Gem-Digold Acetylide Complexes for Catalytic Intermolecular [4 + 2] Cycloaddition: Having Two Gold Centers Is Better for Asymmetric Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b02263

Gold(I)-catalyzed highly enantioselective intermolecular [4 + 2] cycloaddition is shown with ynones and cyclohexadiene. Various bicyclo[2.2.2]octadiene derivatives are produced in high yields (up to 99%) with good enantioselectivity (up to 96% ee). Key to the success is generation of the gem-digold terminal alkyne as a catalytic on-cycle species. As proof of the gem-digold catalysis, a positive nonlinear effect is clarified between the ee's of the ligand and the cycloadduct.

Full text of DOI:10.1021/acs.orglett.8b02263

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
gem-Digold Acetylide Complexes for Catalytic Intermolecular [4 + 2]
Cycloaddition: Having Two Gold Centers Is Better for Asymmetric
Catalysis
Masaki Nanko, Satoru Shibuya, Yuya Inaba, Soichiro Ono, Shigekazu Ito, and Koichi Mikami*
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology,
Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: Gold(I)-catalyzed highly enantioselective in-
termolecular [4 + 2] cycloaddition is shown with ynones and
cyclohexadiene. Various bicyclo[2.2.2]octadiene derivatives
are produced in high yields (up to 99%) with good
enantioselectivity (up to 96% ee). Key to the success is
generation of the gem-digold terminal alkyne as a catalytic on-
cycle species. As proof of the gem-digold catalysis, a positive
nonlinear effect is clarified between the ee’s of the ligand and
the cycloadduct.
simple ynones 1 and cyclohexa-1,3-diene (2) using gold(I)
catalysts containing BINOL-based phosphoramidites L. The
gem-digold species 4 is found to be the key to success in the
intermolecular cycloaddition7 as a catalytic on-cycle species.
The Diels−Alder reaction of ynone 1a with 2 was executed
in the presence of a 5 mol % loading of the chiral
phosphoramidite−gold(I) complex (R)-L1-AuOTf, and the
reaction proceeded smoothly to give the desired adduct 3a
(Table 1, entry 1). Screening of solvents showed that CH2Cl2
was the best solvent to obtain the highest yield of the desired
product 3a (entries 1−4).
A variety of BINOL-derived phosphoramidites were then
evaluated to give higher enantioselectivity. Bulky 4′-tert-butyl-
2′,6′-dimethyl-1,1′-biphenyl (L2) afforded the product with
lower enantioselectivity (entry 5). A ligand with a less bulky
phenyl substituent (L3) was also ineffective for enantiocontrol
(entry 6). To our delight, the ligand with Ar = 4′-tert-
butylphenyl (L4) resulted in much increased enantioselectivity
(entry 7). Surprisingly, however, a highly bulky substituent
such as 2,4,6-triisopropylphenyl (L5) resulted in the formation
of racemic product (entry 8). When the dimethylamino group
in the phosphoramidite was replaced with a bulkier
diisopropylamino substituent (L6; entry 9) or a chiral bis(1-
phenylethyl)amino group (L7, entry 10), however, very low
enantioselectivities were obtained. These results show that a
suitable size of the amino group could exert a striking influence
to give higher enantioselectivity. Thus, use of a cyclic amino
group (L8, n = 1) afforded higher enantioselectivity up to 80%
ee (entry 11). Incidentally, both ring-expanded piperidylamine
(L9, n = 2; entry 12) and chiral pyrrolidylamine (L10, n = 1;
n the past decade, homogeneous gold(I) catalysts have
Iexhibited their power for novel carbon−carbon and
carbon−heteroatom bond formation by virtue of π activation
of a carbon−carbon multiple bond toward nucleophilic attack
in atypical catalytic pathways.1 However, a gold(I)-catalyzed
enantioselective version has been quite limited even in an
intramolecular reaction.2 Therefore, enantioselective gold(I)
catalysis poses a formidable challenge because of the
propensity of gold(I) to form a linear two-coordinate complex:
the reacting center is distally far from the chiral ligand. For this
reason, the development of gold(I)-catalyzed asymmetric
reactions, especially intermolecular ones, is difficult. Recently,
several intermolecular cycloadditions have been reported with
highly reactive unsaturated substrates. Mascarenas described
̃
the [4 + 2] cycloaddition of allenamides,3 and Gong reported
