Oxidative addition of ether O-methyl bonds at a Pt(0) centre

Article abstract of DOI:10.1039/c4cc00853g

Pt(PCyp₃)₂ (Cyp = cyclopentyl) undergoes C-O oxidative addition with 2,3,5,6-tetrafluoro-4-methoxypyridine, pentafluoroanisole, 2,3,5,6-tetrafluoroanisole and 2,3,6-trifluoroanisole yielding platinum methyl derivatives. The reactions occur in preference to C-H or C-F activation.

Full text of DOI:10.1039/c4cc00853g

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Oxidative addition of ether O-methyl bonds
at a Pt(0) centre†‡
Cite this: Chem. Commun., 2014,
50, 3914
Naseralla A. Jasim, Robin N. Perutz,* Barbara Procacci and Adrian C. Whitwood
Received 31st January 2014,
Accepted 6th February 2014
DOI: 10.1039/c4cc00853g
www.rsc.org/chemcomm
Pt(PCyp3)2 (Cyp = cyclopentyl) undergoes C–O oxidative addition with anisoles and demonstrated rearrangement to Ir–O metallacycles
2,3,5,6-tetrafluoro-4-methoxypyridine, pentafluoroanisole, 2,3,5,6- containing IrQcarbene linkages. Goldman used fluorinated anisoles
tetrafluoroanisole and 2,3,6-trifluoroanisole yielding platinum and isolated the products of ‘‘simple’’ O-methyl oxidative addition.
methyl derivatives. The reactions occur in preference to C–H or Following the initial C–H cleavage, aryloxy migration and 1,2-hydride
C–F activation.
migration generated the product. Cross-coupling reactions involving
unsupported aryl methyl ether C–O bonds have been described at
We have shown that Pd0(PR3)2 and Pt0(PR3)2 complexes can nickel.22,23 In contrast to the iridium reactions, the cross-coupling
undergo C–F oxidative addition, and the resulting square-planar results in cleavage of the aryl C–O bond although this bond is
Pt(II) complexes are stable to C–F reductive elimination.1–3 How- stronger than the O–CH3 bond.
ever, C–F reductive elimination is possible from 3-coordinate
Here we report the reactions of Pt0(PCyp3)2 1 (Cyp = cyclo-
Pd(II)4 and from the metal aryl fluoride complexes in oxidation pentyl) with 2,3,5,6-tetrafluoro-4-methoxypyridine, pentafluoro-
state IV.5–8 The related oxidative addition of ether C–O bonds anisole, 2,3,5,6-tetrafluoroanisole and 2,3,6-trifluoroanisole and
appears to be missing from the literature although examples of show that ArFO–CH3 oxidative addition occurs in preference
C–O reductive elimination of ethers from Pd(II) are known.9,10 to C–H or C–F oxidative addition. We also compare reactions
In contrast, C–O (ester) oxidative addition at Pt(0) has recently with pentafluorophenol and 2,3,5,6-tetrafluoro-4-ethoxypyridine
been described11 and reductive elimination of ester C–O bonds (Scheme 1). These reactions offer competition between C–F,
from Pd(IV) and Pt(IV) is well known.12–14 We demonstrate the C–H, C–O and O–H activation.
importance of electron withdrawing aryl groups for achieving
oxidative addition of methyl aryl ethers at Pt(0) and probe the methoxypyridine a (1 equiv., hexane, 60 1C, 2 days) generated
mechanism of C–O activation. one product quantitatively (Scheme 2). The colourless crystalline
The thermal reaction of 1 in the presence of tetrafluoro-
Activation of C–O bonds of ethers at transition metals, first material was characterised by multinuclear NMR spectroscopy,
described many years ago,15–17 has been rejuvenated by the work LIFDI mass spectrometry24 and X-ray crystallography. The 1H NMR
of Carmona, Paneque18,19 and Goldman20,21 who investigated spectrum shows a triplet resonance at d 0.89 with 195Pt satellites
the reactivity of iridium tris(pyrazolyl)borate and iridium pincer ( JPH = 6.3, JPtH = 83.8 Hz) with integration 3H relative to cyclo-
complexes, respectively. In both cases, the C–O activation pentyl protons and no signals to higher field. The 195Pt-1H
was associated with initial methyl C–H bond activation (see HMQC spectrum shows that this resonance correlates with a
Scheme S1 in ESI‡). Carmona and Paneque used methylated platinum triplet at d À4060. The 1H peaks of the tricyclopentyl-
phosphine also show cross-peaks to the platinum resonance.
