Rearrangements and stereomutations of metallacycles derived from allenes and imidozirconium complexes

Article abstract of DOI:10.1021/ja045607k

The mechanisms of the rearrangements and Stereoinversion of azametallacyclobutenes generated via [2+2] cycloaddition of allenes and imidozirconium complexes have been studied. Metallacycles derived from allenes bearing β-hydrogen atoms racemize at room temperature by reversible β-hydride elimination, a process which is also responsible for their eventual conversion to monoazadiene complexes. Metallacycles derived from diarylallenes racemize by reversible thermal bond homolysis at 95°C; racemization of these metallacycles is also catalyzed by mild oxidants.

Full text of DOI:10.1021/ja045607k

Published on Web 01/20/2005
Rearrangements and Stereomutations of Metallacycles
Derived from Allenes and Imidozirconium Complexes
Forrest E. Michael, Andrew P. Duncan, Zachary K. Sweeney, and
Robert G. Bergman*
Contribution from the Department of Chemistry, UniVersity of California,
Berkeley, California 94720
Received July 21, 2004; E-mail: bergman@cchem.berkeley.edu
Abstract: The mechanisms of the rearrangements and stereoinversion of azametallacyclobutenes generated
via [2+2] cycloaddition of allenes and imidozirconium complexes have been studied. Metallacycles derived
from allenes bearing â-hydrogen atoms racemize at room temperature by reversible â-hydride elimination,
a process which is also responsible for their eventual conversion to monoazadiene complexes. Metallacycles
derived from diarylallenes racemize by reversible thermal bond homolysis at 95 °C; racemization of these
metallacycles is also catalyzed by mild oxidants.
Introduction
treatment of the metallacycle with parent allene (C3H4) and was
found to be considerably enantioenriched (84% ee). More
The hydroamination of alkynes and allenes is a powerful
method for transforming unsaturated hydrocarbons into substi-
tuted amines or imines.1-5 A variety of titanium and zirconium
complexes have been shown to efficiently catalyze this
transformation.6-14 Prior work in this laboratory established that
a key step in zirconium-catalyzed hydroamination reactions is
a [2+2] cycloaddition of the unsaturated organic substrate with
a zirconium imido complex (LnZr)NR) to generate an azamet-
allacyclobutane.13,14 Understanding the factors that govern the
formation and rearrangement of these metallacycles is important
in controlling the regio- and enantioselectivity of hydroamination
reactions and in the development of new reactions based on
this mechanism.
recently, we communicated preliminary results of a study of
the mechanism of this remarkable stereoinversion.16 In the
present report, we describe a complete synthetic and mechanistic
investigation of the formation, racemization, and rearrangements
of allene metallacycles derived from ebthi imido complex 1 and
achiral zirconocene imido complex 3 (Scheme 1). Consideration
Scheme 1. Synthesis and Reactions of Enantiopure Allenes with
Zirconocene Imido Complex 3a
Recently, we reported an unusual selective stereoinversion
of allenes promoted by enantioenriched ethylenebis(tetrahy-
droindenyl) (ebthi) imidozirconium complexes.15 In that report,
enantioenriched ebthi imidozirconium complex 1 reacted with
1 equiv of racemic 1,2-cyclononadiene to generate only a single
diastereomer of the corresponding metallacycle (eq 1). The 1,2-
a BuLi, MsCl, then RMgBr, CuBr, LiBr.
of both new and previously proposed mechanisms by which
the multifold rearrangements of allene-derived metallacycles
proceed follows the presentation of the data.
Results
Cycloaddition. To ascertain the stereospecificity of the
cycloaddition step, as well as to study the subsequent metalla-
cyclononadiene could be released from the metal center by
(1) Muller, T. E.; Beller, M. Transition Met. Org. Synth. 1998, 2, 316.
(2) Pohlki, F.; Doye, S. Chem. Soc. ReV. 2003, 32, 104.
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(4) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003, 935.
(5) Roesky, P. W.; Mueller, T. E. Angew. Chem., Int. Ed. 2003, 42, 2708.
(6) Li, Y.; Shi, Y.; Odom, A. L. J. Am. Chem. Soc. 2004, 126, 1794.
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(11) Ackermann, L.; Bergman, R. G.; Loy, R. N. J. Am. Chem. Soc. 2003, 125,
11956.
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(13) Johnson, J. S.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123, 2923.
(14) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc. 1992,
114, 1708.
(15) Sweeney, Z. K.; Salsman, J. L.; Andersen, R. A.; Bergman, R. G. Angew.
Chem., Int. Ed. 2000, 39, 2339.
(16) Michael, F. E.; Duncan, A. P.; Sweeney, Z. K.; Bergman, R. G. J. Am.
Chem. Soc. 2003, 125, 7184.
(8) Khedkar, V.; Tillack, A.; Beller, M. Org. Lett. 2003, 5, 4767.
(9) Knight, P. D.; Munslow, I.; O’Shaughnessy, P. N.; Scott, P. Chem. Commun.
2004, 894.
(10) Ackermann, L. Organometallics 2003, 22, 4367.
9
1752
J. AM. CHEM. SOC. 2005, 127, 1752-1764
10.1021/ja045607k CCC: $30.25 © 2005 American Chemical Society

Rearrangements and Stereomutations of Metallacycles
A R T I C L E S
Figure 1. X-ray crystal structure of diphenylallene metallacycle 10. 50%
probability boundaries are indicated. Bond distances: Zr1-N1 2.081 Å,
Zr1-C12 2.348 Å, N1-C11 1.430 Å, C11-C12 1.503 Å, C11-C25 1.357
Å. Bond angles: N1-Zr1-C12 64.1°, Zr1-N1-C11 95.8°, N1-C11-
C12 107.2°.
Figure 2. Representative CD and UV spectra from the racemization of
metallacycle 8.
membered ring is substantially puckered (angle between the
planes ) 33°). The nitrogen atom is planar (sum of bond angles
) 359.5°), but this plane is canted by 29° relative to the Zr1-
N1-C12 plane. This lessens the overlap between the filled p
orbital on N and the exocyclic double bond, but it allows partial
overlap between this filled p orbital and an empty in-plane
acceptor orbital on Zr.
Scheme 2. E/Z Isomerization and Racemization of Metallacycles
In contrast, treatment of imido complex 3 with dialkylallenes
5b,c initially formed metallacycles 7 and 8 with Z exocyclic
double bond geometries. These assignments were also deter-
mined by NOESY spectroscopy. Due to the complexity of the
NMR spectrum of dicyclohexylmetallacycle 9, the configuration
of the double bond could not be definitively determined, but is
presumed to be Z by analogy. Instead of affording the corre-
sponding E isomers, heating the dialkylmetallacycles provided
an unexpected rearrangement product, the formation of which
will be discussed in detail below.
Inversion (Racemization). To determine if the initial [2+2]
cycloaddition was concerted, the CD spectra of a dilute hexane
solution of metallacycles 6-9 were measured. A solution of
isolated diphenylmetallacycle 6 displayed a strong CD signal
with maxima at 234, 257, and 294 nm, demonstrating that this
metallacycle is optically active. Although the maxima at 234
and 257 nm coincide closely with the spectrum of free (R)-
diphenylallene, the CD signal at 294 nm is new, and NMR
spectroscopy shows no free allene is present. Heating this
solution at 45 °C for 18 h resulted in no change in the CD
spectrum, indicating no racemization under these conditions.
However, after heating at 95 °C for 24 h, the CD spectrum
indicated that the metallacycle had lost all of its optical activity.
It was established by 1H NMR and UV spectroscopy that
metallacycle decomposition does not occur under these condi-
tions.
cycle isomerizations in a simpler system, the reactions of
enantioenriched 1,3-disubstituted allenes with achiral zir-
conocene imido complex 3 were examined. Enantioenriched
allenes 5a-d were prepared by stereoselective SN2′ displace-
ment of the appropriate propargylic alcohols 4a-d (Scheme
1).17 As a control, heating a solution of enantiopure allene (R)-
5a at 95 °C for 24 h resulted in no loss of enantioenrichment.
Enantiopure 1,3-disubstituted allenes underwent [2+2] cycload-
dition with achiral zirconocene imido complex 3 to generate
deep blue or purple azametallacyclobutanes 6-9. Metallacycles
6-9 possess two types of stereoisomerism: the configuration
of the alpha stereocenter (R or S), and the orientation of the
exocyclic double bond (E or Z).
E/Z Isomerization. The configuration of the exocyclic double
bond in the initially formed metallacycles was strongly depend-
ent on the allene substituents. Diphenylmetallacycle 6 was
initially formed exclusively as the E isomer, as determined by
NOESY spectroscopy. After being heated at 95 °C for 24 h,
however, (E)-6 was transformed to an equilibrium mixture of
both isomers, favoring (Z)-6 3:1 (Scheme 2). The E and Z
isomers of this complex were consistently distinguishable by
the 1H NMR chemical shift of the vinyl proton of the exocyclic
olefin. This resonance appears at 5.72 ppm for the E isomer,
but is shifted downfield to 6.38 ppm for the Z isomer due to
deshielding of this proton from the R-phenyl group. X-ray
crystallographic analysis of the closely related metallacycle 10
confirmed the structure of the kinetically formed metallacycle
and its assignment as the E isomer. An ORTEP diagram of 10
and relevant metric parameters are given in Figure 1. The X-ray
crystal structure of diphenylmetallacycle 10 reveals that the four-
Solutions of diisopropyl- and dicyclohexylmetallacycles 8 and
9 generated by treatment of a suspension of imido complex 3
with an excess of enantioenriched allene displayed CD signals
at approximately 300 and 480 nm (Figure 2). The free
dialkylallenes display no CD signal at wavelengths greater than
240 nm. In contrast to the high temperatures required for the
(17) Elsevier, C. J.; Vermeer, P. J. Org. Chem. 1989, 54, 3726.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1753

A R T I C L E S
Michael et al.
Table 1. Racemization Kinetics for Metallacycle 8
Scheme 4. Synthesis and Racemization of Deuterated
Diisopropylmetallacycle
1
a
1
temp (
°
C)
k
H (s-
)
k
D (s-
)
kH/kD
25
35
45
1.48 × 10-4
4.25 × 10-4
1.16 × 10-3
ND
NA
2.8
2.8
1.49 × 10-4
4.08 × 10-4
a Two runs for the first entry gave kobs ) 1.46 and 1.51 × 10-5 s-1, so
the error was estimated to be (5%, and errors in activation parameters and
isotope effect were determined on the basis of this value.
racemization of diphenylmetallacycle 6, solutions of these
metallacycles lost their optical activity over the course of 18 h
at room temperature. Even more dramatically, treatment of imido
complex 3 with di-n-propylallene resulted in a solution of
metallacycle 7 that was optically inactive after only 15 min at
room temperature.
Scheme 5. MAD Complex Formation
Interestingly, most disubstituted allenes could be released
from the metal center by treatment of the metallacycles with
either allene (C3H4) or diisopropylcarbodiimide (DIC) under
mild heating. The enantiopurity of the allenes recovered in this
fashion was measured to confirm the results of the CD
experiments above (Scheme 3). As expected, treatment of
Scheme 3. Enantiopurity of Recovered Allenes
method of Myers.21 Deuteration of the N-methylephedrine ligand
in the alkyne addition reaction was required to avoid loss of
label in the aldehyde coupling partner. The racemization of the
deuterated metallacycle 8-d2 was also monitored by CD
spectroscopy at 35 and 45 °C, and the first-order rate constants
are given in Table 1. A normal primary isotope effect of kH/kD
) 2.8 ( 0.3 was observed at both temperatures.
Monoazadiene Complex Formation. Dialkylmetallacycles
7-9 rearranged slowly to new isomeric complexes upon mild
heating (45 °C) in the absence of trapping agent (Scheme 5).
The rate of this rearrangement was always slower than that of
racemization and depended on the size of the alkyl substituents,
being fastest with the smallest substituents (n-Pr > (CH2)6 >
i-Pr ≈ c-Hex). These new complexes were identified as the
monoazadiene (MAD) complexes 14-16 by 2D NMR spec-
troscopy (NOESY, TOCSY, HMQC). Cyclononadiene-derived
metallacycle 17 also rearranged at 45 °C, but in this case a 3:1
mixture of two MAD complex isomers 18a,b was observed.
