Reactions of ruthenium benzylidene complexes with cyclic and acyclic imines: Oligomerization of 1-pyrroline and metathesis via tautomerism

Article abstract of DOI:10.1021/om0301029

At room temperature, NMR spectroscopy indicates that the ruthenium benzylidene complex (Cl)₂(PCy₃)₂Ru=CHPh reacts with 1-pyrroline to yield (Cl)₂(PCy₃)(1-pyrroline)Ru=CHPh. Heating a solution of (Cl)₂(PCy₃)₂Ru=CHPh with excess 1-pyrroline to 90°C results in ring-opening oligomerization of the cyclic imine. The combination of ruthenium carbene complexes (Cl)₂(PCy₃)₂Ru=CHPh and (Cl)₂(PCy₃)(H₂IMes)Ru=CHPh (H₂IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) with acyclic imines of the type (R)N=CH(R′) results in metathesis reactions when the imine possesses a C-H bond α to the imine carbon. Imines that lack C-H bonds α to the imine carbon do not react with (Cl)₂(PCy₃)₂Ru=CHPh. The primary products from the reactions of (Cl)₂(PCy₃)₂Ru=CHPh and (Cl)₂(PCy₃)(H₂IMes)Ru=CHPh with acyclic imines are olefins and new Fischer carbene complexes of the type (Cl)₂(L)(L′)Ru=CH{N(H)R} (L = L′ = PCy₃; L = PCy₃ L′ = H₂IMes). The ruthenium complex (Cl)₂(PCy₃)₂Ru=CH{N(H)Pr} has been isolated from the reaction of (Cl)₂(PCy₃)₂ Ru=CHPh with (Pr)N=CH(i-Pr) and has been fully characterized. A possible pathway for the reactions of (Cl)₂(PCy₃)₂Ru=CHPh and (Cl)₂(PCy₃)(H₂IMes)Ru=CHPh with acyclic imines that involves imine to enamine tautomerism followed by C=C bond metathesis reactions is discussed. The failure of the ruthenium benzylidene complexes (Cl)₂(PCy₃)₂Ru=CHPh and (Cl)₂- (PCy₃)(H₂IMes)Ru=CHPh to react with the C=N bonds of a cyclic imines in combination with observed reactivity between Ru(Cl)₂(PPh₃)₃ and 1-pyrroline indicates that the oligomerization of 1-pyrroline with (Cl)₂(PCy₃)₂Ru=CHPh likely proceeds via a Lewis acid catalyzed mechanism.

Full text of DOI:10.1021/om0301029

Organometallics 2003, 22, 2291-2297
2291
Rea ction s of Ru th en iu m Ben zylid en e Com p lexes w ith
Cyclic a n d Acyclic Im in es: Oligom er iza tion of
1-P yr r olin e a n d Meta th esis via Ta u tom er ism
J ubo Zhang and T. Brent Gunnoe*
Department of Chemistry, North Carolina State University,
Raleigh, North Carolina 27695-8204
Received February 12, 2003
At room temperature, NMR spectroscopy indicates that the ruthenium benzylidene complex
(Cl)2(PCy3)2RudCHPh reacts with 1-pyrroline to yield (Cl)2(PCy3)(1-pyrroline)RudCHPh.
Heating a solution of (Cl)2(PCy3)2RudCHPh with excess 1-pyrroline to 90 °C results in ring-
opening oligomerization of the cyclic imine. The combination of ruthenium carbene complexes
(Cl)2(PCy3)2RudCHPh and (Cl)2(PCy3)(H2IMes)RudCHPh (H2IMes ) 1,3-dimesityl-4,5-
dihydroimidazolylidene) with acyclic imines of the type (R)NdCH(R′) results in metathesis
reactions when the imine possesses a C-H bond R to the imine carbon. Imines that lack
C-H bonds R to the imine carbon do not react with (Cl)2(PCy3)2RudCHPh. The primary
products from the reactions of (Cl)2(PCy3)2RudCHPh and (Cl)2(PCy3)(H2IMes)RudCHPh with
acyclic imines are olefins and new Fischer carbene complexes of the type (Cl)2(L)(L′)Rud
CH{N(H)R} (L ) L′ ) PCy3; L ) PCy3, L′ ) H2IMes). The ruthenium complex (Cl)2(PCy3)2-
RudCH{N(H)Pr} has been isolated from the reaction of (Cl)2(PCy3)2RudCHPh with (Pr)Nd
CH(i-Pr) and has been fully characterized. A possible pathway for the reactions of
(Cl)2(PCy3)2RudCHPh and (Cl)2(PCy3)(H2IMes)RudCHPh with acyclic imines that involves
imine to enamine tautomerism followed by CdC bond metathesis reactions is discussed.
The failure of the ruthenium benzylidene complexes (Cl)2(PCy3)2RudCHPh and (Cl)2-
(PCy3)(H2IMes)RudCHPh to react with the CdN bonds of acyclic imines in combination
with observed reactivity between Ru(Cl)2(PPh3)3 and 1-pyrroline indicates that the oligo-
merization of 1-pyrroline with (Cl)2(PCy3)2RudCHPh likely proceeds via a Lewis acid
catalyzed mechanism.
