Palladium-oxygen and palladium-arene interactions in complexes derived from biaryl aminocarbenes: Comparison with biaryl phosphanes

Article abstract of DOI:10.1002/anie.200704927

(Chemical Presented) Similar yet very different: Biaryl aminocarbenes were prepared, characterized spectroscopically, and coordinated to an allylpalladium fragment (see scheme). In the resulting Pd complexes of these bidentate hemilabile ligands, secondar

Full text of DOI:10.1002/anie.200704927

Angewandte
Chemie
DOI: 10.1002/anie.200704927
Aminocarbene Ligands
Palladium–Oxygen and Palladium–Arene Interactions in Complexes
Derived from Biaryl Aminocarbenes: Comparison with Biaryl
Phosphanes**
Joan Vignolle, Heinz Gornitzka, Bruno Donnadieu, Didier Bourissou,* and Guy Bertrand*
The spectacular progress in homogeneous catalysis can be
attributed largely to the development of a diverse range of
ligand frameworks. The fine-tuning of steric and electronic
factors has led to extremely active and selective catalysts. A
good example is the recent advances that have been made in
Pd-catalyzed cross-coupling reactions with biaryl phosphanes
(Scheme 1).[1] Efforts to rationalize the unique behavior of
tigate the possibility of preparing monoaminocarbenes
(MACs)[9] that feature the same topology as that of biaryl
phosphanes.[10–12] Herein we report the synthesis and charac-
terization of the MACs 1 and 2, which are related to A
(SPhos: R = cyclohexyl) and MeO-MOP (B),[13] respectively.
We show that the coordination of these carbenes to the
Pd(allyl)+ fragment is accompanied by secondary interac-
tions, albeit of a markedly different nature to those encoun-
tered with the corresponding biaryl phosphanes. A rare Pd–
oxygen contact was authenticated structurally by using the
stable biphenylcarbene 1, and spectroscopic evidence for an
unprecedented, reversible Pd–arene interaction was obtained
when the persistent binaphthylcarbene 2 was used as a ligand.
The carbene 1 was obtained readily by treatment of the
corresponding iminium triflate[14] with 2,2,6,6-tetramethylpi-
peridinolithium (TMPLi) in THF at À808C (Scheme 2). The
MAC 1 is stable indefinitely in solution at room temperature
and was characterized unambiguously on the basis of a
Scheme 1. Structures of the biaryl phosphanes A and B, and of the
related biaryl aminocarbenes 1 and 2.
these ligands have led to the substantiation both experimen-
tally and computationally of the critical role of weak Pd–
arene and Pd–X (X = O or N) interactions.[2–6]
The development of N-heterocyclic carbenes as ligands
also represents a major breakthrough in homogeneous
catalysis.[7] Our interest in increasing further the structural
variety of available stable carbenes[8] prompted us to inves-
Scheme 2. Neutral and cationic allyl palladium(II) complexes derived
fromthe biphenyl MAC 1. Tf=trifluoromethanesulfonyl.
[*] Dr. J. Vignolle, Dr. H. Gornitzka, Dr. D. Bourissou
UniversitØ Paul Sabatier
Laboratoire HØtØrochimie Fondamentale et AppliquØe
UMR–CNRS 5069
118, route de Narbonne, 31062 Toulouse cedex 09 (France)
Fax: (+33)5-6155-8204
13C NMR signal at d = 305 ppm.[11a,b] The in situ coordination
of 1 upon treatment with [{Pd(allyl)(m-Cl)}2] afforded the
corresponding air-stable complex 3 in 63% yield.[14] In the
E-mail: dbouriss@chimie.ups-tlse.fr
[15]
Dr. J. Vignolle, B. Donnadieu, Prof. G. Bertrand
UCR–CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: gbertran@mail.ucr.edu
À
solid state,
the MAC complex 3 exhibits typical Pd C
[12,16]
À
(2.03 Š) and C N (1.31 Š) bond distances,
but no
secondary interaction. To favor the participation of the
biphenyl skeleton in the coordination, complex 3 was treated
with AgBF4 in CH2Cl2. The pronounced desymmetrization of
the two methoxy groups (dH = 4.12/3.68 ppm, dC = 71.6/
56.1ppm) upon cationization strongly supports the formation
of a Pd–O contact. This hypothesis was confirmed by an X-ray
diffraction study (Figure 1). The short Pd–O distance
[**] We thank the NIH (grant R01 GM 68825), CNRS, UPS, and RHODIA
for financial support of this research.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
Angew. Chem. Int. Ed. 2008, 47, 2271 –2274
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2271

Communications
low temperature to give the air-stable complex 5 in 47%
yield.[14] X-ray diffraction analysis indicated the absence of a
secondary interaction.
