A 1-norbornene adduct of a transition metal and an electrochemical study of the bridgehead olefin complex

Article abstract of DOI:10.1021/om0503388

Treatment of a slurry of (PCy₃)₂NiCl₂ in pentane with 1-norbornyllithium did not lead to the expected bis(1-norbornyl) nickel complex but instead led to formation of a 1-norbornene adduct, which has been structurally characterized. The X-ray data and the electrochemistry of the 1-norbornene complex suggest a metallacyclopropane extreme of olefin binding, stemming from the highly reactive nature of the bridgehead double bond.

Full text of DOI:10.1021/om0503388

Organometallics 2005, 24, 3821-3823
3821
A 1-Norbornene Adduct of a Transition Metal and an
Electrochemical Study of the Bridgehead Olefin
Complex
Gavin D. Jones and David A. Vicic*
Department of Chemistry and Biochemistry, University of Arkansas,
Fayetteville, Arkansas 72701
Received April 29, 2005
Summary: Treatment of a slurry of (PCy3)2NiCl2 in
pentane with 1-norbornyllithium did not lead to the
expected bis(1-norbornyl) nickel complex but instead led
to formation of a 1-norbornene adduct, which has been
structurally characterized. The X-ray data and the
electrochemistry of the 1-norbornene complex suggest a
metallacyclopropane extreme of olefin binding, stemming
from the highly reactive nature of the bridgehead double
bond.
trans-cycloalkene.11 Indirect evidence of the anti-Bredt12
olefin 1 has been obtained by trapping transiently
generated 1 with a variety of organic reagents or by
monitoring the thermal rearrangement products of
precursor derivatives.13-17 A number of computational
methods have also been applied to 1 in order to better
understand the structure, energetics, and reactivity of
this highly sensitive molecule.18-23 However, as no
direct observation of 1 in solution has ever been reported
and no reversible trapping agent for this molecule has
ever been prepared, its chemistry has been difficult to
study.
A transition-metal binding approach, similar to what
has been employed in benzyne chemistry,24-29 could be
one possible strategy to obtain stable precursors of these
short-lived compounds for synthetic and mechanistic
studies. While transition-metal complexes of larger
bridgehead double bonds are known30-35 or speculated
Double bonds located at a bridgehead position of
fused-ring systems have fascinated chemists for many
years.1-3 The distortion of the π bond in these molecules
caused by twisting often imparts reactivity atypical of
a normal C-C double bond.3 The appearance of bridge-
head double bonds in important natural products such
as taxol4-6 has also sparked a great deal of interest in
learning about the susceptibility of these olefins to a
variety of reaction conditions that might be employed
in a total synthesis. Additionally, there is much impetus
for developing methods to systematically study the
reaction chemistry of the bridgehead double-bond func-
tionality, as the removal of a bridgehead double bond
in the calicheamicin/esperamicin family of enediyne
toxins is believed to be a primary step in the activation
process leading to arene-1,4-diyl formation, H-atom
abstraction, and DNA cleavage.7-10
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263.
While bridgehead double bonds in larger ring systems
are known to be relatively stable, the existence of
smaller ring systems with bridgehead double bonds such
as 1-norbornene (1) has been harder to evaluate, as
Wiseman predicted that the strain in bridgehead alk-
enes is closely related to the strain of the corresponding
(17) Dorok, S.; Ziemer, B.; Szeimies, G. Chem. Eur. J. 2002, 8, 4506-
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* To whom correspondence should be addressed. E-mail:
dvicic@uark.edu.
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Eur. J. Org. Chem. 1999, 3057-3066.
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Kaszynski, P.; Miller, R. D. Pure Appl. Chem. 1987, 59, 1613-1626.
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10.1021/om0503388 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/09/2005

