Room-temperature β-H elimination in (P2P)Pt(OR) cations: Convenient synthesis of a platinum hydride

Article abstract of DOI:10.1021/om061105z

In situ-generated [(BINAP)(PMe₃)Pt][BF₄]₂ reacts with benzyl alcohol at RT to yield [(BINAP)(PMe₃)Pt-H][BF ₄] and benzaldehyde. This reactivity contrasts similarly ligated platinum-alky I species, which are stable to β-hydride elimination even at elevated temperatures. Protonolysis of the platinum hydride leads to a species that is readily substituted by weakly coordinating ligands (acetone, pentafluorobenzonitrile).

Full text of DOI:10.1021/om061105z

2788
Organometallics 2007, 26, 2788-2790
Notes
Room-Temperature â-H Elimination in (P2P)Pt(OR) Cations:
Convenient Synthesis of a Platinum Hydride
Alison N. Campbell and Michel R. Gagne´*
Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill,
North Carolina 27599-3290
ReceiVed December 5, 2006
Summary: In situ-generated [(BINAP)(PMe3)Pt][BF4]2 reacts
with benzyl alcohol at RT to yield [(BINAP)(PMe3)Pt-H][BF4]
and benzaldehyde. This reactiVity contrasts similarly ligated
platinum-alkyl species, which are stable to â-hydride elimina-
tion eVen at eleVated temperatures. Protonolysis of the platinum
hydride leads to a species that is readily substituted by weakly
coordinating ligands (acetone, pentafluorobenzonitrile).
a bidentate/monodentate (P2P) ligand array could similarly block
migratory deinsertion and improve diene cycloisomerization
reaction profiles (e.g., yield, diastereo- and enantioselectivity)
compared to first-generation PPP catalysts.8
Introduction
â-Hydride elimination is a well-established reaction in P2-
PtR2,1 P2Pt(OR)R,2 and P2Pt(OR)22 complexes.3 In the case of
Pt alkyls it has been possible to inhibit these reactions using
bidentate ligands (e.g., dppf and dppe)1a,2c that inhibit phosphine
dissociation and thus block low-energy migratory deinsertion
from three-coordinate intermediates.1a It has been possible to
similarly inhibit â-H elimination in cationic structures using
tridentate ligands (e.g., triphos (PPP) or pyridyl bisphosphine),
which block the cis positions required for low-energy migratory
deinsertion.4 In fact, this strategy has been key to a number of
processes where â-H elimination is undesirable,5 including some
catalytic Pt(II) alkene activation reactions.4b,6,7 Our group
recently reported that dicationic platinum catalysts containing
* Corresponding author. E-mail: mgagne@unc.edu.
(1) (a) Whitesides, G. M.; Gaasch, J. F.; Stedronsky, E. R. J. Am. Chem.
Soc. 1972, 94, 5258-5270. (b) Foley, P.; DiCosimo, R.; Whitesides, G.
M. J. Am. Chem. Soc. 1980, 102, 6713-6725. (c) Whitesides, G. M. Pure
Appl. Chem. 1981, 53, 287-292. (d) McCarthy, T. J.; Nuzzo, R. G.;
Whitesides, G. M. J. Am. Chem. Soc. 1981, 103, 3396-3403. (e) Nuzzo,
R. G.; McCarthy, T. J.; Whitesides, G. M. J. Am. Chem. Soc. 1981, 103,
3404-3410. (f) Komiya, S.; Morimoto, Y.; Yamamoto, A.; Yamamoto, T.
Organometallics 1982, 1, 1528-1536.
(2) (a) Bryndza, H. E.; Kretchmar, S. A.; Tulip, T. H. J. Chem. Soc.,
Chem. Commun. 1985, 977-978. (b) Bryndza, H. E. J. Chem. Soc., Chem.
Commun. 1985, 1696-1698. (c) Bryndza, H. E.; Joseph, C. C.; Marsi, M.;
Roe, D. C.; Tam, W.; Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 4805-
4813.
Results and Discussion
A particularly useful experiment for probing the mechanism
of the original (PPP)Pt+2-catalyzed cycloisomerization reaction
was to include in situ traps (benzyl alcohol) for putative
carbocation intermediates (eq 1). Similar trapping experiments
on second-generation P2P cyclopropanation catalysts unexpect-
edly diverged, and no Pt alkyl species were observed. Instead,
a new (BINAP)(PMe3)Pt species was generated with a JPt-P of
2200 Hz (31P NMR) for the phosphorus trans to the new ligand
(eq 2). When the alcohol was changed to either phenol or
methanol, the same species was observed. Particularly informa-
tive was the H NMR, which showed a diagnostic platinum
hydride resonance at -5.2 ppm, suggesting [(BINAP)(PMe3)-
Pt-H][BF4]2, 1, as the structure (Figure 1).
