Unprecedented 1,3-migration of the aryl ligand in metallacyclic aryl α-naphthyl Pt(iv) difluorides to produce β-arylnaphthyl Pt(ii) complexes

Article abstract of DOI:10.1039/c3cc41079j

Electrophilic fluorination of aryl α-naphthyl Pt(ii) complexes leads to an unprecedented 1,3-migration of the aryl ligand to the β-position of the naphthyl group. The reaction proceeds via the initial oxidative addition of two fluoro ligands to the Pt cen

Full text of DOI:10.1039/c3cc41079j

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Unprecedented 1,3-migration of the aryl ligand in
metallacyclic aryl a-naphthyl Pt(IV) difluorides to
produce b-arylnaphthyl Pt(II) complexes†
Ina S. Dubinsky-Davidchik,a Israel Goldberg,a Arkadi Vigalok*a and
Andrei N. Vedernikov*b
Cite this: Chem. Commun., 2013,
49, 3446
Received 7th February 2013,
Accepted 8th March 2013
DOI: 10.1039/c3cc41079j
www.rsc.org/chemcomm
Electrophilic fluorination of aryl a-naphthyl Pt(II) complexes leads to an
unprecedented 1,3-migration of the aryl ligand to the b-position of the
naphthyl group. The reaction proceeds via the initial oxidative addition
of two fluoro ligands to the Pt center followed by C(sp2)–C(sp2)
coupling and aryl migration.
Electrophilic fluorination of the platinum group organometallic
complexes is a rapidly developing area of research.1,2 Typically, the
initially formed high-oxidation state species can react further leading
to the formation of new bonds, particularly C–C or C–F bonds,3–5
with the reaction outcome depending on the nature of the ligands,
Scheme 1 Reagent-dependent reactivity of Pt(II) aryl complexes.
The naphthalene-based ligand 1 was designed and prepared to
explore the possibility of the benzylic C–H bond functionalization
under the electrophilic fluorination conditions. In this ligand,
both C(sp3)–H and C(sp2)–H bonds may be involved in cyclo-
platination after the initial coordination of the phosphine 1 to a
Pt(II) center. In practice, only cyclometallated Pt(II)–aryl complex 2
was obtained upon the reaction with (COD)PtCl2 (Scheme 2).
Complex 2 can be readily converted to the diaryl derivatives
3a–d by the action of the corresponding Grignard reagents in
the presence of pyridine. The X-ray structure of 3a (see ESI†)
confirms the formation of a square-planar Pt(II) complex with the
aryl ligands cis to each other.9 The reaction of 3 with XeF2 in
CH2Cl2 or CH3CN leads to the formation of moderately unstable
Pt(IV) difluorides 4a–d (Scheme 3). Complexes 4a–d exhibit the
characteristic multiplets in the 19F{1H} NMR spectra between
ꢀ200 and ꢀ230 ppm due to two inequivalent fluoro ligands
at the Pt(IV) center. The 31P{1H} NMR spectra of complexes 4
show the signal at B75 ppm with a Pt–P coupling constant of
ca. 2900 Hz. In the case of the 3,5-difluorophenyl complex 3b Pt(IV)
difluoride 4b could be prepared in a pure form and characterized
using single crystal X-ray diffraction (Scheme 3). The platinum
atom in 4b has an octahedral environment with both fluoro
geometry of the metal complex and the source of the electrophilic
s
´
fluorine. For example, Gagne and co-workers used Selectfluor for
fluorination of cationic trisphosphine aryl Pt(II) complexes to pro-
duce the corresponding arylfluorides, whereas the use of XeF2 was
less efficient.6 On the other hand, the reaction of cyclometallated
Pt(II) complexes with XeF2 can lead to benzylic C–H fluorination,
whereas the use of the cationic N-fluorocollidinium salt as a
fluorinating agent resulted in the C(sp2)–C(sp2) bond formation with
subsequent cyclometallation of the resulting biphenylylphosphine
(Scheme 1).7 Similarly, competitive C(sp2)–H and C(sp3)–H activation
at the Pt(II) center in the products resulting from the C–C reductive
elimination from the Pt(IV) precursor was reported very recently by
Crespo et al.8 In all these cases, the newly formed C–X bond resulted
from a classical C–X reductive elimination at the high valent metal
center. Herein, we report the first example of the C(sp2)–C(sp2)
coupling of metallacyclic aryl a-naphthyl Pt(IV) difluorides including
an unprecedented 1,3-aryl migration from the Pt center to the
b-carbon of the naphthyl fragment.