We anticipated that the anti orientation of the methoxy
substituent with respect to the palladium center would
preclude a Pd–O interaction in the corresponding cationic
complex. To determine whether the binaphthyl skeleton may
engage in Pd–arene interactions, the cationic complex 6 was
prepared by chloride abstraction with AgBF4 in CH2Cl2. The
signals for the methoxy groups remained virtually unchanged
in the NMR spectra (DdH = 0.16 ppm, DdC = 1.9 ppm), in
agreement with the absence of a Pd–O contact in 6. Careful
inspection of the 13C NMR spectrum revealed that two of the
signals for the carbon atoms of the binaphthyl skeleton are
shifted upfield to a significant extent in this cationic complex
(d = 99.3 and 112.0 ppm in 6 versus d > 122 ppm in 5). This
situation is reminiscent of that observed for some binaph-
thylphosphane complexes[4d–f] and strongly suggests weak
h2 coordination of the naphthyl backbone to the Pd center.
The two diagnostic high-field signals observed for 6 were
assigned unambiguously to C9’ and C10’ on the basis of 2D
NMR experiments (see Scheme 3 for atom numbering). This
interaction contrasts with the h2 bonding through C1’/C2’
encountered in the related MeO-MOP complexes. In fact,
Pd–arene interactions involving the remote ring of the
binaphthyl skeleton, as observed in 6, are unknown in biaryl
phosphanes. In the case of 6, the interaction most likely
results from the configuration of the binaphthyl axis.
Crystals of 6 suitable for X-ray diffraction analysis could
only be obtained upon cooling a solution of 6 in THF to
À308C. The modest quality of the crystallographic data
precludes discussion of the geometric parameters, but the
connectivity of the complex was established unambiguously
(Figure 1). Thus, a 1:1 adduct 7 was formed by the coordina-
tion of one THF molecule to the Pd center and removal of the
Pd–arene interaction. The dissolution of 7 in CDCl3 returned
6 along with free THF in a further illustration of the
hemilabile character of the aminobinaphthylcarbene 2.
Thus, as observed for the related biaryl phosphanes SPhos
and MeO-MOP, the coordination of the biaryl aminocarbenes
1 and 2 to the {Pd(allyl)}+ fragment is accompanied by
secondary interactions. The formal replacement of the
phosphane group with an aminocarbene moiety influences
markedly the relative positions of the metal fragment and the
biaryl skeleton, so that unusual Pd–O and Pd–arene inter-
actions are found with the hybrid ligands 1 and 2. Biaryl
aminocarbenes further expand the structural diversity of
bidentate ligands based on aminocarbenes.[18] Their hemi-
labile character,[19] which has been substantiated spectroscop-
ically by reversible competition between h2 naphthyl coordi-
nation and coordination by THF, may offer interesting
perspectives in catalysis.
Figure 1. Molecular views (thermal ellipsoids at 50% probability) of
complexes 4 (left) and 7 (right) in the solid state. Hydrogen atoms are
omitted for clarity.
