A Gold(I) Complex with a 1,1′-Binaphthyl-Substituted Diphosphene: Synthesis, Structure, and Catalytic Application to Intramolecular Hydroarylation Reactions

Article abstract of DOI:10.1021/acs.organomet.9b00665

A gold(I) complex with a 1,1′-binaphthyl-substituted diphosphene was synthesized and fully characterized. A phosphorus atom with a less-hindered binaphthyl group coordinates to a gold(I) moiety in an η¹ fashion. Both experiment and theoretical calculations supported the facile rotation around the C(Naph)-P bond in neutral and cationic diphosphene-gold(I) complexes. The newly obtained complex 2 was applied to the intramolecular hydroarylation of aryl propynyl ethers 5, and 2H-chromenes 6 were formed in 83-94% yield. Furthermore, the chiral diphosphene-gold(I) complex (S)-2 was used in the atropselective reaction to furnish 6b,c with up to 9% ee, which is the first example of the use of a diphosphene as a ligand for the transition-metal-catalyzed organic transformation.

Full text of DOI:10.1021/acs.organomet.9b00665

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
A Gold(I) Complex with a 1,1′-Binaphthyl-Substituted Diphosphene:
Synthesis, Structure, and Catalytic Application to Intramolecular
Hydroarylation Reactions
Akihiro Tsurusaki,* Rikako Ura, and Ken Kamikawa*
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
S
* Supporting Information
ABSTRACT: A gold(I) complex with a 1,1′-binaphthyl-substituted
diphosphene was synthesized and fully characterized. A phosphorus atom
with a less-hindered binaphthyl group coordinates to a gold(I) moiety in an
η1 fashion. Both experiment and theoretical calculations supported the facile
rotation around the C(Naph)−P bond in neutral and cationic diphosphene−
gold(I) complexes. The newly obtained complex 2 was applied to the
intramolecular hydroarylation of aryl propynyl ethers 5, and 2H-chromenes 6
were formed in 83−94% yield. Furthermore, the chiral diphosphene−gold(I)
complex (S)-2 was used in the atropselective reaction to furnish 6b,c with up
to 9% ee, which is the first example of the use of a diphosphene as a ligand
for the transition-metal-catalyzed organic transformation.
INTRODUCTION
■
Phosphorus chemistry has rapidly advanced in the last two
decades due to a plethora of intriguing properties, such as
coordination ability to transition metals and their chiral
environments.1 Trivalent organophosphorus compounds, such
as phosphine, phosphite, and phosphoramidite, are some of the
most intensely investigated compounds as ligands because of
their strong coordination ability to transition metals and/or
their chiral environments derived from the high inversion
barrier. The development of new ligands that exhibit high
catalytic activities has posed a continuous challenge in modern
organic syntheses.
In contrast to conventional phosphorus(III) compounds
with coordination number 3, ligands exploiting low-coordinate
phosphorus species, such as phosphaalkenes and phosphinines,
are still underdeveloped.2 These are promising candidates due
to the low-lying LUMO level derived from the π* orbital of the
PC bond. Diphosphinidenecyclobutenes (DPCBs, A),2d,3
Figure 1. Examples of phosphaalkenes and phosphinines utilized as
PN-bidentate (B), or PNP pincer-type (C) ligands with
ligands for catalytic organic transformations.
phosphaalkenes4 and phosphinine ligands (E and F) combined
with oxazoline, pyridine, and phosphine to give bidentate-type
ligands2e,3 are utilized in transition-metal-catalyzed organic
Diphosphene having a PP double bond in the molecule is
transformations, including asymmetric reactions4b,5a,b,6 (Figure
1). As has been illustrated for gold(I) complexes, Ito, Yoshifuji,
also a promising candidate due to its significantly low-lying
PP π* orbital,9,10 which should enhance the π-accepting
character of the transition metal. A variety of diphosphene
transition-metal complexes featuring η1- or η2-type coordina-
tion modes have been explored,9a,e,11 even before the isolation
of the first example of diphosphene, Mes*PPMes* (Mes* =
and co-workers reported the cycloisomerization of 1,6-enynes
and the lactonization of pent-4-ynoic acids catalyzed by
phosphaalkene−chlorogold(I) complexes with A or D,7 and
Muller and co-workers indicated that two phosphinine−
̈
gold(I) complexes with G showed catalytic activity for the
cycloisomerization of a 1,6-enyne and a propynyl-tethered
benzamide.8
Received: October 9, 2019
© XXXX American Chemical Society
A
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2,4,6-tri-tert-butylphenyl).12 In 2009, Protasiewicz and co-
workers reported the first isolation of the gold(I) complexes of
Mes*PPMes*, where one or both lone pairs on the
phosphorus atoms coordinate to the gold atoms in an η1
fashion.13 However, to the best of our knowledge, a
diphosphene complex has never been used as a ligand for
catalytic organic transformations.14 We recently reported the
synthesis and optical properties of the first chiral diphosphene
1 with a 1,1′-binaphthyl group.15 We wish to report herein the
complexation of 1 with a gold(I) atom and the application of
the obtained diphosphene−gold(I) complex 2 to the intra-
molecular hydroarylation of aryl propynyl ethers.
