Synthesis and Structures of Bis(indolyl)-Coordinated Titanium Dichlorido Complexes and Their Catalytic Application in the Cyclotrimerization of Alkynes

Abstract

The impact of the terminal ligands on the titanium center on the coordination features of deprotonated 2,2′-bis­(indolyl)­methanes (henceforth: bis­(indolyl)­s) was studied via a structural comparison between {bis­(indolyl)}­Ti­(NEt2)2 complexes and the corresponding dichlorido complexes. As a result, several flexible aspects of bis­(indolyl) coordination were found. For example, it was revealed that an η1-coordinated indolyl moiety can change its coordination mode to coordination via the five-membered ring of indolyl when the terminal diethylamido ligands are replaced by chlorido ligands. Moreover, we found that the methoxy group in the central aromatic ring of the bis­(indolyl) ligand can coordinate to the titanium center. The synthesized dichlorido complexes were applied for catalytic alkyne cyclotrimerization reactions, as Ti-based catalyst systems are less developed than Co-, Ni-, Ru-, Rh-, and Ir-based systems. During this study, the cyclotrimerization of HCCSiMe3 was found to preferentially produce the 1,3,5-form (1,3,5-form:1,2,4-form = 79:21), contrary to the typical trend of transition-metal-mediated alkyne cyclotrimerization, and the isolated yield (72%) is the highest among the known 1,3,5-favoring reactions using Ti-based catalyst systems. Furthermore, the reaction mechanism was experimentally verified to proceed through a typical stepwise mechanism involving monomeric species.

A Mixed-Valence Ti(II)/Ti(III) Inverted Sandwich Compound as a Regioselective Catalyst for the Uncommon 1,3,5-Alkyne Cyclotrimerization

“…In evaluating catalytic performance, our titanium catalytic system demonstrates enhanced activity, evidenced by turnover number (TON = 40) and turnover frequency (TOF = 13 h –1 ) values, when compared to those reported for the in situ reduced [CpTiCl 3 ] (TON = 15; TOF = 1 h –1 ) and [bis­(indolyl)­TiCl 2 ] (TON = 6; TOF = 1 h –1 ) . However, our results do not reach the higher performance metrics of titanium alkoxide catalysts described by Ladipo (TON = 99; TOF = 396 h –1 ), Rothwell (TON = 235), and Morohashi and Hattori (TON = 380) .…”

“…Compared to previous reports on cyclotrimerization reactions mediated by titanium compounds, , our catalytic system eliminates the need for reductants such as magnesium or zinc metal. As a result, a broader functional group spectrum is tolerated, being compatible with trifluoromethyl groups (Table , entry 7) which as far as we are aware, the desired cyclotrimerization product has not been isolated using a titanium-based catalytic system .…”

“…In contrast, using smaller alkynes leads to a mixture of both isomers, with 1,2,4isomer being the predominant. Similarly, Ohta 19 described a [bis(indolyl)TiCl 2 ] (indolyl = 2,2′-bis(indolyl)-methanes) compound which in combination with magnesium metal favors the formation of the symmetric 1,3,5-arene product only for the trimethylsilyl-substituted terminal alkyne. High selectivity for the 1,3,5-isomer is consistently achieved by the catalytic system utilizing mono-or dinuclear titanium ptert-butylthiacalix [4]arene and sodium metal described by Morohashi and Hattori.…”

“…[TiCl 4 (thf) 2 ], 45 N,N′bis(2,4,6-trimethylphenyl)-o-phenylenediamine ( Mes PDAH 2 ), 46 [Li 2 ( Mes PDA)(thf) 3 ], 47 and cyclopentyl lithium 48 were synthesized as described in the literature. NMR spectra were recorded on a Varian Mercury-VX spectrometer operating at 300 MHz for 1 H, 75 MHz for 13 C{ 1 H}, and 376 MHz for 19 F-{ 13 C}, on a Bruker Neo spectrometer operating at 400 MHz for 1 H, 100 MHz for 13 C{ 1 H], and on a Bruker 400 Ultrashield system operating at 400 MHz for 1 H for kinetic studies. 1 H, 13 C{ 1 H}, and 19 F chemical shifts are expressed in parts per million (δ, ppm) and referenced to residual solvent peaks.…”

“…NMR spectra were recorded on a Varian Mercury-VX spectrometer operating at 300 MHz for 1 H, 75 MHz for 13 C{ 1 H}, and 376 MHz for 19 F-{ 13 C}, on a Bruker Neo spectrometer operating at 400 MHz for 1 H, 100 MHz for 13 C{ 1 H], and on a Bruker 400 Ultrashield system operating at 400 MHz for 1 H for kinetic studies. 1 H, 13 C{ 1 H}, and 19 F chemical shifts are expressed in parts per million (δ, ppm) and referenced to residual solvent peaks. All coupling constants (J) are expressed in absolute values (Hz), and resonances are described as s (singlet), d (doublet), and m (multiplet).…”