Electrophilic fluorination of cationic Pt-aryl complexes

Article abstract of DOI:10.1039/c1cc15006e

The electrophilic fluorination of several (triphos)Pt-aryl⁺ establishes the first example of aryl-F coupling from a Pt center.

Full text of DOI:10.1039/c1cc15006e

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Electrophilic fluorination of cationic Pt-aryl complexesw
a
b
a
c
a
´
Shu-Bin Zhao, Rui-Yao Wang,z Ha Nguyen, Jennifer J. Becker and Michel R. Gagne*
Received 11th August 2011, Accepted 29th October 2011
DOI: 10.1039/c1cc15006e
The electrophilic fluorination of several (triphos)Pt-aryl+ establishes
the first example of aryl–F coupling from a Pt center.
Sp2-carbon–halogen bond forming reactions from Pt(IV) centers
are rare,1g,9,10 with the few known examples restricted to C–I and
C–Br couplings.11 Extending our efforts on Pt–C bond fluorination
reactions, we have examined the electrophilic fluorination of
(triphos)Pt-aryl+ complexes. Herein, we report these reactions
and provide evidence that supports the intermediacy of Pt(IV)–F
complexes in the C–F reductive coupling reaction.
The demand for organofluorine compounds has stimulated
much recent effort to develop metal mediated fluorination
reactions.1 Despite the versatility of available C–X (X = C,
N, O, S, Cl, Br, I, etc.) coupling methodologies,2 C–F couplings via
reductive elimination remain challenging.1d,f Metal catalyzed C–F
couplings that utilize fluoride sources encounter additional
challenges due to the intrinsically low polarizability and
nucleophilicity, pronounced hydration power, and high basicity
of Fꢀ. Nevertheless, several notable Pd0/II catalyzed nucleophilic
fluorinations have been recently reported.3 More fruitful have
been recent metal-catalyzed electrophilic fluorination reactions,5–8
wherein high-valent metal fluoro intermediates (e.g. Pd(IV),4
Ag(II)ꢁꢁꢁAg(II),5 Au(III),6 etc.) are more prone to productive
reactivity, including C–H activation, cross-coupling, and C–F
reductive elimination.1f,7
Complexes 1–4 were synthesized by ligand displacement of
(COD)PtAr(X) (COD = cycloocta-1,5-diene, X = Cl, or I) with
triphos, followed by salt metathesis with NaBF4.12,13 Complex 5
was prepared by treating chloro(2-phenylpyridine)[2-(2-pyridyl)-
phenyl-C,N]Pt14 with triphos, while its dicationic isostere 6
15
was obtained by reacting [(triphos)Pt(NCC6F5)](BF4)2 with
2-phenylpyridine.12
These compounds were characterized by NMR and HRMS,
with the molecular structure of 4y being verified by X-ray
analysis (Fig. 1).12 Consistent with the solid state structure of
4, NOESY analysis suggested that the ortho-substituent in 2,
4–6 preferentially oriented syn to the central P-Ph group of the
triphos ligand. While 2, 4, 5 and 6 exist exclusively in this
syn-rotamer, both syn- and anti-forms (2.7 : 1) were observed
for 3.12 The preference for the syn- over the anti-form suggests
that the face of the square plane containing the apical P-Ph
group is less congested and may be more kinetically accessible.
When subjected to electrophilic fluorination conditions, these
(triphos)Pt-aryl+ complexes were found to be much less reactive
than their Pt-alkyl+ analogs.8b When screening common ‘‘F+’’
sources including N-fluorobenzenesulfonimide, several N-fluoro-
pyridinium salts, Selectfluors and XeF2, only the latter two
exhibited reasonable reactivity with 1, for which the optimal
solvent was identified to be acetonitrile. 31P and 19F NMR
spectroscopy proved most advantageous for in situ monitoring
To explore Pt analogues of these electrophilic reactions, we
recently demonstrated a system that efficiently fluorinates
Pt–Csp3 bonds.8b As illustrated in eqn (1), wherein PPP =
bis(2-diphenylphosphinoethyl)phenylphosphine (i.e., triphos),
the C–F coupling proved to be stereoretentive and was
proposed to occur by concerted reductive elimination of a
putative dicationic Pt(IV)–F intermediate (A). The reaction
was accelerated by increased steric congestion around Pt,8b
however, information on the short-lived Pt(IV)–F species was
lacking.
