Allyl Titanium Complexes Research Articles

A Straightforward Route to Homoallyl‐Homocrotylamines Promoted by a Titanium Complex

Abstractπ‐Allyltitanium complexes, generated in situ from 1,3‐dienes and [Cp2TiH], react with benzotriazole derivatives to give homoallylic amines in good yields. Under similar conditions, triple cascade reactions (allyltitanation followed by cationic 2‐aza‐Cope rearrangement followed by a second allyltitanation) occur from bis(benzotriazolyl) compounds affording a straightforward route to homoallyl‐(E)‐homocrotylamines. A theoretical study provides further insight into the factors that govern the selectivity of this sequence of reactions.

Novel η 3-allyl complexes of titanium

Treatment of TiCl 4(dmpe) ( 1a) and TiCl 4(depe) ( 1b) with two equivalents of C 3H 5MgCl in toluene at −30°C affords two novel allyl–titanium complexes: a dinuclear mixed-valent Ti 2 5+ species Ti 2(μ-Cl) 2Cl 2(dmpe) 2(μ 2-η 3-C 3H 5) ( 2) and a mononuclear Ti III complex, TiCl(depe)(η 3-C 3H 5) 2 ( 3). Syntheses and crystal structures of 1b, 2 and 3 are presented. In complex 2, in addition to two bridging chloride ligands a η 3-allyl group spans two titanium atoms with a Ti…Ti separation of 2.908(1) Å. The allyl bridge is symmetrical with each terminal carbon atom having a Ti–C distance of 2.154(6) Å, while the central carbon atom is equidistant from the titanium atoms, Ti–C cent, 2.430(6) Å. Each Ti atom in 2 is further coordinated by a chelating dmpe ligand and a chloride ion. In the mononuclear complex 3 two η 3-allyl groups are bound to the Ti III center and the coordination is completed by a chelating diphosphine ligand and one chloride ion. The Ti–C distances in 3 vary from 2.286(3) to 2.478(3) Å.

Regiocontrolled functionalization of seven-membered ring via nucleophilic cycloheptenyl-η3-allyltitanium complexes

A new "electron-reversed" method for the functionalization of a seven-membered ring is developed. Cycloheptenyl-η3-allyltitanocene complexes derived from cycloheptatriene react with aldehydes or carbon dioxide to afford, respectively, the mixture of isomeric 1,4- and 1,3-cycloheptadienyl carbinols 3a–e and 2a–e or methyl esters 7 and 6. The 3/2 and 7/6 ratios increase with increasing steric hindrance in the electrophile. Moreover, these may be significantly increased by introducing tert-butyl substituents on the Cp rings of the starting titanocene dichloride. The regiochemistry of the reaction was rationalized in terms of the equilibrium between the two envisageable allylic complexes. The reaction constitutes the first direct entry to functionalized 1,4-cycloheptadienes, some of which are related to biologically active compounds. Keywords: η3-allyltitanium complexes, cycloheptadiene derivatives, regiocontrol, electron reversal.

Steric effect of allyl substituent on the molecular structures of allyltitanium complexes

The crystal structures of a series of allyltitanium complexes have been determined by X-ray crystallography. Crystals of (C 5H 5 2Ti(1,3-dimethylalyl) ( 1) are monoclinic, space group P2 1/ n, with a = 13.353(2), b = 7.674(1), c = 25.543(2) Å, β = 90.92(1)°, and Z = 8. Crystals of (C 5H 5) 2Ti(2-methylallyl) ( 2_ are orthorhombic, space group Pbca, with a = 15.299(4), b = 25.797(2), c = 12.014(1) Å, and Z = 16. The unit cells of 1 and 2 each contain two molecules in a crystallographic asymmetric unit. The coordination geometry of a non-substituted allyltitanium compound was determined for (C 5H 5(C 5Me 5)-Ti(allyl) ( 3) with the mixed ancillary ligands since (C 5H 5) 2Ti(Allyl) ( 4) includes the packing disorder. Crystals of 3 are orthorhombic, space group Pnma, with a = 9.633(1), b = 12.743(1), c = 12.718(1) Å, and Z = 4. The molecular structure exhibits a crystallographic mirror symmetry. All the crystal structures were refined by the full-matrix least-squares to the R indices of 0.062, 0.051, and 0.065 for 1, 2, and 3, respectively. The allyl groups are η 3-coordinated to the metal with a mirror symmetry. Detailed comparisons of these complexes revealed the remarkable steric effect by the methyl substituents of the allyl ligands on the coordination geometries of allyl ligands to titanium atom. Complex 1 has the longest TiC(1) and TiC(3) bonds, while complex 2 has the longest TiC(2) bond. The shortest Tiallyl bonds were seen in complex 3.