In tr od u ction
There is significant interest in the ability to introduce
heteroatomic functionalities into polymers,13 and the
extension of transition-metal-catalyzed metathesis
polymerization reactions to carbon-heteroatom mul-
tiple bonds would allow the direct incorporation of
functionality into polymer backbones. Catalysts for the
cross-metathesis of acyclic imines that utilize zirconium,
molybdenum, tantalum, titanium, and niobium systems
have been reported.14-21 However, with a few excep-
tions, the mechanisms of these reactions are not well
defined, and only a single family of transition-metal
imido systems that catalyze imine metathesis has been
Transition-metal carbene complexes that mediate
bond-breaking and bond-forming reactions of carbon-
carbon multiple bonds have become valuable synthetic
tools and have been applied toward a variety of reac-
tions, including olefin isomerization, metathesis polym-
erization, and the synthesis of small organic molecules
via ring-closing metathesis sequences.1-9 Synthetic ap-
plications of group VI alkylidene complexes have been
driven primarily by high catalyst activities, while the
tolerance of diverse functionality by ruthenium
complexes has increased the scope of accessible
reactions.4-6,10-12
(11) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413-4450.
(12) Grubbs, R. H.; Miller, S. J .; Fu, G. C. Acc. Chem. Res. 1995,
28, 446-452.
* To whom correspondence should be addressed. E-mail:
brent_gunnoe@ncsu.edu.
(1) Ivin, K. J .; Mol, J . C. Olefin Metathesis and Metathesis Polym-
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1998.
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10.1021/om0301029 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 05/01/2003

2292 Organometallics, Vol. 22, No. 11, 2003
Zhang and Gunnoe
Sch em e 1. Regioselective Meta th esis of
Ca r bon -Nitr ogen Mu ltip le Bon d s Ba sed on Meta l
Ca r ben e Liga n d s P ossibly Lea d in g to Ca ta lytic
Tu r n over
Ch a r t 1. Ru th en iu m Ben zylid en e Com p lexes
demonstrated to proceed via diazametallacycle inter-
mediates that are analogous to olefin metathesis
intermediates.20-23 In contrast, mechanistic studies of
other imine metathesis catalysts have revealed the
possibility of multiple reaction pathways.18,24,25 In ad-
dition to imine metathesis, reactions of imido complexes
with isocyanates or carbodiimides, stoichiometric metal
insertion into pyridine, metal-catalyzed metathesis of
PdP bonds, and intermolecular metal imido exchange
reactions have been observed.26-31
Although imido ligands are often inert toward me-
tathesis and other reactivity, carbene ligands are fre-
quently active for metathesis transformations.1,3-7,9,10,32-35
Thus, it might be anticipated that catalytic metathesis
of C-N multiple bonds based on transition-metal car-
bene ligands would be more viable than reactions that
incorporate imido ligands (Scheme 1). However, reac-
tions of transition-metal carbene systems with imines,
nitriles, or carbonyl compounds typically yield CdC and
MdNR or MdO bond formation with catalytic turnover
disrupted by the formation of reaction-inert imido or oxo
complexes.15,36-41 The group VI complex (CO)5WdCHPh
reacts with carbodiimides with atypical regioselectivity
to yield imines and (CO)5WdCdNR complexes.42 We
report herein the ring-opening oligomerization of 1-pyr-
roline with (Cl)2(PCy3)2RudCHPh (1) as catalyst as well
as reactions of the series of metathesis catalysts (Cl)2-
(PCy3)2RudCHPh (1), (Cl)2(PPh3)2RudCHPh (2), and
(Cl)2(PCy3)(H2IMes)RudCHPh (3) (H2IMes ) 1,3-di-
mesityl-4,5-dihydroimidazolylidene) with acyclic imines
(Chart 1). Complexes 1 and 3 do not react with the Cd
N bonds of acyclic imines but rather with the CdC
bonds of enamine tautomers.6,43
Resu lts a n d Discu ssion
Rea ction s w ith 1-P yr r olin e. Ligands that engage
in multiple bonding with transition-metal complexes are
known to exhibit variable reactivity (i.e., electrophilic
vs nucleophilic character) as a function of metal identity,
metal oxidation state, and the identity of ancillary
ligands. Increased ligand-based electrophilic character
has been observed as a progression from left to right in
the transition series is made. An explanation for this
phenomenon is the localization of the HOMO and
LUMO that results from metal-ligand π-bonding,44,45
and it might be anticipated that transition-metal-
carbene bonds of later and more electronegative transi-
tion metals would serve to reverse the reaction regio-
selectivity of metathesis reactions with imines. Thus,
we supposed that the regioselectivity of metathesis
reactions of ruthenium carbene complexes might differ
from that observed with earlier transition-metal sys-
tems (i.e., CdN bond formation might be accessible).
The combination of (Cl)2(PCy3)2RudCHPh (1) with
1-pyrroline in C6D6 results in an immediate ligand
exchange at room temperature to yield (Cl)2(PCy3)(1-
pyrroline)RudCHPh (4), as indicated by the decrease
in intensity for the carbene CH singlet of complex 1 and
the appearance of a new carbene CH resonance (doublet,
(22) Krska, S. W.; Zuckerman, R. L.; Bergman, R. G. J . Am. Chem.
Soc. 1998, 120, 11828-11829.
(23) Meyer, K. E.; Walsh, P. J .; Bergman, R. G. J . Am. Chem. Soc.
1994, 116, 2669-2670.
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2002, 21, 1933-1941.
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J ohnson, M.; Toporek, S. Organometallics 1993, 12, 1023-1025.
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1999, 584, 200-205.
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nometallics 1998, 17, 322-329.
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(32) Schrock, R. R. In Metathesis Polymerization of Olefins and
Polymerization of Alkynes; Imamoglu, Y., Ed.; Kluwer Academic:
Dordrecht, The Netherlands, 1998; pp 357-380.
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40.
1
3J PH ) 12 Hz) at 20.50 ppm in the H NMR spectrum
(eq 1). Coupling between the carbene proton and the
(35) Schmalz, H.-G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1833-
1836.
