Permethyltitanocene-bis(trimethylsilyl) acetylene, an efficient catalyst for the head-to-tail dimerization of 1-alkynes
摘要:
In the series of the (C(5)H(5-n)Me(n))(2)Ti[eta(2)-C-2(SiMe(3))(2)] (n=0-5) (1A-1F) complexes only (C(5)Me(5))(2)Ti[eta(2)-C-2(SiMe(3))(2)] (1F) catalyzed the linear head-to-tail dimerization of 1-alkynes. A selectivity of better than 98% and a turnover number (TN) in the range 1200-1500 mmol of 1-alkyne per mmol of Ti was obtained after 14 d at 30 degrees C for 1-pentyne, 1-hexyne, cyclohexylethyne, phenylethyne and trimethylsilylethyne. Among other complexes in the series, 1A-1D attained TN less than or equal to 25 after 5 d at 30 degrees C and produced mainly a mixture of 1,3,5- and 1,2,4-trisubstituted benzene derivatives. The complex (C(5)HMe(4))(2)Ti[eta(2)-C-2(SiMe(3))(2)] (1E) afforded ca. 20% of the head-to-tail dimers in addition to cyclotrimers but its activity was extremely low, corresponding to TN similar to 7. The rate of dimerization by 1F is controlled by the slow exchange rate of 1-alkynes with bis(trimethylsilyl)acetylene.
A mixed decamethyltitanocene(III) isonitrile alkynyl complex (7) was synthesized by the sequential introduction of the isonitrile and alkynyl ligands. Direct synthesis results in the formation of the diamagnetic decamethyltitanocene bis(isonitrile) (2) and bis(alkynyl) (3) complexes. Compound 7 undergoes disproportionation at room temperature to give 2 and 3. All complexes were fully characterized