a School of Chemistry, The Sackler Faculty of Exact Sciences, Tel Aviv University,
Tel Aviv 69978, Israel. E-mail: avigal@post.tau.ac.il; Fax: +972-3-6406205;
Tel: +972-3-6408617
b Department of Chemistry and Biochemistry, University of Maryland, College Park,
Maryland 20742, USA. E-mail: avederni@umd.edu; Fax: +1-301-314-9121;
Tel: +1-301-405-2784
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of compounds 1–7 and computational details. CCDC 922512–922520. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc41079j
Scheme 2 Synthesis of the Pt(II) diaryl complexes 3.
c
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Noteworthily, transient platinum phosphine complex 6 (Fig. 1a)
having the metal-coordinated HF2ꢀ ligand11 was observed almost in
all the cases during the conversion of 4 to 5. The fraction of complex
6 in such solutions depends on the aryl substituent, solvent and the
reaction time. The addition of (i-Pr)2EtN or pyridine to a solution
containing 6b in CH3CN leads to the disappearance of the signal of
Scheme 3 Electrophilic fluorination of Pt(II) diaryl complexes 3.
the coordinated HF2 in the 19F NMR spectrum and formation of
ꢀ
the Pt(IV) monofluoride 6b0 having the cyclometallated tert-butyl
group (Fig. 1b). The addition of bases led to the complete suppres-
sion of the C(sp2)–C(sp2) coupling suggesting that 6 is not an
intermediate of the reaction in Scheme 4. In the case of the
perfluorophenyl complex 3e no 4e was observed upon treatment
of 3e with XeF2. Instead, the intermediate 6e was detected which
decomposed very slowly to form several products including the
crystallographically characterized 7 in which the initially cyclometal-
lated tert-butyl group is coupled to the a-naphthyl group (Fig. 1c).
To get further insight into the unprecedented C–C coupling of 4
leading to the formation of the 7-aryl-1-naphthylphosphine core
present in 5, transformation of the representative 3,5-difluorophenyl
Pt(IV) complex 4b was studied using the DFT calculations.12 In
these calculations acetonitrile was modeled as the reaction
medium; the Gibbs energies for the gas phase reactions are
also provided. The reaction sequence that we considered is
presented in Scheme 5. Pyridine dissociation from 4b to form 8
and subsequent isomerization of 8 lead to the formation of
a relatively low-energy five-coordinate transient 9 with the
coordination vacancy trans to the phenyl ligand, which is
required for facile C–C coupling13 of the phenyl and the
a-naphthyl ligands. Interestingly, the C–C coupling of 9 is
thermodynamically unfavorable. The coupling product 10 is a
Scheme 4 Aryl migration to the 7 position of the naphthalene ligand core. See
ESI† for X-ray structures of 5a and 5c.
ligands trans to the aryl ligands, which is typical for the difluoro
diaryl platinum complexes bearing chelating donors. The Pt–F
distances of 2.043(4) Å and 2.071(4) Å are within the typical
range for the Pt(IV) complexes.6,10
Intriguingly, the aryl a-naphthyl Pt(IV) difluorides 4a–d dis-
solved in MeCN or CH2Cl2 reacted at 20–60 1C to form cyclo-
metallated Pt(II) complexes 5a–d (>90% NMR yield), the
products of the unprecedented C(sp2)–C(sp2) coupling of the
phenyl and the naphthyl groups, which includes the 1,3-aryl
migration between the Pt center and the b-carbon of the
naphthyl fragment (Scheme 4). The structures of complexes
5a–d have been established unequivocally using single crystal
X-ray diffraction. The reactions are fast enough already at 20 1C
for the electron-rich 4c,d but require heating at 40–60 1C for the
electron-poor 4a,b. The 31P{1H} NMR spectra of complexes 5
exhibited one signal at B88 ppm (JPtP B 2100–2500 Hz)
whereas the 1H NMR spectra were consistent with the presence
of Pt-bound aryl groups derived from the original naphthyl and
phenyl ligands in 4. In turn, the 19F{1H} spectrum of 5b showed
two broad singlets due to two non-equivalent carbon-bound
fluorine atoms and no Pt-bound fluorides.
Scheme 5 DFT-calculated Gibbs energy profile for reaction of 4b in acetonitrile
solution and gas phase to form the 7-aryl-1-naphthyl-bis(tert-butyl)phosphine
core present in 5.