Attempted recovery of di-n-propylallene from metallacycle 7
by treatment with diisopropylcarbodiimide resulted in quantita-
tive formation of MAD complex 14, indicating that rearrange-
ment to 14 is faster than retrocyclization to form free allene.
To obtain more information about the MAD complexes and
their structures, complex 18a was crystallized from THF/Et2O
and an X-ray crystal structure was determined. An ORTEP
diagram of the structure is given in Figure 3. The MAD moiety
is strongly bent with respect to the Zr-N-C plane, but Zr-
C11 and Zr-C12 bond distances of 2.65 and 2.54 Å, respec-
tively, reveal that 18 is best described as a metallacyclopentene,
rather than as an η4-MAD complex. This observation has been
made in other reports from the literature.22-30 The strain of the
optically active diphenylmetallacycle 6 with DIC at 45 °C
generated 1,3-diphenylallene of unchanged enantiopurity (>95%
ee). Also, after heating 6 at 95 °C for 24 h to effect its
racemization, the diphenylallene recovered upon treatment with
DIC was racemic. Furthermore, 1,3-diisopropylallene was
recovered from reaction of metallacycle 8 with DIC, but only
after several hours at 45 °C, conditions that were sufficient to
fully racemize 8, and the recovered allene in this case was
racemic.
The racemization of diisopropylmetallacycle 8 was monitored
by CD spectroscopy at 25, 35, and 45 °C. This transformation
displayed clean first-order kinetics at all three temperatures, and
the first-order rate constants are given in Table 1. The rate of
racemization of 8 was independent of the concentration of excess
free allene. Activation parameters of ∆Hq ) 19 ( 1 kcal/mol
and ∆Sq ) -13 ( 3 cal/mol‚K were obtained from an Eyring
plot of the data in Table 1.
To provide information about the racemization mechanism,
enantiopure deuterium-labeled allene 5c-d2 was synthesized via
the route depicted in Scheme 4. The synthesis was accomplished
by the enantioselective coupling of appropriately labeled alde-
hyde (11-d1) and alkyne (12-d2) partners by the method of
Carreira,18-20 followed by stereospecific rearrangement of the
propargylic alcohol (13-d2) to the enantioenriched allene by the
(21) Myers, A. G.; Zheng, B. J. Am. Chem. Soc. 1996, 118, 4492.
(22) Davis, J. M.; Whitby, R. J.; Jaxa-Chamiec, A. J. Chem. Soc., Chem.
Commun. 1991, 1743.
(23) Scholz, J.; Nolte, M.; Krueger, C. Chem. Ber. 1993, 126, 803.
(24) Scholz, J.; Kahlert, S.; Goerls, H. Organometallics 1998, 17, 2876.
(25) Scholz, J.; Kahlert, S.; Goerls, H. Organometallics 2004, 23, 1594.
(26) Scholz, J.; Goerls, H. Organometallics 2004, 23, 320.
(27) Enders, D.; Kroll, M.; Raabe, G.; Runsink, J. Angew. Chem., Int. Ed. 1998,
37, 1673.
(18) Frantz, D. E.; Faessler, R.; Carreira, E. M. J. Am. Chem. Soc. 2000, 122,
1806.
(19) Anand, N. K.; Carreira, E. M. J. Am. Chem. Soc. 2001, 123, 9687.
(20) Boyall, D.; Frantz, D. E.; Carreira, E. M. Org. Lett. 2002, 4, 2605.
9
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Rearrangements and Stereomutations of Metallacycles
A R T I C L E S
Scheme 7. Rearrangement of Mismatched Metallacycle 20 at
45-65 °C
Figure 3. X-ray structure of MAD complex 18a. 50% probability
boundaries are indicated. Bond distances: Zr1-N1 2.160 Å, Zr1-C13 2.254
Å, Zr1-C11 2.652 Å, Zr1-C12 2.497 Å, N1-C11 1.385 Å, C11-C12
1.386 Å, C12-C13 1.441 Å. Bond angles: N1-Zr1-C13 83.7°.
Scheme 6. Formation of Matched and Mismatched Metallacycles
and Establishment of the Equilibrium Mixture
Table 2. First-Order Rate Constants and Eyring Parameters for
Rearrangement of Metallacycle 20
1
1
1
rate constanta @ 45
°
C (s-
)
@ 55
°
C (s-
)
@ 65
°
C (s-
)
∆
Hq (kcal/mol)
∆
Sq (eu)
k1
k-1
k2
k3
k4
4.08 × 10-5 1.51 × 10-4 5.72 × 10-4
4.28 × 10-5 1.62 × 10-4 6.17 × 10-4
1.54 × 10-5 5.52 × 10-5 2.07 × 10-4
6.52 × 10-6 2.68 × 10-5 1.00 × 10-4
3.00 × 10-6 1.06 × 10-5 4.20 × 10-5
27.5
27.8
27.1
28.5
27.5
+8
+9
+4
+7
+3
a The error in isolated rate constants is estimated to be (5%, and the
error in activation parameters is estimated to be (1 kcal/mol and (3 eu.
no erosion of enantiopurity (>95% ee). Likewise, cleavage of
mismatched metallacycle 20 with carbodiimide formed (S)-
diphenylallene (>95% ee), indicating that no racemization
occurred during the cycloaddition/cycloreversion sequence for
either antipode of the allene.
bridgehead double bond is evident in the reduced torsion angle
about that bond (157°). When crystals of 18a were redissolved
in C6D6, the same 3:1 mixture of 18a and 18b was observed by
1H NMR spectroscopy. NOESY spectroscopy established that
the crystal structure corresponds to the major isomer 18a and
that the minor isomer 18b is likely to be that depicted in Figure
3. Interestingly, the proton on C13 resonates at 3.0 ppm in 18a,
but at -0.4 ppm in 18b, a large difference that has also been
observed previously.22 This difference presumably reflects a
large shielding of this proton in 18b from the adjacent
cyclopentadienyl ring which is lacking in 18a.
Matched metallacycle 19 did not rearrange at temperatures
lower than 95 °C. However, after 24 h at 105 °C, an equilibrium
mixture of the starting material and its Z isomer 21 was formed,
favoring the original metallacycle 3:1 (Scheme 6). The same
equilibrium mixture of 19 and 21 was also obtained upon heating
mismatched metallacycle 20 at 105 °C. Under milder conditions,
20 rearranged at 45 °C to give a mixture of all four possible
metallacycle diastereomers. Initially, first-order decay of 20 to
give a 2.5:2.5:1 mixture of 21, 22, and 19 was observed. Newly
formed metallacycle 22 then decayed more slowly to give a
mixture of 19 and 21. Metallacycles 19 and 21 do not
interconvert appreciably at this temperature, but the same 3:1
equilibrium ratio of 19 and 21 was observed upon heating to
105 °C. The concentrations of all four metallacycles were
measured as a function of time at 45, 55, and 65 °C. These
data were fitted to the pathways illustrated in Scheme 7 with
the aid of the Gepasi computer modeling program.31-33 The
RMS error of each fit was <0.9%, and the estimated standard
deviation for most rate constants was <2%.34 When a rate
constant (k5) for the conversion of 22 to 21 was included in the
modeling, its optimum value was essentially zero, and leaving
this parameter out of the modeling had an insignificant effect
on the total error. Approximate activation parameters for each
rate constant were obtained from Eyring plots and are listed in
Table 2.
Matched and Mismatched Metallacycles. Metallacycles
derived from enantiopure ebthi imido complexes were synthe-
sized to simultaneously monitor the rates of epimerization and
E/Z isomerization by NMR spectroscopy in the same system.
Treatment of imido complex (R,R)-1 with (R)-diphenylallene
gave exclusively metallacycle diastereomer 19 (Scheme 6). This
is the diphenylallene enantiomer that reacts more rapidly with
the (R,R)-imido complex, that is, the “matched” metallacycle.
In contrast, the reaction of (S)-diphenylallene with imido
complex (R,R)-1 yielded an equilibrium mixture of starting
imido complex (R,R)-1 and “mismatched” metallacycle 20, again
as a single diastereomer (Keq ≈ 1). Removal of the released
THF under vacuum drove the cycloaddition to completion and
allowed for the isolation of the pure compound. Both diaster-
eomers displayed proton resonances at approximately 5.81 ppm,
which suggest an E geometry of the exocyclic olefin, and the
configuration of both metallacycles was further confirmed by
NOESY spectroscopy. Treatment of matched metallacycle 19
with diisopropylcarbodiimide generated (R)-diphenylallene with
Catalytic Isomerization. Attempted functionalization of
diphenylmetallacycle 6 by treatment with benzophenone did not
(31) Mendes, P. Comput. Appl. Biosci. 1993, 9, 563.
(28) Lorenz, V.; Gorls, H.; Scholz, J. Angew. Chem., Int. Ed. 2003, 42, 2253.
(29) Erker, G.; Kruger, C.; Muller, G. AdV. Organomet. Chem. 1985, 24, 1.
(30) Yasuda, H.; Tatsumi, K.; Nakamura, A. Acc. Chem. Res. 1985, 18, 120.
(32) Mendes, P. Trends Biochem. Sci. 1997, 22, 361.
(33) Mendes, P.; Kell, D. B. Bioinformatics 1998, 14, 869.
(34) Hoyt, H.; Michael, F.; Bergman, R. J. Am. Chem. Soc. 2004, 126, 1018.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1755

A R T I C L E S
Michael et al.
Scheme 8. Catalytic Racemization and Isomerization of
Metallacycle 6
Scheme 9. Insertion Reactions of Diphenylmetallacycle 6
Figure 4. X-ray crystal structure of complex 23. 50% probability boundaries
are indicated. Bond distances: Zr1-Sn1 2.969 Å, Zr1-N1 2.133 Å, Zr1-
C14 3.00 Å, Zr1-C15 3.28 Å, N1-C14 1.416 Å, C14-C15 1.347 Å, C14-
C22 1.525 Å. Bond angles: N1-Zr1-Sn1 107.9°, Zr1-N1-C14 113.9°.
Instead of the expected insertion product, treatment of
cyclononadiene metallacycle 14 with benzaldehyde for 1 h at
room temperature gave benzyloxide product 25 (eq 3). This
product presumably results from â-hydride elimination followed
by hydride insertion. Acetophenone and 4,4′-dichlorobenzophe-
none yielded similar insertion products.
result in insertion of the carbonyl moiety into the Zr-C bond;
rather, the 1:3 equilibrium mixture of E and Z isomers was
formed within 30 min at room temperature (Scheme 8). This
E/Z isomerization was slower in the presence of 4,4′-dimeth-
ylbenzophenone, and faster in the presence of 4,4′-dichloroben-
zophenone. Catalytic quantities of these reagents were sufficient
to promote the isomerization, and the presence or absence of
light had no effect on its rate. Pivaldehyde and di-tert-butyl
ketone failed to promote the isomerization, but di-p-tolyl
disulfide, and ferricenium and cobaltocenium salts, also cata-
lyzed the E/Z isomerization of the metallacycle, suggesting that
these isomerizations proceed by an electron-transfer mechanism.
CD spectroscopy revealed that racemization of the metallacycle
took place concomitantly.
Dicyclopropylallene. In an additional effort to detect radical
intermediates, zirconocene imido complex 3 was treated with
dicyclopropylallene. This did not yield the (presumably initially
formed) metallacycle 28; instead, a new metallacycle was
formed quantitatively in 15 min at room temperature (eq 4).
This species was identified as the seven-membered ring met-
allacycle 29 by 2D NMR spectroscopy. The unusual trans double
bond in a seven-membered ring was confirmed by the large
coupling constant (J ) 15 Hz) and by X-ray crystallographic
analysis of related complex 30. An ORTEP diagram and relevant
metric parameters are given in Figure 5. The double bond is
not coordinated to the Zr, as shown by the Zr-C13 and Zr-
C14 distances of 2.88 and 3.06 Å, and is somewhat strained, as
shown by the C12-C13-C14-C15 torsion angle of 155°
(rather than 180°) and the Zr1-C11 bond distance of 2.41 Å.
