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

The
complex (PNP)­TiCHtBu­(CH2tBu) (PNP = N­[2-PiPr2-4-methyl­phenyl]2–) dehydrogenates cyclo­hexane to cyclo­hexene
by forming a transient low-valent titanium-alkyl species, [(PNP)­Ti­(CH2tBu)], which reacts with 2 equiv
of quinoline (Q) at room temperature to form H3CtBu and a Ti­(IV) species where the less
hindered C2N1 bond of Q is 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(CH2tBu)]
results in regio­selective cleavage of the C1N2 and C1H bonds, which eventually couple
to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)­Ti­(CH2tBu)] complex can ring-open and
couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)­TiN­[C10H9N] (4). Multi­nuclear 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 where 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.