Synthesis, structure and reactivity of titanium(IV) and titanium(II) complexes supported by proximally bridged p-t-butylcalix[4]arene ligands

The chemistry of Ti supported by p-t-butylcalix[4]arene ligands proximally bridged by divalent phosphorus- or silicon-based groups was investigated. Complexes (DMSC)TiCl₂ (170), (t-BuPQTiCl₂ (171), (PhPC)TiCI2 (172) and (PhPSC)TiCI2 ( 173) (see Section 2.2 for definitions) are modestly active as ethylene polymerization catalysts when activated by MAO. NMR spectroscopic methods were used for the determination of the conformation of the bridged cahx[4]arene compounds in solution. All of the DMSC-based Ti complexes were found to exist in 1,2-alternate conformation. Such a conformation imposes a unique stereochemical environment around Ti in (DMSC)TiX₂ complexes. The endo -substituent X is located inside the crowded setting of the calix[4]arene cavity, while the exo-substituent X is located in a fairly unencumbered steric environment. This is reflected in the reactivity of the DMSC-based Ti complexes. The bulkier moieties are restricted from the endo-side and the transformations preferentially take place at the exo-side. Reduction of (DMSC)TiCl₂ by Mg* in the presence of Me₃SiC≡CH afforded (DMSC)Ti(η6-1,2,4-(Me ₃Si)₃-C₆H₃) (193) in a good yield. The solid-state structure of 193 was determined by X-ray crystallography. The η6-arene ligand in 193 is strongly reduced and adopts a puckered cyclohexadiene-diyl structure with the fold angle of 29.7(7)°. 193 acts as a (DMSC)Ti II equivalent in catalytic and stoichiometric reactions. 193 is an active catalyst of highly regioselective terminal alkyne cyclotrimerization to 1,2,4-substituted benzenes. Kinetic and mechanistic investigations revealed that the rate-limiting step in Me₃SiC≡CH cyclotrimerization is the displacement of the coordinated arene by Me₃SiC≡CH The activation parameters of this process ΔH‡ = +14.4(10) kcal/mol and ΔS‡ = –10.8(10) cal/(mol×K) were determined. The origin of the 1,2,4-selectivity is the directional influence of the DMSC ligand Reactions with ketones proceed ultimately with a loss of the coordinated arene and formation of the corresponding diolate complexes. Intermediate products of insertion of aliphatic ketones into the Ti-C bond of 193 were isolated. 193 also mediates the coupling of one molecule of an alkyne and two molecules of a ketone to give 2,6-dioxa-l-titana-4-cycloheptenes. This reaction can be viewed as interception of the transient (DMSC)Ti(η2-alkyne) complex by two ketone molecules.