Experimental modeling of selective alkene oligomerization: Evidence for facile metallacyclopentane dehydrogenation mediated by a transannular β-hydrogen agostic interaction

Article abstract of DOI:10.1021/om900132u

Ring-expansion of the zirconacyclopropane (n⁵-C ₅Me₅)Zr[N(i-Pr)C(Me)N(i-Pr)](n²-CH ₂CHC₆H₅)(1) was accomplished through insertion of ethene, propene, and styrene into a metal-car

Full text of DOI:10.1021/om900132u

2520
Organometallics 2009, 28, 2520–2526
Experimental Modeling of Selective Alkene Oligomerization:
Evidence for Facile Metallacyclopentane Dehydrogenation Mediated
by a Transannular ꢀ-Hydrogen Agostic Interaction
Albert Epshteyn, Emily F. Trunkely, Denis A. Kissounko, James C. Fettinger, and
Lawrence R. Sita*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
ReceiVed February 18, 2009
Ring-expansion of the zirconacyclopropane (η5-C5Me5)Zr[N(i-Pr)C(Me)N(i-Pr)](η2-CH2CHC6H5) (1)
was accomplished through insertion of ethene, propene, and styrene into a metal-carbon bond to yield
the series of crystalline zirconacyclopentane derivatives 2-4, respectively, for which solid-state molecular
structures have been obtained through single-crystal X-ray analyses. The metallacyclopentane rings of 2
and 4 adopt half-chair conformations that place all ꢀ-hydrogens distant from the metal center (cf.
nonbonded distances of 3.2-4.0 Å). On the other hand, a twisted-envelope conformation for the
zirconacyclopentane ring of 3 features a ꢀ-hydrogen agostic interaction with the metal center [cf. a
Zr1-H21B distance of 2.38(2) Å]. Unlike 2 and 4, compound 3 decomposes rapidly in solution at 25 °C
to provide the structurally characterized zirconacyclopent-3-ene derivative 5. Ring-expansion of 1 with
1-(trimethylsilyl)ethene directly provides the zirconacyclopent-3-ene derivative 7. A mechanism for this
facile metallacyclopentane dehydrogenative process is proposed that features an intramolecular allylic
ꢀ-hydrogen abstraction within a 3-butenyl metal hydride intermediate as a key step. It is further proposed
that similar metallacyclopentane to metallacyclopent-3-ene dehydrogenations may be more common than
previously thought.
Scheme 1
Introduction
Several early transition metal (Ti, Ta, and Cr)-based catalysts
have now been developed that are extremely active and highly
selective for the trimerization of ethene.1 Mechanistically, this
process is thought to operate through a “metallacycle” mech-
anism in which the control point for dimerization versus
trimerization rests with the relative rates for metallacyclopentane
expansion to a metallacycloheptane through alkene insertion
(path A) versus that for formal reductive five-membered ring-
opening to form a 1-butene metal complex via intramolecular
ꢀ-hydrogen transfer followed by reductive elimination (path
B/C) as presented in Scheme 1.2 Although isotopic labeling
studies,3 as well as more indirect circumstantial evidence,4 now
provide strong support in favor of this mechanism, to date,
experimental precedent for the critical ring-expansion of a
metallacyclopentane to a metallacycloheptane through direct
alkene insertion into a metal-carbon single bond is still lacking.5
Furthermore, only scant experimental data still currently exist
regarding the structures, thermal stabilities, and decomposition
pathways of medium-sized-ring metallacycles containing groups
4-6 metals.6 Finally, it has long been proposed that metalla-
cyclopentanes lack the degree of conformational flexibility
required to engage in transannular ꢀ-hydrogen agostic interac-
* Corresponding author. E-mail: lsita@umd.edu.
(1) (a) Hessen, B. J. Mol. Catal. A: Chem. 2004, 213, 129–135. (b)
Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21,
5122–5135. (c) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem.,
Int. Ed. 2001, 40, 2516–2519. (d) You, Y.; Girolami, G. S. Organometallics
2008, 27, 3172–3180. (e) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler,
K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423–7424. (f) Briggs, J. R.
J. Chem. Soc., Chem. Commun. 1989, 674–675. (g) Dixon, J. T.; Green,
M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004, 689, 3641–
3668. (h) Wass, D. F. Dalton Trans. 2007, 816–819.
(2) For computational mechanistic studies of metallacycloalkane ring-
fragmentation relevant to selective ethene trimerization, see: (a) Yu, Z.-X.;
Houk, K. N. Angew. Chem., Int. Ed. 2003, 42, 808. (b) Blok, A. N. J.;
Budzelaar, P. H. M.; Gal, A. W. Organometallics 2003, 22, 2564–2570.
(c) de Bruin, T. J. M.; Magna, L.; Raybaud, P.; Toulhoat, H. Organome-
tallics 2003, 22, 3404–3413. (d) Tobisch, S.; Ziegler, T. Organometallics
2004, 23, 4077–4088.
