Reactions of Titanium Hydrazinediido Complexes with Unsaturated Organic Substrates

Abstract

Reaction of [Cp*Ti(NXylN){N-NPh2}(NH2tBu)] (1a) with 1 molar equiv of phenylallene resulted in the formation of a mixture of the two metallacyclic compounds [Cp*Ti(NXylN){κ2N(NPh2)C(CHPh)CH2}] (2a and 2b) in a molar ratio of 3:1. Comparison of the 13C and 15N NMR resonances with the computed chemical shifts, along with an X-ray diffraction study of 2a, allowed the assignment of the diastereomers as the cycloadducts derived from a (3,2)-cycloaddition of the allene to the Ti═N bond. Upon reaction of 1a with phenylisothiocyanate, two complexes were formed in a 3:1 ratio. The major isomer, [Cp*Ti(NXylN){κ2-N(NPh2)C(NPh)S}] (4a), was isolated and identified by X-ray diffraction to result from a [2+2] cycloaddition of the C═S bond in PhNCS to the Ti═N bond of the hydrazinediido unit, giving a four-membered metallacyclic ring with an S,N-coordination to the titanium center and an exocyclic C═N double bond. In the minor compound (4b) the two nitrogen atoms of the thiourea unit formed in the cycloaddition are coordinated to the titanium center. Isomerization between the cycloadducts occurs only in the direction 4b → 4a, and crossover experiments indicate a dissociative mechanism (via a retro-cycloaddition) for the rearrangement. The reaction of 1a with PhNCO gave analogous N,O- and N,N-coordinated cycloadducts. Finally, the N,N-dimethyl hydrazinediido complexes reacted unspecifically with most organic azides at ambient temperature. However, complex [Cp*Ti(NXylN){N-NMe2}(dmap)] (1c) underwent a clean and fast reaction with trimethylsilylazide to give the N-silylated η2-hydrazido(1−) titanium azide [Cp*Ti(NXylN)(η2-NMe2-NSiMe3)(N3)] (8), in which the trimethylsilyl group of the azide is transferred to the hydrazido ligand, while the azide is terminally coordinated to the titanium center.