Generation and coordinating properties of a carbene bearing a phosphorus ylide: An intensely electron-donating ligand

Article abstract of DOI:10.1002/anie.200704746

A generous giver: Rh and Pd complexes of an ylide-substituted carbene were synthesized. The carbonyl stretching vibration of the Rh complex (see picture; Rh purple, C turquoise, N blue, O red, P orange, Cl green) was observed at the lowest wave-number of all cis-[RhCl(CO)₂(carbene)] complexes known to date, which indicates that this is the strongest known electron-donating carbene.

Full text of DOI:10.1002/anie.200704746

Angewandte
Chemie
DOI: 10.1002/anie.200704746
Carbenes
Generation and Coordinating Properties of a Carbene Bearing a
Phosphorus Ylide: An Intensely Electron-Donating Ligand**
Shin-ya Nakafuji, Junji Kobayashi, and Takayuki Kawashima*
Dedicated to Professor Renji Okazaki on the occasion of his 70th birthday
Since the first synthesis of stable N-heterocyclic carbenes
high p-donating ability and low electronegativity at the same
(NHCs) A (Scheme 1) was reported by Arduengo and co-
time. Thus,we focused our attention on a phosphorus ylide
moiety as a carbene-stabilizing substituent. Aminoylidecar-
bene (AYC) D[7] is expected to have a high s-donating ability
towards transition metals because the inductive effect of the
phosphorus ylide moiety is smaller than that of the amino
groups. In addition, p-donation of the ylide carbanion to the
carbene center might stabilize a carbene similar to the case of
amino groups in NHCs.[8] Although this unknown carbene D
can be classified as a kind of cyclic alkylamino carbenes
(CAAC),[6b,c] the substituent of an sp2-hybridized anionic
carbon atom,instead of an sp 3-hybridized neutral carbon
atom,will enhance the donating ability of the carbene.
Scheme 1. Stabilized carbenes.
workers in 1991,[1] their chemistry as stable carbene species
has been extensively studied.[2] Furthermore,the spectacular
success of NHCs as a new class of ligands for transition-metal
complexes and their application to catalytic systems[3] have
proved to be a tremendous advantage of using NHCs. One of
the major features of NHCs is their high electron-donating
ability to the metal center,by which catalytically active
species are electronically stabilized and catalytic processes
are accelerated. From this viewpoint,transition-metal com-
plexes bearing a carbene with higher electron-donating ability
are attractive target molecules. Modification of the scaffold is
an important method to improve the donating ability of
NHCs,and replacement of one or both nitrogen atoms
adjacent to the carbene center with other main-group
elements has provided a dramatic effect.[4] For example,
Bertrand and co-workers recently reported diphosphinocar-
benes (PHC) B[5] and alkylamino carbenes C,[6] which exhibit
enhanced electron-donating ability. The electron-donating
ability of carbenes is considered to be dominated by p-
electron-donating ability and the electronegativities of the
two adjacent atoms. To develop the electron-donating ability
of NHCs,the neighboring substituents are required to have
Several complexes (Pt,Cr,Mo,W) of AYC (R
= H) have
already been reported by Michelin and co-workers.[9] These
complexes were synthesized from the corresponding transi-
tion-metal isocyanide complexes bearing a phosphorus ylide
at the ortho position through intramolecular cyclization;
unfortunately,extension to a wide variety of transition-metal
complexes is limited. Moreover,most of the complexes were
obtained as an inseparable mixture with imido complexes;
therefore,the unambiguous investigation of properties of
carbene–metal complexes is difficult. Herein,we report the
development of general synthetic methods for AYC (R =
CH3) and its complexes with Rh and Pd,along with their
properties.
Phosphonium salt 1 was prepared according to a literature
procedure.[10] To improve the solubility of 1 in THF,the
counteranion of 1 was exchanged to tetraphenylborate
(Scheme 2). Tetraphenylborate salt 2 was treated with
mesityllithium (MesLi) at ꢀ788C,and the reaction mixture
was allowed to warm to room temperature to give diphenyl-
phosphine 4 as a major product. Formation of 4 suggested the
[*] S. Nakafuji, Dr. J. Kobayashi, Prof. Dr. T. Kawashima
Department of Chemistry
Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5800-6899
E-mail: takayuki@chem.s.u-tokyo.ac.jp
[**] This work was partly supported by Grants-in-Aid for The 21st
Century COE Program for Frontiers in Fundamental Chemistry (T.K.)
and for Scientific Research (T.K.) from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan. We also thank
Tosoh Finechem Corporation for the generous gifts of alkyl lithiums.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 2. Generation of AYC 3. M es=2,4,6-trimethylphenyl.
Angew. Chem. Int. Ed. 2008, 47, 1141 –1144
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1141

