Competitive ArC-H and ArC-X (X = Cl, Br) activation in halobenzenes at cationic titanium centers

Article abstract of DOI:10.1021/ja0583618

The titanium methyl cation [Cp*(^tBu₃P=N) TiCH₃]⁺[B(C₆F₅)₄] ^- reacts rapidly with H₂ to give the analogous cationic hydride [Cp*(^tBu₃P=N)TiH(THF)_n] ⁺[B(C₆F₅)₄]^- (n = 0, 1), which can be trapped and isolated as its THF adduct 1·THF (n = 1). When generated in the presence of chloro or bromobenzene, 1 undergoes C-X activation or ortho-C-H activation, depending on the amount of dihydrogen present in the reaction medium. At ～4 atm of H₂, C-X activation is preferred, giving the halocations [Cp*(^tBu₃P= N)TiX] ⁺[B(C₆F₅)₄]^- (2X) and C₆H₆/biphenyl mixtures. At lower pressures of H ₂ (> 1 atm), the β-halophenyl cations [Cp*(Bu ₃P=N)Ti(2-X-C₆H₄)]⁺[B(C ₆F₅)₄]^- (3X) are the products isolated. In the absence of H₂, these compounds are quite thermally stable, but undergo β-halogen elimination upon moderate heating, to give 2X (～20%) and compounds 4X which are the result of reaction between 2X and benzyne via addition of the benzyne C-C triple bond across the Ti-N bond of the phosphinimide ligand. Thus, three separate bond activation processes are operative in this system: direct C-X activation, ortho-C-H activation, and indirect C-X activation via β-halogen elimination. Mechanistic studies on all three processes have been done and support a radical pathway for direct C-X cleavage, σ-bond metathesis of the ortho-C-H bond of η¹- coordinated C₆H₅X, and β-halogen elimination from base-free compound 3X.

Full text of DOI:10.1021/ja0583618

Published on Web 02/18/2006
Competitive ArC-H and ArC-X (X ) Cl, Br) Activation in
Halobenzenes at Cationic Titanium Centers
Kuangbiao Ma, Warren E. Piers,* and Masood Parvez
Contribution from the Department of Chemistry, UniVersity of Calgary,
2500 UniVersity DriVe N.W., Calgary, Alberta, Canada T2N 1N4
Received December 9, 2005; E-mail: wpiers@ucalgary.ca
Abstract: The titanium methyl cation [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- reacts rapidly with H2 to give the
analogous cationic hydride [Cp*(tBu3PdN)TiH(THF)n]+[B(C6F5)4]- (n ) 0, 1), which can be trapped and
isolated as its THF adduct 1‚THF (n ) 1). When generated in the presence of chloro or bromobenzene, 1
undergoes C-X activation or ortho-C-H activation, depending on the amount of dihydrogen present in
the reaction medium. At ∼4 atm of H2, C-X activation is preferred, giving the halocations [Cp*(tBu3Pd
N)TiX]+[B(C6F5)4]- (2X) and C6H6/biphenyl mixtures. At lower pressures of H2 (>1 atm), the â-halophenyl
cations [Cp*(tBu3PdN)Ti(2-X-C6H4)]+[B(C6F5)4]- (3X) are the products isolated. In the absence of H2, these
compounds are quite thermally stable, but undergo â-halogen elimination upon moderate heating, to give
2X (∼20%) and compounds 4X which are the result of reaction between 2X and benzyne via addition of
the benzyne C-C triple bond across the Ti-N bond of the phosphinimide ligand. Thus, three separate
bond activation processes are operative in this system: direct C-X activation, ortho-C-H activation, and
indirect C-X activation via â-halogen elimination. Mechanistic studies on all three processes have been
done and support a radical pathway for direct C-X cleavage, σ-bond metathesis of the ortho-C-H bond
of η1-coordinated C6H5X, and â-halogen elimination from base-free compound 3X.
Introduction
highly reactive toward aryl halides, particularly those with metal
hydride functions. For example, both neutral4 and cationic5
The selective activation of aryl halide bonds is a key step in
dozens of transition metal catalyzed coupling protocols or ArX
bond activations.1 Despite its importance, the intimate mecha-
nism of the Caryl-X cleavage step is not generally known in
detail2 and may involve concerted oxidative addition, nucleo-
philic aromatic substitution (SNAr2), or some mechanism
involving radicals. In addition, competitive processes involving
the activation of the Caryl-H bonds present in these substrates
can further complicate the chemistry involved in the reactions
of aryl halides with transition metal complexes.3 A better
understanding of the interplay between these various pathways
will lead to more selective and active catalysts for transformation
of these plentiful substrates.
zirconocene hydrides have been observed to react with fluoro-
and chloroaromatic substrates. Lanthanocene hydrides, isoelec-
tronic to the cationic group 4 metallocenium ions, also react
with aryl halides to give often ill-defined mixtures of products,6
stemming at least in part from â-halo elimination processes
involving the kinetic products of ortho-C-H bond activation
via σ-bond metathesis.7 This nonredox pathway is generally
thought to be dominant, although observations that the ther-
modynamic products where the halogen atom is transferred to
the electrophilic metal are sometimes assisted by light5 indicating
that one-electron redox processes may be involved for those
metals that can access lower oxidation states.8
Monomeric d0 metal hydrides, because of their high reactivity,
are generally rare and difficult to study and often must be
generated in situ via hydrogenolysis or dissociation from dimeric
or oligomeric9 structures. Thus, harnessing their potential for
Although catalytic protocols involving late metal based
catalyst systems are more widely practiced,1 it has been known
for some time that d0 early transition metal compounds are
(1) (a) Metal Catalyzed Cross-Coupling Reactions, 2nd ed.; de Miejere, A.,
Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (b) Negishi,
E. I., Ed. Handbook of Organopalladium Chemistry for Organic Synthesis;
Wiley-Interscience: New York, 2002.
(2) Recent mechanistic studies on Ar-X activation: (a) Barrios-Landeros, F.;
Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 6944. (b) Littke, A. F.; Fu, G.
C. Angew. Chem., Int. Ed. 2002, 41, 4176. (c) Lewis, A. K. de K.; Caddick,
S.; Cloke, F. G. N.; Billingham, N. C.; Hitchcock, P. B.; Leonard, J. J.
Am. Chem. Soc. 2003, 125, 10066. (d) Espinet, P.; Echavarren, A. M.
Angew. Chem., Int. Ed. 2004, 43, 4704. (e) Galardon, E.; Ramdeehul, S.;
Brown, J. M.; Cowley, A.; Hii, K. K.; Jutand, A. Angew. Chem., Int. Ed.
2002, 41, 1760.
(3) Recent examples employing late metals: (a) Fan, L.; Parkin, S.; Ozerov,
O. V. J. Am. Chem. Soc. 2005, 127, 16772. (b) Ben-Ari, E.; Gandelman,
M.; Rozenberg, H.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2003,
125, 4714. (c) Zhang, X.; Kanzelberger, M.; Emge, T. J.; Goldman, A. S.
J. Am. Chem. Soc. 2004, 126, 13192.
(4) (a) Jones, W. D. Dalton Trans. 2003, 3991 and references therein. (b)
Turculet, L.; Tilley, T. D. Organometallics 2002, 21, 3961. (c) Kraft, B.
M.; Jones, W. D. J. Am. Chem. Soc. 2002, 124, 8681. (d) Clot, E.; Megret,
C.; Kraft, B. M.; Eisenstein, O.; Jones, W. D. J. Am. Chem. Soc. 2004,
126, 5647.
(5) Wu, F.; Dash, A.; Jordan, R. F. J. Am. Chem. Soc. 2004, 126, 15360.
(6) Booij, M.; Deelman, B.-J.; Duchateau, R.; Postma, D. S.; Meetsma, A.;
Teuben, J. H. Organometallics 1993, 12, 3531.
(7) Maron, L.; Werkema, E. L.; Perrin, L.; Eisenstein, O.; Andersen, R. A. J.
Am. Chem. Soc. 2005, 127, 279.
(8) Other recent examples of ArX reactions with d0 metal hydrides: (a)
Gavenonis, J.; Tilley, T. D. Organometallics 2004, 23, 31. (b) Deutsch, P.