Fig. 1 Proposed structure of complex 6 and X-ray structure of the derived Pt(IV)
monofluoride, 6b0.
c
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stabilized Pt(II) naphthylidene transient and deprotonation of
the latter carbene complex leading to the restoration of the
naphthalene aromatic system. These results also point to the
high potential for discoveries of new reactions induced by
sterically bulky and/or rigid ligands present in a transition
metal coordination sphere.17
Notes and references
Scheme 6 The analogy of the 11 - 12 rearrangement and the pinacol
1 K. M. Engle, T.-S. Mei, X. Wang and J.-Q. Yu, Angew. Chem., Int. Ed.,
2011, 50, 1478.
rearrangement.
2 A. Vigalok, Organometallics, 2011, 30, 4802.
3 C–C bond formation: (a) A. Yahav, I. Goldberg and A. Vigalok, J. Am.
Chem. Soc., 2003, 125, 13634; (b) C. F. Rosewall, P. A. Sibbald,
D. V. Liskin and F. E. Michael, J. Am. Chem. Soc., 2009, 131, 9488;
(c) N. D. Ball, J. W. Kampf and M. S. Sanford, J. Am. Chem. Soc., 2010,
132, 2878; (d) N. D. Ball, J. B. Gary, Y. Ye and M. S. Sanford, J. Am.
Chem. Soc., 2011, 133, 7577.
4 C–F bond formation: (a) K. L. Hull, W. Q. Anani and M. S. Sanford,
J. Am. Chem. Soc., 2006, 128, 7134; (b) J. M. Racowski, J. B. Gary and
M. S. Sanford, Angew. Chem., Int. Ed., 2012, 51, 3414; (c) X. Wang,
T.-S. Mei and J.-Q. Yu, J. Am. Chem. Soc., 2009, 131, 7520;
(d) K. S. L. Chan, M. Wasa, X. Wang and J.-Q. Yu, Angew. Chem.,
Int. Ed., 2011, 50, 9081; (e) T. Furuya and T. Ritter, J. Am. Chem. Soc.,
2008, 130, 10060; ( f ) T. Furuya, D. Benitez, E. Tkatchouk,
A. E. Strom, P. Tang, W. A. Goddard, III and T. Ritter, J. Am. Chem.
Soc., 2010, 132, 3793; (g) S.-B. Zhao, J. J. Becker and M. R. Gagne,
Organometallics, 2011, 30, 3926; (h) A. W. Kaspi, A. Yahav-Levi,
I. Goldberg and A. Vigalok, Inorg. Chem., 2008, 47, 5.
5 Formation of other bonds: (a) K. Sun, X. T. Li, J. Zhang and
Q. Zhang, J. Am. Chem. Soc., 2011, 133, 1694; (b) T. Xu, S. Qiu and
G. Liu, J. Organomet. Chem., 2011, 696, 46; (c) I. S. Dubinsky-David-
chik, S. Potash, I. Goldberg, A. Vigalok and A. N. Vedernikov, J. Am.
Chem. Soc., 2012, 134, 14027.
6 (a) S.-B. Zhao, R.-Y. Wang, H. Nguyen, J. J. Becker and M. R. Gagne,
Chem. Commun., 2012, 48, 443; (b) In a similar system, XeF2 was more
efficient in the Pt-catalyzed cyclization–fluorination: N. A. Cochrane,
H. Nguyen and M. R. Gagne, J. Am. Chem. Soc., 2013, 135, 628.
7 A. W. Kaspi, I. Goldberg and A. Vigalok, J. Am. Chem. Soc., 2010,
132, 10626.
