Addition of 2.0 equiv of Na(silox) to
TiCl4(THF)2 afforded
(silox)2TiCl2 (1), which
yielded (silox)2(tBu3SiNH)TiCl (2-Cl) upon
treatment with tBu3SiNLi.
Grignard or alkyllithium additions to 2-Cl or
1,2-RH-addition
to transient
(silox)2TiNSitBu3
(3) produced
(silox)2(tBu3SiNH)TiR
(2-R; R = Me, Et, CH2Ph = Bz,
CHCH2 =
Vy, cBu, nBu, Ph, H,
cPr, cPe,
CH2-3,5-Me2C6H3 =
Mes, neoHex, cHex,
η3-H2CHCH2,
η3-H2CCHCHMe).
Insertions
of C2H4, butadiene, HC2H, and
HC2
tBu into the titanium−hydride bond of
2-H generated
(silox)2(tBu3SiNH)TiR
(2-R; R = Et, η3-H2CCHCHMe,
Vy, E-CHCHtBu). Trapping of
3 by donors L afforded
(silox)2LTiNSitBu3
(3-L; L = OEt2, THF (X-ray, two independent
molecules: d(TiN) = 1.772(3), 1.783(3) Å),
py, PMe3, NMe3,
NEt3) and
RC2R‘ =
HC2H, MeC2Me, EtC2Et,
HC2
tBu) and
Kinetics of
1,2-RH-elimination from 2-R revealed a first-order process
(24.8
°C): R = Bz < Mes < H < Me (1.54(10) ×
10-5 s-1) <
neoHex < Et < nBu <
cBu < cPe < cHex
< cPr < Vy <
Ph. Kinetics data, large 1,2-RH/D-elimination KIE's (e.g., MeH/D,
13.7(9), 24.8 °C), and Eyring parameters (e.g.,
2-Me, ΔH
⧧ = 20.2(12)
kcal/mol, ΔS
⧧ = −12(4) eu) portray
a four-center, concerted transition state where the
N···H···R
linkage is nearly linear. Equilibrium measurements led to the
following relative standard free energy scale:
2-cHex
> 2-cPe >
2-nPr ∼ 2-nBu
> 2-neoHex > 2-Et >
2-cBu >
2-CH2SiMe3 > 2-Ph
> 2-Me > 2-Bz >
2-cPr ∼ 2-Mes >
2-Vy > 3-C2H4 >
3-NEt3 > 2-H >
3-OEt2 > 3-EtC2Et
>3-MeC2Me > 3-THF >
3-NMe3 > 3-PMe3 >
3-py. A
correlation of D(TiR)rel to D(RH)
revealed greater differences in titanium−carbon bond energies.
THF loss from
3-THF allowed a rough estimate of
ΔG°(3). Using thermochemical
cycles, relative activation energies for 1,2-RH-addition were assessed: cHexH >
cPeH > nBuH >
neoHexH > EtH > BzH > cBuH > MesH
> MeH > PhH >
cPrH > VyH >
3-C2H4 formation > H2.
On the basis of a parabolic model, C−H bond activation
selectivities are
influenced by the relative ground state energies of 2-R and
a parameter representing the reaction coordinate. A
more compressed reaction coordinate for sp2- vs
sp3-substrates eases their activation.
