Isocyanide-promoted ylidene transfer to phosphorus: Rearrangement in the first-generation grubbs complex

Article abstract of DOI:10.1021/om8003652

Carbon monoxide or an aryl isocyanide promotes a benzylidene carbene transfer from ruthenium to tricyclohexy-lphosphine in the first-generation Grubbs carbene complex. The resulting ruthenium(II) complexes have been isolated and structurally characterized by X-ray crystal structure analysis; aprobable mechanism for the ligand-promoted transformation is also discussed.

Full text of DOI:10.1021/om8003652

3630
Organometallics 2008, 27, 3630–3632
Isocyanide-Promoted Ylidene Transfer to Phosphorus:
Rearrangement in the First-Generation Grubbs Complex
Brandon R. Galan, Mateusz Pitak, Jerome B. Keister,* and Steven T. Diver*
Department of Chemistry, UniVersity at Buffalo, the State UniVersity of New York, Buffalo,
New York 14260-3000
ReceiVed April 25, 2008
coordination sphere of the metal to the phosphorus atom of
tricyclohexylphosphine (Scheme 1, eqs 2 and 3).
Summary: Carbon monoxide or an aryl isocyanide promotes a
benzylidene carbene transfer from ruthenium to tricyclohexy-
lphosphine in the first-generation Grubbs carbene complex. The
resulting ruthenium(II) complexes haVe been isolated and
structurally characterized by X-ray crystal structure analysis;
a probable mechanism for the ligand-promoted transformation
is also discussed.
Reaction of the Grubbs first-generation carbene complex with
carbon monoxide gave a ruthenium complex that showed loss
of the benzylidene fragment from the metal. Treatment of a
benzene solution of 5 with carbon monoxide (60 psig) for 15
min resulted in a color change from purple to dark yellow. Slow
diffusion of pentanes overnight resulted in precipitation of
yellow crystals of complex 6. The pure material gave a CH2
resonance as a doublet in the 1H NMR at δ 4.12 ppm, with JHP
) 14.0 Hz. The coupling suggested that this methylene was
attached to phosphorus directly. The downfield chemical shift
also suggested that it was deshielded, possibly bound to a
phosphonium species. The 31P NMR spectrum of complex 6
revealed two phosphorus atoms at δ 29.5 and 16.6 ppm. The
31P resonance at δ 29.5 ppm correlated with the proton CH2
Decomposition and carbene insertion processes in the Grubbs
ruthenium carbene complexes are of great interest.1 We recently
reported a method to quench the Grubbs carbenes in order to
easily purify organic products obtained from olefin metathesis
reactions.2 The procedure offers a very practical solution to a
common problem in metathesis chemistry: removing the ruthe-
nium-containing byproducts. The isocyanides 3 and 4 (or carbon
monoxide) promoted a Buchner cyclopropanation/ring expansion
process into an aromatic ring of the N-heterocyclic carbene
ligand (NHC), as depicted in eq 1 (Scheme 1).1e The aromatic
ring of the NHC is found in the second-generation Grubbs
complex 1. The isocyanide-promoted ligand insertion process
using 4 gave the polar ruthenium(II) complex 2 (L )
CNCH2CO2K), which proved to be easy to remove. Interest-
ingly, the first-generation Grubbs complex also gave a polar
product that was conveniently separated from the organic
reaction products.3 Since the first-generation Grubbs complex
has no aromatic rings to give the Buchner reaction, we were
interested to learn what was happening in this system. In the
presence of strongly coordinating ligands (such as CO), apical
coordination to the open binding site on ruthenium and ligand
displacement appear to be the most likely available reaction
pathways. In this report, we have identified the product of
ligand-promoted rearrangement in the Grubbs first-generation
carbene complex using carbon monoxide or isocyanides. The
transformation is a ligand-promoted ylidene transfer from the
1
resonance at δ 4.12 ppm by H-31P HETCOR analysis. The
remaining 31P signal found at δ 16.6 ppm correlated with a CH
at δ 2.9 ppm. The crystals of complex 6 proved suitable for
X-ray crystal structure determination, which revealed the solid-
state structure depicted in Figure 1. The carbon monoxide
reaction produced the salt 6 in 27% yield (based on 5). The
solid -state structure shows that two molecules of CO added to
ruthenium, which bears three chloride ligands. Two different
tervalent phosphorus atoms can be found in the salt 6.