P.; Maguire, J. A.; Jones, W. D.; Eisenberg, R. Inorg. Chem. 1990, 29,
686.
(9) (a) Ephritikhine, M. Chem. ReV. 1997, 97, 2193. (b) Emslie, D. J.; Piers,
W. E. Coord. Chem. ReV. 2002, 233-34, 129.
9
10.1021/ja0583618 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 3303-3312
3303

A R T I C L E S
Scheme 1
Ma et al.
Scheme 2
chloro- or bromobenzene solvent resulted in quantitative
production of the titanium halocations [Cp*(tBu3PdN)TiX]+-
[B(C6F5)4]- (X ) Cl, 2Cl; X ) Br, 2Br) as the titanium-
containing products (Scheme 2). In the reaction involving
C6H5Br, formation of 2Br is accompanied by generation of
benzene and biphenyl14 in the ratios shown in the scheme,
accounting for a total of ∼1 equiv of bromobenzene used in
the reaction. The identity of the organic byproducts was verified
by 1H NMR spectroscopy and GCMS analysis; when the
reaction is carried out using D2, benzene-d1 is produced. These
reactions are facile at room temperature when vigorously stirred
(reaction complete in ∼5 min) and occur equally well in the
absence of light, with no significant change in the benzene:
biphenyl ratio.
mediating useful chemical transformations has been hampered
by the difficulties in performing detailed mechanistic studies
utilizing well-defined hydride complexes. Recently, we prepared
and fully characterized a reasonably stable half metallocene
titanium(IV) hydrido species, [Cp*(tBu3PdN)TiH(THF)n]+-
[B(C6F5)4]- (n ) 0, 1; n ) 1, 1‚THF),10 stabilized by a bulky
phosphinimide donor.11 While this material can be very cleanly
generated in solution or isolated as a THF adduct, it undergoes
a number of facile bond activation reactions, including ones
involving chloro- and bromobenzene. Here we describe in detail
the chemistry involved in these reactions and expose the
operation of diverse mechanisms of C-X and C-H bond
activations depending on the conditions employed.
The halocations 2X are isolable as solids and appear to be
monomeric, although this has not been conclusively demon-
strated. ESI-MS analysis of 2Br showed that the water adduct
was the major species (M+ ) 497), but a minor peak at M+ )
479 has an isotope pattern consistent with a monomeric structure
for 2Br. A C5H5-supported analogue of 2Cl has been shown to
be dimeric with trans disposed phosphinimide ligands across
the Ti2Cl2 core.15 However, the sterically more bulky Cp*
ligands employed here, in conjunction with the electrostatic
destabilization inherent in a dicationic dimer, may be enough
to render compounds 2X monomeric. Furthermore, there is no
evidence for rac/meso diastereomers, which might be expected
Results and Discussion
Synthetic Studies. As previously reported, the cationic
methyl complex [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- can be
cleanly generated in toluene, bromobenzene, or chlorobenzene
by activation of the neutral dimethyl precursor with [Ph3C]+-
[B(C6F5)4]-.12 In nonhalogenated solvents, such as toluene, this
species reacts rapidly with H2 to give a monomeric cationic
titanium hydride complex, 1, which can be trapped as its THF
adduct, 1‚THF, and fully characterized by NMR spectroscopy
and X-ray crystallography. This chemistry is summarized in
Scheme 1. Solutions of base-free 1 are stable in the presence
of an excess of H2 for at least several hours, but upon removal
of the dihydrogen atmosphere, 1 decomposes to a mixture of
products, some derived from reaction with the toluene solvent,13
and thus is not isolable as a base-free titanium hydrido complex.
In the presence of haloarenes, however, hydride 1 effects bond
activation of either the C-X or the ortho-C-H bonds via two
competing processes that are partitioned by the amount of
dihydrogen present in the system. For example, hydrogenolysis
of [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- using 4 atm of H2 in
t
in dimers with more equisteric ligands (Cp* and Bu3PdN).16
In any case, both of these compounds readily form monomeric
THF adducts 2X‚THF when treated with THF. The identities
of 2Cl and 2Cl‚THF were also confirmed by their independent
syntheses via treatment of Cp*(tBu3PdN)Ti(Cl)CH3 with
[Ph3C]+[B(C6F5)4]-, cleanly generating solutions of 2Cl that
were spectroscopically identical to those obtained by reaction
of 1 with C6H5Cl. Furthermore, the X-ray structure of 2Br‚
THF was obtained, and a Crystalmaker depiction of the cationic
portion of this molecule is shown in Figure 1, along with
(10) Ma, K.; Piers, W. E.; Gao, Y.; Parvez, M. J. Am. Chem. Soc. 2004, 126,
5668.
(11) Stephan, D. W. Organometallics 2005, 24, 2548 and references therein.
(12) Chien, J. C. W.; Tsai, W.-M.; Rausch, M. D. J. Am. Chem. Soc. 1991,
113, 8570.
(14) Small amounts of what we presume to be cyclohexylbenzene (∼3%) and
trace quantities (<1%) of bromobiphenyl were also observed in the C6H5-
Br reaction. The observation of small amounts of partially hydrogenated
biphenyl is curious and under further investigation. The GCMS data do
not allow us to conclusively identify the product as cyclohexylbenzene,
but it is clear from the use of D2/C6H5X or H2/C6D5X that six hydrogen or
deuterium atoms are incorporated. Cationic hydride 1 has also been observed
to rapidly hydrogenate anthracene to tetrahydroanthracene (Ma, K.; Piers,
W. E. Unpublished results).
(13) (a) The major product observed is the benzyl cation [Cp*(tBu3PdN)TiCH2-
Ph]+[B(C6F5)4]-, which can be trapped as its THF adduct [Cp*(tBu3-
PdN)TiCH2Ph(THF)]+[B(C6F5)4]-. The identity of this compound was
confirmed by separate synthesis. The mechanism of its formation is
unknown, but the preference for the benzyl species is not consistent with
a σ-bond metathesis mechanism13b unless this is the thermodynamic isomer
in this system. (b) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger,
B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J.
Am. Chem. Soc. 1987, 109, 203.
(15) (a) Cabrera, L.; Hollink, E.; Stewart, J. C.; Wei, P.; Stephan, D. W.
Organometallics 2005, 24, 1091. (b) Yue, N. L. S.; Stephan, D. W.
Organometallics 2001, 20, 2303.
(16) See for example: Zhang, S.; Piers, W. E. Organometallics 2001, 20, 2088.
9
3304 J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006

ArC−H and ArC−X (X
)
Cl, Br) Activation in Halobenzenes
A R T I C L E S
Figure 1. Crystalmaker depiction of the molecular structure of the cation
in 2Br‚THF (hydrogens omitted for clarity). Selected bond distances (Å):
Ti(1)-Br(1), 2.4365(12); Ti(1)-N(1), 1.780(5); Ti(1)-O(1), 2.084(4);
P(1)-N(1), 1.629(5). Selected bond angles (°): N(1)-Ti(1)-O(1), 103.1(2);
N(1)-Ti(1)-Br(1), 104.4(2); O(1)-Ti(1)-Br(1), 93.8(2); Ti(1)-N(1)-
P(1), 165.7(3); C(23)-O(1)-Ti(1), 125.1(3); C(26)-O(1)-Ti(1), 125.6(3).
Figure 2. Partial 1H NMR spectra of the proceeding reaction between
[Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- (bottom spectrum) and H2 (4 atm) in
bromobenzene-d5 at room temperature. The spectra show the initial
appearance of signals for 3Br, formed competitively with 2Br, and eventual
complete conversion to 3Br.
selected metrical data.17 The Ti center has a three-legged piano
stool geometry with no close contacts to the borate anion. The
Ti(1)-N(1) distance, which does not vary much in a range of
titanium phosphinimido compounds,11 is typical at 1.780(5) Å.