(36) Rocklage, S. M.; Schrock, R. R. J . Am. Chem. Soc. 1982, 104,
3077-3081.
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7808-7809.
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19, 3562-3568.
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1979, 101, 3210-3222.
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PCy3 ligands of complex 1 is not observed. The lack of
significant coupling is a result of the trans arrangement

Reactions of Ru Benzylidene Complexes with Imines
Organometallics, Vol. 22, No. 11, 2003 2293
of the phosphine ligands, large P-Ru-P bond angle,
and perpendicular carbene orientation.46 In addition, a
resonance consistent with a metal-bound imine CH
appears at 7.82 ppm as well as a new resonance
assigned as the metal-bound N-CH2 moiety (3.5 ppm,
triplet) and a new doublet due to the carbene phenyl
ortho protons (8.48 ppm). Other resonances appear in
1
the upfield region of the H NMR spectrum, but defini-
tive assignment is complicated by the multiple reso-
nances for the reaction mixture (i.e., the mixture of
complexes 1 and 4 and the presence of free 1-pyrroline
monomer and trimer and tricyclohexylphosphine result
in a complex spectrum). In addition to resonances due
to the formation of free PCy3 (10.9 ppm) and the Grubbs
carbene complex 1, a new resonance is observed at 30.1
ppm in the 31P NMR spectrum of the reaction mixture.
An equilibrium between the pyrroline complex 4 and
PCy3 with complex 1 and 1-pyrroline is observed (Keq
F igu r e 1. COSY spectrum of poly(pyrroline).
1
≈ 0.5 at room temperature, as determined by H NMR
Sch em e 2. Mon om er ic 1-P yr r olin e in Equ ilibr iu m
w ith Cyclic Tr im er
spectroscopy), and the addition of excess 1-pyrroline
pushes the equilibrium toward bound pyrroline (as
determined by 1H NMR spectroscopy). However, all
attempts to isolate complex 4 have failed (possibly due
to rapid imine dissociation in the absence of excess
1-pyrroline leading to decomposition). Two minor dou-
blets (ratio of the carbene CH of 4 to minor doublets is
∼2:1) observed at 20.15 and 19.15 ppm could be due to
isomers of η2-bound 1-pyrroline (a minor resonance at
25.6 ppm is observed in the 31P NMR spectrum). To our
knowledge, no precedent for η2 coordination of a cyclic
imine to a transition metal exists; however, η2-bound
acyclic imines and η1-cyclic imines have been re-
ported.47-50 A second ligand substitution of PCy3 with
1-pyrroline to yield trans-(Cl)2(1-pyrroline)2RudCHPh
is possible; however, the carbene CH would resonate as
a singlet (not observed). At room temperature, the
mixture of complex 1 and 1-pyrroline does not undergo
significant change for at least 24 h.
in isolation of the oligomerized product. A COSY experi-
ment reveals data that are consistent with the assign-
ments of resonances due to polypyrroline (Figure 1). The
13C NMR spectrum of the isolated product displays
resonances at 170.7, 63.2, 37.1, and 21.9 ppm (see the
Supporting Information). The cyclic imine 1-pyrroline
exists as a cyclic trimer (Scheme 2) but can be reverted
to the monomer upon heating,38,51 and the total amount
of monomer, trimer, and polypyrroline remains constant
throughout the reaction. Use of an internal standard
indicates that approximately 3.6 equiv of 1-pyrroline/
equiv of catalyst is oligomerized. GPC analysis of the
polymeric product reveals no high-molecular-weight
polymer, and MALDI mass spectroscopy is consistent
with low-molecular-weight polypyrroline (i.e., no com-
pounds with molecular weight >830 are observed). TGA
analysis results in 50% weight loss at 200 °C. Thus, the
isolated poly(pyrroline) contains a significant amount
of ruthenium impurities (consistent with elemental
analysis). Results from reactions of complex 3 with
1-pyrroline are similar to those observed with complex
1.
Heating complex 1 with excess 1-pyrroline to 90 °C
results in a decrease in intensity for resonances due to
the free 1-pyrroline monomer. A similar reaction also
occurs at 60 °C, albeit at a reduced rate. After 1 h at 90
°C, use of an internal standard indicates that ap-
proximately 20% of the 1-pyrroline has been consumed
with concomitant formation of a new triplet at 8.18 ppm
as well as multiplets at 4.22, 2.15, and 1.55 ppm in the
1H NMR spectrum. These results are consistent with
ring-opening oligomerization of 1-pyrroline (eq 2). Sol-
Heating the reaction solution for 24 h results in only
minor amounts of additional polymerization. The dis-
continuation of polymerization could be due to catalyst
decomposition; however, it is also possible that the lack
of significant ring strain for 1-pyrroline could result in
a thermodynamic inhibition to further polymerization.
For example, the ROMP of cyclopentene to cis polymer
is only slightly exergonic with ∆G° ) -0.3 kJ /mol.1
vent removal followed by washing with pentane results
(41) Pine, S. H.; Zahler, R.; Evans, D. A.; Grubbs, R. H. J . Am. Chem.
Soc. 1980, 102, 3270-3272.
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630.
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2001, 123, 6543-6554.
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F. A. Inorg. Chim. Acta 1982, 65, L91-L93.
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Wiley: New York, 1988.
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1993, 115, 3527-3532.

