Azaheteroalkene metathesis: Reaction of imines with molybdenum(VI) bis(imide) complexes

Article abstract of DOI:10.1039/a702780j

Bis(imide) complexes (dme)Cl₂Mo(=NR)₂ (R = Bu^t, C₆H₃Prⁱ₂-2,6) and (Bu^tO)₂Mo(=NAr)₂ (Ar = C₆H₃Prⁱ₂-2,6

Full text of DOI:10.1039/a702780j

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Azaheteroalkene metathesis: reaction of imines with molybdenum(vi)
bis(imide) complexes
Gidget K. Cantrell and Tara Y. Meyer*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
Bis(imide) complexes (dme)Cl2Mo(NNR)2 (R
=
But,
increase in rate; the optimum reaction conditions have not yet
been determined.
C6H3Pri2-2,6) and (ButO)2Mo(NNAr)2 (Ar = C6H3Pri2-2,6)
undergo imide/imine metathesis with PhNNCH(But) or
(Prn)NNCHPh.
These reactions proceeded cleanly. No evidence for pre-
coordination or for metallacyclic intermediates was observed by
1H NMR spectroscopy for any of the stoichiometric combina-
tions;¶ nor were alkenes detected. Competing intermolecular
imide/imide metathesis,11 which in these systems equilibrates
all bis(imide) complexes, prevented isolation and complete
characterization of the mixed imide product (dme)Cl2Mo(N
Transition-metal-mediated reactions offer many potential ad-
vantages over traditional approaches to carbon–heteroatom
bond formation including selectivity, mild reaction conditions,
varied functional group tolerances, and increased reaction rates.
A promising strategy for the development of these coupling
reactions involves the extension of metal–alkylidene-mediated
alkene metathesis, which is used extensively in polymer
synthesis and increasingly in the preparation of natural
products,1–3 to heteroalkenes. We are particularly interested in
carbon–nitrogen bond formation through metal–imide-medi-
ated imine metathesis.
1
NAr)(NNPh) 3. H NMR resonances associated with 3 were,
however, identified in the reaction mixture by comparison with
those from a spectrum of an authentic sample.∑,**
Imide/imine metathesis is remarkably general for this class of
bis(imides). The reaction proceeds despite changes in substrate
imine, imide-nitrogen substituent, or ancillary ligand (Table 1).
Specifically, imides 1 and (dme)Cl2Mo(NNBut)2, 5,10 react with
imine 2. Imides 1, 5, and (ButO)2Mo(NNAr)2, 6,12 also react
with N-propylbenzaldimine 8, to give the expected exchange
products. Only the sterically crowded (ArO)2Mo(NNAr)2, 7, is
unreactive.†† The fact that imide/imine metathesis occurs in all
but one of the examples investigated to date suggests that these
catalysts can be ‘tuned’ by varying the ancillary ligands.
One goal of this work was to identify a catalyst that does not
undergo significant termination reactions. Compelling evidence
for a lack of such termination reactions in the systems we have
described can be found by examining the 1H NMR spectra; no
significant amount of any species other than those described
above was present. Specifically, we did not observe any stable
Reports of catalytic metathesis of azaheteroalkenes are rare
and no system of general utility has been described.4,5
Carbodiimides have been metathesized by Cl4MoNNR6 and
isocyanates by [(h-C5H4Me)Mo(NNR)2]2.7 Only one example
exists, however, of the metathesis of the less activated, but more
versatile imine substrates. The early transition-metal imide
complex,
(h-C5H5)2Zr(NNBut)(thf),
reacted
with
N-phenylimines to give diazametallacycles that underwent
further metathetical exchange with external imines, presumably
via imide intermediates.8,9 Although this reagent is an imine
metathesis catalyst, it suffers from significant limitations, the
reaction is inhibited by high imine concentrations and the
reagent is reported to undergo a competing dimerization that
deactivates it towards further reaction. Herein, we identify a
class of metal complexes that exhibit the key step of catalytic
imine metathesis, imide/imine metathesis, without such sub-
strate inhibition or terminal dimer formation.
Table 1. Imide/imine metathesis
NR1
NR1
NR3
NR1
R2
H
R2
H
NR3
+
NR1
+
(dme)Cl2Mo
(dme)Cl2Mo
Treatment of (dme)Cl2Mo(NNAr)210 (Ar = C6H3Pri2-2,6) 1,
with 0.8 equiv. of tert-butylanil 2† in C6D6 at 100 °C produced
metathesis product 4 over the course of several days as observed
by 1H NMR spectroscopy (Scheme 1). After 18 days the ratio of
2 to 4 was ca. 1:1.‡ When the reaction was repeated with 12
equiv. of imine 2 to 1 equiv. of bis(imide) 1, the reaction
proceeded much more rapidly, producing 1.4 equiv. of imine 4
after 8 days at 85 °C.§ Inversion of the stoichiometry, 9 equiv.
of 1 to 1 equiv. of imine 2, slowed the reaction to the degree that
only a trace of product imine 4 was observed after 8 days at
85 °C. Unambiguous identification of product imine 4 was
achieved by comparison of the 1H NMR spectrum with that of
an authentic sample and by GC–MS analysis of the reaction
mixture. Increasing the temperature results in a concomitant
Bis(imide)
Substrate imine
But
Product iminea
But
NAr
H
(dme)Cl2Mo(=NAr)2
1
2
8
NPh
H
Ph
NAr
H
Ph
N
H
But
NBut
H
But
(dme)Cl2Mo(=NBut)2
5
2
8
NPh
H
Ph
Ph
NBut
N
H
H
Ph
NAr
H
Ph
(dme)Cl2Mo(=NAr)2 + PhN=CHBut
(ButO)2Mo(=NAr)2
(ArO)2Mo(=NAr)2
8
8
N
N
6
7
1
2
H
Ph
(dme)Cl2Mo(=NAr)(=NPh) + ArN=CHBut
No reaction
H
3
4
Scheme 1
a Characterized by 1H NMR spectroscopy and/or GC–MS analysis.
Chem. Commun., 1997
1551

