A zwitterionic palladium(II) η3-boratoxypentadienyl complex: Cooperative activation of dibenzylideneacetone between palladium and a phosphine/thioether/borane ligand

Article abstract of DOI:10.1021/om0602052

phosphine/thioether/borane ligand (5) suitable to position a Lewis acidic borane in close proximity to a coordinated late transition metal has been prepared. Reaction with 0.5 [Pd₂dba)₃] gave a zwitterionic palladium(II) η³boratoxypentadienyl complex with the empirical formula [Pd(dba)(5)].

Full text of DOI:10.1021/om0602052

2412
Organometallics 2006, 25, 2412-2414
A Zwitterionic Palladium(II) η3-Boratoxypentadienyl Complex:
Cooperative Activation of Dibenzylideneacetone between Palladium
and a Phosphine/Thioether/Borane Ligand
David J. H. Emslie,* James. M. Blackwell, James F. Britten, and Laura E. Harrington
Department of Chemistry, McMaster UniVersity, 1280 Main Street West, Hamilton,
Ontario L8S 4M1, Canada
ReceiVed March 2, 2006
Summary: A free phosphine/thioether/borane ligand (5) suitable
to position a Lewis acidic borane in close proximity to a
coordinated late transition metal has been prepared. Reaction
with 0.5 [Pd2(dba)3] gaVe a zwitterionic palladium(II) η3-
boratoxypentadienyl complex with the empirical formula [Pd-
(dba)(5)].
Several Lewis acid-Lewis base molecules have also been
proposed as in situ generated bifunctional catalysts, but the exact
nature of these complexes has not been established.10
We are interested in the preparation of Lewis acid-Lewis
base molecules as ligands suitable to position a strongly Lewis
acidic group in close proximity to a coordinated late transition
metal. Once in the coordination sphere of the metal, the borane
can be expected to interact either with the metal to form a rare8,11
transition-metal borane complex (MfBR3) or with coligands,
resulting in their coordination, abstraction, or activation. To this
end, 2,7-di-tert-butyl-5-(diphenylboryl)-4-(diphenylphosphino)-
9,9-dimethylthioxanthene (5) (Scheme 1), which incorporates
a phosphine, a thioether, and a borane group in a rigid chelating
framework, was prepared. A molecule containing two donor
groups was chosen to ensure that a coordinated metal is
positioned in close proximity to the pendant borane, and phenyl
substituents on boron and phosphorus were chosen to prevent
intramolecular adduct formation as well as impart sufficient
Lewis acidity to the borane.12
Molecules which contain both a group 13 Lewis acid and a
Lewis base have recently received increased interest, largely
due to their photophysical and nonlinear optical properties or
potential as cooperative Lewis acid and Lewis base catalysts
for organic transformations. For the former application, the
Lewis acid is typically rendered unreactive with the use of
extremely bulky (usually 2,4,6-trimethylphenyl (Mes)) substit-
uents.1 In the latter applications, various molecules have been
prepared, but many suffer from inter- or intramolecular Lewis
acid-Lewis base adduct formation which can shut down
reactivity,2-4 and those which do not employ either a poor Lewis
base (e.g. a triarylamine) or a relatively poor Lewis acid (e.g.
ArB(OR)2) to circumvent such problems.3,5 Exceptions are bis-
and tris(2-thienyl)boranes,6 1-phenylthieno[3,4-d]borepin,7 bis-
(o-(diisopropylphosphino)phenyl)phenylborane,8 and boron-
containing polythiophenes reported recently by Ja¨kle et al.9
Ligand 5 was prepared in five steps from commercially
available thioxanthone (Scheme 1). Reaction of thioxanthone
with AlMe3 in toluene gave 9,9-dimethylthioxanthene (1) in a
manner analogous to the preparation of 9,9-dimethylxanthene.13
* To whom correspondence should be addressed. Fax: (905) 522-2509.
Tel: (905) 525-9140. E-mail: emslied@mcmaster.ca.
(10) (a) France, S.; Wack, H.; Hafez, A. M.; Taggi, A. E.; Witsil, D. R.;
Lectka, T. Org. Lett. 2002, 4, 1603. (b) Gro¨ger, H. Chem. Eur. J. 2001, 7,
5247. (c) Ichikawa, E.; Suzuki, M.; Yabu, K.; Albert, M.; Kanai, M.;
Shibasaki, M. J. Am. Chem. Soc. 2004, 126, 11808. (d) Takamura, M.;
Hamashima, Y.; Usuda, H.; Kanai, M.; Shibasaki, M. Angew. Chem., Int.