(5) Insertion into a metal-carbon bond of a conformationally restricted
dibenzo-annulated chromacyclopentane (the formal equivalent of a metal-
lacyclopenta-1,3-diene) has been observed; see, ref 3b.
(6) For instance, see: (a) McLain, S. J.; Wood, C. D.; Schrock, R. R.
J. Am. Chem. Soc. 1979, 101, 4558–4570. (b) Knight, K. S.; Wang, D.;
Waymouth, R. M.; Ziller, J. J. Am. Chem. Soc. 1994, 116, 1845–1854. (c)
Takahashi, T.; Fischer, R.; Xi, Z. F.; Nakajima, K. Chem. Lett. 1996, 357–
358. (d) Jolly, P. W. Acc. Chem. Res. 1996, 29, 544. (e) Hagadorn, J. R.;
Arnold, J. J. Chem. Soc., Dalton Trans. 1997, 3087–3096. (f) Mansel, S.;
Thomas, D.; Lefeber, C.; Heller, D.; Kempe, R.; Baumann, W.; Rosenthal,
U. Organometallics 1997, 16, 2886–2890. (g) Warren, T. H.; Erker, G.;
Fröhlich, R.; Wibbeling, B. Organometallics 2000, 19, 127–134. (h) Sun,
H.; Burlakov, V. V.; Spannenberg, A.; Baumann, W.; Arndt, P.; Rosenthal,
U. Organometallics 2001, 20, 5472–5477. (i) Wang, S. Y. S.; VanderLende,
D. D.; Abboud, K. A.; Boncella, J. M. Organometallics 1998, 17, 2628–
2635. (j) Ison, E. A.; Abboud, K. A.; Boncella, J. M. Organometallics 2006,
25, 1557–1564.
(3) (a) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 808–811. (b) Agapie, T.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 2007, 129, 14281–14295.
(4) Emrich, R.; Heinemann, O.; Jolly, P. W.; Kru¨ger, C.; Verhovnik,
G. P. J. Organometallics 1997, 16, 1511–1513.
10.1021/om900132u CCC: $40.75
2009 American Chemical Society
Publication on Web 03/27/2009

Modeling of SelectiVe Alkene Oligomerization
Organometallics, Vol. 28, No. 8, 2009 2521
Results and Discussion
A. Synthesis and Structural Characterization of Zircona-
cyclopentanes. As Scheme 2 reveals, formal ring-expansion of
the previously reported zirconacyclopropane 110 could be
accomplished in modest to excellent yields through insertion
of ethene, propene, and styrene into a metal-carbon bond to
yield the new zirconacyclopentane derivatives 2, 3, and 4,
respectively. Here it is important to note that due to significant
differences in thermal stabilities of the compounds, a much
lower temperature for the synthesis, isolation, and subsequent
handling of solutions of propene-derived 3 is required (Vide
1
infra). In each case, preliminary H NMR spectra indicated
formation of a mixture of stereoisomers; however, through
recrystallization at -30 °C, a single, analytically pure, crystalline
stereoisomer could be isolated and subjected to single-crystal
X-ray analysis. Figures 1-3 provide the derived solid-state
molecular structures and selected geometric parameters for the
isolated stereoisomers of 2, 3, and 4, respectively. Intriguingly,
for compounds 2 and 4, the relative configurations of the
exocyclic phenyl substituents appear to enforce half-chair
conformations of the five-membered ring that place all ꢀ-hy-
drogens of the zirconacyclopentane ring distant from the metal
center (cf. nonbonded distances of 3.2-4.0 Å) (see Figures 1
and 3). On the other hand, the solid-state twisted-envelope
conformation for the zirconacyclopentane ring of 3 is notable
in that it appears to feature a transannular ꢀ-hydrogen agostic
interaction with the metal center [cf. a Zr1-H21B distance of
2.38(2) Å].7 It is reasonable to assume that the magnitude of
nonbonded steric interactions that are associated with the relative
configurations of the exocyclic substituents for this specific
stereoisomer of 3 might be responsible for enforcing a ring-
conformation that is conducive for formation of this transannular
interaction. Unfortunately, attempts to isolate any other stereo-
isomer of 3 were not successful owing to the limited stability
of this compound in solution (Vide infra).