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§
Typical NMR experiment: reaction of (dme)Cl2Mo(NAr)2 1 and
imide-bridged dimers, a particular concern since similar dimer
formation limited the utility of (h-C5H4Me)2Zr(NNBut)(thf) as
an imine metathesis catalyst.8,9 Previous workers, who have
examined these molybdenum bis(imide) complexes for other
applications, have also found them to be primarily monomeric
in solution. In a particularly relevant and elegant study, Gibson
and coworkers observed intermolecular imide/imide exchange
of (ButO)2Mo(NNBut)2 with (ButO)2Mo(NNAr)2.11 Although
the most likely mechanism for exchange would involve an
imide-bridged dimer, no spectral evidence for a significant
concentration of this intermediate could be found. Also
germane is the observation by Chisholm et al. that an analogous
bis(imide) dimer [(ButO)2Mo(NNPh)(m-NPh)]2,13 characterized
by X-ray crystallography, exhibited a monomeric solution
molecular mass.14 Based on our data and these reports we can
conclude that, although imide-bridged dimers are likely to be
present in our reaction mixture, they do not represent a
termination step for imide/imine metathesis.
Although these preliminary studies do not provide a complete
picture of the metathesis, the data do provide some insights into
the mechanism. It is clear, for example, that the imine products
do not arise from a pathway involving initial decomposition of
the metal imide. In control experiments, catalysts 1 and 5 were
observed to be stable for weeks at 100 °C in C6D6. Moreover,
integration of imide 1H NMR resonances vs. an internal
standard showed that there was very little diversion of metal–
imide complexes to insoluble or unidentified species.
We also do not believe that the reaction we are observing is
water catalyzed. No acceleration was observed when trace
water was added to the reaction mixture, nor was the reaction
retarded when the reaction mixture was pre-dried for 3 days
over molecular sieves before it was decanted and combined
with bis(imide) complex 1.
PhNNCH(But) 2. Complex 1 (0.005 g, 0.008 mmol, 1 equiv.) was dissolved
in ca. 0.5 ml C6D6 in an NMR tube equipped with a Teflon stopcock.
Hexamethylbenzene in C6D6 and imine 2 (ca. 12 equiv.) were added. The
sample was maintained for 24 h at room temp. to verify the stoichiometry
and stability of the reaction mixture. The sample was heated and maintained
at 85 °C for 18 d. The progress of the reaction was monitored by 1H NMR
spectroscopy. H NMR (C6D6) of ArNNCH(But) (unisolated) d 7.25 (s, 1,
1
CHBut), 3.05 (spt, 2, CHMe2, JCH 6.7 Hz), 1.17 (d, 12, CHMe2, JCH 6.6 Hz),
1.07 (s, 9, But); mass spectrum (EI) m/z 245 (M+), 188 (M+ 2 But). 1H NMR
(C6D6) (dme)Cl2Mo(NAr)(NPh) (unisolated) d 4.40 (spt, 2, CHMe2, JCH
7.1 Hz), 1.30 (d, 12, CHMe2, JCH 6.8 Hz). Phenyl and aryl resonances could
not be definitively assigned owing to the complexity of the spectrum in this
region.
¶ The diazametallacycle, if present, exists only in trace amounts since the
total imine concentration (substrate + product) remains nearly constant
throughout the reaction.
∑ Compound 3 was independently prepared by the reaction of bis(imide) 1
with phenyl isocyanate.
** (dme)Cl2Mo(NNPh)2 should be present as well, arising from metathesis
of both imido groups and, also, from intermolecular imide/imide metathesis.
The complexity of the phenyl region of the spectrum precludes the
assignment of resonances for this species.
