Reactions of gold(III) oxo complexes with cyclic alkenes

Article abstract of DOI:10.1002/anie.200501754

(Chemical Equation Presented) An auraoxetane, obtained from the reaction of norbornene with a gold (III) oxo complex, has been isolated and fully characterized (see structure). Action of the olefin leads to elimination of the epoxide from the auraoxetane.

Full text of DOI:10.1002/anie.200501754

Communications
Gold Chemistry
DOI: 10.1002/anie.200501754
Reactions of Gold(iii) Oxo Complexes
with Cyclic Alkenes**
Maria A. Cinellu,* Giovanni Minghetti, Fabio Cocco,
Sergio Stoccoro, Antonio Zucca, and Mario Manassero
As pointed out in a recent review, metal-mediated olefin
oxidation is still a subject of great interest.[1] Various classes of
mechanisms have been envisaged for different metal systems.
Among these, that involving metallaoxetanes (1-metalla-2-
oxacyclobutanes) as a key intermediate in the oxygen-transfer
reaction has been recently corroborated by experimental
evidence.[2] While an oxametallacycle has been identified as
the crucial surface intermediate in the ethylene epoxidation
on Ag,[3] a platinaoxetane, obtained from the reaction of a
platinum(ii) oxo complex with norbornene, has been structur-
ally characterized.[4] The same oxo complex has been found to
stoichiometrically oxidize ethylene to acetaldehyde, thus
providing a rare example of alkene oxidation by isolated
late-transition-metal oxo complexes.[5] Metallaoxetanes have
also been obtained from the reaction of iridium(i) and
rhodium(i) alkene complexes with molecular oxygen or
hydrogen peroxide.[6]
Gold oxo species are likely to be involved in oxidations
catalyzed by gold supported on metal oxides,[7] as, for
example, in the direct epoxidation of propene with molecular
oxygen.[7d,8] As soluble oxo complexes are valuable models for
species involved in heterogeneous catalytic oxidation systems,
we studied the reactions of a series of gold(iii) oxo com-
plexes[9] with olefins.[10] The model reaction of [Au2(bipyR)2(m-
O)2](PF6)2 (bipyR = 6-R-2,2’-bipyridine; see Scheme 1 for R)
with styrene yielded novel cationic gold(i) alkene complexes
[Au(bipyR)(h2-CH2=CHPh)](PF6) and oxygenated styrene
derivatives.[11,12] After these encouraging results, we searched
[*] Prof. M. A. Cinellu, Prof. G. Minghetti, F. Cocco, Dr. S. Stoccoro,
Dr. A. Zucca
Dipartimento di Chimica
Università di Sassari
via Vienna 2, 07100 Sassari (Italy)
Fax: (+39)079-229-559
E-mail: cinellu@uniss.it
Prof. M. Manassero
Dipartimento di Chimica Strutturale e Stereochimica Inorganica
Università di Milano
Centro CNR
via Venezian 21, 20133 Milano (Italy)
[**] Support from the University of Sassari (60%) is gratefully
acknowledged. We thank Mrs. E. Azara (ICB, CNR, Sassari) and Dr.
A. Furesi (PMP, USSL, Sassari) for performing GC-MS and LC-MS
analyses. Thanks are also due to Dr. G. A. Chelucci (Dipartimento di
Chimica, Università di Sassari) for support in the organic syntheses
and simulation of NMR spectra.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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for the supposed oxametallacyclic intermediate.[13] A growing
number of papers have appeared on the homogeneous gold-
the main feature of the coordination geometry has been
established.[16] The second species, 4a-PF6 , proved to be the
auraoxetane, [Au(bipyMe)(k2-O,C-2-oxynorbornyl)](PF6).[17]
The structure consists of the packing of 4a cations and PF6
anions in the molar ratio of 1:1 with normal van der Waals
contacts. The molecular structure of 4a is shown in Figure 1.
