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Synlett 2007(11): 1715-1719  
DOI: 10.1055/s-2007-984512
LETTER
© Georg Thieme Verlag Stuttgart · New York

Construction of Extended π-Conjugation Systems Utilizing Novel Multicarbene Complexes of Titanium

Akitoshi Ogata, Shintaro Anno, Takeshi Kurata, Song Xu, Akira Tsubouchi, Takeshi Takeda*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
Fax: +81(42)3887034; e-Mail: takeda-t@cc.tuat.ac.jp;
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Publication History

Received 10 April 2007
Publication Date:
25 June 2007 (online)

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Abstract

Titanium-multicarbene complexes, namely organo­titanium species having a plurality of titanium-carbene complex substructures, were easily prepared by the reaction of aromatic ­nuclei possessing spatially separated thioacetal moieties with the ­titanocene(II) reagent Cp2Ti[P(OEt)3]2. Reaction of these multi­carbene complexes with aromatic ketones gave various highly conjugated compounds in good yields.

Key words

carbene complexes - conjugation - olefination - thioacetals - titanium

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General Procedure
Cp2TiCl2 (398 mg, 1.6 mmol), magnesium turnings (43 mg, 1.76 mmol), and finely powdered 4 Å MS (128 mg) were placed in a flask and dried by heating with a heat gun in vacuo (2-3 mmHg). After cooling, THF (2.4 mL) and P(OEt)3 (0.55 mL, 3.2 mmol) were added successively with stirring under argon. During the addition, the reaction mixture was cooled in a water bath so that the temperature was maintained between 20 °C and 30 °C. After stirring for 3 h at 25 °C, a THF (1.0 mL) solution of the thioacetal 3c (108 mg, 0.2 mmol) was added. Then, a THF (4.0 mL) solution of 7b (261 mg, 0.8 mmol) was added dropwise over 10 min and the reaction mixture was stirred for 3 h under reflux. The reaction was quenched by addition of 1 M NaOH and the insoluble materials were filtered off through Celite® and washed with CHCl3. The layers were separated, and the aqueous layer was extracted with CHCl3. After the combined organic extracts were dried with Na2SO4 and concentrated, the remaining triethyl phosphate, formed by the oxidation of triethyl phosphite, was removed by azeotropic distillation with MeOH. Purification of the residue by PTLC on silica gel (hexane-CHCl3, 96:4) gave 8h as yellow crystals (102 mg, 64%), mp 130-132 °C. 1H NMR (300 MHz, CDCl3): δ = 0.96 (t, J = 7.4 Hz, 6 H), 0.99 (t, J = 7.4 Hz, 6 H), 1.41-1.59 (m, 8 H), 1.67-1.85 (m, 8 H), 3.94 (t, J = 6.2 Hz, 4 H), 3.96 (t, J = 6.1 Hz, 4 H), 6.70 (s, 2 H), 6.75-6.89 (m, 12 H), 7.07 (d, J = 8.4 Hz, 4 H), 7.20 (d, J = 8.4 Hz, 4 H). 13C NMR (75 MHz, CDCl3): δ = 13.8, 13.9, 19.2, 19.3, 31.3, 31.4, 67.6, 67.7, 114.1, 114.4, 125.8, 128.8, 129.0, 131.6, 136.0, 136.4, 141.6, 158.5, 158.8. IR (KBr): ν = 2956, 2932, 2871, 1604, 1570, 1509, 1467, 1390, 1284, 1247, 1175, 1146, 1111, 1070, 1027, 1010, 973, 834, 813, 617 cm-1. Anal. Calcd for C50H58O4: C, 83.06; H, 8.09. Found: C, 82.82; H, 8.12.