However, distances from Ti(1) to the O(1) and Br(1) atoms are
shorter than most comparable literature bond lengths, due to
the cationic charge of the complex and its low formal coordina-
tion number. For example, the Ti(1)-Br(1) distance in 2Br‚
THF is ∼0.1 Å shorter than the distances in a neutral titanocene
dibromide,18 and the Ti(1)-O(1) length of 2.084(4) Å is shorter
than that observed in [Cp*2TiCH3(THF)]+[BPh4]- (2.154(6)
Å).19
Scheme 3
When 1 is generated in C6D5X under 4 atm of H2 and the
1
reaction monitored by H NMR spectroscopy (Figure 2, il-
lustrated for X ) Br), two important observations are made.
First, essentially complete conversion to 2Br is accomplished
in less than 6 min under these conditions. Second, another
prominent species is observed to form early in the reaction,
which eventually gets converted to the bromotitanium cation
product. This species can be generated quantitatively (by NMR
spectroscopy) and isolated in good yields by carrying out the
reaction at pressures of less than 1 atm of dihydrogen. In fact,
use catalytic amounts of dihydrogen or the titanium hydride 1
converts [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- into these prod-
ucts, which were identified as the ortho-halophenyl cations
[Cp*(tBu3PdN)Ti(2-X-C6H4)]+[B(C6F5)4]- (X ) Cl, 3Cl; X
) Br, 3Br) by NMR spectroscopy and the solid-state charac-
terization of their THF adducts 3X‚THF (Scheme 3). The
â-haloaryl cations 3X exhibit the expected ligand resonances
1
in the H and 31P NMR spectra, as well as four characteristic
1
resonances in the H NMR spectra for the remaining C-H
aromatic protons. When solutions of 3X are exposed to 4 atm
of H2 at room temperature, conversion to the halide cations 2X
is observed, presumably via regenerated 1 and ArX.
The X-ray structures of 3Cl‚THF and 3Br‚THF were both
obtained, and the molecular structures are shown in Figures 3
and 4, respectively. The two compounds are isostructural and
exhibit three-legged piano stool (tetrahedral) geometry about
the titanium center; both crystallize with a second, noncoordi-
nated THF molecule in the crystal lattice, which is apparent
also from the elemental analysis data (see Experimental Section).
Ti(1)-N(1) and Ti(1)-O(1) distances are slightly elongated in
comparison to those in 2Br‚THF, reflecting greater steric
congestion about the metal in compounds 3X‚THF. Most
(17) Full details on this particular structure can be found in the Supporting
Information of ref 10.
(18) Klouras, N.; Nastopoulos, V.; Demekopoulos, I.; Leban, I. Z. Anorg. Allg.
Chem. 1993, 619, 1927.
(19) Bochmann, M.; Jaggar, A. J.; Wilson, L. M.; Hursthouse, M. B.; Motevalli,
M. Polyhedron 1989, 8, 1838.
9
J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006 3305

A R T I C L E S
Ma et al.
Table 1. Summary of Data Collection and Structure Refinement
Details for 3Cl‚THF, 3Br‚THF, and 4Cl
3Cl
‚THF
3Br
‚
THF
4Cl
formula
C60H62BClF20
NO2PTi
1334.24
173(2)
triclinic
P1h
12.165(2)
14.823(3)
17.749(3)
71.616(15)
77.542(12)
84.788(8)
2964.8(9)
2
-
C60H62BBrF20
NO2PTi
1378.70
173(2)
triclinic
P1h
12.244(2)
14.765(6)
17.718(4)
71.772(7)
77.525(6)
84.712(7)
2969.5(8)
2
-
C52H46BClF20
-
NPTi
fw
1190.03
173(2)
triclinic
P1h
12.343(2)
14.797(3)
14.932(4)
83.933(9)
89.154(9)
67.536(15)
2505.0(9)
2
temp, K
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
d
calcd, mg m-3
1.495
0.324
1.542
0.951
1.578
0.370
µ, mm-1
cryst size, mm 0.16 × 0.12 × 0.08 0.14 × 0.13 × 0.06 0.20 × 0.10 × 0.06
no. of rflns
measd
no. of unique
rflns
25213
13325
25258
13405
16510
8781
Figure 3. Crystalmaker depiction of the molecular structure of the cation
in 3Cl‚THF (hydrogens and the THF of crystallization omitted for clarity).
Selected bond distances (Å): Ti(1)-C(11), 2.170(3); Ti(1)-N(1), 1.791(2);
Ti(1)-O(1), 2.103(2); P(1)-N(1), 1.616(2); C(12)-Cl(1), 1.767(4). Selected
nonbonded distance (Å): Ti(1)‚‚‚Cl(1), 3.657. Selected bond angles (°):
N(1)-Ti(1)-O(1), 101.96(10); N(1)-Ti(1)-C(11), 98.82(12); O(1)-
Ti(1)-C(11), 103.15(11); Ti(1)-N(1)-P(1), 171.65(16); Ti(1)-C(11)-
C(12), 129.7(3); C(11)-C(12)-Cl(1), 120.3(3).
R1/wR2
gof
0.058/0.134
0.996
0.076/0.206
0.97
1.53/-1.66
0.0417/0.1003
0.999
0.282/-0.506
res density, e/Å3 0.55/-0.51
an interaction may be at play in the base-free compounds 3X,
it appears that ligation of the THF easily displaces this weaker
intramolecular dative bond. While it might be reasonable to
suggest that these â-haloaryl cations are intermediates along
the way to the thermodynamic products 2X via the process
described above in Scheme 2, several lines of evidence suggest
that this is not the case. In the absence of H2, products 3X are
relatively stable species at ambient temperature, but convert to
the thermodynamically favored halocations 2X at a rate that
(qualitatively) depends on the amount of hydrogen present. In
the complete absence of H2, compounds 3X (and 3X‚THF) do
undergo conversion to the halocations 2X, but at a much slower
rate than observed in the presence of H2. Furthermore, 2X
cations are the minor products in this reaction, the major species
being the benzyne adducts 4X, strongly implicating a â-halogen
elimination pathway (Scheme 4) for this chemistry. Adducts
4X comprise ∼80% of the product mixture, while compounds
2X form in roughly 20% yield. Formation of 2Cl or 2Br is
accompanied by various side products arising from the reaction
of “C6H4”, benzyne, with the haloarene solvent employed
(primarily regioisomeric Diels-Alder adducts). These products
were identified by GCMS in the reaction mixture, which also
included trace amounts of a product arising from benzyne/Cp*
coupling.
The major products of â-halogen elimination, 4X, were
identified by their NMR spectra, and the structure was confirmed
by X-ray crystallography on suitable crystals of 4Cl; the
molecular structure is shown in Figure 5. To produce compounds
4, the benzyne that is eliminated upon â-X elimination is trapped
by the Ti-N bond in 2X, forming a new Ti-Caryl bond and
converting the phosphinimido ligand into a neutral phosphin-
imine ligand. This is indicated by the significant lengthening
of the Ti(1)-N(1) bond to 2.051(2) Å20 and the bending of the
Ti(1)-N(1)-P(1) angle from ∼170 to 139.45(14)°. The 31P
Figure 4. Crystalmaker depiction of the molecular structure of the cation
in 3Br‚THF (hydrogens and the THF of crystallization omitted for clarity).
Selected bond distances (Å): Ti(1)-C(11), 2.274(14); Ti(1)-N(1), 1.787(15);
Ti(1)-O(1), 2.113(12); P(1)-N(1), 1.619(4); C(12)-Br(1), 1.931(6).
Selected nonbonded distance (Å): Ti(1)‚‚‚Br(1), 3.841. Selected bond angles
(°): N(1)-Ti(1)-O(1), 101.4(6); N(1)-Ti(1)-C(11), 95.8(7); O(1)-Ti(1)-
C(11), 101.1(6); Ti(1)-N(1)-P(1), 171.1(4); Ti(1)-C(11)-C(12), 129.8-
(5); C(11)-C(12)-Cl(1), 121.6(4).
notably, there does not appear to be any interaction between
the titanium center and the â-halogens as indicated by the long
Ti(1)-X(1) distances (3.657 Å, X ) Cl; 3.841 Å, X ) Br),
and in contrast to observations for a related [Cp*2Zr(η2-C,Cl-
2-Cl-C6H4)(CH3CN)]+[B(C6F5)4]- reported by Jordan et al. in
which there is a â-chloro metal interaction, 2.831(1) Å.5
Furthermore, the C(11)-C(12)-X(1) angles are close to 120°,
and the Ti(1)-C(11)-C(12) angles are ∼129°; both of these
angles (particularly the latter) would be expected to contract in
the event of significant metal-â-halo interaction. While such
(20) A similar lengthening in the complex [(Ph3PdN)(Ph3PdNH)TiF3]2, which
contains both phosphinimido (Ti-N ) 1.777(4) Å) and phosphinimine
(Ti-N ) 2.134(4) Å) ligands, was observed: Grun, M.; Harms, K.; zu
Kocker, R. M.; Dehnicke, K.; Goesmann, H. Z. Anorg. Allg. Chem. 1996,
622, 1091.
