Zirconium-mediated metathesis of imines: A study of the scope, longevity, and mechanism of a complicated catalytic system

Article abstract of DOI:10.1021/ja992869r

By kinetically stabilizing imidozirconocene complexes through the use of a sterically demanding ligand, or by generating a more thermodynamically stable resting state with addition of diphenylacetylene, we have developed transition metal-catalyzed imine metathesis reactions that are mechanistically analogous to olefin metathesis reactions catalyzed by metal carbene complexes. When 5 mol% of Cp*Cp(THF)Zr = N'Bu is used as the catalyst precursor in the metathesis reaction between PhCH = NPh and p-TolCH = N-p-Tol, a 1:1:1:1 equilibrium mixture with the two mixed imines p-TolCH = NPh and PhCH = N-p-Tol is generated in C₆D₆ at 105 °C. The catalyst was still active after 20 days with an estimated 847 turnovers (t(1/2) 170 m; TON = 1.77 h^-1). When the azametallacyclobutene Cp₂Zr(N(Tol)C(Ph) = C(Ph)) is used as the catalyst precursor under similar reaction conditions, a total of 410 turnovers are obtained after 4 days (t(1/2) 170 m; TON = 4.3 h^-1). An extensive kinetic and equilibrium analysis of the metallacyclobutene- catalyzed metathesis of PhCH = N-p-Tol and p-F-C₆H₄CH = N-p-F-C₆H₄ was carded out by monitoring the concentrations of imines and observable metal- containing intermediates over time. Numerical integration methods were used to fit these data to a detailed mechanism involving coordinatively unsaturated (16-electron) imido complexes as critical intermediates. Examination of the scope of reaction between different organic imines revealed characteristic selectivity that appears to be unique to the zirconium-mediated system. Several zirconocene complexes that could generate the catalytically active 'CpCp'Zr = NAr' (Cp' = Cp or Cp*) species in situ were found to be effective agents in the metathetical exchange between different N-aryl imines. N-Alkyl aldimines were found to be completely unreactive toward metathesis with N-aryl aldimines, and metathesis reactions involving the two N-alkyl imines TolCH = NPr and PhCH = NMe gave slow or erratic results, depending on the catalyst used. Metathesis was observed between N-aryl ketimines and N-aryl aldimines, but for N-aryl ketimine substrates, the catalyst resting state consists of zirconocene enamido complexes, generated by the formal C-H activation of the α position of the ketimine substrates.

Full text of DOI:10.1021/ja992869r

VOLUME 122, NUMBER 5
FEBRUARY 9, 2000
©
Copyright 2000 by the
American Chemical Society
Zirconium-Mediated Metathesis of Imines: A Study of the Scope,
Longevity, and Mechanism of a Complicated Catalytic System
Rebecca L. Zuckerman, Shane W. Krska, and Robert G. Bergman*
Contribution from the Department of Chemistry, UniVersity of California, Berkeley, California, 94720
ReceiVed July 23, 1999
Abstract: By kinetically stabilizing imidozirconocene complexes through the use of a sterically demanding
ligand, or by generating a more thermodynamically stable resting state with addition of diphenylacetylene, we
have developed transition metal-catalyzed imine metathesis reactions that are mechanistically analogous to
t
olefin metathesis reactions catalyzed by metal carbene complexes. When 5 mol % of Cp*Cp(THF)ZrdN Bu
is used as the catalyst precursor in the metathesis reaction between PhCHdNPh and p-TolCHdN-p-Tol, a
1
:1:1:1 equilibrium mixture with the two mixed imines p-TolCHdNPh and PhCHdN-p-Tol is generated in
C6D6 at 105 °C. The catalyst was still active after 20 days with an estimated 847 turnovers (t1/2 170 m; TON
-
1
)
1.77 h ). When the azametallacyclobutene Cp2Zr(N(Tol)C(Ph)dC(Ph)) is used as the catalyst precursor
under similar reaction conditions, a total of 410 turnovers are obtained after 4 days (t1/2 170 m; TON ) 4.3
-
1
h ). An extensive kinetic and equilibrium analysis of the metallacyclobutene-catalyzed metathesis of PhCHd
N-p-Tol and p-F-C6H4CHdN-p-F-C6H4 was carried out by monitoring the concentrations of imines and
observable metal-containing intermediates over time. Numerical integration methods were used to fit these
data to a detailed mechanism involving coordinatively unsaturated (16-electron) imido complexes as critical
intermediates. Examination of the scope of reaction between different organic imines revealed characteristic
selectivity that appears to be unique to the zirconium-mediated system. Several zirconocene complexes that
could generate the catalytically active “CpCp′ZrdNAr” (Cp′ ) Cp or Cp*) species in situ were found to be
effective agents in the metathetical exchange between different N-aryl imines. N-Alkyl aldimines were found
to be completely unreactive toward metathesis with N-aryl aldimines, and metathesis reactions involving the
two N-alkyl imines TolCHdNPr and PhCHdNMe gave slow or erratic results, depending on the catalyst
used. Metathesis was observed between N-aryl ketimines and N-aryl aldimines, but for N-aryl ketimine substrates,
the catalyst resting state consists of zirconocene enamido complexes, generated by the formal C-H activation
of the R position of the ketimine substrates.
Introduction
applications in organic synthesis1,2 and polymer chemistry.3,4
In contrast, analogous metal-catalyzed methods for selectively
metathesizing double bonds to nitrogen (e.g., imines, diazenes)
Olefin metathesis has emerged as an important class of
reactions in chemistry for the formation of new carbon-carbon
double bonds. These transformations have found widespread
5
-8
have been virtually unexplored until recently.
1-4
(3) Noels, A. F.; Demonceau, A. J. Phys. Org. Chem. 1998, 11, 602.
*
(
Author to whom correspondence should be addressed.
1) Schuster, M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36,
036.
2) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413.
(4) Oskam, J. H.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 11631.
(5) Cantrell, G. K.; Meyer, T. Y. Organometallics 1997, 16, 5381.
(6) Cantrell, G. K.; Meyer, T. Y. J. Chem. Commun. 1997, 1551.
(7) T o´ th, G.; Pint e´ r, I.; Messmer, A. Tetrahedron Lett. 1974, 735.
2
(
1
0.1021/ja992869r CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/25/2000

752 J. Am. Chem. Soc., Vol. 122, No. 5, 2000
Zuckerman et al.
t
Scheme 1
of diazametallacycle 3b and the imine PhCHdN Bu (2b). While
stoichiometric imine metathesis is feasible with this system,
attempts to carry out this reaction using catalytic amounts of
1
a were hindered by the irreversible formation of catalytically
inactive dimeric species [Cp2Zr(NR)]2 (e.g., R ) Ph, 5a).
We report here our efforts to develop long-lived zirconium-
based catalysts for the metathesis of imines. The activity and
longevity of several catalysts have been examined. In addition,
the scope of the metathesis reaction has been investigated with
respect to the imine substrates. Finally, a series of kinetic
experiments have been conducted in order to more fully
understand the mechanism of the catalytic imine metathesis
reaction. The results obtained from these studies are consistent
1
1
with the mechanistic model shown in Scheme 1.
Results
Catalytic Imine Metathesis of PhCHdNPh and p-TolCHd
N-p-Tol. A study was carried out aimed at examining the
activities and longevities of several different zirconium-based
catalysts for the metathesis reaction between equimolar amounts
of PhCHdNPh (2a) and p-TolCHdN-p-Tol (2c) to generate
equilibrium mixtures (1:1:1:1) of 2a, 2c, PhCHdN-p-Tol (2d),
and p-TolCHdNPh (2e) (Table 1; for a list of all imines utilized
in the study, see Table 2). When 5 mol % of Cp2(THF)Zrd
N(2,6-Me2C6H3) (1b) was used as the catalyst in C6D6 at 105
Scheme 2
°
C, the t1/2 of the reaction was measured to be 8 min. Based
1
upon the appearance of the H NMR spectrum of the reaction
mixture, the resting state for the catalyst under these conditions
was determined to be the statistical mixture of diazametallacy-
clobutane complexes that form from the reaction of free Cp2-
ZrdNPh and Cp2ZrdN-p-Tol with the reactant and product
imines. This conclusion is supported by the intense purple color
of the metathesis reaction mixture, which is the same color as
that observed for solutions of isolated diazametallacyclobutane
complexes (vide infra).
Upon continued heating, the metathesis reaction mixture
developed a green color, with accompanying new signals in the
1
H NMR spectrum. After 24 h at 105 °C, the formation of a
green precipitate was noted. At this point the solution was no
longer catalytically active, as demonstrated by the observed lack
of reaction upon addition of further amounts of the reactant
imines. The green precipitate was assumed to be a mixture of
dimeric bridging imido complexes that have partial solubility
in C6D6. To confirm the identity of one of the zirconium-
containing species in solution, the dimer [Cp2ZrN-p-Tol]2 (5b)
was independently synthesized by heating Cp2Zr(N(H)(-p-Tol))-
Scheme 1 illustrates a potential mechanism for imine me-
tathesis catalyzed by zirconium imido complexes that is
analogous to the pathway established for olefin metathesis
catalyzed by metal alkylidene complexes. A ZrdNR species
can react with an organic imine in an overall [2 + 2]
cycloaddition reaction to generate a diazametallacyclobutane
intermediate. Cycloreversion of this intermediate can occur in
one of two directions, resulting either in a degenerate reaction
or in the production of a new ZrdNR′ species along with a
product imine. The newly formed imido species can react with
a second reactant imine and undergo a similar pathway to
regenerate the original ZrdNR species and release a second
product imine.
