A Novel Approach to Stilbenoid Dendrimer Core Synthesis

Abstract

A new synthetic protocol for the one-pot, stereoselective synthesis of 1,3,5-tris[(E)-4-halostyryl]benzene and 1,2,4,5-tetrakis[(E)-4-halostyryl]benzene derivatives as stilbenoid dendrimer cores via palladium-catalyzed Hiyama cross-coupling of aryl tri- or tetrahalides with 1,3-bis[(E)-4-halostyryl]disiloxanes is described.

cross-coupling - palladium catalyst - stilbenoid dendrimer cores - silicon derivatives

References and Notes

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<A NAME="RG24008ST-17">17</A>

Typical Procedure for the Synthesis of 1,3-Bis[( E )-4-halostyryl]disiloxanes: The glass reactor (10-mL, two-necked, round-bottomed flask equipped with a magnetic stirring bar, reflux condenser, argon bubbling tube and thermostated oil bath) was evacuated and flushed with argon. [RuH(Cl)(CO)(PPh₃)₃] (47.6 mg, 0.05 mmol), 1,3-divinyltetramethyldisiloxane (0.5 g, 2.5 mmol), styrene or
4-bromo(or chloro)styrene (10 mmol) and anhyd dioxane (5 mL) were added to the reactor. Then the reaction mixture was stirred and heated at 100 ˚C under argon flow. After 5 min, copper(I) chloride (CuCl; 9.9 mg, 0.1 mmol) was added as a co-catalyst. The synthesis process was carried out for the next 16 h. After the reaction was completed (GC-MS analysis) the volatiles were evaporated under vacuum and the crude product was chromatographed on silica gel (eluent: hexane-EtOAc, 10:1) to afford the analytically pure products.
1,3-Bis[( E )-4-bromostyryl]tetramethyldisiloxane (2): mp 56-60 ˚C. IR (KBr): 799.5, 844.8, 985.3, 1055.5, 1253.5, 1485.7, 1604.6, 2956.9, 3020.3 cm^-¹. ^¹H NMR (CDCl₃): δ = 0.24 (s, 12 H, SiMe), 6.41 (d, J = 19.1 Hz, 2 H, SiCH=), 6.86 (d, J = 19.2 Hz, 2 H, PhCH=), 7.27 (d, J = 7.6 Hz, 4 H, BrC₆H₄), 7.43 (d, J = 8.3 Hz, 4 H, BrC₆H₄). ^¹³C NMR (CDCl₃): δ = 0.9, 121.9, 127.9, 129.4, 131.5, 136.9, 142.9. MS (EI): m/z (%rel. int.) = 496 (7) [M⁺], 415 (32), 297 (60), 133 (100), 117 (37), 73 (50). HRMS: m/z [M⁺] calcd for C₂₀H₂₄ ⁷⁹Br^8¹BrOSi₂: 495.9712; found: 495.9685.
1,3-Bis[( E )-4-chlorostyryl)tetramethyldisiloxane (3): mp 51-54 ˚C. IR (KBr): 800.6, 845.1, 985.6, 1056.4, 1254.2, 1488.9, 1564.5, 1606.2, 2957.6, 3024.3 cm^-¹. ^¹H NMR (CDCl₃): δ = 0.25 (s, 12 H, SiMe), 6.40 (d, J = 19.2 Hz, 2 H, SiCH=), 6.88 (d, J = 19.2 Hz, 2 H, PhCH=), 7.28 (d, J = 8.8 Hz, 4 H, ClC₆H₄), 7.34 (d, J = 8.8 Hz, 4 H, ClC₆H₄). ^¹³C NMR (CDCl₃): δ = 0.9, 127.6, 128.6, 129.2, 133.7, 136.5, 142.8. MS (EI): m/z (%rel. int.) = 406 (21) [M⁺], 281 (90), 253 (100), 227 (59), 133 (98), 117 (62), 73 (98). HRMS:
m/z [M⁺] calcd for C₂₀H₂₄ ^³5Cl₂OSi₂: 406.0743; found: 406.0746.

<A NAME="RG24008ST-18">18</A>

Synthesis of PPV from 1,3-Bis[( E )-4-bromostyryl]tetramethyldisiloxane: [Pd_{2 (}dba)₃] (9.16 mg, 0.01 mmol), dioxane (4 mL), 1,3-bis[(E)-4-bromostyryl]tetramethyldisiloxane (2; 248 mg, 0.5 mmol), and tetrabutylammonium fluoride (320 mg, 1.2 mmol) were placed in an evacuated and flushed with argon, 10-mL flask. The mixture was heated at 80 ˚C for 12 h under an argon atmosphere. The degree of conversion of the substrates was estimated by GC and TLC analyses. Then the reaction mixture was cooled and the precipitated solid was filtered and washed extensively with acetone.

