A novel ylide-stabilized carbene; Formation and electron donating ability of an amino(sulfur-ylide)carbene

Article abstract of DOI:10.1039/b813454e

A novel N-heterocyclic carbene, amino(sulfur-ylide)carbene, which is stabilized by electron donation from the carbanion of a sulfur ylide as well as a nitrogen atom, was generated and its Rh complex was synthesized; the electron donating properties of the amino(sulfur-ylide)carbene were investigated using the IR carbonyl stretching frequencies of the Rh complex, indicating a relatively strong electron-donating ability among reported carbenes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b813454e

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A novel ylide-stabilized carbene; formation and electron donating ability
of an amino(sulfur-ylide)carbenew
Junji Kobayashi, Shin-ya Nakafuji, Atsushi Yatabe and Takayuki Kawashima*
Received (in Cambridge, UK) 5th August 2008, Accepted 11th September 2008
First published as an Advance Article on the web 14th October 2008
DOI: 10.1039/b813454e
A novel N-heterocyclic carbene, amino(sulfur-ylide)carbene,
which is stabilized by electron donation from the carbanion of
a sulfur ylide as well as a nitrogen atom, was generated and its
Rh complex was synthesized; the electron donating properties of
the amino(sulfur-ylide)carbene were investigated using the IR
carbonyl stretching frequencies of the Rh complex, indicating
a relatively strong electron-donating ability among reported
carbenes.
donating property of APYC. On the other hand, ylide species
of sulfur have also been well known since the 19th century as
well as phosphorus ylides, the ylide stability being higher in
sulfur ylides than in phosphorus ylides.7 For these reasons, we
expected that the sulfur ylide would also be a good candidate
for a substituent to enhance the electron-donating ability of
carbenes. Herein we report the generation of a novel sulfur
ylide-substituted carbene (amino(sulfur-ylide)carbene; ASYC)
and the electron-donating properties of ASYC.
The synthesis of a stable carbene, in which the carbene center
is stabilized by two adjacent nitrogen atoms, (N-heterocyclic
carbene; NHC) was a very important landmark,1 because the
utilization of NHCs as a new class of ligands for transition
metals and their application to catalytic systems has been
widely investigated since their isolation in 1991.2
In NHCs, the carbene center is in an sp2 hybrid state with a
localized lone pair and a vacant p-orbital. The lone pair of the
carbene center has similar properties to those of the phosphine
ligand, and NHCs can coordinate to transition metals. The
vacant p-orbital is stabilized by the mesomeric effect from the
lone pair of the nitrogen atom, and at the same time, this
electronic donation also contributes to the prevention of back
donation from the d-orbital of the transition metal when it
coordinates to the transition metal. For these reasons, NHCs
usually show strong s-donating ability to the transition metal
and NHC-transition metal complexes achieve high catalytic
activity.2 From such a viewpoint, the s-donating ability of
NHCs should be more and more improved. The most success-
ful strategy for enhancing the s-donating ability of NHCs is
modification of the NHC scaffold, especially replacement of
the nitrogen atom by other main group elements.3 Recently,
we reported a novel NHC (amino(phosphorus-ylide)carbene;
APYC), in which a phosphorus ylide moiety is at the position
adjacent to the carbene center instead of another nitrogen
atom, and showed that APYC has the strongest electron-
donating ability among known carbenes to date.4–6 Because
phosphorus ylides have a resonance structure with both a
carbanion and a phosphonium cation, the ylide carbon can
function as an electron donor, and the carbon has a much
lower electronegativity than the nitrogen atom. These char-
acteristics are the main features giving the strong electron-
The precursor 2 of ASYC was synthesized by modification of
the reported procedure. In the literature procedure, unsubsti-
tuted pyrrole was used as a starting material, and the reaction
gave a mixture of 2- and 3-sulfoniopyrrole (Scheme 1). How-
ever, introduction of the sterically hindered mesityl group on
the nitrogen atom prevented substitution at the 2-position and
gave the desired product 1 in moderate yield. After counter-
anion exchange, 2 was isolated as a colorless solid. The
structure of 2 was confirmed by X-ray crystallographic analysis.
An ORTEP drawing of 2 is shown in Fig. 1.8
In the previous report, we confirmed the generation of
APYC by the reaction of the corresponding precursor with
MesLi and succeeded in trapping APYC with elemental sulfur
and in synthesizing its transition metal complexes. ASYC was
also generated from 2 using MesLi as a base and trapped by
elemental sulfur at À40 1C (Scheme 2). The formation of
thioamide 3 was suggested by mass spectroscopy, but its
isolation by GPC or PTLC failed due to decomposition.
However, subsequent treatment of thioamide 3 with MeI led
Department of Chemistry, Graduate School of Science, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan.