ð1Þ
a Caudill Laboratories, Department of Chemistry,
University of North Carolina at Chapel Hill, Chapel Hill,
NC 27599-3290, USA. E-mail: mgagne@unc.edu
b Department of Chemistry, Queen’s University, Kingston,
ON K7L 3N6, Canada
c U.S. Army Research Office, P.O. Box 12211,
Research Triangle Park, NC 27709, USA
w Electronic supplementary information (ESI) available: Experimental
details, characterization data, and complete X-ray diffraction data.
CCDC 838071–838073. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1cc15006e
z The author to whom inquiries on the X-ray structures should be
directed.
Fig.ꢀ1 Left: complexes 1–6; right: X-ray structure of 4 (H atoms and
BF4 anion are omitted for clarity).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 443–445 443

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of these reactions and Selectfluors proved to be cleaner and more
productive than XeF2.
With 1, a complex mixture of phenyl Pt(IV)–F species was
obtained upon reacting with XeF2 (RT, o20 min). By contrast,
Selectfluors provided one main phenyl Pt(IV)–F complex
(RT, B2 h) (dF = ꢀ360.3 ppm, JPt–F = 1453 Hz).12 These
Pt(IV)–F species, however, failed to reductively eliminate PhF
even after prolonged heating (80 1C, 430 h).
The ortho-substituents considerably slowed down the reactions
of 2 and 3 with XeF2 and Selectfluors, however, their presence
proved beneficial for achieving the desired sp2 C–F coupling.
In the case of 2, XeF2 provided one major Pt(IV)–F complex
(d = ꢀ352.8 ppm, JPt–F = 1442 Hz) in B75% NMR yield
(RT, 12 h).12 However, the precise structure of this product
remains unclear, as all attempts to crystallize it failed and
spectroscopic data were not conclusive. Heating a freshly
prepared reaction mixture containing this Pt(IV)–F complex
at 80 1C led to only traces of the aryl–F coupling product
(o5% GC-MS yield). Similar results were obtained when
directly reacting 2 with XeF2 at 80 1C. In contrast, reactions
of 3 with XeF2 (RT, 15 h) directly generated a substantial
amount of the aryl–F coupling product 1-fluoro-2,4-dimethyl-
benzene (B55% NMR yield), along with the corresponding
[(triphos)Pt-NCMe]2+ by-product. The formation of a Pt(IV)–F
complex (dF = ꢀ351.9 ppm, JPt–F = 1146 Hz) in B25% NMR
yield and other unidentified Pt species was also observed.12 To
our knowledge, this reaction represents the first example of
aryl–F coupling from a Pt center.
Scheme 1 Generation of complex 7; inset: X-ray structure of 7
(H atoms and anion are omitted for clarity).
in melting acetonitrile.8b This reactivity mode apparently reflects
the intermediacy of Pt(IV) fluorides in both cases.8b The propensity
of Pt(IV) and Pd(IV) centers in metalating aromatic C–H bonds has
been demonstrated and exploited recently in several coupling
strategies.7,8a
Heating an acetonitrile solution of 7 at 80 1C resulted in slow
Fꢀ extrusion and the concomitant formation of a dicationic
Pt(IV)–MeCN adduct, 8 (eqn (2)). No C–F reductive elimination
was observed during the process, and X-ray diffractiony revealed
that the MeCN ligand coordinates syn to the triphos ligand’s
central P-Ph group (eqn (2)).12 Consistent with the increase in the
net charge of the Pt(IV) center are large downfield shifts of the
31P NMR signals as compared to 7 (e.g., Dd = +22.7 ppm for
the central P) and 1H NMR signals of the biphenyl moiety.