9
3306 J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006

ArC−H and ArC−X (X
)
Cl, Br) Activation in Halobenzenes
A R T I C L E S
Scheme 4
NMR chemical shifts for the phosphinimine ligands of com-
pounds 4 appear at ∼81 ppm, shifted downfield by ap-
proximately 24-25 ppm in comparison to the anionic ligands.
The angles within the Ti(1)-C(12)-C(11)-N(1) ring are as
expected, and the angle between the planes defined by C(12)-
C(11)-N(1) and C(12)-Ti(1)-N(1) is 20.8(2)°, indicating a
slightly puckered ring; the Ti(1)-C(11) distance is 2.421(3) Å.
Mechanistic Considerations and Studies. The above ob-
servations suggest that compounds 3X are kinetic products of
the reaction of cationic hydride 1 with C6H5X that are not
intermediates on the low energy pathway to the thermodynamic
products 2X observed in the presence of H2. Thus, three
mechanistically distinct bond activation processes are operational
in the chemistry involving cationic hydride 1 with chloro- and
bromobenzene: direct C-X bond activation, ortho-C-H bond
activation, and â-halo elimination from compounds 3. To test
this hypothesis and probe the mechanisms of the various
processes involved, further studies were performed as described
in the following sections.
Scheme 5
Direct Activation of ArX. Proposed pathways for the direct
reaction of ArX with M-H to give M-X and ArH include
σ-bond metathesis-like transition states (I) or stepwise pathways
that invoke nucleophilic attack on the aryl halide followed by
â-halo transfer to the metal. The latter path is likely for systems
where the metal hydride is hydridic in character (e.g., Cp*2-
ZrH2) and the aryl halide is highly electrophilic, for example,
C6F6.21 This is not a high probability scenario where 1 and C6H5-
Cl or C6H5Br are concerned. Furthermore, σ-bond metathesis
transition states, such as I, are computed to be rather high in
energy7 due to the positioning of carbon in the â-position of
the kite-shaped transition state and thus are also improbable
for such a transformation.
Given the biphenyl product observed (Scheme 2) in this
reaction, it appears most likely that conversion of 1 to 2X is
mediated by trace amounts of Ti(III) species generated from 1
in the presence of H2 (Scheme 5). Previously, we have observed
that the putative C5H5 (Cp) analogue of 1, generated by
treatment of [Cp(tBu3PdN)TiCH3]+[B(C6F5)4]- with H2, is
rapidly reduced to “[Cp(tBu3PdN)Ti]+[B(C6F5)4]-” presumably
by homolysis of the Ti-H bond.10 Although the more donating
and sterically protecting Cp* ligand stabilizes the Ti(IV) hydride
1, it is conceivable that the Ti(III) cation shown in the scheme
is generated in enough quantities to initiate the cycle shown.
Once generated, abstraction of X• from ArX by the Ti(III) cation
II22 (Scheme 5) yields the products 2X and a phenyl radical.
This is likely the rate-limiting step since, in a competition
Figure 5. Crystalmaker depiction of the molecular structure of the cation
in 4Cl (hydrogens omitted for clarit)y. Selected bond distances (Å): Ti(1)-
Cl(1), 2.2642(10); Ti(1)-C(12), 2.037(3); Ti(1)-N(1), 2.051(2); P(1)-
N(1), 1.644(2). Selected nonbonded distance (Å): Ti(1)-C(11), 2.421(3).
Selected bond angles (°): N(1)-Ti(1)-Cl(1), 116.15(7); N(1)-Ti(1)-
C(12),71.51(10);Cl(1)-Ti(1)-C(12),99.76(9);Ti(1)-N(1)-P(1),139.45(14);
Ti(1)-C(12)-C(11), 57.18(15); Ti(1)-N(1)-C(11), 85.31(14); C(12)-
C(11)-N(1), 112.5(2).
(21) Kraft, B. M.; Jones, W. D. J. Organomet. Chem. 2002, 658, 132.
9
J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006 3307

A R T I C L E S
Ma et al.
experiment in which 1 is generated in the presence of a mixture
of C6H5Cl/C6F5Br, 2Br is formed preferentially (2Br:2Cl ≈ 27:
1) as would be expected given the weaker C-Br versus C-Cl
bond.23,24 The phenyl radicals produced rapidly abstract a
hydrogen atom from 1, regenerating [Cp*(tBu3PdN)TiIII]+-
[B(C6F5)4]-; as the concentration of 1 is depleted, phenyl
radicals dimerize, yielding biphenyl.25 High concentrations of
H2 are necessary to convert the kinetically favored compounds
3X back to 1, effectively maintaining high concentrations of
the hydride, the source of Ti(III).
The above scenario is supported by a series of experiments
wherein the hydrogenolysis of [Cp*(tBu3PdN)TiCH3]+-
[B(C6F5)4]- in chlorobenzene is monitored by ESR spectro-
scopy. While solutions of neutral starting material Cp*(tBu3Pd
N)TiCH3)2 are ESR silent, upon activation with [Ph3C]+-
[B(C6F5)4]-, detectable amounts of trityl radical are present, as
shown in Figure 6a,26 suggesting that the trityl reagent activates
the titanium dimethyl by both abstractive27 and oxidative28
pathways. While no titanium(III) species are observed in these
solutions, upon hydrogenolysis, evidence for the trityl radical
disappears and a strong signal (295 K, g ) 1.984 G, Figure 6b)
assignable to a titanium-centered paramagnet emerges in the
spectrum and grows in intensity as the reaction proceeds. The
line width of the signal is reasonably narrow (∼5 G), but no
discernible hyperfine coupling is observed either to the ligand
nuclei or titanium isotopes (47Ti, I ) /2, 7.28%; 49Ti, I ) /2,
5.51%). The lack of hyperfine coupling is common in related
metallocene Ti(III) complexes, although in some cases, coupling
to the spin-active Ti isotopes is discernible.29 Nonetheless, given
that the g value is similar to others reported in the literature,29
these experiments provide strong evidence that a Ti(III) species
is generated under these conditions. It is likely, therefore, that
the small quantities of trityl radical present in these solutions30
initiate the direct C-X bond activation chemistry; upon close
Figure 6. ESR spectra (295 K) of (a) Ph3C• formed during the in situ
generation of [Cp*(tBu3PdN)TiCH3][B(C6F5)4] in chlorobenzene (g )
2.003). (b) Tube ≈ 40 min after H2 (4 atm) was admitted to the sample.
The intense signal (g ) 1.984) is assigned to a Ti(III) species generated
upon H2 generation.
5
7
examination of the GCMS traces of these reactions, small
amounts of Ph3CH are indeed observed. Once conversion to
2Cl is complete, this signal for the Ti(III) species (presumably
[Cp*(tBu3PdN)TiIII]+[B(C6F5)4]-) diminishes in intensity as the
titanium speciation returns to mostly Ti(IV), although some
Ti(III) persists in these solutions.
(22) Halogen carbon bond cleavage in aliphatic RX substrates by Ti(III): (a)
Yanlong, Q.; Guisheng, L.; Huang, Y. Z. J. Organomet. Chem. 1990, 381,
29. (b) Qian, Y.; Guisheng, Z.; Xiaofan, H.; Huang, Y. Z. Synlett 1991,
489. (c) Covert, K. J.; Mayol, A.-R.; Wolczanski, P T. Inorg. Chim. Acta
1997, 263, 263. (d) Agapie, T.; Dianescu, P. L.; Mindiola, D. J.; Cummins,
C. C. Organometallics 2002, 21, 1329. (e) C-O bond cleavage: Rajanbabu,
T. V.; Nugent, W. A. J. Am. Chem. Soc. 1994, 116, 986.
(23) Lagoa, A. L. C.; Diogo, H. P.; Dias, M. P.; Minas da Piedade, M. E.;
Amaral, L. M. P. F.; Ribeiro da Silva, M. A. V.; Martinho Simoes, J. A.;
Guedes, R. C.; Costa Cabral, B. J.; Schwarz, K.; Epple, M. Chem.sEur.
