Atropisomeric [(diphosphine)Au2Cl2] complexes and their catalytic activity towards asymmetric cycloisomerisation of 1,6-enynes

Article abstract of DOI:10.1002/chem.201404496

X-ray crystal structures of two [(diphosphine)-Au₂Cl₂] complexes (in which diphosphine = P-Phos and xylyl-P-Phos; P-Phos = [2,2′,6,6′-Tetramethoxy-4,4′-bis(diphenylphosphino)-3,3′-bipyridine]) were determined and compared to the reported structures of similar atropisomeric gold complexes. Correlations between the Au...Au distances and torsional angles for the biaryl series of ligands (MeOBIPHEP, SEGPhos, and P-Phos; BIPHEP = 2,2′-bis(diphenylphosphino)-1,1′-biphenyl, SEGPhos = [(4,4′-bi-1,3-benzodioxole)-5,5′-diyl]-bis[diphenylphosphine]) can be made; these measurements appear to be very dependent upon the phosphorous substituent. Conversely, the same effect was not observed for ligands based on the binaphthyl (BINAP) series. The catalytic activity of these complexes was subsequently assessed in the enantioselective cycloisomerisation of 1,6-enynes and revealed an over-riding electronic effect: more-electron-rich phosphines promote greater enantioselectivity. The possibility of silver acting as a (co-)catalyst was ruled out in these reactions.

Full text of DOI:10.1002/chem.201404496

DOI: 10.1002/chem.201404496
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Gold |Hot Paper|
Atropisomeric [(diphosphine)Au2Cl2] Complexes and their
Catalytic Activity Towards Asymmetric Cycloisomerisation of 1,6-
Enynes
Elena M. Barreiro,[a] Ekaterina V. Boltukhina,[b] Andrew J. P. White,[a] and King Kuok
(Mimi) Hii*[a]
Abstract: X-ray crystal structures of two [(diphosphine)-
Au2Cl2] complexes (in which diphosphine=P-Phos and xylyl-
appear to be very dependent upon the phosphorous
substituent. Conversely, the same effect was not observed
for ligands based on the binaphthyl (BINAP) series. The
catalytic activity of these complexes was subsequently ass-
essed in the enantioselective cycloisomerisation of 1,6-
enynes and revealed an over-riding electronic effect: more-
electron-rich phosphines promote greater enantioselectivity.
The possibility of silver acting as a (co-)catalyst was ruled
out in these reactions.
P-Phos;
P-Phos=[2,2’,6,6’-Tetramethoxy-4,4’-bis(diphenyl-
phosphino)-3,3’-bipyridine]) were determined and compared
to the reported structures of similar atropisomeric gold com-
plexes. Correlations between the Au···Au distances and tor-
sional angles for the biaryl series of ligands (MeOBIPHEP,
SEGPhos, and P-Phos; BIPHEP=2,2’-bis(diphenylphosphino)-
1,1’-biphenyl, SEGPhos=[(4,4’-bi-1,3-benzodioxole)-5,5’-diyl]-
bis[diphenylphosphine]) can be made; these measurements
Introduction
The recent upsurge of interest to explore the medicinal
properties of three-dimensional molecular structures and frag-
ments[1–4] has driven the need for synthetic methodologies
that can generate complex scaffolds from acyclic precursors in
a single step. An exemplar of such reactions is the cycloisomer-
isation of 1,6-enynes, which generates carbo- and heterobicy-
clo[4.1.0]heptene derivatives with up to three stereocentres,
thus provides extremely valuable compounds for organic
synthesis.[5–7]
The reaction was first discovered in 1995 by Blum and co-
workers by using a catalytic amount of PtCl4.[8] Since then,
a number of other p-acidic transition-metal catalysts were re-
ported, including Rh,[9,10] Ir,[11] Pt[12–14] and Au catalysts.[15,16] The
first enantioselective examples were provided by Michelet and
co-workers in 2009, by using a gold(I) complex of the chiral
diphosphine ligand L1 (Scheme 1).[17,18] Subsequently, the en-
antiomeric excess (ee) values obtained have been superseded
Scheme 1. Enantioselective Au-catalysed cycloisomerisation of 1,6-enynes
[Z=O, N-Tosyl (Ts)].
by other transition-metal catalysts (Pt,[19] Rh[20–22] and Ir[23]), but
gold is the only reported system that is catalytically active at
room temperature.