Interestingly, this ruthenium(II) species is anionic and is paired
with a phosphonium counterion.
The formation of the anionic ruthenium complex requires
further comment. First, salt 6 comprised only 27% of the
ruthenium mass. Overall, about 60% of the ruthenium mass
could be accounted for by 31P NMR spectroscopy of the crude
reaction mixture. In addition to complex 6, the two complexes
ttt-Ru(CO)2(Cl)2(PCy3)2 and cct-Ru(CO)2(Cl)2(PCy3)2 were
identified in the crude reaction mixture, on the basis of their
31P chemical shifts at δ 27.5 and 27.2 ppm, as compared to the
shifts for authentically prepared samples. Complex 6 shows that
the benzylidene fragment was transferred from ruthenium to
Cy3P to make the phosphonium salt. A simple ligand displace-
ment mechanism would produce a phosphorus ylide. Loss of
CH2dPPh3 in decomposition of the second-generation Grubbs
methylidene (H2IMes)(Cy3P)Cl2RudCH2 has been reported by
Grubbs.1h Though ylidene transfer would result in formation
of a phosphorus ylide, an ylide was not detected in the crude
reaction mixture (on the basis of comparison to an authentic
sample) and the product 6 observed requires that the ylide
obtained a proton. The proton source has not been identified. It
is plausible that the ylide deprotonated a CO adduct of carbene
* To whom correspondence should be addressed. E-mail: diver@
buffalo.edu.
(1) Ruthenium carbene decomposition studies: (a) Ulman, M.; Grubbs,
R. H. J. Org. Chem. 1999, 64, 7202–7207. (b) Dinger, M. B.; Mol, J. C.
Organometallics 2003, 22, 1089–1095. Bond insertion: (c) Trnka, T. M.;
Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T.-L.;
Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546–
2558. (d) Jazzar, R. F. R.; Macgregor, S. A.; Mahon, M. F.; Richards, S. P.;
Whittlesey, M. K. J. Am. Chem. Soc. 2002, 124, 4944–4945. (e) Galan,
B. R.; Gembicky, M.; Dominiak, P. M.; Keister, J. B.; Diver, S. T. J. Am.
Chem. Soc. 2005, 127, 15702–15703. (f) Hong, S. H.; Chlenov, A.; Day,
M. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 2007, 46, 5148–5151.
Methylidene decomposition: (g) Hong, S. H.; Day, M. W.; Grubbs, R. H.
J. Am. Chem. Soc. 2004, 126, 7414–7415. (h) Hong, S. H.; Wenzel, A. G.;
Salguero, T. T.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2007, 129,
7961–7968.
(2) Galan, B. R.; Kalbarczyk, K. P.; Szczepankiewicz, S.; Keister, J. B.;
Diver, S. T. Org. Lett. 2007, 9, 1203–1206.
(3) The isocyanide treatment of crude metathesis reactions promoted
by the Grubbs complex 5 effectively quenches metathesis activity.2 Crude
metathesis reactions contain several different metal carbene species.
10.1021/om8003652 CCC: $40.75
2008 American Chemical Society
Publication on Web 07/09/2008

Communications
Organometallics, Vol. 27, No. 15, 2008 3631
Scheme 1. Dichotomous Ligand-Promoted Insertion in the First- and Second-Generation Grubbs Carbenes
Figure 1. ORTEP drawing of Cy3PCH2Ph+[Ru(CO)2Cl3(PCy3)]-
(6). Two ion pairs were found in the asymmetric unit cell, with
one ion pair being shown in the figure. Thermal ellipsoids are drawn
at the 50% probability level. The Supporting Information gives
complete structural details.
Figure 2. ORTEP drawing of the tris(isocyanide) ruthenium
complex 7. Thermal ellipsoids are drawn at the 50% probability
level. One molecule of d-chloroform appears in the unit cell.
complex 5.4 For example, Schrock has shown that phosphorus
presence of two isocyanide sp carbons as doublets coupled to
phosphorus (see the Supporting Information); similarly, IR
ylides are sufficiently basic to deprotonate the Schrock molyb-
showed two isocyanide absorptions at 2168 and 2106 cm-1
.
denum(IV) carbene, giving an alkylidyne.5 Recently, Johnson
found that a germylene abstraction of HCl from the Grubbs
complex 5 gave a ruthenium alkylidyne.6 So far, efforts to
identify other ruthenium-containing coproducts have been
unsuccessful.