Regarding the two different possible regiochemical paths for
alkene insertion into the metal-carbon bonds of zirconacyclo-
propane 1, it is interesting to note that compound 2 arises from
formal insertion of ethene into the phenyl-substituted side of
the three-membered ring bond through a pathway that also
permits epimerization of the original stereocenter associated with
the R-phenyl substituent of 1. In contrast, the stereoisomer
isolated for compound 3 clearly arises from formal alkene
insertion into the opposite, unsubstituted zirconium-carbon
bond of 1, which further proceeds with retention of stereo-
chemistry of the R-carbon bearing the phenyl substituent. For
zirconacyclopentane 4, the issue of regiochemistry centers on
the task of determining the precursor origins of the two different
phenyl substituents. Fortunately, this question could be quickly
addressed by utilizing 1,1-[2H]2-labeled styrene (99%, d2) for
insertion. More specifically, after isolation of the corresponding
4-d2 material, a 1H NMR spectrum revealed that ring-expansion
of 1 is indeed regiospecific and that it proceeds through styrene
insertion into the more substituted zirconium-carbon bond of
the metallacyclopropane according to Scheme 3. Furthermore,
monitoring of solution dynamic processes involving 4-d2 by 1H
NMR at room temperature served to establish that, while rapid
reversible epimerization between major and minor stereoisomers
(ca. 1.2:1 at 25 °C) is rapidly occurring on the NMR time scale,
no exchange of the two styrene fragments of the five-membered
ring ever transpires. In dilute solution, thermal decomposition
Figure 1. Molecular structure (30% thermal ellipsoids) of com-
pound 2. Hydrogen atoms have been removed for the sake of clarity
except those for C19-C22, which are represented by small spheres
of arbitrary size. Selected bond lengths (Å) and bond angles (deg):
Zr1-C22, 2.2839(15); C22-C21, 1.527(2); C21-C20, 1.529(2);
C20-C19, 1.548(2); C19-Zr1, 2.3073(14); C19-Zr1-C22,
77.21(5); Zr1-C22-C21, 111.39(10); C22-C21-C20, 109.63(12);
C21-C20-C19, 106.97(12); C20-C19-Zr1, 105.54(9).
Chart 1
tions that are a prerequisite for lowering the barrier to intramo-
lecular ring-fragmentation according to path B in Figure 1.2,7,8
Accordingly, with a growing body of work now supporting the
ability of the monocyclopentadienyl, monoamidinate (CpAm)
ligand environment to impart a high degree of kinetic stability
to alkyl substituents bearing ꢀ-hydrogens within mid- and high-
valent early transition metal complexes,9 we initiated a program
to synthesize and investigate the structures, stabilities, and
reaction profiles of group 4 metallacycles of general formula
(η5-C5R5)M[-CH2(CH2)nCH2-][N(R1)C(R2)N(R3)] where n )
2 (I) and n ) 4 (II) (see Chart 1). Herein, we now report results
for a series of substituted zirconacyclopentane derivatives
(M ) Zr in I) that include support for a ground-state transan-
nular ꢀ-hydrogen agostic interaction that further appears
responsible for promoting a conformationally dependent facile
metallacyclopentane to metallacyclopent-3-ene dehydrogenation
pathway in solution. These results further provide a possible
alternative explanation for the origin of an anomalous Cꢀ-Cꢀ′
bond-length contraction sometimes reported for the solid-state
structures of metallacyclopentanes and, significantly, for chroma-
and zirconacyclopentanes.6c,d,f
(7) For reviews of metal-hydrogen agostic interactions, see: (a)
Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 1988,
36, 1–124. (b) Scherer, W.; McGrady, G. S. Angew. Chem., Int. Ed. 2004,
43, 1782–1806. (c) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl.
Acad. Sci. U.S.A. 2007, 104, 6908–6914.
(8) Huang, X.; Zhu, J.; Lin, Z. Organometallics 2004, 23, 4154–4159,
and references therein.
(9) (a) Keaton, R. J.; Koterwas, L. A.; Fettinger, J. C.; Sita, L. R. J. Am.
Chem. Soc. 2002, 124, 5932–5933. (b) Harney, M. B.; Keaton, R. J.;
Fettinger, J. C.; Sita, L. R. J. Am. Chem. Soc. 2006, 128, 3420–3432. (c)
Epshteyn, A.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2006, 128, 16052–
10653.
(10) Kissounko, D. A.; Epshteyn, A.; Fettinger, J. C.; Sita, L. R.
Organometallics 2006, 25, 531–535.

2522 Organometallics, Vol. 28, No. 8, 2009
Epshteyn et al.
Scheme 2
of 4-d2 was also observed to proceed through regiospecific
elimination of deuterated styrene to regenerate 1. On the basis
of this information, we tentatively assign the major stereoisomer
as being the 2,4-cis isomer of 4, for which the solid-state
structure was obtained (see Figure 3), and with the minor isomer
then having the corresponding 2,4-trans configuration as shown
epimerization of the ꢀ-carbon stereocenter during ethene inser-
tion to provide compound 2. Although hard experimental
evidence for the Zr(III) species postulated in Scheme 3 has not
yet been obtained, we have recently confirmed that the CpAm
ligand environment can support the synthesis and isolation of a
series of closely related Ti(III) alkyl complexes.11
Metallacyclopentanes have long been proposed, or known,
to be products of formal oxidative [2+2+1] cyclization of two
alkenes coming together at a reduced metal center.12 Surpris-
ingly, however, the total number of solid-state structures of
isolated unsubstituted and substituted metallacyclopentane
derivatives remains small.4,6 Furthermore, within this database
of structures, there has sometimes been noted an anomalous
1
in Scheme 3. Since H NMR spectra for 4-d2 confirm that
dynamic epimerization does not occur at the ꢀ-carbon stereo-
center, it is further postulated that facile epimerization of the
R-carbon stereocenter in 4 might proceed through reversible
scission of a zirconium-carbon “weak bond” with generation
of a transient “Zr(III)-stabilized” radical species such as that
depicted in Scheme 3. A similar transient species derived from
reversible bond scission in 1 might then also account for
Figure 2. Molecular structure (30% thermal ellipsoids) of com-
pound 3. Hydrogen atoms have been removed for the sake of clarity
except those for C19-C22, which are represented by small spheres
of arbitrary size. Selected bond lengths (Å) and bond angles (deg):
Zr1-C22, 2.326(2); C22-C21, 1.529(3); C21-C20, 1.545(3);
C20-C19, 1.511(3); C19-Zr1, 2.296(2); C19-Zr1-C22, 75.17(8);
Zr1-C22-C21,85.10(12);C22-C21-C20,111.70(18);C21-C20-C19,
108.53(17); C20-C19-Zr1, 103.24(13).