Despite being unreactive at RT, Selectfluors readily fluorinated
2 and 3 at 80 1C to produce the aryl fluoride;12 no Pt(IV)–F species
was observable during in situ monitoring of these reactions. These
results are summarized in Table 1.
Surprisingly, the reaction of 4 with XeF2 preferentially yielded
the ortho-cyclometalated complex 7 (Scheme 1). NMR monitoring
of the reaction revealed its gradual conversion to the Pt(IV)–F
complex, 7y, which was characterized by NMR, HRMS and X-ray
diffraction.12 In contrast to the aforementioned Pt(IV)–F species,
this complex exhibits a 19F NMR resonance at d = ꢀ299.9 ppm
with a considerably diminished 195Pt–19F coupling (B173 Hz).
As shown in Scheme 1, the Pt center in 7 adopts an
octahedral coordination geometry, with the Pt–F bond
(2.099(2) A) oriented anti to the central P-Ph group of the
triphos ligand, and the biphenyl moiety adopting a C,C0-chelating
mode. Similar cyclometalation of an ortho sp2-C–H bond was
previously noted upon fluorinating (triphos)Pt-CH2Ph+ with XeF2
ð2Þ
In addition to 7, reactions of 4 with XeF2 at RT (Scheme 1)
also yielded traces of 8 (o5%).12 By contrast, reactions of 4 with
Selectfluors directly provided 8 (85%, B5 h), along with 15% of
2-fluorobiphenyl and the corresponding [(triphos)Pt-NCMe]2+
(Scheme 2).12 We reason that the formation of both 7 and 8
implies the presence of Pt(IV) intermediates.
Table 1 Fluorination of complexes 1–3 with Selectfluorsa
Complex
Product
Time
NMR yieldb (%)
60–70
The contrasting outcomes for reactions of 2–4 with XeF2
and Selectfluors presumably stem from the presence of a basic
fluoride anion in the former case, though a size difference in
the ‘‘F+’’ source is also conceivable.16 Shown in Scheme 1 is
one way wherein Fꢀ could accelerate ortho-metalation vs.
reductive elimination. Since the two Pt(II) faces were shown to
be sterically different, it is also possible that these reactions evolve
differently based on which face F+ attacks.16 Recently, Vigalok
and co-workers have also reported an F+ reagent-dependent
reaction behavior when fluorinating Pt-aryl complexes.8a
1
[(PPP)PtIV(Ph)(F)]2+
o20 min
2
3
a
1 h
2 h
91
495
Conditions: complexes 1–3 (0.02 mmol), 1.5 equiv. of Selectfluors
,
b
dry CD3CN (0.5 mL), 80 1C. Mass balance: structurally unidentified
organometallic Pt species.
c
444 Chem. Commun., 2012, 48, 443–445
This journal is The Royal Society of Chemistry 2012

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%
P1, a = 13.666(1) A, b = 17.484(2) A, c = 22.340(2) A, a =
95.002(1)1, b = 113.180(1)1, g = 96.172(1)1, V = 4829.3(8) A3,
Z = 4, T = 180(2) K, 18 829 collected reflections, 18 829 unique
reflections (Rint = 0.0454); R1 = 0.0339, wR2 = 0.0841 for data with
I 4 2s(I), and R1 = 0.0483, wR2 = 0.0876 for all unique data.
1 (a) T. Hiyama, Organofluorine Compounds, Springer, Berlin, 2000;
(b) K. Muller, C. Faeh and F. Diederich, Science, 2007, 317, 1881;
¨
(c) K. L. Kirk, Org. Process Res. Dev., 2008, 12, 305;
(d) V. V. Grushin, Acc. Chem. Res., 2010, 43, 160; (e) T. Furuya,
C. A. Kuttruff and T. Ritter, Curr. Opin. Drug Discovery Dev.,
2008, 11, 803; (f) T. Furuya, A. S. Kamlet and T. Ritter, Nature,
2011, 473, 470; (g) A. Vigalok, Organometallics, 2011, 30, 4802.
2 J. F. Hartwig, Organometallic Metal Chemistry, University Science
Books, Sausalit, CA, 2010, ch. 19.
