The interaction of gold(I) cations with 1,3-dienes

Article abstract of DOI:10.1002/anie.201102750

Slip-sliding away: Gold(I) cations bind to only one double bond of a 1,3-diene unit but can glide over the C₄ backbone on a low-activation-energy pathway. However, the binding mode is influenced by substituents on the diene backbone, which also induces a significantly higher barrier to migration. Copyright

Full text of DOI:10.1002/anie.201102750

Communications
DOI: 10.1002/anie.201102750
Gold Cations
The Interaction of Gold(I) Cations with 1,3-Dienes**
Rajashekharayya A. Sanguramath, Thomas N. Hooper, Craig P. Butts, Michael Green,*
John E. McGrady, and Christopher A. Russell*
In memory of F. Gordon A. Stone
ꢀ
The activation of C C multiple bonds by substituted gold
cations [R3PAu]+ is an important theme in contemporary
homogeneous catalysis.[1] However, it is interesting to note
that studies of the reactivity of conjugated 1,3-dienes using
gold catalysts are relatively rare, although Au/ZrO2 catalysts
were found to be effective in the heterogeneous selective
hydrogenation of 1,3-butadiene.[2] Furthermore, homoge-
neous gold catalysts have been used to synthesize function-
alized cyclopentadienes[3] and have been employed, for
example, in both hydrothiolation[4] and hydroamination[5] of
conjugated dienes. In the last of these reactions, a mechanism
was suggested whereby the gold center binds h4 to the diene,
which is thus activated towards nucleophilic attack by the
amine. Subsequent theoretical studies[6] have refined the
mechanism suggesting that the gold actually binds h2 to the
diene prior to attack of the N-nucleophile on the coordinated
double bond; protodeauration by proton transfer from the
NH2 group to the unsaturated carbon atom regenerates the
catalyst. For further advances to be made, a detailed under-
standing of the interaction of gold centers with 1,3-dienes
would of course be beneficial.[7] Since the submission of this
manuscript, a related study of gold cation interactions with
1,3-dienes has been published.[8]
highly reactive 2,3-dimethoxybuta-1,3-diene did not show any
evidence of reaction (1H NMR spectroscopy), which sug-
gested to us that the interaction of this substrate and gold may
be rather different to its counterparts.
The nature of the interaction between gold and the 1,3-
diene substrate was then probed using the room-temperature
reaction of a slurry of [LAuCl] (L = tBu3P; tBu2(o-biphe-
nyl)P) and AgSbF6 in CH2Cl2 with a series of symmetric
=
ꢀ
=
conjugated dienes R2C CR’ CR’ CR2 (Scheme 1). Filtration
Scheme 1. Synthesis of compounds 1–3.
of the resulting white precipitate of AgCl and recrystallisation
of the product yielded the cationic gold butadiene products
[(h2-1,3-diene)Au(L)]+. The mixing time for the initial
reaction was found to be critical in determining the outcome
of the reaction: Whereas the reaction using 2,3-dimethylbuta-
1,3-diene was invariant to long reaction times (typically 16 h),
side reactions were seen (by 31P NMR spectroscopy) for 2,5-
dimethylhexa-2,4-diene after only 15 minutes. Most strikingly,
immediate filtration of the mixture using 2,3-dimethoxylbuta-
1,3-diene (typically within 30 seconds) was necessary to avoid
intense colorations typically associated with the formation of
colloidal gold solutions.
