Catalytic C=N bond formation by metal-imide-mediated imine metathesis

Article abstract of DOI:10.1021/ja981272t

Molybdenum bis(imide) complexes of the general formula (DME)Cl₂Mo(=NR)₂ catalytically metathesize acyclic imine substrates. This C=N bond-forming reaction has significant implications for the synthesis of small molecules by ring-closing metathesis and polymers by ring-opening metathesis. A series of closely related =NR transfer reactions were examined in an effort to fully understand the process. (RO)₂Mo(=CHR')(=NAr) (1a, Ar = 2,6-diisopropylphenyl in all cases; R = C(CF₃)₂CH₃) underwent alkylidene/imine exchange with a variety of imines in situ to give a mixed bis(imide) product and an olefin. The reactivity decreased if the alkoxide substituents were changed for more electron donating ones. (DME)Cl₂Mo(=NR)₂ (3a, R = 2,6-diisopropylphenyl; 4, R = t-Bu) reacted with imines in an imide/imine metathesis to give mixed bis(imide) complexes and new imines. The reaction rates depended strongly on the steric demands of the imide NR substituent. Replacement of the chlorides with more electron donating alkoxide ligands inhibited the reactions. Compounds 3a and 4 also exchanged imide ligands with each other (imide/imide metathesis). Finally, compounds 3a and 4 were found to catalytically metathesize mixtures of two imines at 80 °C in 105 and 22 h, respectively. It was established that the reaction is mediated by the presence of unique initiation products in catalytic mixtures. A catalytic cycle is proposed and the nature of the mechanism is discussed in terms of the data gathered on all of the metatheses.

Full text of DOI:10.1021/ja981272t

J. Am. Chem. Soc. 1998, 120, 8035-8042
8035
Catalytic CdN Bond Formation by Metal-Imide-Mediated Imine
Metathesis
Gidget K. Cantrell and Tara Y. Meyer*
Contribution from the Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
ReceiVed April 15, 1998
Abstract: Molybdenum bis(imide) complexes of the general formula (DME)Cl2Mo(dNR)2 catalytically
metathesize acyclic imine substrates. This CdN bond-forming reaction has significant implications for the
synthesis of small molecules by ring-closing metathesis and polymers by ring-opening metathesis. A series
of closely related dNR transfer reactions were examined in an effort to fully understand the process. (RO)2Mo-
(
dCHR′)(dNAr) (1a, Ar ) 2,6-diisopropylphenyl in all cases; R ) C(CF3)2CH3) underwent alkylidene/imine
exchange with a variety of imines in situ to give a mixed bis(imide) product and an olefin. The reactivity
decreased if the alkoxide substituents were changed for more electron donating ones. (DME)Cl2Mo(dNR)2
(3a, R ) 2,6-diisopropylphenyl; 4, R ) t-Bu) reacted with imines in an imide/imine metathesis to give mixed
bis(imide) complexes and new imines. The reaction rates depended strongly on the steric demands of the
imide NR substituent. Replacement of the chlorides with more electron donating alkoxide ligands inhibited
the reactions. Compounds 3a and 4 also exchanged imide ligands with each other (imide/imide metathesis).
Finally, compounds 3a and 4 were found to catalytically metathesize mixtures of two imines at 80 °C in 105
and 22 h, respectively. It was established that the reaction is mediated by the presence of unique initiation
products in catalytic mixtures. A catalytic cycle is proposed and the nature of the mechanism is discussed in
terms of the data gathered on all of the metatheses.
Catalytic double bond metathesis, a reaction that has been
used extensively in the synthesis of polymers and small
molecules, is no longer limited to the formation of CdC bonds.
With our recent discovery that known molybdenum bis(imide)
1
complexes of the type (DME)Cl2Mo(dNR)2 metathesize imines
catalytically we have established that this approach can be used
to make CdN bonds.2 In this paper we present evidence that
the imine metathesis is mediated by metal imides and we discuss
the mechanistic implications of a series of stoichiometric dNR
transfer reactions. Ultimately these studies will enable the
exploitation of imine substrates for living polymer synthesis by
3
ring-opening metathesis polymerization (ROMP), condensation
4,5
Figure 1. Four classes of dNR transfer by metathesis
imide) complexes, but also the well-known Schrock-type olefin
polymer synthesis by acyclic diene metathesis (ADMET), and
6
,7
small molecule synthesis by ring-closing metathesis (RCM).
(
To understand the catalytic metathesis of imines, we have
found it essential to concurrently examine several classes of
dNR transfer reactions within the same metal/ligand framework.
The selected class of compounds, which can be generalized as
a molybdenum(VI) system with two multiply bonded ligands
and two singly bonded ancillary ligands, includes not only bis-
metathesis catalysts of the general formula (RO) Mo(dNAr)-
(dCHR′). Within this framework, we consider four reac-
2
1
tions: three stoichiometric and one catalytic (Figure 1). The
stoichiometric reactions are alkylidene/imine, imide/imine, and
imide/imide metathesis, and the catalytic reaction is imine/imine
metathesis (henceforth referred to simply as imine metathesis).
Alkylidene/imine metathesis is a Wittig-like reaction of an imine
with an alkylidene to give a metal imide and an olefin as
products. Imide/imine metathesis is the stoichiometric metath-
esis of a metal imide with a single imine. A regiospecific
reaction of this type that favors MdN bond formation will
produce a new metal imide and a new imine. Imide/imide
metathesis is a reaction in which the dNR groups between metal
centers are exchanged. Finally, catalytic imine metathesis,
which is analogous to olefin metathesis, is the catalytic reaction
of two different imines to give a statistical mixture of all possible
dNR exchange products. Although this final reaction differs
from the stoichiometric ones, in that the imine substrates do
not react directly with each other, it is useful to consider them
(
1) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare,
M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875-3886.
