Heterocumulene metathesis by iridium guanidinate and ureylene complexes: Catalysis involving reversible insertion to form six-membered metallacycles

Article abstract of DOI:10.1021/ja026178y

The four-membered metallacycles Cp*Ir((NAr)₂C=Z) have been prepared for Z = NAr and Z = O. These complexes undergo facile exchange with free heterocumulenes. This exchange process can be employed to effect the rapid, catalytic metathesis of aryl carbodiimides at room temperature and the metathesis of aryl carbodiimides with aryl isocyanates at slightly elevated temperature. The exchange process appears to proceed via a novel associative mechanism involving ring expansion to form a six-membered metallacycle rather than cycloreversion to give an imido complex. Observation of key intermediates and the results of crossover experiments support this hypothesis. Copyright

Full text of DOI:10.1021/ja026178y

Published on Web 07/10/2002
Heterocumulene Metathesis by Iridium Guanidinate and Ureylene Complexes:
Catalysis Involving Reversible Insertion To Form Six-Membered Metallacycles
Andrew W. Holland and Robert G. Bergman*
Department of Chemistry and Center for New Directions in Organic Synthesis, UniVersity of California,
Berkeley, California 94720
Received March 12, 2002
Scheme 1. Carbodiimide Exchange
Since the development of modern catalysts for alkene and alkyne
metathesis a decade ago,1 these reactions have come into widespread
use.2 Such advances have spawned increasing interest in the
metathesis of other types of double bonds, and several catalysts
for imine3 and carbodiimide4 metathesis have now been reported.
These metathesis processes have long been understood to proceed
by reversible [2 + 2] cycloaddition of an olefin (or CdN bond) to
a catalytically active metal alkylidene (or imido) complex.3b,5 We
report herein the development of a new, highly active catalyst for
the metathesis of aryl carbodiimides that departs from this
mechanistic scheme. The iridium guanidinate complex 1a catalyzes
the rapid metathesis of aryl-substituted carbodiimides at room
temperature and that of carbodiimides with isocyanates at 45 °C.
Strong evidence suggests that the mechanisms for these transforma-
tions involve ring expansion to six-membered metallacycles and
not cycloreversion to metal-imido species.
mM in each) with 5 mol % guanidinate complex 1a at room
temperature leads to complete equilibration (Keq ) 1.8 ( 0.1)
between the starting carbodiimides and the mixed species 3c within
3 min (eq 2). The rate of this metathesis stands in stark contrast to
those of previously reported carbodiimide metathesis systems, most
of which require temperatures above 100 °C. The catalyst remains
active at the end of these runs, as equilibrium is rapidly re-
established when additional substrate is added.
The pentamethylcyclopentadienyl iridium guanidinate complex
1a can be prepared most conveniently by treatment of [Cp*IrCl2]2
with N,N′,N′′-tri-p-tolyl guanidine and base in THF at room
temperature followed by recrystallization of the green product from
toluene and pentane (eq 1). A similar ureylene complex 2a can be
produced by the same general method, and the crystal structure of
this complex was confirmed by X-ray diffraction. While late metal
ureylenes have been reported previously,6 the complexes reported
here are unique in that they are sterically unencumbered, 16 e-
species.
Complex 1a also reacts with just 1 equiv of p-tolylisocyanate,
yielding the ureylene complex 2a in 83% yield (eq 3). One
equivalent of free carbodiimide 3a is also observed in this
transformation. This led us to explore the metathesis of 3a and
isocyanate p-CH3O(C6H4)NCO (yielding TolNCO and carbodi-
imides 3b and 3c), which cannot be achieved by known hetero-
cumulene metathesis catalysts without concomitant disproportion-
ation of isocyanate to CO2 and carbodiimide. This transformation
can indeed be achieved without disproportionation using 5 mol %
of either 1a or 2a at 45 °C (eq 4).
The guanidinate complex 1a undergoes exchange reactions with
a variety of heterocumulenes to form new heterocycles and liberate
carbodiimides. For example, treatment of 1a with a large excess
of methoxyphenyl-substituted carbodiimide 3b at room temperature
produces upon mixing the newly substituted guanidinate complex
1b, as well as 1 equiv each of di-p-tolylcarbodiimide (3a) and the
mixed carbodiimide species 3c bearing one tolyl group and one
p-methoxylphenyl substituent (Scheme 1). We presume that this
transformation occurs via two separate exchange processes through
intermediate 1c, and treatment of 1a with 1 or 2 equiv of 3b does
yield an equilibrium mixture containing multiple guanidinate
complexes bearing both p-tolyl and p-methoxylphenyl substituents.
The observation of mixed carbodiimide 3c above led us to