Treatment of the metallacycle 6 with a full equivalent of
ferricenium tetrakis(pentafluorophenyl)borate in chlorobenzene-
d5 resulted in the formation of a red solution that displayed no
sharp resonances by NMR or EPR spectroscopy, except for that
corresponding to the quantitative formation of ferrocene (δ 4.05
ppm). Immediate addition of 1 equiv of cobaltocene to this
mixture regenerated a blue solution of the metallacycle as an
equilibrium mixture of E and Z isomers. However, if the red
solution is allowed to stand at room temperature for 2 h, addition
of cobaltocene fails to generate any recognizable products.
Insertions. To further investigate possible electron-transfer
reactions, the metallacycle 6 was treated with known radical
traps. For example, metallacycle 6 reacted with Me3SnH within
18 h at room temperature to give the product of formal insertion
into the Zr-C bond 23 (Scheme 9). The 1H NMR spectrum of
23 displayed broad resonances due to hindered rotation about
the N-C bond. The two protons on the benzylic methylene
group are diastereotopic at room temperature (δ 4.1 and 2.57
ppm), but their signals coalesce at high temperatures. The
structure of 23 was confirmed by X-ray crystallographic
analysis; an ORTEP diagram is displayed in Figure 4. In
contrast, the poorer radical trap Et3SiH failed to react with
metallacycle 6, even at 105 °C. Treatment of metallacycle 6
with bis(4-trifluoromethylphenyl)disulfide at room temperature
gave insertion product 24, which displayed similar hindered
rotation.
Discussion
The results presented above provide insight into the mech-
anisms of the isomerization and racemization of allene-derived
metallacycles, and a discussion of these mechanisms follows.
9
1756 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

Rearrangements and Stereomutations of Metallacycles
A R T I C L E S
Scheme 11. Possible Isomerization and Racemization
Mechanisms
Figure 5. X-ray structure of dicyclopropylallene metallacycle 30. 50%
probability boundaries are indicated. Bond distances: Zr1-N1 2.235 Å,
Zr1-C11 2.410 Å, C13-C14 1.327 Å, C15-C16 1.333 Å. Bond angles:
N1-Zr1-C11 117.9°, Zr1-C11-C12 108.1°.
Scheme 10. Stepwise versus Concerted Cycloaddition Processes
First, it is important to establish whether the racemization of
allenes occurs during the initial cycloaddition step or is due to
a subsequent isomerization event. After answering this question,
potential mechanisms for E/Z isomerization and racemization
will be presented, and the kinetic and thermodynamic selectivi-
ties in the cycloaddition process and subsequent isomerizations
will be discussed.
Two different types of stereoisomerization were observed: E/Z
isomerization and epimerization of the alpha stereocenter.
Isomerization of either the alpha stereocenter or the exocyclic
double bond results in net inversion of the stereochemistry of
the allene fragment of the metallacycle and thus could be
responsible for the overall allene stereoinversion. Several
mechanistic possibilities for the various isomerization processes
are depicted in Scheme 11. Although diradical (or zwitterionic)
species 31 is not an intermediate in the initial cycloaddition
process, simple reversible fragmentation of the Zr-C bond of
the metallacycle to generate intermediate 31 can explain both
the racemization and the E/Z isomerization processes (eq 5).
Alternatively, coordination of the exocyclic double bond to the
Zr center to generate η4-azatrimethylenemethane intermediate
(or transition state) 32,35-37 followed by collapse to the opposite
terminus, can also explain both types of isomerization processes
(eq 6). Reversible â-hydride elimination, with transient forma-
tion of achiral Zr hydride 33, would result in racemization of
the metallacycle, but not E/Z isomerization, and is only available
to dialkylmetallacycles (eq 7). Net E/Z isomerization can be
accomplished by simple rotation around the double bond (eq
8) or by retrocyclization followed by cycloaddition to the
opposite face of the allene (eq 9), but neither pathway allows
for racemization of the metallacycle. These potential mecha-
nisms will serve as a guide for the following discussion of the
experimental results.
Stereospecificity of Cycloaddition. In our initial report
discussing the selective stereoinversion of cyclononadiene,15 we
proposed that this stereoinversion could be the result of a
stepwise [2+2] cycloaddition mechanism which proceeds
through an achiral diradical or zwitterionic intermediate 31 (path
a, Scheme 10). If this were the case, reaction of enantiopure
allenes with achiral imido complexes should yield racemic
products because intermediate 31 is achiral. However, the
initially formed zirconocene metallacycles (with the exception
of di-n-propylmetallacycle 7) in this study exhibited strong CD
signals and were therefore optically active. Furthermore, the
structures of the cycloadducts of (R)- and (S)-diphenylallene
with enantiopure ebthi imido complex (R,R)-1, 19 and 20,
respectively, reveal that the original stereochemistry of the allene
is retained through the cycloaddition/cycloreversion process.
Therefore, the [2+2] cycloaddition process is almost certainly
concerted and stereospecific for all allenes, initially generating
enantiopure metallacycles that subsequently racemize by another
mechanism (path b). This racemization is too rapid to observe
in the case of the di-n-propylmetallacycle 7, but is slow enough
for the other metallacycles to allow observation of optical
activity.
Kinetic and Thermodynamic Olefin Geometries. On the
basis of a simple analysis of the steric factors in the [2+2]
Potential Metallacycle Rearrangement Mechanisms. After
establishing that the cycloaddition is concerted and stereospe-
cific, we began investigating the nature and mechanism of the
subsequent isomerizations of the allene-derived metallacycles.
(35) Bazan, G. C.; Rodriguez, G.; Cleary, B. P. J. Am. Chem. Soc. 1994, 116,
2177.
(36) Keaton, R. J.; Koterwas, L. A.; Fettinger, J. C.; Sita, L. R. J. Am. Chem.
Soc. 2002, 124, 5932.
(37) Kissounko, D. A.; Sita, L. R. J. Am. Chem. Soc. 2004, 126, 5946.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1757

A R T I C L E S
Michael et al.
Scheme 12. Mechanisms for MAD Complex Formation
substantially slower (t1/2 > 1 h) than the hypothetical E/Z
isomerization (t1/2 < 5 min).
Monoazadiene Complex Formation. All metallacycles with
available hydrogen atoms beta to the metal center eventually
rearrange to monoazadiene (MAD) complexes. The rate of this
rearrangement is affected by the size of the substituents (n-Pr
> (CH2)6 > i-Pr ≈ c-Hex). The failure to isolate free allene
upon treatment of di-n-propylmetallacycle 7 with diisopropyl-
carbodiimide indicates that rearrangement of 7 to the MAD
complex is faster than retrocycloadditon. A mechanism con-
sistent with these observations is depicted in Scheme 12 (path
a). Initial â-hydride elimination to generate zirconium hydride
complex 34 is followed by intramolecular hydrozirconation of
the enamide double bond to give η2-imine complex 35. A [1,3]-
shift of the zirconium center then generates the MAD complex.38
An alternative mechanism initiated by dissociation of the allene
followed by intermolecular C-H activation does not fit the
observation that metallacycle 7 forms MAD complex 14 even
in the presence of DIC (path b).
Racemization. The rate of racemization of metallacycles
formed from 1,3-dialkylallenes parallels the rate of MAD
complex formation (n-Pr > i-Pr ≈ c-Hex . Ph). The initial
â-hydride elimination step in the proposed mechanism for MAD
complex formation suggested a possible route for racemization
of dialkyl-substituted metallacycles and an explanation for their
relative rates. Reversible â-hydride elimination would result in
loss of optical activity via achiral intermediate 33 (Scheme 11,
eq 7). To test this hypothesis, the racemization rate of enan-
tiopure deuterated metallacycle 8-d2 was determined. This
process exhibited a normal primary isotope effect (kH/kD ) 2.8),
similar in magnitude to that of reported â-hydride elimina-
tions.39-41 The formation of the alkoxide complexes 25-27 upon
treatment of cyclononadiene metallacycle 14 with carbonyl
compounds (eq 3) provides additional evidence for the inter-
mediacy of Zr hydride 33. Insertion of the ketone or aldehyde
into the transient Zr hydride complex is evidently much faster
than insertion into the Zr-C bond of the metallacycle. These
data strongly indicate that the racemization of dialkylmetalla-
cycles proceeds by a reversible â-hydride elimination mecha-
nism (eq 7).
Figure 6. Analysis of the kinetic and thermodynamic selectivities leading
to E and Z isomers.
cycloaddition, it was anticipated that the substituent on the
exocyclic double bond would prefer to be oriented away from
the bulky imido substituent and, therefore, that the E isomers
of the metallacycles would be favored (Figure 6). This model
holds for the kinetic cycloadducts of diphenylallene with both
zirconocene and ebthi imido complexes. In fact, even in the
reaction of imido complex (S,S)-1 with (R)-diphenylallene,
where an unfavorable interaction between the bulk of the ebthi
ligand and the R-phenyl group could be avoided by forming
the Z isomer, only the E isomer was observed. This preference
also explains the observed rate difference in the formation of
the matched and mismatched isomers.
In contrast to the strong kinetic preference for formation of
E metallacycles from diphenylallene, the energies of the E and
Z isomers are fairly similar. In fact, for the zirconocene
metallacycle 6, the Z isomer is slightly more stable than the E
isomer (Keq ) 3 at 95 °C). In the ebthi series, the equilibrium
between 20 and 22 also favors the Z isomer (k1/k-1 ) Keq ) 6
at 45 °C), but in the epimeric pair (19 and 21), the E isomer is
more stable (Keq ) 3 at 105 °C). However, in all cases, the
difference in energy between the isomers is small (∆G° < 1.2
kcal/mol). There appears to be a subtle balance between two
types of allylic strain involving orientation of the exocyclic
phenyl group toward either the R-phenyl substituent or the imido
substituent (Figure 6). Although the imido substituent is larger,
the X-ray crystal structure of metallacycle 10 reveals that, due
to puckering of the metallacycle, the R-phenyl substituent is
closer to being coplanar with the exocyclic phenyl group (26°
vs 39°, Figure 1), thereby increasing the magnitude of steric
interaction for this substituent, and compensating for its smaller
size.
For the dialkylmetallacycles 7-9, only the Z isomer was ever
observed, so it is unclear whether the stereochemistry about the
exocyclic double bond is due to kinetic or thermodynamic
preference. DFT calculations indicate that the Z isomer is
thermodynamically more stable than the E isomer for all
dialkylmetallacycles. We cannot say, however, whether the Z
isomer is formed upon initial cycloaddition or if the E isomer
is kinetically preferred and a rapid E/Z isomerization follows.
If such a rapid E/Z isomerization is occurring, it must do so
without racemization of the metallacycle, as this process is
Rapid reversible â-hydride elimination explains the relative
ease with which dialkylmetallacycles racemize, but such a
(38) This method has been used to synthesize MAD complexes from allylamido-
(methyl)zirconium complexes, see ref 23.
(39) Alibrandi, G.; Scolaro, L. M.; Minniti, D.; Romeo, R. Inorg. Chem. 1990,
29, 3467.
(40) Bryndza, H. E. J. Chem. Soc., Chem. Commun. 1985, 1696.
(41) Evans, J.; Schwartz, J.; Urquhart, P. W. J. Organomet. Chem. 1974, 81,
C37.
9
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Rearrangements and Stereomutations of Metallacycles
A R T I C L E S
one-electron oxidation of zirconium alkyl complexes has been
described by Jordan et al.,42 and reversible one-electron oxida-
tion of a zirconaazacyclobutene has been reported by Norton
et al.43 These precedents suggest that oxidation could occur
directly from the Zr-C bond or from the enamide π system. In
either case, rapid fragmentation of the Zr-C bond should take
place to form intermediate 36. Because this intermediate has
lost the original stereochemical information of the allene,
collapse to regenerate the metallacycle upon reduction occurs
to give an equilibrium mixture of all possible isomers.