2) Portions of this work have appeared in a preliminary form. (a)
(
Cantrell, G. K.; Meyer, T. Y. J. Chem. Soc., Chem. Commun. 1997, 1551-
1
5
552. (b) Cantrell, G. K.; Meyer, T. Y. Organometallics 1997, 16, 5381-
383.
(
3) For reviews of olefin metathesis see: (a) Grubbs, R. H.; Tumas, W.
Science 1989, 243, 907. (b) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158.
c) Amass, A. J. In New Methods of Polymer Synthesis; Ebdon, J. R., Ed.;
Chapman and Hall: New York, 1991; p 76.
4) Lindmark-Hamberg, M.; Wagener, K. B. Macromolecules 1987, 20,
949-2951.
5) For a leading reference see: Wolfe, P. S.; G o´ mez, F. J.; Wagener,
(
(
2
(
K. B. Macromolecules 1997, 30, 714-717.
(
(
6) Fu, G. C.; Grubbs, R. H. J. Am. Chem. Soc. 1992, 114, 5426-5427.
7) Fu, G. C.; Grubbs, R. H. J. Am. Chem. Soc. 1992, 114, 7324-7325.
S0002-7863(98)01272-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/30/1998

8036 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Cantrell and Meyer
Table 1. Imine Designations for R(H)CdNR′ Comprise a Roman
Numeral Portion That Specifies the C-Substituent and a Standard
Group Abbreviation That Identifies the N-Substituent
Scheme 1
R′ )
n-Pr
Bn
t-Bu
Ph
Ara
Ph
I-Pr
I-Bu
I-Ph
I-Ar
NR′
I
1
type olefin metathesis catalysts of the general formula (RO)2-
t-Bu
n-Pr
n-Pent
II-Pr
II-Bu
II-Ph
II-Ar
Mo(dCHR′)(dNAr) (Ar ) 2,6-diisopropylphenyl in all cases;
NR′ II
1
a, R ) C(CF3)2CH3, R′ ) t-Bu; 1b, R ) C(CF3)(CH3)2, R′ )
III-Pr
IV-Pr
III-Bn
NR′ III
t-Bu; 1c, R ) t-Bu, R′ ) C(Ph)(CH3)2)), the study of their
reactions with imines provides a natural bridge between dCR
and dNR transfer. Alkylidene/imine exchange would be
expected to be facile since an analogous exchange has been
IV-Bu
NR′ IV
a
Ar ) 2,6-diisopropylphenyl.
1
6
reported with aldehydes.
We have studied this class of
reactions and examined the role of ancillary ligands and imine
substrates.
together since the overall metathetical transformation of reagents
to products parallels that of the stoichiometric reactions.
Prior to our work only a few, isolated examples of these four
classes of metatheses were known. Alkylidene/imine metathesis
The reaction of alkylidene complex 1a with imine I-Pr (see
Table 1 for imine designations) to give the alkylidene/imine
exchange product trans-Ph(H)CdC(H)(t-Bu) is representative
of this class and will be described in detail (Scheme 1). Two
doublets associated with the olefin product at δ 6.34 and 6.20
was first reported by Schrock et al. for a series of tantalum
alkylidene complexes.8 Both imide/imine and imine metatheses
have been described by Bergman and co-workers for Cp*2Zr-
1
were observed by H NMR spectroscopy after 4 h when complex
(dN-t-Bu)(THF). This zirconium imide complex reacts with
1
a was heated at 60 °C in C6D6 with 11 equiv of imine I-Pr.
imines to form diazametallacycles that undergo further meta-
An excess of imine was necessary to inhibit bimolecular
thetical exchange with external imines.9
,10
Although catalytic
decomposition of 1a, a reaction that has previously been
and relevant to our studies, the tendency of the zirconium imide
products to dimerize decreases the practical utility of the system.
All other examples of metathetical reactions involving a CdN
1
observed by Schrock and co-workers. As the reaction pro-
ceeded, growth of product olefin resonances was accompanied
by a reduction in intensity of the alkylidene resonance of 1a at
δ 12.05. After 20 h all of the alkylidene had been consumed.
The identity of the olefin was confirmed by GC/MS analysis
of the reaction mixture. By comparing the intensities of both
reactant and product resonances to an internal standard, we
verified that >90% of the alkylidene was converted to the
product olefin.
1
1
bond involve the more activated substrates, isocyanates and
12,13
carbodiimides.
Imide/imide metathesis was originally docu-
mented for molybdenum bis(imide) complexes by Gibson et
14,15
al.
In this report we endeavor to combine these important
precedents with our studies on reactions of complexes of the
general formula X2Mo(dY)2 with imines to form a compre-
hensive picture of the molybdenum-imide-mediated metathesis
of imines.
Reaction of complex 1b with I-Pr also gave the expected
exchange products, although the rate was significantly slower
(Scheme 1). After 20 h at 60 °C, the reaction of alkylidene 1b
with 7-10 equivalents of imine I-Pr was less than 10%
complete. An additional 7 d at 60-80 °C was necessary to
consume all of alkylidene 1b. A significant amount of the
bimolecular decomposition product, (t-Bu)HCdCH(t-Bu), was
also produced. The nonfluorinated derivative 1c did not react
with I-Pr.
Results
Throughout the course of our studies, several imines were
used and produced. To avoid the ambiguity that might arise
by assigning arbitrary compound numbers to each imine, we
have adopted a labeling system that emphasizes the dNR′ group
that is transferred. Each R(H)Cd group is given a Roman
numeral designation, while each dNR′ group is labeled as its
R′ substituent. Thus, imines II-Pr and II-Bu have different
dNR′ groups (n-Pr vs t-Bu) but share a common carbon
substituent. The designations are summarized in Table 1.