The gold atom has a square-planar coordination environment
with a slight square-pyramidal distortion; maximum devia-
tions from the best plane are + 0.024(8) and À0.027(1) Š for
O and Au atoms, respectively. Carbon atom C13 lies in the
metal coordination plane, and the oxametallacycle is strictly
planar, as reported for other late-transition-metal metal-
À
catalyzed addition of oxygen nucleophiles to C C multiple
bonds of alkynes or alkenes.[14] Active gold–alkyne (gold–
alkene) and cyclic organogold species, which have been
À
suggested as key intermediates in the C O bond formation,
[14f,i–n,q] have never been isolated.
Herein we describe the reaction of [Au2(bipyR)2(m-O)2]-
(PF6)2 [1-(PF6)2: R = Me (1a), CHMe2 (1b), CH2CMe3 (1c),
C6H3Me2-2,6 (1d)] with the strained cyclic alkenes norbor-
nene (nb) and 2,5-norbornadiene (nbd) to give alkene
complexes
and
unprecedented
metallaoxetanes
(Scheme 1).[15] A dinuclear complex [Au2(bipyR)2(m-h2,h2-
laoxetanes.[18] The Au O bond length of 1.967(7) Š is very
À
À
similar to the Au O bond lengths
of 1.971(4) and 1.960(6) Š found in
[Au(bipy)(OMe)2](PF6),[19] and the
À
Au C12 separation of 2.055(8) Š
À
is comparable with the Au C bond
length of 2.028(7) Š found in
[Au{N2C10H7(CMe2CH2)-6}Cl]+
(5), in which N2C10H7(CMe2CH2)-6
is a cyclometalated 6-tert-butyl-
2,2’-bipyridine ligand.[20] The Au
À
À
N1 and Au N2 bonds (2.026(6)
and 2.183(6) Š, respectively) can
be compared with corresponding
interactions in 5 (1.976(5) and
2.151(5) Š, respectively).
As the isolation of oxametalla-
cycles is unprecedented in the
chemistry of gold, we tried to
synthesize 4a-PF6 in better yields
in order to study its reactivity.[21] In
Scheme 1. Oxo complexes 1-(PF6)2 (only the trans isomer is depicted; 1a and 1b are mixtures of the
cis and trans isomer) and products of the reaction with 2,5-norbornadiene, 2-(PF6)2, and with
norbornene, 3-PF6 and 4-PF6.
nbd)](PF6)2 [2-(PF6)2: R = Me (2a), CHMe2 (2b), CH2CMe3
(2c)] with a bridging nbd ligand is the main product of the
reaction as shown by NMR spectra and other analytical data;
signals attributable to trace amounts of a second species are
1
sometimes observed in the H NMR spectra.
1
In the case of the nb derivatives, the H NMR spectra
indicate the presence of two species, the ratio of which
depends on the preparative conditions and the nature of the
substituent of the bipyridine ligand. The main product is the
alkene complex [Au(bipyR)(nb)](PF6) [3-PF6: R = Me (3a),
CHMe2 (3b), CH2CMe3 (3c), C6H3Me2-2,6 (3d)]. When
signals from the minor species, 4, are not observed in the
1H NMR spectrum, as in the case of the 6-(2,6-xylyl)-2,2’-
bipyridine derivative, 4 is detected in the mass spectrum in
which, besides the molecular ion, M+, corresponding to the
nb complex (3d), a peak of low to medium intensity is found
at [M+16]+. Under comparable preparative conditions,
significant amounts of the latter species are obtained from
1a-(PF6)2.[15] To separate the two species, different methods
were employed in the case of the 6-methyl-2,2’-bipyridine
(bipyMe) derivatives 3a-PF6 and 4a-PF6. Crystals of 3a-PF6
and 4a-PF6, obtained by slow diffusion of diethyl ether into an
acetonitrile solution of the mixture, were separated and
subjected to X-ray diffraction analyses. Although full refine-
ment of the structure of 3a-PF6 has not been accomplished,
Figure 1. Molecular structure (ORTEP) of cation 4a. Ellipsoids are
drawn at the 30% probability level. Principal bond lenghts [Š] and
angles [8]: Au–N1 2.026(6), Au–N2 2.183(6), Au–O 1.967(7), Au–C12
2.055(8), C12–C13 1.550(14), C13–O 1.433(12); N1-Au-N2 78.2(3),
N1-Au-C12 103.9(3), N1-Au-O 172.9(3), N2-Au-O 108.2(3), N2-Au-C12
177.7(3), C12-Au-O 69.6(3), Au-C12-C13 91.2(5), C12-C13-O 100.7(7),
C13-O-Au 98.5(6).