The 31P{1H} NMR spectrum gives a singlet at d 22.3 with Pt
Department of Chemistry, University of York, York, UK YO10 5DD.
E-mail: robin.perutz@york.ac.uk
† This article is published in celebration of the 50th anniversary of the opening of
the Chemistry Department at the University of York.
‡ Electronic supplementary information (ESI) available: Scheme of reactions of
ethers at iridium, general procedures, syntheses, HSQC of 1a and 1c, the kinetic
isotope effect including spectra, in situ monitoring of formation of 1b, the kinetic
plot and spectra of intermediates, NMR data, crystallographic methods and data
for 1a, 1b, 1e and 1f. CCDC 966767, 966768, 983216 and 983215. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
Scheme 1 Substrates.
c4cc00853g
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Scheme 2 Reactivity of 1.
Fig. 1 Molecular structure of 1a (above) and 1b (below). H atoms omitted.
Anisotropic displacement parameters shown as 50% ellipsoids.
satellites ( JPtP 2962 Hz). The 19F NMR spectrum shows two sets
of inequivalent fluorines at d À168.3 (m, F meta to N) and at d
À99.0 (m, F ortho to N) in a 1 : 1 integration ratio. 13C{1H} NMR
spectroscopy reveals a triplet at d À34.3 with 195Pt satellites investigated the reaction of 1 with a mixture of a and a-d3 in
1
( JPC = 7 Hz, JPtC = 695 Hz) which correlates with the H NMR hexane. The reaction was performed with a 10-fold excess of each
resonance at d 0.89. This pair of correlated resonances is consistent substrate with the consequence that no heating was required to
with a methyl group bound to platinum (a methoxy group would be generate a mixture of 1a and 1a-d3. The 31P NMR spectrum and
expected at lower field with small couplings to Pt). Compound 1a the 195Pt NMR spectrum showed clear isotopic shifts to low field
was therefore assigned as trans-Pt(PCyp3)2(CH3)(OC5F4N). Colourless of 0.17 ppm (35.3 Hz) and 14.2 ppm (1527 Hz), respectively.
crystals were grown by slow diffusion of hexane into a concentrated Determination of the initial substrate ratio by mass spectrometry
benzene solution. The crystal structure was determined and the and the product ratio by quantitative 31P{1H} NMR spectroscopy
nature of 1a (Fig. 1) was confirmed demonstrating conclusively yielded a kinetic isotope effect kH/kD of 1.11 Æ 10%.
that the ArFO–CH3 bond of the ether undergoes selective
oxidative addition at the Pt(0) centre (Scheme 2).
The progress of the reaction of 1 with two equiv. b at 50 1C
was monitored in situ by 31P NMR spectroscopy in protio
The reaction of 1 with pentafluoroanisole b proceeded solvents. The half-life of 1 proved to be ca. 3 times shorter in
analogously to that with tetrafluoromethoxypyridine generating THF than in hexane (t1/2 200 and 620 min, respectively). An
1b as the only product (Scheme 2). The product 1b exhibits intermediate was detected in the early stages of reaction in both
similar spectroscopic features to those for 1a and was therefore solvents which declined with time. It exhibited a 31P resonance
assigned as trans-Pt(PCyp3)2(CH3)(OC6F5). A crystal structure of with JPtP = 3956 Hz characteristic of Pt(0) (d 26.9, s, in THF) and
1b (crystals grown from hexane) confirmed its identity (Fig. 1). five distinct 19F resonances, consistent with a provisional
The Pt–O and Pt–C bond lengths for 1a are 2.1586(15) and assignment as Pt(PCyp3)2(Z2-CQC-C6F5OMe) (see ESI‡).