(Me) in THF at 95 °C for several days (eq 1). Comparison of
Previous work in our laboratories demonstrated that stoichio-
metric imine metathesis reactions may be mediated by mono-
1
9,10
the H NMR spectrum of 5b with that of the metathesis reaction
mixture confirmed the presence of the dimeric species 5b in
the mixture. Additionally, the green precipitate was isolated and
MS analysis confirmed the identity of 5b. To determine whether
this dimer was capable of cycloreversion to generate the
catalytically active zirconocene imido complex under the
reaction conditions, 5 mol % of 5b was heated to 105 °C in the
meric imidozirconocene complexes (Scheme 2).
Treatment
of N-tert-butylimidozirconocene complex 1a with 2 equiv of
benzaldehyde N-phenylimine (2a) results in a stoichiometric
exchange of imine N substituents accompanied by the formation
(8) (a) McInnes, J. M.; Mountford, P. Chem. Commun. 1998, 1669. (b)
McInnes, J. M.; Blake, A. J.; Mountford, P. J. Chem. Soc. Dalton Trans.
1
1
1
998, 3623.
9) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1995,
17, 974.
(
(11) Portions of this work have been communicated previously. Krska,
S. W.; Zuckerman, R. L.; Bergman, R. G. J. Am. Chem. Soc. 1998, 120,
11828.
(10) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1994,
16, 2669.

Zirconium-Mediated Metathesis of Imines
J. Am. Chem. Soc., Vol. 122, No. 5, 2000 753
Table 1. Half-Lives (t1/2 Values) for Metathesis Reactions
t
1/2 (min)
starting
imines
starting
imines
temp (°C)
R
1
R
2
R
3
R
4
R
5
R
6
1a
1b
6a
10a
105
135
135
135
150
2a
2i
2l
2m
2i
Ph
Ph
Ph
t-Bu
Ph
H
H
Me
H
Ph
Me
Ph
p-Tol
Me
2c
2h
2c
2a
2f
p-Tol
p-Tol
p-Tol
Ph
H
H
H
H
H
p-Tol
14
10
8
NR
12
NR
NR
70
NR
780
b
n-Pr
p-Tol
Ph
NR
NR
257
NR
NR
NR
H
p-Tol
6 4
p-MeOC H
a
Catalyst ) 1a, 1b, 6a, or 10a. b Incomplete reaction. NR ) no reaction.
Table 2. Imines Utilized in the Study
which afforded the desired azametallacyclobutene products 6a,b
in good yields after recrystallization.
When complex 6a was used as the catalyst under the standard
metathesis reaction conditions, metathesis occurred, but the rate
was slowed by a factor of 9 relative to the reaction with 1b as
catalyst (Table 1). The reaction mixture was a deep green color,
typical of azametallacyclobutene complexes 6. Analysis of the
1
reaction mixture by H NMR spectroscopy under working
metathesis conditions revealed metallacyclobutene complexes
6
a and 6b as the predominant (>90%) Zr-containing species,
although minor amounts of diazametallacyclobutane complexes
could also be detected. No dimeric species were observed, even
1
6
after heating for 4 days at 105 °C. In contrast to experiments
with 1a, the catalyst was still active at this time (total turnovers
ca. 410), which was demonstrated by adding more imine starting
materials to the system and observing the reinitiation of
metathesis. Similar results could be obtained by generating the
metallacyclobutene catalyst in situ using an equimolar mixture
of 1a and diphenylacetylene.
The equilibrium constant for the azametallacyclobutene/
diazametallacyclobutane interchange under the metathesis reac-
tion conditions was determined by heating complexes 6a,b in
the presence of excess amounts of the corresponding imine
1
presence 2a and 2c. There was no metathesis observable by H
NMR after 51 h.
(
2a,d), and measuring the relative concentrations of the species
To increase the longevity of the imidozirconocene catalysts,
some method was sought to inhibit or, if possible, eliminate
the formation of the catalytically inactive dimeric species.
Diphenylacetylene has been shown to bind strongly to imi-
1
involved by H NMR spectroscopy (eq 3). For the intercon-
dozirconocene complexes, forming azametallacyclobutene
adducts.1
2-15
Because the dimerization reaction is necessarily
second-order in the free imidozirconocene species, the addition
of such a strongly binding ligand might be expected to slow
the dimerization process dramatically by sequestering the active
species in a more stable resting state. Azametallacyclobutene
complexes 6a,b were therefore synthesized as outlined in eq 2
-
3
version of 6a and 3b, Keq ) 1.8 × 10 at 75 °C; for 6b and
-
3
17
3
d, Keq ) 1.2 × 10 at 105 °C. To confirm the presence of
diazametallacycles 3b,d in the equilibrium mixtures, authentic
samples were synthesized independently from zirconocene imido
1
a and 2 equiv of the corresponding imine (eq 4), and their
and tested as catalysts for the metathesis reaction. Their synthesis
involved treatment of diazametallacycles 3a,c (generated in situ
by the reaction of 1a with 1 equiv of the corresponding imine,
2
a,c), with 1 equiv of diphenylacetylene at 45 °C for ca. 7 h,
(16) A slow side reaction between diphenylacetylene and the reactant
and product imines was catalyzed by 6b. This resulted in the complete
consumption of diphenylacetylene over a period of 4 days, leaving
diazametallacycles 3 as the predominant Zr-containing species.
(17) The previously reported Keq value of 7.4 for the 6a/3a intercon-
version at 70 °C is incorrect because of the misinterpretation of spectroscopic
data. The equilibrium value was remeasured at 75 °C at three different
concentrations of 3a, and the spectral data were compared with those for
authentic samples of 6a and 3a.
(
12) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. Organometallics 1993,
1
1
1
2, 3705.
(
993, 115, 2753.
13) Baranger, A. M.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc.
(
14) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc.
992, 114, 1708.
15) Lee, S. Y.; Bergman, R. G. Tetrahedron 1995, 51, 4255.
(

754 J. Am. Chem. Soc., Vol. 122, No. 5, 2000
Zuckerman et al.
Figure 1. ORTEP diagram of diazametallacycle 3d. The thermal
ellipsoids are scaled to represent the 50% probability surface.
t
Figure 2. ORTEP digram of CpCp*(THF)ZrdN Bu (10a). The thermal
ellipsoids are scaled to represent the 50% probability surface.
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for
Compounds 3d, 10a, 10b, and 13b
Scheme 3
bond lengths
compd
Zr1-N1
Zr1-N2
Zr1-O1
N2-C26
3
d
2.093(4)
1.846(2)
1.881(3)
2.070(3)
2.091(4)
1
0a
2.285(2)
2.281(2)
1
1
0b
3b
2.239(2)
1.426(4)
bond angles
compd N1-Zr1-N2 N1-Zr1-O1 Zr1-N1-C16 Zr1-N2-C26
3
d
66.1(1)
analyzed by X-ray diffraction, and the ORTEP diagram is shown
in Figure 2. The Zr-N bond distance in 10a was found to be
10a
92.75(9)
93.52(10)
169.5(2)
168.4(2)
1
1
0b
3b
1
.846(2) Å, 0.02 Å longer than the corresponding distance
100.8(1)
121.9(2)
1
2
(
1.826(4) Å) in 1a. In addition, the imido linkage in 10a was
slightly more bent (∠ Zr-N-C ) 169.5(2)°) than that of 1a
174.4(3)°) (Table 3). Both of these structural features imply a
NMR spectroscopic data were compared to those obtained in
the equilibrium experiments.9,10
Crystals of purple diazametallacycle 3d suitable for X-ray
diffraction were obtained by cooling a hexane solution at -35
C for 1 day. The ORTEP diagram for 3d is shown in Figure
. The Zr-N bond distances are equal (2.09 Å) (Table 3), and
are typical for zirconocene amido complexes.
metallacyclobutane ring is slightly puckered, with C11 deviating
from the mean plane calculated through Zr1, N1, and N2 by
(
slightly lower Zr-N bond order in 10a than 1a. This may be a
consequence of the more highly electron-donating Cp* ligand
in 10a which reduces the extent of π-donation from the imido
21
°
1
2
lone pair to the empty metal-centered b antibonding orbital.
Imido complex 10a reacts with stoichiometric amounts of
organic imines to give diazametallacyclobutane complexes 12
(eq 5). Upon mixing 10a and an imine such as p-CH OC H -
12,18,19
The diaza-
3
6
4
0
.2 Å in the direction opposite to the phenyl ring.
Synthesis and Catalytic Activity of (η -C5H5)(η -C5Me5)-
5
5
(THF)Zr(dNR) Complexes. A second strategy for increasing
the lifetime of the imidozirconocene catalyst involves using
increased steric bulk around the metal center to inhibit the
dimerization reaction. This may be accomplished by replacing
one of the Cp ligands of the imido complex with a Cp*
(
pentamethylcyclopentadienyl) ligand. The synthetic pathway
t
to prepare CpCp*(THF)Zr(dN Bu), 10a, is outlined in Scheme
3
20
. Treatment of the previously reported dimethyl complex 7
+
-
with 1 equiv of Me3NH Cl afforded methyl chloride complex
in high yield and 95% purity (the balance of the material was
8
CHdN-p-Tol (2f) in C6D6, there was an immediate color change
from yellow to rose. Analysis of the reaction mixture by H
CpCp*ZrMe2 and CpCp*ZrCl2). The reaction of complex 8 with
LiNH Bu gave the methyl amide complex 9, which upon
thermolysis in THF at 95 °C for 3 days generated imido complex
1
t
NMR spectroscopy revealed what is tentatively identified as
the two diastereomers of the expected metallacycle 12, resulting
from an overall [2 + 2] cycloaddition reaction between 10a
and 2f. Over a matter of hours at 25 °C, the solution began to
1
1
0a. Recrystallization from toluene layered with hexanes gave
0a as yellow blocks in 55% yield. One of these crystals was
1
assume a brick-red color. H NMR spectroscopy showed the
(
18) Cardin, D. J.; Lappert, M. F.; Raston, C. L.Chemistry of Organo-
Zirconium and -Hafnium Compounds; Ellis Horwood: New York, 1986.