2294 Organometallics, Vol. 22, No. 11, 2003
Zhang and Gunnoe
Ta ble 1. Meta th esis Rea ction s of Acyclic Im in es
Fresh catalyst precursor 1 (approximately the same
amount as the initial reaction) was added to the NMR
solution, and after heating to 90 °C polymerization
activity resumed. After an additional 1 h of heating,
approximately 37% of the original 1-pyrroline monomer/
trimer had been polymerized (compared with 20% after
the initial catalyst consumption; see above). These
results demonstrate that the observed loss of catalyst
activity after approximately 1 h at 90 °C is not due to
thermodynamic inhibition of polymerization.
Rea ction s w ith Acyclic Im in es. Simple Lewis and
protic acids are known to catalyze imine metathesis
reactions.52 To study the feasibility of metathesis reac-
tions of ruthenium carbene complexes with CdN bonds,
a series of reactions of acyclic imines with complexes
1-3 was explored. The reaction of complex 1 and the
N-alkylimine (Pr)NdCH(i-Pr) (Pr ) propyl) was moni-
tored at 75 °C in an NMR tube (C6D6). After the mixture
was heated for approximately 10 h, the formation of
2-methyl-1-phenyl-1-propene and the new Fischer car-
bene complex (Cl)2(PCy3)2RudCH{N(H)Pr} (5) was
observed by 1H NMR spectroscopy (eq 3). Small amounts
w ith Com p lexes 1-3a
com-
imine
plex
reacn productb
Me2CdCH(Ph)
trans-(Et)HCdCH(Ph) 13.68, 8.96 (dd)
trans-(Me)HCdCH(Ph) 13.60, 9.15 (dt)
trans-(Et)HCdCH(Ph) 13.60, 9.15 (dt)
1H NMRc
(Pr)NdCH(i-Pr)
(i-Pr)NdCH(Pr)
(Pr)NdCH(Et)
(Pr)NdCH(Pr)
(i-Pr)NdCH(t-Bu)
(t-Bu)NdCH(Pr)
(Ph)NdCHPh
(i-Pr)NdCH(Ph)
(Ph)NdCH(Et)
(Pr)NdCH(Pr)
(Pr)NdCH(Pr)
1
1
1
1
1
1
1
1
1
2
3
13.60,d 9.15 (dt)
no reacn
N/A
trans-(Et)HCdCH(Ph) 13.88, 9.25 (d)
no reacn
no reacn
trans-(Me)HCdCH(Ph) 15.03, 11.26 (d)
decomposition N/A
trans-(Et)HCdCH(Ph) 12.98, 8.80 (dt)
N/A
N/A
a
b
All reactions performed in C6D6 between 70 and 75 °C. In
addition to olefin, the formation of new Ru carbene complexes is
observed. c Chemical shifts of the resonances due to carbene proton
and amine proton for the new ruthenium carbene complexes are
d
listed consecutively. Each carbene proton resonates as a doublet.
Sch em e 3. Com p lex 1 Rea cts w ith N-t-Bu Im in e
bu t Does Not Rea ct w ith Im in es Th a t P ossess a
t-Bu Gr ou p a t th e Im in e Ca r bon
of other organic products are observed (possibly due to
metathesis of the resulting olefins). After approximately
24 h, the conversion of 1 to complex 5 is complete. The
carbene complex 5 can be isolated pure after workup
and has been characterized by multinuclear NMR
spectroscopy and elemental analysis. Salient features
(Pr) (all reactions at approximately 70 °C) result in
transformations closely related to that observed with
(Pr)NdCH(i-Pr). In all reactions the formation of a new
olefin as the primary organic product is observed by 1H
NMR spectroscopy (Table 1). In addition, the appear-
ance of olefin is accompanied by the formation of new
Ru carbene complexes, as indicated by doublets between
13 and 14 ppm (due to the carbene proton) and multi-
plets between 8 and 12 ppm (due to the amine proton)
1
of the H NMR spectrum include a downfield doublet
due to the carbene proton at 13.60 ppm (J ) 14 Hz) and
a doublet of triplets (J ) 14, 7 Hz) due to the amine
proton that resonates at 9.15 ppm. COSY and proton-
decoupling experiments are consistent with the assigned
structure (see Supporting Information). The 13C NMR
spectrum reveals the carbene resonance at 240.1 ppm,
and a resonance due to the new Ru complex is observed
at 32.5 ppm in the 31P NMR spectrum. The syntheses
of other ruthenium carbene complexes that possess
heteroatomic functionality (Fischer carbene complexes)
have been previously reported.53-58
1
in the H NMR spectra. In contrast, heating a solution
of complex 1 with (i-Pr)NdCH(t-Bu) at 70 °C results in
no observable reaction after 24 h. To determine if the
failure of complex 1 to react with (i-Pr)NdCH(t-Bu) is
due to steric inhibition by the bulky tert-butyl substitu-
ent, (t-Bu)NdCH(Pr) was reacted with 1. After 10 h at
70 °C, the formation of trans-â-ethylstyrene and a new
Ru carbene complex was observed (Scheme 3; Table 1).
Thus, the incorporation of a tert-butyl group at the imine
nitrogen does not inhibit metathesis reactivity, and the
failure of (i-Pr)NdCH(t-Bu) to undergo reaction is not
likely attributable to steric factors.
The reactions of complex 1 with the acyclic and alkyl
imines (i-Pr)NdCH(Pr), (Pr)NdCH(Et), and (Pr)NdCH-
(52) To´th, G.; Pinte´r, I.; Messmer, A. Tetrahedron Lett. 1974, 735-
738.
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684.
Metathesis reactions are also observed with N-
arylimines. For example, the formation of trans-â-
methylstyrene and a new Ru carbene complex is ob-
served when complex 1 is reacted with (Ph)NdCH(Et)
(Scheme 4). Although the presence of an N-aryl sub-
stituent does not disrupt the observed metathesis reac-
tions, the incorporation of a phenyl substituent at the
(55) Katayama, H.; Urushima, H.; Nishioka, T.; Wada, C.; Nagao,
M.; Ozawa, F. Angew. Chem., Int. Ed. 2000, 39, 4513-4515.
