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

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) formation of A in
an overall endergonic step by α-hydrogen abstraction, (ii) an
exergonic binding of pyridine, and (iii) concerted, exergonic [2 +
2] cycloaddition followed by (iv) exergonic ring-opening metathesis
and finally (v) a concerted hydrogen atom migration. Complexes 2−4 can denitrogenate, that is, completely
remove N of the heterocycle at 65 °C over 72 h, when treated
with silyl chlorides such as ClSiR3 (R = Me, iPr, Ph) to cleanly afford the titanium silylimides (PNP)TiNSiR3(Cl) (R = Me (8), iPr (9), Ph (10)) and the corresponding tBu-arene
organic byproduct. [Et3Si][B(C6F5)4] also promotes denitrogenation of 2 to
yield tBu-benzene, but the metal complex could not be characterized
from such a reaction. The conversion 2 → 8 was found to have activation parameters ΔH⧧ = 30(6) kcal/mol and ΔS⧧ = 10(2) cal/(mol K), therefore
yielding ΔG⧧ ≈
27 kcal/mol at 298.15 K. A KIE of 1.6(2) at 85 °C was observed
when 2/2-d5 were denitrogenated to 8 in the presence of ClSiMe3, with the rate of
the reaction being insensitive to both the steric nature and concentration
of the trialkylsilyl chloride. Denitrogenation leading to 8−10 is proposed to occur via a series of steps
including a 1,3-hydrogen migration, an electrocyclic rearrangement,
a retrocycloaddition, and a Si−Cl addition. The transformations 1 → 2/3/4 and 2/3/4 → 8 can
be made cyclic by a series of steps such as deimination of the imide
moiety in 8 with 2 equiv of MoCl5, followed
by reduction and transmetalation with LiCH2tBu and then oxidatively induced α-hydrogen abstraction. The
reactivity of 1 with other heterocycles such as THF,
thiophene, and piperidine is also discussed.