Ed. 2000, 39, 1650.
(1) (a) Liu, Z.; Fang, Q.; Wang, D.; Cao, D.; Xue, G.; Yu, W.; Lei, H.
Chem. Eur. J. 2003, 9, 5074. (b) Agou, T.; Kobayashi, J.; Kawashima, T.,
Org. Lett. 2005, 7, 4373. (c) Albrecht, K.; Kaiser, V.; Boese, R.; Adams,
J.; Kaufmann, D. E. J. Chem. Soc., Perkin Trans. 2 2000, 2153. (d) Yuan,
Z.; Taylor, N. J.; Sun, Y.; Marder, T. B.; Williams, I. D.; Cheng, L.-T. J.
Organomet. Chem. 1993, 449, 27. (e) Yuan, Z.; Taylor, N. J.; Marder, T.
B.; Williams, I. D.; Kurtz, S. K.; Cheng, L.-T. J. Chem. Soc., Chem.
Commun. 1990, 1489.
(2) Dimeric [(Me2PCH2AlMe2)2] has been shown to act as a source of
Me2PCH2AlMe2 in solution: Fontaine, F.-G.; Zargarian, D. J. Am. Chem.
Soc. 2004, 126, 8786.
(3) (a) Weis, N.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 1999,
393. (b) Go¨rz, D.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2003,
2783.
(4) (a) Mu¨ller, G.; Lachmann, J. Z. Naturforsch., B 1993, 48, 1248. (b)
Rathke, J.; Schaeffer, R. Inorg. Chem. 1972, 11, 1150. (c) Sugihara, Y.;
Miyatake, R.; Takakura, K.; Yano, S. Chem. Commun. 1994, 1925.
(5) (a) Giles, R. L.; Howard, J. A. K.; Patrick, L. G. F.; Probert, M. R.;
Smith, G. E.; Whiting, A. J. Organomet. Chem. 2003, 680, 257. (b) Roesler,
R.; Piers, W. E.; Parvez, M. J. Organomet. Chem. 2003, 680, 218. (c)
Herberich, G. E.; Fischer, A. Organometallics 1996, 15, 58. (d) Karsch, H.
H.; Appelt, A.; Ko¨hler, F. H.; Mu¨ller, G. Organometallics 1985, 4, 231.
(e) Labinger, J. A.; Bonfiglio, J. N.; Grimmett, D. L.; Masuo, S. T.; Shearin,
E.; Miller, J. S. Organometallics 1983, 2, 733.
(6) (a) Wrackmeyer, B.; Milius, W.; Molla, E. Z. Naturforsch., B 1996,
51, 1811. (b) Kohler, T.; Faderl, J.; Pritzkow, H.; Siebert, W. Eur. J. Inorg.
Chem. 2002, 2942.
(7) Sugihara, Y.; Yagi, T.; Murata, I. J. Am. Chem. Soc. 1992, 114, 1479.
(8) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou,
D. Angew. Chem., Int. Ed. 2006, 45, 1611.
(11) (a) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Willis, A. C.
Organometallics 2005, 24, 4083. (b) Hill, A. F.; Owen, G. R.; White, A. J.
P.; Williams, D. J. Angew. Chem., Int. Ed. 1999, 38, 2759. (c) Foreman,
M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J.
Organometallics 2003, 22, 4446. (d) Foreman, M. R. St.-J.; Hill, A. F.;
White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 913. (e) Crossley,
I. R.; Hill, A. F. Organometallics 2004, 23, 5656. (f) Crossley, I. R.; Hill,
A. F.; Willis, A. C. Organometallics 2005, 24, 1062. (g) Crossley, I. R.;
Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Chem.
Commun. 2005, 221. (h) Mihalcik, D. J.; White, J. L.; Tanski, J. M.;
Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.; Rheingold, A. L.;
Rabinovich, D. Dalton Trans. 2004, 1626. (i) Gamboa, J. A.; Sundararaman,
A.; Kakalis, L.; Lough, A. J.; Ja¨kle, F. Organometallics 2002, 21, 4169. (j)
Duchateau, R.; Lancaster, S. J.; Thornton-Pett, M.; Bochmann, M. Orga-
nometallics 1997, 16, 4995. (k) Carpenter, B. E.; Piers, W. E.; Parvez, M.;
Yap, G. P. A.; Rettig, S. J. Can. J. Chem. 2001, 79, 857.