Alkylidene/Imine Metathesis. Since the imide metathesis
catalysts that we are using are closely related to the Schrock-
Of the imines surveyed, I-Pr, III-Pr, III-Bn, and IV-Pr were
found to react at room temperature with 1a over 4-5 d to give
the expected products (summarized in Table 2). The olefins
produced in the C-alkyl imine reactions (imine classes III and
1
IV) were identified by their distinctive H NMR signatures, a
doublet and doublet of triplets in the δ 5-6 region due to the
olefinic protons, and GC/MS analysis of the reaction mixtures.
Imine IV-Bu also underwent alkylidene/imine metathesis with
(
8) (a) Rocklage, S. M.; Schrock, R. R. J. Am. Chem. Soc. 1980, 102,
1
a to give the expected olefin product. In this case, however,
7
1
808-7809. (b) Rocklage, S. M.; Schrock, R. R. J. Am. Chem. Soc. 1982,
the rate was significantly slower (9 d at 80 °C) and the products
were found to decompose under the reaction conditions. All
olefin product resonances exhibited trans couplings. Imines
I-Bu, I-Ph, and II-Ph did not react with 1a even after 1 week
at 60-80 °C. No indication of a competing reaction of the
imine substrate with the ancillary aryl imide ligand was observed
in any of the reactions described above.
04, 3077-81. (c) Turner, H. W.; Fellmann, J. D.; Rocklage, S.; Schrock,
R. R. J. Am. Chem. Soc. 1980, 102, 7809-7811.
(
9) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1994,
16, 2669-2670.
10) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1995,
17, 974-85.
11) Green, M. L. H.; Hogarth, G.; Konidaris, P. C.; Mountford, P. J.
1
1
(
(
Organomet. Chem. 1990, 394, C9-C15.
(
12) Meisel, I.; Hertel, G.; Weiss, K. J. Mol. Catal. 1986, 36, 159-162.
Evidence for pre-coordination of the imine substrate to 1a
prior to metathesis was observed only for imines III-Bn, III-
Pr, and IV-Pr. For example, when 1a was treated with III-
Bn the original alkylidene resonance at δ 12.1 decreased as
new resonances at δ 13.2 and 13.6 appeared. An accompanying
(13) Birdwhistell, K. R.; Gross, R.; Harris, S.; Toporek, S. presented at
the 203rd National Meeting of the American Chemical Society; 1993; INOR
O152.
(
14) Jolly, M.; Mitchell, J. P.; Gibson, V. C. J. Chem. Soc., Dalton Trans.
992, 1331-1332.
15) Bell, A.; Clegg, W.; Dyer, P. W.; Elsegood, M. R. J.; Gibson, V.
C.; Marshall, E. L. J. Chem. Soc., Chem. Commun. 1994, 2247-2248.
1
(
2
downfield shift in the sp C-H imine resonance, from δ 7.28

Catalytic CdN Bond Formation
J. Am. Chem. Soc., Vol. 120, No. 32, 1998 8037
Table 2. Alkylidene/Imine Metathesis
imine
III-Pr
R/coordc
alkylidene
complex
I-Pr
Ra
R/slow
NR
I-Bu
NRb
I-Ph
NRb
II-Ph
NRb
III-Bn
R/coordc
IV-Pr
R/coordc
IV-Bu
R/decd
1a
1b
1c
e
a
R ) reaction. b NR ) no reaction. c Precoordinates prior to reaction. d Products decompose. e Reacts significantly slower than 1a.
Scheme 2
to 7.45, was also observed. Similar alkylidene shifts were
observed in the presence of imines III-Pr and IV-Pr. The
resonances associated with these putative intermediates de-
creased at the same rate as product olefin was produced.
Imide/Imine Metathesis. Bis(imides) of the type (DME)-
Cl2Mo(dNR)2 (3a, R ) 2,6-diisopropylphenyl; 4, R ) t-Bu)
react metathetically with imines to give a mixed bis(imide) and
a new imine. This exchange is more relevant to catalytic imine
metathesis than alkylidene/imine exchange because the nature
of the active site is not changed; it remains an imide and could,
therefore, be part of the cycle. A representative reaction of this
type will be described in detail. Treatment of bis(imide) 3a
with 0.8 equiv of imine II-Ph in C6D6 at 100 °C yielded
1
metathesis products II-Ar and 5 as observed by H NMR
spectroscopy (Scheme 2). After 18 d the ratio of II-Ph and
1
II-Ar was ca. 1:1. H NMR data for a reaction of this type are
shown in Figure 2. The identity of II-Ar was confirmed by
GC/MS analysis of the reaction mixture and by comparison of
1
the H NMR spectrum with that of an authentic sample. The
identity of the mixed bis(imide) product 5 was confirmed by
1
comparison of the H NMR spectrum with that of an indepen-
dently prepared sample (prepared by reaction of 3a with phenyl
2
a
isocyanate).
Complex 3a was then treated with an excess of imine since
the slow rate of this nearly stoichiometric experiment made the
determination of an equilibrium point difficult. The reaction
of 3a with 12 equiv of II-Ph at 85 °C reached equilibrium,
producing 1.4 equivalents of product imine II-Ar, after 8 days.
If each imide ligand is assumed to react independently, a Keq
of 1.1 ( 0.2 can be calculated for this process. Inversion of
the stoichiometry, 9 equiv of 3a to 1 equiv of imine II-Ph,
slowed the reaction to the extent that only a trace of product
imine II-Ar was observed after 8 days at 85 °C. Bis(arylimide)
3
a also reacted metathetically with imine I-Pr (Scheme 2).
Bis(tert-butylimide) 4 reacted with imines II-Ph and I-Pr at
1
Figure 2. Isopropyl and tert-butyl region of the H NMR spectra in
of an in situ reaction of bis(imide) 3a with 4 equiv of II-Ph. (A)
immediately after mixing and (B) after 3 d at 85 °C. Approximately
10% of the imine has been consumed. Spinning sidebands (ss) are
visible in spectrum B.
a faster rate than bis(arylimide) 3a (Scheme 3). To qualitatively
determine relative reaction rates of complexes 3a and 4 each
catalyst was treated in parallel experiments with 14 equiv of
6 6
C D
1
imine I-Pr. The reactions were monitored by H NMR

8038 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Cantrell and Meyer
Scheme 4
(imide) 3b showed no reactivity toward imines (nor decomposi-
tion), even though precedent would predict that it should be
the most activated in the series, confirms the anomalous nature
of the behavior of 3c.