The catalytic activity of cationic gold complexes continues
to be a subject of considerable intrigue and debate.[24] Diphos-
phine ligands were frequently found to be effective for many
asymmetric transformations catalysed by gold. More often
than not, silver salts are required to activate [(diphosphi-
ne)Au2Cl2] precursors towards catalysis. This is thought to gen-
erate a cationic species I (Scheme 2) with sufficient p-Lewis
acidity for catalysis, but the characterisation of such cationic
gold complexes remains elusive; no X-ray crystal structure has
been determined yet. Most diphosphine ligands do not form
[a] Dr. E. M. Barreiro, Dr. A. J. P. White, Dr. K. K. Hii
Department of Chemistry, Imperial College London
Exhibition Road, South Kensington
London SW7 2AZ (UK)
E-mail: mimi.hii@imperial.ac.uk
[b] Dr. E. V. Boltukhina
Current address: Schyolkovo Agrochim, CJSC
2 Zavodskaya Street, Schyolkovo
Moscow Region 141101 (Russia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404496.
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Table 1. Selected structural parameters for [(diphosphine)Au2Cl2]
complexes.
Scheme 2. Formation of cationic gold complexes.
chelate rings with gold(I), instead linear geometries that may
contain short gold–gold distances, indicative of aurophilic
interactions, are favoured.[25] Thus, a significant gold–gold co-
operative effect may be important for the catalytic activity. In
reactions catalysed by [AuCl(PR3)]/AgX systems, chloro-bridged
species II (Scheme 2) has been isolated and identified. Also,
the participation of silver in the catalytic reaction (presumably
via bimetallic complexes) cannot be ruled out.[26,27]
Diphosphine Au···Au [ꢁ] AuÀCl [ꢁ]
q
[a] [8] CCDC code Reference
L1
5.323
5.316
5.296
5.820
3.147
4.131
4.984
2.994
5.661
3.785
5.482
5.422
5.480
5.976
5.466
3.670
3.182
4.792
2.266, 2.269 104.27 AJOHIP
2.275 105.74 AJOHIR01
2.290, 2.283 103.62 AJOHIP02
2.282
2.2932
2.263
2.275, 2.274 99.58
2.292, 2.279 68.08 TADSIC
2.286, 2.289 114.47 LITKAB
[29]
[30]
[31]
[32]
[31]
[33]
L2
L3
105.05 EKOLIA
95.58 ELOCUE
In this paper, we report the results of a preliminary study
conducted with different diphosphine–gold(I) complexes for
the cycloisomerisation reactions of 1,6-enynes. We have
attempted to correlate the structural parameters of the gold
complexes to the observed enantioselectivity and to
investigate the role of the silver salt on the outcome of these
reactions.
96.54
YUZLUB
L4
L5
L6
L7
[30]
[34]
[31]
[35]
[36]
[36]
[37]
[38]
[b]
2.278
2.290
2.282
101.42 ELOCOY
98.50 KIKBAI
98.44 NUXBUE
2.277, 2.299 90.68 NUXCAL
2.288 110.58 YAQGIH
2.303, 2.284 94.98 NANPUP
2.277, 2.282 88.93
2.290, 2.295 102.76
2.281, 2.274 114.20
L8
L9
L10
Results and Discussion
–
The study began with a search in the CCDC database for
reported structures of chiral atropisomeric biaryl [(diphos-
phine)Au2Cl2] complexes, from which 14 discreet structures
corresponding to 9 compounds can be found (diphosphine=
L1–L9, Table 1).[28] The major conformational parameters that
vary between the complexes are given in Table 1; namely, the
torsional twist about the central aryl–aryl bond (q), the AuÀCl
bond length and the intramolecular Au···Au separation.