(2.0266(9) Å) is slightly shorter than that of 1 (2.0323(6)
Å)15 but is longer than those of diphosphene−gold(I)
complexes 3a (1.975(5) Å) and 3b (2.003(4) Å) (Figure
3).13 In addition, the P−Au and Au−Cl bond lengths of 2
Figure 3. Diphosphene−gold(I) complexes 3.
(2.2107(7) and 2.2753(7) Å, respectively) are both also longer
than those of 3a (2.180(4) and 2.222(4) Å, respectively) and
3b (2.201(2) and 2.250(2) Å, respectively). The reason for the
difference between 2 and 3 is unclear at present, although both
compounds have aryl groups on the phosphorus atoms. The
geometry around gold is almost linear with a P1−Au−Cl angle
of 177.53(2)°. The Au−C22 distance of 3.59 Å implies a weak
gold−aryl interaction in 2.
RESULTS AND DISCUSSION
■
We applied a similar method to that reported by Protasiewicz
and co-workers for the synthesis of the gold(I) complex of 1.13
Treatment of binaphthyl-substituted diphosphene 1 with 1
equiv of (tht)AuCl (tht = tetrahydrothiophene) in CH2Cl2
resulted in the formation of the diphosphene−gold(I) complex
2 in 66% yield as yellow crystals (Scheme 1). The 31P NMR
In our previous report, we showed the facile rotation around
the C(Naph)−P bond of 1 at room temperature.15 The
transition energy was estimated to be lower than 5 kcal mol−1
in both rotations in the vicinity of the methyl group at the 3-
position of the naphthyl group (TSA) and the naphthyl group
(TSB) (Scheme 2). To reveal the effect for the C(Naph)−P
Scheme 1. Synthesis of Binaphthyl-Substituted
Diphosphene−Gold(I) Complex 2
Scheme 2. Interconversion between Syn and Anti Isomers of
1, 2, and [4]+
signals of 2 were observed upfield (δP 405.9 and 315.7) relative
to those of 1 (δP 451.5 and 525.7).15 The coupling constant of
1JPP = 535 Hz indicated an η1 coordination with retention of
the double-bond character of the PP bond after complex-
ation (cf. 1, JPP = 570 Hz; Mes*(ClAu)PPMes* (3a),13 δP
1
338.8 and 386.0, JPP = 538 Hz).16
1
The X-ray crystallographic analysis of 2 revealed that the
phosphorus atom with a binaphthyl group coordinates to a
gold(I) moiety (Figure 2), probably due to the relatively lesser
Figure 2. Molecular structure of 2.
bond rotation in the gold complex (syn−anti isomerization),
the energy barriers of (S)-2 and the cationic gold complex
[(S)-4]+ without chloride were investigated by theoretical
calculations at the B3LYP functional with the basis set of
LanL2DZ (for Au) and 6-31G(d) (for the other atoms) level
in conjunction with the PCM model (CH2Cl2).17 With
reference to the previous report, the syn configuration
indicates that the P atom with a Mes* group and benzene
ring A in the naphthyl group at the 1-position have the same
orientation, whereas the anti configuration indicates an inverse
orientation (Scheme 2). The calculated energies of TSA-(S)-2,
TSB-(S)-2, [TSA-(S)-4]+, and [TSB-(S)-4]+ are 8.7, 9.2, 5.3,
steric hindrance of the binaphthyl group in comparison to the
bulky Mes* group. The P2 atom and benzene ring A of the
naphthyl group at the 1-position take the syn configuration as
in 1.15 Theoretical calculations revealed that the syn isomer is
slightly more stable than the anti isomer, but the energy
difference is less than 1 kcal mol−1 depending on the applied
functionals (Table S2). The intermolecular π−π interaction
between naphthyl plane with a methyl group was observed in
the crystal packing (Figure S20), while no intermolecular Au−
Au contact was observed. The PP bond length of 2
B
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Article
and 7.2 kcal mol−1, respectively, and the energies slightly
increase in the order of 1, [4]+, and 2. It should be noted that
the anti isomer is slightly more stable (∼1 kcal mol−1) than the
syn isomer in [(S)-4]+, in contrast to (S)-2, which is likely due
to the effective gold−aryl (Naph) interaction (the distance
between gold and the carbon atom at the 8′-position is 3.05
Å).