19) Arney, D. J.; Bruck, M. A.; Huber, S. R.; Wigley, D. E. Inorg.
Chem. 1992, 31, 3749.
20) Wolczanski, P. T.; Bercaw, J. E. Organometallics 1982, 1, 793.
appearance of a new product with a concurrent disappearance
of the metallacycles. From the spectroscopic data, it appeared
(
(
(21) Green, J. C. Chem. Soc. ReV. 1998, 27, 263.

Zirconium-Mediated Metathesis of Imines
J. Am. Chem. Soc., Vol. 122, No. 5, 2000 755
Scheme 4
Figure 3. ORTEP diagram of CpCp*(THF)ZrdNTol (10b). The
thermal ellipsoids are scaled to represent the 50% probability surface.
that cycloreversion of the metallacycle was occurring with the
extrusion of imine 2g and the formation of N-p-Tol imido
complex 10b. This is in marked contrast to the behavior of the
bis(cyclopentadienyl) system, where only the dimeric species
of the general formula [Cp2Zr(NAr)]2 are isolated when there
are no ortho substituents on the aryl ring to sterically prevent
the dimerization reaction.
Repeating the reaction between 10a and 2f on a preparative
scale resulted in the isolation of imido complex 10b in 67%
yield. X-ray quality crystals were obtained by slow diffusion
of hexanes into a toluene solution of 10b at -35 °C. An ORTEP
diagram of 10b is shown in Figure 3. The imido Zr-N bond
length is 1.881(3) Å, 0.035 Å longer than that of 10a. Consistent
with the longer Zr-N bond in 10b was its Zr-N-C angle of
Complete metathesis was observed in the latter case after 30
min of heating, but it was completely suppressed in the former
case, even after extended heating (19 h) with ca. 50% of the
1
added Cp2ZrMe2 still remaining by H NMR. Performing the
imidozirconocene-catalyzed metathesis reactions of 2a and 2b
in the presence of Cp2ZrMe2 gave different results, depending
on the catalyst. Complexes 1b and 6a exhibited shorter lifetimes
in the presence of Cp2ZrMe2 (giving a total of three and nine
turnovers, respectively). However, both of these catalysts were
independently shown to react with Cp2ZrMe2 to give several
unidentified products. On the other hand, complex 10a was
shown to react much more slowly with Cp2ZrMe2 and demon-
strated undiminished catalytic activity in its presence.
1
1
68.4(2)°, which deviated further from linearity than that of
0a (Table 3). That imido complex 10b, which has a smaller N
substituent than 10a, should exhibit a longer Zr-N bond length
and a more bent Zr-N-C angle is consistent with the notion
that electronic factors as opposed to steric factors are responsible
for these subtle structural variations in the zirconium imido
Scope of the Metathesis Reaction. Reactions of N-Alkyl
Aldimines. Among the various classes of imines tested as
substrates for the metathesis reaction were N-alkyl aldimines
(Table 1). These exhibited very different reactivity than their
N-aryl counterparts. For example, N-alkyl aldimines were found
to be completely unreactive toward metathesis with N-aryl
aldimines using imidozirconocene-based catalysts. Treating an
equimolar mixture of p-TolCHdNPr (2h) and PhCHdNPh (2a)
with the various catalysts shown in Table 1 and Scheme 4
resulted in no observable metathesis, even at elevated temper-
atures (up to 150 °C). In addition, a green precipitate formed
during heating when 1a, 1b, or 6a were used as catalysts,
indicative of dimer formation. When an equimolar mixture of
two N-aryl aldimines (2a and 2c) and an N-alkyl aldimine (2h)
was heated (105 °C) in the presence of catalyst (6a), metathesis
occurred only between the two N-aryl aldimines, leaving the
N-alkyl aldimine unreacted (Scheme 4). Thus, the imidozir-
conocene catalysts were found to be active in the presence of
N-alkyl imines, but unable to catalyze the metathesis of N-alkyl
with N-aryl imines.
2
2
subunit.
When complex 10a was used as the catalyst under the
standard metathesis reaction conditions (using imines 2a and
2
c), a slow but constant rate was observed (Table 1). Analysis
1
of the purple reaction mixture by H NMR spectroscopy
indicated the presence of a statistical mixture of diazametalla-
cyclobutane complexes similar to 12. The catalyst was still
active after 20 days at 105 °C (giving a total of ca. 850 TO),
and there was no spectroscopic evidence for the formation of
dimeric species.
7
8
Control Experiments. Since both acids and bases have
been shown to catalyze imine metathesis, several control
experiments were carried out to establish that the metathesis
observed in the present system was not due to the presence of
trace amounts of acidic or basic impurities. In the first control
experiment, equimolar amounts of imines 2a and 2c were
subjected to the usual reaction conditions in the absence of
catalyst; no metathesis was observed, even after 51 h of heating.
This result ruled out the possibility that impurities in the solvent
or on the surface of the glassware were responsible for the
observed metathesis.
Metathesis reactions involving two N-alkyl imines, TolCHd
NPr (2h) and PhCHdNMe (2i) gave different results depending
on the catalyst used. No metathesis was observed under any
conditions using 1b or 6a as catalysts. When 10a was employed
as catalyst, metathesis occurred at 135 °C, but the reaction
ceased at less than 50% conversion. In contrast to the poor
activities found for 1b, 6a, and 10a, complex 1a effectively
catalyzed the metathesis of N-alkyl imines 2h and 2i in C6D6
at 135 °C (5 mol % of 1a) to give an equilibrium mixture of
A second set of control experiments involved the use of Cp2-
2
3
ZrMe2 as a trap for adventitious water or acidic impurities.
As a test of this method, an equimolar mixture of 2a and 2c
containing 5 mol % of p-toluic acid was heated in C6D6 to 105
°
C in the presence and absence of 10 mol % of Cp2ZrMe2.
(
(
22) Williams, D. S.; Korolev, A. V. Inorg. Chem. 1998, 37, 3809.
23) Proulx, G.; Bergman, R. G. Organometallics 1996, 15, 684.

756 J. Am. Chem. Soc., Vol. 122, No. 5, 2000
Zuckerman et al.
Scheme 5
2
h, 2i, TolCHdNMe (2j), and PhCHdNPr (2k). The measured
24
t1/2 for the reaction was <10 min.
Figure 4. ORTEP diagram of 13b. The thermal ellipsoids are scaled
to represent the 50% probability surface.
Reactions of the Ketimine Ph(Me)CdNPh. Several of the
imidozirconocene catalysts in Table 1 were found to effectively
catalyze the metathesis reaction between N-aryl ketimine Ph-
Table 4. Initial Concentrations of Reactants and Catalyst Used in
Kinetic Runs
(Me)CdNPh (2l) and N-aryl aldimine 2c. However, two unusual
run [2d]
0
(M) [2n]
0
(M) [6b]
0
0
(M) [Ph-CtC-Ph] (M)
features of these reactions were noted. First, the metathesis
reaction mixtures did not display the purple color typical of
diazametallacyclobutane complexes 3, and instead were orange
in color. In addition, the reactions were very slow at 105 °C
and required heating to 135 °C to proceed at reasonable rates.
To understand better the nature of the reactivity of ketimine
1
2
3
4
5
6
7
8
0.302
0.134
0.268
0.399
0.200
0.302
0.0973
0.151
0.101
0.297
0.165
0.237
0.464
0.333
0.281
0.101
0.0381
0.0254
0.0316
0.0252
0.0271
0.0297
0.0173
0.0183
0.0409
0.0304
0.0360
0.0295
0.0310
0.0322
0.0209
0.0212
2l with imidozirconocene complexes, stoichiometric reactions
of complexes 1b and 10a with 2l were carried out (Scheme 5).
These reactions did not afford diazametallacyclobutane com-
plexes, but instead gave enamido complexes 13a,b in quantita-
competent catalysts for the metathesis of 2l with 2c (5 mol %
catalyst, 135 °C).