The isocyanide complex 7 was crystallized from CDCl3 by slow
diffusion of pentanes, to give yellow crystals (75% yield). The
crystals proved suitable for an X-ray crystal structure determi-
nation, yielding the structure shown in Figure 2. The structure
shows that the benzylidene fragment was lost from the metal
and three isocyanides added to the metal center in a mer
arrangement. The isocyanide carbon atoms are located 1.96-1.99
Å from the ruthenium atom, whereas the phosphorus is located
2.45 Å away. Some deformation of the linear isocyanides is
seen, as a result of steric compression against the tricyclohexy-
lphosphine ligand and crystal packing forces.
We posited that a phosphorus ylide could explain the loss of
the benzylidene fragment, similar to the observation in the CO-
promoted reaction above. To test this hypothesis, repetition of
the reaction with isocyanide 3 using freshly distilled C6D6
resulted in the appearance of three peaks in the 31P NMR
spectrum at δ 29.5, 22.0, and 9.8 ppm, assigned to complex 7,
PhCHdPCy3, and free Cy3P, respectively. The chemical shift
of the phosphorus ylide was determined by comparison to an
authentically prepared sample and by a spiking experiment (see
The reaction of Grubbs’ complex 5 with the aryl isocyanide
p-ClC6H4NC (eq 3, Scheme 1) showed similar loss of the
benzylidene from the metal. This reaction proved to be high-
yielding and was much cleaner than the CO reaction described
above. Reaction of 5 with 3.3 equiv of p-ClC6H4NC in benzene
gave complex 7. The isolated complex 7 showed a single 31P
resonance at δ 23.9 ppm. The 13C NMR spectrum showed the
(4) In a control experiment, the phosphorus ylide Cy3PdCHPh did not
react with the Grubbs complex 5 in the absence of CO. In the presence of
CO, it is possible that the 18-electron CO complex could serve as a Brønsted
acid, though further experiments are needed and will be reported in the
full paper.
(5) Tonzetich, Z. J.; Schrock, R. R.; Mueller, P. Organometallics 2006,
25, 4301–4306.
(6) Caskey, S. R.; Stewart, M. H.; Ahn, Y. J.; Johnson, M. J. A.; Rowsell,
J. L. C.; Kampf, J. W. Organometallics 2007, 26, 1912–1923.

3632 Organometallics, Vol. 27, No. 15, 2008
Communications
Scheme 2. Probable Mechanism of Isocyanide-Promoted
Ylidene Transfer to Phosphorus
the Supporting Information). We repeated the reaction in the
presence of a protic solvent (CDCl3, undegassed) and observed
the crude reaction mixture by 31P NMR spectroscopy. In the
31P NMR, three resonances were seen at δ 50.0, 29.3, and 29.0
ppm. The downfield resonance was assigned to Cy3PdO,7 and
that at δ 29.3 ppm was due to the isocyanide complex 7. The
new resonance observed at δ 29.0 ppm was assigned to
[Cy3PCH2Ph]+, on the basis of comparison to an authentic
sample.
The mechanism of ylidene transfer could be intermolecular
or intramolecular. The first isocyanide binds trans to the
benzylidene to give A. Presumably, the π-acceptor properties
of the ligand serve to increase the electrophilic character of the
carbene center, setting the stage for nucleophilic attack by the
phosphine. In the first case, the ylide formation in eq 3 of
Scheme 1 could involve ligand-promoted dissociation of Cy3P
(to solution) and subsequent capture of the benzylidene by free
Cy3P (intermolecular ylidene transfer). In this case, a second
ligand binding would prevent Cy3P from rebinding to the metal.
As a result, the displaced phosphine attacks the electrophilic
carbene center. Alternatively, apical donor ligand binding to
give A may cause the Cy3P to migrate intramolecularly by a
1,2-shift (intramolecular ylidene transfer) to give the intermedi-
ate B. This would be provoked by a combination of enhanced
electrophilicity of the carbene and would alleviate ligand-ligand
crowding in complex A. A second ligand binding would prevent
the phosphine from returning to the metal, giving intermediate
C, ultimately suffering elimination of the ylide to give the
observed products 6 and 7.
