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Synlett 2008(19): 3026-3030  
DOI: 10.1055/s-0028-1083630
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

A Novel Approach to Stilbenoid Dendrimer Core Synthesis

Wiesław Prukała*
Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Fax: +48(61)8291508; e-Mail: wprukala@amu.edu.pl;
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Publication History

Received 24 July 2008
Publication Date:
12 November 2008 (online)

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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.

Key words

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

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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 79Br8¹BrOSi2: 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.

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Synthesis of PPV from 1,3-Bis[( E )-4-bromostyryl]tetramethyldisiloxane: [Pd2 (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.

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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 [Pd2 (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.