Platinum-mediated aryl-aryl bond formation and sp3 C-H bond activation

Article abstract of DOI:10.1039/c0dt00631a

A novel process is described in which intramolecular oxidative addition is followed by aryl-aryl bond formation and sp³ C-H bond activation leading to a six-membered platinacycle. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00631a

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Platinum-mediated aryl–aryl bond formation and sp3 C–H bond activation†
Margarita Crespo,*a Teresa Calvetb and Merce` Font-Bardiab
Received 23rd April 2010, Accepted 15th June 2010
First published as an Advance Article on the web 23rd June 2010
DOI: 10.1039/c0dt00631a
A novel process is described in which intramolecular oxidative
addition is followed by aryl–aryl bond formation and sp3 C–H
bond activation leading to a six-membered platinacycle.
The biaryl motif is a predominant feature in many pharmaceuti-
cals and bioactive compounds. For this reason, the development
of new methods for aryl–aryl bond formation, often involving
transition-metal-mediated reactions, is a field of great interest.1
We have recently reported a process involving C–C coupling
and formation of seven-membered platinum(II) metallacycles
in the reactions of cis-[Pt2(4-MeC6H4)4(m-SEt2)2] with ligands
=
ArCH NCH2(4-ClC6H4) (Ar = 2-BrC6H4 (1a); 2,6-Cl2C6H3 (1b)
shown in Scheme 1) or with the corresponding potentially
=
terdentate ligands ArCH NCH2CH2NMe2. It has been shown
that these reactions take place through initial intramolecular
C–X (X = Br or Cl) bond activation to produce a cyclometallated
platinum(IV) compound.2 In this case, as well as in related C–C
coupling processes leading to either seven-3 or five-membered4
platinacycles, the activation of the C–X bond gives a metallacycle
containing the imine functionality (endo-metallacycle) and the
C–C coupling takes place between a freely rotating aryl group
and a rigid aryl ring of a metallacycle.
Scheme 2 Synthetic method.
ligand and one of the para-tolyl groups leading to a biaryl linkage.
However, in contrast to previous results, the subsequent C–H
activation does not take place at the biaryl system to yield either
seven or five-membered metallacycles but instead an aliphatic C–H
bond of the mesityl group is activated to produce a six-membered
endo-metallacycle (compound 2c). Due to the low stability of
2c, the triphenylphosphine derivative 3c was also prepared and
fully characterised.5 Crystals of 3c just good enough for structure
resolution were obtained and allow for definitive confirmation
of the formation of an aryl–aryl bond and of a six-membered
platinacycle (Fig. 1).6 The resulting biaryl linkage is pointing
away from the platinum centre and the two phenyl rings are tilted
66.3(4)◦ from each other.
Scheme 1 Formation of seven-membered platinacycles (Ref. 2).
In order to progress in understanding the factors that govern
these processes, we envisaged analogous reactions with imines such
=
as 2,4,6-C6H2(CH3)3CH NCH2(2-BrC6H4) (1c) for which C–Br
bond activation should lead to more flexible exo-platinacycles. The
mesityl group was chosen in order to block the ortho positions of
the benzylidene group and drive the reaction towards the benzyl
ring of the ligand. As shown in Scheme 2, C–C bond formation
does indeed take place between the benzyl group of the imine
=
Ligand 2,4,6-C6H2(CH3)3CH NCH2(2-ClC6H4) (1d) was also
tested and produced an analogous reaction to that reported for
=
1c while ligand 2,4,6-C6H2(CH3)3CH NCH2C6H5 (1e) failed to
react. These results indicate that C–X bond activation (X =
Br or Cl) to produce a platinum(IV) metallacycle is required
for the process to take place. On the other hand, formation of
a six-membered platinacycle through sp3 C–H bond activation
is remarkable since a strong tendency to form five- versus six-
membered rings as well as a preference for the activation of sp2
over sp3 C–H bonds are generally observed in cyclometallation
reactions.7 In particular, platinacycles formed through aliphatic
C–H bond activation are uncommon.8 While activation of a methyl
C–H bond of a mesityl group with formation of six membered
aDepartament de Qu´ımica Inorga`nica, Universitat de Barcelona, Diag-
onal 647, 08028, Barcelona, Spain. E-mail: margarita.crespo@qi.ub.es;
Fax: +34934907725; Tel: +34934039132
bDepartament de Cristal·lografia, Mineralogia i Dipo`sits Minerals, Univer-
sitat de Barcelona, Mart´ı i Franque`s s/n, 08028, Barcelona, Spain
† Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds. CCDC
reference number 774868. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt00631a
6936 | Dalton Trans., 2010, 39, 6936–6938
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Scheme 3 Proposed reaction pathway.
membered platinacycles through aromatic C–H bond activation
(at positions indicated in Scheme 3 as a and b, respectively) or
to an endo-six-membered platinacycle arising from aliphatic C–H
bond activation (indicated as c). The results here reported reveal
that the latter option is the most favoured and this can be related