Catalytic Imine Metathesis. Both of the bis(imide) com-
plexes that underwent imide/imine metathesis were also found
to be imine metathesis catalysts. For example, when imines
I-Pr and II-Ph were combined with 4 mol % of bis(tert-
butylimide) 4, at 85 °C in C6D6, the metathesis products, imines
I-Ph and II-Pr, were produced (Scheme 4). After 22 h,
equilibrium was achieved and the imines were present in a 1:1:
Figure 3. Relative rates of reaction for bis(imides) 3 and 4.
Scheme 3
1
:1 ratio. The imide/imine metathesis products, I-Bu and II-
Bu, were also identified in the reaction mixture. Bis(arylimide)
a also catalyzed the metathesis of I-Pr and II-Ph. However,
3
the reaction rate was much slower, requiring 4 days at 85 °C to
reach equilibrium. The alkoxy-substituted bis(imide) com-
plexes, 3b-d, did not catalyze the metathesis of imines, nor
did the alkylidene complexes 1a or 1b. The relative catalytic
activity of the mixed bis(imide) products 6 and 8 was also
investigated by generating them in situ from 3a and 4. Since
the reactions to generate these species require a significant
amount of time to reach equilibrium, we employed 50/50
mixtures of the homo- and mixed-bis(imide) complexes. Imines
I-Pr and II-Ph were added to these mixtures (3a + 6 or 4 + 8,
prepared as in relative rate studies; see Figure 3). In each case
imine metathesis with the catalyst mixtures was much faster
than with the simple homo bis(imides). Catalysts 3a + 6
equilibrated imines I-Pr and II-Ph in 10 h, and catalysts 4 +
spectroscopy until 0.5 equiv of product imine, I-Ar or I-Bu,
respectively, was produced. Bis(imide) 4 required only 20 h
at 85 °C while 3a required ∼4 days to reach the same point
(Figure 3).
It is important to note that, in general, the reaction mixtures
containing 3a and 4 were stable under the reaction conditions
employed and no significant concentration of any species other
than the reagents and products was present. In particular, no
resonances were observed that would be consistent with the
presence of olefins, diazametallacycles, or imines coordinated
to the metal center. Also significant is the fact that, without
reagents present, the bis(imide) catalysts 3a and 4 were stable
at 100 °C in C6D6 for >2 weeks. In the presence of imines,
decomposition to form insoluble materials was observed in the
reaction mixtures only after prolonged periods of heating (>10
days at 85 °C).
8
required only 5 h.
In control experiments the uncatalyzed and acid-catalyzed
reactions of imines I-Pr and II-Ph were studied for comparison
to the metal-mediated reactions. We found that in C6D6, under
reaction conditions designed to mimic those used for the metal-
18
mediated reaction, these imines undergo Brønsted and Lewis
acid (ZnI2 and TiCl4) catalyzed exchange. HCl-catalyzed
exchange reached equilibrium in days at room temperature; ZnI2
required days at 60 °C; and TiCl4 required hours at 80 °C.
Without either molybdenum or acid catalyst only a small amount
of exchange (<5%) was observed when the imines were heated
at 80 °C for 2 weeks.
The DME ligand is somewhat labile; addition of DME to
either 3a or 4 resulted in broadening and coalescing of the free
1
and bound DME resonances in the H NMR spectrum. In
Imide/Imide Metathesis. Not surprisingly, we observed
concurrent imide/imide exchange in the reaction mixtures for
both imide/imine and alkylidene/imine metatheses. This type
of reaction was originally reported for alkoxide derivatives of
reaction mixtures of bis(imide) 3a with imines, however, no
broadening and only minor changes in chemical shift were
observed. In the case of bis(imide) 4, some differences in line
width and chemical shift were noted although free DME was
not observed.
14
3 and 4 by Gibson et al. To verify that this comproportionation
was also possible for the DME-ligated dichloride complexes
3a and 4, equimolar amounts of these two were mixed.
Exchange of the dNR groups was observed within 3 h at 70
°C (Scheme 5). Equilibrium was attained within 24 h. The
equilibrium constant was ca. 12 at 25 °C, which is similar in
magnitude to the K60°C ) 25 reported by Gibson and co-workers
for alkoxide analogues. To complete the series, the metathesis
of the alkoxide complex ((CF ) CH CO) Mo(dNAr) (3b) with
To investigate electronic effects of the ancillary ligands,
several bis(imide) bis(alkoxide) complexes, (RO)2Mo(dNAr)2
(3b, R ) C(CF3)2CH3; 3c, R ) t-Bu; 3d, R ) 2,6-diisopropy-
lphenyl), were prepared. Complexes 3b and 3d exhibited no
reaction with imine I-Pr, while complex 3c, as reported
previously, does react to give imine I-Ar.2 This latter reaction
proceeds through an initial decomposition of 3c that involves
the tert-butoxide ligands and is not, therefore, relevant to the
current discussion. The details of this reaction pathway will
be described elsewhere.17 The fact that the fluoroalkoxide bis-
a
3
2
3
2
2
(DME)Cl2Mo(dN-t-Bu)2 (4) was also examined. As expected,
the reaction proceeded to an equilibrium mixture of the imide
19
exchange products.
(
16) Bazan, G.; Schrock, R. R.; O’Regan, M. Organometallics 1991,
0, 1062-1067.
17) Yahner, A. C.; Cantrell, G. K.; Meyer, T. Y. Manuscript in
preparation.
(18) Toth, G.; Pinter, I.; Messmer, A. Tetrahedron Lett. 1974, 9, 735-
38.