(2.21Š) [17] indicated unambiguously the presence of a bond-
ing interaction. This Pd–O contact contrasts markedly with
the Pd–arene interactions commonly encountered in the
related SPhos and MeO-MOP complexes,[3c,4a,d,f] a difference
that may be attributed to the different orientations of the
À
À
Ccarbene Pd and P Pd bonds with respect to the biphenyl
skeleton (Pd-Ccarbene-Cipso-Cortho: 788 in 3, 868 in 4 versus Pd-P-
C
ipso-Cortho: 0–108). Surprisingly, evidence for such Pd–O
contacts has been observed only once for biaryl phosphanes,
by NMR spectroscopy.[4e]
We then turned our attention to the binaphthylaminocar-
bene 2, which is related to MeO-MOP (B). The corresponding
iminium salt was deprotonated with KHMDS at À808C in
THF (Scheme 3).[14] Carbene 2 is only moderately stable at
1
room temperature (t= ꢀ 30 min) and was therefore charac-
2
terized at À788C. Its structure was confirmed unambiguously
on the basis of the diagnostic 13C NMR signal observed at d =
307 ppm. In situ coordination of 2 to {Pd(allyl)Cl} occurred at
Complex 3 derived from the biphenylaminocarbene 1 was
evaluated as a catalyst for the a arylation of propiophe-
none.[20,21] With a precatalyst loading of 2 mol%, bromoben-
zene underwent quantitative coupling with propiophenone at
room temperature within 15 h. Although the reaction pro-
ceeded more slowly, chlorobenzene also acted as an effective
arylating agent under the same conditions.[14] According to
Scheme 3. Neutral and cationic allyl palladium(II) complexes derived
fromthe binaphthyl MAC 2. HMDS=hexamethyldisilazane.
2272
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 2271 –2274

Angewandte
Chemie
[2] For an o-phenanthrylphosphane, see: J. Yin, M. P. Rainka, X.-X.
these preliminary results, the MAC complex 3 competes with
the most active phosphane-based catalysts reported to date
for the coupling of aryl bromides. Indeed, binap (2,2’-
bis(diphenylphosphanyl)-1,1’-binaphthyl) ligands,[20] bis(di-
arylphosphanyl)ferrocene derivatives,[21] and biphenylphos-
phanes[22a,b] are typically used at temperatures higher than
508C, and only PtBu3,[21b] DtBPF (1,1’-bis(di-tert-butylphos-
phanyl)ferrocene),[21b] and binaphthyl(dialkyl)phosphanes[22c]
were found to be suitable for the coupling of aryl bromides at
room temperature. The reaction of chlorobenzene at room
temperature is also encouraging, as, until now, aryl chlorides
have only been coupled at high temperatures,[20,21b,22a,b,23]
except with cyclic alkyl aminocarbenes.[24] Given the superi-
ority of cyclic carbenes over their acyclic counterparts as
robust ligands for transition metals,[25] better catalytic results
can be expected with cyclic biaryl aminocarbenes, the
preparation of which is under active investigation.
Zhang, S. L. Buchwald, J. Am. Chem. Soc. 2002, 124, 1162 – 1163.
[3] For biphenylphosphanes, see: a) S. M. Reid, R. C. Boyle, J. T.
Mague, M. J. Fink, J. Am. Chem. Soc. 2003, 125, 7816 – 7817;
b) J. W. Faller, N. Sarantopoulos, Organometallics 2004, 23,
2008 – 2014; c) S. D. Walker, T. E. Barder, J. R. Martinelli, S. L.
Buchwald, Angew. Chem. 2004, 116, 1907 – 1912; Angew. Chem.
Int. Ed. 2004, 43, 1871 – 1876; d) T. E. Barder, M. R. Biscoe, S. L.
Buchwald, Organometallics 2007, 26, 2183 – 2192.
ˇ
´
ˇ
[4] For binaphthylphosphanes, see: a) P. Kocovsky, S. Vyskocil, I.
ˇ
ˇ
ˇ
Císarovµ, J. Sejbal, I. Tislerovµ, M. Smrcina, G. C. Lloyd-Jones,
S. C. Stephen, C. P. Butts, M. Murray, V. Langer, J. Am. Chem.
Soc. 1999, 121, 7714 – 7715; b) Y. Wang, X. Li, J. Sun, K. Ding,
Organometallics 2003, 22, 1856 – 1862; c) W. J. Marshall, V. V.