E-mail: takayuki@chem.s.u-tokyo.ac.jp; Fax: +81-3-5800-6899;
Tel: +81-3-5841-4338
w Electronic supplementary information (ESI) available: Synthesis of
the compounds. CCDC 695652 and 695653. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
b813454e
Scheme 1 Synthesis of carbene precursor 2. Reagents and conditions:
(a) (CF3CO)2O, Tol2SQO, CH2Cl2; (b) aq. LiClO4 (22% yield for 2
steps); (c) NaBPh4, MeOH (83%).
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Scheme 3 Formation of ASYC–Rh complexes 5 and 6. Reagents and
conditions: (a) MesLi, THF, À40 1C; (b) [Rh(cod)Cl]2, THF, À40 1C
(40% yield for 2 steps); (c) CO(gas), THF (70%).
shifted upfield compared with that of [(APYC)Rh(CO)2Cl]
1
(dC 187.8, JCRh = 35.7 Hz). However, it is very close to
1
that of the [(NHC)Rh(CO)2Cl] complex 7 (dC 170.2, JCRh
=
43 Hz).9
Single crystals of 6 were obtained by slow evaporation of an
Et2O–THF solution of 6. The ORTEP drawing of 6 is shown
in Fig. 2.8 The Rh center has a square-planar structure (SRh
359.881) and the heterocyclic moiety is also planar (Sring
=
=
540.01). These two planes are twisted relative to each other by
63.881. The Rh–Ccarbene bond length (2.058(4) A) is a normal
value. The Rh–CCO bond lengths are 1.912(5) A for trans CO
against ASYC and 1.816(5) A for cis CO, indicating the trans
influence of the ASYC ligand. The Ccarbene–Cylide bond length
(1.397(6) A) lies between those of single and double bonds,
whereas Ccarbene–N (1.382(5) A) is slightly longer than those in
the [(NHC)Rh(CO)2Cl] complex (1.349–1.350 A). These ten-
dencies were also observed in APYC–Rh complexes, meaning
Fig. 1 ORTEP drawing of carbene precursor 2 (50% probability).
Selected bond lengths and angles: C(1)–C(4) 1.371(3) A; C(1)–N(1)
1.355(2) A; S(1)–C(4) 1.730(2) A; N(1)–C(1)–C(6) 107.24(17) A.
Scheme 2 Generation and trapping of ASYC. Reagents and condi-
tions: (a) MesLi, THF, À40 1C; (b) S8, THF, À40 1C (not isolated); (c)
MeI (90% yield for 3 steps).
to the isolation of methyl thioether 4 in 90% yield. These
results indicate that ASYC can certainly be generated in good
yield and has sufficient stability to be trapped by appropriate
reagents at low temperature. We have also attempted the
direct observation of ASYC at a lower temperature (À40 1C).
Although we could observe the signal assignable to the
carbene carbon by 13C NMR (dC 198.9), unambiguous identi-
fication of ASYC failed because it was observed as a mixture
with the starting material and an unidentified product (prob-
ably due to the decomposition of ASYC).
To elucidate the coordination properties of ASYC, the
synthesis of the Rh complex of ASYC was examined. ASYC
was generated by the method described above, and subsequent
treatment with [Rh(cod)Cl]2 afforded [(ASYC)Rh(cod)Cl] 5
(Scheme 3). In the 13C NMR spectrum, the carbene carbon
was observed at 179.7 ppm as a doublet (1JCRh = 37.1 Hz) and
the signal was shifted upfield compared with [(APYC)Rh(cod)Cl]
1
(dC 200.6, JCRh = 40.3 Hz). 5 was converted to dicarbonyl
complex 6 by reaction with carbon monoxide. 6 was charac-
terized by 1H and 13C NMR and its structure was finally
determined by X-ray crystallographic analysis. The 13C NMR
showed a chemical shift corresponding to the carbene center at
Fig. 2 ORTEP drawing of [(ASYC)Rh(CO)2Cl] 6 (50% probability).
Selected bond lengths and angles: C(1)–C(6) 1.397(6) A; C(1)–N(1)
1.382(5) A; C(1)–Rh(1) 2.058(4) A; S(1)–C(6) 1.727(4) A; Rh(1)–C(2)
1.912(5) A; Rh(1)–C(3) 1.816(5) A; N(1)–C(1)–C(6) 102.9(3)1.
172.5 ppm with one-bond coupling to the Rh center (1JCRh
=
37.1 Hz). The chemical shift of the carbene center of 6 is also
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that p-donation from the ylide carbon to the carbene center is
working effectively and p-donation from the nitrogen atom to
the carbene center is smaller than observed in other NHC
complexes.
Notes and references
1 A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am. Chem.
Soc., 1991, 113, 361.
2 (a) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290;
(b) C. M. Crudden and D. P. Allen, Coord. Chem. Rev., 2004, 248,
2247; (c) N. M. Scott and S. P. Nolan, Eur. J. Inorg. Chem., 2005,
1815; (d) L. Cavallo, A. Correa, C. Costabile and H. Jacobsen,
J. Organomet. Chem., 2005, 690, 5407; (e) R. H. Crabtree,
J. Organomet. Chem., 2005, 690, 5451; (f) E. A. B. Kantchev,
C. J. O’Brien and M. G. Organ, Angew. Chem., Int. Ed., 2007, 46,
2768.