Figure 3. Molecular structure (30% thermal ellipsoids) of com-
pound 4. Hydrogen atoms have been removed for the sake of clarity
except those for C19-C22, which are represented by small spheres
of arbitrary size. Selected bond lengths (Å) and bond angles (deg):
Zr1-C22, 2.375(2); C22-C21, 1.540(3); C21-C20, 1.516(3);
C20-C19, 1.538(3); C19-Zr1, 2.292(2); C19-Zr1-C22, 76.67(8);
Zr1-C22-C21,107.09(14);C22-C21-C20,111.98(18);C21-C20-C19,
107.13(18); C20-C19-Zr1, 112.00(16).

Modeling of SelectiVe Alkene Oligomerization
Organometallics, Vol. 28, No. 8, 2009 2523
Scheme 3
Chart 2
Scheme 4
Scheme 4. The identity of 5 was unequivocally established
through single-crystal X-ray analysis, and Figure 4 provides the
solid-state molecular structure of this compound. The most
prominent structural feature of 5 is the obvious double bond
that exists between the two ꢀ-carbon atoms [cf. C20-C21,
1.376(7) Å] that enforces coplanarity with C29 of the exocyclic
contraction of the carbon-carbon bond between the two cyclic
ꢀ-carbons that replicates the distance expected for a carbon-
carbon double bond, rather than a carbon-carbon single bond
and, most notably, for those structures presented in Chart 2.6c,d,f
In this regard, the bond lengths associated with the five-
membered rings of the zirconacyclopentanes 2-4 are largely
unremarkable. More specifically, as presented in Figures 1-3,
all three compounds possess Cꢀ-Cꢀ′ (i.e., C20-C21) bond
lengths for the five-membered rings that are within the range
expected for carbon-carbon single bonds [cf. for 2, 1.529(2)
Å; 3, 1.545(3) Å; 4, 1.516(3) Å]. Furthermore, while it is
tempting to assign the slightly longer Zr1-C22 bond length of
2.326(2) Å relative to the Zr1-C19 of 2.296(2) Å in 3 as arising
in concert with the previously noted ꢀ-hydrogen agostic
interaction, a similar disparity in the Zr-CR bond lengths of 4
suggests that the origin is likely, and largely, steric in nature.
B. Zirconacyclopent-3-enes via Facile Dehydrogenation.
Given the wealth of seminal investigations conducted by Negishi
and co-workers12 regarding the reversibility of zirconacyclo-
pentane formation via ring-fragmenting alkene elimination, the
relative stabilities of the isolated zirconacyclopentane derivatives
2-4 in solution were of significant interest. To begin, 1H NMR
(400 MHz, benzene-d6, 25 °C) spectra quickly revealed that,
when monitored within an NMR tube, sealed under an inert
atmosphere of dinitrogen, solutions of the three zirconacyclo-
pentanes presented the following relative ordering of stability
at room temperature: 2 (indefinitely) . 4 (days) . 3 (hours).
Indeed, solutions of compound 3 were found to darken to brown
within a few minutes at room temperature, and within hours, a
1H NMR spectrum confirmed that significant decomposition had
occurred, as evidenced by new resonances appearing for several
different species. Cooling a pentane solution of this mixture to
-30 °C surprisingly provided, after several days, a 28% yield
of the red-purple zirconacyclopent-3-ene derivative 5 shown in
Figure 4. Molecular structure (30% thermal ellipsoids) of com-
pound 5. Hydrogen atoms have been removed for the sake of clarity
except for those for C19-C22, which are represented by small
spheres of arbitrary size. Selected bond lengths (Å) and bond angles
(deg): Zr1-C22, 2.377(5); C22-C21, 1.463(7); C21-C20, 1.376(7);
C20-C19, 1.454(8); C19-Zr1, 2.347(5); C19-Zr1-C22, 75.47(19);
Zr1-C22-C21, 75.8(3); C22-C21-C20, 122.6(5); C21-C20-C19,
120.0(5); C20-C19-Zr1, 78.3(3).