Figure 7. [1,3]-Shift mechanism for E/Z isomerization and racemization.
mechanism is not available to diarylmetallacycles, which lack
available beta hydrogen atoms. An alternative mechanism is
therefore required to explain the racemization of these com-
plexes. The E/Z isomerization and racemization of the diaryl-
metallacycles occur simultaneously in both the zirconocene
system and the ebthi system, so mechanisms that explain the
E/Z isomerization but do not allow for racemization (eqs 8 and
9) are unlikely. On the basis of preliminary DFT calculations,
we proposed a [1,3]-shift mechanism involving coordination of
the exocyclic olefin to the zirconium to form an η4-azatrim-
ethylenemethane complex 32 as either an intermediate or a
transition state (Scheme 11, eq 6). Collapse of this species to
the other terminus of the allyl system would result in racem-
ization of the metallacycle. Analogous [1,3]-shifts proceeding
through alternative azatrimethylenemethane isomers can also
explain the E/Z isomerization (Figure 7). Each [1,3]-shift results
in either inversion of the alpha stereocenter or E/Z isomerization,
but not both. However, as discussed below, additional evidence
has led us to postulate a different mechanism for the diaryl-
metallcycle isomerization.
Matched and Mismatched Rearrangements. To try to
observe individual [1,3]-shifts, matched and mismatched met-
allacycles 20 and 21 were synthesized by treatment of enan-
tiopure ebthi imido complex (R,R)-1 with (R)- and (S)-
diphenylallene, respectively. These isomers correspond to
structures A and B in Figure 7. The rearrangement of the
mismatched metallacycle 20 was observed to form all possible
isomers of this complex through a complex series of reactions.
Kinetic modeling revealed that this rearrangement followed the
kinetic scheme depicted in Scheme 7. These rearrangements
correspond to the conversions of B to C (k1), B to D (k2), B to
A (k3), C to B (k-1), and C to A (k4) in Figure 7. The observed
direct rearrangements of 20(B) to 21(D) and 22(C) to 19(A)
(k2 and k4) are inconsistent with any of the simple [1,3]-shifts
depicted in Figure 7.
Intermediate 36 in the oxidative catalysis pathway suggested
an alternative mechanism for the uncatalyzed racemization and
E/Z isomerization processes. Thermal homolysis of the Zr-C
bond in 6 would result in the formation of a structure (31)
containing the same stabilized diphenylallyl radical and a Zr-
(III) center (Scheme 11, eq 5). Radical recombination would
regenerate the metallacycle, but because the original stereo-
chemistry is lost in the diradical intermediate, an equilibrium
mixture of the four isomers would result. Although none of these
constitutes conclusive proof, several observations support this
mechanism, rather than the [1,3]-shift mechanism for the
racemization of diarylmetallacycles. First, the rearrangements
of mismatched metallacycle 20 are not consistent with the
predictions of the [1,3]-shift mechanism; that is, the direct
rearrangements of 20 to 21 and 22 to 19 (k2 and k4) should not
occur via a simple [1,3]-shift (see above). Second, the entropies
of activation for these rearrangements are moderately positive
(∆Sq ) 3-9 eu), which is more consistent with a mechanism
in which a bond is breaking, than with one in which a concerted
[1,3]-shift is taking place.44-46 Third, the Zr-C bond length in
diphenylmetallacycle 10 is 0.1 Å longer47 than the Zr-C bond
in Cp2ZrMe2 (BDE ) 67 kcal/mol),48,49 indicating that it may
also be substantially weaker. Fourth, preliminary DFT calcula-
tions indicate that allylic Zr-C bonds have bond dissociation
energies similar to the enthalpies of activation measured for
the rearrangements (calc. BDE ) 29 kcal/mol).50 Fifth, diphe-
nylmetallacycle 6 undergoes insertion reactions with stannanes
and disulfides that are reminiscent of radical trapping reactions
(Scheme 9). Finally, the rearrangement product of the dicyclo-
propylallene metallacycle 29 is formally the result of a radical
(42) Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett, R. J.
Am. Chem. Soc. 1987, 109, 4111.
(43) Harlan, C. J.; Hascall, T.; Fujita, E.; Norton, J. R. J. Am. Chem. Soc. 1999,
121, 7274.
(44) Brown, K. L.; Zhou, L. Inorg. Chem. 1996, 35, 5032.
(45) Brown, K. L.; Li, J. J. Am. Chem. Soc. 1998, 120, 9466.
(46) Amaudrut, J.; Wiest, O. J. Am. Chem. Soc. 2000, 122, 3367.
(47) Hunter, W. E.; Hrncir, D. C.; Bynum, R. V.; Penttila, R. A.; Atwood, J. L.
Organometallics 1983, 2, 750.
(48) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701.
(49) Diogo, H. P.; De Alencar Simoni, J.; Minas da Piedade, M. E.; Dias, A.
R.; Martinho Simoes, J. A. J. Am. Chem. Soc. 1993, 115, 2764.
(50) DFT calculations at the BP86/LACVP level of theory were performed on
the compounds in the following equation using Jaguar 5.5, Schro¨dinger,
Inc., Portland, OR, 1991-2000.
Oxidative Isomerization. Mild oxidants (benzophenone,
disulfides, cobaltocenium, ferricenium) were found to catalyze
the E/Z isomerization and racemization of diphenylmetallacycle
6. This process could be reproduced in a stepwise fashion by
treatment of the metallacycle with ferricinium cation, followed
by reduction with cobaltocene. The proposed mechanism for
this transformation involves removal of an electron from the
metallacycle, resulting in fragmentation of the Zr-C bond to
generate a structure (36) containing a Zr cation and a highly
stabilized diphenylallyl radical (eq 10). Ample precedents for
this type of reactivity exist: bond fragmentation induced by
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1759

A R T I C L E S
Michael et al.
6, an analytical solution of the metallacycle in hexanes was prepared
in the glovebox. The solution was transferred to a 1 cm quartz glass
cuvette, to which a Kontes vacuum stopcock had been sealed. For
metallacycles 7 and 8, the imido complex 3 (2 mg) was stirred in
hexanes (20 mL) for 30 min to allow it to dissolve, and then an excess
(5-10 equiv) of the appropriate enantioenriched allene (5b or 5c) was
added. The solution took on a pale purple color within 5 min at room
temperature. The metallacycle solutions were filtered and transferred
to an airtight cuvette, as described above. Initial spectra were taken
within 30 min of mixing the reagents.
ring-opening process. All of these observations taken together
argue forcibly for reversible bond homolysis (Scheme 11, eq
5) as the mechanism of racemization and E/Z isomerization of
diarylmetallacycles.
Conclusion
The stereoinversion of allenes promoted by imidozirconium
complexes has been thoroughly investigated. The [2+2] cy-
cloaddition of allenes and imido complexes is concerted and
stereospecific, leading to the formation of metallacycles in which
the original configuration of the allene has been retained. These
metallacycles undergo racemization and E/Z isomerization
processes, as well as subsequent rearrangement to monoazadiene
(MAD) complexes. Reversible â-hydride elimination is the
initial step in the racemization of dialkylmetallacycles and their
rearrangements to MAD complexes. Diarylmetallacycles un-
dergo racemization and E/Z isomerization processes via a
reversible homolysis of the unusually weak Zr-C bond. The
racemization and E/Z isomerization of diarylmetallacycles is
also catalyzed by weak oxidants, proceeding through a radical
cation intermediate. We hope to use the mechanistic insights
disclosed here in the development of new catalytic and
stoichiometric transformations of allenes mediated by imidozir-
conium complexes, especially potentially asymmetric versions.
CD Kinetics of Racemization of Metallacycle 8. Solutions of
metallacycles 8 and 8-d2 were prepared as described above, and the
CD signal at 300 nm was recorded as a function of time at 25, 35, and
45 °C. Clean first-order kinetics were observed in all cases, and the
observed rate constants were obtained from a plot of log(CD) versus
time.
(E)-Cp2ZrN(2,6-Me2C6H3)C(dCHPh)C(H)Ph ((E)-6). To a solu-
tion of 3 (0.097, 0.24 mmol) in benzene (3 mL) was added a solution
of (R)-5a (0.45 g, 0.24 mmol). After 10 min at room temperature, the
resulting deep blue solution was frozen (-30 °C). The solvent was
lyophilized, yielding a purple solid. Crystallization by diffusion of
pentane into a concentrated THF solution at -30 °C afforded pure (E)-6
as a brown powder (0.082 mg, 58%) (approximately 1 equiv of THF
cocrystallized with (E)-6 and could not be removed, even after extended
exposure of the crushed crystalline material to dynamic vacuum, as
determined by 1H NMR spectroscopy). 1H NMR (C6D6, 500 MHz): δ
6.79-7.13 (m, 13H, ArH), 5.77 (s, 5H, C5H5), 5.72 (br d, J ) 2 Hz,
dCH), 5.63 (s, 5H, C5H5), 4.48 (br d, J ) 2 Hz, 1H, ZrCH), 3.56 (br
s, 4H, ((CH2CH2)2O), 2.22 (s, 3H, ArCH3), 2.14 (s, 3H, ArCH3), 1.40
(br s, 4H, ((CH2CH2)2O) ppm. 13C NMR (C6D6, 125 MHz): δ 151.4
(NCdC), 149.9 (Caryl), 140.6 (Caryl), 136.9 (Caryl), 132.8 (Caryl), 132.5
(Caryl), 130.7 (Caryl), 129.1 (Caryl), 129.0 (Caryl), 128.2 (Caryl), 127.7 (Caryl),
126.8 (Caryl), 123.9 (Caryl), 123.1 (Caryl), 121.4 (Caryl), 115.2 (C5H5),
113.6 (C5H5), 106.2 (dC(H)Ph), 68.2 ((CH2CH2)2O), 53.7 (ZrC), 26.1
((CH2CH2)2O), 22.3 (ArCH3), 20.5 (ArCH3) ppm. FT-IR (Nujol): 3106
(w), 1590 (m), 1543 (s), 1376 (m), 1263 (m), 1213 (m), 1147 (m),
Experimental Section
General Procedures. Unless otherwise noted, reactions and ma-
nipulations were performed at ambient temperature in an inert
atmosphere (N2) glovebox, or using standard Schlenk and high vacuum
line techniques. Glassware was dried overnight at 150 °C or flame dried
under vacuum immediately prior to use. All NMR spectra were obtained
at ambient temperature using Bruker AMX-300, AMX-400, or DRX-
1
500 spectrometers. H NMR chemical shifts (δ) are reported in parts
per million (ppm) downfield of TMS and are referenced relative to
residual protiated solvent. 13C NMR chemical shifts (δ) are reported
in ppm relative to the carbon resonance of the deuterated solvent. In
cases where assignment of 13C resonances was ambiguous, standard
DEPT 45, 90, and/or 135 pulse sequences or 13C-1H HMQC experi-
ments were used. Infrared (IR) spectra were recorded as a Nujol mull
or a thin film between NaCl plates. Elemental analyses were performed
at the University of California, Berkeley Microanalytical facility on a
Perkin-Elmer 2400 Series II CHNO/S Analyzer. Optical rotations were
measured on a Perkin-Elmer model 241 polarimeter at room temper-
ature.
Materials. Unless otherwise noted, reagents were purchased from
commercial suppliers and were used without further purification.
Pentane, hexanes, benzene, and toluene (Fisher) were passed through
a column of activated alumina (type A2, size 12 × 32, Purifry Co.)
under nitrogen pressure and sparged with N2 prior to use. Diethyl ether
and tetrahydrofuran (Fisher) were distilled from sodium/benzophenone
ketyl under N2 prior to use. Deuterated solvents (Cambridge Isotope
Laboratories) were purified by vacuum-transfer from the appropriate
drying agent (Na/Ph2CO or CaH2) prior to use. Imido complexes 1
and 3 were prepared according to literature procedures.15,51 (rac)-1,2-
Dicyclopropylallene was a generous gift of Dr. Armin DeMeijere.
General Procedures for Circular Dichroism Experiments. All
circular dichroism spectra were recorded at room temperature on a Jasco
J-810 spectropolarimeter. In experiments to determine the change in
optical activity with time and/or temperature, benzene or hexanes
solutions of metallacycles were stored and/or heated in glass ampules
to which a Kontes vacuum stopcock had been sealed. For metallacycle
1067 (w), 1015 (m), 912 (m), 797 (s), 762 (m), 701 (m), 637 (m) cm-1
.
Anal. Calcd for ZrNC33H31(C4H8O): C, 73.46; H, 6.50; N, 2.32.
