Synthetic and Mechanistic Studies of the Ring Opening and Denitrogenation of Pyridine and Picolines by Ti−C Multiple Bonds

Abstract

The neopentylidene−neopentyl complex (PNP)Ti═CHtBu(CH2tBu) (1; PNP− = N[2-P(CHMe2)2-4-methylphenyl]2) extrudes neopentane in neat pyridine or picoline (3- or 4-picoline) under mild conditions (25 °C), to generate the transient titanium alkylidyne intermediate (PNP)Ti≡CtBu (A), which subsequently ring-opens the pyridine by ring-opening metathesis of the aromatic N═C bond across the Ti≡C linkage, generating the metallaazabicycles (PNP)Ti(C(tBu)C5H3RNH) (R = H (2), 3-Me (3), 4-Me (4)). Kinetic studies suggest that the C−N activation process obeys a pseudo-first-order process in titanium, with α-hydrogen abstraction being the rate-determining step (the KIE for 1/1-d3 conversion to 2 was 3.8(3) at 25 °C). The activation parameters are ΔH⧧ = 23(3) kcal/mol and ΔS⧧ = −4(3) cal/(mol K). The intermolecular kH/kD ratio is close to unity, 1.07(3) at 25 °C, for the conversion of 1 to 2 in pyridine versus pyridine-d5. Detailed theoretical studies suggest the 1 → 2 transformation proceeds in the following order: (i) fo...