Initially we sought confirmation that the ligand and diene
substrates which we employed were active in the hydro-
amination of 1,3-dienes. Pleasingly, 2,3-dimethylbuta-1,3-
diene undergoes near-quantitative hydroamination (by
NMR spectroscopy) with both benzylcarbamate and p-
toluenesulfonamide in 1,2-dichloroethane in the presence of
10 mol% [tBu3PAuCl]/AgSbF6 catalyst under mild conditions
to form an allylic amine; similar results were observed under
the same conditions using the combination of 2,5-dimethyl-
hexa-2,4-diene with [(tBu2(o-biphenyl)PAuCl]/AgSbF6. We
also noted that under the same conditions, the normally
A series of cationic gold–h2-alkene complexes has been
recently reported which were prepared from the interaction
of an alkene with either “free” [Au]+ or [AuL]+ centers and
employing a suitable weakly-coordinating anion;[9] similar
methods have also been used to generate the corresponding
silver salts.[10] In common with these limited examples of
cationic gold(I) alkene complexes, both 1 and 2 are remark-
ably stable, being ostensibly air- and moisture-stable and
showing no sign of decomposition (1H and 31P NMR spec-
troscopy) after 2 h reflux in CDCl3. The onset of decom-
position upon heating in the solid state does not occur until
1588C for 1 and 2188C for 2. Futhermore, both compounds
are indefinitely stable to light (daylight) and show no sign of
decomposition when the solid is subjected to a vacuum (ca.
10ꢀ2 torr, 30 min). In contrast, 3 is sensitive to air and
moisture, decomposing upon brief exposure to air to form
unidentified product(s).
[*] R. A. Sanguramath, Dr. T. N. Hooper, Dr. C. P. Butts, Prof. M. Green,
Dr. C. A. Russell
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS (U.K.)
E-mail: chris.russell@bristol.ac.uk
Homepage: http://www.inchm.bris.ac.uk/people/russell.shtm
Prof. J. E. McGrady
Department of Chemistry, Inorganic Chemistry Laboratory
University of Oxford, South Parks Road, Oxford OX1 3QR (U.K.)
[**] We thank the University of Bristol (M.G., C.A.R., T.N.H., R.A.S.) for
financial support. We thank Umicore AG & Co. KG for the generous
donation of HAuCl4.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102750.
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Single crystals of 1–3 suitable for analysis by X-ray
diffraction[11] were grown from saturated CH2Cl2 solutions
layered with either Et2O (1) or n-hexane (2 and 3). Whereas
the diffraction data for 2 and 3 are very good, the data for 1
showed that the crystals were non-merohedrally twinned and
displayed significant disorder in the diene unit. Whilst the
disorder in the diene unit for 1 means a detailed analysis of
the bond lengths and angles would be inappropriate, it is
sufficient to show that the diene adopts a planar trans-
conformation with the gold binding in h2-fashion to only one
1.384(4) ꢀ, C4–C5 1.325(4) ꢀ). Moreover the linking C2–C4
bond (1.492(4) ꢀ) approaches the value associated with non-
ꢀ
conjugated C C single bonds.
The most striking observation, however, relates to the
orientation of the gold center. As with 1 and 2, the heavy
=
metal is associated with only one of the C C bonds, but in this
case it has slipped so significantly (45%) towards the CH2 end
of the olefin so that it is perhaps better described as being s-
bonded to the carbon atom, giving a near linear geometry at
gold (C1-Au1-P1 171.89(8)8). This description is also reflected
in the 1H NMR spectrum of 3, which shows four peaks for the
=
of the C C double bonds (Supporting Information, Fig-
ure S1).
