A stable four-membered N-heterocyclic carbene

Article abstract of DOI:10.1021/ja047892d

The synthesis of the first four-membered N-heterocyclic carbene is described. Depending on the substituents on the nitrogen atoms, it is possible to characterize at room temperature the carbene dimer or the free carbene. Crystallographic analyses are prov

Full text of DOI:10.1021/ja047892d

Published on Web 08/03/2004
A Stable Four-Membered N-Heterocyclic Carbene
Emmanuelle Despagnet-Ayoub and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratories for Chemical Synthesis, DiVision of Chemistry and Chemical Engineering,
California Institute of Technology, 1200 East California BouleVard, 164-30, Pasadena, California 91125
Received April 13, 2004; E-mail: rhg@its.caltech.edu
Since the discovery of the first stable carbenes in the 1980s,1
significant advances have been realized in organometallic chemistry
and catalysis2 using N-heterocyclic carbene (NHC) ligands due to
their unique coordination properties (good σ-donor, poor π-accep-
tor). However, most of the diaminocarbenes reported in the literature
are composed of five-membered rings such as imidazole and
triazole. Only two other frameworks have been described: a six-
membered ring and an acyclic diaminocarbenes,3,4 both of which
afford increased steric hindrance around the carbene center and
electron-donating ability. Thus, slight changes to the NHC archi-
tecture have a dramatic effect on the electronic properties. We
propose to extend the diaminocarbene family with a more strained
system and report here the first synthesis and structural character-
ization of a four-membered NHC.
The iminium salt 2a was prepared in 66% yield from the
silylamidine 15 as a white powder by addition of (diethylamino)-
dichlorophosphine and trimethylsilyl-trifluoromethanesulfonate at
room temperature (Scheme 1). The 1H and 13C NMR chemical shifts
of the NCHN fragment (δ 1H 9.1 ppm, δ 13C 165 ppm) were in the
same range as those observed for imidazolium salts.
Figure 1. Thermal ellipsoid diagram of 2a (50% probability). Selected
bond distances (Å) and angles (deg): C1-N1 1.3306(11), C1-N2
1.33279(11), P1-N1 1.8237(8), P1-N2 1.8136(8), P1-N3 1.6219(8),
N1-C1-N2 104.29(7), N1-P1-N2 70.49(3).
Scheme 1
By X-ray diffraction,6 two conformers were revealed in the same
single crystal in a 95/5 ratio. They differed by an inversion of the
tetrahedral geometry around the phosphorus atom but shared the
same position for N1, N2, and the aromatic substituents. Only the
major conformer will be described in detail (Figure 1). The structure
of 2a is similar to that of all 1,3-diaza-2-phosphetine cations
reported,7 exhibiting delocalization of the π-electrons along the
N1-C1-N2 fragment [N1-C1 1.3306(11) Å, N2-C1 1.3279(11)
Å], a small intracyclic N1-P1-N2 angle [70.49(3)°], and longer
endocyclic P-N bonds [P1-N1 1.8237(8) Å, P1-N2 1.8136(8)
Å] than the exocyclic one [P1-N3 1.6219(8) Å].
Attempts to generate the free carbene under standard conditions
led to ring opening by P-N bond cleavage. Even with protection
of the phosphino group with bulkier substituents such as diisopro-
pylamino, the addition of potassium tert-butoxide to 2b generated
product 3 as a result of nucleophilic attack on the phosphorus. Such
reactions demonstrate the electrophilic character of the phosphorus
atom in these compounds.
Deprotonation of 2b can be achieved with more hindered and
less nucleophilic bases such as mesityllithium or potassium hexa-
methyldisilazide in toluene at room temperature. After workup, the
carbene dimer 4 was isolated in 45% yield as a yellow powder.
The structure of 4 was established by single-crystal X-ray diffraction
(Figure 2).6 The small intracyclic angles [N1A-C1A-N2A
95.30(10)° and N1B-C1B-N2B 95.12(10)°] led to a large C(1A,
1B)-N-Cmesityl angle [125-134°], decreasing the steric hindrance
around the nitrogen atoms and allowing facile dimerization. In five-
membered N-heterocyclic carbenes, the mesityl substituents were
sufficient to stabilize the carbene due to a smaller Ccarbene-N-
Cmesityl angle [122-123°].
To avoid this reaction, iminium 2c was synthesized with ortho
isopropyl substituents. Deprotonation of 2c with mesityllithium or
potassium hexamethyldisilazide at room temperature generated the
carbene 5. The 13C NMR signal for the carbene center appeared at
285 ppm as a doublet (2JCP ) 13 Hz). To our knowledge, this
chemical shift is the most downfield reported for a diaminocarbene.8
Single crystals6 were obtained from a saturated pentane solution at
-50 °C (Figure 3). Surprisingly, the structure shows that 5 does
not possess C2 symmetry; the mesityl groups are nonsymmetric