Reactions of the C-Alkyl Aldimine BuCHdN-p-Tol. The
1
tive yield (by H NMR spectroscopy). Complexes 13a,b were
t
isolated as crystalline solids in high yields and identified by
their characteristic 1H NMR spectra. These contained two
olefinic resonances in the δ 5.6 to 4.5 ppm range along with a
broad NH resonance at 6.4 (for complex 13a) and 4.6 ppm (for
complex 13b). Complex 13b was further characterized by X-ray
crystallography (as a hexane solvate). An ORTEP diagram is
shown in Figure 4. All bond lengths and angles are consistent
with the structure drawn in Scheme 5. One notable feature of
the structure is the somewhat large disparity between the Zr1-
N1 and Zr1-N2 bond lengths (2.070(3) and 2.239(2) Å,
respectively) (Table 3), which can be attributed to delocalization
of the N2 lone pair into the π-system of the enamido ligand
final class of imine substrate examined in the metathesis reaction
t
bore an alkyl substituent ( Bu) on the imine carbon atom in place
of the usual aryl group. Metathesis between this sterically
t
hindered substrate ( BuCHdN-p-Tol; 2m) and 2a occurred only
when complex 6a (5 mol %) was used as the catalyst (Table
1). The reaction required elevated temperatures (135 °C) and
proceeded much more slowly (t1/2 ) 257 min) than the
corresponding metathesis reaction involving only C-aryl aldi-
mines, consistent with the high steric demand of the C- Bu
substituent of 2m.
t
Kinetic Studies. To gain further insight into the mechanism
of the catalytic imine metathesis reaction, a series of kinetic
experiments were undertaken. Azametallacyclobutene 6b was
employed as catalyst for these studies due to its stability under
(sum of angles about N2 ) 359.6°).
When either 13a or 13b was heated in C6D6 at 105 °C in the
presence of diphenylacetylene, the corresponding azametalla-
cyclobutene complex was formed in quantitative yield along
2
5
the reaction conditions. Reactions were run in sealed NMR
1
1
with 1 equiv of 2l, according to H NMR spectroscopy (Scheme
tubes and monitored by H NMR spectroscopy. The imines
5
). The identity of the product metallacyclobutenes was
PhCHdN-p-Tol (2d) and p-F-C6H4CHdN-p-F-C6H4 (2n) were
chosen as starting materials because the CH resonances for these
compounds and their metathesis products, PhCHdN-p-F-C6H4
(2o) and p-F-C6H4CHdN-p-Tol (2p), are resolved well enough
to guarantee accurate quantification of concentration by NMR
integration. To maintain a constant concentration of diphenyl-
acetylene throughout the reaction, 1 equiv of alkyne was added
confirmed by comparison with the spectral data of independently
synthesized samples (e.g., see Scheme 3). This provides
evidence that complexes 13a,b reversibly generate the corre-
sponding free imidozirconocene species at elevated tempera-
tures, and these are subsequently trapped by diphenylacetylene
to form azametallacyclbutene complexes 6c and 11. In accord
with this observation, complexes 13a,b were also found to be
26
relative to the catalyst. Reactions were run at different catalyst
concentrations using several different ratios of the two reactant
(24) The stoichiometric reaction between 1a and 2i was carried out to
gain further understanding of the metathesis reaction between 2i and 2h in
the presence of 5 mol % 1a. Unlike the N-aryl cases, a diazametallacycle
was spectroscopically observed as a possible intermediate in the generation
of a new dinuclear zirconmium species that was crystallographically
characterized as (Cp2Zr)2(m-NH Bu)(m-NdCH2). The details of this study
have been published independently (J. Organomet. Chem., 1999, 591, 2).
(25) Complex 10a was not chosen as the catalyst precursor for the kinetic
measurements because its imine adducts analogous to 3 exist as pairs of
diastereomers. It was thought that this would further complicate the already
complex system.
(26) The Keq measurements in eq 3 predict that, at equilibrium, less than
10% of 6b should be converted to diazametallacyclobutanes 3.
t

Zirconium-Mediated Metathesis of Imines
J. Am. Chem. Soc., Vol. 122, No. 5, 2000 757
Scheme 6
Figure 5. Experimentally determined (points) and simulated (lines)
concentration vs time profiles for the reactant and product imines and
the two azametallacyclobutene complexes 6b and 6d for a representative
kinetic run. Error bars assume 10% uncertainty in NMR integration.
imines (Table 4). The concentrations of imines, the two
azametallacyclobutenes, and diphenylacetylene were monitored
with time.
Based upon the mechanistic evidence presented in the
preceding sections, the metathesis reaction involving catalyst
6b is proposed to occur according to the mechanism presented
in Scheme 6. The complexity of this kinetic model necessitated
the use of kinetic simulation software27 to derive rate constants
from the experimental kinetic data. For this purpose, the 20
microscopic rate constants shown in Scheme 6 were reduced
to six distinguishable parameters, kR, kT, k′RT, k′RF, k′TT, and
k′TF, based on reasonable assumptions about the relative rates
of these elementary steps. The rates of reversion (kR) of
metallacyclobutene complexes 6b and 6d were assumed to be
equal, as were the rates of trapping (kT) of the transient imido
species 4b and 4c with diphenylacetylene. The rates of cyclo-
reversion of diazametallacycles 3d,f,g,i to release N-p-Tol
imines were set equal to the parameter k′RT. Likewise, the rates
of cycloreversion of complexes 3e,g-i to release N-p-F-C6H4
imines were set equal to k′RF. The rates of trapping of the
transient imido complexes 4b and 4c with N-p-Tol (k′TT) and
N-p-F-C6H4 (k′TF) imines constituted the final two parameters.
in the rate constant. The fit of the model showed a smaller,
although not negligible, dependence on the parameters k′ and
RT
2
9
k′ . In contrast, the model was insensitive to the absolute
RF
magnitude of the trapping rate constants k , k′ , k′ , although
T
TT
TF
the ratio k /(k′ + k′ ) determined the relative rates of imine
T
TT
TF
metathesis and 6b/6d equilibration. Finally, the ratio k (k′
R
TT
+
k′ )/k (k′ + k′ ), which is essentially equivalent to the
TF
T
RT
RF
equilibrium constants described in eq 3, strongly affected the
fit of the model, because it controlled the absolute concentrations
of metallacyclobutenes 6b and 6d in the simulated reaction
mixture. Because of these different responses of the fit to
changes in the rate constants, we estimate that the tolerances in
k and k as approximately ( 10%, k ′ as (15% and k ′ as
2
7
By application of a nonlinear least-squares fitting routine,
the kinetic model described above was simultaneously fit to
the experimental concentration vs time data for all eight kinetic
runs. The rate constants derived from this optimization are given
in Scheme 6. The tolerances associated with these values were
estimated by varying each parameter independently and deter-
mining the point at which the goodness-of-fit of the simulation
was significantly reduced. For most of the kinetic runs, the
agreement between the experimental data and the simulated
curves was found to be satisfactory; a sample plot is shown in
Figure 5.28
The dependence of the goodness-of-fit of the model on the
six parameters was examined by varying them independently.
The overall fit of the model was most sensitive to the parameter
kR, changing significantly even with small variations (<10%)
R
T
TT
TF
(35%.
Discussion
Development of Robust Imine Metathesis Catalysts. Previ-
ous studies indicated that monomeric imidozirconocene com-
plexes were capable of mediating stoichiometric imine metath-
9,10
esis reactions (Scheme 2).
However, to develop a useful
catalytic version of these reactions, the inherent tendency of
the imidozirconocene species to dimerize irreversibly had to
be overcome.
Two approaches were explored to eliminate this deleterious
pathway. The first involved generating a more thermodynami-
cally stable resting state for the catalyst by adding the ligand
diphenylacetylene. This strategy took advantage of the fact that
(27) The FORTRAN programs GEAR and GIT (v. 2.1) were used for
all kinetic simulations. (a) McKinney, R. J.; Weigert, F. J. Project
SERAPHIM. Program number 1b-1407.8. (b) Weigert, F. J. Comput. Chem.
(29) Changing the parameters kR (ca. 10- s-1) and k′RT (10-2 s-1) also
resulted in a reasonable fit for the experimental versus simulated data.
However, these rate constants are not consistent with the measured Keq value
for the interconversion of 6b and 3d.
3
1
987, 11, 273. (c) Stabler, R. N.; Chesick, J. Int. J. Chem. Kinet. 1978, 10,
4
61.
(28) Further examples of concentration vs time plots for the experimental
and kinetic data are available as Supporting Information.

758 J. Am. Chem. Soc., Vol. 122, No. 5, 2000
Zuckerman et al.
the unwanted dimerization reaction is necessarily second-order
in the active imido species, whereas the imine metathesis
reaction is presumably first-order in this species. By decreasing
the effective concentration of the active imidozirconocene
species through its complexation with diphenylacetylene, we
expected the dimerization reaction to be slowed, much more
so than the catalytic metathesis reaction. This proved to be the
case. Although the metathesis reaction using catalyst 6a was
slowed by a factor of 9 relative to that using catalyst 1a (Table
Scheme 7
complexes. The latter are not observed but lead to imine
metathesis (Scheme 7).
The metathesis reaction of the C-alkyl aldimine 2m demon-
1), the dimerization reaction was no longer observed, increasing
the catalyst lifetime. The dynamic equilibrium between the new
metallacyclobutene catalyst resting state and the diazametalla-
cyclobutane complexes responsible for imine metathesis was
t
strated the effect of placing a sterically demanding Bu sub-
1
stituent on the C position of the imine. This substrate reacted
only with the less-hindered bis(cyclopentadienyl) catalyst 6a,
and then only at a very sluggish rate at elevated temperatures
confirmed by H NMR spectroscopic studies (eq 3). Based on
the measured equilibrium constants, the difference in stability
for the two species was on the order of 5 kcal/mol.
(see Table 1).