J. 2001, 7, 483 and references therein.
(24) (a) This step may proceed with initial electron transfer to ArX, followed
by C-X bond cleavage. The selectivity observed is also consistent with
this path since bromobenzene is more easily reduced than chlorobenzene.24b
(b) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J.-M.; Saveant, J.-M.
J. Am. Chem. Soc. 1979, 101, 3432.
(25) (a) Alternatively, it is conceivable that Ph• can react with H2 to yield
benzene; this reaction is thermodynamically favorable,25b,c but it is not clear
that it would be kinetically significant under our conditions, especially in
light of the significant quantities of biphenyl produced. (b) Mebel, A. M.;
Lin, M. C.; Yu, T.; Morokuma, K. J. Phys. Chem. A 1997, 101, 3189. (c)
Park, J.; Dyakov, I. V.; Lin, M. C. J. Phys. Chem. A 1997, 101, 8839.
(26) (a) The spectra obtained do not match exactly the literature ESR spectrum
of the trityl radical,26b which was acquired in toluene at -20 °C. However,
a spectrum of genuine trityl radical (made by dissolving the trityl radical
dimer26c in chlorobenzene) under our reaction conditions matched the trace
we observed in solutions of [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- generated
as described. (b) Chesnut, D. B.; Sloan, G. J. J. Chem. Phys. 1960, 33,
637. (c) Volz, H.; Lotsch, W.; Schnell, H.-W. Tetrahedron 1970, 26, 5343.
(27) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391.
(28) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc. 1986,
108, 1718. (b) Jordan, R. F.; Bajgur, C. S.; Willett, R.; Scott, B. J. Am.
Chem. Soc. 1986, 108, 7410. (c) Jordan, R. F.; Lapointe, R. E.; Bajgur, C.
S.; Willett, R. J. Am. Chem. Soc. 1987, 109, 4111.
ortho-C-H Activation. Kinetically competitive with the
above process at low pressures of H2 is an ortho-C-H bond
activation reaction leading to the â-haloaryl cations 3X (Scheme
3). Compounds 3X are likely formed via a σ-bond metathesis
reaction between Ti-H and an ortho-C-H bond of the
haloarene substrate to eliminate hydrogen, which if not removed,
can render this transformation reversible. Evidence for this was
provided by a measured primary kinetic isotope effect on the
formation of 3Cl‚THF using the experiment outlined in Scheme
6. Cationic hydride 1 was generated in toluene by treatment of
[Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- with hydrogen to give 1
as an insoluble liquid clathrate-like oil. This solution was
degassed to remove hydrogen, and an excess of ortho-1-Cl-
C6H4D-d1 was added at low temperature. The sample was again
degassed and allowed to warm, converting 1 to a mixture of
isotopomers of 3Cl, which were quenched by addition of THF;
the ratio of isotopomers was determined by integration methods
in the 1H and 2H NMR spectra. A 3.4:1 preponderance of 3Cl‚
THF-d1 indicated a kH/kD of up to 3.4(2).31 This is slightly
greater than isotope effects observed in other σ-bond metathesis
reactions,13b but is in line with a more linear transition state in
the six-membered geometry expected for H2 elimination from
(29) (a) Chase, P. A.; Piers, W. E.; Parvez, M. Organometallics 2000, 19, 2040.
(b) Luinstra, G. A.; ten Cate, L. C.; Heeres, H. J.; Pattiasina, J. W.; Meetsma,
A.; Teuben, J. H. Organometallics 1991, 10, 3227. (c) Pattiasina, J.; Heeres,
H. J.; van Bolhuis, F.; Meetsma, A.; Teuben, J. H.; Spek, A. L.
Organometallics 1987, 6, 1004.
(30) The intensity of the ESR signal for the trityl radical, in equilibrium with
its dimer, is ∼30 times lower than the signal observed for the Ti(III) species
present during the reaction.
9
3308 J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006

ArC−H and ArC−X (X
)
Cl, Br) Activation in Halobenzenes
A R T I C L E S
Scheme 6
η1-XC6H5 adducts.7 Dihydrogen elimination is thus likely
preceded by rapid coordination of the haloarene to the cationic
titanium center via a lone pair of electrons from the halogen to
form an η1-XC6H5 complex, depicted as shown in Scheme 6.
Recently, excellent models for such η1-XC6H5 adducts of
cationic early metal compounds5,32 have been reported, and their
formation is kinetically facile and likely not heavily influenced
by the nature of X. Consistent with this is the observation that,
when 1 is generated with less than 1 atm of hydrogen in the
presence of an excess of C6H5Cl/C6H5Br (1:1), 3Cl and 3Br
are formed at essentially the same rate, with a final ratio of
∼1.0:0.7 3Cl:3Br observed.33 This observation contrasts with
the above-mentioned selectivity in the direct activation of Ar-
X, where the activation of bromobenzene is strongly favored
over chlorobenzene. Interestingly, when the roughly 1:1 mixture
of 3Cl/3Br formed in this experiment is treated with 4 atm of
H2 (still in the presence of an excess of C6H5Cl/C6H5Br), the
bromocation 2Br is again formed predominantly, indicating that,
under these conditions, compounds 2X are formed by hydro-
genolysis of â-haloaryl cations 3X to regenerate 1, which reacts
in the Ti(III) manifold with C6H5X as described above.
The â-halogen elimination reaction depicted in Scheme 4 was
subjected to quantitative kinetic analysis by monitoring the
conversion of 0.0328 M solutions of 3Cl and 3Cl‚THF to the
product 2Cl‚THF and 4Cl via 1H NMR spectroscopy; â-bromo
elimination was studied for a 0.0328 M solution of 3Br at 292
K. Loss of compounds 3 was observed to be first order in [3]
over several half-lives at various temperatures in the range of
292-332 K. At 292 K, a first-order rate constant of 7.43(8) ×
10-5 s-1 was observed for 3Br, while the analogous rate for
3Cl was not significantly different, 9.76(8) × 10-5 s-1. Eyring
analysis of the rates for 3Cl (Figure 7a) reveals a substantial
enthalpic barrier of 19.7(4) kcal mol-1 and a ∆Sq of -10(1)
eu, indicating that some ordering is required to achieve the four-
centered transition state for â-chloro elimination.
Not surprisingly, the presence of THF hampers the elimina-
tion; kobs (292 K) for 3Cl‚THF is 7.2 × 10-6 s-1 36
. A parallel
Eyring analysis of this elimination (Figure 7b) gives a ∆Hq of
28.9(8) kcal mol-1 and a ∆Sq of 16(1). That ∆Sq is now positive
is suggestive of a mechanism which requires partial or complete
THF dissociation prior to â-halogen elimination, as shown in
Scheme 7. Addition of further equivalents of THF results in
more severe rate suppression, and by assuming that base-free
3Cl is present in a steady-state concentration in the presence of
THF, an expression for kobs as a function of [THF] (eq 1a) is
derivable. Thus, a plot of
â-Halogen Elimination. Early transition metal alkyl com-
plexes containing â-halogens are rare,34 but those that are known
are confined to ortho-haloarene complexes that are kinetically
stable by virtue of the high energy benzyne intermediate
produced upon transfer of the halogen to the metal.5,35 Nonethe-
less, â-halogen elimination from such compounds is thermo-
dynamically favorable when the eliminated benzyne is trapped.
k1k2
kobs
)
(1a)
(1b)
k-1[THF] + k2
k-1
(31) Although the experiment was designed to minimize the amount of H2/HD/
D2 present, its complete removal from the medium was not possible. The
ratio of isotopomeric products was measured as soon as possible after
conversion to 3Cl‚THF to minimize and skewing of the kinetic ratio by
equilibration/scrambling involving evolved H2/HD. The fact that the
observed ratio of isotopomers did not significantly change when the
measurement was performed again after 30 min is indicative that, under
these conditions, conversion of the kinetic ratio of 3Cl‚THF and 3Cl‚THF-
d1 to a thermodynamic mixture governed by an equilibrium isotope effect
is slow under these conditions.
