A facile route to functionalized N-heterocyclic carbenes (NHCs) with NHC base-stabilized dichlorosilylene

Article abstract of DOI:10.1021/ja104386g

Reaction of IPr·SiCl₂ (1) [IPr) 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene] with 1-azidoadamantane leads to functionalized N-heterocyclic carbene (NHC) 2. Silyl-substituted NHC 2 reacts easily with 1-azidoadamantane to form triazene 3, in which the exocyclic CdN bond is slightly shorter than those of regular NHC-derived triazines. 2 could serve as a promising ligand for transition metals.

Full text of DOI:10.1021/ja104386g

Published on Web 07/07/2010
A Facile Route to Functionalized N-Heterocyclic Carbenes (NHCs) with NHC
Base-Stabilized Dichlorosilylene
Rajendra S. Ghadwal, Herbert W. Roesky,* Markus Granitzka, and Dietmar Stalke
Institut fu¨r Anorganische Chemie, UniVersita¨t Go¨ttingen, Tammannstrasse 4, 37077 Go¨ttingen, Germany
Received May 27, 2010; E-mail: hroesky@gwdg.de
Scheme 1
Abstract: Reaction of IPr ·SiCl2 (1) [IPr ) 1,3-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene] with 1-azidoadamantane leads to
functionalized N-heterocyclic carbene (NHC) 2. Silyl-substituted
NHC 2 reacts easily with 1-azidoadamantane to form triazene 3,
in which the exocyclic CdN bond is slightly shorter than those of
regular NHC-derived triazines. 2 could serve as a promising ligand
for transition metals.
N-heterocyclic carbenes (NHCs) have been extensively used as
versatile ligands in organometallic chemistry1 and transition-metal
catalysis.2 Moreover, they have found widespread applications as
organocatalysts.3 The extraordinary binding properties of NHCs
as strong σ-donating and weak π-accepting ligands have resulted
in the isolation of metal complexes in unusual oxidation states.1,4-6
Further progress in this field is driven by the design and synthesis
of new NHCs.7 The NHCs typically form metal complexes in which
the carbene carbon (the C2 position) coordinates to the metal center
and the C4 and C5 positions remain unsubstituted. (The numbering
of the elements within the imidazole ring of NHCs follows the
IUPAC nomenclature). NHCs in which one of these positions is
substituted by a metal center are called abnormal NHCs and have
of an NHC at the C4 and C5 positions has an influence on the
electronic properties of the carbene atom. The C4,C5-dichloro-
substituted analogue of IPr is extremely stable16 and can be handled
in air. Therefore, C4 or C5 functionalization of NHCs is extremely
important in NHC chemistry.
been reported for d- and f-block elements.1,8,9
Recently, abnormal NHCs have been synthesized by Bertrand
and co-workers.10 The abnormal behavior of NHCs for p-block
elements11-13 is rare and only observed especially when the C2
carbon is not blocked or bound to a metal. Gates and co-workers12
have reported 4-phosphino-substituted NHCs prepared by the direct
reaction of phosphaalkenes with NHCs. Silyl-substituted NHCs have
been formed13 by the reactions of NHCs with transient silylenes.
Silylation13b-d at the C4 or C5 position of metal-coordinated NHCs
has been reported. In order to increase the electronic stabilization
effect, functionalization of NHCs at the C4 and C5 positions has
attracted attention due to broader applications. In spite of extensive
research work in functionalizing NHCs, to date there have been
no reports on generating NHCs from stable NHC complexes.
Herein, we report the formation of C4-silyl-substituted NHC 2
by oxidative addition and C-H activation of the NHC-stabilized
dichlorosilylene5 IPr·SiCl2 (1) [IPr ) 1,3-bis(2,6-diisopropylphe-
nyl)imidazol-2-ylidene] with 1-azidoadamantane (AdN3). Most
silylene reactions with organic azides14 led to the isolation of
silaimines, azidosilanes, or silatetrazolines. In contrast, the formation
of 2 shows the unique reactivity of NHC-stabilized dichlorosilylenes
and offers a new route to functionalized NHCs. The mechanism15
is believed to involve the initial formation of the unstable silaimine
2′ under evolution of nitrogen. Silaimine 2′ consists of an easily
polarizable SidN bond and hence abstracts the proton from the
C4 position of the coordinated IPr ligand to form 2′′. 2′′ rearranges
to the stable silyl-substituted NHC 2, which was isolated as colorless
crystals in 87% yield (Scheme 1). It has been shown that substitution
1
NHC 2 was characterized by elemental analysis and H, 13C,
1
and 29Si NMR spectroscopic studies. The H NMR spectrum of 2
shows resonances that are quite different from those of 1 but close
to those of free IPr.16 The backbone proton of NHC 2 appears as
a singlet (δ ) 7.45 ppm) that is shifted downfield by ∆δ ) 0.83
and 1.08 ppm relative to those of IPr (6.62 ppm)16 and 1 (6.37
ppm), respectively.5 The new broad resonance appearing at 2.61
ppm is tentatively assigned to the amino proton. In addition to the
other expected resonances, the appearance of a new resonance in
the 13C NMR spectrum of 2 at 224.21 ppm (C2, carbene carbon)16
clearly revealed the formation of the new NHC 2.