The second approach to increasing the catalyst lifetime
involved disfavoring dimer formation through increased steric
bulk around the zirconium metal center. Imidozirconocene
complex 10a bearing a sterically bulky Cp* ligand (Scheme 3)
was found to be an effective catalyst for the metathesis of N-aryl
aldimines. The mechanism of imine metathesis involving this
species appears to be the same as that of catalysts 1a and 1b,
involving diazametallacyclobutane intermediates. These com-
plexes imparted a distinctive purple color to metathesis reaction
Kinetic Studies. The most notable aspect of the previously
reported stoichiometric imine metathesis reaction (shown in
Scheme 2) was that it operated by a well-defined, kinetically
characterizable mechanism that was directly analogous to olefin
2
metathesis (Scheme 1). The extension of this chemistry to a
catalytic imine metathesis reaction would constitute the first
example of catalytic CdX (X ) heteroatom) metathesis
demonstrated to proceed in an analogous manner to CdC
metathesis catalyzed by metal alkylidene complexes. Preliminary
spectroscopic studies indicated that catalytic imine metathesis
did indeed occur by a mechanism similar to that of the
1
mixtures involving 10a, and were directly observable by H
NMR spectroscopy. Catalyst 10a was extremely long-lived,
retaining its catalytic activity even after 20 days at 105 °C. There
was no spectroscopic evidence for the formation of any dimeric
species.
1
stoichiometric reaction. For example, H NMR analysis of the
N-aryl aldimine metathesis reactions catalyzed by complex 6a
suggested that azametallacyclobutene complexes 6 were in
equilibrium with diazametallacyclobutane complexes 3 (eq 3);
these latter complexes were presumably responsible for the
Scope of Metathesis Reaction. The scope of the metathesis
reaction involving the various imidozirconocene catalysts was
examined with respect to several classes of imines (Table 1).
Although all of the catalysts examined were effective for the
metathesis of N-aryl aldimines, none of these catalysts were
able to effect the metathesis of N-alkyl imines with N-aryl
imines. Only one of the complexes examined (1a) was able to
effectively catalyze the metathesis of two N-alkyl imines, and
this catalyst, when mixed with two N-aryl imines and one
N-alkyl imine, selectively metathesized only the N-aryl imines,
leaving the N-alkyl imine unreacted (Scheme 4). The reasons
for this selectivity are not entirely clear but may be due to a
much stronger binding affinity of N-aryl imines for the active
imido species, coupled with a preference for N-alkyl, N-aryl
diazametallacyclobutanes to preferentially cyclorevert to gener-
ate N-alkyl imines and N-aryl imidozirconocene species. It
should be mentioned that other reported imine metathesis
9,10
observed imine metathesis (cf. Scheme 2). To provide more
evidence for this mechanism, kinetic studies were undertaken
of the metathesis reaction between PhCHdN-p-Tol (2d) and
p-F-C6H4CHdN-p-F-C6H4 (2n) catalyzed by complex 6b.
The kinetic runs listed in Table 4 were conducted using
widely varying concentrations of imines and catalyst. The
concentrations of five separate species (2d, 2o, 2p, 6b, and 6d)
in the reaction mixtures were monitored over time. These data
were simultaneously fit to the model shown in Scheme 6 using
six parameters (kR, kT, k′RT, k′RF, k′TT, and k′TF; number of data
points/parameter ) 53). The result was a satisfactory agreement
between the model’s predictions and the experimentally deter-
28
mined concentration vs time profiles (e.g., Figure 5).
The magnitudes of the rate constants derived from the model
are summarized in the form of a free-energy diagram in Figure
5,6
7
catalysts (e.g., (DME)Cl2Mo(dNR)2, and various acids ) do
6
. Moving from left to right in this diagram, the slowest step in
not show this selectivity.
the catalytic cycle (kR) was found to be the cycloreversion of
azametallacyclobutene complexes 6b and 6d (whose ∆Grel at
105 °C in Figure 6 was arbitrarily set to 0 kcal/mol), giving the
free imido species 4b,c. The free energy of activation for this
R
Imine metathesis reactions involving N-aryl ketimines ex-
hibited different catalyst resting states than the reactions
involving only N-aryl aldimines. These new resting states were
identified as enamido complexes 13a,b, generated by the
stoichiometric reactions between imidozirconocene complexes
process was calculated from k to be 29.3 kcal/mol, in good
1
7
qualitative agreement with the corrected value obtained from
the previous stoichiometric studies (29.9 kcal/mol at 70 °C).9
Since this is the rate-determining step of the catalytic cycle, its
,10
1b and 10a and ketimine 2l (Scheme 5). Complexes 13a,b arise
from formal C-H activation of the R position of ketimine 2l.
The reversibility of this reaction was demonstrated by heating
associated parameter (k ) had the strongest effect on the
R
1
3a,b in the presence of diphenylacetylene to give azametal-
lacyclobutene complexes 6c and 11 in quantitative yields
Scheme 5). Isolated complexes 13a,b were also found to be
goodness-of-fit of the simulation.
The rates of trapping of the transient imido species 4b,c with
alkyne (kT) and imines (k′TT, k′TF) were of the same order of
magnitude, consistent with prior results determined for the
stoichiometric system.
The rates of cycloreversion of diazametallacyclobutane
complexes 3d-i (k′RT, k′RF) depended only weakly on the nature
(
effective catalysts for imine metathesis. Thus, it appears that
ketimines exhibit a dual mode of reactivity with imidozir-
conocene species: C-H activation to give stable enamido
complexes and cycloaddition to give diazametallacyclobutane

Zirconium-Mediated Metathesis of Imines
J. Am. Chem. Soc., Vol. 122, No. 5, 2000 759
of sufficient stability relative to the free imido species to prevent
the rapid formation of the catalytically inert dimeric bridging
imido complexes. For the pentamethylcyclopentadienyl ana-
logues, however, the dimerization reaction is sterically inhibited,
and the catalysts are extremely long-lived. For the bis-
(cyclopentadienyl) system, in the presence of the additive
diphenylacetylene, the catalyst resting state shifts to the more
stable azametallacyclobutene complexes, and the lifetime of the
catalyst is greatly increased. For N-aryl ketimine substrates, the
catalyst resting state consists of zirconocene enamido complexes,
generated by the formal C-H activation of the R position of
the ketimine substrates.
Experimental Section
General Procedures. Unless otherwise noted, reactions and ma-
nipulations were performed using standard drybox or Schlenk tech-
niques. Glassware was dried overnight at 150 °C before use. Unless
1
otherwise noted, H NMR spectra were recorded at 500 MHz and
chemical shifts (δ) are reported in parts per million (ppm) relative to
Figure 6. Free-energy diagram for catalytic imine metathesis (for
explanation of compound labels, see Scheme 6).
6 6 8
residual protiated solvent: C D (7.15 ppm), THF-d (3.58 ppm). Unless
13
1
otherwise noted, C{ H} NMR were recorded at 100 MHz and are
reported in ppm relative to the carbon resonance of the deuterated
solvent: C D (128.0 ppm), THF-d (67.57 ppm). Elemental analyses
6 6 8
were performed at the University of CaliforniasBerkeley Microana-
lytical facility.
Materials. Unless otherwise noted, reagents were purchased from
commercial suppliers and used without further purification. Solvents
of the N-aryl substituent of the imine. The free energies of
activation for the extrusion of N-p-Tol and N-p-F-C6H4 imines
from the corresponding diazametallacyclobutanes 3 were cal-
culated (from k′RT and k′RF) to be 23.6 and 24.1 kcal/mol,
respectively. These values compare favorably to the value
previously determined for the cycloreversion of metallacycle
3
were distilled from the appropriate drying agents under N
2
or passed
prior to
6c, and 7 were synthesized
b (24.9 kcal/mol at 70 °C).9 That the rates of trapping and
,10
through a column of activated alumina and sparged with N
2
use. Complexes 1a,12 1b, 6a,
12
9,10
13
20
cycloreversion involving imines were found to be only weakly
dependent on the electronic nature of the imine substituents
provided justification for the simplification of the kinetic model
in Scheme 6 to include only 6 distinguishable parameters instead
of the 20 possible rate constants. As a final note, it should be
emphasized that the difference in stability between the metal-
lacyclobutene complexes 6b,d and the diazametallacyclobutane
complexes 3d-i shown in Figure 6 (on the order of 5 kcal/
mol) was in close agreement with the experimentally determined
value of 5.0 kcal/mol for the 6b/3d interconversion shown in
according to literature procedures. All lithium amides, LiNHR, were
prepared by deprotonating the parent amine with 1.0 equiv of n-BuLi.
3
1
Imines were synthesized according to the standard procedure of
condensing the parent aldehyde or ketone and amine in C over dried
Å molecular sieves and either recrystallized (solids) or dried over 4
6 6
H
4
Å molecular sieves (liquids) prior to use. Product imines generated
from the metathesis reactions between parent imines were independently
1
synthesized and their H NMR data were compared with literature
values,31 or their identity was confirmed by GC-MS analysis.
31,32
6 4
Characterization data were not found in the literature for p-F-C H -
eq 3.17,30
CHdN-p-F-C H4 (2n). This imine was prepared by the standard
6
procedure stated above, and characterization data are listed below.
Diazametallacycles 3a and 3b were synthesized according to
literature procedures.9 Diazametallacycles 3c and 12 were generated
in situ as intermediates in the synthesis of 6b and 10b, respectively,
and were tentatively identified by their H NMR data. Diazametallacycle
3d was characterized by X-ray crystallographic analysis, and spectro-
Conclusions
,10
Long-lived catalysts for the metathesis of N-aryl aldimines
and ketimines have been developed based on imidozirconocene
complexes. The mechanism of action of these catalysts involves
formal [2 + 2] cycloaddition reactions between the imine
substrates and the active imidozirconocene catalysts, giving
diazametallacyclobutane intermediates. A series of kinetic and
spectroscopic experiments has convincingly demonstrated that
the mechanism of the catalytic metathesis is directly analogous
to that of the well-known and widely used olefin metathesis
reaction.