To differentiate the two mechanistic possibilities, the fol-
lowing competition study was conducted: 3.3 equiv of
p-ClC6H4NC and 3 equiv of Bu3P in CDCl3 were added to a
solution of Grubbs’ complex 5. A control reaction showed that
the phosphine did not pre-exchange over the course of 20-30
min. Use of protic solvent served to protonate the phosphorus
ylides in situ to give the corresponding phosphonium salts. The
crude reaction mixture was monitored by 31P NMR spectros-
copy. Within minutes, the phosphonium salts Cy3P+CHDPh
(65% of trapped benzylidene) and Bu3P+CHDPh (34% of
trapped benzylidene) had appeared, along with complex 7.8 If
the phosphine migration had occurred by a completely inter-
molecular pathway, then at least 75% trapping by Bu3P would
have been expected. Instead, a predominance of Cy3P trapping
took place, despite its lower relative abundance. This experiment
suggests that the isocyanide addition results in a predominantly
intramolecular 1,2-shift of Cy3P to the carbene carbon, A to B
(Scheme 2).
Instead, the carbene is intercepted by the phosphine nucleophile.9
Only ylidene transfer to phosphine was observed, despite the
fact that isocyanides are capable nucleophiles.10 The “migra-
tions” of the metal-bound carbene fragment are different in the
two ligand environments of the first- and second-generation
Grubbs carbenes but are entirely consistent with the behavior
of electrophilic singlet carbenes.
In summary, a ligand-promoted transfer of the benzylidene
to tricyclohexylphosphine has been observed in the first-
generation Grubbs carbene complex. Owing to the importance
of the Grubbs catalyst in olefin metathesis, this reaction provides
insight into the reactivity of the carbene moiety and shows how
donor ligands can mediate the transfer of the bound carbene
ligand. Each of the ligands, carbon monoxide and p-chlorophe-
nyl isocyanide, provokes ylidene transfer. The products from
each of the two ligand-promoted reactions were structurally
characterized. In the isocyanide case, the phosphine migration
is primarily an intramolecular process. The data presented here
provides an interesting contrast to the Buchner insertion/ring
expansion observed in the Grubbs second-generation complexes.
Further studies on the use of different ligands to promote similar
carbene insertions in both the first- and second-generation
Grubbs carbene complexes are in progress and will be reported
in due course.
Acknowledgment. This work was supported by the donors
of the Petroleum Research Fund, administered by the
American Chemical Society (Grant No. PRF AC-44202, to
S.T.D. and J.B.K.), and the NSF (Grant No. CHE-601206,
to S.T.D. and J.B.K.). We thank Professor Philip Coppens
(SUNY Buffalo) for use of his X-ray diffractometer, Dr.
Mark Messerschmidt (Coppens Group) for the structure
determination of 6, and Materia, Inc. (Pasadena, CA), for
generously supplying Grubbs’ first-generation catalyst.
The coordination of carbon monoxide or isocyanide triggers
loss of the benzylidene carbene fragment from the metal.
Depending on the existing ligands in the Grubbs complex,
dichotomous reactivity is observed. In the case of the second-
generation Grubbs complex 1, this coordination results in
cyclopropanation of the mesityl ring of the 1,3-bis(mesityl)dihy-
droimidazolylidene ligand. In the present study conducted with
the first-generation Grubbs complex 5, the absence of the
aromatic moiety does not permit the cyclopropanation pathway.
Supporting Information Available: Text and tables giving
experimental details and characterization data and details of the
X-ray structure determination and a CIF file giving X-ray structural
data for 7. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM8003652
(9) Interestingly, Grubbs used phosphorus ylides to transfer the ylidene
to tungsten(IV) imido complexes to form the metal carbenes; see: Johnson,
L. K.; Frey, M.; Ulibarri, T. A.; Virgil, S. C.; Grubbs, R. H.; Ziller, J. W.
J. Am. Chem. Soc. 1993, 115, 8167–8177.
(10) Mayr et al. showed that phosphines are approximately 10 orders
of magnitude more nucleophilic than aryl isocyanides in their reactions with
stabilized carbenium ions; see: Tumanov, V. V.; Tishkov, A. A.; Mayr, H.
Angew. Chem., Int. Ed. 2007, 46, 3563–3566.
(7) Some of our samples of the complex 5 showed small amounts of
Cy3PdO. Most of the oxide can be attributed to this source, rather than
oxidation under the reaction conditions.
(8) Other major peaks seen included Bu3P and Bu3PdO. Control studies
showed that there was no exchange of phosphonium salts (R3PR′ + PR′′3
to R′PR′′3) on the time scale of these experiments.