2524 Organometallics, Vol. 28, No. 8, 2009
Epshteyn et al.
Scheme 5
Scheme 6
C. Synthesis and Stability of Methyl ω-Alkenyl Zirco-
nium Complexes. In order to provide more weight to the
mechanistic hypothesis of Scheme 5, the series of methyl,
ω-alkenyl zirconium complexes 9-11 were prepared in straight-
forward fashion according to Scheme 7. As mentioned previ-
ously, the ability of the CpAm ligand environment to impart a
high degree of kinetic stability to complexes with alkyl
substituents bearing ꢀ-hydrogens clearly comes into play to
provide for the successful synthesis, isolation, and structural
characterization of 9–11. Indeed, Figure 6 presents the solid-
state molecular structure as obtained from single-crystal X-ray
analysis for the most challenging of these compounds: the
3-butenyl derivative 9. Regarding the thermal stabilities of these
compounds in solution, perhaps not unexpectedly, while com-
pounds 10 and 11 proved to be quite robust in solution toward
ꢀ-hydrogen transfer processes, 9 was found to quantitatively
convert in solution at 25 °C to the zirconacyclopent-3-ene
derivative 12 with coproduction of methane according to
Scheme 7. For a final comparison, Figure 7 presents the solid-
state molecular structure of 12 as obtained from single-crystal
X-ray analysis.
methyl substituent. Furthermore, in keeping with other zircona-
cyclopent-3-ene derivatives based on the CpAm ligand environ-
ment that we have previously reported.,9a,13 the five-membered
ring adopts an envelope conformation that places the endocyclic
double bond within proximity to the metal center, and thereby,
in possible support of a secondary π-donor interaction.9a
1
Upon closer inspection, the initial H NMR spectrum of the
solution decomposition mixture derived from 3 also revealed
formation of the zirconanorbornadiene complex 6, which is
depicted in Scheme 4. Compound 6 was previously prepared
in near quantitative yield through the low-pressure hydrogena-
tion of zirconacyclopropane 1 in pentane at room temperature.10
Accordingly, although we do not spectroscopically observe
formation of dihydrogen during the decomposition of 3, facile
formation of 6, which is concomitant with the appearance of 5,
can be viewed as providing circumstantial evidence for transient
H2 production during the 3 f 5 process. On the basis of this
hypothesis, together with the greatly reduced stability of 3 vis-
a`-vis zirconacyclopentanes 2 and 4, we tentatively conclude that
the presence of the conformationally allowed, transannular
agostic interaction in 3 significantly lowers the barrier for a ring-
fragmenting intramolecular ꢀ-hydrogen transfer to the metal to
generate a transient 3-butenyl metal hydride intermediate
according to Scheme 5. However, in contrast to undergoing
reductive elimination according to the B/C pathway of Scheme
1, in the present case, subsequent intramolecular ꢀ-hydrogen
atom abstraction at the “activated” allylic position of the
3-butenyl substituent leads directly to loss of H2 and formation
of an η2-(1,3-butadiene) metal complex that can rearrange to a
metallacyclopent-3-ene according to Scheme 5. Surprisingly,
to the best of our knowledge, transformation of a 3-butenyl metal
hydride by the process of Scheme 5 has not been previously
proposed, but it certainly provides a viable catalyst deactivation
process to consider for selective ethene oligomerization.2,14 The
generality of this dehydrogenative process is further supported
by observation that reaction of 1 with 5 equiv of 1-(trimethyl-
silyl)ethene directly provides the structurally characterized
zirconacyclopent-3-ene 7 in modest yield according to Scheme
6. The identity and molecular structure of 7 was once again
confirmed by single-crystal X-ray analysis, and Figure 5 presents
the solid-state molecular structure of this compound along with
selected geometric parameters.
Conclusion
In summary, the present results provide considerable support
for a simple, but fundamental, low-energy pathway involving
3-butenyl metal hydrides not previously considered as part of
Figure 5. Molecular structure (30% thermal ellipsoids) of com-
pound 7. Hydrogen atoms have been removed for the sake of clarity
except for those for C19-C22, which are represented by small
spheres of arbitrary size. Selected bond lengths (Å) and bond angles
(deg): Zr1-C22, 2.422(2); C22-C21, 1.432(3); C21-C20, 1.378(3);
C20-C19, 1.464(3); C19-Zr1, 2.295(2); C19-Zr1-C22, 76.58(8);
Zr1-C22-C21,75.42(13);C22-C21-C20,125.4(2);C21-C20-C19,
119.2(2); C20-C19-Zr1, 79.46(13).
(11) Trunkely, E. F.; Epshteyn, A.; Zavalij, P. Y.; Sita, L. R., results to
be published.
(12) (a) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124–130.
(b) Negishi, E. Dalton Trans. 2005, 827–848.