Pt(II) s-complex derived from the 8-aryl-1-naphthyl-bis(tert-butyl)-
phosphine where the newly formed C–C bond is coordinated to
the Pt(II) center. The biaryl unit in 10 is heavily distorted due to the
rigidity of the Pt-coordinated 8-arylnaphthylphosphine and its
inability to accommodate the agostic C–C bond perpendicular
to the Pt(II) coordination plane. These geometric constraints make
the C–C coupling of 9 unfavorable, whereas, typically, the C–C
coupling of Pt(IV) diaryl complexes is exothermic.14 The reaction
coordinate analysis shows that the C–C reductive coupling of 9 to
form 10 involves the low-lying transition state TSPt–C8. Subsequent
isomerization of 10 via another low energy transition state TSs–p
produces the highly geometrically distorted p-complex 11. Inter-
estingly, the transient 11 undergoes a cleavage of the C–C bond
formed initially at the C–C coupling step to form a moderately
stable Pt(II) carbene complex 12. This reaction includes migration
of a formally carbanionic aryl from the C8 to the C7 position of the
naphthalene fragment via the transition state TSC8–C7. As a result,
the geometric distortion of 11 is relieved and the planarity of the
Pt(II)–naphthylphosphine fragment is regained thus contributing
to the exothermicity of this transformation. The formation of 12
from 11 may, formally, be compared with pinacol rearrangement
in organic chemistry (Scheme 6).15
8 (a) M. Crespo, C. M. Amderson, N. Kfoury, M. Font-Bardia and
T. Calvet, Organometallics, 2012, 31, 4401. See also: (b) L. Keyes,
T. Wang, B. O. Patrick and J. A. Love, Inorg. Chim. Acta, 2012,
380, 284.
9 For related Pt(II) complexes see: K. A. Grice, W. Kaminsky and
K. I. Goldberg, Inorg. Chim. Acta, 2011, 369, 76.
10 (a) A. Yahav, I. Goldberg and A. Vigalok, Inorg. Chem., 2005, 44, 1547;
(b) A. Yahav-Levi, I. Goldberg and A. Vigalok, J. Fluorine Chem., 2010,
131, 1100.
The 11 to 12 transformation is the rate limiting step of the
whole reaction sequence in Scheme 6, both in MeCN solution and
in the gas phase.16 The aromaticity of the former naphthalene
fragment present in 12 is restored and complex 13 is formed as a
result of an intramolecular low-barrier proton transfer from
the C7 position of the naphthalene fragment to the fluoride 11 See for example, N. A. Jasim and R. N. Perutz, J. Am. Chem. Soc.,
2000, 122, 8685.
12 Jaguar, version 7.9, Schrodinger, LLC, New York, NY, 2012.
ligand cis- to the naphthyl ligand. The elimination of HF from
¨
13 allows production of a C–H agostic transient 14 required for
the cycloplatination of the 7-aryl-1-naphthylphosphine residue.
The cycloplatination leads to a Pt(IV) hydride intermediate 15 via
the transition state TSCH–Pt. Finally, the observed reaction product
5b results from the reductive elimination of HF from 15 and the
formation of a pyridine–(HF)2 adduct.
In summary, we have discovered an unprecedented 1,3-aryl
migration in aryl a-naphthyl Pt(IV) complexes. The reaction
produces unexpected C(sp2)–C(sp2) coupling products contain-
ing the aryl b-naphthyl rather than the expected aryl a-naphthyl
fragment. Based on the results of the experimental study
and the DFT modeling the reaction mechanism was proposed
that includes a tandem C–C coupling of Pt(IV) aryl naphthyl
complexes, a 1,2-aryl shift in the naphthalene ring to produce a
13 R. H. Crabtree, The Organometallic Chemistry of the Transition Metals,
John Wiley & Sons, 5th edn, 2009.
14 A. Yahav-Levi, I. Goldberg, A. Vigalok and A. N. Vedernikov, J. Am.
Chem. Soc., 2008, 130, 724.
15 M. B. Smith and J. March, March’s Advanced Organic Chemistry:
Reactions, Mechanisms, and Structure, Wiley, New York, 5th edn, 2001.
16 The calculated Gibbs activation energy for the rate limiting step of
the reaction involving the p-methoxyphenyl analog 4c is 19.1/
27.2 kcal molꢀ1 (MeCN solution/gas phase, all at 25 1C), lower than
for the 3,5-difluorophenyl complex 4b (23.1/31.8 kcal molꢀ1
;
Scheme 5), in a qualitative agreement with the trend found in the
experiment. The experimentally observed Gibbs activation energy in
MeCN is 23.0 kcal molꢀ1 for the reaction 4c - 5c at 25 1C (t1/2
=
2.5 h; see the 1st order kinetics plot in Fig. S1, ESI†); the reaction
4b - 5b is complicated by the formation of large amounts of 6b; its
kinetics are more complex with a half-life of about 22 h at 70 1C.
17 P. J. Milner, T. J. Maimone, M. Su, J. Chen, P. Mu¨ller and
S. L. Buchwald, J. Am. Chem. Soc., 2012, 134, 19922.
c
3448 Chem. Commun., 2013, 49, 3446--3448
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