(56) Schaaf, P. A. v. d.; Kolly, R.; Kirner, H.-J .; Rime, F.; Mu¨hlebach,
A.; Hafner, A. J . Organomet. Chem. 2000, 606, 65-74.
(57) Wu, Z.; Nguyen, S. T.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem.
Soc. 1995, 117, 5503-5511.
(58) Coalter, J . N., III; Bollinger, J . C.; Huffman, J . C.; Werner-
Zwanziger, U.; Caulton, K. G.; Davidson, E. R.; Ge´rard, H.; Clot, E.;
Eisenstein, O. New J . Chem. 2000, 24, 9-26.

Reactions of Ru Benzylidene Complexes with Imines
Organometallics, Vol. 22, No. 11, 2003 2295
Sch em e 4. Com p lex 1 Rea cts w ith N-Ar yl Im in es
bu t Does Not Rea ct w ith Im in es Th a t P ossess Ar yl
Gr ou p s a t th e Im in e Ca r bon
Sch em e 5. P r op osed Rea ction P a th w a y for th e
Con ver sion of Ru Ben zylid en e Com p lexes a n d
Acyclic Im in es to New Olefin a n d F isch er Ca r ben e
System s
imine carbon results in no reaction. For example,
neither N-benzylideneaniline nor (i-Pr)NdCH(Ph) re-
acts with complex 1 after 24 h at 70 °C (Scheme 4).
To compare the abilities of the ruthenium complexes
to impact the reaction, complexes 2 and 3 were reacted
with (Pr)NdCH(Pr). Complex 2 decomposes before
reaction with the imine is observed, and similar results
were obtained upon reacting complex 2 with (Ph)Nd
CH(Ph) or (i-Pr)NdCH(Ph). In contrast, complex 3
reacts with (Pr)NdCH(Pr) to yield identical organic
products and a new Fischer carbene complex, as ob-
served with the reaction of complex 1 (Table 1).
Rea ction P a th w a y. No evidence for the formation
of Ru imido complexes is observed upon reaction of
complexes 1-3 with acyclic imines. The preparation of
Ru imido complexes is relatively rare.59-63 Although
complex 1 does not appear to react directly with the Cd
N bond of acyclic imines, reactions with acyclic imines
that possess C-H bonds R to the imine carbon to form
new CdC bonds and new Fischer carbene complexes are
observed. In contrast, imines that lack C-H bonds R to
the imine carbon fail to react with the ruthenium
carbene complexes. This reactivity trend along with the
identities of the olefin products are consistent with a
reaction pathway that involves imine tautomerism prior
to CdC metathesis reactivity (Scheme 5). Tautomerism
to enamine results in the formation of a carbon-carbon
double bond with an amine functionality,64,65 and the
metathesis reaction of the ruthenium carbene moiety
with the new CdC bond accounts for the formation of
the new olefins (R′)(R′′)CdCH(Ph) (Scheme 5). Meta-
thesis reactions between ruthenium carbene complexes
and olefins that possess electron-donating heteroatomic
functionalities have been reported.53,57,66,67 The incor-
poration of a phenyl group or a tert-butyl group at the
imine carbon disrupts the formation of the enamine
tautomer, and the presence of these substituents at the
imine carbon results in no observed reaction between
the ruthenium benzylidene complexes and acyclic imi-
nes. Thus, in contrast to Ta and Mo carbene complexes
(see above), the ruthenium carbene complexes 1 and 3
do not react with the carbon-nitrogen bonds of acyclic
imines under the observed reaction conditions.
The observation of acyclic imines reacting with com-
plexes 1 and 3 as enamines suggests that the polym-
erization of 1-pyrroline catalyzed by complex 1 might
proceed via a mechanism in which ruthenium catalyzes
ring-opening polymerization by acting as a Lewis acid.
However, RuCl3 hydrate does not polymerize 1-pyrroline
at 90 °C. The combination of RuCl2(PPh3)3 with 1-pyr-
roline in C6D6 at room temperature results in PPh3/1-
pyrroline ligand exchange, as determined by 1H and 31P
NMR spectroscopy (Scheme 6). The observation of a
broad singlet at 8.31 ppm and a multiplet at 3.78 ppm
(1H NMR) are consistent with an η1-bound 1-pyrroline
ligand (free 1-pyrroline monomer resonates at 7.27 and
3.68 ppm), and the 31P NMR spectrum reveals new
resonances at -4.4 ppm for free PPh3 and 38.3 ppm,
consistent with the formation of RuCl2(PPh3)2(η1-1-
pyrroline). Heating this solution to 60 °C for ap-
proximately 30 min results in the observation of new
resonances at 8.47 ppm (broad singlet) and 4.25 ppm
(59) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239-482.
(60) Danopoulos, A. A.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse,
M. B. J . Chem. Soc., Dalton Trans. 1996, 3771-3778.
(61) Burrell, A. K.; Steedman, A. J . Organometallics 1997, 16, 1203-
1208.
(62) Che, C.-M.; Yam, V. W.-W. Adv. Inorg. Chem. 1992, 39, 233-
325.
(63) Danopoulos, A. A.; Hay-Motherwell, R. S.; Wilkinson, G.;
Cafferkey, S. M.; Sweet, T. K. N.; Hursthouse, M. B. J . Chem. Soc.,
Dalton Trans. 1997, 3177-3184.
(64) Hickmott, P. W. Tetrahedron 1982, 38, 1975-2050.