Silyl-substituted NHC 2 reacts in a similar way as a nonsubsti-
tuted NHC17 with AdN3 to form triazene 3, which can also be
prepared by treatment of 1 with 2 equiv of AdN3 (Scheme 2).
Formation of triazene 3 was accompanied by a large upfield shift
1
of the NCH resonance in the H NMR spectrum (δ ) 5.88 ppm;
1
∆δ ) 1.57 ppm). Other H NMR resonances were observed as
expected.
Single-crystal X-ray structure analysis18 confirmed the insertion
of the aminosilyl group into the C-H bond at the C4 position of
IPr to afford new NHC 2, which crystallizes in the triclinic space
j
group P1 with a toluene molecule in the asymmetric unit (Figure
1). The observed imidazol geometry in 2 is very similar to that
reported for free IPr.16 For example, the C1-N1 and C1-N2 bond
9
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10.1021/ja104386g  2010 American Chemical Society

C O M M U N I C A T I O N S
shorter than those in unsubstituted NHC-derived triazenes,17
indicating better acceptor properties of C1.
Scheme 2
Compounds 2 and 3 show 29Si NMR resonances (-30.01 and
-34.34 ppm, respectively) consistent19 with those reported for
aminosilanes.
NHC-stabilized dichlorosilylene IPr ·SiCl2 1 reacts in a unique
way with 1-azidoadamantane to form aminosilyl-substituted NHC
2, providing a new method for C4 or C5 functionalization of NHCs.
This is the first example of a unique silylene reaction that might
be associated with initial formation of unstable silaimine 2′, which
abstracts a proton from the IPr backbone to afford silyl-substituted
NHC 2 via the intermediate 2′′. 2 behaves as a free NHC and yields
triazene 3 upon reaction with AdN3 via end-on addition of AdN3.
distances are close to those in IPr. The N1-C1-N2 angle of
101.40(12)° is identical to that in IPr.16
Acknowledgment. Financial support from the Deutsche Fors-
chungsgemeinschaft (DFG) is gratefully acknowledged.
Supporting Information Available: Full experimental details; X-ray
crystallographic data for 2 and 3 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Bourissou, D.; Guerret, O.; Gabba¨ı, F. P.; Bertrand, G. Chem. ReV.
2000, 100, 39–91. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41,
1290-1309; Angew. Chem. 2002, 114, 1342-1363. (c) Liddle, S. T.;
Edworthy, I. S.; Arnold, P. L. Chem. Soc. ReV. 2007, 36, 1732–1744. (d)
Ku¨hl, O. Chem. Soc. ReV. 2007, 36, 592–607. (e) Dragutan, V.; Dragutan,
I.; Delaude, L.; Demonceau, A. Coord. Chem. ReV. 2007, 251, 765–794.
(f) Arnold, P. L.; Pearson, S. Coord. Chem. ReV. 2007, 251, 596–609. (g)
D´ıez-Gonza´lez, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874–883.
(h) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord.
Chem. ReV. 2009, 253, 687–703. (i) de Fre´mont, P.; Marion, N.; Nolan,
S. P. Coord. Chem. ReV. 2009, 253, 862–892.
(2) (a) N-Heterocyclic Carbenes in Transition Metal Catalysis; Glorius, F.,
Ed.; Topics in Organometallic Chemistry, Vol. 21; Springer-Verlag: Berlin,
2007. (b) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47,
3122-3172; Angew. Chem. 2008, 120, 3166-3216. (c) McGuinness, D.
Dalton Trans. 2009, 6915–6923.
(3) (a) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004, 37, 534–541. (b)
Marion, N.; D´ıez-Gonza´lez, S.; Nolan, S. P. Angew. Chem., Int. Ed. 2007,
46, 2988-3000; Angew. Chem. 2007, 119, 3046-3058. (c) Enders, D.;
Niemeier, O.; Henseler, A. Chem. ReV. 2007, 107, 5606–5655.