Communications
generation of AYC 3 and subsequent formal 1,3-phenyl
rearrangement of 3 to 4. However,numerous attempts to
observe AYC 3 at low temperature failed because of its
instability. In contrast,it was observed by 31P NMR spectros-
copy that treatment of the reaction mixture with elemental
sulfur at ꢀ788C gave thioamide 5 quantitatively (Scheme 3),
thus indicating AYC has sufficient stability to be trapped
under the reaction conditions.
Figure 1. ORTEP drawings (50% probability) of 6 (left) and 7 (right)
(H atoms are omitted). Selected bond lengths (Š) and angles (8), 6:
C1–C2 1.417(3), C2–P1 1.744(2), C1–N1 1.378(3), C1–Rh1 2.036(2),
Rh1–Cl1 2.4258(6), Rh1–C10 2.097(2), Rh1–C11 2.101(2), Rh1–C14
2.202(2), Rh1–C15 2.231(2), N1-C1-C2 105.0(2), C2-C1-Rh1 137.70(18),
Rh1-C1-N1 117.29(15); 7: C1–C2 1.396 (3), C2–P1 1.740(2), C1–N1
1.370(3), C1–Rh1 2.075(2), Rh1–Cl1 2.4235(6), Rh1–C3 1.914(3), Rh1–
C4 1.828(3), C3–O1 1.126(4), C4–O2 1.069(4), N1-C1-C2 105.7(2), C2-
C1-Rh1 133.5(2), Rh1-C1-N1 120.72(18).
Scheme 3. Trapping of AYC 3 with S8.
We synthesized transition-metal complexes of AYC 3 to
investigate its coordinating properties. Treatment of 2 with
MesLi at ꢀ788C and subsequent reaction with [{Rh(cod)Cl}2]
led to the formation of Rh complex 6 (Scheme 4),which was
analysis (Figure 1). The average carbonyl stretching fre-
quency of 7 (2012 cmꢀ1) was observed at the lowest wave-
number compared with IR data for known cis-[RhCl(CO)2-
(carbene)] complexes (Table 1). This result indicated the high
electron-donating ability of AYC.[14]
Table 1: IR carbonyl frequencies [cmꢀ1
] of cis-[(carbene)Rh(CO)2Cl]
complexes.[a]
n(CO) I
n(CO) II
Average
2012
Ref.
2062
1962
this work
Scheme 4. Synthesis of Rh complexes. cod=1,5-cyclooctadiene.
2062
2057
1976
1984
2018
2021
[13a]
[13b]
1
characterized by H, 13C,and 31P NMR spectroscopy and X-
ray crystallographic analysis (Figure 1).[11] The 13C NMR
chemical shift for the carbene center of 6 was observed at
dC = 200.6 ppm (1JCRh, JCP = 40.8,40.3 Hz),which is slightly
2
low-field shifted relative to benzimidazol-2-ylidene [Rh-
(cod)Cl] complexes (dC = 195–196 ppm).[12] The Rh Ccarbene
bond length (2.036(2) Š) and other parameters around the
ꢀ
2059
2070
2076
1985
1989
1996
2022
2030
2036
[5a]
Rh atom are within the range of those reported for
[(NHC)Rh(cod)Cl].[13] The Ccarbene
bond length
ꢀ
C
(1.417(3) Š) lies between those of single and double bonds,
whereas the Ccarbene N bond length (1.378(3) Š) is slightly
longer than those of [(NHC)Rh(cod)Cl] complexes (1.32–
1.37 Š),[13] meaning that p-donation of the amino group to the
carbene center is weaker than observed in other NHC
complexes.
[6a]
ꢀ
[13d]
The carbonyl stretching frequencies of cis-[RhCl(CO)2L]
complexes are recognized as an index of the electron-
donating properties of the ligand L. Complex 7 was obtained
by bubbling CO through a solution of 6 in CHCl3 (Scheme 4),
and its structure was determined by X-ray crystallographic
2081
1996
2039
[13b]
[a] Mes*=2,4,6-tBu3C6H2.
1142
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 1141 –1144