(19) Cl-alkoxide exchange is also possible. We cannot rule out this
pathway based on the data from this single experiment.
1
(

Catalytic CdN Bond Formation
J. Am. Chem. Soc., Vol. 120, No. 32, 1998 8039
Scheme 5
Figure 4. Possible intermediate metallacycles for imide/imine me-
tathesis.
Scheme 6
Scheme 7
pathway, though the lack of space and the competition of imide
lone pairs for the empty orbital reduce its probability.22
In the first step imine coordination is also postulated.
Although we do not observe this in the imide/imine metathesis
reaction, there is clear evidence for precoordination prior to
alkylidene/imine metathesis by complex 1a. The NMR spectra
of the observed imine adducts are qualitatively similar to those
reported for coordination of PMe3 to this type of complex.23
The fact that precoordination was not observed in the bis(imide)
systems can be explained in terms of effective competition by
chelating DME. If DME is slow to disassociate and the imine
does not act as a good trap for the unsaturated metal center,
then the imine-coordinated intermediate would not be observ-
able. We do not have sufficient data to comment on the exact
order of imine coordination and DME dissociation.
After coordination, the olefin analogy would predict a [2+2]
addition to give a diazametallacycle. Although we do not
directly observe such a product, the regiospecificity of the
reaction can be explained in terms of this type of intermediate.
Alkylidene/imine and imide/imine metatheses proceed to give
exclusively the imide product. This regiospecificity is consistent
9
with prior observations in related systems and is not surprising
given the electrophilic nature of these early transition metal
complexes. If we assume that some sort of metallacyclic species
lies on the reaction coordinate for this reaction we can explain
this preference in terms of intermediate/transition state stabiliza-
tion. A 2,4-diazametallacycle (A) would be able to stabilize
the expected distribution of partial charges more effectively than
would a 2,3-diazametallacycle (B) (Figure 4). Similar argu-
ments have been offered by Crowe and co-workers to explain
the selectivity observed in the cross metathesis of polar acyclic
Discussion
Mechanism. The olefin metathesis reaction provides a
reasonable starting point for thinking about imine metathesis.
Based on the substantial body of research in this area,
generally accepted that the olefin metathesis reaction proceeds
via initial coordination of olefin, followed by a [2+2] addition.
The resulting metallacycle may then cleave either degeneratively
or productively to give the new alkylidene and olefin products
3
,20
it is
2
4
olefins.
The fact that no metallacycle is observed in the
reaction mixture does not contradict this mechanism since in
(
Scheme 6).
olefin metathesis by complexes 1a-c metallacyclobutanes are
1
rarely observed. It is reasonable to postulate that bis(imide)
An analogous mechanism can be proposed for heteroolefin
complexes would exhibit a similar preference for an unsaturated
resting state. Since the final steps of the reaction are simply
the reverse of the first two steps, they will not be discussed
separately.
We also note that the steric requirements of the imide/imine
reaction are consistent with the direct reaction of the imide and
imine groups to form a metallacycle. The 5-fold difference in
the time required for bis(imide) complexes 3a and 4 to reach
equilibrium in an imide/imine exchange is most probably due
to the variation in size of the two imide nitrogen substituents.
This type of steric dependence would not be expected if the
mechanism involved disassociation of a dNR unit from the
metal center as the first, rate-determining step.
metathesis. The imide/imine exchange of bis(imide) 4 will be
used for illustration. Imine coordination, accompanied by partial
DME dissociation, is followed by formation of a metallacycle.
Productive cleavage of the metallacycle then gives the imide
and imine products. DME can then recoordinate to give the
resting state of the imide complexes (Scheme 7). The mech-
anism for alkylidene/imine metathesis would be essentially the
same, except for the steps involving DME.
By examining the results obtained from the entire set of
dNR transfer reactions, we can evaluate the applicability of
this mechanism to our reaction. In the first step, DME
1
dissociation is postulated. Although no direct H NMR evidence
supports this supposition, the examination of a space-filling
model of the closely related mixed bis(imide) (DME)Cl2Mo-
To definitively determine the mechanism it would be desirable
to undertake detailed kinetic studies. Unfortunately, kinetics
(
dN-t-Bu)(dNAr), which was prepared and structurally char-
21
acterized by Gibson and co-workers, shows little space for
an incoming ligand. We cannot rule out a purely associative
22) A coordination site could also be generated by Cl- dissociation.
Experiments to date do not distinguish between Cl and DME dissociation.
23) Schrock, R. R.; Crowe, W. E.; Bazan, G. C.; DiMare, M.; O’Regan,
(
(
(
20) H e´ risson, J.-L.; Chauvin, Y. Macromol. Chem. 1970, 141, 161.
M. B.; Schofield, M. H. Organometallics 1991, 10, 1832-1843.
(24) Crowe, W. E.; Goldberg, D. R. J. Am. Chem. Soc. 1995, 117, 5162-
5163.
(21) Copley, R. C. B.; Dyer, P. W.; Gibson, V. C.; Howard, J. A. K.;
Marshall, E. L.; Wang, W.; Whittle, B. Polyhedron 1996, 15, 3001-3008.

8040 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Cantrell and Meyer
are challenging with this bis(imide) system due to the presence
of a second reactive imide functionality. Upon activation of
both sites the metal is partitioned among three very similar
species in solution; it is very difficult to quantify them due to
Scheme 8
1
the overlap of H NMR resonances. More problematic for the
interpretation of kinetic results is the fact that bimolecular imide/
imide exchange, which equilibrates the mixed and homo bis-
(imides), is on a time scale similar to that of the imide/imine
metathesis reaction. This disproportionation/comproportionation
reaction also precludes the isolation of any of the mixed bis-
(imide) products from the reaction mixtures and dictates that
these reactions be monitored in situ.