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reactions using different ratios of nb to 1a-(PF6)2 and
at different concentrations of 1a-(PF6)2 , in both
CH3CN and in CH3CN/H2O, we established a general
trend.[22] During the reaction of trans-[1c-(PF6)2] (the
most soluble complex) with nb in CD3CN, we
1
observed by H NMR spectroscopy that the ratio of
3 to 4 remains unchanged. Signals for 3c-PF6 and 4c-
PF6 were detected after about 40 h (3c-PF6/4c-PF6 =
2:1); a resonance at d = 9.58 ppm (d, JH-H = 1.9 Hz),
which is typical of an aldehyde proton, appeared at
the same time. Isolation of the organic fraction
obtained from the same reaction in MeCN identified
exo-2,3-epoxynorbornane (6)[23] as the main product
accompanied by at least three aldehydes, 7, 8, and
9.[24] The main aldehyde species was cyclopentane-
1,3-dicarbaldehyde (7, M+ 126)[25], the others were
likely to be 3-methylene-cyclopentane carbaldehyde
(8, M+ 110)[26] and 3-methyl-2-cyclopentene carbal-
dehyde (9, M+ 110). The epoxide and the dialdehyde
were also the main products of the same reaction in
MeCN in the presence of a small amount of water
(3%). When a larger amount was added (ꢀ 10%), a
6:1 mixture of cis-endo-2,3-norbonanediol (10) and
trans-2,3-norbonanediol (11) is obtained. GC–MS
and LC–MS analyses showed peaks of M+ = 128,
which correspond to C7H12O2.
Formation of the epoxide in solution and direct
loss of a C7H10O fragment from the molecular ion of
[Au(bipyMe)(C7H10O)]+ in the gas phase—shown by
the FAB mass spectrum of 4a—point to the oxaaura-
cycle 4 being the origin of both the oxygenated
products and the olefin complex 3. Accordingly, the
olefin complex 3a-PF6, 2,3-epoxynorbornane (6), and
small amounts of aldehydes 8 and 9 were slowly
Scheme 2. Proposed reaction pathways for the formation of 3, 4, and oxy-
genated organic products 6–11: A) in MeCN/H2O and B) in MeCN. I and II are
supposed intermediates.
formed when a MeCN solution of 4a-PF6 was treated with
excess nb.[27] To the best of our knowledge, elimination of an
epoxide from an isolated oxametallacycle has never been
previously observed.[1,2b] Aldehydes or ketones are formed by
isolated RhIII and IrIII 2-metalla-oxetanes.[6e,f,28] Plausible
reaction pathways for the formation of the auraoxetane, 4,
and the alkene complex, 3, are given in Scheme 2.
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Complexes 4a–4c are the first isolated auraoxacyclobu-
tanes. Their formation by the reaction of gold oxo complexes
with alkenes may lead to a better understanding of the
oxidation of olefins mediated by late-transition-metal com-
plexes. Moreover, both the gold(i) alkene complex and the
auraoxetane provide evidence for intermediates in the gold-
catalyzed addition of oxygen nucleophiles to alkenes and
alkynes.
Received: May 20, 2005
Revised: July 28, 2005
Published online: October 5, 2005
[7] a) M. Haruta, Gold Bull. 2004, 37, 27 – 36; b) G. C. Bond, D. T.
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Keywords: alkene ligands · gold · oxidation · oxo ligands
.