Figure 1. Molecular structure of 2·PhMe (ellipsoids are shown at the 50%
probability level; solvent toluene, isopropyl groups at the ortho positions,
and H atoms have been omitted for clarity). Selected bond lengths (Å) and
angles (deg): C1-N2, 1.3654(19); C2-C3, 1.356(2); C2-N2, 1.4124(19);
C2-Si1, 1.8417(16); Si1-Cl1, 2.0553(6); N3-Si1, 1.6726(14); N2-C1-N1,
101.40(12); N3-Si1-C2, 109.63(7); C2-Si1-Cl1, 107.50(5); Cl1-Si1-Cl2,
101.93(3).
(4) (a) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King, R. B.;
Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc.
2007, 129, 12412–12413. (b) Masuda, J.; Schoeller, W. W.; Donnadieu,
B.; Bertrand, G. J. Am. Chem. Soc. 2007, 129, 14180–14181. (c) Masuda,
J.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed.
2007, 46, 7052-7055; Angew. Chem. 2007, 119, 7182-7185. (d) Dyker,
C. A.; Bertrand, G. Science 2008, 321, 1050–1051. (e) Wang, Y.; Xie, Y.;
Wei, P.; King, R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson,
G. H. Science 2008, 321, 1069–1071. (f) Wang, Y.; Xie, Y.; Wei, P.; King,
R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem.
Soc. 2008, 130, 14970–14971. (g) Dyker, C. A.; Lavallo, V.; Donnadieu,
B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 3206-3209; Angew.
Chem. 2008, 120, 3250-3253. (h) Fu¨rstner, A.; Alcarazo, M.; Goddard,
R.; Lehmann, C. W. Angew. Chem., Int. Ed. 2008, 47, 3210-3214; Angew.
Chem. 2008, 120, 3254-3258. (i) Wang, Y.; Quillian, B.; Wei, P.; Xie,
Y.; Wannere, C. S.; King, R. B.; Schaefer, H. F., III; Schleyer, P. v. R.;
Robinson, G. H. J. Am. Chem. Soc. 2008, 130, 3298–3299. (j) Wang, Y.;
Robinson, G. H. Chem. Commun. 2009, 5201–5213. (k) Back, O.;
Kuchenbeiser, G.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2009,
48, 5530-5533; Angew. Chem. 2009, 121, 5638-5641. (l) Filippou, A. C.;
Chernov, O.; Schnakenburg, G. Angew. Chem., Int. Ed. 2009, 48, 5687-
5690; Angew. Chem. 2009, 121, 5797-5800. (m) Sidiropoulos, A.; Jones,
C.; Stasch, A.; Klein, S.; Frenking, G. Angew. Chem., Int. Ed. 2009, 48,
9701-9704; Angew. Chem. 2009, 121, 9881-9884. (n) Abraham, M. Y.;
Wang, Y.; Xie, Y.; Wei, P.; Schaefer, H. F., III; Schleyer, P. v. R.;
Robinson, G. H. Chem.sEur. J. 2010, 16, 432–435.
j
Triazine 3 crystallizes in the triclinic space group P1 with two
toluene molecules in the asymmetric unit (Figure 2). Comparison
of the crystallographic data for 3 with those for reported
donor-acceptor triazines17 revealed that 3 consists of a shorter
N-N single bond [N4-N5, 1.380(3) Å] and a longer N-N double
bond [N5-N6, 1.257(3) Å]. The C1-N4 bond in 3 is slightly
(5) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Angew. Chem.,
Int. Ed. 2009, 48, 5683-5686; Angew. Chem. 2009, 121, 5793-5796.
(6) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Stalke, D. Chem.sEur. J.
2010, 16, 85–88.
(7) (a) Fu¨rstner, A.; Alcarazo, M.; Ce´sar, V.; Lehmann, C. W. Chem. Commun.
2006, 2176–2178. (b) Fu¨rstner, A.; Alcarazo, M.; Ce´sar, V.; Krause, H.;
Hopkins, C. Org. Synth. 2008, 85, 34–44. (c) Urban, S.; Tursky, M.;
Fro¨hlich, R.; Glorius, F. Dalton Trans. 2009, 6934–6940. (d) Mendoza-
Espinosa, D.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc. 2010, 132,
7264–7265.
Figure 2. Molecular structure of 3·2PhMe (ellipsoids are shown at the
50% probability level; solvent toluene, isopropyl groups at the ortho
positions, and H atoms have been omitted for clarity). Selected bond lengths
(Å) and angles (deg): C1-N2, 1.382(4); C1-N4, 1.321(4); N4-N5,
1.380(3); N5-N6, 1.257(3); N6-C38, 1.480(4); C2-C3, 1.344(4); C3-N1,
1.406(4); C3-Si1, 1.847(3); Si1-Cl1, 2.0508(13); N3-Si1, 1.665(3);
N1-C1-N2, 105.0(2); C1-N4-N5, 111.6(2); N4-N5-N6, 110.9(2);
N5-N6-C38, 112.6(2); N3-Si1-C3, 111.51(14); C3-Si1-Cl1, 105.48(10);
Cl1-Si1-Cl2, 102.75(6).