Grushin, Organometallics 2003, 22, 555 – 562; d) P. Dotta,
P. G. A. Kumar, P. S. Pregosin, A. Albinati, S. Rizzato, Organo-
metallics 2003, 22, 5345 – 5349; e) P. Dotta, P. G. A. Kumar, P. S.
Pregosin, A. Albinati, S. Rizzato, Organometallics 2004, 23,
4247 – 4254; f) P. G. A. Kumar, P. Dotta, R. Hermatschweiler,
P. S. Pregosin, A. Albinati, S. Rizzato, Organometallics 2005, 24,
1306 – 1314.
[5] For Pd–arene interactions in biaryl phosphane palladium(I)
dimers, see: a) U. Christmann, D. A. Pantazis, J. Benet-Buch-
holz, J. E. McGrady, F. Maseras, R. Vilar, J. Am. Chem. Soc.
2006, 128, 6376 – 6390; b) T. E. Barder, J. Am. Chem. Soc. 2006,
128, 898 – 904.
[6] Pd–arene interactions have also been evidenced in arylated-
alkyl palladium(II) complexes; see, for example: a) J. Cµmpora,
J. A. López, P. Palma, P. Valerga, E. Spillner, E. Carmona,
Angew. Chem. 1999, 111, 199 – 203; Angew. Chem. Int. Ed. 1999,
38, 147 – 151; b) M. Catellani, C. Mealli, E. Motti, P. Paoli, E.
Perez-Carreæo, P. S. Pregosin, J. Am. Chem. Soc. 2002, 124,
4336 – 4346.
[7] a) W. A. Herrmann, Angew. Chem. 2002, 114, 1342 – 1363;
Angew. Chem. Int. Ed. 2002, 41, 1290 – 1309; b) C. M. Crudden,
D. P. Allen, Coord. Chem. Rev. 2004, 248, 2247 – 2273; c) E. A. B.
Kantchev, C. J. OꢀBrien, M. G. Organ, Angew. Chem. 2007, 119,
2824 – 2870; Angew. Chem. Int. Ed. 2007, 46, 2768 – 2813.
[8] a) A. J. Arduengo III, Acc. Chem. Res. 1999, 32, 913 – 921; b) D.
Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem. Rev.
2000, 100, 39 – 92; c) Y. Canac, M. Soleilhavoup, S. Conejero, G.
Bertrand, J. Organomet. Chem. 2004, 698, 3857 – 3865.
[9] a) S. SolØ, H. Gornitzka, W. W. Schoeller, D. Bourissou, G.
Bertrand, Science 2001, 292, 1901 – 1903; b) X. Catton, H.
Gornitzka, D. Bourissou, G. Bertrand, J. Am. Chem. Soc. 2004,
126, 1342 – 1343.
Experimental Section
All reactions and manipulations were carried out under an atmos-
phere of dry argon by using standard Schlenk techniques. All NMR
spectra were recorded at 293 K unless otherwise stated.
Carbenes 1 and 2: A 1:1 mixture of the iminium precursor and
TMPLi or KHMDS was cooled to À788C, and [D8]THF was added.
The suspension was warmed to room temperature and stirred for
30 min. 13C NMR ([D8]THF): dC(carbene) = 304.6 (for 1) and
307.1ppm (for 2).
Neutral complexes 3 and 5: [{Pd(allyl)Cl}2] (0.45 equiv with
respect to the iminium precursor) was added at À788C to a freshly
prepared solution of carbene 1 or 2 in THF (10 mL). The resulting
solution was warmed to room temperature and stirred for 2 h.
Workup and recrystallization from CH2Cl2/Et2O afforded 3 or 5 as
yellow crystals. 13C NMR (CDCl3): dC(carbene) = 253.9 (for 3) and
252.6 ppm (for 5).
