Photoinduced generation of catalytic complexes from substituted-titanocene-bis(trimethylsilyl)ethyne complexes: Contribution to the mechanism of the catalytic head-to-tail dimerization of terminal alkynes

A number of substituted-titanocene-alkynyl-alkenyl complexes, [(eta(5)-C5Me 4R1)(2)Ti(eta(1)-C equivalent to CR2)(eta(1)-(E)-CH=CHR2)] (R-1 = H, Me, Ph , Bz; R-2 = CMe3, SiMe3, ferrocenyl; A type complexes), were obtained by re acting the corresponding bis(trimethylsilyl)ethyne complexes [(eta 5-C5Me4R 1)(2)Ti(eta(2)-Me3SiC equivalent to CSiMe3)] with 1-alkynes (RC)-C-2 equiva lent to CH in the dark at 60 degrees C. The complexes undergo a coupling of the carbyl ligands upon exposure to sunlight to give titanocene complexes with 1,4-disubstituted but-1-en-3-ynes, [(eta 5-C5Me4R1)(2)Ti(3,4-eta-(RC)- C-2 equivalent to CCH=CHR2)] (B type complexes). In contrast to A type comp lexes that do not react further with an excess of tert-butylethyne and (tri methylsilyl)ethyne in the dark, B type complexes induce rapid dimerization of these terminal alkynes to 2,4-disubstituted but-1-en-3-ynes (head-to-tai l dimers). This implies that the known dimerization of l-alkynes in the pre sence of [(eta(5)-C5Me4R1)(2)Ti(eta(2)-Me3SiC equivalent to CSiMe3)] (R-1 = H, Me) performed in diffuse daylight is initiated by the B type complexes originating from photoinduced conversion of the initially formed A type pro ducts. Titanocene complexes with 2,4-disubstituted but-1-en-3-ynes, [(eta(5 )-C5Me4R1)(2)Ti(3,4-eta-(RC)-C-2 equivalent to CC(R-2)=CH2)] (R-1 = H, Me, Ph; R2 SiMe3), were also prepared and their participation in the catalytic cycle was demonstrated.