[C1-N1-C2 126.21(13)°, C1-N2-C14 121.87(13)°]. As ex-
pected, the N1-C1-N2 angle [96.72(13)°] is significantly smaller
than the analogous angles observed in isolated diaminocarbenes
[100-121°]. The N-C1 bonds [C1-N1 1.373(2) Å, C1-N2
9
10198
J. AM. CHEM. SOC. 2004, 126, 10198-10199
10.1021/ja047892d CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
for 2-6. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
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Figure 2. Thermal ellipsoid diagram of 4 (50% probability), (H atoms are
omitted). Selected bond distances (Å) and angles (deg): C1A-C1B
1.3335(17), N1A-C1A-N2A 95.30(10), N1B-C1B-N2B 95.12(10),
C2A-N1A-C1A 134.14(10), C11A-N2A-C1A 127.79(10), C2B-N1B-
C1B 132.99(11), C11B-N2B-C1B 125.49(10).
(4) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1121-1123.
(5) Silylamidines 1 were prepared from the N,N′-diarylformamidine.9 n-
Butyllithium solution in hexanes (7.1 mmol) was added slowly to a THF
(30 mL) solution of formamidine (7.1 mmol) at room temperature followed
by trimethylsilyl chloride (0.77 g, 0.71 mmol) after 30 min. Evaporation
and extraction with ether gave the desired product.
(6) Crystal data for 2a: C24H33F3N3O3PS, M ) 531.56, triclinic, space group
P1h, a ) 10.5668(3) Å, b ) 11.8758(4) Å, c ) 12.7952(4) Å, R ) 67.7490-
(10)°, â ) 65.7880(10)°, γ ) 67.4890(10)°, V ) 1305.19(7) Å3, Z ) 2,
µ(Mo KR) ) 0.71073 Å, crystal size ) 0.37 × 0.26 × 0.25 mm3, 50465
reflections collected (15049 independent, Rint ) 0.0566), 348 parameters,
R1 [I > 2σ(I)] ) 0.0450, wR2 [all data] ) 0.0850. For 4: C50H72N6P2,
M ) 819.08, monoclinic, space group P21/c, a ) 13.7626(6) Å, b )
16.2123(8) Å, c ) 21.7085(10) Å, â ) 101.3420(10)°, V ) 4749.1(4)
Å3, Z ) 4, µ(Mo KR) ) 0.71073 Å, crystal size ) 0.33 × 0.31 × 0.21
mm3, 76278 reflections collected (14587 independent, Rint ) 0.0787), 811
parameters, R1 [I > 2σ(I)] ) 0.0500, wR2 [all data] ) 0.0770. For 5:
C31H48N3P, M ) 493.69, monoclinic, space group P21/n, a ) 8.9994(10)
Å, b ) 26.366(3) Å, c ) 12.6593(14) Å, â ) 94.240(2)°, V ) 2995.5(6)
Å3, Z ) 4, µ(Mo KR) ) 0.71073 Å, crystal size ) 0.41 × 0.22 × 0.07
mm3, 26662 reflections collected (6905 independent, Rint ) 0.0643), 508
parameters, R1 [I > 2σ(I)] ) 0.0525, wR2 [all data] ) 0.0846. Data were
collected at 98(2) K for 2a, 100(2) K for 4 and 100(2) K for 5 on Bruker
SMART 1000 diffractometer. The structures were solved by direct methods
(SHELXS-97),10 and refined using the least-squares method on F2.11
Crystallographic data have been deposited at the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK, and copies can be obtained on request, free of
charge, by quoting the publication citation and the deposition number
228866 (2a), 231890 (4), 234638 (5).
Figure 3. Thermal ellipsoid diagram of 5 (50% probability), (H atoms are
omitted). Selected bond distances (Å) and angles (deg): C1-N1 1.373(2),
C1-N2 1.387(2), N1-C1-N2 96.72(13), C1-N1-C2 126.21(13),
C1-N2-C14 121.87(13).
1.387(2) Å] are short, suggesting interaction of the nitrogen with
the carbene center, but the geometry around the nitrogen atoms is
not strictly planar [sum of the bond angles ) 355.1° and 348.2°],
presumably due to the ring strain.
This report demonstrates the preparation of the first stable four-
membered ring diaminocarbene. Studies of its coordination proper-
ties with transition metals are currently in progress.
Acknowledgment. We thank Lawrence M. Henling and Michael
W. Day for X-ray crystallography studies. E.D.-A. is grateful to
the Ministe`re des Affaires Etrange`res (France) for a Lavoisier
postdoctoral fellowship. This work was supported by the National
(7) Roques, C.; Mazie`res, M.-R.; Majoral, J.-P.; Sanchez, M.; Jaud, J. Inorg.
Chem. 1989, 28, 3931-3933.
13
(8) Range for δ
C
in diaminocarbenes: five-membered ring 205-245
carbene
ppm, six-membered ring 236-242 ppm, acyclic 236-256 ppm. In the
case of monoaminocarbenes, more downfield chemical shifts have been
reported: (a) Sole´, S.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.;
Bertrand, G. Science 2001, 292, 1901-1903. (b) Merceron-Saffon, N.;
Baceiredo, A.; Gornitzka, H.; Bertrand, G. Science 2003, 301, 1223-
1225.
(9) Taylor, E. C.; Ehrhart, W. A. J. Org. Chem. 1963, 28, 1108-1112.
(10) Sheldrick, G. M. Acta Crytallogr. 1990, A46, 467.
(11) Sheldrick, G. M. SHELXL-97. Program for Crystal Structure Refinement,
Science Foundation.
University of Go¨ttingen, 1997.
Supporting Information Available: Full experimental details
and spectroscopic data for 2-6. X-ray crystallographic data (CIF)
JA047892D
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J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10199