Abstract:The ligands 2,3,5,64etraphenylphenoxide and 3,5-dimethyl-2,6-diphenylphenoxide undergo intramolecular aromatic C-H bond activation by tantalum alkylidene groups at rates 20 and 100 times slower than simple 2,6-diphenylphenoxide.
“…There is a correlation between the logarithms of the relative metalation rates and the Hammett σ constants for arenes bearing substituents X (ρ = −5.43). A ruthenium(II) complex metalates arenes in the presence of a methyl derivative of aluminum 66b and an interesting C-metalation of a pyrrole ligand coordinated to rhenium has been described 66c66d…”
Section: A Electrophilic Metalation Of C−h
Compounds1 Metalation Of A...mentioning
Although alkanes are undoubtedly much less reactive than other organic compounds including unsaturated hydrocarbons, the number of known alkanes reactions is large. In this section we will briefly survey the main types of hydrocarbon transformations that occur without the participation of metal complexes. These reactions are presented for comparison to the metal complex activation reactions.
“…There is a correlation between the logarithms of the relative metalation rates and the Hammett σ constants for arenes bearing substituents X (ρ = −5.43). A ruthenium(II) complex metalates arenes in the presence of a methyl derivative of aluminum 66b and an interesting C-metalation of a pyrrole ligand coordinated to rhenium has been described 66c66d…”
Section: A Electrophilic Metalation Of C−h
Compounds1 Metalation Of A...mentioning
Although alkanes are undoubtedly much less reactive than other organic compounds including unsaturated hydrocarbons, the number of known alkanes reactions is large. In this section we will briefly survey the main types of hydrocarbon transformations that occur without the participation of metal complexes. These reactions are presented for comparison to the metal complex activation reactions.
“…Concerted 1,2-RH-additions across MX (X = O (R = OH, H, R‘‘NH), NR‘ (R = hydrocarbyl, − R‘‘NH), , CR‘ 2 ) , multiple bonds comprise a relatively recent class of C−H bond cleavage reactions of which d 0 metal imido complexes constitute the most prevalent subgroup. Hydrocarbon reactivity is typified by 1,2-additions of RH to imido functionalities of electrophilic transients Cp 2 ZrN t Bu (R = arene, sp 2 -substrates), ( t Bu 3 SiNH) 2 ZrNSi t Bu 3 (R = hydrocarbyl), , ( t Bu 3 SiNH) 2 TiNSi t Bu 3 (R = benzene), ( t Bu 3 SiNH)Ta(NSi t Bu 3 ) 2 (R = methane, benzene), ( t Bu 3 SiNH)V(NSi t Bu 3 ) 2 (R = hydrocarbyl), and (silox) 2 TiNSi t Bu 3 ( 3 , silox = t Bu 3 SiO, R = hydrocarbyl) .…”
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.
“…High-valent (e.g., d 0 ) metal compounds with MC or MN double bonds can effect the intermolecular activation of saturated hydrocarbons. For example, alkylidene or imido intermediates such as (RO) 2 MCHR, Cp 2 MCHR, and Cp 2 MNR, where M can be Ti, Zr, V, or Ta, generated by 1,2 alkane elimination from dialkyl or alkylamido precursors, can add a second hydrocarbon R‘−H across the double bond. ,,− Intermolecular addition of a hydrocarbon across an MCHR (or M=NR) bond constitutes the microscopic reverse of α-hydrogen abstraction from metal−dialkyl (or metal−alkylamido) complexes. …”
Studies of the thermolysis of
Ti(CH2CMe3)4 in solution
have been carried out in parallel with studies of
the chemical mechanism responsible for its conversion to titanium
carbide under CVD conditions. In hydrocarbon
solutions, the neopentyl complex thermolyzes to eliminate 2.1 equiv of
neopentane as the principal organic product.
A deuterium kinetic isotope effect
(k
α
(H)/k
α
(D)
= 5.2 ± 0.4) upon deuterating the alkyl groups at the α
positions
provides clear evidence that the initial step in the thermolysis is an
α-hydrogen abstraction reaction to form neopentane.
The activation parameters for this α-hydrogen abstraction
process are ΔH
⧧ = 21.5 ± 1.4 kcal/mol
and ΔS
⧧ = −16.6
± 3.8 cal/(mol K). The titanium-containing product of this
reaction is a titanium alkylidene, which in solution
activates C−H bonds of both saturated and unsaturated hydrocarbon
solvents such as benzene and cyclohexane. No
activation of the C−F bonds of hexafluorobenzene is seen, however.
Under special circumstances, a second thermolysis
pathway for TiNp4 can be detected, γ-hydrogen activation,
but this pathway is intrinsically about 25 times slower
than the α-hydrogen abstraction process.
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