To determine the source of the hydride, [(BINAP)(PMe3)-
PtI][I] (2) was taken with 2.5 equiv of AgBF4 and 20 equiv of
benzyl alcohol in nitromethane. The reaction cleanly generated
1 and benzaldehyde, indicating that benzyl alcohol likely served
as the hydride source; ethanol and 2-propanol similarly afforded
(3) Davies, J. A.; Hartley, F. R. Chem. ReV. 1981, 81, 79-90.
(4) (a) Cucciolito, M. E.; D’Amora, A.; Vitagliano, A. Organometallics
2005, 24, 3359-3361. (b) Hahn, C.; Morvillo, P.; Herdtweck, E.; Vitagliano,
A. Organometallics 2002, 21, 1807-1818. (c) Oestereich, M.; Dennison,
P. R.; Kodanko, J. J.; Overman, L. E. Angew. Chem., Int. Ed. 2001, 40,
1439-1442. (d) Zhang, L.; Zetterberg, K. Organometallics 1991, 10, 3806-
3816. (e) Arnek, R. Zetterberg, K. Organometallics 1987, 6, 1230-1235.
(f) Arai, I.; Daves, G. D. J., Jr. J. Am. Chem. Soc. 1981, 103, 7683.
(5) (a) Zhou, J.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 14726-14727.
(b) Fischer, C.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 4594-4595. (c)
Arp, F. O.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 10482-10483.
(6) (a) Hahn, C.; Cucciolito, M. E.; Vitagliano, A. J. Am. Chem. Soc.
2002, 124, 9038-9039. (b) Koh, J. H.; Gagne´, M. R. Angew. Chem., Int.
Ed. 2004, 43, 3459-3461.
1
(7) (a) Kerber, W. D.; Gagne´, M. R. Org. Lett. 2005, 7, 3379-3381. (b)
Kerber, W. D.; Koh, J. H.; Gagne´, M. R. Org. Lett. 2004, 6, 3013-3015.
(8) Feducia, J. A.; Campbell, A. N.; Doherty, M. Q.; Gagne´, M. R. J.
Am. Chem. Soc. 2006, 128, 13290-13297.
10.1021/om061105z CCC: $37.00 © 2007 American Chemical Society
Publication on Web 04/07/2007

Notes
Organometallics, Vol. 26, No. 10, 2007 2789
Scheme 2. Proposed Mechanism for Hydride Formation
from Ph2NMe
Figure 1. 1H NMR spectrum of 1 in the hydride region (δ ) -5.2
ppm).
Table 1. Selected JP-Pt Coupling Constants for the
Phosphine trans to X/L
chemical shift of
(BINAP)(PMe3)Pt2+-X/L
JP-Pt (Hz)
PMe3 (ppm)a,b
X ) alkyl-
1700
2200
3510
3650
4020
4100
-10
-15.6
-13.9c
-3.5
2.1
H-
I-
L ) NCC6F5
acetone
“MeOH”d
3.8
-
a In CD3NO2 unless otherwise noted; the counterion is BF4
.
b Externally
c
d
referenced to 85% H3PO4. In CDCl3. Tentative assignment based on JP-Pt
.
PMe3, and SEGPHOS/PMe3 ligand arrays all successfully
generated the respective platinum hydrides on reacting with
benzyl alcohol, while dppm/PMe3, dppe/PMe3, and triphos did
not. Phosphorus-platinum coupling constants suggest that these
species weakly coordinate the alcohol13 but do not facilitate the
subsequent â-elimination. Table 1 collects the diagnostic JP-Pt
for the phosphine trans to the variable site; the PMe3 chemical
shift was sensitive to the charge on the fourth ligand and is
also included.
-
Figure 2. Chem3D representation of 1. BF4 counterion is not
shown. Selected bond lengths (Å): Pt-H ) 1.682, Pt-P1 ) 2.2944-
(12), Pt-P2 ) 2.3472(12), Pt-P3 ) 2.2966(12). Selected bond
angles (deg): P1-Pt-P2 ) 92.17, P2-Pt-P3 ) 101.45.
Scheme 1. Proposed Mechanism for Hydride Formation
from BnOH
The utility of 1 to function as a convenient (Ag+-free)
precursor to highly reactive (P2P)Pt2+ catalysts was investigated.
HBF4 and HNTf2 were each able to protonolyze off the hydride
in the presence of a suitable trapping ligand (acetone or
pentafluorobenzonitrile) (Scheme 3), though similar protonolysis
experiments on (P2P)Pt+-CH3 were very sluggish under these
conditions.14 [Ph2NH2][BF4] (pKa ) 0.8)15 was not acidic
enough to initiate similar reactivity.