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Chemie
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[10] D. Rais, Dissertation, Università di Sassari, 1998.
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ent 5795 with Rint = 0.043. Final R2 (F2, all reflections) = 0.113,
wR2 = 0.165, conventional R1 = 0.052 for 262 variables. Bruker
SMART CCD area detector, MoKa radiation (l = 0.71073 Š), w-
scan mode, qmin = 38, qmax = 268. Structure solved by direct
methods and refined by full-matrix least squares. The program
used to refine the structure was Personal SDP. CCDC-272640
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[18] A. A. Zlota, F. Frolow, D. Milstein, J. Am. Chem. Soc. 1990, 112,
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[19] M. A. Cinellu, G. Minghetti, M. V. Pinna, S. Stoccoro, A. Zucca,
M. Manassero, J. Chem. Soc. Dalton Trans. 2000, 1261 – 1265.
[20] M. A. Cinellu, G. Minghetti, S. Stoccoro, A. Zucca, M. Manas-
sero, M. Sansoni, J. Chem. Soc. Dalton Trans. 1996, 4217 – 4225.
[21] 4a-PF6 was isolated in low yield after elaborate workup of the
mixture. 4a-BPh4 which conversely can be obtained in fairly
good yields, is not stable enough in solution for carrying out a
reactivity study.[15]
[12] Analogous alkene complexes are likewise obtained with other 1-
alkenes, whereas no reaction occurs with linear internal alkenes;
a small amount of an oxygenated gold species is also detected by
1H NMR spectroscopy and FAB-MS of the a-methylstyrene
derivative. Oxygenated organic products were characterized.
Styrene derivatives: phenylacetaldehyde (main product) and
benzaldehyde, reaction in MeCN; phenylacetaldehyde dimethyl
acetal, reaction in MeCN/MeOH; styrene glycol, reaction in
MeCN/H2O. a-Methylstyrene derivatives: acetophenone, reac-
tion in MeCN; 2-phenyl-1,2-propanediol, reaction in MeCN/
H2O; this diol converts almost completely into methylbenzylke-
tone after a prolonged reaction time.
[13] Part of this work was presented in oral form: M. A. Cinellu, F.
Cocco, G. Minghetti, S. Stoccoro, A. Zucca, M. Manassero, III
Euchem Conference on Nitrogen Ligands in Organometallic
Chemistry and Homogeneous Catalysis, Camerino, Italy, Sep-
tember 8 – 12, 2004, O3.
[14] For selected reviews on homogeneous catalysis by gold, see:
a) A. Hoffmann-Röder, N. Krause, Org. Biomol. Chem., 2005, 3,
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c) A. S. K. Hashmi, Gold Bull. 2003, 36, 3 – 9; d) G. Dyker,
Angew. Chem. 2000, 112, 4407 – 4409; Angew. Chem. Int. Ed.
2000, 39, 4237 – 4239; selected examples of alkyne transforma-
tions: e) Y. Fukuda, K. Utimoto, J. Org. Chem. 1991, 56, 3729 –
3731; f) J. H. Teles, S. Brode, M. Chabanas, Angew. Chem. 1998,
110, 1475 – 1478; Angew. Chem. Int. Ed. 1998, 37, 1415 – 1418;
g) A. S. K. Hashmi, T. M. Frost, J. W. Bats, Org. Lett. 2001, 3,
3769 – 3771; h) E. Mizushima, K. Saro, T. Hayashi, M. Tanaka,
Angew. Chem. 2002, 114, 4745 – 4747; Angew. Chem. Int. Ed.
2002, 41, 4563 – 4565; i) R. Casado, M. Contel, M. Laguna, P.
Romero, S. Sanz, J. Am. Chem. Soc. 2003, 125, 11925 – 11935;
j) T. Yao, X. Zhang, R. C. Larock, J. Am. Chem. Soc. 2004, 126,
11164 – 11165; k) A. S. K. Hashmi, J. P. Weyrauch, W. Frey, J. W.
