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

Abstract

A number of substituted-titanocene−alkynyl−alkenyl complexes, [(η5-C5Me4R1)2Ti(η1-C⋮CR2)(η1-(E)-CHCHR2)] (R1 = H, Me, Ph, Bz; R2 = CMe3, SiMe3, ferrocenyl; A type complexes), were obtained by reacting the corresponding bis(trimethylsilyl)ethyne complexes [(η5-C5Me4R1)2Ti(η2-Me3SiC⋮CSiMe3)] with 1-alkynes R2C⋮CH in the dark at 60 °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, [(η5-C5Me4R1)2Ti(3,4-η-R2C⋮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 1-alkynes in the presence of [(η5-C5Me4R1)2Ti(η2-Me3SiC⋮CSiMe3)] (R1 = 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, [(η5-C5Me4R1)2Ti(3,4-η-R2C⋮CC(R2)CH2)] (R1 = H, Me, Ph; R2 = SiMe3), were also prepared and their participation in the catalytic cycle was demonstrated.

Synthesis and Structures of ansa‐Titanocene Complexes with Diatomic Bridging Units for Overall Water Splitting

“…[24] The molecular structure of compound 3-alkenyl is shown in Figure 5. Some evidence for the formation of the corresponding dihydroxido complex 3-(OH) 2 was found by ].…”

Stabile Cyclopentine – Metalle machen's möglich!?

“…Auch die gemischte reduktive Eliminierung von einem Acetylid und einer Vinylgruppe zu But-1-en-3-inen wurde beobachtet (siehe Abschnitt 6). [12] Die 1-Metallacyclopent-3-ene II und 1-Metallacyclopenta-2,3,4-triene V, alle gebildet durch Kupplung von zwei Anionen, sind thermodynamisch stabiler als die Ausgangsverbindungen. Dies wurde für [Cp 2 M(CH=CHR) 2 ] [13] und [Cp 2 M(C CR) 2 ] berechnet.…”

“…[12] Für Zirconocene ist eher eine h 4 -p,p-Koordination (ähnlich zu den Butatrienen und Butadiinen) unter Bildung von unsymmetrischen 1-Zirconacyclopenta-2,3-dienen (3) zu erwarten, denn einige 1,3-Butadiine bilden mit Permethyltitanocen h 2 -p-Alkinkomplexe und mit dem größeren Permethylzirconocen h 4 -p,p-Komplexe (1-Zirconacyclopenta-2,3,4-triene, V). Es ist fraglich, ob die oben genannten (bisher nicht isolierten) unsymmetrischen 1-Zirconacyclopenta-2,3-diene (But-1-en-3-in-Komplexe) entweder zu symmetrischen 1-Zirconacyclopenta-2,4-dienen III [9] oder 1-Zirconacyclo-3-pentinen IV isomerisieren können.…”

Transition Metal Acetylides

“…Five-membered metallacyclocumulenes Cpl 2 M(h 4 -1,2,3,4-RC 4 R) (18) are the key intermediates both in reactions of CsC single bond cleavage of RCaCsCaCR to produce acetylide groups and in the opposite reaction of CsC single bond formation from acetylide groups [RCaC] -with formation of 1,3-butadiynes (Scheme 4.14) [2].…”

“…It is worth mentioning that the reverse catalytic effect of B(C 6 F 5 ) 3 was found in the case of the permethylzirconocene systems, where the CsC bond of the complexed diyne in [Cp* 2 Zr(h 4 -1,2,3,4-Me 3 SiC 4 SiMe 3 )](18) was cleaved with formation of the bis-alkynyls 5, such as [Cp* 2 Zr(s-CaCSiMe 3 ) 2 ] [2d, 6l]. Apparently, an equilibrium between the zirconacycle and the bis-alkynyl is shifted by B(C 6 F 5 ) 3 to the cycle for Cp/Me and to the bis-alkynyl complex for Cp*/ Me 3 Si.…”