†† (ArO)2Mo(NAr)2 was prepared by analogy to (ButO)2Mo(NAr)2 by
reaction of (dme)Cl2Mo(NAr)2 with 2 equiv. of LiOAr. 1H NMR (C6D6) d
6.87–7.08 (m, 12, aryl), 3.82 (spt, 4, CHMe2, JCH 6.8 Hz), 3.48 (spt, 4,
CHMe2, JCH 6.8 Hz), 1.25 (d, 24, CHMe2, JCH 6.8 Hz), 1.00 (d, 24, CHMe2,
JCH 6.8 Hz); 13C NMR (C6D6) d 160.7 (Cipso, OAr), 154.4 (Cipso, NAr),
143.6, 138.2, 127.6, 124.3, 123.9, 123.1 (aryl), 29.4 (CHMe2, OAr), 28.0
(CHMe2, NAr), 24.5 (CHMe2, OAr), 24.2 (CHMe2, NAr). Anal. Calc. for
C
48H68N2O2Mo: C, 71.87; H, 8.56; N, 3.50. Found: C, 71.91; H, 8.74; N,
3.50%.
1 For reviews of alkene metathesis see: (a) R. H. Grubbs and W. Tumas,
Science, 1989, 243, 907; (b) R. R. Schrock, Acc. Chem. Res., 1990, 23,
158; (c) A. J. Amass, in New Methods of Polymer Synthesis, ed. J. R.
Ebdon, Chapman and Hall, New York, 1991, p. 76.
2 S.-H. Kim, N. Bowden and R. H. Grubbs, J. Am. Chem. Soc., 1994, 116,
10801.
3 G. C. Fu and R. H. Grubbs, J. Am. Chem. Soc., 1992, 114, 7324.
4 W. A. Nugent, Metal-Ligand Multiple Bonds, Wiley, New York,
1988.
Like many of the alkylidene catalysts, the bis(imide) catalysts
described herein do not exhibit precoordinated nor metalla-
cyclic intermediates in their reaction with unsaturated sub-
strates.1c Further studies are required to ascertain if, in fact, the
metathesis proceeds through a [2 + 2] mechanism as would be
expected by analogy to alkene metathesis systems. These
studies are currently being pursued.
In summary, we have identified the first potential catalysts
for imine metathesis. These bis(imide) catalysts undergo a
general metathesis with imines without termination by dimer
formation.
5 For
a recent review of transition metal imido complexes, see
D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239.
6 I. Meisel, G. Hertel and K. Weiss, J. Mol. Catal., 1986, 36, 159.
7 M. L. H. Green, G. Hogarth, P. C. Konidaris and P. Mountford,
J. Organomet. Chem., 1990, 394, C9.
8 K. E. Meyer, P. J. Walsh and R. G. Bergman, J. Am. Chem. Soc., 1994,
116, 2669.
9 K. E. Meyer, P. J. Walsh and R. G. Bergman, J. Am. Chem. Soc., 1995,
117, 974.
10 H. H. Fox, K. B. Yap, J. Robbins, S. Cai and R. R. Schrock, Inorg.
Chem., 1992, 31, 2287.
We thank the National Science Foundation (CHE-9624138)
and The University of Pittsburgh (CRDF) for financial support.
G. K. C. thanks the Pennsylvania Space Grant Consortium for
fellowship support.
11 M. Jolly, J. P. Mitchell and V. C. Gibson, J. Chem. Soc., Dalton Trans.,
1992, 1331.
12 M. Jolly, J. P. Mitchell and V. C. Gibson, J. Chem. Soc., Dalton Trans.,
1992, 1329.
13 M. H. Chisholm, K. Folting, J. C. Huffman, C. C. Kirkpatrick and
A. L. Ratermann, J. Am. Chem. Soc., 1981, 103, 1305.
14 M. H. Chisholm, K. Folting, J. C. Huffman and A. L. Raterman, Inorg.
Chem., 1982, 21, 978.
Footnotes and References
* E-mail: tmeyer+@pitt.edu
† Imines were prepared by condensation of the parent aldehyde and amine
in benzene over molecular sieves, followed by purification by vacuum
distillation. The procedure was analogous to that described for imine
preparation by S. R. Sandler, and W. Karo, in Organic Functional Group
Preparations, Academic Press, New York, 1986, vol. II, p. 302.
‡ The ratio of products reported reflects the equilibrium reached after
complete metathesis of the first and partial metathesis of the second imide
reactive site.
Received in Bloomington, IN, USA, 23rd April 1997; 7/02780J
1552
Chem. Commun., 1997