Angewandte
Chemie
To gain more insight into the origin of the donating
property of the carbene,theoretical calculations were per-
formed on AYC 3 and several NHCs.[15] The donor and
acceptor molecular orbitals of AYC and other NHCs are
shown in the Supporting Information. The p-acceptor orbital
of 3 (0.60 eV),which mainly consists of the p( p) orbital of the
carbene center,is not located at a higher energy level relative
to those of NHCs (ꢀ0.24 eV to 1.69 eV). In contrast,the s-
donating HOMO orbital of 3 (ꢀ4.4 eV) is higher than those of
NHCs (ꢀ5.8 eV to ꢀ5.2 eV). These results suggest that the
enhanced electron-donating ability of the present carbene is
derived from high s-donating ability,not from low p-accept-
ing ability.
Scheme 6. Arylation of morpholine with Pd complex 8. DM E=1,2-
dimethoxyethane.
among carbenes known to date. Such a high donating ability
promises a good catalytic activity in transition-metal-cata-
lyzed systems and further catalytic reactions are now under
investigation.
Received: October 13,2007
Published online: January 4,2008
Among transition-metal complexes bearing NHCs,palla-
dium complexes are particularly useful for various catalytic
reactions.[3f] We could obtain Pd complex 8 as a 2:1 mixture of
stereoisomers by using phosphonium salt 1 (Scheme 5). These
Keywords: carbene ligands · carbenes · palladium · rhodium ·
.
ylides
[1] A. J. Arduengo III,R. L. Harlow,M. Kline, J. Am. Chem. Soc.
1991, 113,361.
[2] a) W. A. Herrmann,C. Köcher, Angew. Chem. 1997, 109,2256;
Angew. Chem. Int. Ed. Engl. 1997, 36,2162; b) A. J. Arduen-
go III, Acc. Chem. Res. 1999, 32,913; c) R. W. Alder,M. E.
Scheme 5. Synthesis of Pd complex 8.
Blake,L. Chaker,J. N. Harvey,F. Paolini,J. Schütz,
Angew.
Chem. 2004, 116,6020; Angew. Chem. Int. Ed. 2004, 43,5896.
[3] a) W. A. Herrmann, Angew. Chem. 2002, 114,1342; Angew.
Chem. Int. Ed. 2002, 41,1290; b) C. M. Crudden,D. P. Allen,
Coord. Chem. Rev. 2004, 248,2247; c) N. M. Scott,S. P. Nolan,
Eur. J. Inorg. Chem. 2005,1815; d) L. Cavallo,A. Correa,C.
Costabile,H. Jacobsen, J. Organomet. Chem. 2005, 690,5407;
e) R. H. Crabtree, J. Organomet. Chem. 2005, 690,5451;
stereoisomers were also found in the crystallographic analysis
as a disordered structure (Figure 2). The chlorine atom on the
f) E. A. B. Kantchev,C. J. OꢀBrien,M. G. Organ,
Angew.
Chem. 2007, 119,2824; Angew. Chem. Int. Ed. 2007, 46,2768.
[4] a) D. Bourissou,O. Guerret,F. P. Gabbaï,G. Bertrand, Chem.
Rev. 2000, 100,39; b) Y. Canac,M. Soleilhavoup,S. Conejero,G.
Bertrand, J. Organomet. Chem. 2004, 689,3857; c) F. E. Hahn,
Angew. Chem. 2006, 118,1374; Angew. Chem. Int. Ed. 2006, 45,
1348.
[5] a) D. Martin,A. Baceiredo,H. Gornitzka,W. W. Schoeller,G.
Bertrand, Angew. Chem. 2005, 117,1728; Angew. Chem. Int. Ed.
2005, 44,1700; b) J. D. Masuda,D. Martin,C. Lyon-Saunier,A.
Baceiredo,H. Gornitzka,B. Donnadieu,G. Bertrand,
Chem.
Asian J. 2007, 2,178.
Figure 2. ORTEP drawings (50% probability) of 8 (major isomer: left,
minor isomer: right) (H atoms are omitted). Selected bond lengths
(Š), C1–C4 1.404(5), C4–P1 1.734(3), C1–N1 1.379(4), C1–Pd1
2.036(3), Pd1–I1 2.6524(8).
[6] a) V. Lavallo,J. Mafhouz,Y. Canac,B. Donnadieu,W. W.
Schoeller,G. Bertrand, J. Am. Chem. Soc. 2004, 126,8670; b) V.
Lavallo,Y. Canac,C. Präsang,B. Donnadieu,G. Bertrand,
Angew. Chem. 2005, 117,5851; Angew. Chem. Int. Ed. 2005, 44,
5705; c) V. Lavallo,Y. Canac,A. DeHope,B. Donnadieu,G.
Bertrand, Angew. Chem. 2005, 117,7402; Angew. Chem. Int. Ed.
2005, 44,7236.
[7] As a related NHC with a similar framework,the stable
benzannulated NHC is known. For the first example,see: F. E.
Hahn,L. Wittenbecher,R. Boese,D. Bläser, Chem. Eur. J. 1999,
5,1931.
[8] . Fekete,L. Nyulµszi, J. Organomet. Chem. 2002, 643–644,278.
[9] a) G. Facchin,R. Campostrini,R. A. Michelin, J. Organomet.
Chem. 1985, 294,C21; b) R. A. Michelin,G. Facchin,D. Braga,P.
Sabatino, Organometallics 1986, 5,2265; c) R. A. Michelin,M.
Mozzon,G. Facchin,D. Braga,P. Sabatino, J. Chem. Soc. Dalton
Trans. 1988,1803; d) G. Facchin,M. Mozzon,R. A. Michelin,
M. T. A. Ribeiro,A. J. L. Pombeiro, J. Chem. Soc. Dalton Trans.
1992,2827; e) M. Tamm,F. E. Hahn, Coord. Chem. Rev. 1999,
palladium atom was replaced by iodine,derived from the
counteranion of phosphonium salt 1. In the [(NHC)Pd-
ꢀ
(allyl)X] complexes,the halogen-exchange reaction Pd Cl!
Pd I is well known to be thermodynamically favored.[16] As a
ꢀ
preliminary investigation,the catalytic reactivity of 8 was
tested in the Buchwald–Hartwig reaction. The reaction was
completed to give the desired product quantitatively,as
analyzed by GC–MS (Scheme 6).
In summary,we have synthesized and characterized novel
Rh and Pd complexes 6–8 containing AYC 3 as a ligand,and
shown that AYC has the highest electron-donating ability
Angew. Chem. Int. Ed. 2008, 47, 1141 –1144
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1143