(9) We are unable, as of yet, to distinguish between these two possibili-
ties, as the reaction is fast both with and without the added weak base (Ph2-
NH). We favor the alkoxide route simply because hydride migration from
a coordinated alcohol would generate aldehyde bound to both an electro-
philic Pt Lewis acid and H+. A counterpoint to this notion is the observation
that dicationic P2Pt2+ Lewis acids can function with a Brønsted co-catalyst
in certain electrophilic activation reactions on aldehydes; see: Mullen, C.
A.; Gagne´, M. R. Org. Lett. 2006, 8, 665-668. Regardless, the discussion
explicitly assumes a key alkoxide intermediate.
the hydride. A generic mechanism for hydride formation is
shown in Scheme 1, with the key step being â-hydride
elimination from an alkoxide (or an alcohol) intermediate.9
In contrast to the in situ carbocyclization trapping experiments
in eq 2, direct reactions of phenol and methanol with (P2P)Pt2+
did not yield 1. The source of the hydride in the former cases
was separately traced to the amine base (Ph2NMe), which, in
the absence of alcohol, generates 1 on reacting with the dication.
In this case a â-H elimination to generate the N,N′-diphenylim-
minium ion is envisioned10 (Scheme 2); switching to Ph2NH
completely suppresses hydride formation (no â-H’s).
An X-ray structure of 1 was obtained (Figure 2) by slow vapor
diffusion of pentane to an acetone solution. Chlorinated solvents
were not suitable for crystallization because 1 was readily
converted to the platinum chloride. Pt-H and Pt-P bond lengths
are similar to related platinum hydride structures.11 The Pt-P
bond trans to the hydride is about 0.1 Å longer than the Pt-P
bond trans to the chloride in the analogous chloride structure.12
Interestingly, this route to the hydrides seems to be limited
to compounds containing biaryl-linked diphosphine ligands.
Platinum complexes containing BINAP/PMe3, xyl-BINAP/
(10) The imminium ion was not observed.
(11) Selected, similar platinum hydride structures: (a) Clark, H. C.;
Dymarski, M. J.; Oliver, J. D. J. Organomet. Chem. 1978, 154, C40-C42.
(b) Manojlovic-Muir, L.; Jobe, I. R.; Ling, S. S. M.; McLennan, A. J.;
Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1985, 1725-1726. (c)
Alonso, E.; Fornies, J.; Fortuno, C.; Martin, A.; Orpen, A. G. Organome-
tallics 2001, 20, 850-859. (d) Jaska, C. A.; Lough, A. J.; Manners, I. Dalton
Trans. 2005, 326-331. (e) Packett, D. L.; Syed, A.; Trogler, W. C.
Organometallics 1988, 7, 159-166.
(12) See supporting information for details on the [(BINAP)(PMe3)PtCl]-
[BF4] X-ray structure.
(13) (a) Alcock, N. W.; Platt, A. W. G.; Pringle, P. G. J. Chem. Soc.,
Dalton Trans. 1989, 139-143. (b) Alcock, N. W.; Platt, A. W. G.; Pringle,
P. J. Chem. Soc., Dalton Trans. 1987, 2273-2280. (c) Alcock, W. N.; Platt,
A. W. G.; Pringle, P. G. Inorg. Chim. Acta 1987, 128, 215-216.
(14) Feducia, J. A.; Campbell, A. N.; Anthis, J. W.; Gagne´, M. R.
Organometallics 2006, 25, 3114-3117.
(15) Measured in aqueous HCl. See: (a) Dolman, D.; Stewart, R. Can.
J. Chem. 1967, 45, 903-910. (b) Stewart, R.; Dolman, D. Can. J. Chem.
1967, 45, 925-928.

2790 Organometallics, Vol. 26, No. 10, 2007
Scheme 3. Protonolysis of Pt-H
Notes
thaw degassed. MeNO2 was purified according to literature
procedures, which removes trace propionitrile from the commercial
material.17 CD3NO2 and Ph2NMe were distilled from CaH2 and
freeze-pump-thaw degassed prior to use. HNTf2 and phenol were
sublimed under vacuum. Anhydrous benzyl alcohol, methanol,
ethanol, and 2-propanol were used as received from Aldrich. P2-
PtI2 was prepared by stirring equimolar quantities of the bidentate
phosphine with (COD)PtI2 (COD ) 1,5-cyclooctadiene) in CH2-
Cl2 and then precipitating with pentane. [(R)-BINAP(PMe3)PtI][I]
was prepared by adding 1 equiv of PMe3 to ((R)-BINAP)PtI2 in
MeNO2 as previously reported.8 NMR spectra were recorded on
either a Bruker 400 MHz DRX or a Bruker 300 MHz AMX
spectrometer; chemical shifts are given in ppm and are referenced
to residual solvent resonances (1H, 13C) or an external 85% H3PO4
standard (31P). Elemental analysis was performed by Robertson
Microlit Labs.