The resting state of the catalysts varies with the nature of
the imine substrates and the presence of additives. In the absence
of additives, the metathesis of N-aryl aldimines involves
diazametallacyclobutane complexes as the catalyst resting state.
For the bis(cyclopentadienyl) system, these complexes are not
1
scopic characterization data for this compound are listed below.
1
Metallacycles 3e-i were tentatively assigned by their H NMR data
and are transient intermediates outlined in Scheme 6 in the metathesis
reaction between PhCHdN-p-Tol (2d) and p-F-C
2n).
p-F-C
6
H
4
CHdN-p-F-C
6
H
4
(
1
6
H
4
CHdN-p-F-C
6
H
4
(2n). White crystals. H NMR (C
6
D
6
):
δ 7.84 (s, 1H, aldimine CH), 7.56 (m, 2H, aryl), 6.89 (m, 2H, aryl),
13
6
.81 (m, 2H, aryl), 6.72 (m, 2H, aryl) ppm. C NMR (C
6 6
D ): δ 164.91
(d, JC-F ) 251 Hz, aryl), 161.64 (d, JC-F ) 245 Hz, aryl), 158.08 (CHd
N), 148.31 (d, JC-F ) 4 Hz, aryl), 132.99 (d, JC-F ) 2.5 Hz, aryl),
122.66 (d, JC-F ) 7 Hz, aryl), 116.05 (d, JC-F )14 Hz, aryl), 115.87
(d, JC-F ) 14 Hz, aryl) ppm. IR (NaCl disk): 756 (w), 844 (vs), 1090
(
m), 1152 (w), 1185 (w), 1218 (m), 1245 (s), 1054 (s), 1590 (m), 1603
s), 1628 (s), 2855 (w). Anal. Calcd C13 N: C, 71.88; H, 4.18; N,
.45. Found: C, 71.89; H, 4.11; N, 6.36.
Cp Zr(N(Tol)CH(Ph)N(Tol)) (3d). A glass reaction vessel equipped
with a Teflon stopcock was charged with Cp
mg, 0.33 mmol) and 5 mL of C . The solution was stirred and
(
9 2
H F
(30) The parameters chosen initially for the experimental data fitting were
6
based on rate constants experimentally determined from previous kinetic
studies of the stoichiometric reactions. These parameters were then adjusted
to obtain the best fit for the eight experimental runs. Final optimization of
the model was accomplished by application of a nonlinear least-squares
fitting routine. Simulations were allowed to converge, then restarted at the
convergence point. This process was repeated until no further variations
were seen in the rate constants. Rate constants obtained after convergence
were still in agreement with independently measured equilibrium and rate
constants.
2
t
2
(THF)ZrdN Bu (1a) (120
6 6
H
PhCHdNTol (2d) (129 mg, 0.66 mmol, 2 equiv) was added dropwise,
(31) Bolognese, A.; Mazzoni, O.; Giordano, F. Tetrahedron 1991, 47,
7417.
(32) Nongkunsan, P.; Ramsden, C. Tetrahedron 1997, 53, 3805.

760 J. Am. Chem. Soc., Vol. 122, No. 5, 2000
Zuckerman et al.
with a noted color change from yellow to purple upon the addition of
PhCHdNTol. The mixture was heated at 45 °C for 7 h. Removing the
solvent at reduced pressure left a purple solid which was dissolved in
obtain a green solid, which was dissolved in Et
2
O (2 mL) and stored
at -35 °C for 1 day. Forest-green crystals (58 mg, 78% yield) were
1
obtained. H NMR (C
6 6
D ): δ 7.20 (m, 4H, aryl), 7.0 (m, 3H, aryl),
Et
140 mg, 81% yield). H NMR (C
aryl), 7.01 (m, 4H, aryl), 6.90 (d, JH-H ) 8.5 Hz, 4H), 6.24 (s, 5H,
), 5.91 (s, 5H, C ), 5.47 (s, 1H, CH), 2.25 (s, 6H, CH ) ppm.
C NMR (C ): δ 50.91, 142.72, 129.86, 128.66, 128.54, 127.15,
25.87, 116.11(aryl), 114.91, 114.68 (C ), 63.27 (CH), 20.72 (CH
2
O (2 mL) and stored at -35 °C for 1 day to give purple crystals
6.92 (m, 3H, aryl), 6.72 (t, JH-H ) 8 Hz, 2H, aryl), 6.10 (m, 2H, aryl),
1
13
(
6
D
6
): δ 7.69 (d, JH-H ) 9.0 Hz, 2H,
5.78 (s, 10H, C
5 5 6 6
H ) ppm. C NMR (C D ): δ 182.3 (aryl), 148.8 (d,
J
C-F ) 235 Hz, aryl), 149.8, 147.9, 132.18, 129.7, 128.6, 128.4, 127.6,
C
5
H
5
5
H
5
3
127.0, 123.7, 121.9 (aryl), 121.2 (d, JC-F ) 6 Hz, aryl), 115.2 (d, JC-F
) 22 Hz, aryl), 112.3 (C
13
6
D
6
5
H
5
) ppm. HRMS (EI): m/z calcd for C30
24
H -
+
1
5
H
5
3
)
FNZr 507.0940 (M ), found 507.0942. Anal. Calcd C30
H24FNZr: C,
ppm. Repeated attempts to obtain elemental analysis on this complex
70.83; H, 4.75; N, 2.75. Found: C, 70.48; H, 4.67; N, 2.62.
were not successful. It has been characterized by X-ray diffraction.
Cp*CpZr(Me)Cl (8). A solution of 7 (3.27 g, 10.2 mmol) in ca. 10
mL of THF was added to a slurry of Me NHCl in 10 mL of THF. The
3
t
Cp
2
Zr[N( Bu)CH(p-F-C
6
H
4
)N(p-F-C
6
H
4
)] (3k). A glass reaction
vessel equipped with a Teflon stopcock was charged with Cp
2
-
reaction mixture was stirred at room temperature for 4 days, and then
the volatile materials were removed at reduced pressure. The remaining
white solid was extracted with ca. 15 mL of toluene, and the resulting
solution was filtered and evaporated to leave a white crystalline solid
t
6 6
(THF)ZrdN Bu (1a) (188 mg, 0.515 mmol) and 7 mL C H . The yellow
solution was stirred and p-F-C CHdN-p-F-C (2n) (112 mg, 0.515
H
6 4
6 4
H
mmol, 1 equiv) was added dropwise with an immediate color change
to maroon. A heterogeneous mixture formed during stirring at 25 °C
for 2 h, and the resulting solid was isolated and triturated with hexanes
1
(3.43 g, 99%). Analysis of this material by H NMR indicated that it
was 95% pure (the balance being Cp*CpZrMe
2 2
and Cp*CpZrCl ).
(
2 × 1 mL). The last traces of solvent were removed at reduced pressure
Analytically pure material could be obtained by repeated crystallization
1
1
to afford 186 mg (71% yield) of a rose powder. H NMR (THF-d
δ 7.51 (m, 2H, aryl), 6.95 (m, 2H, aryl), 6.67 (m, 2H, aryl), 6.60 (s,
8
):
from toluene/hexanes. H NMR (C
15H, C (CH ), 0.21 (s, 3H, ZrCH
(CH ), 113.2 (C ), 34.9 (ZrCH
6
D
3
6
): δ 5.78 (s, 5H, C
5
H
5
), 1.69 (s,
). 1 C NMR (C
): δ 120.1 (C
3 5
), 11.8 (C (CH ) ). Anal. Calcd
3
5
3
)
5
6
D
6
5
-
t
5
H, C
5
H
5
), 6.30 (s, 5H, C
ppm. C NMR (THF-d ): δ 163.0 (d, JC-F ) 242 Hz, aryl), 156.2 (d,
C-F ) 230 Hz, aryl), 150.4 (aryl), 141.5 (d, JC-F ) 15.1 Hz, aryl),
H
5 5
), 4.94 (s, 1H, CH), 0.96 (s, 9H, Bu)
)
3 5
H
5 5
3
5
1
3
8
C
16
H
23ClZr: C, 56.19; H, 6.78. Found: C, 56.15; H, 6.91.
t
J
1
5
6
Cp*CpZr(Me)NH Bu (9). Added to a stirred solution of 8 (0.81 g,
t
29.2, 116.5, 115.4, 115.3 (aryl), 115.1 (C
H
5 5
),114.3 (C
) ppm. Anal. Calcd C27
3.62; H, 5.54; N, 5.50. Found: C, 63.34; H, 5.73; N, 5.39.
Cp Zr(µ-p-Tol)] (5b). Method A. Cp Zr(Me)(Cl) (678 mg, 2.43
mmol) and p-TolC NHLi (267 mg, 2.43 mmol, 1 equiv) were
5
H
5
), 64.55 CH),
28 2 2
2.4 mmol) in 5 mL of THF was a solution of LiNH Bu (0.20 g, 2.6
mmol) in 5 mL of THF. The mixture was stirred for 20 h at room
temperature, and then the solvent was removed at reduced pressure.