(65) Shainyan, B. A.; Mirskova, A. N. Russ. Chem. Rev. (Engl.
Transl.) 1979, 48, 107-117.
(66) Katayama, H.; Urushima, H.; Ozawa, F. Chem. Lett. 1999, 369-
370.
(67) Katayama, H.; Urushima, H.; Ozawa, F. J . Organomet. Chem.
2000, 606, 16-25.

2296 Organometallics, Vol. 22, No. 11, 2003
Zhang and Gunnoe
Sch em e 6. Liga n d Su bstitu tion Rea ction s u p on
Com bin a tion of Ru Cl2(P P h 3)3 w ith Excess
1-P yr r olin e
conclusion is that the polymerization proceeds via a
Lewis acid ring-opening polymerization, the results do
not conclusively rule out a metathesis reaction.
Su m m a r y a n d Con clu sion s
The ruthenium complexes 1 and 3 undergo metathesis
reactions with acyclic imines that proceed via an imine/
enamine tautomerism mechanism. The observed reac-
tion pathways are in contrast with reactions of carbene
ligands bound to earlier transition metals (i.e., Ta and
Mo) and further illustrate the potential mechanistic
complications of metathesis reactions involving C-N
multiple bonds. The failure to observe reactions with
the CdN bonds could be due to the inability of the
ruthenium complexes to coordinate the acyclic imines.
In contrast, the cyclic imine 1-pyrroline rapidly coordi-
nates to complex 1 via a ligand exchange reaction in
which PCy3 dissociates, and heating a solution of excess
1-pyrroline with 1 results in ring-opening oligomeriza-
tion. The mechanism of the 1-pyrroline oligomerization
has not been definitively determined; however, a Lewis
acid catalyzed mechanism seems likely, since RuCl2-
(PPh3)3 catalyzes a similar reaction.
(multiplet) in the 1H NMR spectrum as well an increase
in the intensity of the resonance for free PPh3 and a
new resonance at 51.3 ppm in the 31P NMR spectrum.
These observations are consistent with the formation
of RuCl2(PPh3)(η1-1-pyrroline)2 and 2 equiv of free PPh3
(Scheme 6). Continuous heating (60 °C) for 2 h leads to
the disappearance of the resonances corresponding to
RuCl2(PPh3)2(η1-1-pyrroline) and a decrease in the
intensity of resonances due to RuCl2(PPh3)(η1-1-pyrro-
line)2. At this point, two resonances are observed in the
31P NMR due to free PPh3 and RuCl2(PPh3)(η1-1-
pyrroline)2. In addition, new resonances in the 1H NMR
are observed at 8.13 ppm (broad singlet) and 3.95 ppm
(multiplet). These results are consistent with PPh3/1-
pyrroline ligand substitution to yield an equilibrium
between RuCl2(η1-1-pyrroline)3 and PPh3 with RuCl2-
(PPh3)(η1-1-pyrroline)2 and 1-pyrroline (Scheme 6). Con-
tinued heating at 60 °C does not result in any observable
changes to the reaction solution after 24 h. However,
heating to 90 °C for 12 h yields resonances consistent
with minor amounts of poly(1-pyrroline) after 12 h
(<10% of the original 1-pyrroline is polymerized at this
time). Clean isolation of the ruthenium complexes with
bound pyrroline was not possible.
During the reactions of complex 1 with acyclic imines,
no evidence of imine coordination to ruthenium is
observed. In contrast, the cyclic imine 1-pyrroline
undergoes a rapid ligand exchange reaction (at room
temperature) with PCy3 when combined with complex
1 in benzene. The different predilection toward imine
coordination between 1-pyrroline and the acyclic imines
is likely due to steric factors. Significant electronic
contributions are unlikely, as indicated by the failure
of the N-alkylimines (Pr)NdCH(Et) and (Pr)NdCH(Pr)
to coordinate to the ruthenium complex 1. The inability
of complex 1 to react with the CdN bonds of acyclic
imines suggests that the ring-opening oligomerization
of 1-pyrroline may not occur via a metathesis reaction
mechanism, and the ability of RuCl2(PPh3)3 to catalyze
a similar polymerization reaction at 90 °C supports this
conclusion. However, it should be noted that the cata-
lytic polymerization reaction with RuCl2(PPh3)3 does not
occur at 60 °C and is slow at 90 °C compared to reactions
with complex 1, and the ability of complex 1 to coordi-
nate 1-pyrroline (in contrast to the acyclic imines) could
lead to alternative reaction pathways for cyclic versus
acyclic imines. In addition, it is perhaps noteworthy that
simple ruthenium complexes that do not bear carbene
ligands serve as catalyst precursors for CdC bond
metathesis reactions.68,69 Thus, while the tentative
Exp er im en ta l Section
Gen er a l Meth od s. All procedures were performed under
an inert atmosphere (nitrogen) in an Innovative Technologies
glovebox or using standard Schlenk techniques. The glovebox
atmosphere was maintained by periodic nitrogen purges and
monitored by an oxygen analyzer (O2(g) <15 ppm for all
reactions). Benzene and toluene were purified by distillation
from sodium/benzophenone. Pentane was refluxed over P
2O5
for a few hours followed by distillation. All solvents were
purged with nitrogen for at least 10 min prior to use. Benzene-
d6 was degassed via three freeze-pump-thaw cycles prior to
use and stored over 4 Å molecular sieves. 1H and 13C NMR
spectra were recorded on a General Electric 300 or Varian
Mercury 400 MHz spectrometer. All 1H and 13C NMR chemical
shifts are reported in ppm and are referenced to tetrameth-
ylsilane using residual proton signals or the 13C signals of the
deuterated solvents. 31P NMR spectra were recorded on a
Varian Mercury 400 MHz instrument and referenced against
external 85% phosphoric acid. MALDI-TOF mass spectroscopy
was performed with a Bruker Perflex system using the matrix
POPOP (1,4-bis(5-phenyloxazol-2-yl)benzene). TGA was per-
formed with a Hi-Res TGA 2950 thermogravimetric analyzer.