Found: C, 73.25; H, 6.77; N, 2.69. Reaction of 3 with a slight excess
of (R)-1,3-diphenylallene, followed by removal of solvent and trituration
with pentane, afforded a quantitative yield of spectroscopically pure
(S,E)-6.
(Z)-Cp2ZrN(2,6-Me2C6H3)C(dCHPh)C(H)Ph ((Z)-6). A glass
ampule was charged with a solution of (E)-6 (0.120 g, 0.23 mmol) in
toluene (3 mL). The ampule was sealed (Kontes Teflon vacuum
stopcock) and heated to 95 °C for 32 h. Upon cooling to room
temperature, the toluene solution was layered with pentane (5 mL) and
stored at -30 °C. After 2 d, blue blocky crystals were isolated and
solvent was removed under reduced pressure to afford pure (Z)-6 as a
blue solid (0.046 g, 33%) (exactly 0.6 equiv of toluene cocrystallized
with (Z)-6 and could not be removed, even after extended exposure of
the crushed crystalline material to dynamic vacuum, as determined by
1H NMR spectroscopy). 1H NMR (C6D6, 500 MHz): δ 7.30-7.36 (m,
4H, ArH), 6.93-7.01 (m, 8H, ArH), 6.93 (m, 1H, ArH), 6.68 (m, 1H,
ArH), 6.63 (m, 1H, ArH), 6.38 (br d, J ) 2 Hz, 1H, dCH), 5.79 (s,
5H, C5H5), 5.72 (s, 5H, C5H5), 4.25 (br d, J ) 2 Hz, 1H, ZrCH), 2.18
(s, 3H, ArCH3), 2.10 (s, 1.8H, CH3C6H5), 1.87 (s, 3H, ArCH3) ppm.
13C NMR (C6D6, 125 MHz): δ 150.5 (NCdC), 149.3 (Caryl), 139.7
(Caryl), 138.2 (Caryl), 132.6 (Caryl), 131.1 (Caryl), 129.7 (Caryl), 128.9 (Caryl),
128.7 (Caryl), 128.0 (Caryl), 127.8 (Caryl), 127.8 (Caryl), 127.6 (Caryl), 127.5
(Caryl), 126.0 (Caryl), 124.2 (Caryl), 122.1 (Caryl), 121.1 (Caryl), 115.2 (Caryl),
114.1 (C5H5), 113.5 (C5H5), 108.7 (dCH), 59.6 (ZrC), 23.6 (ArCH3),
21.8 (CH3C6H5), 20.8 (ArCH3) ppm. FT-IR (Nujol): 3057 (w), 1590
(m), 1545 (w), 1377 (m), 1292 (w), 1259 (m), 1232 (m), 1199 (w),
1115 (w), 1013 (m), 1069 (w), 1013 (m), 929 (w), 877 (w), 804 (s),
(51) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. Organometallics 1993, 12,
3705.
9
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Rearrangements and Stereomutations of Metallacycles
A R T I C L E S
763 (m), 736 (m), 694 (m) cm-1. Anal. Calcd for ZrNC33H31(C7H8)0.6
C, 75.97; H, 6.14; N, 2.39. Found: C, 75.71; H, 6.22; N, 2.30.
:
pentane (1 mL/9 mL) was added a solution of 5a (0.033 g, 0.18 mmol)
in pentane (2 mL). After 20 min at room temperature, the resulting
deep blue solution was concentrated to dryness. Crystallization of the
residue by diffusion of pentane into a concentrated toluene solution at
-30 °C afforded pure 10 as blue blocky crystals (0.062 g, 60%). One-
sixth of an equivalent of toluene remained trapped in the crystals of
10, even after extended exposure to high vacuum, as determined by
1H NMR spectroscopy. 1H NMR (C6D6, 500 MHz): δ 6.92-7.15 (m,
9H, ArH), 6.77-6.82 (m, 4H, ArH), 5.87 (s, 5H, C5H5), 5.69 (s, 5H,
C5H5), 5.58 (br d, J ) 2 Hz, 1H, dCH), 4.56 (br d, J ) 2 Hz, 1H,
ZrCH), 3.14 (sept, J ) 7 Hz, 1H, CH(CH3)2), 3.04 (sept, J ) 7 Hz,
1H, CH(CH3)2), 2.10 (s, 0.5 H, C6H5CH3), 1.37 (d, J ) 7 Hz, 3H,
CH(CH3)2), 1.35 (d, J ) 7 Hz, 3H, CH(CH3)2), 1.90 (d, J ) 7 Hz, 3H,
CH(CH3)2), 1.12 (d, J ) 7 Hz, CH(CH3)2) ppm. 13C NMR (C6D6, 125
MHz): δ 149.7 (NCdC), 148.2 (Caryl), 144.3 (Caryl), 144.1 (Caryl), 140.6
(Caryl), 139.0 (Caryl), 129.7 (Caryl), 129.2 (Caryl), 128.9 (Caryl), 128.0 (Caryl),
127.7 (Caryl), 127.1 (Caryl), 125.9 (Caryl), 124.8 (Caryl), 123.9 (Caryl), 123.6
(Caryl), 121.4 (Caryl), 115.1 (C5H5), 113.6 (C5H5), 107.9 (dC(H)Ph),
53.1 (ZrC), 21.1 (CH(CH3)2), 28.7 (CH(CH3)2), 26.1 (CH(CH3)2), 26.0
(CH(CH3)2), 26.0 (CH(CH3)2, 25.9 (CH(CH3)2) ppm. FT-IR (Nujol):
3046 (w), 1592 (s), 1536 (m), 1379 (m), 1320 (s), 1254 (s), 1204 (m),
1115 (w), 1065 (w), 1015 (w), 910 (m), 801 (s), 768 (m), 688 (m),
629 (m) cm-1. Anal. Calcd for ZrNC37H39(C7H8)1/6: C, 75.86; H, 6.73;
N, 2.32. Found: C, 76.20; H, 6.92; N, 2.21.
(Z)-Cp2ZrN(2,6-Me2C6H3)C(dC(H)C3H7)CH(C3H7) (7). Although
it is initially formed cleanly, azazirconacyclobutane 7 rearranges slowly
(t1/2 ≈ 10 h) to a new product at room temperature; therefore, it could
not be isolated in analytically pure form. Rather, 7 was generated in
1
situ and characterized by H and 13C NMR spectroscopy only. To a
solution of 3 (0.016 g, 0.04 mmol) in C6D6 (0.5 mL) was added a
solution of (S)-5b (0.007 g, 0.06 mmol) in C6D6 (0.5 mL). The resulting
deep purple reaction mixture was transferred to an NMR tube, which
was flame-sealed under static vacuum. 1H NMR (C6D6, 500 MHz): δ
5.93 (s, 5H, C5H5), 5.83 (s, 5H, C5H5), 5.02 (dt, J ) 7 Hz, 2.5 Hz, 1H,
dCH), 2.78 (m, 1H, ZrCH), 2.58 (m, 1H, ZrCHCH2), 2.34 (s, 3H,
ArCH3), 2.06 (s, 3H, ArCH3), 2.05 (m, 1H, ZrCHCH2), 1.79 (m, 2H,
dCHCH2), 1.37 (m, 4H, dCHCH2CH2, ZrCHCH2CH2), 1.18 (t, J ) 7
Hz, 3H, ZrCH(CH2)2CH3), 0.83 (t, J ) 7 Hz, dCH(CH2)2CH3) ppm.
13C NMR (C6D6, 125 MHz): δ 152.9 (NCdC), 131.3 (C6H3), 130.4
(C6H3), 129.2 (C6H3), 128.8 (C6H3), 120.5 (C6H3), 114.1 (dCH), 111.7
(C5H5), 110.8 (C5H5), 58.5 (ZrC), 36.1 (CH2), 31.4 (CH2), 25.7 (CH2),
23.8 (CH2), 23.3 (ArCH3), 20.4 (ArCH3), 15.5 (CH3), 14.8 (CH3) ppm.
One of the aryl resonances in the 13C spectrum could not be resolved.
(Z)-Cp2ZrN(2,6-Me2C6H3)C(dC(H)CH(CH3)2)CH(CH(CH3)2) (8).
To a solution of 3 (0.095 g, 0.23 mmol) in benzene (5 mL) was added
a solution of (S)-5c (0.034 g, 0.28 mmol) in benzene (1 mL). The
resulting deep purple solution was stirred for 6 h at room temperature.
The solvent was removed under reduced pressure, leaving a viscous
purple material. This was taken up in pentane (10 mL) and filtered.
Concentration of this solution afforded analytically pure 8 as a purple
(R)-2,6-Dideutero-2,6-dimethyl-4-heptyn-3-ol (13-d2). A flask
containing Zn(OTf)2 (364 mg, 1 mmol) was heated with a heat gun
under vacuum until gas evolution ceased and was then allowed to cool
to room temperature. (+)-N-Methylephedrine (198 mg, 1.1 mmol) was
added, and the flask was evacuated for 5 min. The flask was refilled
with N2, and toluene (5 mL) and NEt3 (250 mg, 2.5 mmol) were added.
After the mixture was stirred for 2 h, 3-methyl-1-butyne (0.767 mL, 5
mmol) was added, followed by isobutyraldehyde (0.460 mL, 5 mmol).
The flask was stoppered, and the mixture heated to 60 °C for 12 h.
After being cooled to room temperature, saturated NH4Cl was added,
and the mixture was extracted with Et2O (3×). The combined organic
phases were washed with brine, dried (MgSO4), concentrated, and
chromatographed (15% Et2O/pentane) to give the product as a colorless
1
solid (0.090 g, 84%). H NMR (C6D6, 500 MHz): δ 7.04 (d, J ) 7
Hz, 1H, m-ArH), 6.98 (d, J ) 7 Hz, 1H, m-ArH), 6.78 (t, J ) 7 Hz,
1H, p-ArH), 5.80 (s, 5H, C5H5), 5.67 (s, 5H, C5H5), 4.68 (dd, J ) 10
Hz, 2.5 Hz, 1H, dCH), 2.34 (dd, J ) 9.5 Hz, 2 Hz, ZrCH), 2.22 (s,
3H, ArCH3), 2.13 (m, 2H, CH(CH3)2), 1.94 (s, 3H, ArCH3), 1.29 (d, J
) 6 Hz, 3H, CH(CH3)2), 0.97 (d, J ) 6 Hz, 3H, CH(CH3)2), 0.78 (d,
J ) 6 Hz, 3H, CH(CH3)2), 0.66 (d, J ) 6 Hz, 3H, CH(CH3)2) ppm.
13C NMR (C6D6, 125 MHz): δ 153.6 (NCdC), 130.6 (C6H3), 129.6
(C6H3), 129.5 (C6H3), 128.8 (C6H3), 124.2 (dCH), 120.7 (C6H3), 67.6
(ZrC), 30.6 (CH(CH3)2), 28.3 (CH(CH3)2), 26.9 (CH(CH3)2), 26.7 (CH-
(CH3)2), 24.9 (CH(CH3)2), 22.9 (ArCH3), 22.6 (CH(CH3)2), 20.5
(ArCH3) ppm. FT-IR (Nujol): 3064 (w), 1590 (m), 1278 (s), 1236 (m),
1175 (w), 1109 (m), 1013 (m), 984 (w), 914 (w), 795 (s) cm-1. Anal.
Calcd for ZrNC27H35: C, 69.77; H, 7.59; N, 3.01. Found: C, 69.40;
H, 7.68; N, 3.33.
1
liquid (491 mg, 71%). H NMR matches literature values for 13-d0,53
2
minus the two methine protons, which were observed in the H NMR
1
spectrum. H NMR (CDCl3, 400 MHz): δ 4.15 (s, 1H, CHOH), 1.69
(s, 1H, CHOH), 1.16 (s, 6H, CH(CH3)2), 0.98 (s, 3H, CH(CH3)2), 0.96
2
(s, 3H, CH(CH3)2) ppm. H NMR (CDCl3, 61.6 MHz): δ 2.65 (br s),
1.91 (br s) ppm.
