Photoinduced Generation of Catalytic Complexes from Substituted-Titanocene-Bis(trimethylsisyl)ethyne Complex: Contribution to the Mechanism of the Catalytic Head-to-Tail Dimerization of Terminal Alkylnes
Abstract
A number of substituted-titanocene-alkynyl-alkenyl complexes, [(eta(5)-C5Me4R1)(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 reacting the corresponding bis(trimethylsilyl)ethyne complexes [(eta 5-C5Me4R1)(2)Ti(eta(2)-Me3SiC equivalent to CSiMe3)] with 1-alkynes (RC)-C-2 equivalent 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 complexes that do not react further with an excess of tert-butylethyne and (trimethylsilyl)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-tail dimers). This implies that the known dimerization of l-alkynes in the presence 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 products.
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.