Direct Epimetallation of π‐Bonded Organic Substrates with Titanium(II) Isopropoxide: Intermediacy of Biradical, Oligomeric Titanium(II) Reagents

Abstract

AbstractExtensive EPR experiments show that a titanium‐containing molecular triplet state is formed in solution by the reaction of two equivalents of butyllithium with one equivalent of titanium(IV) isopropoxide. At higher concentrations this product, titanium(II) isopropoxide, admixed with two equivalents of lithium isopropoxide, is accompanied by the formation of a variety of nontitanium‐containing side products. The powder EPR spectrum of the molecular triplet state in frozen solution is consistent with an asymmetric molecular chain of three Ti centers on which the unpaired electron centers are three metal atoms apart. Dilution experiments show that at lower concentrations, where the nontitanium‐containing side products have dissipated, the intensity of the molecular triplet spectrum varies approximately linearly with concentration. Thus there is no evidence that the observed triplet molecule is one component in a series of concentration‐dependent oligomerization steps. The bulky isopropoxy substituents and the coordination of the isopropoxide anions from the LiOiPr present appear to prevent closure of the Ti3 centers into an equilateral triangular diamagnetic structure. This steric hindrance, operative at the terminal diisopropoxytitanium centers and preventing closure to a ring, seems not to be observed with TiCl2·2THF, which is diamagnetic and may thus be expected to exist as an equilateral triangular cluster of three units of TiCl2·2THF, a structural model currently under further investigation. The smaller steric demand of the chloro and THF units would seem to permit octahedral coordination about each Ti center in such an equilateral trigonal array of Ti3 atoms. Chemical reactions carried out individually with diphenylacetylene, cis‐stilbene or cis‐stilbene oxide and titanium(II) diisopropoxide provide stoichiometric and stereochemical evidence that the attacking titanium(II) reagent is in fact the trimeric biradical. The role of the lithium isopropoxide byproduct in fostering the course of the previously reported SET reactions of titanium(II) isopropoxide and in determining the detailed structure of the open‐chain Ti trimer biradical has been explicated. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)