(S)-2,6-Dideutero-2,6-dimethyl-3,4-heptadiene (5c-d2). To a solu-
tion of PPh3 (1.55 g, 6 mmol) in THF (20 mL) at 0 °C was added
diethylazodicarboxylate (0.932 mL, 6 mmol), and the resulting mixture
was stirred for 10 min. A solution of (R)-13-d2 (691 mg, 5 mmol) in
THF (15 mL) was added, and the resulting mixture was stirred for 10
min. A solution of o-nitrobenzenesulfonylhydrazide54 (1.285 g, 6 mmol)
in THF (15 mL) was added, and the resulting mixture was stirred for
1 h. The mixture was warmed to room temperature and stirred for 16
h. The solution was concentrated and chromatographed (pentane) to
give the product as a colorless liquid (350 mg, 56%). 1H NMR (CDCl3,
300 MHz): δ 5.14 (s, 2H, dCH), 0.98 (s, 12H, CD(CH3)2). GC-MS
(CI) m/z 126 (M+), 111 (MCH3+), >90% d2; GC: >95% ee (B-DM
(Z)-Cp2ZrN(2,6-Me2C6H3)C(dC(H)C6H11)CH(C6H11) (9). To a
solution of 3 (0.107 g, 0.25 mmol) in benzene (4 mL) was added a
solution of (S)-5d (0.052 g, 0.26 mmol) in benzene (1 mL). After 9.5
h at room temperature, the solvent was removed under vacuum, leaving
a dark purple solid. Precipitation from a concentrated ether solution at
-30 °C afforded spectroscopically pure 9 (0.063 g, 45%). Several
attempts to crystallize 9 to analytical purity were unsuccessful. 1H NMR
(C6D6, 500 MHz): δ 7.05 (d, J ) 7 Hz, 1H, m-ArH), 7.00 (d, J ) 7
Hz, 1H, m-ArH), 6.81 (t, J ) 7 Hz, 1H, p-ArH), 5.86 (s, 5H, C5H5),
5.73 (s, 5H, C5H5), 4.84 (dd, J ) 9 Hz, 2 Hz, 1H, dCH), 2.50 (br d,
1H, CH(CH2)5), 2.47 (dd, J ) 10 Hz, 2 Hz, 1H, ZrCH), 2.28 (s, 3H,
ArCH3), 2.06 (s, 3H, ArCH3), 1.70-1.94 (m, 6H, C6H11), 0.770-1.47
(m, 15H, C6H11) ppm. 13C NMR (C6D6, 125 MHz): δ 154.0 (NCdC),
130.5 (C6H3), 129.9 (C6H3), 129.7 (C6H3), 129.0 (C6H3), 122.7 (dCH),
120.8 (C6H3), 111.8 (C5H5), 109.5 (C5H5), 66.2 (ZrC), 40.8 (C6H11),
38.8 (C6H11), 38.4 (C6H11), 37.7 (C6H11), 35.3 (C6H11), 33.3 (C6H11),
28.3 (C6H11), 28.1 (C6H11), 27.6 (C6H11), 27.5 (C6H11), 27.0 (C6H11),
27.0 (C6H11), 22.9 (ArCH3), 20.6 (ArCH3) ppm. FT-IR (Nujol): 3108
(w), 1590 (w), 1376 (m), 1283 (m), 1268 (m), 1235 (m), 1104 (w),
1012 (m), 800 (s), 759 (m), 718 (w) cm-1. MS (EI) m/z ) 543 (M+).
HRMS (EI): calcd, m/z ) 543.2439; found, m/z ) 543.2443.
(E)-Cp2ZrN(2,6-i-Pr2C6H3)C(dCHPh)C(H)Ph (10). To a solution
of Cp2ZrdN(2,6-i-Pr2C6H3)(THF)52 (0.082 g, 0.18 mmol) in toluene/
(30 m); 65 °C; 0.8 mL/min) tr (R) ) 5.6 min, tr (S) ) 5.9 min. [R]D
+91° (EtOH, c ) 0.45).
)
Cp2ZrN(2,6-Me2C6H3)C(C4H9)dC(H)C(H)C2H5 (14). To a solu-
tion of 3 (0.103 g, 0.25 mmol) in benzene (5 mL) was added a solution
of 5b (0.031 g, 0.25 mmol) in benzene (2 mL). Evolution of a deep
purple color occurred over several minutes at ambient temperature,
followed by a slow fading to orange-brown over 2 d. Concentration of
the reaction mixture gave a dark oil, which was extracted into pentane
(52) Zuckerman, R. L.; Bergman, R. G. Organometallics 2000, 19, 4795.
(53) Baudin, J.-B.; Julia, S. A.; Wang, Y. Bull. Soc. Chim. Fr. 1995, 132, 739.
(54) Myers, A. G.; Zheng, B.; Movassaghi, M. J. Org. Chem. 1997, 62, 7507.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1761

A R T I C L E S
Michael et al.
(10 mL). Filtration followed by solvent removal under reduced pressure
2 mL) to give pure 17 by 1H NMR spectroscopy. An analytically pure
sample was obtained by recrystallizing from saturated pentane. 1H NMR
(C6D6, 500 MHz): δ 7.07 (d, J ) 8 Hz, 1H, m-ArH), 7.06 (d, J ) 8
Hz, 1H, m-ArH), 6.88 (t, J ) 8 Hz, 1H, p-ArH), 5.82 (s, 5H, C5H5),
5.70 (s, 5H, C5H5), 4.87 (t, J ) 8 Hz, 1H, dCH), 2.6-2.5 (m, 2H),
2.29 (t, J ) 10 Hz, 1H, ZrCH), 2.23 (s, 3H, ArCH3), 2.2-2.1 (m, 2H),
2.00 (s, 3H, ArCH3), 2.0-1.9 (m, 2H), 1.9-1.8 (m, 1H), 1.8-1.7 (m,
1H), 1.5-1.3 (m, 4H) ppm. 13C NMR (C6D6, 125 MHz): δ 151.7 (NCd
C), 136.3 (C6H3), 131.0 (C6H3), 129.8 (C6H3), 128.3 (C6H3), 121.6
(C6H3), 111.2 (C5H5), 110.9 (C5H5), 110.5 (dCH), 57.5 (ZrC), 36.0
(CH2), 33.0 (CH2), 32.1 (CH2), 27.7 (CH2), 25.2 (CH2), 25.0 (CH2),
21.4 (ArCH3), 20.1 (ArCH3), 15.5 (CH3), 14.8 (CH3) ppm. Anal. Calcd
for ZrNC27H33: C, 70.1; H, 7.2; N, 3.0. Found: C, 70.36; H, 7.10; N,
2.99.
afforded 14 as a yellow-brown oily solid (0.100 g, 83%). Several
1
attempts to crystallize 14 to analytical purity were unsuccessful. H
NMR (C6D6, 500 MHz): δ 7.09 (d, J ) 7 Hz, 1H, m-ArH), 6.96 (d, J
) 7 Hz, 1H, m-ArH), 6.90 (t, J ) 7 Hz, 1H, p-ArH), 5.50 (s, 5H,
C5H5), 5.34 (s, 5H, C5H5), 4.55 (d, J ) 7 Hz, 1H, dCH), 2.45 (m, 1H,
ZrCH), 2.37 (s, 3H, ArCH3), 2.12 (m, 1H, CH2), 1.86 (m, 2H, CH2),
1.74 (s, 3H, ArCH3), 1.38 (m, 2H, CH2), 1.27 (t, 3H, 7 Hz, CH3), 1.09
(m, 2H, CH2), 0.73 (t, J ) 7 Hz, CH3), 0.49 (m, 1H, CH2) ppm. 13C
NMR (C6D6, 125 MHz): δ 149.4 (NCdC), 133.2 (C6H3), 132.7 (C6H3),
132.3 (C6H3), 129.3 (C6H3), 129.1 (C6H3), 124.0 (C6H3), 107.0 (C5H5),
105.4 (dCH), 104.1 (C5H5), 69.0 (ZrC), 34.2 (CH2), 31.1 (CH2), 30.8
(CH2), 23.4 (CH3), 21.6 (CH2), 19.3 (CH3), 18.1 (CH3), 15.9 (CH3)
ppm. FT-IR (Nujol): 1592 (w), 1522 (w), 1375 (s), 1281 (m), 1233
(m), 1189 (w), 1111 (w), 1095 (w), 1015 (m), 793 (s), 763 (m) cm-1
.
Cyclononadiene-Derived MAD Complex 18. A solution of 17 (115
mg, 0.25 mmol) in benzene (4 mL) was heated at 75 °C for 11 h.
Solvent was removed under vacuum, and the resulting solid was
crystallized by slow diffusion of Et2O into a THF solution to give pure
bright yellow solid 18 as a 3:1 mixture of two isomers (65 mg, 57%).
MS (EI) m/z ) 463 (M+). HRMS (EI) calcd, m/z ) 463.1821; found,
m/z ) 463.1817.
Cp2ZrN(2,6-Me2C6H3)C(CH2CH(CH3)2)dC(H)CH(CH3)2 (15). To
a solution of 3 (0.092 g, 0.22 mmol) in benzene (8 mL) was added a
solution of 5c (0.033 g, 0.27 mmol) in benzene (2 mL). The mixture
was heated at 75 °C for 11 h. Solvent was removed under reduced
pressure, and the resulting solid was precipitated from a concentrated
ether solution at -30 °C to afford spectroscopically pure 15 as a bright
yellow solid (0.033 g, 33%). Several attempts to crystallize 15 to
analytical purity were unsuccessful. 1H NMR (C6D6, 500 MHz): δ 7.04
(d, J ) 7.5 Hz, 1H, m-ArH), 6.94 (d, J ) 7.5 Hz, 1H, m-ArH), 6.89 (t,
J ) 7.5 Hz, 1H, p-ArH), 5.54 (s, 5H, C5H5), 5.42 (s, 5H, C5H5), 4.29
(s, 1H, dCH), 2.20 (s, 3H, ArCH3), 2.08 (s, 3H, ArCH3), 1.89 (s, 3H,
ZrC(CH3)2), 1.68 (m, 3H, CH2CH), 1.34 (s, 3H, ZrC(CH3)2), 0.88 (d,
J ) 6.5 Hz, 3H, CH(CH3)2), 0.80 (d, J ) 6.5 Hz, 3H, CH(CH3)2) ppm.
13C NMR (C6D6, 125 MHz): δ 148.9 (NCdC), 138.6 (C6H5), 133.5
(C6H5), 132.8 (C6H5), 129.7 (C6H5), 129.4 (C6H5), 124.3 (C6H5), 108.3
(C5H5), 106.3 (dCH), 106.1 (C5H5), 60.0 (ZrC), 44.0 (CH2), 34.4 (ZrC-
(CH3)2), 29.1 (ZrC(CH3)2), 27.2 (CH(CH3)2), 23.8 (CH(CH3)2), 23.4
(CH(CH3)2), 22.6 (ArCH3), 20.6 (ArCH3) ppm. FT-IR (Nujol): 3089
(w), 1591 (w), 1264 (m), 1236 (m), 1097 (w), 1080 (w), 1022 (w),
796 (s), 769 (m) cm-1. MS (EI) m/z ) 463 (M+). HRMS (EI): calcd,
m/z ) 463.1814; found, m/z ) 463.1817.