(12) While steric effects and issues of hardness/softness have a pro-
nounced effect on the strength of Lewis acid-base interactions, the order
AlCl3 > BF3 ≈ B(C6F5)3 > BPh3 > BEt3, PhB(OH)2, B(OMe)3, B(NMe2)3
is well accepted: (a) Farfa´n, N.; Contreras, R. J. Chem. Soc., Perkin Trans.
2 1987, 771. (b) Bradley, D. C.; Harding, I. S.; Keefe, A. D.; Motevalli,
M.; Zheng, D. H. J. Chem. Soc., Dalton Trans. 1996, 3931. (c) Beckett,
M. A.; Brassington, D. S.; Coles, S. J.; Hursthouse, M. B. Inorg. Chem.
Commun. 2000, 3, 530. (d) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P.
Organometallics 2005, 24, 1685. (e) Holleman, A. F.; Wiberg, E. Inorganic
Chemistry, 101st ed.; Academic Press: San Diego, CA, 2001. (f) Childs,
R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982, 60, 801.
(9) Sundararaman, A.; Victor, M.; Varughese, R.; Ja¨kle, F. J. Am. Chem.
Soc. 2005, 127, 13748.
10.1021/om0602052 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/07/2006

Communications
Organometallics, Vol. 25, No. 10, 2006 2413
Scheme 1. Preparation of Ligand 5 from Thioxanthone
Scheme 2. Reaction of Ligand 5 with [Pd2(dba)3]a
a Two limiting structures of the product are shown.
tert-Butyl groups were then installed in the 2- and 7-positions
to give 2 by a Friedel-Crafts reaction with 2-chloro-2-
methylpropane and AlCl3. Subsequent reaction of 2 with Br2
in AcOH furnished 2,7-di-tert-butyl-4,5-dibromo-9,9-dimeth-
ylthioxanthene (3), which can be selectively monolithiated using
n-butyllithium at -45 °C and then reacted with Ph2PCl to give
the desired bromo/phosphine precursor (4). Lithiation of 4 using
tert-butyllithium and then quenching with Ph2BBr afforded
crude 5, which could not be purified directly due to its extremely
high solubility. However, dissolution of crude 5 in MeCN
resulted in precipitation of the acetonitrile adduct of the ligand
(5‚NCMe) as a white solid. Under dynamic vacuum, the adduct
regenerates base-free 5, which is isolated in 66% yield as an
air-sensitive lemon yellow powder.
Figure 1. ORTEP drawing of [Pd(dba)(5)]‚3CH2Cl2 with solvent
and most hydrogen atoms omitted for clarity (50% thermal
ellipsoids). Selected bond distances (Å) and angles (deg): Pd-P
) 2.268(1), Pd-S ) 2.344(1), Pd-C(48) ) 2.139(3), Pd-C(49)
) 2.149(3), Pd-C(50) ) 2.337(3), C(48)-C(49) ) 1.425(5),
C(49)-C(50) ) 1.397(5), C(50)-C(51) ) 1.473(5), C(51)-C(52)
) 1.334(5), C(50)-O ) 1.319(4), B-O ) 1.537(4); P-Pd-S )
87.15(3).
Ligand 5 was characterized by NMR spectroscopy, HRMS,
and CHS elemental analysis. In solution, 5 shows a sharp singlet
at -8.65 ppm in the 31P NMR spectrum and a broad singlet at
+69 ppm in the 11B NMR spectrum. The 11B chemical shift is
characteristic of a free triarylborane, and the sharp 31P signal is
indicative of a free triarylphosphine. The monomeric nature of
5 is further supported by mass spectrometry and its high
at -30 °C.15 The solid-state structure (Figure 1)16 shows that
ligand 5 is coordinated to palladium via phosphorus and sulfur
and the borane group of 5 is bonded to the oxygen atom of a
dba molecule ligated to palladium. Considerable distortion of
ligand 5 is clearly required in order to allow boron-oxygen
bond formation; boron lies 1.14 Å below the S-C(9)-C(13)
plane, and the S-C(12)-C(5)-B torsion angle is 17.7(5)°.