to the higher stability of the endo versus exo-metallacycles (the so-
called endo effect)7,9 which allows to overcoming the low tendency
to form six-membered rings and to activate a sp3 C–H bond.
In summary, we have succeeded in achieving platinum-mediated
C–C coupling using a different type of imine ligand. Moreover, in
spite of the fact that the imine functionality is not required for
aryl–aryl bond formation, it is decisive for the regioselectivity
of the cyclometallation step. Further work aimed at a better
understanding and at expanding the scope of the process is
currently in progress.
Fig. 1 Molecular structure of compound 3c showing 50% probability
ellipsoids.
Acknowledgements
metallacycles is well known at palladium,9 such process has not
We thank the Ministerio de Ciencia y Tecnolog´ıa (project
CTQ2009-11501) for financial support.
been observed in the reactions of imines 1d or 1e with platinum
11
substrates cis-[PtCl2(dmso)2]10 or cis-[PtMe2(m-SMe2)]2 which
gave instead exo-metallacycles through activation of aromatic C–
H or C–Cl bonds.
Notes and references
As a whole, the results here reported indicate that intramolecular
C–X (X = Br or Cl) bond activation at the saturated arm of
the benzylidene-benzylamines may promote, upon reaction with
cis-[Pt2(4-MeC6H4)4(m-SEt2)2], the formation of a biaryl linkage
between one of the tolyl groups and the benzyl group of the
ligand. In contrast to previously reported reactions, the biaryl
linkage is not involved in the subsequent metallation which leads
to an unusual six-membered platinacycle. These results suggest
that the C–C coupling and the cyclometallation are independent
steps of the process and allow us to propose the reaction path
shown in Scheme 3. In this process, the initial C–X bond activation
to produce a platinum(IV) derivative is followed by reductive
elimination with formation of an aryl–aryl bond. The final
cyclometallation step could lead to either exo-five- or exo-seven-
1 (a) D. Alberico, M. E. Scott and M. Lautens, Chem. Rev., 2007, 107,
174–238; (b) G. P. Mc Glacken and L. M. Bateman, Chem. Soc. Rev.,
2009, 38, 2447–2464.
2 R. Mart´ın, M. Crespo, M. Font-Bardia and T. Calvet, Organometallics,
2009, 28, 587–597.
3 (a) M. Crespo, M. Font-Bard´ıa and X. Solans, Organometallics, 2004,
23, 1708–1713; (b) A. Capape´, M. Crespo, J. Granell, A. Vizcarro, J.
Zafrilla, M. Font-Bardia and X. Solans, Chem. Commun., 2006, 4128–
4130; (c) M. Font-Bard´ıa, C. Gallego, M. Mart´ınez and X. Solans,
Organometallics, 2002, 21, 3305–3307.
4 T. Calvet, M. Crespo, M. Font-Bardia, K. Go´mez, G. Gonza´lez and
M. Mart´ınez, Organometallics, 2009, 28, 5096–5106.
5 Experimental procedures and characterization data for all the new
compounds here reported are given in the ESI.† For 3c, in the methyl
region of the 1H NMR spectrum, in addition to the methyl resonance
corresponding to the para-tolyl group (2.35 ppm), only two resonances
integrating 3 hydrogen atoms (1.92 and 2.02 ppm) corresponding to
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the mesityl group are observed. The resonance assigned to Pt–CH2 is
observed at 2.06 ppm (J(H–Pt) = 96 Hz) in the 1H NMR spectrum and
at 14.70 ppm (J(C–Pt) = 630 Hz) in the 13C NMR spectrum in good
agreement with previously reported data for analogous compounds.8
The large J(P–Pt) value observed in the 31P and 195Pt spectra indicates
that the phosphine is trans to N rather than to C atom.
8 (a) S. H. Crosby, G. J. Clarkson and J. P. Rourke, J. Am. Chem. Soc.,
2009, 131, 14142–14143; (b) A. Zucca, S. Stoccoro, M. A. Cinellu, G.
Minghetti, M. Manassero and M. Sansoni, Eur. J. Inorg. Chem., 2002,
3336–3346.
9 See for instance: (a) J. Albert, J. Granell, J. Sales, X. Solans and M.
Font-Altaba, Organometallics, 1986, 5, 2567–2568; (b) J. Albert, R. M.
Ceder, M. Gomez, J. Granell and J. Sales, Organometallics, 1992, 11,
1536–1541; (c) R. Bosque, C. Lo´pez, J. Sales, D. Tramuns and X. Solans,
J. Chem. Soc., Dalton Trans., 1995, 2445–2452; (d) C. L. Chen, Y. H.
Liu, S. M. Peng and S. T. Liu, Tetrahedron Lett., 2005, 46, 521–523;
(e) J. Albert, J. M. Cadena, A. Gonzalez, J. Granell, X. Solans and M.
Font-Bardia, Chem.–Eur. J., 2006, 12, 887–894.
6 Compound 3c crystallized as an hydrate and the water molecule
displays positional disorder. C42H39BrNPPt·H2O, MW = 881.73,
¯
˚
˚
triclinic, space gro◦up P1, a = 12.032(5) A, b = 12.230(4) A, c = 12.715(3)
◦
◦
˚
A, a = 81.08 (2) , b = 72.52 (2) , g = 79.01 (2) , V = 1742.3(10)
3
˚
A , Z = 2, number of reflections collected/unique = 16 366/8758
[R(int) = 0.0627], final R indices [I > 2s(I)] R1 = 0.0550, wR2
=
10 A. Capape´, M. Crespo, J. Granell, M. Font-Bardia and X. Solans,
Dalton Trans., 2007, 2030–2039.
11 M. Crespo, M. Mart´ınez and J. Sales, Organometallics, 1992, 11, 1288–
1295.
0.1579, R indices (all data) R1 = 0.0551, wR2 = 0.1579, GOF =
1.087.
7 For a recent review on cyclometallation using d-block transition metals,
see: M. Albrecht, Chem. Rev., 2010, 110, 576–623.
6938 | Dalton Trans., 2010, 39, 6936–6938
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The Royal Society of Chemistry 2010
©