alkene hydrogen atoms, two in the region normally associated
2
ꢀ
The same bonding mode is also observed in the structure
of 2 (Figure 1), with the gold bonding in an asymmetric
fashion to one alkene (8% slippage towards the C H end of a
with C(sp ) H (d = 5.21 and 4.60 ppm), and two which are
significantly upfield (d = 3.56 and 2.68 ppm). This extreme
slippage can be rationalized in terms of the p-donor OMe
groups on the diene backbone, which can stabilize the
carbocationic center generated by the p-bonded gold frag-
ment, an observation which has precedent in gold–alkene
chemistry.[12]
ꢀ
=
=
C C double bond). The coordinated C C bond (1.391(4) ꢀ)
Whereas the experimental evidence suggests that the
solid-state structure of 3 is maintained in solution, a rather
different picture emerges for 1 and 2. The room-temperature
13C NMR spectrum of 1 and 2 showed only three and four
resonances respectively for the 1,3-diene component, sug-
gesting dynamic behavior consistent with one of three
possible scenarios: 1) In solution, the gold–phosphine unit is
bound to both alkene sites creating a symmetric cis-1,3-diene
and therefore giving three resonances; 2) the gold–phosphine
unit is only bound to one alkene but is involved in intra-
molecular exchange with the uncoordinated alkene site,
which is fast on the NMR timescale; and 3) the gold–
phosphine unit is only bound to one alkene but is involved
in an intermolecular exchange process with uncoordinated
alkenes, which is fast on the NMR timescale. To differentiate
these possibilities, variable-temperature 1H and 13C NMR
spectra of 1 and 2 were recorded. The resonances for the 1,3-
diene unit of 2 remain invariant in both the 1H and 13C NMR
spectra upon cooling to ꢀ808C. However, the room-temper-
Figure 1. Molecular structure of [Au{PtBu2(o-biphenyl)}-
{Me2CC(H)C(H)CMe2}]+ (2); the [SbF6]ꢀ anion and all hydrogen atoms
are omitted for clarity. Selected bond lengths [ꢀ]: Au1–C4 2.270(3),
Au1–C2 2.335(3), C2–C4 1.391(4), C4–C5 1.464(5), C5–C6 1.349(5).
is longer than its uncoordinated counterpart (1.349(5) ꢀ).
Although superficially similar, the structure of 3 is rather
different to those of 1 and 2 (Figure 2). The C C distances
within the diene backbone show more pronounced variations
ꢀ
1
compared to 2, with quite distinct alkene groups (C1–C2
ature H NMR spectrum of 1 shows three resonances (one
each for the cis- and trans-alkene protons and one for the
methyl group) that broaden upon cooling and ultimately (at
ꢀ808C) separate into five signals, three in the alkene region
(in ratio 1:1:2) and two for the now distinct methyl groups.
The alkene signals were assigned to the cis and trans hydrogen
atoms of the metal-coordinated alkene. The chemically
independent uncoordinated hydrogen atoms, assigned based
on the similarity of their chemical shift compared to the free
diene, are unresolved owing to the broadness of the signals at
this low temperature.
At ꢀ908C, the 13C{1H} NMR spectrum showed four
signals in the alkene region, two for the uncoordinated alkene
(in similar positions to those observed for free 2,3-dimethyl-
=
ꢀ
1,3-butadiene) at d = 140.14 ( CMe ) and 124.81 ppm
=
( CH2) and two for the coordinated alkene at d 153.14
=
ꢀ
=
( CMe ) and 93.03 ppm ( CH2). Separate peaks were also
observed for each methyl group. These findings suggest the
structure in solution at ꢀ908C is similar to that determined by
X-ray crystallography and therefore that the gold is coordi-
nated to only one of the alkenes with a rapid exchange
Figure 2. Molecular structure of [Au{PtBu2(o-biphenyl)}{H2CC(OMe)C-
(OMe)CH2}]+ (3); the [SbF6]ꢀ anion and all hydrogen atoms are
omitted for clarity. Selected bond lengths [ꢀ]: Au1–C1 2.175(2), Au1–
C2 2.543(2), C1–C2 1.384(4), C2–C4 1.492(4), C4–C5 1.325(4), O1–C2
1.319(3), O2–C4 1.353(3).
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Figure 3. Equilibrium and transition structures and migration barriers for compounds 1’–3’.
=
process. Furthermore, the low-temperature NMR spectra at
two further concentrations of 1 (CD2Cl2) showed the rate of
exchange of coordinated and uncoordinated arms to be
independent of concentration, suggesting the exchange pro-
cess proceeds by an intra- rather than intermolecular process.
