Iva Tišlerová

Reduction of Bis[η5-(ω-alkenyl)tetramethylcyclopentadienyl]titanium Dichlorides: An Efficient Synthesis of Long-Chainansa-Bridged Titanocene Dichlorides by Acidolysis of Cyclopentadienyl-Ring- Tethered Titanacyclopentanes

The reduction of symmetric, fully-substituted titanocene dichlorides bearing two pendant ω-alkenyl groups, [TiCl2(η5-C5Me4R)2], RCH(Me)CH=CH2 (1 a), (CH2)2CH=CH2 (1 b) and (CH2)3CH=CH2 (1 c), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [TiIV{η1:η1:tlsb&endash;3%>η5:η5-C5Me4CH(Me)CH(Ti)CH2CH(CH2(Ti))CH(Me)C5Me4}] (2 a), [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)(CH2)2CH(Ti)(CH2)2C5Me4}] (2 b) and [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)CH(Me)CH(Me)CH(Ti)(CH2)2C5Me4}] (2 c), respectively, as the products of oxidative coupling of the double bonds across a titanocene intermediate. For the case of complex 1 c, a product of a double bond isomerisation is obtained owing to a preferred formation of five-membered titanacycles. The reaction of the titanacyclopentanes with PbCl2 recovers starting materials 1 a from 2 a and 1 b from 2 b, but complex 2 c affords, under the same conditions, an isomer of 1 c with a shifted carbon-carbon double bond, [TiCl2{η5-C5Me4(CH2CH2CH=CHMe)}2] (1 c′). The titanacycles 2 a-c can be opened by HCl to give ansa-titanocene dichlorides ansa-[{η5:η5-C5Me4CH(Me)CH2CH2CH(Me)CH(Me)C5Me4}TiCl2] (3 a), ansa-[{η5:η5-C5Me4(CH2)8C5Me4}TiCl2] (3 b), along with a minor product ansa-[{η5:η5-C5Me4CH2CH=CH(CH2)5C5Me4}TiCl2] (3 b′), and ansa-[{η5:η5-C5Me4(CH2)3CH(Me)CH(Me)CH=CHCH2C5Me4}TiCl2] (3 c), respectively, with the bridging aliphatic chain consisting of five (3 a) and eight (3 b, 3 b′ and 3 c) carbon atoms. The course of the acidolysis changes with the nature of the pendant group; while the cyclopentadienyl ring-linking carbon chains in 3 a and 3 b are fully saturated, compounds 3 c and 3 b′ contain one asymetrically placed carbon-carbon double bond, which evidently arises from the β-hydrogen elimination that follows the HCl addition.

Syntheses and structures of doubly tucked-in titanocene complexes with tetramethyl(aryl)cyclopentadienyl ligands

Titanocene–bis(trimethylsilyl)ethyne complexes [Ti(η5-C5Me4R)2(η2-Me3SiCCSiMe3)], where R=benzyl (Bz, 1a), phenyl (Ph, 1b) and p-fluorophenyl (FPh, 1c), thermolyse at 150–160°C to give products of double CH activation [Ti(η5-C5Me4Bz){η3:η4-C5Me3(CH2)(CHPh)}] (2a), [Ti(η5-C5Me4Bz){η3:η4-C5Me2Bz(CH2)2}] (2a′), [Ti(η5-C5Me4Ph){η3:η4-C5Me2Ph(CH2)2}] (2b), and [Ti(η5-C5Me4FPh){η3:η4-C5Me2FPh(CH2)2}] (2c). In the presence of 2,2,7,7-tetramethylocta-3,5-diyne (TMOD) the thermolysis affords analogous doubly tucked-in compounds bearing one η3:η4-allyldiene and one η5-C5Me4R ligand having TMOD attached by its C-3 and C-6 carbon atoms to the vicinal methylene groups adjacent to the substituent R (R=Bz (3a), Ph (3b), and FPh (3c)). Compound 3a is smoothly converted into air-stable titanocene dichloride [TiCl2{η5-C5Me2Bz(CH2CH(t-Bu)CHCHCH(t-Bu)CH2)}(η5-C5Me4Bz)] (4a) by a reaction with hydrogen chloride. Yields in both series of doubly tucked-in complexes decrease in the order of substituents: Bz≫Ph>FPh. Crystal structures of 1c, 2a, 2b, and 3b have been determined.

Nickel-catalyzed cyclization of α,ω-dienes: formation vs. cleavage of C–C bonds

A combination of a catalytic amount of a Ni–phosphine complex and triethylaluminium or chlorodiethylaluminium is able to selectively cyclize a number of 1,7-heptadienes to methylidene(methyl)cyclopentanes and cyclopentenes even in cases where the dienes are prone to deallylation.

Spectroscopic, Morphological, and Mechanistic Investigation of the Solvent‐Promoted Aggregation of Porphyrins Modified in meso‐Positions by Glucosylated Steroids

Solvent-driven aggregation of a series of porphyrin derivatives was studied by UV/Vis and circular dichroism spectroscopy. The porphyrins are characterised by the presence in the meso positions of steroidal moieties further conjugated with glucosyl groups. The presence of these groups makes the investigated macrocycles amphiphilic and soluble in aqueous solvent, namely, dimethyl acetamide/water. Aggregation of the macrocycles is triggered by a change in bulk solvent composition leading to formation of large architectures that express supramolecular chirality, steered by the presence of the stereogenic centres on the periphery of the macrocycles. The aggregation behaviour and chiroptical features of the aggregates are strongly dependent on the number of moieties decorating the periphery of the porphyrin framework. In particular, experimental evidence indicates that the structure of the steroid linker dictates the overall chirality of the supramolecular architectures. Moreover, the porphyrin concentration strongly affects the aggregation mechanism and the CD intensities of the spectra. Notably, AFM investigations reveal strong differences in aggregate morphology that are dependent on the nature of the appended functional groups, and closely in line with the changes in aggregation mechanism. The suprastructures formed at lower concentration show a network of long fibrous structures spanning over tens of micrometres, whereas the aggregates formed at higher concentration have smaller rod-shaped structures that can be recognised as the result of coalescence of smaller globular structures. The fully steroid substituted derivative forms globular structures over the whole concentration range explored. Finally, a rationale for the aggregation phenomena was given by semiempirical calculations at the PM6 level.