Chart 1
[((R)-BINAP)(PMe3)PtH][BF4] (1). To a solution of 70 mg of
[((R)-BINAP)(PMe3)PtI][I] (61 µmol) in 0.5 mL of MeNO2 was
added 126 µL of benzyl alcohol (1.22 mmol) and 30 mg of AgBF4
(152 µmol). The mixture was stirred for 15 min at 23 °C, diluted
with CH2Cl2, and filtered through a 0.45 µm PTFE syringe filter.
The solution was then washed three times with distilled water. The
organic fraction was dried over Na2SO4, filtered, and concentrated
in Vacuo. The crude material was dissolved in CH2Cl2 and
precipitated with n-pentane five times until no benzyl alcohol
remained by 1H NMR. The purified solid was dried under vacuum
to yield 45 mg (64%) of a white solid: 1H NMR (400 MHz, CDCl3)
δ 7.97-6.49 (m, 32H), 1.33 (m, 9H); 31P NMR (161 MHz, CDCl3)
δ 28.56 (dd, 1P, JP-P ) 21, 352 Hz, 1JP-Pt ) 2616 Hz), 17.46 (dd,
The rapid â-hydride elimination of the described platinum-
alkoxides significantly contrasts with comparably ligated plati-
num-alkyl compounds, which do not â-eliminate up to 70
°C.6a,7 Bercaw and Bryndza have previously noted the polarizing
influence of electronegative substituents on â-H elimination
from 16-electron neutral P2PtX2 complexes: (dppe)Pt(OMe)2
(25 °C) . (dppe)Pt(OMe)Et (100 °C) > (dppe)PtEt2 (160 °C).2c
Mechanistic studies implicated a pre-equilibrium â-H elimina-
tion to an 18-electron, five-coordinate (dppe)Pt(H)OMe(H2Cd
O) intermediate, which reacted by competitive loss of MeOH
or formaldehyde. Similarly, Strukul has shown that â-H
elimination is rapid at RT in (dppe)(CF3)Pt(OR) complexes,
which act as efficient oxidation catalysts in the presence of
H2O2.16 The electron-deficient CF3 ligand presumably increased
the metal’s electrophilicity, which increased the rate of â-H
elimination (cf. (dppe)Pt(OMe)Et, which does react until 100
°C). We hypothesize, on the basis of these studies and our own,
that the combination of a cationic metal and an alkoxide ligand
generates a sufficiently electrophilic complex to enable rapid
â-H eliminate and provide 1 at RT. The analogous alkyl
complexes lacking such an electronegative substituent do not
readily â-eliminate. The situation may be more complex than
this, since not all (P2P)Pt2+ complexes generated the hydride.
The complexes known to â-hydride eliminate at RT are collected
in Chart 1.
1P, JP-P ) 20, 21 Hz, 1JP-Pt ) 2020 Hz), -17.65 (dd, 1P, JP-P
)
1
20, 352 Hz, JP-Pt ) 2464 Hz). Anal. Calcd for C47H42BF4P3Pt:
C, 57.51; H, 4.31. Found: C, 57.23; H, 4.12.
[((rac)-BINAP)(PMe3)PtCl][BF4] (2). Slow vapor diffusion of
n-pentane to a solution of 1 in CDCl3 yielded X-ray quality crystals
of [((rac)-BINAP)(PMe3)PtCl][BF4].18
Platinum-Hydride Cleavage Reactions. In a typical reaction,
to 15 mg of [((R)-BINAP)(PMe3)PtH][BF4] (15.3 µmol) in CD3-
NO2 was added 1 equiv of acid (HNTf2, HBF4, [Ph2NH2][BF4], or
[Ph3C][BF4]) and 5 equiv of NCC6F5 or acetone. Disappearance of
the hydride resonance was monitored by 1H NMR, and the
appearance of a new platinum species (either the nitrile or acetone
adduct) was observed by 31P NMR.
In summary, we report a convenient method for the synthesis
of chiral (P2P)Pt-H cations and additionally extend the
compound types known to â-H eliminate at RT to several
cationic triphosphine structures.
Acknowledgment. The National Institute of General Medi-
cine (GM-60578) is gratefully acknowledged for support.
Supporting Information Available: X-ray data and tables and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
Experimental Section
General Methods. Synthetic procedures were performed in a
dinitrogen-filled MBraun Labmaster 100 glovebox. CH2Cl2 was
sparged with dry argon and passed through a column of activated
alumina. Acetone was distilled from CaSO4 and freeze-pump-
OM061105Z
(17) Parrett. F. W.; Sun, M. S. J. Chem. Educ. 1977, 54, 448-449.
(18) During crystallization attempts of 1, the presence of chlorinated
solvents (particularly chloroform and dichloromethane) led to the formation
of 2, which selectively crystallized.
(16) Zennaro, R.; Pinna, F.; Strukul, G. J. Mol. Catal. 1991, 70, 269-
275.