Kinetic control and multiple mechanisms for C-H bond activation by a Zr=N complex

Article abstract of DOI:10.1002/anie.200701816

The [Cp*CpZr=NCMe₃(thf)] system provides the first direct measurement of kinetic selectivity in sp, sp², and sp³ C-H bond activation with Group 4 imido complexes. This feature allows the design of selectivity and mechanistic experiments to probe the 1,2-RH-addition event. Substrates reacting with the highest relative rates generally form the most thermodynamically stable products. Cp* = η⁵-C ₅Me₅, Cp = η⁵-C₅H₅.

Full text of DOI:10.1002/anie.200701816

Communications
DOI: 10.1002/anie.200701816
À
C H Activation
À
Kinetic Control and Multiple Mechanisms for C H Bond Activation by
a Zr N Complex**
=
Helen M. Hoyt and Robert G. Bergman*
A promising approach to the functionalization of unactivated
found that the transient species 2 reacted with benzene to
À
À
hydrocarbons (RH) involves a 1,2-RH-addition across metal
produce the C H activation product [Cp*CpZr(Ph)-
(NHCMe3)] (3 f) in both cases. In contrast to most Lewis
heteroatom multiple bonds [MX] (M = Groups 4–6, X =
NR,[1,2] CR2,[3] CR,[4] O[5]) to afford products of general
structure [M(XH)(R)]. Extensive selectivity studies have
been performed on Group 4 imido [M = NR’] systems that
activate hydrocarbons under reversible conditions.[1l–m] How-
ever, isolation of kinetically controlled product ratios has not
yet been achieved owing to the reverse reaction: competitive
extrusion of RH from the resulting [M(NHR’)(R)] products.
One process requiring kinetic control of the product distri-
=
base (LB) complexes of the type [LnZr NR(LB)], the use of
complex 1 resulted in the activation of benzene at a lower
temperature than did its corresponding methyl amide com-
plex 4.[7] That methyl amide 4 appeared more thermally stable
than imide 1 suggested that the analogous hydrocarbyl amide
À
products of C H activation (e.g., 3 f) may form irreversibly
from 1 at 458C.
Given the unique reactivity of 1, kinetic control would
require that products [Cp*CpZr(R)(NHCMe3)] (3-R) form
irreversibly under the reaction conditions. Complex 1 reacted
with tert-butylacetylene and (E)-tert-butylethylene at ambient
temperature to generate products 3a and 3d, respectively,
whereas gentle heating (458C) was required to activate other
substrates depicted in Table 1 (forming 3b–c,[6b] 3e–k).[6c]
These preliminary data suggested that the hydrocarbon
substrate was involved in the rate-determining step of the
reaction.
À
bution is diastereoselective activation of a C H bond, a
desirable transformation wherein kinetic selectivity will play
a critical role. Herein, we report the first Zr NR system
=
capable of generating kinetic product distributions in the
selective C H bond activation of unactivated sp and sp3
2
À
hydrocarbons. The results provide important information
regarding the selectivity and mechanism of the 1,2-RH-
addition event.
In an ongoing study in our laboratory,[1a–e] zirconium
complexes bearing Cp*Cp ligands emerged as ideal candi-
À
To determine the thermal stability of C H activation
À
dates for the investigation of kinetic control in C H bond
products 3a–k, these complexes were thermolyzed with
10 equivalents of an imido trapping agent. Upon extrusion
of RH from complexes 3a–k, intermediate 2 was generated
and trapped irreversibly with di-p-tolylacetylene to produce
metallacycle 5, which subsequently rearranged to cyclometal-
lated complex 6 upon extended heating [Eq. (3)].[8] Elevated
activation (Cp* = h5-C5Me5, Cp = h5-C5H5). Specifically,
=
imido precursors [Cp*CpZr NCMe3(thf)] (1) and
[Cp*CpZr(Me)(NHCMe3)] (4) extrude THF or methane,
respectively, on thermolysis to form the transient imido
[6a]
=
complex [Cp*CpZr NCMe3] (2) [Eq. (1) and (2)].
We
temperatures (75–1508C) were generally required for extru-
sion of RH from 3-R.[8,9a] Given the high thermal stability of
À
these products, we presume that C H bond activation from 1
À
is effectively irreversible at 458C. This confirms that C H
[*] H. M. Hoyt, Prof. R. G. Bergman
Department of Chemistry
activation occurred under kinetic control at 458C.
University of California, Berkeley
Berkeley, CA 94720 (USA)
Fax: (+1)510-642-7714
To gain mechanistic insight into this process, we per-
formed a crossover labeling study to determine whether the
1,2-RH-addition event proceeded in an inter- or intramolec-
ular fashion. Reaction of complex 1 with a mixture of
[D0]mesitylene/[D6]benzene solely furnished products 3h/
[D6]-3 f. The lack of crossover products ([D1]-3h/[D5]-3 f)
indicated that 1,2-RH(D)-addition to 2 placed the R and
H(D) groups from the reacting hydrocarbon on the same
metal center in the final product: an intramolecular process.
E-mail: rbergman@berkeley.edu
[**] This work was supported by the NIH through grant no. GM-25459.
We thank Dr. Herman van Halbeek for assistance with 2D NMR
spectroscopy experiments.
Supportinginformation for this article (includingfull experimental
procedures) is available on the WWW under http://www.ange-
wandte.orgor from the author.
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Chemie
À
Table 1: Relative kinetic (kRH/kR’H) and thermodynamic (KRH/R’H) selectivity of C H bond activation for
substrates RH by complex 1.
Arenes bearing electron-withdraw-