investigate the activity of complex 1a as a catalyst for carbodiimide
metathesis. Treatment of carbodiimides 3a and 3b (solutions 10
Attempts to observe stoichiometric exchange reactions involving
isocyanates and ureylenes revealed the formation of a new type of
complex in mixtures of these reagents. Treatment of the ureylene
complex with 1 equiv of p-tolylisocyanate causes a color change
from green to brown-red and the appearance of new 1H NMR
resonances at δ 2.39 (3H), 2.28 (6H), and 0.95 (15H) ppm in
addition to those of the remaining starting material and free
isocyanate. We attribute these resonances to complex 4a, a six-
membered metallacycle (featuring a dianionic biureto ligand)
resulting from isocyanate insertion into the chelate ring (Scheme
* To whom correspondence should be addressed. E-mail: bergman@
cchem.berkeley.edu.
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10.1021/ja026178y CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Heterocumulene Insertion
of the tolyl-substituted ureylene complex 2a with the p-CF3C6H4-
substituted analogue 2b at room temperature yields no trace of the
mixed ureylene 2c below 75 °C, and the analogous experiment with
the guanidinate complexes 1a and 1b produces similar results. In
both reactions, crossover occurs rapidly at 25 °C when a trace of
the appropriate heterocumulene is added. These results clearly rule
out the dissociative exchange mechanism (by which crossover
would be as facile as catalysis) and suggest a catalytic cycle
involving six-membered metallacycles rather than imido complexes.
Scheme 3. Exchange Mechanisms
In conclusion, we have found that guanidinate and ureylene
complexes 1 and 2 serve as highly active catalysts for the metathesis
of aryl carbodiimides with each other and for the more difficult
metathesis of aryl carbodiimides with aryl isocyanates. These
transformations appear to proceed via interconversion of four- and
six-membered metallacycles, and the intermediates in such processes
have been observed. While both the structural motifs and the basic
reaction steps of the associative exchange have precedent, this is
the first catalytic metathesis system in which such a mechanism
has been implicated.
Acknowledgment. We thank the NSF (Grant No. CHE-
0094349) for funding this work, and Dr. Fred Hollander and Dr.
Allen Oliver for performing the X-ray diffraction study. The Center
for New Directions in Organic Synthesis is supported by Bristol-
Meyers Squibb as a Sponsoring Member and Novartis as a
Supporting Member.
2). Such late metal birueto complexes,7 and even insertion reactions
to form them,6d have been reported.
Although complexes 2a and 4a equilibrate rapidly at room
temperature, and their relative solubilities have prevented us from
isolating 4a cleanly, experiments at low temperature have allowed
us to study 4a where equilibration with 2a is very slow. In
particular, we sought to confirm that 4a is an insertion product
rather than a dative isocyanate adduct of the ureylene. Addition of
p-methoxyphenyl-substituted isocyanate to the p-tolyl ureylene
1
complex 2a at -20 °C yields complex 4b, the H NMR spectrum
of which exhibits two distinct tolyl resonances (δ 2.40 (3H), 2.29
(3H) ppm), as well as one signal for the methoxy group of the
newly bound isocyanate moiety (δ 3.81 (3H) ppm) (eq 5). Also,
Supporting Information Available: Complete synthesis and
characterization of 1a, 1b, 2a, 2b, 3c, and details of catalytic, low-
temperature, and crossover experiments (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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On closer examination, complex 1a exhibits similar reversible
insertion chemistry with carbodiimides. Although treatment of the
guanidinate complex 1a with additional carbodiimide 3a at room
temperature yields no evidence of reaction, cooling this mixture to
-78 °C causes its color to change from green to brown. Examina-
1
tion of the brown solution by H NMR spectroscopy revealed a
new complex 6, with five incorporated tolyl group resonances at δ
2.38 (6H), 2.30 (3H), and 2.07 (6H) ppm (Scheme 2). This complex,
like 4a above, appears to be an insertion product and forms PMe3
adduct 7.
The observation of these insertion products demonstrates that
the exchange reactions described above likely proceed via reversible
insertion reactions to form six-membered metallacycles 4 and 6
(Scheme 3, upper path).8 To rule out the possibility of exchange
via reversible extrusion of heterocumulene to form imido com-
plexes9 (Scheme 3, lower path), which one would expect by analogy
to other metathesis systems, crossover experiments were conducted
in both the guanidinate and the ureylene systems (eq 6). Treatment
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