(32) Bouwkamp, M. W.; Budzelaar, P. H. M.; Gercama, J.; Del Hierro Morales,
I.; de Wolf, J.; Meetsma, A.; Troyanov, S. I.; Teuben, J. H.; Hessen, B. J.
Am. Chem. Soc. 2005, 127, 14310.
(33) (a) This observation suggests that C6H5Cl and C6H5Br have similar
thermodynamic base strength toward 1. Although little is known about the
relative base strength of haloarenes, the fact that they are relatively weak
donors suggests33b that differences in donor strength to a given Lewis acid
should be relatively small. Furthermore, of the halobenzene series, these
two members have the most similar physical properties.33b (b) Kulawiec,
R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89.
1
1
k1
)
[THF] +
kobs k1k2
1/kobs versus [THF] is linear (eq 1b, Figure 7c) and allows
extraction of k1 from the y-intercept; since k2 was measured
independently in the base-free system, estimates for k-1 and
Keq for THF dissociation from 3Cl‚THF are also obtainable.
(34) (a) Stockland, R. A., Jr.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 6315.
(b) Shen, H.; Jordan, R. F. Organometallics 2003, 22, 2080. (c) Stockland,
R. A., Jr.; Foley, S. R.; Jordan, R. F. J. Am. Chem. Soc. 2003, 125, 796.
(d) Strazisar, S. A.; Wolczanski, P. T. J. Am. Chem. Soc. 2001, 123, 4728.
(35) (a) Kraft, B. M.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 2001,
123, 10973. (b) van Rooyen, P. H.; Schindehutte, M.; Lotz, S. Organo-
metallics 1992, 11, 1104.
(36) This reaction was followed only to 65% completion.
9
J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006 3309

A R T I C L E S
Ma et al.
Scheme 7
The system illustrates the range of possible pathways for the
activation of such substrates by d0 metal hydrides.
Experimental Section
General Procedures: All operations were performed under a purified
argon atmosphere using glovebox or vacuum line techniques. Toluene,
hexanes, and THF were dried and purified by passing through activated
alumina and Q5 columns. Bromobenzene, chlorobenzene, and bro-
mobenzene-d5 (C6D5Br) were dried over CaH2 and distilled under
reduced pressure. Deuterated NMR solvents toluene-d8 (C7D8) and THF-
1
d8 were dried and distilled from sodium/benzophenone ketyl. H, 11B,
13C, 19F, and 31P NMR experiments were performed on Bruker AMX-
300 and DRV-400 spectrometers. Data are given in parts per million
1
(ppm) relative to residual solvent signals for H and 13C spectra. 11B,
19F, and 31P spectra were referenced to external BF3‚Et2O, C6H5F, and
H3PO4, respectively. The 11B and 19F NMR spectra of the anion
[B(C6F5)4]- in ionic complexes do not significantly change and appear
at the following positions: 11B NMR (C6D5Br 128.2 MHz): δ -16.9.
19F NMR (C6D5Br, 282.4 MHz): δ - 132.8 (ortho-F), -163.2 (para-
F), -167.0 (meta-F). Elemental analyses were performed in the
microanalytical laboratory of the Department of Chemistry (University
of Calgary). X-ray crystallography was performed on suitable crystals
coated in paratone oil and mounted on a Rigaku AFC6S diffractometer
(University of Calgary). [Ph3C]+[B(C6F5)4]- was supplied as a generous
gift by Nova Chemicals Corp. The compounds Cp*(tBu3PdN)TiCl2,
Figure 7. (a) Eyring plot of the thermolysis of 3Cl (292-332 K) in C6D5-
Br. (b) Eyring plot of the thermolysis of 3Cl‚THF (292-332 K) in C6D5-
Br. (c) Plot of 1/kobs versus THF concentration (0.075-0.484 M) with [Ti]
) 0.0328 M for the thermolysis of 3Cl‚THF. The reactions were carried
out in C6D5Br at 325 K. Intercept ) 542.9; slope ) 4349; linear correlation
coefficient ) 0.997.
These data are given in the legend for Scheme 7 and provide a
quantitative basis for the observed rate suppression in the
presence of THF. At concentrations of THF beyond the points
shown in Figure 7c, rate suppression is extreme, such that the
equilibrium is effectively saturated in favor of 3Cl‚THF and
the rate of â-chloro elimination becomes negligible under these
conditions.
Cp*(tBu3PdN)Ti(CH3)2,37 [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- 10 and
,
2-2H-chlorobenzene38 were prepared by literature procedures.
Synthesis of 2Cl and 2Cl‚THF. A chlorobenzene solution (0.4 mL)
of Cp*(tBu3PdN)Ti(CH3)2 (28 mg, 0.065 mmol) was added dropwise
to a chlorobenzene solution (0.4 mL) of [Ph3C]+[B(C6F5)4]- (60 mg,
0.067 mmol) in a J-Young NMR tube at room temperature. The
resulting solution was shaken for about 1 min before the NMR tube
was evacuated and recharged with H2 (ca. 4 atm). With agitation, the
Conclusions. The cationic titanium complex 1 is a rare
monomeric hydrido species that while stable enough to generate
and characterize is nonetheless highly reactive. This study has
demonstrated its rapid reactivity with chloro- and bromobenzene
by diverse pathways of bond activation, including direct C-X
bond cleavage mediated by catalytic amounts of Ti(III), σ-bond
metathetical elimination of H2 for â-haloaryl cations, and â-halo
elimination (indirect C-X activation) from these aryl cations.
1
reaction was monitored by H NMR spectroscopy; when conversion
(37) Stephan, D. W.; Stewart, J. C.; Gue´rin, F.; Courtney, S.; Kickam, J.; Hollink,
E.; Beddie, C.; Graham, T.; Wei, P.; Spence, R. E. v. H.; Xu, W.; Koch,
L.; Gao, X.; Harrison, D. G. Organometallics 2003, 22, 1937.
(38) Roberts, J. D.; Semenow, D. A.; Simmons, H. E.; Carlsmith, L. A. J. Am.
Chem. Soc. 1956, 78, 601.
9
3310 J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006

ArC−H and ArC−X (X
)
Cl, Br) Activation in Halobenzenes
A R T I C L E S
was complete, the volume of the reaction mixture was reduced to ca.
0.2 mL under vacuum. Hexanes (ca. 4 mL) were added, and the product
was precipitated as a red solid. The supernatant was decanted, and the
resulting red solid was dried under reduced pressure to afford 2Cl.
Yield, 66 mg (91%). 2Cl was dissolved in THF to form the THF adduct
2Cl‚THF quantitatively. NMR data (C6D5Br) for 2Cl: 1H NMR δ 1.85
precipitation and the solid isolated by decanting the supernatant and
washing with cold hexanes (2 mL). The solid was dried under reduced
pressure to afford 3Cl. Yield, 232 mg (85%). The compound 3Cl was
dissolved in THF to form quantitatively the THF adduct 3Cl‚THF.
