Insertions into Azatitanacyclobutenes: New Insights into Three-Component Coupling Reactions Involving Imidotitanium Intermediates

Reaction of the titanium imido complexes [Ti(NR)(N2XylNpy)(L)] (N2XylNpy = MeC(2-C5H4N){CH2N(3,5-C6H3Me2)}2) with aryl acetylenes afforded the {2+2} cycloaddition products [Ti(N2XylNpy){κ2-N(tBu)CHCAr}] (Ar = Ph: 1a, Tol: 1b (Tol = 4-C6H4Me)). Complex 1a was found to insert sulfur and selenium atoms into the Ti−C bond of the strained azatitanacyclobutene unit to give the five-membered metallacyclic systems [Ti(N2XylNpy){κ2-N(tBu)CHC(Ph)E}] (E = S: 2, Se: 3). Likewise, isonitriles were found to insert into the Ti−C bonds to give [Ti(N2XylNpy){κ2-N(tBu)CHC(Ar)CNR}] (Ar = Ph, R = xylyl: 4, Cy: 5, tBu: 6a; Ar = C6H5Me, R = tBu: 6b), one example of which (4) has been characterized by X-ray crystallography. The latter species is thought to be a key intermediate in the titanium-mediated three-component coupling of primary amines with acetylenes and isonitriles (“iminoamination”). Reaction of [Ti(N2XylNpy){κ2-N(tBu)CHC(Ph)CNR}] with tBuNH2 gave tBuNCHC(Ph)CHNHR, which have been previously identified as the major reaction products in the catalytic aminoamination of alkynes.