the hetero-Diels−Alder reaction of diazenes.4 However, an
intermolecular [4 + 2] cycloaddition involving alkynes has no
precedent (Scheme 1),5 although the bicyclo[2.2.2]octadiene
products 3 obtained from 1 and 2 are in synthetic demand.6
Herein we report the asymmetric Diels−Alder cycloaddition of
Scheme 1. Chiral Gold(I)-Catalyzed Intermolecular [4 + 2]
Cycloaddition with Ynones
Received: August 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02263
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
Table 2. Intermolecular Diels−Alder Reaction of Various
a
Aryl Ynones
b
c
entry
[Au]
solvent
yield (%)
ee (%)
b
c
entry
1
Ar
3
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
(R)-L1-AuOTf
(R)-L1-AuOTf
(R)-L1-AuOTf
(R)-L1-AuOTf
(R)-L2-AuOTf
(R)-L3-AuOTf
(R)-L4-AuOTf
(R)-L5-AuOTf
(R)-L6-AuOTf
(R,R,R)-L7-AuOTf
(R)-L8-AuOTf
(R)-L9-AuOTf
(R,R)-L10-AuOTf
(R)-L8-AuNTf2
(R)-L8-AuSbF6
(R)-L8-AuBF4
(R)-L8-AuPF6
(R)-L8-AuPF6
CH2Cl2
THF
EtOH
54
nr
nr
36
−
−
−
4
8
65
6
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
4-MeO-C6H4
C6H5
3a
3b
3c
3d
3e
3f
−
3h
3i
3j
95
97
99
99
99
99
trace
99
99
99
98
99
90
99
94 (+)
94 (+)
90 (+)
94 (+)
84 (+)
96 (+)
−
92 (+)
87 (+)
90 (+)
87 (+)
84 (+)
83 (+)
94 (+)
4-Me-C6H4
4-Ph-C6H4
4-t-Bu-C6H4
4-OH-C6H4
4-NMe2-C6H4
4-F-C6H4
4-Cl-C6H4
4-Br-C6H4
4-I-C6H4
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
trace
55
51
55
38
31
16
41
50
40
13
17
23
1
1
10
11
12
13
14
15
16
17
80
63
63
76
40
89
3k
3l
3m
3n
4-CF3-C6H4
4-CN-C6H4
4-MeO2C-C6H4
a
Ynone 1 (0.1 mmol), cyclohexadiene 2 (2 equiv), and (R)-L8-AuPF6
(5 mol %) were used in 1 mL of CH2Cl2 at 0 °C for 24 h. Isolated
yields. The enantiomeric excess was determined by chiral HPLC.
b
d
95
59
94 (+)
94
c
e
18
a
Ynone 1a (0.1 mmol), cyclohexadiene 2 (2 equiv), and LAuX (5 mol
b
%) were used in 1 mL of solvent at 0 °C for 24 h. Yields were
determined by 1H NMR analysis using tetrachloroethane as an
c
groups on the aryl ynones, such as H (1b), Me (1c), Ph
(1d), t-Bu (1e), and even free OH (1f) groups, were excellent,
giving high yields and enantioselctivities (entries 2−6).
However, the amino substituent (NMe2, 1g) gave only a
trace amount of the product because it trapped the conjugate
acid of the catalyst counteranion (entry 7), so protodeauration
did not take place to give the product. Halide substituents (F,
1h; Cl, 1i; Br, 1j; I, 1k) afforded the corresponding products in
similarly high yields and enantioselectivities (entries 8−11).
The electron-withdrawing groups CF3 (1l), CN (1m), and
CO2Me (1n) gave the desired bicyclooctadienes in virtually
quantitative yield with high levels of stereoselectivity (entries
12−14).
internal standard. The enantiomeric excess was determined by chiral
HPLC. Remarkable enantioinversion was not observed. Isolated
yield. The reaction time was 48 h at −10 °C.
d
e
The scope of this intermolecular asymmetric reactions was
substantially expanded to various substituted ynones 1o−y
(Table 3). The π-conjugated 1-naphthyl (1o, entry 1) and 2-
naphthyl (1p, entry 2) products were obtained in good yields
with high enantioselectivities. Meta-disubstituted (1q, entry 3),
and heterocyclic 2-furyl (1r, entry 4) and 2-thienyl (1s, entry
5) ynones effectively gave 88, 84, and 90% ee, respectively.
However, alkyl substituents at the acetylenic terminus such
as cyclohexyl (1t, entry 6) and hexyl (1u, entry 7) gave low
yields and enantioselectivities. Benzyl ester (1v, entry 8) gave a
high yield but low enantioselectivity. With a silylated terminal
alkyne (1w, entry 9), the sole product 3a was obtained in
desilylated form with good enantioselectivity. In contrast,
internal alkynes 1x (entry 10) and 1y (entry 11) gave no
products. These results imply that terminal alkyne functionality
is indispensable for the progress of this reaction. The absolute
configuration of the product 3b was determined to be (1S,4R)
by comparison of the specific optical rotation with that of
(1R,4S)-3b synthesized from the known compound (1R,4S)-
methyl bicyclo[2.2.2]octa-2,5-diene-2-carboxylate (see the
Supporting Information). The configurations of the other
products were also determined to be (1S,4R) by comparison of
their specific optical rotations with that of (1R,4S)-3b.
entry 13) gave lower enantioselectivity than the simple
pyrrolidyl ligand L8.