Despite the difficulties in acquiring accurate kinetic data, we
have qualitatively determined that the rate of alkylidene/imine
and imide/imine metathesis decreases with increasing electron-
donating character of the ancillary alkoxide ligands. The fact
that the same activity order (1a > 1b > 1c) was reported by
Schrock and co-workers for olefin metathesis with these
complexes is further evidence for some correlation of the olefin
and imine mechanisms.1 There is a significant difference in
the ancillary ligand dependence, however. The alkylidene/imine
exchange does not occur for 1c and imide/imine exchange will
not proceed for any of the three; only complexes with the least
electron-donating ligands, chlorines, underwent metathesis. The
deactivating effect of the alkoxide ligands explains the inertness
of the “ancillary” imide group of the alkylidene complexes 1a-c
toward imide/imine exchange.
solutions of two imines with catalysts 3a and 4 were reduced
by factors of 1/10 and 1/4, respectively, when the catalysts were
preinitiated by reaction with an imine with a smaller N
substituent. In the reaction depicted in Scheme 8, for example,
the reaction time decreased when 4 was partly converted to 7
prior to the addition of the other imine. Moreover, the decrease
in reaction times concurs qualitatively with the relative decreases
in N-substituent size; the change from 2,6-diisopropylphenyl
to n-propyl (105 h to 10 h) is more dramatic than the change
from tert-butyl to n-propyl (22 h to 5 h).
The preference for a more electrophilic center raises serious
questions about the validity of adopting the olefin metathesis
mechanism to explain imide/imine exchange. Although pre-
coordination and metallacycle intermediates appear viable, there
may very well be significant differences in their nature and
formation. An imine, by virtue of its lone pair and its inherent
polarity, is quite different from an olefin and would be expected
to exhibit differences in coordination mode and in reaction
pathway. There may, for example, be significantly less syn-
chronicity in the formation of the metallacycle than is apparently
the case for alkylidene/olefin metathesis. The data collected
to date on these systems do not yet allow us to make these
distinctions nor do they rule out alternate mechanisms involving
other low-concentration intermediates or catalysts.
One reaction that does not appear to affect the chemistry
significantly is dimerization of the catalyst. Although it is likely
that such dimers are present, both Gibson and Chisholm have
concluded that, despite the imide/imide exchange, this class of
complexes are primarily monomeric in solution.1
4,25,26
Our
observations concur with theirs. The lack of an irreversible
dimerization is significant since, as was observed in the
zirconacene imide system reported by Bergman and co-
9
,10
workers, it is one of the most likely termination steps for an
unsaturated imide catalyst. The bis(imide) catalysts described
in this paper have two forms of steric protection, chelating DME
and the other imide substituent. We have already discussed
the role of DME, but the steric protection offered by the second
imide is more subtle since this group is also an active catalytic
site. We speculate that after initial activation of imide 1 on a
metal center, it becomes the active site while imide 2, which is
sterically less accessible, becomes ancillary. Only at long
reaction times will significant conversion of imide 2 to an active
site occur. Until this second exchange happens, the catalyst
enjoys a exceptionally sterically protected environment. Un-
fortunately, the same steric protection may also contribute to
the low overall rate of metathesis.
Catalysis. Based on our understanding of the imide/imine
metathesis, we have proposed a catalytic cycle for imine
metathesis (Scheme 8). In an initiation step, the initial bis-
(
imide) complex 4 undergoes imide/imine metathesis with II-
Ph and I-Pr to generate one of the catalytically active species,
or 8. The mixed bis(imide) complexes 7 and 8 then react
7
with the complementary imine in the propagation step to give
the observed products. All reactions are reversible such that
an equilibrium mixture of all possible imine combinations is
expected. Also, the second site must be assumed to participate
in the cycle to some degree although for the sake of clarity
both the second site reaction and imide/imide metathesis have
been omitted from the cycle.
A key question arises when thinking about catalytic imine
metathesis: How do we know that it is metal mediated? Since
we know that metathesis is catalyzed by conventional acids, it
could be hypothesized that adventitious acid is responsible for
the observed reactions. Both the results of the initiation studies,
which show the presence of imide/imine metathesis products,
and the reproducibility of reaction times in independently
prepared samples are inconsistent with this hypothesis. It is
The accuracy of our proposed catalytic cycle is supported
by experimental observations. The most important of these is
the presence of initiation products, I-Bu and II-Bu in Scheme
8
, in the reaction mixtures. Since these imines can only arise
through imide/imine metathesis, and since the initiation studies
show that they are the slowest of the imide/imine combinations,
we can be confident that the proposed catalytic pathway is
operating in the reaction mixture.
(
25) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C.;
Ratermann, A. L. J. Am. Chem. Soc. 1981, 103, 1305-1306.
26) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Raterman, A. L. Inorg.
Chem. 1982, 21, 978-982.
The proposed initiation step in this pathway is also consistent
with our data. The times required to equilibrate identical
(

Catalytic CdN Bond Formation
J. Am. Chem. Soc., Vol. 120, No. 32, 1998 8041
Scheme 9
work is necessary before the relationship between dNR transfer
and dCR transfer by metathesis will be understood.
Conclusion
The metathesis reaction can now be used to form CdN bonds
catalytically. We have discovered that molybdenum bis(imide)
complexes of the type (DME)Cl2Mo(dNR)2 metathesize imines
catalytically, but do not suffer from termination by irreversible
dimerization. We can, therefore, begin to pursue the potential
applications of this reaction to the synthesis of polymeric
materials and small molecules. In addition, by studying
alkylidene/imine, imide/imine, imine, and imide/imide metathe-
ses, we have discovered that the catalytic reaction is part of a
general class of reactions that involve the transfer of the dNR
group.
clear that the reaction does involve the metal center and is not
due simply to external acid.
Experimental Section
There does exist another potential Lewis acid-catalyzed
pathway that cannot be as easily dismissed, however. In this
other process, it is assumed that the molybdenum complex itself
is the Lewis acid, activating the coordinated imine for nucleo-
philic attack by an external imine or trace amine (Scheme 9).