Scheme 2 Competitive cyclometalation and C–F coupling pathways.
To gain more insights into the Pt(IV)–F species proposed in
Schemes 1 and 2, the fluorination of 5 and 6 by XeF2 was
examined. In particular, we hoped that the ortho-pyridyl
3 For recent examples of metal-catalyzed nucleophilic fluorinations,
see: (a) D. A. Watson, M. Su, G. Teverovskiy, Y. Zhang, J. Garcıa-
´
group in
5 could trap the coordinatively unsaturated
Fortanet, T. Kinzel and S. L. Buchwald, Science, 2009, 325, 1661;
(b) M. H. Katcher and A. G. Doyle, J. Am. Chem. Soc., 2010,
132, 17402; (c) C. Hollingworth, A. Hazari, M. N. Hopkinson,
M. Tredwell, E. Benedetto, M. Huiban, A. D. Gee, J. M. Brown
and V. Gouverneur, Angew. Chem., Int. Ed., 2011, 50, 2613.
4 For recent examples of C–F couplings through Pd(IV) intermediates,
see: (a) K. L. Hull, W. Q. Anani and M. S. Sanford, J. Am. Chem.
Soc., 2006, 128, 7134; (b) T. Furuya, H. M. Kaiser and T. Ritter,
Angew. Chem., Int. Ed., 2008, 47, 5993; (c) T. Furuya and T. Ritter,
J. Am. Chem. Soc., 2008, 130, 10060; (d) N. D. Ball and
M. S. Sanford, J. Am. Chem. Soc., 2009, 131, 3796; (e) X. Wang,
T. S. Mei and J. Q. Yu, J. Am. Chem. Soc., 2009, 131, 7520;
(f) T. Wu, G. Yin and G. Liu, J. Am. Chem. Soc., 2009, 131, 16354;
(g) 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; (h) W. Wang, J. Jasinski, G. B. Hammond and B. Xu,
Angew. Chem., Int. Ed., 2010, 49, 7247.
Pt(IV)–F intermediate. Instead, XeF2 converted 5 into the
Pt(II) complex 9 (eqn (3)), whose configuration was deduced
from 31P NMR data (e.g., dF = ꢀ37.9 ppm, JP1-F = 652 Hz;
JPt-P3 = 3745 Hz vs. JPt-P2 = 1863 Hz).12 The formation of
this complex presumably occurred via associative displacement
of one triphos phosphine arm (P1, eqn (3)) in 5 by the pyridyl
ligand, followed by oxidation of the unligated phosphine ligand.
We have previously shown that phosphine fluorination by XeF2
is rapid.8b Despite its structural analogy to 4 and 5, complex 6
failed to react with XeF2, indicating that a dicationic Pt(II)
center may be too electron deficient to generate a tricationic
Pt(IV) structure.
5 For Ag(I)-catalyzed electrophilic fluorination reactions, see:
(a) T. Furuya, A. E. Strom and T. Ritter, J. Am. Chem. Soc.,
2009, 131, 1662; (b) T. Furuya and T. Ritter, Org. Lett., 2009,
11, 2860; (c) P. Tang, T. Furuya and T. Ritter, J. Am. Chem. Soc.,
2010, 132, 12150; (d) T. Xu, X. Mu, H. Peng and G. Liu, Angew.
Chem., Int. Ed., 2011, 50, 8176.
ð3Þ
6 J. A. Akana, K. X. Bhattacharyya, P. Muller and J. P. Sadighi,
¨
J. Am. Chem. Soc., 2007, 129, 7736.
7 For a review on the use of F+ to enable coupling and activation
reactions from high-valent metal centers, see: K. M. Engle,
T.-S. Mei, X. Wang and J.-Q. Yu, Angew. Chem., Int. Ed., 2011,
50, 1478.
In summary, we report the first sp2 C–F coupling from a
Pt center. Like Pt–Csp3 bonds, steric congestion is a key factor, as
is F+ source. We have also demonstrated that ortho-metalation
may be competitive with C–F reductive elimination. The inter-
mediacy of Pt(IV)–F complexes, the product of direct F+ addition
to Pt(II), is supported by the direct spectroscopic observation of
several Pt(IV)–F species and the isolation of ortho-metalation
products.
