Room-Temperature Ring-Opening of Quinoline, lsoquinoline, and Pyridine with Low-Valent Titanium

Abstract

The complex (PNP)Ti=(CHBu)-Bu-t((CH2Bu)-Bu-t) (PNP = N[2-P(i)p(r2)-4-methylphenyl](2-)) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti((CH2Bu)-Bu-t)], which reacts with 2 equiv of quinoline (Q) at room temperature to form (H3CBu)-Bu-t and a Ti(IV) species where the less hindered C-2=N-1 bond of Qis ruptured and coupled to another equivalent of Q, The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti=N[C18H13N] (1). Under photolytic conditions, intramolecular C-H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti((CH2Bu)-Bu-t)] results in regioselective cleavage of the C-I=N-2 and C-1-H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti((CH2Bu)-Bu-t)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti=N[C10H9N] (4). Multinudear and multidimensional NMR spectra confirm structures for complexes 1-4, whereas solid-state structural analysis reveals the structures of 2, 3, and 4. DFT calculations suggest an unprecedented Mechanism for ring-opening of Q wheat the reactive intermediate in the low-spin manifold crosses over to the high-spin surface to access a low-energy transition state but returns to the low-spin surface immediately. This double spin-crossover constitutes a rare example of a two-state reactivity, which is key for enabling the reaction at room temperature. The regioselective behavior of Iq ring-opening is found to be due to electronic effects, where the aromatic resonance of the bicycle is maintained during the key C-C coupling event.