Synthesis of Titanium Borylimido Compounds Supported by Diamide-Amine Ligands and Their Reactions with Alkynes

Abstract

We report a combined synthetic, mechanistic, and computational (DFT) study of the synthesis of new diamide-amine-supported titanium borylimides and their reactions with TolCCH and ArFCCH (Tol = 4-C6H4Me, ArF = C6F5). Reaction of Ti­{NB­(NAr′CH)2}­Cl2(py)3 (Ar′ = 2,6-C6H3 iPr2) with Li2N2 RNMe (N2 RNMe = MeN­(CH2CH2NR)2) or Li2N2Npy (N2Npy = (2-C5H4N)­CMe­(CH2NSiMe3)2) afforded the borylimides Ti­(N2 RNMe)­{NB­(NAr′CH)2}­(py) (R = SiMe3 (9), ArF (10), or iPr (11)) and Ti­(N2Npy)­{NB­(NAr′CH)2}­(py) (21). Compounds 9 and 10 reacted with ArCCH (Ar = Tol or ArF) via [2 + 2] cycloaddition to form the azatitanacyclobutenes Ti­(N2 RNMe)­{N­{B­(NAr′CH)2}­C­(H)­C­(Ar)}. In the case of R = ArF these underwent subsequent intramolecular C–F bond cleavage/C–C coupling processes. Reaction of 11 and 21 with TolCCH also formed azatitanacyclobutenes, whereas ArFCCH formed borylamide-acetylides via a C–H bond activation process which is endergonic in the case of TolCCH. On heating, these kinetic products rearranged via alkyne elimination to form the corresponding azatitanacyclobutenes as the thermodynamic outcomes.

Isolation and Reactivity of an Iron Azametallacyclobutene Complex

“…Several examples of isolable azametallacyclobutenes have been reported, ,,,− the vast majority of which are based on zirconium ,− and titanium. ,,− Reports from Bergman and co-workers revealed important insights into the mechanism of [2 + 2] cycloaddition in zirconium imide complexes, including that metallacycle formation is reversible . The reactivity of group 4 azametallacyclobutenes has been extensively studied. − Particularly, these complexes have been found to undergo migratory insertion of unsaturated organic substrates including aldehydes, , imines, carbodiimides, alkynes, carbon monoxide, and isonitriles .…”