3822 Organometallics, Vol. 24, No. 16, 2005
Communications
critical in removing HCl that could possibly result from
a â-hydride elimination reaction, as described in eq 2.
An alternative mechanism may involve formation of a
Ni(norbornyl)2 species, which then eliminates hydro-
carbon en route to 2. One equivalent of norbornane
relative to nickel starting material is produced in the
reaction described in eq 2. Repeating the lithiation in a
3:1 mixture of pentane and furan did not lead to any
furan adducts of norbornene,15 consistent with an in-
tramolecular process in which olefin 1 is never released
from the metal.
Of note, the C(1)-C(2) bond length of 1.454(6) Å is
especially long for a carbon-carbon double bond. (PCy3)2-
Ni(η2(C,C)-methyl methacrylate) (3) is the only other
olefin complex containing the (PCy3)2Ni fragment that
has been crystallographically characterized. Even with
the presence of an electron-withdrawing ester group, the
coordinated double bond in 3 (1.410(13) Å)41 was found
to be shorter than that observed in 2. DFT calculations
predict a CdC bond length of 1.3584 Å for uncomplexed
1.23
Figure 1. ORTEP diagram of the 1-norbornene adduct 2.
Ellipsoids are shown at the 50% level. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Ni(1)-C(1)
) 1.949(4), Ni(1)-C(2) ) 1.957(5), C(1)-C(2) ) 1.454(6),
Ni(1)-P(1) ) 2.186(3), Ni(1)-P(2) ) 2.1878(16).
to have formed in metal-mediated reactions,36-38 the
trapping of smaller, more reactive bridgehead double
bonds remains much more problematic, especially since
trapping must usually be competitive with dimer forma-
tion resulting from formal 2 + 2 cycloaddition reactions.
Here we report a new method to prepare a transition-
metal adduct of 1-norbornene that overcomes the prob-
lems associated with competitive trapping and also
describe the unexpected electrochemistry of this new
transition-metal adduct.
Chart 1
Treatment of a slurry of (PCy3)2NiCl2 in pentane with
2 equiv of 1-norbornyllithium was found to afford
(PCy3)2NiHCl39 and a new product (2; eq 1) that
displayed two doublets in the 31P{1H} NMR spectrum.
The crystallographic data thus support the metalla-
cyclopropane extreme of olefin coordination for com-
pound 2 (A; Chart 1) instead of the Dewar-Chatt
description, which does not affect the oxidation state of
the metal (B; Chart 1).42 To determine if the bonding is
more a result of a strongly π-basic metal or of the
unusual nature of the bridgehead double bond of 1-nor-
bornene, electrochemical measurements were performed
on a series of related olefin adducts of (PCy3)2Ni (Table
1). Both (PCy3)2Ni(η2-1-hexene) and (PCy3)2Ni(η2-eth-
ylene) display three sequential oxidations in the cyclic
voltammograms, consistent with stepwise oxidation of
a Ni(0) species to Ni(III). Compound 2, on the other
hand, displays only one irreversible oxidation. One
possible interpretation of these data is that the irrevers-
ible oxidation of 2 is in fact an oxidation of Ni(II) to Ni-
(III), consistent with the metallacyclopropane character
The ratio of (PCy3)2NiHCl to 2 was 1:22 after 10 min.
Since the 31P{1H} NMR data for 2 could also be
consistent with a cis-(PCy3)2Ni(norbornyl)Cl complex,
crystals of 2 were grown to determine the true identity
of the new compound. X-ray analysis clearly identifies
the new product as an η2-bound adduct of 1-norbornene,
and the ORTEP diagram is shown in Figure 1.
The reaction conditions appear to be key in the overall
transformation to the η2-bound adduct.40 The starting
material (PCy3)2NiCl2 is highly insoluble in pentane;
thus, it can be expected that any alkylated nickel
complex formed in situ is then exposed to a large excess
of norbornyllithium. These basic conditions may be
(40) Preparation of (PCy3)2Ni(η2-1-norbornene) (2): 1-nor-