<A NAME="RG24008ST-19">19</A>

Typical Procedure for the One-Pot Synthesis of 1,3,5-Tris- or 1,2,4,5-Tetrakis[( E )-4-halostyryl)benzenes and Spectroscopic Data of Selected Products: The glass reactor (10-mL, two-necked, round-bottomed flask equipped with a magnetic stirring bar, reflux condenser, argon bubbling tube and thermostated oil bath) was evacuated and flushed with argon. [RuH(Cl)(CO)(PPh₃)₃] (9.52 mg, 0.01 mmol), 1,3-divinyltetramethyldisiloxane (0.1 g, 0.5 mmol), styrene or 4-bromo(or chloro)styrene (2.0 mmol) and anhyd dioxane (2 mL) were added to the reactor. Then the reaction mixture was stirred and heated at 100 ˚C under argon flow. After 5 min, CuCl (1.98 mg, 0.02 mmol) was added as a co-catalyst. The synthesis process was carried out for the next 24 h. After the reaction was completed (GC-MS or GC and TLC analyses), palladium catalyst [Pd_{2 (}dba)₃] (9.16 mg, 0.01 mmol), TBAF (320 mg, 1.2 mmol), dioxane (3 mL) and respective haloarene [1,3,5-tribromobenzene (78.7 mg, 0.25 mmol) or 1,2,4,5-tetra-iodobenzene (116 mg, 0.2 mmol)] were added and the mixture was heated at 80 ˚C (30 ˚C for 1,2,4,5-tetra-iodobenzene) for 16-48 h under an argon atmosphere. The degree of conversion of the substrates was estimated by GC and TLC analyses. The final product was separated using chromatography column with silica (THF-EtOAc).
1,3,5-Tris[( E )-4-chlorostyryl]benzene (6): mp 216-220 ˚C. IR (KBr): 806.3, 841.7, 960.2, 1090.4, 1490.7, 1585.1, 1668.2, 2924.2, 2957.7, 3024.9 cm^-¹. ^¹H NMR (CDCl₃): δ = 7.09 (d, J = 15.9 Hz, 3 H, C₆H₃CH=), 7.20-7.45 (m, 15 H, ClC₆H₄CH=), 7.65 (s, 3 H, C₆H₃). ^¹³C NMR (CDCl₃): δ = 124.8, 125.4, 127.6, 128.1, 128.8, 133.3, 135.5, 137.7. MS (EI): m/z (%rel. int.) = 486 (8) [M⁺], 364 (56), 350 (47), 220 (49), 205 (100), 73 (57). HRMS: m/z [M⁺] calcd for C₃₀H₂₁ ^³5Cl₃: 486.0709; found: 486.0694.
1,2,4,5-Tetrakis[( E )-4-bromostyryl]benzene (8): mp 265-268 ˚C. IR (KBr): 798.9, 845.4, 956.6, 1008.5, 1072.1, 1258.5, 1487.9, 1587.4, 1682.4, 1725.1, 2924.1, 2957.9, 3049.3 cm^-¹. ^¹H NMR (THF-d ₈): δ = 7.19 (d, J = 16.0 Hz, 4 H, C₆H₂CH=), 7.50-7.58 (m, 16 H, BrC₆H₄), 7.67 (d, J = 16.1 Hz, 4 H, BrC₆H₄CH=), 7.98 (s, 2 H, C₆H₂). ^¹³C NMR (THF-d ₈): δ = 121.9, 125.0, 127.2, 129.1, 130.9, 132.4, 136.3, 137.6. MS (EI): m/z (%rel. int.) = 802 (5) [M⁺], 633 (14), 308 (20), 196 (32), 185 (64), 91 (95), 57 (100). Anal. Calcd for C₃₈H₂₆Br₄: C, 56.89; H, 3.27. Found: C, 56.58; H, 3.03.
1,2,4,5-Tetrakis[( E )-4-chlorostyryl]benzene (9): mp 242-246 ˚C. IR (KBr): 808.2, 853.3, 960.3, 1012.2, 1091.7, 1492.4, 1592.4, 1667.7, 1686.8, 2924.6, 2955.8, 3027.2 cm^-¹. ^¹H NMR (C₆D₆): δ = 6.96 (d, J = 16.2 Hz, 4 H, C₆H₂CH=), 7.06-7.17 (m, 16 H, ClC₆H₄), 7.44 (d, J = 16.2 Hz, 4 H, ClC₆H₄CH=), 7.87 (s, 2 H, C₆H₂). ^¹³C NMR (C₆D₆): δ = 126.4, 127.1, 129.3, 129.7, 130.2, 134.2, 136.1, 138.9. MS (EI, %rel. int.): m/z = 622 (6) [M⁺], 248 (32), 178 (51), 139 (68), 125 (100). Anal. Calcd for C₃₈H₂₆Cl₄: C, 73.09; H, 4.20. Found: C, 72.81; H, 4.01.