L11
–
[b]
[a] Dihedral angle of the biaryl backbone, C2-C1-C1’-C2’ torsion angle,
obtained from crystal structures of the [(diphosphine)Au2Cl2] structures.
[b] This work.[39]
approximate C2 symmetry, whereas [(L10)Au2Cl2]-B does not.
The consequence of these differing twists is markedly varied
intramolecular Au1···Au2 separations of 3.6701(3), 3.1821(2)
and 4.7921(4) ꢁ in [(L10)Au2Cl2]-A, [(L10)Au2Cl2]-B and
[(L11)Au2Cl2], respectively (Table 1).
Manifestation of aurophilicity is generally indicated by
Au···Au distances that are shorter than the sum of two van der
Waals radii of 3.7 ꢁ.[40] For the atropisomeric diphosphine
ligands (MeOBIPHEP (L1–L3), BINAP (L7–L9), SEGPHOS (L4–L5)
and P-Phos) it is expected that the Au···Au distance is
dependent upon the degree of rotational freedom between
the aromatic rings that defines the axial chirality, which is, in
turn, determined by the nature of the biaryl that constitutes
the scaffold, as well as the P substituents. With this in mind,
a scatter plot of the torsional angle between the two central
aryl rings against the Au···Au distance was produced (Figure 3)
and revealed some interesting trends.
Additionally, two complexes that contained P-Phos (L10)
and xylyl-P-Phos (L11) were also prepared and their structures
were determined by X-ray crystallography (Figures 1 and 2).
Complex [(L10)Au2Cl2] was found to crystallise with two inde-
pendent molecules (A and B) in the asymmetric unit (Figure 1).
The compositions of [(L10)Au2Cl2] (Figure 1) and [(L11)Au2Cl2]
(Figure 2) differ only in the substituents on the phosphorus
atoms; phenyl in [(L10)Au2Cl2] and 3,5-dimethylphenyl in
[(L11)Au2Cl2], yet all three molecules have noticeably different
conformations.
The two independent molecules of [(L10)Au2Cl2] have similar
twists about the central p–p bond (ꢀ888 and 828), whereas
that for [(L11)Au2Cl2] is substantially different (ꢀ668). For
[(L10)Au2Cl2]-A the orientation of the PÀAu bond relative to its
adjacent pyridyl ring is the same in both instances (ꢀ478),
whereas in [(L10)Au2Cl2]-B they differ markedly (ꢀ888 and 298
for P1 and P2, respectively). In [(L11)Au2Cl2] the twists are
again similar to each other, but at approximately 258 they are
Greater conformational flexibility was found in the biaryl
series MeOBIPHEP, SEGPHOS and P-Phos, for which it is
common to find more than one independent molecule for
each complex. Nevertheless, the Au···Au distance tends to
very different to those seen in [(L10)Au2Cl2]-A. As
a
consequence, both [(L10)Au2Cl2]-A and [(L11)Au2Cl2] have
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Figure 3. Scatter plot of torsional angles between axially-chiral aromatic
rings (q) [8] against the Au···Au distances [ꢁ] for atropisomeric diphosphine–
~
*
^
Au complexes. BINAP series ( ), MeOBIPHEP series ( ), SEGPHOS series ( ),
&
P-Phos series ( ).
appears to lengthen the Au···Au distance quite significantly.
The same trend can be observed in the P-Phos series (L12 and
&
L13, , Figure 3), in which the presence of electron-donating
xylyl groups increased the Au···Au distance. For the SEGPHOS
~
series (L4 and L5, , Figure 3), substitution of PPh2 by PCy2
(Cy=cyclohexyl) groups also has a dramatic effect on the in-
termetallic distance. As a result, Au···Au distances of <3.7 ꢁ
were only observed in four of these biaryl diphosphine ligated
complexes, which all contained PPh2 substituents. Thus, it
appeared that intermetallic interactions are discouraged by
more-electron-rich P donors.