GPC analyses of polymer molecular weights were performed
on a J asco system comprised of a PU-1580 intelligent HPLC
pump, a RI-1530 intelligent refractive index detector, and a
Borwin-GPC control system. The molecular weight was cali-
brated with polystyrene standards. Two PL-Gel mixed columns
were used for analysis with chloroform at a flow rate of 1.0
mL/min. The Grubbs catalysts (Cl)2(PCy3)2RudCHPh (1), (Cl)2-
(PPh3)2RudCHPh (2), and (Cl)2(PCy3)(H2IMes)RudCHPh (3)
were prepared according to published procedures.70,71 The
procedure for the preparation of 1-pyrroline was reported by
Meyer et al.38 Ruthenocene was purchased from Aldrich
Chemical Co. and used as the internal standard for NMR tube
reactions without further purification. Imines were prepared
by condensation of the appropriate aldehyde and amine as
reported.72 Aldehydes and amines were obtained from Aldrich
(69) Novak, B. M.; Grubbs, R. H. J . Am. Chem. Soc. 1988, 110,
7542-7543.
(70) Schwab, P.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc. 1996,
118, 100-110.
(68) Novak, B. M.; Grubbs, R. H. J . Am. Chem. Soc. 1988, 110, 960-
961.
(71) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953-956.

Reactions of Ru Benzylidene Complexes with Imines
Organometallics, Vol. 22, No. 11, 2003 2297
Chemical Co. and used without further purification. Elemental
analysis was performed by Atlantic Microlab, Inc.
C6D6. The resulting solution was transferred to a screw-cap
NMR tube, and a 1H NMR spectrum was acquired of the
homogeneous solution. Next, the reaction solution was heated
to 75 °C in an oil bath for 12 h with periodic monitoring by 1H
NMR spectroscopy. Another 1H NMR spectrum was acquired
and compared with the spectrum that preceded heating. The
1H NMR spectrum revealed that the Grubbs carbene complex
1 had been consumed and that the olefin trans-(Et)HCdCH-
(Ph) was produced as the primary organic product, as indicated
by a characteristic doublet at 6.25 ppm and a doublet of triplets
at 6.05 ppm. In addition, new resonances appeared at 13.68
NMR Tu be Rea ction of (Cl)2(P Cy3)2Ru dCHP h (1) w ith
1-P yr r olin e. The ruthenium complex (Cl)2(PCy3)2RudCHPh
(1; 0.0200 mg) was combined with 10-20 equiv of 1-pyrroline
(as a mixture of monomer and trimer) in C6D6. Upon addition
of 1-pyrroline the solution color changed from purple to green.
To this solution was added 1-2 mg of Cp2Ru as internal
standard. A 1H NMR spectrum was acquired using a 90° pulse
and long pulse delay (10 s). The appropriate pulse delay was
determined by incrementally increasing the delay until inte-
gration remained constant. The NMR spectrum showed a
singlet corresponding to the carbene CH of the Grubbs complex
1 (20.58 ppm) and three new carbene resonances (major
resonance at 20.50 (d) ppm and two minor resonances at 20.15
(d) and 19.15 (d) ppm; all P-H coupling constants are 12 Hz).
A new doublet at 8.48 ppm (carbene phenyl ortho resonance),
a singlet at 7.82 ppm (bound imine CH), and a multiplet at
3.5 ppm were also observed. Complete assignment of reso-
nances due to the proposed 1-pyrroline complex 4 is impossible
due to the presence of multiple resonances as a result of the
complex equilibria between the Grubbs catalyst 1, the 1-pyr-
roline adduct, free PCy3, and isomers of bound 1-pyrroline. The
31P NMR spectrum showed a resonance corresponding to the
Grubbs catalyst 1 at 37.2 ppm, free PCy3 at 10.9 ppm, and a
new resonance at 30.1 ppm designated as the 1-pyrroline
adduct. In addition, a minor resonance is observed at 25.6 ppm
in the 31P NMR spectrum. Heating the reaction solution to 90
°C in an oil bath for 1 h resulted in a color change to brown.
3
ppm (d, J PH ) 14 Hz) and 8.96 ppm (dd, J ) 14 and 7 Hz)
due to the formation of (Cl)2(PCy3)2RudCH{N(H)i-Pr)}.
Rea ction of Ru Cl2(P P h 3)3 w ith 1-P yr r olin e. In a screw-
cap NMR tube, 0.0200 g of RuCl2(PPh3)3 was combined with
an excess of 1-pyrroline in C6D6. A broad singlet at 8.31 ppm
and a multiplet at 3.78 ppm (1H NMR) are observed and are
consistent with an η1-bound 1-pyrroline ligand, and the 31P
NMR reveals new resonances at -4.4 ppm for free PPh3 and
38.3 ppm consistent with the formation of (Cl)2(PPh3)2Ru(η1-
1-pyrroline). Heating this solution to 60 °C for 30 min results
in the observation of new resonances at 8.47 ppm (broad
1
singlet) and 4.25 ppm (multiplet) in the H NMR spectrum as
well as an increase in the resonance for free PPh3 and a new
resonance at 51.3 ppm in the 31P NMR spectrum. Continuous
heating for 2 h leads to disappearance of the resonances
corresponding to (Cl)2(PPh3)2Ru(η1-1-pyrroline) and (Cl)2(PPh3)-
Ru(η1-1-pyrroline)2 with only a single resonance in the 31P
1
NMR due to free PPh3. In addition, new resonances in the H
1
NMR are observed at 8.13 ppm (broad singlet) and 3.95 ppm
(multiplet). These results are consistent with complete PPh3/
1-pyrroline ligand substitution to yield (Cl)2Ru(η1-1-pyrroline)3.
