcd for C₂₈H₃₆B₂O₄ [M⁺]: 458.2800; found: 458.2794. Anal. Calcd for C₂₈H₃₆B₂O₄: C, 73.40; H, 7.92. Found: C, 73.41; H, 7.97
• 6 Stereochemical assignment of 2b-n is deduced from that of 2a
• 8 Milne JE. Buchwald SL. J. Am. Chem. Soc.  2004,  126:  13028 
• 9 In contrast, a cyclohexane or CHCl₃ solution of 3a exhibited no fluorescence. See: InTong H. Dong Y. Häusler M. Lam JWY. Sung HH.-Y. Williams ID. Sun J. Tang BZ. Chem. Commun.  2006,  1133 
• 10a Mitschke U. Bäuerle P. J. Mater. Chem.  2000,  10:  1471 
• 10b Shirota Y. J. Mater. Chem.  2005,  15:  75 

4

Typical Procedure
A solution of 1a (0.10 g, 0.22 mmol), iodobenzene (47 mg, 0.23 mmol), Pd(PPh₃)₄ (13 mg, 0.011 mmol), and 3 M KOH aq (0.22 mL, 0.65 mmol) in THF (5 mL) was stirred at r.t. for 8 h before quenching with sat. aq NH₄Cl (20 mL). The aqueous layer was extracted with EtOAc (3 × 20 mL), washed with sat. NaCl aq (20 mL), and dried over anhyd MgSO₄. Removal of the organic solvents in vacuo followed by PTLC (hexane-EtOAc, 10:1) gave 2a (72 mg, 76% yield) as a colorless solid; mp 98.0 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.00 (s, 12 H), 6.22 (d, J = 16.0 Hz, 1 H), 7.21-7.41 (m, 15 H). ¹³C NMR (67.8 MHz, CDCl₃): δ = 24.4, 83.4, 126.5, 126.5, 126.7, 127.3, 127.6, 127.9, 128.1, 128.5, 128.8, 129.5, 130.2, 134.2, 137.4, 140.4, 141.6, 148.5. IR (KBr): 3051, 3023, 2976, 2928, 2362, 2343, 1952, 1730, 1595, 1576, 1491, 1442, 1360, 1308, 1267, 1213, 1142, 1074, 1024, 974, 859, 752, 700, 579 cm^-1. MS (EI, 70 eV): m/z (%) = 409 (21) [M⁺ + 1], 408 (71) [M⁺], 280 (100). HRMS: m/z calcd for C₂₈H₂₉BO₂ [M⁺]: 408.2261; found: 408.2268. Anal. Calcd for C₂₈H₂₉BO₂: C, 82.36; H, 7.16. Found: C, 82.34; H, 7.16.

5

Crystal data of 2a and 3a are deposited at Cambridge Crystallographic Data Centre as CCDC-619246 and CCDC-619193.

7

Typical Procedure
A solution of 2a (10 mg, 0.023 mmol), β-iodostyrene (8.0 mg, 0.035 mmol), Pd[P(t-Bu)₃]₂ (1.0 mg, 1.8 µmol, 8 mol%) and 3 M aq KOH solution (23 µL, 0.069 mmol) in THF (1 mL) was stirred at 60 °C for 12 h before quenching with sat. aq NH₄Cl solution (10 mL). The aqueous layer was extracted with EtOAc (3 × 10 mL), washed with sat. aq NaCl solution (10 mL). The organic layer was separated, dried over anhyd MgSO₄, and concentrated in vacuo. The residue was purified by preparative TLC (hexane-EtOAc, 10:1) to give 3a (8.5 mg, 96% yield) as a 95:5 mixture of 1E,3E,5E- and 1Z,3Z,5E-isomers as a pale yellow solid. The recrystallization of the mixture from hexane gave (1E,3E,5E)-3a; mp 173.1 °C (hexane). R _f = 0.63 (hexane-EtOAc, 10:1). ¹H NMR (400 MHz, CDCl₃): δ = 6.07 (d, J = 16.0 Hz, 2 H), 6.92 (d, J = 16.0 Hz, 2 H), 7.10-7.53 (m, 20 H). ¹³C NMR (100 MHz, CDCl₃): δ = 126.5, 127.3, 127.4, 128.3, 128.5, 130.4, 130.8, 132.4, 137.6, 138.9, 141,0. IR (KBr): 3030, 1595, 1574, 1489, 1442, 1315, 1146, 1070, 1024, 961, 908, 750, 704, 689, 577, 556, 525 cm^-1. MS (EI, 70 eV): m/z (%) = 386 (7) [M⁺ + 2], 385 (31) [M⁺ + 1], 384 (100) [M⁺]. HRMS: m/z calcd for C₃₀H₂₄ [M⁺]: 384.1878; found: 384.1880. Anal. Calcd for C₃₀H₂₄: C, 93.71; H, 6.29. Found: C, 93.72; H, 6.53.