reaction is driven to completion by only 0.001 mol% of 4 in 12 h
which corresponds to at least 105 TON.
Scheme 2 Diels–Alder approach toward PC(sp3)P-complexes.
To conclude, we have demonstrated that the unprecedented
Diels–Alder reaction of organometallic dienes with organic
dienophiles is indeed possible and appears to be a straight-
forward synthetic route toward dibenzobarrelene-based
C(sp3)-metalated pincer complexes. More detailed studies on
the new family of compounds will follow.
the H-10 signal (from ca. 8.3 ppm in 1–3 to ca. 5.8 ppm in 4–6)
that takes place over the course of the reaction. Other NMR
data also match the proposed structure (see supporting
informationw).
Despite the unambiguity in identification of the new com-
pounds, the single crystals of 4 and 5 were subjected to X-ray
analysis10 (ORTEP pictures are given in Fig. 1). As expected,
unlike in the square planar 1, the palladium center in C(sp3)-
metalated 4 is strongly distorted from planarity toward a
butterfly-like environment. For example, the observed
P(1)–Pd–P(2), C(1)–Pd–Cl(1) angles and P(1)–P(2) intramole-
cular distance between the two phosphorus atoms are
150.43(4)1, 171.86(11)1 and 4.427 A, respectively. As expected,
the Pd–Cl bond in 4 is slightly longer than the corresponding
bond in the previously reported 1 due to a stronger trans
influence exerted by the C(sp3) ligand.9a Very similar structural
features were observed for the nickel analog 5.
We thank the Israel Science Foundation (Grant No. 866/06)
for financial support. We also thank Dr Shmuel Cohen for
solving X-ray structures.
Notes and references
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B. L. Shaw and R. J. Goodfellow, J. Chem. Soc., Chem. Commun.,
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N. Veldman, A. L. Spek, J.-P. Sauvage and G. van Koten, Angew.
Chem., Int. Ed. Engl., 1994, 33, 1282; (c) M. Albrecht, M. Lutz,
A. L. Spek and G. van Koten, Nature, 2000, 406, 970; (d) M. E. van
der Boom and D. Milstein, Chem. Rev., 2003, 103, 1759;
(e) K. B. Renkema, Y. V. Kissin and A. S. Goldman, J. Am. Chem.
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Chem. Res., 2008, 41, 201; (g) W. Leis, H. A. Mayer and
W. C. Kaska, Coord. Chem. Rev., 2008, 252, 1787; (h) The Chemistry
of Pincer Compounds, ed. D. Morales-Morales and C. M. Jensen,
Elsevier, Amsterdam, 2007.
2 (a) G. R. Clark, T. R. Greene and W. R. Roper, J. Organomet.
Chem., 1985, 293, C25; (b) M. McLoughlin, H. A. Mayer, R. Flesher
and W. C. Kaska, Organometallics, 1994, 13, 3816; (c) W. Lesueur,
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J. Chem. Soc., Dalton Trans., 2002, 1396; (e) M. Kanzelberger,
X. W. Zhang, T. J. Emge, A. S. Goldman, J. Zhao, C. Incarvito
and J. F. Hartwig, J. Am. Chem. Soc., 2003, 125, 13644; (f) J. Zhao,
A. S. Goldman and J. F. Hartwig, Science, 2005, 307, 1080;
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2007, 5427; (h) V. Pandarus and D. Zargarian, Chem. Commun., 2007,
978.
Thermogravimetric tests showed that the thermal stability
of the new compounds is exceptional and in some cases
exceeds the stability of C(sp2)-metalacycles. For example, the
first weight loss detected for 4 takes place at 370 1C and this is
120 1C higher than for the parent 1. 5 and 6 also demonstrated
decomposition points far over 280 1C.
Since very often an extreme stability may indicate chemical
inertness, it was important for us to demonstrate at early stages
of the study that this is not the case for the new family of
compounds. We tested 4 as a promoter in the well-explored
Mizoroki–Heck reaction which typically serves as a model reac-
tion for evaluating the catalytic activity of pincer complexes.11
We were pleased to find that despite the exceptional thermal
stability the new complexes are catalytically active. The following
3 (a) A. Vigalok and D. Milstein, Organometallics, 2000, 19, 2061;
(b) D. G. Gusev and A. J. Lough, Organometallics, 2002, 21, 5091;
(c) V. F. Kuznetsov, K. Abdur-Rashid, A. J. Lough and
D. G. Gusev, J. Am. Chem. Soc., 2006, 128, 14388.
4 M. Ohff, A. Ohff, M. E. van der Boom and D. Milstein, J. Am.
Chem. Soc., 1997, 119, 11687.
5 It is noteworthy that bridgehead carbon atoms of bicyclo[2.2.2]octane
have no effect on the charge transfer rates in donor–bridge–acceptor
systems which highlights the compounds as attractive candidates
for construction of conducting materials: R. H. Goldsmith,
J. Vura-Weis, A. M. Scott, S. Borkar, A. Sen, M. A. Ratner and
M. R. Wasielewski, J. Am. Chem. Soc., 2008, 130, 7659.
6 O. Grossman, C. Azerraf and D. Gelman, Organometallics, 2006,
25, 375.
7 The lack of reactivity of transition metals toward carbometalation
is not very surprising considering the low acidity of the methine
hydrogen: L. H. Schwartz, J. Org. Chem., 1968, 33, 3977.
8 C. Azerraf and D. Gelman, Chem.–Eur. J., 2008, 14, 10364.
9 (a) M. W. Haenel, D. Jakubik, C. Krueger and P. Betz, Chem. Ber.,
1991, 124, 333; (b) M. W. Haenel, S. Oevers, K. Angermund,
W. C. Kaska, H.-J. Fan and M. B. Hall, Angew. Chem., Int. Ed.,
2001, 40, 3596; (c) M. Yamashita, K. Kamura, Y. Yamamoto and
K.-Y. Akiba, Chem.–Eur. J., 2002, 8, 2976; (d) N. Solin,
J. Kjellgren and K. J. Szabo, J. Am. Chem. Soc., 2004, 126, 7026.
Fig. 1 ORTEP drawing (50% probability ellipsoids) of the structures
4 and 5. Hydrogen atoms and solvent molecules are omitted for
clarity. Selected bond lengths (A) and angles (1) for 4: Pd1–C1
(2.057(3)), Pd1–Cl1 (2.3694(9)), Pd1–P1 (2.2908(9)), Pd1–P2
(2.2874(9)), Cl1–Pd1–C1 (173.14(10)), P1–Pd1–P2 (148.53(3)),
C1–Pd1–P1 (96.96(3)). Selected bond lengths (A) and angles (1) for
5: Ni1–C1 (1.961(4)), Ni1–Cl1 (2.2069(12)), Ni1–P1 (2.1872(12)),
Ni1–P2 (2.1977(12)), Cl1–Ni1–C1 (165.57(12)), P1–Ni1–P2
(146.64(5)), C1–Ni1–P1 (86.60(12)).
10 Crystallographic data for 4: C44H33ClO4P2Pd,
M
¼
829.49,
orthorhombic, a ¼ 8.3159(5), b ¼ 11.2104(7), c ¼ 38.813(2) A,
ꢀc
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Chem. Commun., 2009, 466–468 | 467