Modeling of SelectiVe Alkene Oligomerization
Organometallics, Vol. 28, No. 8, 2009 2525
Scheme 7
transferred, and were stored cold under an inert atmosphere prior
the potential energy surface for metallacyclopentane decomposi-
tion. Access to this pathway is guaranteed by the conformational
flexibility of the metallcyclopentane ring that can further engage
in ground-state transannular ꢀ-hydrogen agostic interactions.8
Furthermore, given the relatively large number of reports of
anomalously short Cꢀ-Cꢀ′ bond lengths for metallacyclopen-
tanes that are similar in magnitude to those of carbon-carbon
double bonds, the occurrence of this dehydrogenative process
may actually be more widespread than previously thought.
Efforts are currently underway to expand the range of early
transition metal-based metallacyclopentanes that are supported
by CpAm ligand environments.
1
to use. H NMR spectra were recorded at 400 MHz and ambient
temperature. Compounds 1 and (η5-C5Me5)Zr[N(i-Pr)C(Me)N-
(i-Pr)]Cl2 (8) were prepared according to previously published
procedures.10 The syntheses of compounds 10 and 11 will be
reported elsewhere.
(η5-C5Me5)Zr(-CH2CH(Ph)CH2CH2-)[N(i-Pr)C(CH3)N(i-Pr)] (2).
In a 50 mL Schlenk tube fitted with a gastight Teflon valve, 0.10 g
(0.21 mmol) of 1 was dissolved in 5 mL of pentane, the dinitrogen
atmosphere removed under vacuum, and the tube pressurized with
10 psi of ethene before being sealed. After 30 min, the atmosphere
was replaced by N2, and the solution transferred to a 20 mL vial,
whereupon the solvent was partially removed in Vacuo until the
solution volume was approximately 2 mL. After cooling the solution
to -30 °C, compound 2 was isolated as yellow crystals. (0.092 g,
87% yield). Anal. Calcd for C29H46N2Zr: C 67.78, H 9.02, N 5.45.
Found: C 66.81, H 9.04, N 5.15. Due to stereochemical complexity
that prevents simple straightforward assignments, NMR spectra are
not presented. The structure of 2 was verified by single-crystal X-ray
analysis (see Table 1, Supporting Information).
(η5-C5Me5)Zr(-CH2CH(Me)CH2CH(Ph)-)[N(i-Pr)C(CH3)N-
(i-Pr)] (3). In a 50 mL Schlenk tube fitted with a gastight Teflon
valve, 0.20 g (0.42 mmol) of 1 was dissolved in 10 mL of pentane,
the dinitrogen atmosphere removed under vacuum, and the tube
pressurized with 45 psi of propene before being sealed. The tube
was then placed at 0 °C for 2 days (until the color of the solution
turned from dark green to light yellow). The solution was then
Experimental Section
All manipulations were performed under an inert atmosphere of
dinitrogen using either standard Schlenk techniques or a Vacuum
Atmospheres glovebox. Dry, oxygen-free solvents were employed
throughout. Diethyl ether and pentane were distilled from sodium/
benzophenone (with a few milliliters of triglyme being added to
the pot in the case of pentane), while toluene was distilled from
sodium. Benzene-d6 was vacuum transferred from NaK prior to
use for NMR spectroscopy. Ethene and propene were purchased
from Matheson Trigas and passed through activated Q5 and
molecular sieves (4 Å) and styrene (and styrene-d2), were vacuumed
Figure 7. Molecular structure (30% thermal ellipsoids) of com-
pound 12. Hydrogen atoms have been removed for the sake of
clarity except for those for C19-C22, which are represented by
small spheres of arbitrary size. Selected bond lengths (Å) and bond
angles (deg): Zr1-C22, 2.358(5); C22-C21, 1.427(9); C21-C20,
1.414(10);C20-C19,1.461(10);C19-Zr1,2.336(6);C19-Zr1-C22,
77.0(2); Zr1-C22-C21, 78.2(4); C22-C21-C20, 121.6(7);
C21-C20-C19, 121.4(7); C20-C19-Zr1, 78.0(4).
Figure 6. Molecular structure (30% thermal ellipsoids) of com-
pound 9. Hydrogen atoms have been removed for the sake of clarity
except for those for C19-C23, which are represented by small
spheres of arbitrary size. Selected bond lengths (Å) and bond angles
(deg): Zr1-C19, 2.2582(18); C19-C20, 1.539(3); C20-C21,
1.487(3); C21-C22, 1.302(3); Zr1-C23, 2.285(2).

2526 Organometallics, Vol. 28, No. 8, 2009
Epshteyn et al.
transferred to a 20 mL vial, whereupon the solvent was partially
removed in Vacuo until the solution volume was approximately 2
mL. After cooling the solution to -30 °C, compound 3 was isolated
as light yellow crystals (0.11 g, 50% yield). Anal. Calcd for
C29H46N2Zr: C 67.78, H 9.02, N 5.45. Found: C 66.81, H 9.04, N
5.15. Due to stereochemical complexity that prevents simple
straightforward assignments, NMR spectra are not presented. The
structure of 3 was verified by single-crystal X-ray analysis (see
Table 1, Supporting Information).