The carbonyl stretching frequencies of cis-[LRh(CO)2Cl]
complexes are recognized as an index of the electron-donating
properties of the ligand L. The average carbonyl stretching
frequency of 6 (2019 cmÀ1) was observed in the very low
wavenumber region compared with IR data for known cis-
[(carbene)Rh(CO)2Cl] complexes (see, Table 1 of ref. 4),
although the wavenumber is slightly higher than those of
[(APYC)Rh(CO)2Cl] (2012 cmÀ1) and 8 (2018 cmÀ1).10 Taking
the result of the APYC complex into consideration, it is
suggested that ylide-stabilized carbenes generally show very
strong s-donating ability.
3 (a) D. Bourissou, O. Guerret, F. P. Gabbaı and G. Bertrand,
¨
Chem. Rev., 2000, 100, 39; (b) Y. Canac, M. Soleilhavoup,
S. Conejero and G. Bertrand, J. Organomet. Chem., 2004, 689,
3857; (c) F. E. Hahn, Angew. Chem., Int. Ed., 2006, 45, 1348.
4 S.-y. Nakafuji, J. Kobayashi and T. Kawashima, Angew. Chem.,
Int. Ed., 2008, 47, 1141.
5 APYC–transition metal complexes have been synthesized by alter-
native intramolecular cyclization methods, see: (a) G. Facchin,
R. Campostrini and R. A. Michelin, J. Organomet. Chem., 1985,
294, C21; (b) R. A. Michelin, G. Facchin, D. Braga and P. Sabatino,
Organometallics, 1986, 5, 2265; (c) R. A. Michelin, M. Mozzon,
G. Facchin, D. Braga and P. Sabatino, J. Chem. Soc., Dalton Trans.,
1988, 1803; (d) G. Facchin, M. Mozzon, R. A. Michelin, M. T.
A. Ribeiro and A. J. L. Pombeiro, J. Chem. Soc., Dalton Trans.,
1992, 2827; (e) M. Tamm and F. E. Hahn, Coord. Chem. Rev., 1999,
182, 175; (f) A. J. L. Pombeiro, J. Organomet. Chem., 2005, 690,
6021.
6 The studies on aminoylidecarbene have been reported, see:
M. Asay, B. Donnadieu, A. Baceiredo, M. Soleilhavoup and
G. Bertrand, Inorg. Chem., 2008, 47, 3949.
7 V. Aggarwal and J. Richardson, in Science of Synthesis, ed.
A Padwa, Georg Thieme, Stuttgart and New York, 2004, ch. 1,
vol. 27, p. 21.
8 CCDC-695653 (2) and CCDC-695652 (6) contain the supplemen-
tary 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. Crystal
data of 2. C51H48BNSÁCH3OH, M = 749.82, orthorhombic, a =
18.673(5), b = 23.053(7), c = 19.701(6) A, U = 8481(4) A3, T =
120 K, space group Pbca (no. 61), Z = 852 084 reflections
measured, 7364 unique (Rint = 0.0390) which were used in all
calculations. The final wR(F2) was 0.1369 (all data). Crystal data of
6. C29H27ClNO2RhS, M = 591.94, triclinic, a = 9.903(5), b =
11.130(5), c = 13.844(7) A, a = 88.329(19), b = 77.738(17), g =
66.558(14) deg, U = 1365.5(12) A3, T = 120 K, space group P–1
In summary, we have designed ASYC, a novel ylide-stabi-
lized carbene, and succeeded in the generation of ASYC as
well as the synthesis of Rh complexes containing ASYC. The
high electron-donating ability of ASYC was demonstrated by
comparison of the IR stretching frequencies of [LRh(CO)2Cl]
complexes. These results indicated that stabilization of the
carbene center by the ylide moiety is a general and remarkable
strategy for enhancement of the electron-donating ability of
singlet carbene ligands. Such strong electron-donating abilities
of ylide-stabilized carbenes promise an enhancement of the
catalytic activity of transition metal complexes. The catalytic
activity of ASYC–transition metal complexes is now under
investigation.
(no. 2), Z = 28 442 reflections measured, 4640 unique (Rint =
0.0315) which were used in all calculations. The final wR(F2) was
0.1242 (all data).
9 S. Burling, M. F. Mahon, S. P. Reade and M. K. Whittlesey,
Organometallics, 2006, 25, 3761.
This work was supported by the Global COE program for
Chemical Innovation and for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology
of Japan (T.K.). We thank Tosoh Finechem Corp. for the
generous gifts of alkyllithiums.
10 M. Mayr, K. Wurst, K.-H. Ongania and M. R. Buchmeiser,
Chem.–Eur. J., 2004, 10, 1256.
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