Communications
182,175; f) A. J. L. Pombeiro, J. Organomet. Chem. 2005, 690,
6021.
K. ꢁfele,D. v. Preysing,E. Herdtweck, J. Organomet. Chem.
2003, 684,235; c) K. Denk,P. Sirsch,W. A. Herrmann, J.
Organomet. Chem. 2002, 649,219; d) A. Neveling,G. R. Julius,S.
[10] E. Zbiral,L. Berner-Fenz, Monatsh. Chem. 1967, 98,666.
[11] CCDC-663135(5),CCDC-663132( 6),CCDC-663133( 7),and
CCDC-663134(8) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Cronje,C. Esterhuysen,H. G. Raubenheimer,
Dalton Trans.
2005,181.
[14] CV measurements of Cr complexes of carbenes with a phos-
phorus ylide showed that they are stronger net electron donors
than usual alkylamino carbenes.[9f]
[12] a) M. F. Lappert, J. Organomet. Chem. 1988, 358,185; b) F. E.
[15] Gaussian03 (Revision B.05): M. J. Rrisch et al.,see the Support-
ing Information.
Hahn,T. v. Fehren,L. Wittenbecher,R. Fröhlich,
Z. Natur-
forsch. B 2004, 59,544.
[13] For examples,see: a) M. Mayr,K. Wurst,K.-H. Ongania,M. R.
[16] Y. Ding,R. Goddard,K.-R. Pörschke, Organometallics 2005, 24,
439.
Buchmeiser, Chem. Eur. J. 2004, 10,1256; b) W. A. Herrmann,
1144
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 1141 –1144