8 (a) A. W. Kaspi, I. Goldberg and A. Vigalok, J. Am. Chem. Soc.,
2010, 132, 10626; (b) S.-B. Zhao, J. J. Becker and M. R. Gagne
´
,
Organometallics, 2011, 30, 3926.
9 (a) A. Vigalok, Chem.–Eur. J., 2008, 14, 5102; (b) A. W. Kaspi and
A. Vigalok, Top. Organomet. Chem., 2010, 31, 19.
10 For recent examples of sp3-carbon–halogen bond forming
reactions from Pt(IV) centers, see: (a) K. I. Goldberg, J. Yan and
E. L. Winter, J. Am. Chem. Soc., 1994, 116, 1573;
(b) K. I. Goldberg, J. Yan and E. M. Breitung, J. Am. Chem.
Soc., 1995, 117, 6889; (c) S.-B. Zhao, R.-Y. Wang and S. Wang,
Organometallics, 2009, 28, 2572; (d) P. F. Oblad, J. E. Bercaw,
N. Hazari and J. A. Labinger, Organometallics, 2010, 29, 789.
11 (a) R. Ettorre, Inorg. Nucl. Chem. Lett., 1969, 5, 45; (b) A. Yahav-
Levi, I. Goldberg, A. Vigalok and A. N. Vedernikov, J. Am. Chem.
Soc., 2008, 130, 724; (c) A. Yahav-Levi, I. Goldberg, A. Vigalok
and A. N. Vedernikov, Chem. Commun., 2010, 46, 3324.
We acknowledge the generous support of the NIH
(GM-60578) and Army Research Office Staff Research Program.
SZ thanks NSERC of Canada for a Postdoctoral Fellowship.
Notes and references
y X-Ray
structure
data:
complex
4
(CCDC
838071),
C47H44BCl2F4P3Pt, M = 1054.53, monoclinic, space group P21/c,
a = 11.1844(10) A, b = 15.4199(2) A, c = 24.7809(3) A, a = g =
901, b = 91.93(1)1, V = 4271.35(8) A3, Z = 4, T = 100(2) K, 36 354
12 See ESIw for details.
collected reflections, 8252 unique reflections (Rint = 0.0182); R1
=
13 Although not discussed herein, the reaction of (COD)Pt(mesityl)(I)
with triphos triggers migratory insertion of the Pt–mesityl bond
into the COD moiety; the product was crystallographically
0.0241, wR2 = 0.0604 for data with I 4 2s(I), and R1 = 0.0245,
wR2 = 0.0607 for all unique data. Complex 7 (CCDC 838072),
C49H48BF5NO2P3Pt, M = 1076.69, monoclinic, space group C2/c,
a = 31.7695(19) A, b = 10.0332(6) A, c = 33.988(3) A, a = g = 901,
b = 116.680(1)1, V = 9680.2(12) A3, Z = 8, T = 180(2) K, 18 908
characterized, see: S.-B. Zhao, R.-Y. Wang and M. R. Gagne
´
,
Acta Crystallogr., Sect. E: Struct. Rep. Online, 2011, E67, m972.
14 N. Godbert, T. Pugliese, I. Aiello, A. Bellusci, A. Crispini and
M. Ghedini, Eur. J. Inorg. Chem., 2007, 32, 5105.
collected reflections, 9401 unique reflections (Rint = 0.0255); R1
=
0.0302, wR2 = 0.0685 for data with I 4 2s(I), and R1 = 0.0374,
wR2 = 0.0707 for all unique data. Complex 8 (CCDC 838073),
C48H46.75BF9.50NO1.38P3.50Pt, M = 1154.41, triclinic, space group
15 J. H. Koh and M. R. Gagne
43, 3459.
16 Being linear, XeF2 is significantly smaller than Selectfluors
´
, Angew. Chem., Int. Ed., 2004,
.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 443–445 445