ing substituents (e.g., 1,3-bis(tri-
fluoromethyl)benzene to form 3e)
reacted only twice as fast as other
arenes, even in cases where the
thermodynamic stabilities of the
products were substantially differ-
ent (see below).
The high relative rate for acti-
vation of tert-butylacetylene led us
to speculate that 1 may activate
alkynes by a mechanism distinct
from that of direct 1,2-RH-addi-
3-R[a]
3a
RH
kRH/kR’H
>106
KRH/R’H
3-R[a]
3 f
RH
kRH/kR’H
KRH/R’H
[b]
[c]
[b]
[c]
tion via transition state
7
n.d.[d]
17
170
(Scheme 1, path a). In analogy to
=
a
Ti O system,[5] alkynes may
3b
700
370
3g
16
160
undergo rate-determining metalla-
cycle formation (Scheme 1, kmet
,
path b) with 2 to form intermediate
metallacycle 8, followed by rear-
rangement (krearr) to provide 3a. In
a KIE competition study to form
3a/[D1]-3a, kH/kD was found to be
0.8. In contrast with the large
primary KIE values determined
for other substrates, this small,
3c
3d
280
220
40
3h
15
30
1
750
3i
3j
1.7
1.5
n.d.[d]
3e
36
26000
3k
1
0.6[e]
[8]
À
[a] Products 3-R formed from 1 and the explicitly drawn C H bond in the correspondingsubstrate RH.
À
inverse value indicated that a C
[b] Relative rates (kRH/kR’H) for RH activation by 1 calculated per reactive RH bond at 458C; kR’H =kRH for
2,3-dimethyl-2-butene. [c] Relative thermodynamic stability (KRH/R’H) of products 3-R relative to 3i
measured at 1508C from 1 and calculated per reactive RH bond. [d] Determination of thermodynamic
selectivity was unsuccessful owingto decomposition of 3-R. [e] Thermodynamic selectivity measured at
1058C.
H bond was not broken on or
before the rate-determining step.
Thus, we propose that alkynes
follow path b with an initial rate-
determining metallacycle-forming
The nature of the transition state was examined by
performing kinetic isotope effect (KIE) studies. Measured at
458C, KIE values for reaction of 1 with [D0]/[D6]benzene,
[D0]/[D12]n-pentane,[9c] [D0]/[D12]mesitylene, and [D0]/[D1]-
(E)-neohexene were kH/kD = 7.4, 8.9, 8.8, and 6.9, respective-
ly.[1n] These large positive values indicated a primary KIE
À
consistent with a direct C H bond-breaking event in the rate-
determining step of the reaction. Most likely, this occurred
through a four-center transition state, wherein transfer of H
from R to N is relatively linear.[10] These values contrasted
with the equilibrium isotope effect for [D0]/[D6]benzene at
1508C, KH/KD = 1.02, confirming that we have been able to
Scheme 1. Mechanistic pathways for activation of tert-butylacetylene.
select independent thermodynamic conditions by varying the
reaction temperature.
Given that the hydrocarbon substrate underwent con-
=
certed, rate-determining 1,2-RH-addition to the Zr N bond
step, whereas other unsaturated substrates react either
of intermediate 2, intermolecular competition studies were
conducted under kinetic conditions (458C) such that the
observed product ratios ([3-R]/[3-R’]) reflected the relative
rates (kRH/kR’H) of RH activation (Table 1). Generally,
analogously to path a or via reversible metallacycle (kmet/
kmetÀ1) formation followed by a rate-determining rearrange-
ment reaction (krearr).
To compare kinetic selectivity data to the relative
thermodynamic stability (KRH/R’H) of products 3-R, intermo-
lecular competition studies were performed at higher temper-
atures (1508C) in which interconversion of the products takes
place: 3e @ 3d, 3b > 3 f, 3g > 3c, 3h > 3i, 3k (Table 1).[8] With
modest exceptions, substrates reacting with the highest
relative rates generally formed the most thermodynamically
À
substrates bearing C H bonds with a greater degree of
s character formed products 3-R with the highest relative
rates:
sp (3a) > alkene sp2 (3b,
3d) ꢀ cPr (3c) > are-
ne sp2 (3e–g) > sp3 (3h–k).[9b] Less sterically hindered sub-
strates reacted up to 15 times faster than larger substrates
bearing similar electronic properties (3b > 3d; 3h > 3k).
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Communications
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In summary, mixed-ring Cp*Cp complex 1 provided the
first example of the isolation of kinetic product ratios in sp2
3
À
=
and sp C H bond activation with Group 4 M NR complexes.
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mechanistic experiments to probe the 1,2-RH-addition event.
ꢁ
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À
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À
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ancillary Cp-based ligands.
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Received: April 24, 2007
Published online: June 19, 2007
ꢁ
[5] Activation of HC CR: J. L. Polse, R. A. Andersen, R. G.
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obtained (83–99%) on thermolysis of 4 in hydrocarbon solvents
RH.[8]
[7] Reaction of 4 requires elevated temperatures (compared with 1)
to extrude CH4.
[8] See the Supporting Information for further details.
[9] a) Product 3j slowly extrudes n-pentane at 458C. b) The kinetic
selectivity for n-pentane remained constant up to 50% con-
version. c) KIE measured at 20% conversion.
À
Keywords: C H activation · hydrocarbons · isotope effects ·
reaction mechanisms · zirconium
.
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5582
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