The resulting yellow oil was extracted with 10 mL of pentane and
filtered through Celite. Removal of pentane at reduced pressure left 9
as an oily yellow solid (0.83 g, 92%). Redissolving this solid in ca. 4
t
t
6.60 ( Bu C), 31.98 ( Bu CH
3
H F N Zr: C,
[
2
2
2
6
H
4
dissolved in THF (5 mL). The solution was stirred overnight and solvent
was removed at reduced pressure to leave a yellow solid. The resulting
mL of pentane and cooling to -35 °C gave analytically pure 9 as light
1
solid was redissolved in C
residual LiCl. The filtrate was concentrated in vacuo to afford 700 mg
88% yield) of the corresponding complex Cp Zr(Me)(NH-p-Tol),
H
6 6
and filtered through Celite to remove
yellow blocks (0.44 g). H NMR (C
6
D
6
): δ 5.83 (s, 5H, C
), 1.14 (s, 9H, C(CH
): δ 115.84 (C (CH
), 20.18 (ZrCH ), 11.73 (C
5
H
5
), 4.64
), -0.14
),
).
(br s, 1H, NH), 1.73 (s, 15H, C
5
(CH
3
)
5
3
)
3
13
(
2
(s, 3H, ZrCH
56.11 (C(CH
Anal. Calcd C20
H, 8.68; N, 3.68.
3
). C NMR (C
), 34.46 (C(CH
6
D
6
5
)
3 5
), 109.64 (C
5
H
5
which was then dissolved in THF (10 mL). The solution was heated
for 2 days at 95 °C in a constant temperature silicon oil bath. During
this time, kelly-green crystals precipitated out of the solution. The
3
)
3
)
3 3
3
5
(CH
3
)
5
H
33NZr: C, 63.43; H, 8.78; N, 3.70. Found: C, 63.08;
t
crystals were isolated (569 mg, 85% yield).
Cp*Cp(THF)ZrdN Bu (10a). A solution of 9 (0.94 g, 2.5 mmol)
in ca. 10 mL of THF was placed in a glass reaction vessel that was
coated with hexamethyldisilazane, heated under a vacuum prior to use,
and sealed with a Kontes stopcock. The reaction mixture was then
heated at 105 °C for 3 days. The resulting solution was evaporated to
leave a yellow solid that was extracted with ca. 5 mL of toluene and
filtered. The filtrate was layered with hexanes and cooled to -35 °C,
t
Method B. Cp
2
(THF)ZrdN Bu (1a) (258 mg, 0.709 mmol) and
TolCHdNTol (2c) (148 mg, 0.709 mmol, 1 equiv) were dissolved in
(7 mL), and the resulting purple solution was heated at 105 °C
6 6
C D
in a constant temperature silicon oil bath for 3 h. The resulting
heterogeneous green mixture was filtered to afford 120 mg (52% yield)
of 5b. The filtrate was evaporated at reduced pressure to leave a green
solid which was then triturated with Et
an additional 75 mg of 5b (84% combined yield). H NMR (CD
δ 6.90 (d, JH-H ) 7.5 Hz, 4H, aryl), 6.44 (s, 20H, C ), 5.67 (d, JH-H
) ppm. C NMR (CD Cl ): δ
55.5, 128.8, 126.5, 120.6 (aryl), 133.0 (C ), 20.7 (CH ) ppm. Anal.
Calcd C34 Zr : C, 62.53; H, 5.25; N, 4.29. Found: C, 62.24; H,
2
O (2 mL) and filtered to obtain
whereupon 10a crystallized as yellow blocks (2 crops, 0.59 g, 55%).
1
1
2
Cl
2
):
H NMR (C
(CH
): δ 116.22 (C
(C(CH ), 35.09 (C(CH
37NOZr: C, 63.54; H, 8.58; N, 3.22. Found: C, 63.66; H, 8.83;
N, 3.11.
Cp*Cp(THF)ZrdN-p-Tol (10b). To a solution of 10a (107 mg,
6
D
6
): δ 6.13 (s, 5H, C
), 1.33 (s, 9H, C(CH
(CH ), 107.78 (C
), 25.64 (CH ), 12.46 (C
5
H
5
), 3.45 (m, 4H, OCH
), 1.14 (m, 4H, CH
). 13C NMR
), 77.94 (OCH ), 62.25
(CH ). Anal. Calcd
2
), 2.02 (s,
H
5 5
15H, C
(C
5
)
3 5
3
)
3
2
1
3
)
1
8.0 Hz, 4H, aryl), 2.70 (s, 6H, CH
3
2
2
D
6 6
5
3
)
5
H
5 5
2
H
5 5
3
3
)
3
)
3 3
2
5
3 5
)
H N
34 2
2
23
C H
4
.96; N, 4.34.
Cp Zr(N(Tol)C(Ph)dC(Ph)) (6b). Diphenylacetylene (148 mg,
.832 mmol, 1 equiv) and Cp
2
t
0
2
(THF)ZrdN Bu (1a) (303 mg, 0.832
. A green color developed
0.25 mmol) in 1 mL of toluene was added a solution of 2d (54 mg,
0.24 mmol) in 2 mL of toluene. A rose color developed immediately.
The reaction mixture was left for 3 days at room temperature, and then
3 mL of THF was added. The resulting mixture was transferred to a
glass reaction vessel sealed with a Kontes stopcock and heated at 45
°C for 2 days. The solvents were removed at reduced pressure and the
resulting brown solid was dissolved in ca. 5 mL of toluene and layered
6 6
mmol) were dissolved in 5 mL of C H
immediately. The solution was stirred for 5 min at 25 °C, and PhCHd
NPh (149 mg, 0.832 mmol, 1 equiv) was added. The mixture was heated
at 105 °C for 3 h and solvent was removed at reduced pressure to
obtain a green solid, which was dissolved in toluene (2 mL) and stored
at -35 °C for 1 day. The solvent was removed in vacuo to afford 337
mg of forest-green crystals (83% yield). H NMR (C
1
6
6
D ): δ 7.27 (d,
with 10 mL of hexanes. After 4 days at -35 °C, compound 10b was
1
J
H-H ) 7.5 Hz, 2H, aryl), 7.21 (t, JH-H ) 7.5 Hz, 2H, aryl), 7.05 (t,
C-H ) 8 Hz, 4H, aryl), 6.97 (d, JH-H ) 8 Hz, 3H, aryl), 6.96 (d, JH-H
7 Hz, 2H, aryl), 5.84 (s, 10H, C ), 2.14 (s, 3H, CH
) ppm. 13C
NMR (C ): δ 151.1, 148.2, 132.7, 128.4, 128.3, 127.8, 127.2, 126.9,
23.5, 122.5, 121.1 (aryl and alkenyl), 112.0 (C ), 20.7 (CH ) ppm.
HRMS (EI): m/z calcd for C31 27NZr 504.1207 (M ), found 504.1200.
Cp Zr(N(p-F-C )C(Ph)dC(Ph)) (6d). Diphenylacetylene (57 mg,
.318 mmol, 2 equiv) and Cp
3k) (80.8 mg, 0.159 mmol) were dissolved in 5 mL of C
obtained as orange prisms (77 mg, 67%). H NMR (C
6
D
6
): δ 7.16 (d,
), 3.38 (m,
), 1.07 (m, 4H,
J
)
5 5
2H, Tol), 6.46 (d, J ) 8 Hz, 2H, Tol), 6.03 (s, 5H, C H
5
H
5
3
4H, OCH
2
), 2.40 (s, 3H, CH
). C NMR (C ): δ 160.31 (C
(o to CH )), 121.98 (C CH (p to CH
)), 117.29 (C (CH ), 108.67 (C
3 5
CH ), 21.08 (C CH ), 11.89 (C )
3
), 1.92 (s, 15H, C
5
(CH
3
)
5
1
3
D
6 6
CH
2
D
6 6
H
6 4
CH (C-CH
3
3
)), 129.39 (C
CH
6 4
H -
1
5
H
5
+
3
CH
3
3
H
6 4
3
3
)), 117.80 (C
), 77.86 (OCH
(CH ). Repeated attempts
H
6 4
3
(m
H
to CH
(OCH
3
5
3
)
5
5
H
5
2
), 25.72
H
6 4
2
2
6
H
4
3
5
2
t
0
(
2
Zr[N( Bu)CH(p-F-C
6
H
4
)N(p-F-C
6
H
4
)]
to obtain elemental analysis on this complex were not successful. It
has been characterized by X-ray diffraction.
6 6
H
. The
t
solution was heated at 45 °C for 10 h, resulting in a color change from
Cp*CpZr(N( Bu)C(Ph)C(Ph)) (11). Diphenylacetylene (29 mg, 0.17
purple to green. The solvent was removed under reduced pressure to
mmol) and 10a (66 mg, 0.15 mmol) were dissolved in ca. 4 mL of

Zirconium-Mediated Metathesis of Imines
J. Am. Chem. Soc., Vol. 122, No. 5, 2000 761
toluene. A green color developed immediately. After standing for 16 h
at room temperature, the reaction mixture was evaporated to dryness
and redissolved in ca. 1 mL of toluene. The resulting solution was
Table 5. Crystallographic Data for Compounds 3d, 10a, 10b, and
13b
3d
10a
10b
13b‚0.5 C6H14
layered with hexanes and stored at -35 °C. After 3 days, 11 was
1
formula
fw
C38H38N2Zr C23H37NOZr C26H35NOZr C33H42N2Zr
obtained as green needles (2 crops, 78 mg, 95%). H NMR (THF-d
8
,
613.95
434.77
468.79
557.94
5
7
00 MHz): δ 7.50 (d, J ) 8 Hz, 1H, Ph), 7.27 (t, J ) 8 Hz, 1H, Ph),
.13 (t, J ) 8 Hz, 1H, Ph), 7.05 (t, J ) 7 Hz, 1H, Ph), 6.83 (d, J ) 8
color
blue, plate yellow, block orange, plate yellow, plate
monoclinic orthorhombic monoclinic monoclinic
cryst syst
lattice type
temp (K)
space group
a, Å
Hz, 1H, Ph), 6.71 (t, J ) 8 Hz, 2H, Ph), 6.49 (t, J ) 8 Hz, 1H, Ph),
primitive
168
primitive
168
primitive
168
primitive
168
6
1
1
1
.35 (d, J ) 8 Hz, 2H, Ph), 6.30 (s, 5H, C
H
5 5
), 1.91 (s, 15H, C
, 125 MHz): δ 165.78, 146.52,
38.15, 133.22, 132.24, 131.93, 128.62, 128.17, 127.29, 127.15, 126.36,
20.61 (aromatic, vinylic), 120.14 (C (CH ), 111.15 (C ), 56.82
), 34.92 (C(CH ), 12.12 (C (CH ). Anal. Calcd C33
NZr: C, 73.28; H, 7.27; N, 2.59. Found: C, 72.92; H, 7.58; N, 2.51.