1H NMR: δ 6.07 (m, 1H), 5.14 (d, J ) 17.1 Hz, 1H), 4.97 (d, J )
9.9 Hz, 1H), 2.99 and 2.87 (two multiplets, 1H (combined
2
3
integration)), 2.84 (dq, J ) 13.9 Hz, J ) 7.2 Hz, 1H), 2.63 (dq,
2J ) 14.3 Hz, 3J ) 7.2 Hz, 1H), 2.28 (m, 1H), 1.99 and 1.98 (two
singlets, 15H (combined integration), 1.68 and 1.67 (two singlets,
3H (combined integration), 1.32 and 1.30 (two singlets, 9H
(combined integration)), 0.79 and 0.77 (two triplets, J ) 7.55 Hz,
3H (combined integration)), 0.74 and 0.65 (two multiplets, 1H
(combined integration)), 0.23 and 0.11 (two multiplets, 1H (com-
bined integration)). This material was used without further purifica-
tion for the synthesis of 9.
(η5-C5Me5)Zr(-CH2CH(Ph)CH2CH(Ph)-)[N(i-Pr)C(CH3)N-
(i-Pr)] (4). In a 50 mL round-bottom Schlenk flask 0.25 g (0.53
mmol) of 1 was dissolved in 4 mL of neat styrene at room
temperature and allowed to react for 1 h (until there was no visible
green color in solution). The reaction mixture was then placed under
vacuum, and most of the styrene removed under reduced pressure
(0.010 mmHg) for 12 h. The remaining solids were dissolved in 2
mL of Et2O and recrystallized at -30 °C, yielding 4 as a dark
yellow-orange crystalline solid (0.25 g, 82% yield). Anal. Calcd
for C34H48N2Zr: C 70.90, H 8.40, N 4.86. Found: C 71.18, H 8.38,
N 4.88. The structure of 4 was verified by single-crystal X-ray
analysis (see Table 1, Supporting Information).
(η5-C5Me5)ZrMe(CH2CH2CHdCH2)[N(Et)C(CH3)N(t-Bu)] (9).
To a solution of 0.10 g (0.2 mmol) of (η5-C5Me5)Zr(Br/
Cl)(CH2CH2CHdCH2)[N(Et)C(CH3)N(t-Bu)] in 10 mL of Et2O,
cooled to -30 °C, was added 127 µL of MeLi (1.6 M in Et2O, 0.2
mmol) via syringe. The reaction mixture was then cooled to -30
°C overnight, after which time, the volatiles were removed in Vacuo.
The crude product was extracted with pentane, and the extract
filtered through a pad of Celite. After concentrating in Vacuo,
cooling the solution to -30 °C yielded 9 as white crystals (0.08 g,
89% yield). For 9, 1H NMR: δ 6.12 (m, 1H), 5.17 (d, J ) 16.7 Hz,
2
3
(η5-C5Me5)Zr(-CH2C(Me)CHCH(Ph)-)[N(i-Pr)C(CH3)N-
(i-Pr)] (5). In a 20 mL glass vial 0.10 g (0.19 mmol) of 3 was
dissolved in 8 mL of pentane and allowed to stand at room
temperature overnight, after which time the solution turned from
light yellow to dark brown in color. The volatiles were then removed
in Vacuo, and the remaining solids dissolved in a minimal amount
of pentane (∼1.5 mL) and cooled to -30 °C, whereupon 5 was
isolated as a purple crystalline material (0.02 g, 28% yield). Anal.
Calcd for C29H44N2Zr: C 68.04, H 8.66, N 5.47. Found: C 66.11,
H 8.51, N 3.65. Due to stereochemical complexity that prevents
simple straightforward assignments, NMR spectra are not presented.
The structure of 5 was verified by single-crystal X-ray analysis
(see Table 1, Supporting Information).
1H), 4.99 (d, J ) 9.9 Hz, 1H), 2.90 (dq, J ) 21.9 Hz, J ) 7.2
Hz, 1H), 2.70 (m, 2H), 2.53 (bm, 1H), 1.97 (s, 15H), 1.73 (s, 3H),
1.16 (s, 9H), 0.86 (t, J ) 7.2, 3H), 0.44 (m, 1H), 0.16 (s, 3H), 0.05
(bm, 1H). Anal. Calcd for C23H42N2Zr: C 63.10, H 9.67, N 6.40.
Found: C 62.85, H 9.52, N 6.37. The structure of 9 was verified
by single-crystal X-ray analysis (see Table 1, Supporting Informa-
tion).
(η5-C5Me5)Zr(-CH2CHdCH-CH2)[N(Et)C(CH3)N(t-Bu)] (12).
A solution of 0.05 g of 9 in 1 mL of pentane was left overnight,
after which time the solution turned from clear to a deep red color.