Cationic complexes 4 and 6: A 1:1 mixture of 3 or 5 and AgBF4
(0.24 mmol) was cooled to À308C, and CH2Cl2 (5 mL) was added.
The resulting solution was warmed to room temperature and stirred
for 30 min. Workup and recrystallization from CH2Cl2/Et2O at 58C
afforded 4 (0.114 g, 85%) as yellow crystals: 13C NMR (CDCl3,
À208C): d = 56.1(OCH ), 71.6 (br, OCH3), 245.7 ppm (br, Ccarbene).
3
Attempts to crystallize complex 6 (13C NMR (CDCl3, 258C): d = 55.9
(OCH3), 99.3 (C10’), 112.0 (C9’), 246.7 ppm (Ccarbene)) from THF at
À308C afforded the adduct 7 (260 mg, 69%) as brown crystals.
[10] For biphenyl NHCs, see: a) L. Delaude, M. Szypa, A. Demon-
ceau, A. F. Noels, Adv. Synth. Catal. 2002, 344, 749 – 756; b) C.
Fliedel, A. Maisse-François, S. Bellemin-Laponnaz, Inorg. Chim.
Acta 2007, 360, 143 – 148.
Received: October 24, 2007
Published online: February 13, 2008
[11] For binaphthyl NHCs, see: a) D. S. Clyne, J. Jin, E. Genest, J. C.
Gallucci, T. V. Rajan Babu, Org. Lett. 2000, 2, 1125 – 1128; b) W.-
L. Duan, M. Shi, G.-B. Rong, Chem. Commun. 2003, 2916 – 2917;
c) J. J. Van Veldhuizen, D. G. Gillingham, S. B. Garber, O.
Kataoka, A. H. Hoveyda, J. Am. Chem. Soc. 2003, 125, 12502 –
12508; d) A. R. Chianese, R. H. Crabtree, Organometallics
2005, 24, 4432 – 4436.
[12] Transition-metal complexes of heteroditopic (phosphane/amino-
carbene) ligands that feature a biphenyl spacer were obtained
from the corresponding cyclic C-amino phosphorus ylides: J.
Vignolle, B. Donnadieu, D. Bourissou, M. Soleihavoup, G.
Bertrand, J. Am. Chem. Soc. 2006, 128, 14810 – 14811.
[13] MOP is a generic term for 2-phosphanyl-2’-substituted 1,1’-
binaphthyl and MeO-MOP stands for 2-phosphanyl-2’-methoxy-
1,1’-binaphthyl; for applications of binaphthylphosphanes in
asymmetric catalysis, see: a) T. Hayashi, Acc. Chem. Res. 2000,
33, 354 – 362; b) G. C. Lloyd-Jones, S. C. Stephen, M. Murray,
Keywords: aminocarbenes · biaryls · coordination modes ·
metal–ligand interactions · palladium
.
À
À
À
À
[1] For selected examples of Pd-catalyzed C C, C N, C O, and C
B cross-coupling reactions with biaryl phosphanes, see: a) E. R.
Strieter, D. G. Blackmond, S. L. Buchwald, J. Am. Chem. Soc.
2003, 125, 13978 – 13980; b) J. E. Milne, S. L. Buchwald, J. Am.
Chem. Soc. 2004, 126, 13028 – 13032; c) T. E. Barder, S. D.
Walker, J. R. Martinelli, S. L. Buchwald, J. Am. Chem. Soc.
2005, 127, 4685 – 4696; d) C. H. Burgos, T. E. Barder, X. Huang,
S. L. Buchwald, Angew. Chem. 2006, 118, 4427 – 4432; Angew.
Chem. Int. Ed. 2006, 45, 4321– 4326; e) K. L. Billingsley, T. E.
Barder, S. L. Buchwald, Angew. Chem. 2007, 119, 5455 – 5459;
Angew. Chem. Int. Ed. 2007, 46, 5359 – 5363.