The X-ray quality crystals of 3Cl‚THF were grown from layering the
THF solution of compound with hexane at -30 °C. Method 2: Excess
chlorobenzene (5 equiv) was added into a toluene solution of
[Cp*(tBu3PdN)TiH)]+[B(C6F5)4]- which was generated quantitatively
by reacting [Cp*(tBu3PdN)TiCH3)]+ [B(C6F5)4]- and H2 (4 atm) at
room temperature. Upon addition, the color of the reaction mixture
was changed from orange to red immediately. The volatiles were
removed, and the resulting red oil was triturated with cold hexanes (3
× 2 mL) to afford 3Cl, which was dried in vacuo. NMR data (C6D5-
Br) for 3Cl: 1H NMR (263 K) δ 7.23 (d, J ) 7.2 Hz, 1H, CarylH), 7.09
3
(s, 15H, C5(CH3)5), 1.14 (d, JH-P ) 14.3 Hz, 27H, C(CH3)3). 13C
1
NMR: δ 128.1 (C5(CH3)5), 40.1 (d, JC-P ) 40.2 Hz, PC), 29.3
(C(CH3)3), 13.2 (C5(CH3)5). 31P{1H} NMR: δ 63.2. Anal. Calcd for
C46H42BClF20NPTi: C, 49.60; H, 3.80; N, 1.26. Found: C, 49.25; H,
3.58; N, 1.58. NMR data (C6D5Br) for 2Cl‚THF: 1H NMR δ 3.68 (m,
4H, OCH2CH2), 1.87 (s, 15H, C5(CH3)5), 1.64 (m, 4H, OCH2CH2), 1.10
3
(d, JH-P ) 14.3 Hz, 27H, C(CH3)3). 13C NMR: δ 127.7 (C5(CH3)5),
76.7 (OCH2CH2), 41.2 (d, 1JC-P ) 42.3 Hz, PC), 28.9 (C(CH3)3), 25.6
(OCH2CH2), 12.2 (C5(CH3)5). 31P{1H} NMR (C6D5Br): δ 60.6. Anal.
Calcd for C50H50BF20NOPClTi: C, 50.63; H, 4.25; N, 1.18. Found:
C, 50.54; H, 4.60; N, 1.10.
(d, J ) 6.4 Hz, 1H, CarylH), 7.04 (t, J ) 6.4 Hz, 1H, CarylH), 6.90 (t,
3
J ) 7.2 Hz, 1H, CarylH), 1.71 (s, 15H, C5(CH3)5), 0.89 (d, JH-P
)
13.4 Hz, 27H, C(CH3)3). 13C NMR (263 K): δ 183.4 (Ti-C), 135.3,
130.2, 129.4 (Caryl), 128.5 (C5(CH3)5), 125.2, 123.1 (Caryl), 40.8 (d, 1JC-P
) 44.6 Hz, PC), 25.29 (C(CH3)3), 11.9 (C5(CH3)5). 31P{1H} NMR (263
K): δ 57.9. Anal. Calcd for C52H46BClF20NPTi: C, 52.48; H, 3.90; N,
1.18. Found: C, 52.31; H, 4.35; N, 1.56. NMR data (THF-d8) for 3Cl‚
THF: 1H NMR δ 7.45 (dd, J ) 7.2 Hz, J ) 2.0 Hz, 1H, CarylH), 7.30
(dd, J ) 7.4 Hz, J ) 1.3 Hz, 1H, CarylH), 7.11 (dt, J ) 7.2 Hz, J ) 2.0
Hz, 1H, CarylH), 7.01 (dt, J ) 7.4, J ) 1.3 Hz, 1H, CarylH), 3.64 (m,
4H, OCH2CH2), 2.10 (s, 15H, C5(CH3)5),1.79 (m, 4H, OCH2CH2), 1.57
(d, 3JH-P ) 13.6 Hz, 27H, C(CH3)3).13C NMR: δ 189.3 (Ti-C), 139.5,
130.7 (Caryl), 129.7 (C5(CH3)5), 129.3, 129.2, 124.6 (CarylH), 68.2 (OCH2-
CH2), 42.4 (d, 1JC-P ) 42.3 Hz, PC), 30.0 (C(CH3)3), 26.4 (OCH2CH2),
13.4 (C5(CH3)5). 31P{1H} NMR (C6D5Br): δ 56.7. Anal. Calcd for
C56H54BF20NOPClTi‚C4H8O: C, 54.01; H, 4.68; N, 1.05. Found: C,
54.68; H, 4.36; N, 1.24.
An analogous procedure was used to produce 2Br and 2Br‚THF in
88% yield. NMR data (C6D5Br) for 2Br: 1H NMR δ 1.87 (s, 15H,
3
C5(CH3)5), 1.17 (d, JH-P ) 14.2 Hz, 27H, C(CH3)3). 13C NMR: δ
127.8 (C5(CH3)5), 42.1 (d, 1JC-P ) 40.2 Hz, PC), 29.1 (C(CH3)3), 13.3
(C5(CH3)5). 31P{1H} NMR: δ 64.4. Anal. Calcd for C46H42BF20-
NPBrTi: C, 47.70; H, 3.65; N, 1.21. Found: C, 48.15; H, 3.76; N,
1.27. NMR data (C6D5Br) for 2Br‚THF: 1H NMR δ 3.70 (m, 4H,
CH2O), 2.18 (s, 15H, C5(CH3)5), 1.70 (m, 4H, CH2CH2O), 1.13 (d,
3JH-P ) 14.0 Hz, 27H, C(CH3)3). 13C NMR: δ 129.7 (C5(CH3)5), 77.3
1
(CH2O), 41.4 (d, JC-P ) 43.3 Hz, PC), 29.6 (C(CH3)3), 25.6
(OCH2CH2), 12.7 (C5(CH3)5). 31P{1H} NMR: δ 52.7. Anal. Calcd for
C50H50BF20NOPBrTi: C, 48.81; H, 4.10; N, 1.14. Found: C, 49.11;
H, 3.92; N, 1.55.
Synthesis of Cp*(tBu3PdN)Ti(CH3)Cl. B(C6F5)3 (11 mg, 0.021
mmol) was added into a toluene solution (15 mL) of Cp*(tBu3PdN)-
Ti(CH3)2 (192 mg, 0.45 mmol) and Cp*(tBu3PdN)TiCl2 (210 mg, 0.45
mmol) at room temperature. The resulting mixture was stirred overnight
and a small amount oily precipitate formed. The supernatant was
decanted into another flask, and its volume was reduced to 2 mL under
reduced pressure. Cooling to -30 °C to afford yellow crystals. Yield,
305 mg (76%). NMR data (C6D6) 1H NMR: δ 2.09 (s, 15H, C5(CH3)5),
Synthesis of 3Br and 3Br‚THF. Analogous procedures to those
described for 3Cl and 3Cl‚THF were employed to prepare 3Br in 88%
yield. NMR data (C6D5Br) for 3Br: 1H NMR (260 K) δ 7.32 (d, J )
7.3 Hz, 1H, CarylH), 7.05 (d, J ) 6.4 Hz, 1H, CarylH), 6.95 (m, 2H,
3
C
arylH), 1.76 (s, 15H, C5(CH3)5), 0.96 (d, JH-P ) 13.7 Hz, 27H,
C(CH3)3). 13C NMR (260 K): δ 185.0 (Ti-C), 135.2 (Caryl), 129.4
(C5(CH3)5), 128.7, 127.9, 125.7, 119.1 (Caryl), 41.0 (d, JC-P ) 40.6
1
Hz, PC), 28.5 (C(CH3)3), 12.3 (C5(CH3)5). 31P{1H} NMR (260 K): δ
57.3. Anal. Calcd for C52H46BBrF20NPTi: C, 50.59; H, 3.76; N, 1.13.
Found: C, 49.96; H, 3.76; N, 1.22. NMR data (THF-d8) for 3Br‚
THF: 1H NMR δ 7.48 (dd, J ) 7.3 Hz, J ) 1.9 Hz, 1H, CarylH), 7.38
(dd, J ) 6.9 Hz, J ) 2.4 Hz, 1H, CarylH), 7.00 (m, 2H, CarylH), 3.61
(m, 4H, OCH2CH2), 2.12 (s, 15H, C5(CH3)5), 1.77 (m, 4H, OCH2CH2),
1.52 (d, 3JH-P ) 13.6 Hz, 27H, C(CH3)3). 13C NMR: δ 189.5 (Ti-C),
132.5, 130.8, 129.9 (Caryl), 129.8 (C5(CH3)5), 129.4, 124.8 (Caryl), 68.4
3
1.26 (d, JH-P ) 14.0 Hz, 27H, C(CH3)3), 0.92(s, 3H, Ti-CH3). 13C
NMR: δ 121.4 (C5(CH3)5), 47.3 (Ti-CH3), 42.0 (d, 1JC-P ) 42.1 Hz,
PC), 30.4 (C(CH3)3), 13.0 (C5(CH3)5). 31P{1H} NMR: δ 37.8. Anal.