After the volatiles were removed in Vacuo, the crude product was
recrystallized from pentane at -30 °C to yield 0.04 g (98% yield).
For 12, 1H NMR: δ 6.17 (dd, 2J ) 18.1 Hz, 3J ) 8.3 Hz, 1H), 6.09
2
3
2
3
(η5-C5Me5)Zr[-CH2C(SiMe3)CHCH(Ph)-][N(i-Pr)C(CH3)N-
(i-Pr)] (7). In a 50 mL Schlenk tube fitted with a gastight Chemglass
Teflon valve, 0.25 g (0.53 mmol) of 1 was dissolved in 8 mL of
pentane, followed by addition of 0.26 g (2.65 mmol) of 1-(tri-
methylsilyl)ethene and the reaction mixture was allowed to stand
at ambient temperature for 4 days. Volatiles were then removed in
Vacuo, and the remaining residue dissolved in 2 mL of Et2O and
then cooled to -30 °C, whereupon 0.060 g of the purple crystalline
product 6 was isolated in a 40% yield. For 6, 1H NMR: δ 7.29 (s,
2H), 7.28 (s, 1H), 6.99 (m, 1H), 6.96 (m, 1H), 3.32 (sept, J ) 6.0
Hz, 1H), 3.25 (sept, J ) 6.2 Hz, 1H), 2.19 (d, J ) 8.2 Hz, 1H),
1.92 (s, 15H), 1.70 (d, J ) 11.2 Hz, 1H), 1.38 (s, 3H), 1.14 (d, J
) 6.1 Hz, 3H), 0.93 (d, J ) 6.7 Hz, 3H), 0.91 (d, J ) 6.5 Hz, 3H),
0.85 (d, J ) 6.2 Hz, 3H), 0.37 (s, 9H). Anal. Calcd for
C31H50N2SiZr: C 65.32, H 8.84, N 4.91. Found: C 65.17, H 8.81,
N 5.02. The structure of 6 was verified by single-crystal X-ray
analysis (see Table 1, Supporting Information).
(dd, J ) 18.1 Hz, J ) 8.0 Hz, 1H), 2.81 (dq, J ) 2.4 Hz, J )
2
3
7.2 Hz, 2H), 2.05 (s, 15H), 1.96 (dd, J ) 9.1 Hz, J ) 8.3 Hz,
1H), 1.57 (dd, 2J ) 9.1 Hz, 3J ) 8.0 Hz, 1H), 1.44 (s, 3H), 1.01 (s,
9H), 0.81 (t, J ) 7.2 Hz, 3H), 0.45 (dd, 2J ) 9.1 Hz, 3J ) 8.3 Hz,
1H), 0.34 (dd, J ) 9.1 Hz, J ) 8.0 Hz, 1H). Anal. Calcd for
C22H38N2Zr: C 62.65, H 9.08, N 6.64. Found: C 62.63, H 8.98, N
6.56. The structure of 12 was verified by single-crystal X-ray
analysis (see Table 1, Supporting Information).
2
3
Acknowledgment. Funding for this work was provided
by the NSF (CHE-061794), for which we are grateful.
Supporting Information Available: Details of single-crystal
X-ray analyses, including tables of bond lengths, angles, and
anisotropic displacement parameters for the solid-state structures
and complete X-ray crystallographic data (CIF) for compounds 2,
3, 4, 5, 7, 9, and 12. This material is available free of charge via
the Internet at http://pubs.acs.org.
(η5-C5Me5)Zr(Cl/Br)(CH2CH2CHdCH2)[N(Et)C(CH3)N-
(t-Bu)]. To a solution of 2.12 g (4.8 mmol) of 8 in 100 mL of
Et2O was added dropwise 7.6 mL of 3-butenylmagnesium bromide
(0.64 M in Et2O, 4.8 mmol) over a period of 1 h. The solution was
then allowed to warm to room temperature and stirred for another
3 h, whereupon the volatiles were removed in Vacuo. The yellow
product was extracted using 10% toluene in pentane solution, and
the extract filtered through a pad of Celite and cooled to -30 °C,
whereupon the desired product was obtained as a yellow crystalline
material (1.84 g, 79% yield, based on 1.4:1 ) Br:Cl product ratio).
OM900132U
(13) Fontaine, P. P.; Epshteyn, A.; Zavalij, P. Y.; Sita, L. R. J.
Organomet. Chem. 2007, 692, 4683–4689.
(14) (a) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc.
1979, 101, 5451–5453. (b) Dorf, U.; Engel, K.; Erker, G. Angew. Chem.,
Int. Ed. Engl. 1982, 21, 914–915. (c) Wielstra, Y.; Gambarotta, S.; Spek,
A. L. Organometallics 1990, 9, 572–577. (d) Thorn, M. G.; Hill, J. E.;
Waratuke, S. A.; Johnson, E. S.; Fanwick, P. E.; Rothwell, I. P. J. Am.
Chem. Soc. 1997, 119, 8630–8641.