Interestingly, imide steric bulk and alkoxide ancillary ligands
would be expected to affect this pathway in a qualitative fashion
similar to that of our proposed catalytic pathway. Which
pathway the reaction follows, metal-mediated imide/imine
metathesis or Lewis acid-assisted metathesis, should depend
simply on the rate of internal imide vs external imine attack on
the coordinated imine. Although at this point the data are
insufficient to rule out some participation of this Lewis acid
pathway, it is clear from the evidence presented above that our
proposed imide-mediated catalytic pathway is competitive, if
not dominant. Moreover, the apparently low disassociation
constant for DME would be expected to favor the intramolecular
imide attack over external imine attack. Future studies with
cyclic imines should enable us to differentiate between the two
pathways since the Lewis acid pathway would produce cyclic
oligomers and would not be “living” in that there would be
more chains than metal catalyst initiators.
dNR Group Transfer. In the course of these studies the
relationships between the four types of dNR transfer reactions
have been established. For example, although imines react with
both the molybdenum alkylidene and molybdenum imide
functionality, the reactions have different ancillary ligand
requirements. The imide/imine exchange apparently requires
a more electrophilic metal center (only proceeds with Cl ligands)
than alkylidene/imine metathesis. Earlier calculations by Rappe´
and Goddard on oxo/olefin metathesis suggest that the more
electrophilic center may be necessary to stabilize the metalla-
cyclic transition state.27 Although that may be an important
factor, we suggest that the electrophilicity is also necessary to
activate the coordinated polar imine substrate and that this
activation is likely to be important based on the known Lewis
acid-catalyzed organic chemistry of heteroolefins.
General Methods. Unless otherwise noted, the following methods
were utilized for all experiments described in this report. Manipulations
were carried out under an inert nitrogen atmosphere in a Vacuum
Atmospheres drybox or with standard Schlenk or vacuum line
techniques. Reagent grade ether and toluene were distilled from sodium
benzophenone ketyl under nitrogen. Reagent grade hexane was washed
with sulfuric acid, passed through a column of activated alumina, and
distilled from sodium benzophenone ketyl under nitrogen. Reagents
were purified by standard methods unless otherwise noted. Benzene-
6
d was distilled from sodium benzophenone ketyl and stored in a glass
1
vessel equipped with a Teflon stopcock. (RO)
Ar ) 2,6-diisopropylphenyl; 1a, R ) C(CF CH
C(CF )(CH , R′ ) t-Bu), (DME)Cl Mo(dNR)
diisopropylphenyl; 4, R ) t-Bu), and (RO)
C(CF CH
prepared as described in the literature. (RO)
2,6-diisopropylphenyl; 1c, R ) t-Bu; R′ ) C(Ph)(CH
2
Mo(dCHR′)(dNAr)
3
(
)
3
)
2
, R′ ) t-Bu; 1b, R
1
3
3
)
2
2
2
(3a, R ) 2,6-
28
2
Mo(dNAr)
2
(3b, R )
2a
3
)
2
3
; 3c, R ) t-Bu; 3d, R ) 2,6-diisopropylphenyl ) were
Mo(dCHR′)(dNAr) (Ar
)) was
2
)
3
)
2
1
purchased from Strem Chemicals. H NMR spectra were obtained on
a Bruker AF 300 MHz spectrometer at 300 MHz. All spectra were
6
obtained in benzene-d at 25 °C. Mass spectroscopic data were obtained
on a Hewlet Packard Series 5890 GC/5971A MS.
NMR Experiments. Reactions were monitored by NMR spectros-
copy since equilibrium mixtures of starting materials and products were
obtained in all cases. Isolation of organometallic products was not
possible. NMR assignments are reported for significant species
although phenyl region overlap prevents complete assignments in many
cases. Hexamethylbenzene was used as an internal standard. With
the exception of a few mixed bis(imide) complexes all species were
identified by a comparison with authentic samples.
Typical Experimental Procedure for Alkylidene/Imine Metath-
esis. Reaction of (CF
PhHCdNPr. Complex 1b (0.009 g, 0.015 mmol, 1 equiv) was
dissolved in ca. 0.5 mL of C in an NMR tube equipped with a Teflon
stopcock. Hexamethylbenzene in C and imine I-Pr (ca. 9 equiv)
3 3 2 2
(CH ) CO) Mo(dCH(t-Bu))(dNAr) with
6 6
D
6 6
D
were added. The sample was heated for 4 d at 60 °C, 1 d at 70 °C,
1
and 2 d at 85 °C. The progress of the reaction was monitored by H
1
NMR spectroscopy. H NMR (C
6
D
6
) of trans-(Ph)HCdCH(t-Bu)
(unisolated) δ 6.34 (d, 1, (Ph)HC, JHH ) 15.6 Hz), 6.19 (d, 1, CH(t-
Bu), JHH ) 16.3 Hz), 1.03 (s, 9, CH(t-Bu)). The resonances for the
phenyl group and for the mixed bis(imide) were not assignable in this
case due to overlap with substrate and product resonances.
We are also intrigued by the relationship of imine to olefin
metathesis. In particular, the relatively slow speed of the imide/
imine reaction compared with the alkylidene/olefin reaction is
of interest if imine metathesis is to be used synthetically.
Although arguments about the relative ground-state stabilization
of molybdenum(VI) imides vs alkylidenes could be made and
are probably relevant, we cannot judge the real reaction rate
until we can factor out or remove competition from DME. More
Typical Experimental Procedure for Imide/Imine Metathesis.