1
Major isomer, H NMR (C6D6, 500 MHz): δ 7.0-6.8 (m, 3H, ArH),
5.58 (s, 5H, C5H5), 5.28 (s, 5H, C5H5), 3.93 (s, 1H, J ) 11 Hz, dCH),
2.98 (ddd, 1H, J ) 5, 11, 13 Hz, ZrCH), 2.24 (s, 3H, ArCH3), 1.90 (s,
3H, ArCH3), 2.3-1.6 (m, 9H), 1.3-1.2 (m, 1H), 1.1-1.0 (m, 1H),
0.87 (q, 1H, J ) 13 Hz, ZrCHCH2) ppm. 13C NMR (C6D6, 125 MHz):
δ 150.4 (NCdC), 149.0 (C6H5), 134.0 (C6H5), 132.6 (C6H5), 129.3
(C6H5), 128.2 (C6H5), 124.5 (C6H5), 106.2 (C5H5), 104.6 (C5H5), 84.1
(dCH), 60.8 (ZrC), 34.4 (CH2), 31.2 (CH2), 31.1 (CH2), 30.0 (CH2),
30.1 (CH2), 21.4 (ArCH3), 18.5 (ArCH3) ppm. Minor isomer, 1H NMR
(C6D6, 500 MHz): δ 7.08 (d, 1H, J ) 8 Hz, m-ArH), 7.0-6.8 (m, 2H,
ArH), 5.46 (s, 5H, C5H5), 5.28 (s, 5H, C5H5), 3.97 (s, 1H, J ) 9 Hz,
dCH), 2.65 (br d, 1H, J ) 12 Hz, ZrCHCH2), 2.4-2.3 (m, 1H), 2.23
(s, 3H, ArCH3), 1.68 (s, 3H, ArCH3), 2.2-1.4 (m, 9H), -0.41 (br t,
1H, J ) 10 Hz, ZrCH) ppm. 13C NMR (C6D6, 125 MHz): δ 148.7
(NCdC), 145.2 (C6H5), 132.6 (C6H5), 131.2 (C6H5), 128.7 (C6H5), 128.5
(C6H5), 123.8 (C6H5), 105.1 (C5H5), 102.0 (C5H5), 87.5 (dCH), 60.8
(ZrC), 36.6 (CH2), 35.4 (CH2), 32.8 (CH2), 32.6 (CH2), 30.1 (CH2),
26.1 (CH2), 21.5 (ArCH3), 21.3 (ArCH3) ppm. Anal. Calcd for
ZrNC27H33: C, 70.1; H, 7.2; N, 3.0. Found: C, 70.39; H, 6.95; N,
2.98.
Cp2ZrN(2,6-Me2C6H3)C(CH2C6H11)dC(H)C(CH2)5 (16). To a
solution of 3 (0.112 g, 0.27 mmol) in benzene (8 mL) was added a
solution of 5d (0.059 g, 0.29 mmol) in benzene (2 mL). The mixture
was stirred at room temperature for 12 h and at 75 °C for 27 h. The
solvent was removed under vacuum to afford a yellow-brown sticky
solid, which was precipitated from a concentrated ether solution to
afford 16 as an orange solid (0.097 g, 65%). Several attempts to
(R,R)-(ebthi)ZrN(2,6-Me2C6H3)Cd(CHC6H5)(CHC6H5) (19). To
a suspension of (R,R)-(ebthi)ZrN(2,6-Me2C6H3)(THF) (1) (102 mg, 0.19
mmol) in benzene (8 mL) was added (R)-1,3-diphenylpropadiene (40
mg, 0.21 mmol), and the mixture was stirred for 1 h. The solvent was
removed under vacuum, and the purple solid was recrystallized from
a CH2Cl2/pentane mixture to yield the product as purple needles (100
mg, 81%). 1H NMR (C6D6, 400 MHz): δ 7.4-7.3 (m, 5H, ArH), 7.2-
6.9 (m, 7H, ArH), 6.75 (t, J ) 7.2 Hz, 1H, p-Ar), 5.80 (s, 1H, d
CHPh), 5.60 (d, J ) 2.8 Hz, 1H, ebthi), 5.48 (d, J ) 2.8 Hz, 1H,
ebthi), 5.28 (d, J ) 2.8 Hz, 1H, ebthi), 4.90 (d, J ) 2.8 Hz, 1H, ebthi),
4.56 (s, 1H, ZrCHPh), 2.67 (s, 3H, ArCH3), 2.6-2.2 (m, 14 H, ebthi),
2.03 (s, 3H, ArCH3), 1.9-1.8 (m, 1H), 1.6-1.5 (m, 1H), 1.3-1.0 (m,
3 H), 0.9-0.8 (m, 1H) ppm. 13C NMR (C6D6, 100 MHz): δ 152.9,
148.2, 140.8, 137.6, 136.4, 133.6, 132.5, 130.9, 130.4, 129.9, 129.6,
128.9, 128.3, 128.1, 126.6, 125.3, 123.4, 122.7, 122.5, 121.8, 121.2,
116.7, 112.7, 110.2, 106.5, 105.6, 60.5, 28.2, 26.9, 24.8, 24.2, 23.7,
23.5, 23.5, 23.1, 22.9, 22.6, 22.5, 21.0 ppm. IR (Nujol): 3066, 2879,
1587, 1507, 1270, 1030, 944, 778, 699 cm-1. Anal. Calcd for C43H45-
ZrN‚CH2Cl2: C, 70.28; H, 6.30; N, 1.86. Found: C, 70.17; H, 6.69;
N, 1.86.
1
crystallize 16 to analytical purity were unsuccessful. H NMR (C6D6,
500 MHz): δ 7.06 (d, J ) 7 Hz, 1H, m-ArH), 6.97 (d, J ) 7 Hz, 1H,
m-ArH), 6.90 (t, J ) 7 Hz, 1H, p-ArH), 5.54 (s, 5H, C5H5), 5.38 (s,
5H, C5H5), 4.41 (s, 1H, dCH), 2.23 (s, 3H, ArCH3), 1.99 (s, 3H,
ArCH3), 0.74-2.15 (m, 23H, C6H11, CH2C6H11) ppm. 13C NMR (C6D6,
125 MHz): δ 149.9 (NCdC), 137.6 (C6H3), 134.1 (C6H3), 133.7 (C6H3),
130.7 (C6H3), 130.5 (C6H3), 125.3 (C6H3), 113.2 (NCdC), 109.2 (C5H5),
106.7 (C5H5), 73.0 (ZrC), 44.1 (dCCH2), 43.6 (CH2), 39.1 (CH2), 38.2
(CH2), 35.8 (CH2), 35.3 (CH2), 34.5 (CH2), 30.2 (CH2), 29.4 (CH2),
28.2 (CH2), 28.1 (CH2), 27.9 (CH2), 23.5 (ArCH3), 21.0 (ArCH3) ppm.
FT-IR (Nujol): 3071 (w), 1591 (m), 1513 (w), 1376 (m), 1265 (m),
1236 (m), 1102 (w), 1013 (m), 981 (w), 795 (s), 765 (m), 718 (w),
692 (w) cm-1. MS (EI): m/z ) 543 (M+). HRMS (EI) calcd, m/z )
543.2442; found, m/z ) 543.2443.
After a solution of this compound was heated in C6D6 at 105 °C for
24 h, an equilibrium mixture of 19 and a new compound, assigned as
21 on the basis of signals at 6.54 (d), 6.49 (d, dCHPh), 5.61 (d), 5.54
(d), 4.84 (d), and 3.81 (d, ZrCHPh), were formed in a 3:1 ratio.
(R,R)-(ebthi)ZrN(2,6-Me2C6H3)Cd(CHC6H5)(CHC6H5) (20). To
a solution of (R,R)-(ebthi)ZrN(2,6-Me2C6H3)(THF) (27 mg, 0.05 mmol)
in toluene (2 mL) was added (S)-1,3-diphenylpropadiene (12 mg, 0.0625
Cyclononadiene Metallacycle 17. To a solution of 3 (0.103 g, 0.25
mmol) in C6D6 (1 mL) was added a solution of 1,2-cyclononadiene55
(0.033 g, 0.27 mmol) in C6D6 (1 mL). The resulting purple solution
was stirred for 1 h at room temperature. The solvent was removed under
vacuum, and the resulting purple solid was washed with pentane (2 ×
(55) Skatteboel, L.; Solomon, S. Org. Synth. 1969, 49, 35.
9
1762 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

Rearrangements and Stereomutations of Metallacycles
A R T I C L E S
mmol), and the mixture was stirred for 1 h. The solvent was removed
under vacuum. The solid was redissolved in toluene (2 mL), and the
solvent was removed under vacuum three more times to remove the
124.1, 114.0 (dCH), 113.0 (dCH), 112.2 (C5H5), 111.9 (C5H5), 111.8
(NCdCH), 75.9 (OCH2Ph), 33.1, 31.3, 28.7, 28.5, 20.3 (ArCH3), 19.9
(ArCH3) ppm. Anal. Calcd for C34H37ZrON: C, 72.0; H, 6.6; N, 2.5.
Found: C, 71.81; H, 6.74; N, 2.49.
1
THF. H NMR (C6D6, 500 MHz): δ 7.45 (d, J ) 7 Hz, 1H, ArH),
7.2-6.8 (m, 12 H, ArH), 6.31 (d, J ) 3 Hz, 1H, ebthi), 5.85 (d, J )
2.5 Hz, 1H, ebthi), 5.81 (s, 1H, dCHPh), 5.30 (d, J ) 2.5 Hz, 1H,
ebthi), 5.01 (d, J ) 3.0 Hz, 1H, ebthi), 4.94 (s, 1H, ZrCHPh), 2.7-2.4
(m, 8H), 2.33 (s, 3H, ArCH3), 2.19 (s, 3H, ArCH3), 2.1-2.0 (m, 4H),
1.3-0.8 (m, 8H).
Kinetic Study of the Rearrangement of 20. After heating at 45
°C for 8 h, in addition to signals corresponding to 19, 20, and 21, signals
at 6.53 (d, dCHPh), 6.36 (d), 6.14 (d), 5.44 (d), 5.07 (d), and 4.92 (d,
ZrCHPh), assigned as 22, appeared. The concentrations of 19, 20, 21,
and 22 were determined by 1H NMR spectroscopy relative to an internal
standard (Cp2Fe) and were monitored as a function of time at 45, 55,
and 65 °C. The data were fitted to the kinetic scheme in Scheme 7
with the aid of the Gepasi kinetic modeling program.31-33
Cp2Zr(OCHMePh)N(2,6-Me2C6H3)C9H13 (26). To a solution of
metallacycle 14 (115 mg, 0.25 mmol) in toluene (2 mL) was added
acetophenone (33 mg, 0.27 mmol), and the resulting solution was stirred
for 1 h at room temperature. The solvent was removed under vacuum
to give an orange oil, which was dissolved in pentane (1 mL) and kept
at -30 °C for 5 d. Orange crystals of 26 were isolated after decanting
1
the solvent and washing with cold pentane (87 mg, 60%). H NMR
(C6D6, 400 MHz): δ 7.31 (d, J ) 7 Hz, 2H, ArH), 7.26 (t, J ) 7 Hz,
2H, ArH), 7.2-7.0 (m, 3H, ArH), 6.99 (t, J ) 7 Hz, 1H, ArH), 5.88
(br s, 5H, C5H5), 5.75 (br s, 5H, C5H5), 5.55 (br s, 1H, dCH), 5.4-5.2
(m, 2H, dCH), 4.89 (q, J ) 6 Hz, 1H, OCHCH3), 2.5-2.4 (m, 1H),
2.36 (s, 3H, ArCH3), 2.28 (s, 3H, ArCH3), 2.1-2.0 (m, 2H), 1.7-1.6
(m, 2H), 1.6-1.4 (m, 3H), 1.37 (br s, 3H, OCHCH3), 1.3-1.1 (m, 2H)
ppm. 13C NMR (343 K, C6D6, 100 MHz): δ 153.6 (NCdCH), 147.9,
135.5, 135.2, 134.9, 134.0, 129.3, 128.5, 127.3, 126.3, 124.1, 111.8
(C5H5), 81.7 (OCHCH3Ph), 33.0, 31.3, 28.8, 28.4, 28.3, 26.9 (OCHCH3-
Ph), 20.1 (ArCH3) ppm. Anal. Calcd for C35H41ZrON: C, 72.1; H, 7.1;
N, 2.4. Found: C, 71.91; H, 7.09; N, 2.70.