Calcd for C23H45NPClTi: C, 61.40; H, 10.08; N, 3.11. Found: C, 61.61;
H, 9.88; N, 3.35.
Generation of 2Cl from Cp*(tBu3PdN)Ti(CH3)Cl. A C6D5Br
solution (0.3 mL) of Cp*(tBu3PdN)Ti(Cl)CH3 (24 mg, 0.053 mmol)
was added dropwise to a C6D5Br solution (0.3 mL) of [Ph3C]+[B(C6F5)4]-
(49 mg, 0.053 mmol) in a NMR tube at room temperature. The resulting
solution was shaken for about 5 min before the NMR spectra were
recorded. The spectra were identical to those described above.
Determination of Organic Byproducts in Generation of 3Br. 3Br
was prepared as described above in C6H5Br. The reaction mixture was
passed rapidly through a short silica column, followed by a small portion
of pure solvent. The filtrate was analyzed by GCMS, using naphthalene
as an internal standard. Products were identified by spiking with
authentic samples. Ratios were determined by integration of the peak
areas in the GC trace.
Synthesis of 3Cl and 3Cl‚THF. Method 1: A chlorobenzene
solution (2 mL) of Cp*(tBu3PdN)Ti(CH3)2 (98 mg, 0.23 mmol) was
added dropwise to a chlorobenzene solution (3 mL) of [Ph3C]+[B(C6F5)4]-
(210 mg, 0.23 mmol) in a 15 mL flask equipped with a Kontes valve
at room temperature. The resulting solution was stirred before the
headspace was evacuated and recharged with H2 (<1 atm). The color
of the reaction mixture changed rapidly from orange to red whereupon
the hydrogen pressure was relieved under vacuum. Hexanes (ca. 10
mL) were condensed into the vessel, precipitating the red product. The
mixture was cooled to -30 °C overnight to complete product
1
(OCH2CH2), 42.6 (d, JC-P ) 42.7 Hz, PC), 30.2 (C(CH3)3), 26.4
(OCH2CH2), 13.6 (C5(CH3)5). 31P{1H} NMR: δ 56.1. Anal. Calcd for
C56H54BF20NOPBrTi‚C4H8O: C, 52.27; H, 4.53; N, 1.02. Found: C,
53.21; H, 4.50; N, 1.48.
Determination of the Primary Kinetic Isotope Effect in the
Formation of 3Cl. [(Cp*)Ti(NPtBu3)H)]+[B(C6F5)4]- was generated
by reacting [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- and H2 (4 atm) at room
temperature in a J-Young NMR tube in toluene-d8. Excess dihydrogen
was removed, and an excess of 2-D-ClC6H4 was added. The NMR tubes
were thoroughly shaken, and residual H2 was removed under vacuum.
THF was condensed into the NMR tube either immediately or after
1
∼6 min at room temperature. The H NMR spectra were recorded,
and the ratio of isotopomers was determined by integration.
Competition Experiments. Formation of 2Cl/2Br: In a typical
experiment, the compound [Cp*(tBu3PdN)TiCH3]+[B(C6F5)4]- was
generated in a J-Young NMR tube in a mixed solvent (C6H5Cl:C6H5-
Br ) 1.9:1.0 or 9.4:1.0). Dihydrogen (4 atm) was admitted into the
tube and the product mixture assayed by 31P NMR spectroscopy. The
ratio of 2Cl and 2Br was determined by integration, and the selectivity
was determined by correcting for the solvent ratio. Formation of 3Cl/
9
J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006 3311

A R T I C L E S
Ma et al.
3Br: A method analogous to method 1 for the preparation of 3X
described above was employed using solvent mixtures of either C6H5-
Br:C6H5Cl ) 1:1.1 or C6H5Br:C6H5Cl ) 1:4. The ratio of products
was determined by integration of 31P NMR spectra and corrected for
the solvent ratio used.
transferred into a J-Young NMR tube and kept at -78 °C prior to
analysis. Solutions of 3Cl (136 mg) and 3Br (142 mg) were prepared
similarly. Samples were warmed in a water bath prior to being
introduced into a thermostated NMR probe at temperatures calibrated
externally using an ethylene glycol or methanol standard. After
equilibration, the reactions were monitored over time, acquiring spectra
with suitable relaxation delays between pulses. For the THF-dependent
studies, additional equivalents of THF were added by syringe into the
NMR tubes in the glovebox in C6D5Br, and the concentrations of THF
Synthesis of 4Cl. A chlorobenzene solution (1 mL) of 3Cl (43 mg,
0.036 mmol) was stirred at room temperature for 3 days, and the color
gradually turned dark. Three milliliters of hexanes was added, and the
resulting solution was cooled to -30 °C to afford brown crystals of
4Cl. Yield, 22 mg (51%). NMR data (C6D5Br) for 4Cl: 1H NMR δ
7.47 (d, J ) 7.3 Hz, 1H, CarylH), 7.32 (d, J ) 8.1 Hz, 1H, CarylH), 7.14
(t, J ) 8.1 Hz, 1H, CarylH), 6.77 (t, J ) 7.3 Hz, 1H, CarylH), 1.66 (s,
1
in the solution were calculated from the integrations of the H NMR
signals of THF with respect to those of 3Cl‚THF, thereby taking into
account the additional equivalent of free THF present in the crystalline
samples of the adduct. The progress of reactions was monitored by
integration of the Cp* peak in 1H NMR spectrum. The reactions were
followed for a period of at least 3 half-lives except the run for 3Cl‚
THF at 292 K, which was stopped after 35 h (∼65% conversion).
3
15H, C5(CH3)5), 1.11 (d, JH-P ) 13.2 Hz, 27H, C(CH3)3). 13C NMR:
δ 196.6 (Ti-C), 132.8, 132.3, 130.3 (Caryl), 129.5 (C5(CH3)5), 124.3(br),
122.7 (CarylH), 42.3 (br, PC), 30.1 (C(CH3)3), 12.8 (C5(CH3)5). 31P-
{1H} NMR: δ 81.9. Anal. Calcd for C52H46BClF20NPTi: C, 52.48; H,
3.90; N, 1.18. Found: C, 52.33; H, 3.74; N, 1.01.
Synthesis of 4Br. This compound was prepared similarly to 4Cl in
55% yield. NMR data (C6D5Br) for 4Br: 1H NMR δ 7.49 (d, J ) 7.2
Hz, 1H, CarylH), 7.34 (d, J ) 7.9 Hz, 1H, CarylH), 7.02 (t, J ) 7.9 Hz,
1H, CarylH), 6.76 (t, J ) 7.2 Hz, 1H, CarylH), 1.68 (s, 15H, C5(CH3)5),
1.22 (d, 3JH-P ) 13.2 Hz, 27H, C(CH3)3). 13C NMR: δ 199.2 (Ti-C),
132.4, 130.3 (Caryl), 128.2 (C5(CH3)5), 128.1, 124.3(br), 122.7 (CarylH),
41.9 (d, J ) 42.7 Hz, PC), 29.1 (C(CH3)3), 13.3 (C5(CH3)5). 31P{1H}
NMR: δ 81.0. Anal. Calcd for C52H46BBrF20NPTi: C, 50.59; H, 3.76;
N, 1.13. Found: C, 49.92; H, 3.76; N, 0.99.
Kinetic Studies of â-Halo Elimination Reactions. Stock solutions
(0.0328 M) of 3Cl‚THF were prepared by dissolving 153 mg of the
compound, freshly crystallized from THF in 3.5 mL of C6D5Br and
storing frozen at -35 °C. In the glovebox, portions (0.5 mL) were
Acknowledgment. Funding for this work was provided by
Nova Chemicals Inc. of Calgary, Alberta, and NSERC of
Canada through a CRD grant to W.E.P. Mr. Edwin van der
Eide is thanked for help with acquiring and interpreting the ESR
spectra reported herein.
Supporting Information Available: ORTEP diagrams for the
structurally characterized compounds (3Cl‚THF, 3Br‚THF,
4Cl, 4 pages, print/PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA0583618
9
3312 J. AM. CHEM. SOC. VOL. 128, NO. 10, 2006