Figure 1. Structures of the two independent molecules present in the crys-
tals of [(L10)Au2Cl2], A (top) and B (bottom). Hydrogen atoms are omitted
for clarity.
In contrast, Au···Au distances in the gold complexes of the
BINAP series (L7–L9) are essentially insensitive to the P sub-
*
stituent ( , Figure 3). This is attributed to the higher barrier of
rotation for the binaphthyl ring. For this particular series of
gold complexes, all Au···Au distances were found to be >5.4 ꢁ.
These observations suggest that aurophilicity is relatively
uncommon for these atropisomeric [(diphosphine)Au2Cl2]
complexes, inhibited by more-electron-rich P-donor groups
and by restriction of the conformational flexibility of the
aryl–aryl bond.
In the next phase of this study, the catalytic activity of se-
lected complexes was tested in the cycloisomerisation of ether
substrates 1a–d (Scheme 3). Selected results are presented in
Table 2.
Figure 2. Crystal structure of [(L11)Au2Cl2]. Hydrogen atoms and co-crystalli-
sation solvents are omitted for clarity.
The cyclisation of ether-tethered enyne substrate 1a was ex-
amined initially as the model reaction, performed in toluene at
room temperature. The product was somewhat unstable,
therefore quick purification by flash chromatography was nec-
essary to achieve good isolated yields. The benchmark for this
study was set by reproduction of the result reported by Mi-
chelet and co-workers by using ligand L1:[17,18] the product 2a
can be obtained in 87% yield with 99% ee (Table 2, entry 1).
Guided by the structural analysis (Table 1), two complexes
increase with a greater torsional angle between the two aro-
^
matic rings. Within the MeOBIPHEP series (L1–L3, , Figure 3)
the introduction of tert-butyl and methoxy substituents on the
P-aryl group appear to augment both of these values. The dif-
ference between L1 and L3 is particularly notable: the pres-
ence of an additional methoxy substituent at the para position
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presence of various amounts of silver salt, with the stoichio-
metric ratio of Au/Ag either 1:1 or 2:1 (Table 2, entries 8–12). In
each case, the product yield was lower when silver salt
(0.5 equiv) was employed but the level of enantioselectivity re-
mained invariant. This is a significant observation. Considering
the different cationic complexes that can be generated in each
case (Scheme 4), the involvement of a bimetallic Au/Ag com-
Scheme 3. Au-catalysed cycloisomerisation of 1,6-enynes 1a–d.
Table 2. Effect of chiral diphosphine ligands and Au/Ag stoichiometry in
the cycloisomerisation of 1,6-enynes (Scheme 3).[a]
Entry
Substrate
L
Au/Ag
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1b
1b
1c
1c
1d
1d
(S)-L1
(R)-L7
(R)-L8
(R)-L10
(R)-L11
(R)-L11
–
(R)-L11
(R)-L11
(R)-L11
(R)-L11
(R)-L11
(R)-L11
1:1
1:1
1:1
1:1
1:1
2:1
0:1
1:1
2:1
1:1
2:1
1:1
2:1
87
70
85
96
92
89
–
86
54
76
69
54
45
99 (+)
46 (À)
75 (À)
38 (À)
60 (À)
64 (À)
–
66 (À)
64 (À)
78 (+)
74 (+)
72 (+)
74 (+)
Scheme 4. Possible complexes generated by using different Au/Ag ratios.
plex as the active catalyst can be ruled out because this can
only be generated in a significant quantity when a sub-stoi-
chiometric amount of silver salt is used.[41] Furthermore, the
result also strongly suggests that each metal centre operates
independently as an active catalytic site, that is, the presence
of any cooperative effect between the two proximal Au metal
centres may be discounted. This is commensurate with the
previous study, in which greater enantioselectivity was
observed with complexes that had larger intermetallic
distance/less propensity to form Au···Au interactions for steric
(L1) and/or electronic (L8, L11) reasons.