Angew. Chem. Int. Ed. 2008, 47, 2271 –2274
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2273

Communications
ˇ
ˇ
´
C. P. Butts, S. Vyskocil, P. Kocovsky, Chem. Eur. J. 2000, 6, 4348 –
towards tungsten fragments. Weak interactions were evidenced
=
ˇ
4357; c) I. J. S. Fairlamb, G. C. Lloyd-Jones, S. Vyskocil, P.
between the metal and a C C bond of one mesityl ring: F. Wu,
ˇ
´
ˇ
´
Kocovsky, Chem. Eur. J. 2002, 8, 4443 – 4453; d) P. Kocovsky, S.
V. K. Dioumaev, D. J. Szalda, J. Hanson, J. A. Franz, R. M.
Bullock, Organometallics 2007, 26, 5079 – 5090.
[20] M. Palucki, S. L. Buchwald, J. Am. Chem. Soc. 1997, 119, 11108 –
11109.
[21] a) B. C. Hamann, J. F. Hartwig, J. Am. Chem. Soc. 1997, 119,
12382 – 12383; b) M. Kawatsura, J. F. Hartwig, J. Am. Chem. Soc.
1999, 121, 1473 – 1478.
[22] a) D. W. Old, J. P. Wolfe, S. L. Buchwald, J. Am. Chem. Soc.
1998, 120, 9722 – 9723; b) J. M. Fox, X. Huang, A. Chieffi, S. L.
Buchwald, J. Am. Chem. Soc. 2000, 122, 1360 – 1370; c) T.
Hamada, A. Chieffi, J. Ahman, S. L. Buchwald, J. Am. Chem.
Soc. 2002, 124, 1261 – 1268.
[23] a) A. Ehrentraut, A. Zapf, M. Beller, Adv. Synth. Catal. 2002,
344, 209 – 217; b) M. S. Viciu, R. F. Germaneau, S. P. Nolan, Org.
Lett. 2002, 4, 4053 – 4056.
[24] V. Lavallo, Y. Canac, C. Prꢃsang, B. Donnadieu, G. Bertrand,
Angew. Chem. 2005, 117, 5851– 5855; Angew. Chem. Int. Ed.
2005, 44, 5705 – 5709.
[25] Cyclic diaminocarbenes and alkyl aminocarbenes proved to be
much more efficient ligands than their acyclic counterparts; see:
a) W. A. Herrmann, K. ꢄfele, D. v. Preysing, E. Herdtweck, J.
Organomet. Chem. 2003, 684, 235 – 248, and Ref. [24].
ˇ
ˇ
Vyskocil, M. Smrcina, Chem. Rev. 2003, 103, 3213 – 3245.
[14] See the Supporting Information for details.
[15] CCDC 664945 (3), CCDC 664946 (4), CCDC 664947 (5), and
CCDC 664948 (7) 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.
[16] D. Kremzow, G. Seidel, C. W. Lehmann, A. Fꢁrstner, Chem. Eur.
J. 2005, 11, 1833 – 1853.
[17] Similar Pd O distances (2.20–2.22 Š) were observed in palla-
À
dium(II) complexes of (2,6-dimethoxyphenyl)diphenylphos-
phane: J.-F. Ma, Y. Kojima, Y. Yamamoto, J. Organomet.
Chem. 2000, 616, 149 – 156.
[18] a) S. J. Roseblade, A. Ros, D. Monge, M. Alcarazo, E. ꢂlvarez,
J. M. Lassaletta, R. Fernandez, Organometallics 2007, 26, 2570 –
2578; b) Y. Canac, C. Duhayon, R. Chauvin, Angew. Chem. 2007,
119, 6429 – 6431; Angew. Chem. Int. Ed. 2007, 46, 6313 – 6315;
c) O. Kꢁhl, Chem. Soc. Rev. 2007, 36, 592 – 607; d) S. T. Liddle,
I. S. Edworth, P. L. Arnold, Chem. Soc. Rev. 2007, 36, 1732 –
1744, and references therein.
[19] IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) was
recently found to behave as a bidentate hemilabile ligand
2274
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 2271 –2274