Reaction of (DME)Cl
plex 4 (0.005 g, 0.012 mmol, 1 equiv) was dissolved in ca. 0.5 mL of
in an NMR tube equipped with a Teflon stopcock. Hexameth-
ylbenzene in C and imine II-Ph (ca. 14 equiv) were added. The
2 2
Mo(dN-t-Bu) with (t-Bu)HCdNPh. Com-
6 6
C D
6 6
D
sample was maintained for 1 d at room temperature to verify the
stoichiometry and stability of the reaction mixture. The sample was
heated and maintained at 85 °C for 11 d. The progress of the reaction
(
27) Rappe, A. K.; Goddard, W. A., III, J. Am. Chem. Soc. 1982, 104,
(28) Jolly, M.; Mitchell, J. P.; Gibson, V. C. J. Chem. Soc., Dalton Trans.
1992, 1329-1330.
4
48-456.

8042 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Cantrell and Meyer
was monitored by H NMR spectroscopy. 1H NMR (C
1
6 6
D ) of t-BuNd
2
ylbenzene (0.075 mL of a 0.5 M standard solution) and (DME)Cl -
CH(t-Bu) (unisolated) δ 7.41 (s, 1, CH(t-Bu), 1.15 (s, 9, t-BuN). The
Mo(dNAr) (0.005 g, 0.008 mmol, 1 equiv) were added. The NMR
2
CH(t-Bu) resonance could not be assigned due to overlap with the tert-
tube was flame sealed. The sample was maintained for 48 h at room
temperature to verify the stoichiometry and stability of the reaction
mixture. The sample was then heated and maintained at 80-85 °C
1
butyl resonance of II-Ph. H NMR (C
(
D
6 6
) of Mo(NPh)(N-t-Bu)Cl
OCH
2
-
-
DME) (unisolated) δ 1.28 (s, 9, N-t-Bu), 3.35 (s, 6,CH
3
2
1
1
CH OCH ), 3.24 (s, 4, CH OCH CH OCH
2
3
3
2
2
3
). Phenyl resonances could
for 8 d. The progress of the reaction was monitored by H NMR. H
NMR (C ) of imide/imine metathesis products: ArNdCHPh (uniso-
lated) δ 8.0 (s, 1, CHPh), 3.13 (sept, 2, CHMe , JHH ) 6.9 Hz), 1.16
(d, 12, CHMe , JHH ) 6.9 Hz); ArNdCH(t-Bu) (unisolated) δ 7.25 (s,
1, CH-t-Bu), 3.05 (sept, 2, CHMe , JHH ) 6.9 Hz), 1.17 (d, 12, CHMe
HH ) 6.9 Hz). (Note that ArNdCH(t-Bu) is produced in smaller
not be definitively assigned due to the complexity of the spectrum in
this region. DME resonances in the reaction mixture collapse to a very
broad singlet at δ 3.25 by the end of the reaction.
6 6
D
2
2
Reaction of (DME)Cl
2
Mo(dNAr)
2
with (t-Bu)HCdNPh. The
2
2
,
experimental procedure was reported in ref 2a.
Typical Experimental Procedure for Imide/Imide Metathesis.
J
amounts than ArNdCHPh possibly due to differences in the rates of
reaction of the starting material imines with the catalyst.) The aryl
resonances could not be assigned due to overlapping resonances of the
Reaction of (DME)Cl
plexes 3a (0.007 g, 0.017 mmol, 1 equiv) and 4 (0.010 g, 0.016 mmol,
equiv) were dissolved in ca. 0.5 mL of C . Hexametheylbenzene
in C was added as an internal standard. The sample was maintained
2 2 2 2
Mo(NAr) with (DME)Cl Mo(N-t-Bu) . Com-
1
D
6 6
other products: 1H NMR (C
6 6
D ) of imine metathesis products PhNd
D
6 6
CHPh (unisolated) δ 8.12 (s, 1, CHPh), 7.81 (m, 2, Ph), 7.0-7.18 (m,
Ph); (n-Pr)NdCH(t-Bu) (unisolated) δ 7.33 (s, 1, CH(t-Bu), 3.26 (t, 2,
for 15 h at room temperature to verify the stoichiometry and stability
of the reaction mixture. The sample was heated and maintained at 70
NCH
2
CH
2
CH
3
, JHH ) 6.9 Hz), 1.61 (m, 2, NCH
2 2 3
CH CH ), 0.86 (t, 3,
°
C for several weeks. The progress of the reaciton was monitored by
NCH
2
CH
2
CH
3
, JHH ) 7.5 Hz), 1.02 (s, 9, CH(t-Bu)).
1
1
H NMR spectroscopy. H NMR (C
Bu) (unisolated) δ 4.32 (sept, 2, CHMe
CH OCH CH OCH ), 3.19 (s, 4, CH OCH
CHMe , JHH ) 6.8 Hz), 1.26 (s, 9, N-t-Bu). Phenyl resonances could
6
D
6
) of (DME)Cl
, JHH ) 6.8 Hz), 3.41 (s, 6,
CH OCH ), 1.42 (d, 12,
2
Mo(NAr)(N-t-
Catalytic Metathesis of PhNdCH(t-Bu) and (n-Pr)-NdCHPh
with (DME)Cl Mo(dN-t-Bu) The experimental procedure was
reported in ref 2b.
2
2
2
.
3
2
2
3
3
2
2
3
2
not be definitively assigned due to the complexity of the spectrum in
this region. Resonances correspond to those reported by Copley and
Gibson.
Typical Experimental Procedure for Catalytic Imine Metathesis.
Catalytic Metathesis of PhNdCH(t-Bu) and (n-Pr)NdCHPh with
Acknowledgment. We thank the National Science Founda-
tion (CHE-9624138) and Dupont for financial support. G.K.C.
thanks the Pennsylvania Space Grant Consortium for fellowship
support. We also thank Robert G. Bergman and Michael C.
Hopkins for valuable discussions.
2
1
(
DME)Cl
2
Mo(dNAr)
2
. Imines I-Pr (12 equiv) and II-Ph (12 equiv)
in an NMR tube. Hexameth-
were dissolved in ca. 0.75 mL of C
D
6 6
JA981272T