Finally, the active catalyst was also generated in situ by pre-
mixing the gold–chloride complex with the silver additive prior
to addition of the substrate, or by introducing an intermediate
operation, in which the AgCl formed was removed by filtration
through a bed of Celite before the introduction of the sub-
strate. These procedures had no effect on the results. Hence,
the involvement of [LÀAuÀClÀAg]+ is also highly unlikely.
9
10
11
12
13
[a] Reaction conditions: Substrate 1 in toluene (0.5m), [Au]2 (5 mol%),
[Ag] (5 or 10 mol%), RT, 24 h. [b] Isolated yield. [c] Determined by chiral
HPLC. Optical activity of the product is given in parentheses.
from the BINAP series (L7 and L8) were tested as catalysts be-
cause they have similar Au···Au distances (>5.3 ꢁ). In both
cases, the product was isolated in good yields. By using the
BINAP ligand L7, a modest 46% ee was obtained, whereas
tolyl-substituted L8 gave a much higher 75% ee (Table 1,
entries 2 and 3). Similarly, two ligands from the P-Phos series
(L10 and L11) were selected for their similar torsional angles
to L1. In these cases, excellent yields of the product were ob-
tained. Once again, the more-electron-rich P-xylyl-substituted
complex L11 offered significantly better enantioselectivity than
L10 (38 vs. 60% ee; Table 1, entries 4 and 5).
Correlations between structural parameters obtained from
solid-state structures and homogeneous catalytic reactions
should always be exercised with caution. Nevertheless, these
preliminary studies seem to suggest that the enantioselectivity
of atropisomeric biaryl diphosphine ligands can be enhanced
by using more-electron-rich P substituents.
Conclusion
We have conducted a structural study on a series of [(diphos-
phine)Au2Cl2] complexes. Even within the small selection of ex-
amples, some trends can be observed within the diaryl diphos-
phine complexes, for instance an increase in the electron-do-
nating character of the P-Ar substituent increases the Au···Au
distance. This effect is not observed, however, in the BINAP
series of ligands, which is attributed to the greater barrier to
rotation/more-rigid structure. The catalytic activity of selected
complexes was tested in the cycloisomerisation of 1,6-enynes;
the incorporation of electron-rich P donors enhanced the ste-
reoselectivity. Critically, the enantioselectivity of the process
was not affected by using different gold/silver ratios or by re-
moval of the silver by-product. This suggests that each metal
centre is likely to operate independently and that silver does
not participate in the process. These observations are useful
for the future design of better catalysts for these reactions.
Next, we examined the importance of the silver salt in these
reactions. In previous studies, the choice of silver salt/counter-
anion often had a profound effect on the catalytic activity and
it has been suggested that the participation of silver catalysis
cannot be discounted in such reactions.[26] In this case, a control
reaction showed that the linear substrate 1a remained un-
changed in the presence of AgBF4 alone at ambient tempera-
ture (Table 2, entry 7), that is, any background racemic process
due to the silver salt can be categorically ruled out. Subse-
quently, by using the L11-ligated complex, the cycloisomerisa-
tions of three other substrates 1b–d were performed in the
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Keywords: aurophilicity
enynes · gold
· catalysis · cycloisomerization ·
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Received: July 21, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
A structural study of a series of biaryl
diphosphine gold(I) complexes (see
figure) suggests that an increase in the
electron-donating character of the PÀAr
substituent increases the Au···Au
distance and enhances the stereo-
selectivity; each metal centre operates
independently.
&
Gold
E. M. Barreiro, E. V. Boltukhina,
A. J. P. White, K. K. Hii*
&& – &&
Atropisomeric [(diphosphine)Au2Cl2]
Complexes and their Catalytic Activity
Towards Asymmetric
Cycloisomerisation of 1,6-Enynes
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&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!