Table 1. Intramolecular Hydroarylation of 5a: Screening for
Reaction Conditions
This conformational change was experimentally investigated
by a VT-NMR study. When a CD2Cl2 solution of 2 was cooled
from 20 to −90 °C, one of the doublet signals in the lower field
region (δ 406.5) became broad, while the other (δ 315.1) still
remained as a doublet in the 31P NMR spectrum (Figure 4).
a
a
entry
1
catalyst
additive
yield (%) of 6a
yield (%) of 6a′
2
2
2
2
2
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgNTf2
AgOTf
AgBF4
94 (89)
6 (4)
6 (5)
b
2
3
4
5
89 (83)
62
16
71
0
c
4
1
2
0
d
e
6
7
8
9
10
11
12
13
14
none
1
2
2
2
0
0
95
22
16
78
81
86
85
3
trace
3
15
8
f
2
NaBArF
4
(tht)AuCl
(Ph3P)AuCl
3a
AgSbF6
AgSbF6
AgSbF6
5
8
a
1
Yields were determined by H NMR spectroscopic analysis using
1,1,2,2-tetrachloroethane as the internal standard. Isolated yields are
b
c
shown in parentheses. At 40 °C. AgSbF6 was initially treated with
Figure 4. Temperature dependence of 31P{1H} NMR spectra (202.5
MHz) of 2 in CD2Cl2. (a) 20 °C; (b) −40 °C; (c) −70 °C; (d) −80
°C; (e) −90 °C.
d
e
gold complex 2 before adding the substrate. 1 mol % of AgSbF6. 4
f
mol
%
of AgSbF6. NaBArF
= sodium tetrakis[3,5-bis-
4
(trifluoromethyl)phenyl]borate.
Since the former signal (δ 406.5) was assignable to the
phosphorus atoms with a Mes* group on the basis of the
GIAO calculations of 2 (Table S3), the observed coalescence
should be due to the C(Naph)−P bond rotation described
above (Scheme 2).18 The activation energy (ΔG⧧198) should
be roughly estimated as 8.2 kcal mol−1 by using the
coalescence temperature (198 K) and the JPP coupling
constant (528 Hz).19 The experimental value of ΔG⧧ (∼8
kcal mol−1) is in good agreement with the calculated value (∼9
kcal mol−1). To our regret, the precise determination of syn/
anti ratio was unsuccessful because the signals did not
apparently split into two signals under the applied conditions.
Subsequently, we investigated the catalytic activity of the
newly obtained diphosphene−gold(I) complex 2.20 We
decided to perform the intramolecular hydroarylation of 2-
bromophenyl butynyl ether 5a as the substrate in the presence
of 2 mol % of gold complex and an additive.21 The reaction
provided 6-endo cyclized product 6a as the major product
together with a small amount of isomeric benzofuran 6a′.21a
Initial screening for solvent (CH2Cl2, 89%; THF, 0%; toluene,
36%; acetonitrile, 52%) at 40 °C with AgSbF6 as the additive
demonstrated that CH2Cl2 was the best solvent. The reaction
at room temperature gave a slightly improved yield (Table 1,
entry 1 vs entry 2). It is worth mentioning that the order of
addition of the reagents markedly affected the yield of product
6a.22 When AgSbF6 was added to the mixture of 5a and
gold(I) complex 2 in CH2Cl2, the reaction proceeded
smoothly to give 6a in 94% yield (entry 1). On the other
hand, the yield of 6a was drastically decreased to 62% when 5a
was added to a solution containing the cationic gold complex
[(1)Au]+[SbF6]−, which would be generated by the initial
mixing of 2 and AgSbF6 in CH2Cl2 for 2 min at room
temperature (entry 3).23 We believe that one of the reasons for
the difference might be the low stability of [(1)Au]+[SbF6]−
without an alkyne. Furthermore, the use of 0.5 or 2 equiv of
silver (relative to gold) led to low yields (entries 4 and 5). No
reaction occurred in the absence of the gold complex 2 (entry
6) or in the combination with diphosphene 1 and AgSbF6
(entry 7). These results indicate the following. (1) The
generation of the cationic gold complex is necessary, but an
excess amount of silver salt precludes the catalytic activity. (2)
The cationic silver complex itself would not work as the
catalytically active species in this reaction. (3) [(Ag)Mes*P
P(BNpMe)(AuCl)]+ (BNpMe = 3-methyl-[1,1′-binaphthalene]-
2-yl), a diphosphene with the additional η1 coordination of a
silver cation onto another phosphorus atom, could be
considered,24 but it would also be unlikely as the catalytically
active species. (4) [(1)Au]+[SbF6]− could be the catalytically
active species, but the heteronuclear species of Au and Ag
should also be considered. In fact, an electrospray ionization
mass spectroscopy (ESI-MS) analysis of a mixture of 2 and
AgSbF6 in CH2Cl2/MeCN indicated the existence of [(1)Au]+
and [(1)AuCl + Ag]+, showing signals at m/z 771.2 and 913.1,
respectively (Figure S17). To our regret, attempts to isolate
and characterize the cationic diphosphene−gold(I) complex
were unsuccessful.