To obtain greater insight into the bonding and dynamics of
these species we have used density functional theory to
explore the potential energy surfaces for migration of the
metal along the butadiene chain. The results of calculations
(where the phosphine ligand is modeled as PMe3) are given in
Figure 3. A parallel series of calculations using the full ligand
(PtBu3 for 1 and PtBu2(o-biphenyl) for 2 and 3) yielded very
similar structural results (see the Supporting Information),
but the large number of low-frequency vibrational modes in
the complete ligands prevented the location of the corre-
sponding transition structures. The optimized bond lengths
summarized in Figure 3 are fully consistent with the trends
observed experimentally for 2 and 3, and suggest that the
carbon is again stabilized by resonance with the residual C C
double bond but also by the inductive effects of the methyl
groups, giving greater asymmetry. We note in this context that
in their study of gold-catalyzed hydroamination of cis-3-
methyl-1,3-pentadiene,[6] Lledꢁs and co-workers also
reported a strongly slipped h2-coordination mode where the
metal binds to the least substituted CH2 CHR moiety. The
optimized Au C bond lengths of 2.283 ꢀ and 2.455 ꢀ in this
system are very similar to those in 1’. In 3’, the resonance with
the C C double bond is supplemented by conjugation with
the methoxy oxygen atoms, leading to the dramatically
slipped structure and also a distinct asymmetry in the
corresponding C O bond lengths (1.32 ꢀ and 1.36 ꢀ), a
feature which is apparent in the solid-state structure of 3
(1.319(3), 1.353(3) ꢀ). We also note that attempts to locate a
minimum where the metal coordinates to both double bonds
of a cis-buta-1,3-diene unit were unsuccessful: in all cases the
=
ꢀ
=
ꢀ
=
metal slipped towards one C C terminus of the ligand to give
complete series 1!2!3 maps a low-energy migration path-
structures analogous to 1’, 2’, and 3’ but with a cis disposition
of the butadiene. The tendency of AuI to adopt linear
geometries is well-established[13] and the resultant preference
+
=
way for the [Au(PR3)] unit along the C C bond.
At one extreme (2’), the gold is symmetrically disposed
ꢀ
=
towards both carbon atoms (Au C 2.31 ꢀ). However, adding
for coordination of only one C C unit of a diene moiety has
electron donating groups at the C2 and C3 positions results in
a shift towards the terminal carbon that is marginal in 1’
(2.21 ꢀ vs. 2.48 ꢀ) and dramatic in 3’ (2.16 ꢀ vs 2.79 ꢀ). In 2’,
the incipient positive charge at the coordinated carbon
centers is stabilized either by the inductive effects of two
methyl groups (C2) or by resonance with the uncoordinated
been noted by Lledꢁs.[6] The calculations also shed some light
on the relationship between gold–alkene bonding in the
ground state and the barriers to migration from one end of the
diene to the other. In all three cases, a C2-symmetric transition
state has been located where the gold center is coordinated to
ꢀ
ꢀ
the central C C unit. The Au C bond lengths of 2.42 ꢀ and
=
C C bond (C4), and it appears that these two factors offset
2.37 ꢀ in 1’ and 2’, respectively, are marginally longer than in
each other, leading to an almost symmetric h2-coordination
the equilibrium structure. The coordinated C C bond short-
=
mode. In 1’, the developing positive charge at the central
ens in the transition structure (1.37 vs. 1.40 ꢀ in 1’) while the
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amount. The changes relative to the equilibrium structure are,
however, not dramatic, and as a result the computed barriers
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ꢀ
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ꢀ
ꢀ
equilibrium structure (C O 1.32 ꢀ) is much reduced in the
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1.35 ꢀ. The net effect of these substantial structural modifi-
cations along the reaction coordinate is a much larger barrier
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conditions used in our experiments.
In summary, we have isolated three gold–buta-1,3-diene
complexes. Two of the complexes have solid-state structures
with h2-bonding to the butadiene for which VT-NMR
spectroscopy indicates fluxionality via an intramolecular
pathway that is frozen out (in one example) at low temper-
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an Au C s bond to one of the alkene groups, the strong
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by the methoxy groups effectively preventing the movement
of the gold unit over the diene manifold.
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ꢀ
ꢀ
Received: April 20, 2011
Published online: July 5, 2011
Keywords: cations · density functional calculations · dienes ·
gold · X-ray diffraction
.
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