Cp2Zr(SnMe3)N(2,6-Me2C6H3)CdCHPh(CH2Ph) (23). To a solu-
tion of metallacycle 6 (133 mg, 0.25 mmol) in benzene (2 mL) was
added Me3SnH (49 mg, 0.3 mmol), and the resulting mixture was stirred
for 18 h at room temperature. The volatile materials were removed
under vacuum to give a yellow powder. Recrystallization from toluene/
pentane gave pure 23 as yellow blocks (89 mg, 51%). 1H NMR (C6D6,
500 MHz): δ 7.48 (d, J ) 7 Hz, 2H, ArH), 7.2-7.1 (m, 2H, ArH),
7.01 (t, J ) 8 Hz, 1H, ArH), 6.97 (br s, 1H, ArH), 6.81 (t, J ) 8 Hz,
1H, ArH), 6.8-6.7 (m, 3H, ArH), 6.52 (br s, 1H, ArH), 6.38 (br s, 2H,
ArH), 5.85 (br s, 1H, dCH), 5.68 (br s, 5H, C5H5), 5.35 (br s, 5H,
C5H5), 4.10 (br d, J ) 11 Hz, 1H, NCCH2), 2.57 (br d, J ) 11 Hz,
NCCH2), 2.26 (br s, 3H, ArCH3), 1.65 (br s, 3H, ArCH3), 0.33 (s, 9H,
JSn-H ) 15 Hz) ppm. 13C NMR (C6D6, 125 MHz): δ 152.0, 151.3,
138.3, 136.1, 132.0, 130.2, 129.0, 128.9, 128.5, 128.4, 128.3, 127.6,
126.1, 125.6, 125.1, 107.6 (C5H5), 105.7 (C5H5), 101.6 (dCHPh), 35.6
(NCCH2), 21.4 (ArCH3), 18.8 (ArCH3), -5.8 (Sn(CH3)3). Anal. Calcd
for C36H41ZrNSn: C, 62.0; H, 5.9; N, 2.0. Found: C, 62.19; H, 6.07;
N, 2.04.
Cp2Zr(OCH(C6H4Cl)2)N(2,6-Me2C6H3)C9H13 (27). To a solution
of metallacycle 14 (58 mg, 0.13 mmol) in toluene (2 mL) was added
4,4′-dichlorobenzophenone (63 mg, 0.25 mmol), and the resulting
mixture was stirred for 1 h at room temperature. The solvent was
removed under vacuum to give an orange oil, which was dissolved in
pentane (1 mL) and kept at -30 °C for 5 d. Orange crystals of 27
were isolated after decanting the solvent and washing with cold pentane
(40 mg, 45%). 1H NMR (C6D6, 500 MHz): δ 7.2-7.1 (m, 3H, ArH),
7.1-7.0 (m, 5H, ArH), 6.99 (t, J ) 8 Hz, 1H, p-ArH), 5.82 (br s, 5H,
C5H5), 5.72 (br s, 5H, C5H5), 5.64 (s, 1H, OCHAr2), 5.4-5.2 (m, 3H,
dCH), 2.36 (s, 3H, ArCH3), 2.30 (s, 3H, ArCH3), 2.0-1.8 (m, 2H),
1.6-1.5 (m, 2H), 1.5-1.3 (m, 3H), 1.2-1.1 (m, 2H) ppm. 13C NMR
(343 K, C6D6, 125 MHz): δ 156.8 (NCdCH), 153.6, 145.5, 135.2,
134.6, 133.3, 131.8, 128.8, 128.6, 128.6, 128.1, 124.4, 119.2, 112.3
(C5H5), 86.2 (OCHAr2), 33.0, 31.1, 28.8, 28.5, 28.4, 20.0 (ArCH3) ppm.
Anal. Calcd for C40H40ZrONCl2: C, 67.4; H, 5.7; N, 2.0. Found: C,
67.65; H, 5.70; N, 2.26.
Cp2Zr(SC6H4CF3)N(2,6-Me2C6H3)CdCHPh(CH(SC6H4CF3)-
Ph) (24). To a solution of metallacycle 6 (67 mg, 0.25 mmol) in benzene
(2 mL) was added bis(4-trifluoromethylphenyl) disulfide (44 mg, 0.3
mmol), and the resulting mixture was stirred for 18 h at room
temperature. The volatile materials were removed under vacuum to
give an orange powder. Recrystallization from toluene/pentane gave
pure 24 as an orange powder (52 mg, 47%). 1H NMR (C6D6, 500
MHz): δ 7.42 (br s, 2H, ArH), 7.28 (s, 1H, ArSCHPh), 7.2-7.0 (m,
5H, ArH), 6.96 (br s, 4H, ArH), 6.9-6.8 (m, 2H, ArH), 6.80 (d, J )
8 Hz, 2H, ArH), 6.74 (br s, 5H, ArH), 6.63 (d, J ) 8 Hz, 2H, ArH),
6.20 (br s, 5H, C5H5), 5.47 (br s, 5H, C5H5), 4.97 (s, 1H, dCH), 2.55
(s, 3H, ArCH3), 2.10 (s, 3H, ArCH3) ppm. 13C NMR (C6D6, 125
MHz): δ 155.9, 151.8, 151.3, 143.8, 138.0, 137.5, 137.4, 134.9, 134.1,
130.2, 129.9, 128.8, 126.5, 126.3, 126.2, 125.8, 125.4, 125.1, 113.6
(C5H5), 112.1 (C5H5), 47.9 (ArSCHPh), 20.1 (ArCH3), 19.9 (ArCH3)
ppm. Anal. Calcd for C47H39ZrNS2F6: C, 63.6; H, 4.4; N, 1.6. Found:
C, 63.61; H, 4.56; N, 1.77.
Dicyclopropylallene Metallacycle 29. To a solution of imido
complex 6 (103 mg, 0.25 mmol) in toluene (2 mL) was added 1,3-
dicyclopropylallene (33 mg, 0.27 mmol), and the resulting mixture was
stirred for 1 h at room temperature. The solvent was removed under
vacuum to give an orange-red powder (136 mg, quant.). An analytically
pure sample of 29 was isolated by crystallization from toluene/pentane.
1H NMR (C6D6, 500 MHz): δ 7.12 (d, J ) 7 Hz, 1H, m-ArH), 7.08
(d, J ) 7 Hz, 1H, m-ArH), 6.97 (t, J ) 7 Hz, 1H, p-ArH), 6.65 (d, J
) 15 Hz, 1H, NCCHdCH), 5.92 (ddd, J ) 3, 12, 15 Hz, 1H, NCCHd
CH), 5.75 (s, 5H, C5H5), 5.37 (s, 5H, C5H5), 3.04 (dd, J ) 1, 8 Hz,
1H, NCdCH), 2.67 (ddd, J ) 3, 9, 12 Hz, 1H, ZrCH2CH2), 2.28 (s,
3H, ArCH3), 2.13 (s, 3H, ArCH3), 2.10 (dq, J ) 7, 12 Hz, 1H,
ZrCH2CH2), 1.26 (quin, J ) 7 Hz, 1H, ZrCH2CH2), 0.64 (q, J ) 11
Hz, 1H, ZrCH2CH2), 0.7-0.6 (m, 1H, C3H5), 0.6-0.5 (m, 1H, C3H5),
0.3-0.2 (m, 1H, C3H5), 0.2-0.1 (m, 1H, C3H5), 0.1-0.0 (m, 1H, C3H5)
ppm. 13C NMR (C6D6, 125 MHz): δ 154.5 (NCdCH), 147.4, 134.3,
134.2, 129.7, 128.9, 128.8, 123.8 (dCH), 120.7 (dCH), 110.7 (C5H5),
108.2 (C5H5), 96.0 (NCdCH), 30.9 (ZrCH2CH2), 20.3 (ArCH3), 20.1
(ArCH3), 17.1 (ZrCH2CH2), 10.2 (CH(CH2)2), 7.9 (CH(CH2)2), 7.7 (CH-
(CH2)2) ppm. Anal. Calcd for C27H31ZrN: C, 70.4; H, 6.8; N, 3.0.
Found: C, 70.09; H, 7.09; N, 3.13.
Cp2Zr(OCH2Ph)N(2,6-Me2C6H3)C9H13 (25). To a solution of
metallacycle 14 (115 mg, 0.25 mmol) in toluene (2 mL) was added
benzaldehyde (27 mg, 0.25 mmol), and the resulting solution was stirred
for 1 h at room temperature. The solvent was removed under vacuum
to give an orange oil, which was dissolved in pentane (1 mL) and kept
at -30 °C for 5 d. Orange crystals of 25 were isolated after decanting
1
the solvent and washing with cold pentane (80 mg, 56%). H NMR
(C6D6, 400 MHz): δ 7.4-7.2 (m, 4H, ArH), 7.2-7.1 (m, 3H, ArH),
6.99 (t, J ) 7 Hz, 1H, p-ArH), 5.97 (br s, 5H, C5H5), 5.80 (br s, 5H,
C5H5), 5.54 (br d, J ) 16 Hz, 1H, dCH), 5.42 (br s, 1H, dCH), 5.28
(br s, 1H, dCH), 4.92 (s, 2H, OCH2Ph), 2.4-2.3 (m, 1H), 2.39 (s, 3H,
ArCH3), 2.31 (s, 3H, ArCH3), 2.1-1.9 (m, 2H), 1.8-1.6 (m, 2H), 1.6-
1.4 (m, 2H), 1.3-1.1 (m, 2H) ppm. 13C NMR (C6D6, 125 MHz): δ
156.8, 153.5, 143.6, 135.6, 135.2, 128.7, 128.5, 128.4, 127.1, 126.8,
Dicyclopropylallene Metallacycle 30. To a solution of Cp2Zrd
N(2,6-i-Pr2C6H3)(THF)52 (117 mg, 0.25 mmol) in toluene (2 mL) was
added 1,3-dicyclopropylallene (33 mg, 0.27 mmol), and the resulting
mixture was stirred for 1 h at room temperature. The solvent was
removed under vacuum to give an orange-red powder. Orange/red
crystals of pure 30 were isolated by crystallization from pentane at
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1763

A R T I C L E S
Michael et al.
1
-30 °C (89 mg, 69%). H NMR (C6D6, 500 MHz): δ 7.22 (dd, J )
3, 7 Hz, 1H, m-ArH), 7.2-7.1 (m, 2H, ArH), 6.62 (d, J ) 15 Hz, 1H,
NCCHdCH), 6.01 (t, J ) 15 Hz, 1H, NCCHdCH), 5.83 (s, 5H, C5H5),
5.53 (s, 5H, C5H5), 3.48 (quin, J ) 7 Hz, 1H, ArCH(CH3)2), 3.17 (quin,
J ) 7 Hz, 1H, ArCH(CH3)2), 2.82 (dd, J ) 1, 8 Hz, 1H, ZrCH2CH2),
2.69 (t, J ) 9 Hz, 1H, ZrCH2CH2), 2.13 (dq, J ) 7, 12 Hz, 1H, ZrCH2-
CH2), 1.41 (d, J ) 7 Hz, 3H, ArCH(CH3)2), 1.32 (d, J ) 7 Hz, 3H,
ArCH(CH3)2), 1.31 (d, J ) 7 Hz, 3H, ArCH(CH3)2), 1.28 (d, J ) 7
Hz, 3H, ArCH(CH3)2), 0.72 (q, J ) 11 Hz, 1H, ZrCH2CH2), 0.7-0.6
(m, 1H, C3H5), 0.6-0.5 (m, 1H, C3H5), 0.3-0.2 (m, 1H, C3H5), 0.2-
0.1 (m, 1H, C3H5), 0.1-0.0 (m, 1H, C3H5) ppm. 13C NMR (C6D6, 125
MHz): δ 151.6, 149.6, 144.8, 144.5, 127.6, 124.8, 124.7, 123.7, 120.5,
110.6 (C5H5), 108.0 (C5H5), 98.3 (NCdCH), 31.1 (ZrCH2CH2), 27.7,
27.0, 26.6, 25.6, 24.6, 23.9, 17.9 (ZrCH2CH2), 10.1 (CH(CH2)2), 7.6
(CH(CH2)2), 7.4 (CH(CH2)2) ppm. Anal. Calcd for C31H39ZrN: C, 72.0;
H, 7.6; N, 2.7. Found: C, 71.83; H, 7.82; N, 2.73.
Acknowledgment. This work was supported by the National
Institutes of Health through Grant no. GM-25459 and a National
Research Service Award Fellowship to Dr. Forrest Michael. We
thank Dr. Anna Davis and Professor Kenneth Raymond for
assistance with CD spectroscopy, Professor Armin deMeijere
for the generous gift of 1,3-dicyclopropylallene, and Dr. Fred
Hollander and Dr. Allen Oliver of the UC Berkeley CHEXRAY
facility for the X-ray structure determinations.
Supporting Information Available: Experimental procedures
and characterization data for synthesis of allenes, CD spectra
of 6-8, and crystallographic data for 10, 18a, 23, and 30 (PDF,
CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA045607K
9
1764 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005