The additives were also surveyed in screening experiments
(Table 1).25 An additive was added last at room temperature in
the following experiments. The use of AgSbF6 or AgNTf2
afforded 6a in excellent yield in comparison to other silver
salts, such as AgOTf and AgBF4 (entries 1 and 8−10).
NaBArF (ArF = 3,5-bis(trifluoromethyl)phenyl) was also
4
C
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Article
applicable as an additive, although the yield was slightly lower
(entry 11). This result indicates that [(1)Au]+ promoted this
transformation and that silver also assisted the improvement of
the activity in gold catalysis.22a The catalytic activity of 2 was
slightly higher than those of (tht)AuCl, (Ph3P)AuCl, and 3a
(entries 12−14).
The successful hydroarylation described above and the
previous isolation of chiral diphosphene 115 encouraged us to
examine the enantioselective reaction. Among the possible
candidates,20a,b atropselective intramolecular hydroarylation of
5b,c was carried out (Table 2),26 where a 2-methoxy-1-
2 and cationic complex [4]+ in solution. Furthermore,
investigation of the catalytic activity of 2 in gold(I)-catalyzed
intramolecular hydroarylation showed that a diphosphene
ligand worked similarly to a phosphine ligand such as PPh3.
The use of the chiral diphosphene−gold(I) complex (S)-2
promoted the atropselective reaction with ee values as high as
9%. These results denote substantial progress in the chemistry
of low-coordinate phosphorus compounds including diphos-
phenes. Our group is actively pursuing further investigations of
coordination to other transition metals and asymmetric organic
transformation and the development of new diphosphenes
with high catalytic activity.
Table 2. Atropselective Intramolecular Hydroarylation of
5b,c
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00665.
■
S
Experimental procedures, characterization data, crystal-
lographic details, and computational results (PDF)
Cartesian coordinates for the calculated structures
(XYZ)
Accession Codes
CCDC 1958207 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for A.T.: tsurusaki@c.s.osakafu-u.ac.jp.
*E-mail for K.K.: kamikawa@c.s.osakafu-u.ac.jp.
■
entry
substrate
catalyst
yield (%) of 6
ee (%)
a
1
5b
5c
5b
5c
(S)-2
(S)-2
(R)-7
(R)-7
83
86
76
72
1
9
5
6
b
2
a
3
b
ORCID
4
a
b
Akihiro Tsurusaki: 0000-0002-9392-539X
Ken Kamikawa: 0000-0002-7844-4993
At −10 °C for 12 h. At 0 °C for 4 h.
Notes
naphthyl (b) or a 2-methyl-1-naphthyl (c) group was attached
to the alkyne instead of a methyl group as in 5a. The reactions
of 5b,c resulted in the formation of cyclized products 6b,c in
high yields, albeit with low ee values (1% ee for 6b and 9% ee
for 6c, entries 1 and 2).27 The use of gold complex (R)-7 with
a MeO-MOP ligand (MeO-MOP = 2-(diphenylphosphino)-2′-
methoxy-1,1′-binaphthyl)28 also resulted in low ee values (5−
6% ee, entries 3 and 4). Thus, the monodentate ligand of the
binaphthyl backbone with both diphosphene and phosphine
does not seem to be suitable for achieving high enantiose-
lectivity for this reaction. On the other hand, it is intriguing
that the ee value of 6c (9% ee) was higher than that of 6b (1%
ee) in (S)-2, whereas the ee values of 6b,c were comparable in
(R)-7 (5−6% ee). The C(Naph)−P bond rotation in (S)-2 in
solution (vide supra) may contribute to the change of the
chiral environment (i.e., syn/anti ratio) depending on the
substrate.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (B; #18H01983 to K.K.) and (C; #18K05090 to
A.T.), and a Grant-in-Aid for Scientific Research on Innovative
Areas “Precisely Designed Catalysts with Customized Scaffold-
ing” (#JP18H04269 to K.K.) from JSPS (Japan). A.T. also
thanks the Sumitomo Foundation (180777) and the Takasago
International Corporation for research fellowship. The authors
thank Professor Ikuko Miyahara (Osaka City University) for
assistance in X-ray crystallographic analysis. All theoretical
calculations were carried out at the Research Center for
Computational Science (Japan).
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CONCLUSION
■
We synthesized gold complex 2 with a binaphthyl-substituted
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phosphorus atom with a less-hindered binaphthyl group
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1
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́
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DOI: 10.1021/acs.organomet.9b00665
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