For a complex with the empirical formula [Pd(dba)(5)], two
limiting structures are conceivable: (1) a palladium(0) alkene
1
solubility in hexanes and hexamethyldisiloxane. In the H and
13C NMR spectra of 5, the two methyl groups of the ligand
backbone are equivalent and there is no change in the 1H NMR
spectrum between +20 and -80 °C. This could result from a
planar thioxanthene ligand backbone or from a bent backbone
with rapid inversion on the NMR time scale. Attempts to grow
X-ray-quality crystals of 5 were unsuccessful, due to the extreme
solubility of 5 in noncoordinating solvents. However, a bent
thioxanthene backbone is more likely, by analogy with related
alkylidene-9H-thioxanthenes and phenothiazines.14
(14) (a) Koumura, N.; Geertsema, E. M.; Gelder, M. B. v.; Meetsma,
A.; Feringa, B. L. J. Am. Chem. Soc. 2002, 124, 5037. (b) Kra¨mer, C. S.;
Zeitler, K.; Mu¨ller, T. J. J. Tetrahedron Lett. 2001, 42, 8619.
(15) In addition to [Pd(dba)(5)]‚3CH2Cl2, crystals of [Pd(dba)(5)] with
a different amount of (disordered) solvent were also formed and investigated
by X-ray crystallography. The structures of the [Pd(dba)(5)] core are to all
intents and purposes identical in both crystals.
(16) Crystal data for [Pd(dba)(5)]‚3CH2Cl2: C67H68BCl6OPPdS, 0.35 ×
0.25 × 0.10 mm3, mol wt 1282.15, triclinic, P1h, a ) 14.4708(9) Å, b )
15.2820(8) Å, c ) 17.4377(11) Å, R ) 69.479(3)°, â ) 77.378(3)°, γ )
61.798(3)°, V ) 3176.3(3) Å3, Z ) 2, Fcalcd ) 1.341 g cm-3, 2θmax ) 55.02°,
λ(Mo KR) ) 0.710 73 Å, T ) 173(2) K, 28 166 measured reflections, 14 111
unique (Rint ) 0.0522), 14 111 reflections included in the refinement, I >
2σ(I), µ ) 0.644 mm-1, minimum transmission 0.719, maximum transmis-
sion 0.938, 703 parameters, final R indices (I > 2σ(I)) R1 ) 0.0522, wR2
) 0.1209, GOF ) 1.029, residual electron density 0.098 (rms). Crystal-
lographic data (excluding structure factors) for the structure reported in
this paper have been deposited with the Cambridge Crystallographic Data
Centre as Supplementary Publication No. CCDC-290380. Copies of the
data can be obtained free of charge on application to the CCDC, 12 Union
Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223-336-033; e-mail,
deposit@ccdc.cam.ac.uk).
To test the efficacy of 5 as a ligand in late-transition-metal
chemistry and the ability of 5 to position the borane in close
proximity to a coordinated transition metal, the reaction of 5
with 0.5 equiv of [Pd2(dba)3] (dba ) trans,trans-dibenzylide-
neacetone) in CH2Cl2 was carried out, forming orange [Pd(dba)-
(5)]‚CH2Cl2 in 79% yield after crystallization from CH2Cl2/
hexanes and drying in vacuo (Scheme 2). This complex was
studied by NMR spectroscopy and X-ray crystallography.
Selected spectroscopic features include a sharp singlet at +30.25
ppm in the 31P NMR spectrum and a broad singlet at +5.3 ppm
in the 11B NMR spectrum, characteristic of a four-coordinate
borane or borate.
X-ray-quality crystals of [Pd(dba)(5)]‚3CH2Cl2 were grown
by diffusion of hexanes into a CH2Cl2 solution of the complex
(13) Nowick, J. S.; Ballester, P.; Ebmeyer, F.; Julius Rebek, J. J. Am.
Chem. Soc. 1990, 112, 8902.

2414 Organometallics, Vol. 25, No. 10, 2006
Communications
complex with the borane unit coordinated to the ketone and (2)
a zwitterionic palladium(II) η3-boratoxypentadienyl complex
(Scheme 2). The crystal structure of [Pd(dba)(5)] is only
consistent with the palladium(II) η3-boratoxypentadienyl struc-
ture, for the following reasons. (1) The C(49)-C(50) distance
of 1.397(5) Å is considerably shorter than a C-C single bond
(cf. C(50)-C(51) at 1.473(5) Å) but is typical for an allyl unit.
(2) The C(50)-O distance of 1.319(4) Å is longer than expected
for a borane-ketone adduct (typically 1.25-1.28 Å) but falls
within the usual range for an alkoxyborate (typically 1.27-
1.33 Å). (3) The Pd-C(50) distance of 2.337(3) Å is only
slightly longer than that expected for a typical palladium(II)
allyl complex (2.0-2.3 Å).