bornyllithium (15.3 mL, 0.11 M in pentane) was added to a slurry of
(PCy3)2NiCl2 (581 mg, 0.84 mmol) and pentane (100 mL) and stirred
for 10 min. The resulting solution was passed over a 1 cm pad of
alumina, and thorough rinsing with pentane yielded an orange filtrate.
The solvents were evaporated, and the crude product was further
purified by extraction with pentane and additional filtration over a 1
cm pad of alumina. Removal of solvents yielded 398 mg (66%) of an
orange solid. Crystals were grown from a concentrated solution of 2
in pentane at -30 °C. 31P{1H} NMR (C6D6, 30 °C, 121.5 MHz): δ 44.14
(33) Bly, R. S.; Silverman, G. S.; Bly, R. K. J. Am. Chem. Soc. 1988,
110, 7730-7737.
(d, J ) 20.7 Hz), 39.63 (d, J ) 20.8 Hz). Anal. Calcd (found) for C43H76
-
(34) Kumar, A.; Lichtenhan, J. D.; Critchlow, S. C.; Eichinger, B.
E.; Borden, W. T. J. Am. Chem. Soc. 1990, 112, 5633-5634.
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1986, 27, 4615-4618.
NiP2: C, 72.36 (72.12); H, 10.73 (10.49).
(41) Jarvis, A. P.; Haddleton, D. M.; Segal, J. A.; McCamley, A. J.
Chem. Soc., Dalton Trans. 1995, 2033-2040.
(42) Crabtree, R. H., The Organometallic Chemistry of the Transition
Metals, 3rd ed.; Wiley: New York, 2001; pp 115-117.
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149-150.
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Communications
Organometallics, Vol. 24, No. 16, 2005 3823
Table 1. Electrochemical and NMR Data for Olefin
Adducts of (PCy3)2Ni and Related Compoundsa
be a facile way to explore the scope of additions of small
molecules to 1. Keese and co-workers reported that
chemically generated 1 in the presence of furan led to
the formation of furan adducts and norbornene dimers
(eq 3).15 We found, however, that oxidation of 2 by Fe-
31P{1H} NMR
oxidn potential (V) chem shift (ppm)
compd
2
-0.23 only
44.1, 39.6 (C6D6)
43
(PCy3)2Ni(η2-1-hexene) -0.59, 0.16, 0.98
35.0 (C6D12
34.4 (C6D6)
77.0 (C6D6)
)
(PCy3)2Ni(η2-ethylene)
(dippe)Ni(CH3)2
-0.45, 0.36, 1.04
-0.02 only
44
a All potentials were measured in THF solution using 0.1 M
Bu4NPF6 and 3 mM Ni complex and referenced vs Ag/Ag+. Scan
rate: 10 mV/s. Values are reported as anodic peak potentials.
of the organometallic fragment. The energy of the
oxidation is not unreasonable for a Ni(II)-dialkyl spe-
cies, and the oxidation of a related compound, (dippe)-
Ni(CH3)2 (dippe ) 1,2-bis(diisopropylphosphino)ethane),
was found to occur irreversibly at -0.02 V. A cis-dialkyl
complex containing alkyl groups more donating than
methyls should therefore occur at more reducing poten-
tials, such as that observed for 2. The 31P{1H} NMR
chemical shifts of 2 also appear much more downfield
than for reported Ni(0) complexes (Table 1), which may
lend additional support for Ni(II) character in 2. As-
suming then a Ni(II) oxidation state for 2, we conclude
that relief of ring strain in the unusual bridgehead
double bond is the primary reason for both the elongated
C-C bond lengths and the anomalous metal oxidation
state.
(III) produced dinorbornyl as the only identifiable
organic compound (eq 4). Further studies directed at
olefin release and functionalization are underway.
Acknowledgment. We thank Emily Clark for her
assistance with the cyclic voltammetry experiments.
D.A.V. thanks the University of Arkansas, the Arkansas
Biosciences Institute, and the NIH (Grant No. RR-
15569) for support of this work.
Supporting Information Available: Text giving general
methods and CIF files giving X-ray data for compound 2. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The preliminary reactivity of 2 was also explored. We
wondered whether oxidation could be used as a trigger
to release reactive 1 in solution. If so, oxidation could
OM0503388