(8) Crabtree, R. H. Pure Appl. Chem. 2003, 75, 435–443.
(9) Heckenroth, M.; Kluser, E.; Neels, A.; Albrecht, M. Angew. Chem., Int.
Ed. 2007, 46, 6293-6296; Angew. Chem. 2007, 119, 6409-6412.
(10) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.;
Frenking, G.; Bertrand, G. Science 2009, 326, 556–559.
9
J. AM. CHEM. SOC. VOL. 132, NO. 29, 2010 10019

C O M M U N I C A T I O N S
(11) (a) Arduengo, A. J., III; Davidson, F.; Dias, H. V. R.; Goerlich, J. R.;
Khasnis, D.; Marshall, W. J.; Prakasha, T. K. J. Am. Chem. Soc. 1997,
119, 12742–12749. (b) Denk, M. K.; Rodezno, J. M. J. Organomet. Chem.
2001, 617, 737–740. (c) Graham, T. W.; Udachin, K. A.; Carty, A. J. Chem.
Commun. 2006, 2699–2701.
(16) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R. Tetrahedron 1999, 55,
14523–14534.
(17) (a) Khramov, D. M.; Bielawski, C. W. Chem. Commun. 2005, 4958–4960.
(b) Khramov, D. M.; Bielawski, C. W. J. Org. Chem. 2007, 72, 9407–
9417.
(12) Bates, J. I.; Kennepohl, P.; Gates, D. P. Angew. Chem., Int. Ed. 2009, 48,
9844-9847; Angew. Chem. 2009, 121, 10028-10031.
(18) Crystallographic details: Shock-cooled crystals were selected and mounted
under a nitrogen atmosphere using X-TEMP2 (see: Kottke, T.; Stalke, D.
J. Appl. Crystallogr. 1993, 26, 615–619). The structures were solved by
direct methods (SHELXS) and refined on F2 using full-matrix least-squares
methods (SHELXL) (see: Sheldrick, G. M. Acta Crystallogr., Sect. A 2008,
64, 112–122). The crystal structure data have been deposited with the
Cambridge Crystallographic Data Centre as CCDC nos. 7772443 (2) and
7772442 (3). Space group, cell parameters and R values with I > 2σ(I) for
(13) (a) Cui, H.; Shao, Y.; Li, X.; Kong, L.; Cui, C. Organometallics 2009, 28,
5191–5195. (b) Arnold, P. L.; Liddle, S. T. Chem. Commun. 2005, 5638–
5640. (c) Dubinina, G. G.; Furutachi, H.; Vicic, D. A. J. Am. Chem. Soc.
2008, 130, 8600–8601. (d) John, A.; Shaikh, M. M.; Ghos, P. Dalton Trans.
2009, 10581–10591.
(14) (a) Denk, M.; Hayashi, R. K.; West, R. J. Am. Chem. Soc. 1994, 116,
10813–10814. (b) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785–
788. (c) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem.
2001, 627, 1048–1054. (d) Hill, N. J.; Moser, D. F.; Guzei, I. A.; West, R.
Organometallics 2005, 24, 3346–3349. (e) Tomasik, A. C.; Mitra, A.; West,
R. Organometallics 2009, 28, 378–381. (f) Xiong, Y.; Yao, S.; Driess, M.
Chem.sEur. J. 2009, 15, 8542–8547.
j
2: P1; a ) 11.4299(9) Å, b ) 12.0820(10) Å, c ) 15.5000(13) Å, R )
106.6130(10)°, ꢀ ) 100.313(2)°, γ ) 90.680(2)°; wR2 ) 0.0843, R1
)
j
0.0378. For 3: P1; a ) 11.9004(15) Å, b ) 15.3835(19) Å, c ) 15.9396(19)
Å, R ) 76.172(2)°, ꢀ ) 73.774(2)°, γ ) 69.913(2)°; wR2 ) 0.1409, R1 )
0.0615.
(19) Wrackmeyer, B.; Stader, C.; Zhou, H. Spectrochim. Acta 1989, 45A,
(15) Alternatively, formation of free unstable silaimine Cl2SidNAd and its
insertion into the C-H bond at the C4 position of the free IPr to afford 2
cannot be ruled out.
1101–1111.
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