The extent to which the zwitterionic palladium(II) structure
dominates in [Pd(dba)(5)] is remarkable in comparison with
structures of complexes formed upon reaction of [(PPh3)2Pd-
(η2-PhHCdCHC(O)R)] (R ) H, Me) with strong Lewis acids
such as BF3, B(C6F5)3, and AlCl3. For this intermolecular variant
studied by Ogoshi and Kurosawa et al., considerably longer Pd-
CO bond lengths were observed in all cases, and a palladium-
(II) boratoxyallyl like complex was only observed by combi-
nation of BF3 with the more easily activated enal (R ) H).17 In
this work, we employ a far less Lewis acidic borane (similar to
BPh3)12 and an enone which should be more difficult to activate
(R ) PhHCdCH versus R ) H, Me).18 The unusual ability of
the pendant borane in [Pd(dba)(5)] to activate enones is perhaps
due to entropic factors which favor initial palladium-alkene
and borane-ketone coordination and steric constraints imposed
by the rigidity of 5, which force the carbonyl carbon into closer
proximity with the metal. The rigidity of 5 is also manifested
in the exclusive formation of [Pd(dba)(5)] with trans-disposed
substituents around the C(49)-C(50) bond. In contrast, the
Ogoshi/Kurosawa complexes are formed as cis isomers with
one exception; the trans isomer of [(PPh3)2Pd{CHPhCHCHOB-
(C6F5)3}] was observed as a minor component in solution (less
than 5%) but crystallized preferentially.17
NMR data for [Pd(dba)(5)] also support an η3-boratoxypen-
tadienyl structure in solution. At -75 °C in CD2Cl2, the 13C-
{1H} NMR spectrum shows a doublet at 144.08 ppm (2JC,P
)
10 Hz) for C(50). The magnitude of this coupling constant falls
within the range observed for phosphine-ligated palladium(II)
alkyl complexes, and the identity of the peak and origin of the
coupling has been confirmed by 1H/13C-HMBC NMR and 13C-
{1H} NMR at both 126 and 75.5 MHz. C(50) is therefore
coordinated to palladium and remains so on the NMR time scale
at -75 °C, which is not consistent with a simple palladium(0)
alkene complex. The C(50) peak is shifted 44 ppm upfield of
that for free dba, which is a larger shift than has been observed
for any of the Ogoshi/Kurosawa complexes,17 in keeping with
the solid-state data, which suggest a more activated enone.
Despite the structural and spectroscopic evidence to suggest
that [Pd(dba)(5)] is best regarded as a complex of palladium-
(II), preliminary reactivity studies have shown that reactivity
typical of a palladium(0) alkene complex is still accessible. For
example, coordinated dba undergoes exchange with free d2-
dba at room temperature. In contrast, no reaction was observed
with PhC2H, norbornylene, or HNEt2, even after several days
in refluxing benzene. Future work will examine the effect of
varying the nature of the enone, the steric and electronic
properties of ligand 5, and the potential for this work to allow
R,â-unsaturated ketones to undergo palladium(II) allyl like
reactivity.
Acknowledgment. Funding for this work was provided by
the NSERC of Canada in the form of a Discovery Grant to
D.J.H.E. and by McMaster University in the form of a
University Start-Up Fund.
(17) A continuum from palladium(0) enone-Lewis acid adducts to
zwitterionic palladium(II) complexes was observed, with increased palla-
dium(II) character for stronger Lewis acids and enones less able to stabilize
the positive charge generated on the carbonyl carbon upon Lewis acid
coordination: Ogoshi, S.; Yoshida, T.; Nishida, T.; Morita, M.; Kurosawa,
H. J. Am. Chem. Soc. 2001, 123, 1944.
(18) The increased ability of dba (R ) CHdCHPh versus R ) Me, H)
to stabilize the positive charge generated on the carbonyl carbon upon Lewis
acid coordination (cf. IR ν(CO) for free RC(O)H > RC(O)Me > RC(O)-
CHdCHPh) will render dba more difficult to activate toward the palladium-
(II) boratoxyallyl extreme than trans-benzylideneacetone or trans-
cinnamaldehyde.
Supporting Information Available: Text giving full experi-
mental details for all new compounds and characterization details
and a CIF file giving X-ray crystallographic data. This material is
available free of charge via the Internet at http:/pubs.acs.org.
OM0602052