Cp Zr(NH-2,6-Me Ph)(N(Ph)(C(Ph)dCH )(toluene) (13a). Cp
Zr(THF)(dN-2,6-Me Ph) (1b) (177 mg, 0.429 mmol) and PhC(Me)d
NPh (83 mg, 0.429 mmol, 1 equiv) were dissolved in 5 mL of C
5 3 5
(CH ) ),
13
.09 (s, 9H, C(CH
)
3 3
). C NMR (THF-d
8
P21/n (#14) Pbcn (#60) P21/c (#14) P21/n (#14)
11.2145(7) 28.7306(3)
13.5719(9) 10.3514(1)
20.1565(13) 15.1662(1)
8.8942(3)
17.2473(6) 20.9552(1)
15.8147(5) 16.9649(2)
8.8470(1)
b, Å
c, Å
5
3
)
5
5 5
H
(C(CH
3
)
3
3
)
3
5
3
)
5
39
H -
R, deg
90.00
95.345(1)
90.00
3054.5(3)
4
90.00
120.00
90.00
4510.46(6)
8
4.98
1.28
21627
90.00
105.3750(10) 94.767(1)
90.00 90.00
2339.17(12) 4510.46(6)
90.00
â, deg
2
2
2
2
-
γ, deg
V, Å3
Z
2
6 6
H .
4
4
-
1
An orange color developed immediately. After stirring at 25 °C for 1
h, the solvent was removed at reduced pressure to leave an orange
solid which was dissolved in toluene (2 mL) and stored at -35 °C.
µ(MoKR) cm 3.89
4.86
1.331
11027
4.29
1.339
14671
Dcalc, g/cm3
total data
residuals:
R; Rw
1.335
13796
0.040; 0.040 0.028; 0.032 0.033; 0.035 0.036; 0.039
Analytically pure 13a‚toluene was obtained as a yellow powder (180
1
mg, 82% yield). H NMR (C
6
D
6
): δ 7.71 (m, 2H, aryl), 7.56 (d, JH-H
GOF
1.21
1.44
1.38
1.47
)
7.5 Hz, 2H, aryl), 7.12 (m, 2H, aryl), 7.11 (m, 4H, aryl), 7.06 (m,
H, aryl), 6.92 (t, JH-H ) 7.5 Hz, 1H, aryl), 6.75 (t, JH-H ) 7 Hz, 1H,
aryl), 6.44 (s, 1H, NH, 5.70 (s, 10H, C ), 5.48 (s, 1H, vinyl CH),
.50 (s, 1H, Vinyl CH), 1.85 (s, 6H, CH ), 1.83 (s, 3H, CH
) ppm. 13
NMR (C ): δ 158.95, 156.61, 156.01, 140.80, 129.28, 128.65, 128.51,
28.43, 128.29, 127.86, 127.81, 127.51, 125.64, 122.44, 122.02, 118.55,
11.88, 107.49, 21.37, 20.68 ppm. Anal. Calcd C39 Zr: C, 74.59;
radiation
Mo KR (λ ) 0.710 69 Å) graphite monochromated
3
5
H
5
4
3
3
C
various classes of indices showed no unusual trends. All calculations
were performed using the teXsan35 crystallographic software package
of Molecular Structure Corp.
Crystallographic data for compounds 3d, 10a, 10b, and 13b are listed
in Table 5, and selected bond lengths and bond angles are given in
Table 3; full details are given in the Supporting Information.
Kinetic Studies. In a typical experiment, a stock solution of 6b
6 6
D
1
1
H
40
N
2
H, 6.42; N, 4.46. Found: C, 74.34; H, 6.56; N, 4.59.
t
Cp*CpZr(NH Bu)(N(Ph)C(Ph)dCH
2
) (13b). A solution of ketimine
2
l (43 mg, 0.22 mmol) in ca. 5 mL of toluene was added to a solution
(
134.7 mg, 0.267 mmol) and diphenylacetylene (47.6 mg, 0.267 mmol)
in 2.00 mL C was prepared in a volumetric flask. The imines
PhCHdN-p-Tol and p-F-C CHdN-p-F-C were weighed into a
of 10a (96 mg, 0.22 mmol) in ca. 5 mL of toluene. The solution slowly
developed an orange color. After 1 day, the solvent was evaporated at
reduced pressure, and the resulting red-brown oil was dissolved in
hexanes, filtered and stored at -35 °C. After 6 months, 13b‚0.5hexane
6
D
6
6
H
4
6 4
H
vial and a 1-mL volumetric flask, respectively. The stock solution of
6b and diphenylacetylene (150 µL) was added to the 1-mL volumetric
flask. The imine PhCHdN-p-Tol was dissolved in C D and transferred
6 6
1
was obtained as orange plates (80 mg, 61%). H NMR (C
6 6
D ): δ 7.65
(
6
5
d, J ) 8 Hz, 2H, Ph), 7.10 (m, 6H, Ph), 6.97 (t, J ) 8 Hz, 1H, Ph),
to the 1-mL volumetric flask, rinsing twice. The volumetric flask was
diluted to the final volume, the solution was mixed well, and 0.5 mL
of the solution was transferred to a NMR tube. The tube was attached
to a cajon adapter and was flame-sealed under vacuum. The tube was
heated in a 105 °C constant temperature silicone oil bath. Heating times
were monitored with a stopwatch. At selected time intervals, the tube
was immersed in an ice water bath (the metathesis reaction was not
observed at ambient temperature or below). Eight data points were
.68 (t, J ) 8 Hz, 1H, Ph), 6.10 (s, 5H, C
5
H
5
), 5.63 (br s, 1H, CH
(CH ), 1.22
, 125 MHz, 60 °C): δ 160.59,
55.70, 141.73, 128.36, 127.92, 127.65, 127.21, 123.37 (br), 117.22
(CH ), 111.95 (C ),
), 12.84 (C (CH ). Anal. Calcd
Zr: C, 71.04; H, 7.59; N, 5.02. Found: C, 71.12; H, 7.34; N,
2
),
.36 (br s, 1H, CH ), 4.6 (v br, 1H, NH), 1.76 (s, 15H, C
2
5
3 5
)
1
3
3 3 6 6
(br s, 9H, C(CH ) ). C NMR (C D
1
(br), 107.92 (br) (aromatic, vinylic), 120.07 (C
5
)
3 5
5 5
H
5
9.13 (C(CH
3
)
3
), 34.06 (C(CH
3
)
3
5
3 5
)
33 42 2
C H N
5
.06.
1
obtained by H NMR spectroscopy over 907 min reaction time.
X-ray Structure Determination of 3d, 10a, 10b, and 13b. All
crystals were mounted on quartz fibers using Paratone N hydrocarbon
oil. For all compounds, measurements were made on a SMART CCD
area detector with graphite monochromated Mo-KR radiation. Data
were integrated by the program SAINT. The data were corrected for
Lorentz and polarization effect, and analyzed for agreement and possible
Acknowledgment. We thank Drs. Frederick Hollander and
Ryan Powers for solving the crystal structures of 3d, 10a, 10b,
and 13b, the National Institutes of Health (Grant no. GM-25459)
for financial support of this work, and Prof. Tara Y. Meyer
(University of Pittsburgh) for her willingness to exchange data
prior to publication.
3
3
absorption using XPREP. An empirical absorption correction was
made based on comparison of redundant and equivalent reflections as
3
4
applied using SADABS (3d, Tmax ) 0.96, Tmin ) 0.70; 10a, Tmax
.897, Tmin ) 0.818; 10b, Tmax ) 0.97, Tmin ) 0.81). The structures
were solved by direct methods and expanded using Fourier techniques.
)
0
Supporting Information Available: Crystallographic data
for 3d, 10a, 10b, and 13b and kinetic plots for the rate and
simulation study. This material is available free of charge via
the Internet at http://pubs.acs.org.
The non-hydrogen atoms were refined anisotropically. Plots of ∑w(|F
o
|
2
-
|F
c
|) versus |F
o
|, reflection order in data collection, (sin θ)/λ, and
(
33) XPREP, v. 5.03 (part of the SHELXTL Crystal Structure Determi-
nation Package); Siemens Industrial Automation, Inc., 1995.
34) Sheldrick, G. SADABS: Siemens Area Detector Absorption cor-
rection program; Siemens Industrial Automation, Inc., 1996.
JA992869R
(
(35) teXsan: Crystal Structure Analysis Package; Molecular Structure
Corp., 1985 and 1992.