Bats, Org. Lett. 2004, 6, 4391 – 4394; l) V. Mamane, T. Gress, H.
Krause, A. Fürstner, J. Am. Chem. Soc. 2004, 126, 8654 – 8655;
m) A. S. K. Hashmi, P. Sinha, Adv. Synth. Catal. 2004, 346, 432 –
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W. Frey, J. W. Bats, Angew. Chem. 2005, 117, 2858 – 2861; Angew.
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T. M. Frost, Angew. Chem. 2000, 112, 2382 – 2385; Angew. Chem.
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[22] General trend: 1) high 1a-(PF6)2 conversions (high 3a + 4a
yields) require a long reaction time (at least 15 days) a temper-
ature of 10–158C and high nb/1a ratio (20:1–30:1) both in MeCN
and in MeCN/H2O. 2) Under the same preparative conditions
the overall yield (3a + 4a) is only slightly higher in MeCN/
H2O; nevertheless, the 3a/4a ratios are remarkably different:
6.5:1 (averaged) in MeCN and 1.5:1 in MeCN/H2O. 3) Under the
same nb/1a ratio and time conditions higher 4a yields are
obtained from concentrated solutions; the overall yield (3a +
4a) was slightly lower than that from dilute solutions.
[23] Signals of the epoxide could not be detected in the spectrum of
the reaction mixture (CD3CN) as they overlapped with those of
1c, 3c, and 4c.
[24] 1H NMR spectra were recorded soon after workup of the
reaction mixture. Aldehyde signals (CDCl3): 7: 9.64 (d, 2.0 Hz;
2H); 8 or 9: 9.63 (d, 2.4 Hz; 1H); 9 or 8: 9.62 ppm (d, 2.4 Hz;
1H). Intensities: 7 ꢀ (8 + 9); 7 ꢁ 6 (molar ratio based on CHH-
7 of the epoxide at d = 0.70 ppm).
[25] 1H NMR and GC-MS spectra were compared to those of an
authentic sample prepared according to: S. Göksu, R. Altundas,
Y. Sütbeyaz, Synth. Commun. 2000, 30, 1615 – 1621.
[26] Oxidation of nb with N2O to give 3-methylenecyclopentane
carbaldehyde (and norbornanone) has recently been reported:
E. V. Starokon, K. A. Dubkov, D. E. Babushkin, V. N. Parmon,
G. I. Panov, Adv. Synth. Catal. 2004, 346, 268 – 274. In default of
1H NMR data for these aldehydes, the experimental spectra
have been compared to the simulated ones obtained by
ChemDraw 9.0 (CambridgeSoft).
[27] Complex 3a and 6 also form upon addition of nb and a catalytic
amount (5%) of HBF4·Et2O to 4a; in this case small amounts of
diols 10 and 11 are formed.
[28] D. Milstein, J. C. Calabrese, J. Am. Chem. Soc. 1982, 104, 3773 –
3774.
[15] For full experimental data on the synthesis, spectroscopy, and
elemental analysis of complexes 2a–2c, 3a–3d, 4a–4c and for
spectroscopic characterization of organic compounds 6–11 see
Supporting Information.
[16] Crystal data for 3a-PF6: C18H20AuF6N2P, Mr = 606.30 gmolÀ1
,
orthorhombic, space group Pna21 (no. 33), a = 16.480(4), b =
16.662(3), c = 6.911(2) Š, V= 1897.7(7) Š3.
[17] Crystal data for 4a-PF6: C18H20AuF6N2OP, Mr = 622.30 gmolÀ1
,
¯
triclinic, space group P1 (no. 2), a = 6.466(1), b = 8.822(1), c =
17.548(1) Š, a = 103.45(1), b = 90.38(1), g = 100.97(1)8, V=
954.4(2) Š3, Z = 2, 1calcd = 2.165 gcmÀ3, T= 150 K, m(MoKa) =
78.4 cmÀ1, F(000) = 596. Reflections measured 19839, independ-
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