Continued heating at 60 °C does not result in any observable
changes to the reaction solution. However, heating to 90 °C
for 12 h yields resonances consistent with minor amounts of
poly(1-pyrroline) after 12 h (<10% of the original 1-pyrroline
is polymerized at this time).
The H NMR spectrum of the resulting mixture showed that
the Grubbs complex 1 had been consumed in addition to a new
triplet at 8.18 ppm (imine CH of oligomer, t, J HH ) 2 Hz) and
multiplets at 4.22, 2.15, and 1.55 ppm. Downfield resonances
due to ruthenium carbene protons were not observed. Exact
changes in the region between 1 and 3 ppm were difficult to
discern to see due to the presence of free PCy3; however,
isolation of the oligomer (see below) allowed confirmation of
the new upfield multiplets. Use of ruthenocene as internal
standard in several different experiments confirmed that 3-4
equiv of 1-pyrroline/equiv of catalyst was consistently con-
verted to oligomer, while the total amount of 1-pyrroline
monomer, trimer, and oligomer remains the same.
Isola tion of P olyp yr r olin e. The ruthenium complex (Cl)2-
(PCy3)2RudCHPh (1; 0.2000 mg) and 10-20 equiv of 1-pyr-
roline were dissolved in benzene. The resulting solution was
heated to reflux overnight, and volatiles were removed in
vacuo. The residue was washed with pentane, and the result-
ing powder was dried in vacuo. Note: 1H and 31P NMR spectra
reveal a small amount of PCy3 impurity. 1H NMR (C6D6; δ,
ppm): 8.18 (1H, t, 3J HH ) 2 Hz, NdCH of oligomer), 4.22 (2H,
m, CH2 of oligomer), 2.15 (2H, m, CH2 of oligomer), 1.55 (2H,
m, CH2 of oligomer). Minor resonances are also observed in
the aromatic region and could be due to the phenyl group from
the original carbene complex 1. 13C NMR (C6D6, δ, ppm): 170.7
(NdCH of oligomer), 63.2 ppm (CH2 of oligomer), 37.1 (CH2 of
oligomer), 21.9 (CH2 of oligomer). For the 13C NMR spectrum
see the Supporting Information. Anal. Calcd for C4H7N: C,
69.52; H, 10.21; N, 20.27 (C:N:H ratio is 6.81:1.98:1.00).
Found: C, 33.36; H, 4.84; N, 9.12 (C:N:H ratio is 6.89:1.88:
1.00). Analysis of the oligomeric product is consistent with the
C:N:H ratio of pyrroline contaminated with ruthenium species.
NMR Tu be Rea ction s of (Cl)2(P Cy3)2Ru dCHP h w ith
Acyclic Im in es. In a representative reaction, the ruthenium
complex (Cl)2(PCy3)2RudCHPh (1; 0.0500 g) was combined
with 0.0100 g of (i-Pr)NdCH(Pr) in approximately 0.5 mL of
(Cl)2(P Cy3)2Ru dCH{N(H)(P r )} (5). A benzene solution (20
mL) of (Cl)2(PCy3)2RudCHPh (1; 0.2000 mg, 0.240 mmol) and
(Pr)NdCH(i-Pr) (0.2000 mg, 1.77 mmol) was refluxed for
approximately 12 h. After the resulting solution was cooled to
room temperature, volatiles were removed under reduced
pressure. The dry residue was washed with methanol (3 × 5
mL), and the purple powder was dried in vacuo overnight. A
1
purple solid was collected in 57% yield (0.1100 mg). H NMR
3
(C6D6; δ, ppm): 13.60 (1H, d, J HH ) 14 Hz, RudCH), 9.15
(1H, dt, J HH ) 14, 7 Hz, NH), 2.76 (2H, m, NHCH2), 2.1-1.2
(overlapping multiplets due to PCy3 ligands), 1.08 (2H, m,
N(H)CH2CH2), 0.56 (3H, t, 3J HH ) 7 Hz, N(H)(CH2)2CH3). 31P-
{1H} NMR (C6D6; δ, ppm): 32.5. 13C{1H} NMR (C6D6; δ,
ppm): 240.1 (s, RudCH), 55.8 (s, NHCH2), 32.7 (t, J ) 9 Hz,
PCy3), 30.2 (s, PCy3), 28.3 (t, J ) 7 Hz, PCy3), 27.2 (s, PCy3),
23.2 (s, NHCH2CH2), 11.3 (s, CH3). Anal. Calcd for C40H75
-
NCl2P2Ru: C, 59.75; H, 9.42; N, 1.74. Found: C, 59.42; H, 9.16;
N, 1.77.
Ack n ow led gm en t. The National Science Founda-
tion (CAREER Award; Grant No. CHE 0238167) and
North Carolina State University are acknowledged for
support of this work. The Novak research group (NCSU)
is acknowledged for use of GPC instrumentation.
Su p p or tin g In for m a tion Ava ila ble: 13C NMR spectrum
of polypyrroline and COSY spectrum of complex 5. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(72) Cantrell, G. K.; Meyer, T. Y. Organometallics 1997, 16, 5381-
5383.
OM0301029