2.044(2) Å, respectively. The Pt–O–C angle is 130.10(15)1 and the
Complex 1, dissolved in hexane, also reacted with tetrafluoro-
1
torsional angle of the ring C(3)–C(2)–O(1)–Pt is 21.2(3)1. The ethoxypyridine c (1.1 equiv., 60 1C), but the H NMR spectrum
corresponding Pt–O and Pt–C bond lengths for 1b are 2.1306(15) showed this time that the principal product is a platinum
and 2.047(2) Å, while the Pt–O–C angle is 126.78(14)1 and the hydride (d À24.6, t JPH 13.1 Hz, JHPt 1234 Hz). The 31P{1H}
torsional angle is 27.3(3)1.
NMR spectrum shows a singlet resonance at d 46.2 ( JPtP =
Goldman showed that the kinetic isotope effect (KIE) for 2940 Hz). The complex exhibits a 195Pt resonance as a triplet
oxidative addition of pentafluoroanisole (kOCH )/(kOCD ) to the of doublets at d À4704 with the corresponding coupling
3
3
iridium pincer complexes is 4.3 Æ 0.3 providing strong support constants. The 195Pt-1H HSQC spectrum links the hydride
for initial C–H bond activation.20,21 We synthesised a-d3 and and the cyclopentyl resonances at d 1.85 to the 195Pt resonance
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by C–H activation as observed for CpRh(PMe3).25,26 Three alter-
native routes are consistent with a low KIE: direct oxidative
addition, phosphine-assisted reaction2,3 and an ion-pair route9
(Scheme 3). In each case, initial coordination of the substrate
would occur, consistent with detection of the Pt(0) intermediate in
the reaction with b. Direct oxidative addition reaction represents
the reverse of the reductive elimination reactions observed by
Buchwald.10,27 The selectivity for breaking the O–CH3 bond
can be understood in the phosphine-assisted route because
of the preference to put the electronegative O–ArF group on
the metallophosphorane.2,28,29 The ion-pair route generates
[Pt(PCyp3)2Me]+ with the ethers and [Pt(PCyp3)2H]+ with penta-
fluorophenol30,31 in parallel to recent C–O reductive elimination
mechanisms.9 The increased rate in the more polar solvent,
THF, lends support to the ion-pair route.
These results demonstrate selective C–O oxidative addition
at fluorinated aromatic methyl ethers. The selectivity for the
O–CH3 bond in b, e and f in preference to the O–Ar bond matches
Goldman’s observations (also with fluorinated arenes). With
Scheme 3 Proposed mechanisms of C–O oxidative addition.
at d À4704. The 19F NMR spectrum shows two multiplets at unfluorinated Ar–O–CH3 substrates, Paneque again observed
d À99.3 (m, F ortho to N) and À169.8 (m, F meta to N). Evidence O–CH3 cleavage19 but Milstein observed Ar–O cleavage at
for free C2H4 was obtained when the reaction was performed in Rh(PCP).17 The reaction of a at Pt(PCyp3)2 contrasts with the
1
C6D6 and monitored in situ by its H NMR resonance at d 5.2. C–F oxidative addition observed with the same substrate at
Complex 1c was assigned as trans-Pt(PCyp3)2(H)(OC5NF4), Ni(PEt3)2 and with the cyclometallation product formed by C–H
in keeping with the similar results obtained by Goldman.20,21 and C–F activation at CpRh(PMe3).24–26
The X-ray structure of a crystal of 1c was consistent with the
This work was supported by EPSRC. We thank the referees
formulation but suffered from disorder both in the cyclopentyl for useful comments and Odile Eisenstein for discussions.
rings and the OC6F5 group.
The formation of platinum aryloxy hydrides can also be
achieved by reaction with the corresponding phenol, as shown
by reaction of 1 with pentafluorophenol d (300 K, 1.1 equiv. in
Notes and references
1 N. A. Jasim, R. N. Perutz, A. C. Whitwood, T. Braun, J. Izundu,
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the O–H oxidative addition product Pt(PCyp3)2(H)(OC6F5).
The hydride resonated at d À24.3 (t, JPH 13.4, JPtH 1178 Hz)
while the 195Pt resonance appeared at d À4696 (td, JPtP 2964,
JPtH 1176 Hz).
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