The dramatic effects of the counteranion were then
uncovered. AuBF4 gave better enantioselectivity than AuOTf
but a lower yield (entry 16). AuPF6 was finally found to be the
best: both the highest yield and the best enantioselectivity were
obtained with (R)-L8-AuPF6 (entry 17). When the reaction
temperature was decreased to −10 °C, the cycloaddition
incompletely proceeded even after 48 h but gave the same level
of enantioselectivity (entry 18). For reproducibility, the
optimized reaction temperature and time were set at 0 °C
and 24 h, respectively.
We then explored the generality with various para
substituents in aryl ynones (Table 2). Electron-donating
B
DOI: 10.1021/acs.orglett.8b02263
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 3. Reaction of Various Ynones
Scheme 3. Confirmation of the gem-Digold Acetylenic
Complex
b
c
entry
1
R1
R2
3
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
1o
1p
1q
1r
1s
1t
1u
1v
1w
1x
1y
1-naph
2-naph
3,5-(CH3)2-C6H3
1-furyl
1-thienyl
Cy
C6H13
H
H
H
H
H
H
H
H
3o
3p
3q
3r
3s
3t
3u
3v
3a
−
99
99
99
84
91
95
98
79
82 (+)
82 (+)
88 (+)
84 (−)
90 (+)
30 (−)
38 (−)
7
OBn
d
4-MeO-C6H4
4-MeO-C6H4
4-MeO-C6H4
TMS
Ph
n-Bu
11
nr
nr
92 (+)
−
−
10
11
−
to δ = 62.4 ppm in 5b. The 13C NMR spectrum of 5b revealed
heteronuclear coupling between the acetylenic carbons and
two phosphorus atoms. The signal for the aurated atom C1
shifted from δC = 142.5 ppm in 4b (d, 2JPC = 127.6 Hz) to δC =
139.3 ppm (t, 2JPC = 64.3 Hz) in gem-digold complex 5b. The
2JPC coupling constant was 64.3 Hz, in good agreement with
a
Ynone 1 (0.1 mmol), cyclohexadiene 2 (2 equiv), and (R)-L8-AuPF6
b
(5 mol %) were used in 1 mL of CH2Cl2 at 0 °C for 24 h. Isolated
c
yields. The enantiomeric excess was determined by chiral HPLC.
d
1
The yield was determined by H NMR analysis using tetrachloro-
ethane as an internal standard.
the previous report by Fu
̈
rstner.12 Another important feature in
We clarified the role of digold intermediates in the catalytic
cycle. Recently, Toste and co-workers proposed a possible gem-
digold product on the basis of DFT calculations.8aThe ipso-
digold complexes were first discovered by Nesmeyanov et al.9
in 1974 and play a role in gold catalysis, but off-cycle.8b,c
However, digold catalysis on-cycle has remained uncovered.10
In the 31P NMR spectrum, the stoichiometric reaction of 1a
with 1 equiv of (R)-L2-AuOTf showed a singlet peak for 5a
together with an equimolar amount of the remaining substrate
1a (Scheme 2). A large downfield shift of the phosphine atom
the 13C NMR spectrum is the shift in the signal for the α-C
atom C2 from δC 3= 100.2 ppm in 4b (d, 3JPC = 21.9 Hz) to δC
= 106.2 ppm (t, JPC = 12.8 Hz) in gem-digold complex 5b.
Addition of cyclohexadiene to gem-digold complex 5b led to
the stable diaurated intermediate 6b. Unfortunately, the
sterically demanding 6b could not provide the protodeaurated
product 3a even at high temperature. With less sterically
demanding triphenylphosphine as the ligand (5c), the reaction
proceeded smoothly to generate gem-diaurated intermediate 6c
in 88% yield. Finally, TfOH was added to protonate 6c and
give the product 3a in quantitative yield (Scheme 4). In
Scheme 2. Stoichiometric Reaction of Ynone 1a and (R)-L2-
AuOTf
Scheme 4. Diels−Alder Reaction of gem-Digold Species
from δ = 107.4 ppm ((R)-L2-AuOTf) to δ = 130.6 ppm (5a)
showed that the gem-digold complex 5a was spontaneously
formed rather than the σ,π complex 5a′ (Scheme 2).11
The “stoichiometric” reaction of 1a with bulky Johnphos-
AuOTf gave the stabilized gold acetylide complex 4b in the
presence of trimethylamine (Scheme 3). The addition of a
second equivalent of Johnphos-AuOTf converted 4b to gem-
contrast, the addition of cyclohexadiene to monogold complex
4c did not provide any Diels−Alder product. These results
imply the importance of the gem-diaurated ynone as an on-
cycle species in this asymmetric catalysis.
As proof of the gem-digold catalysis, an asymmetric
amplification, namely, a positive nonlinear effect ((+)-NLE)
was validated between the ee’s of ligand L8 and cycloadduct 3a
(Figure 1).13 A convex deviation from the linear relationship
was indeed observed through a wide range of ee values of the
catalyst (R)-L8-AuPF6 to give highly enantioselective cyclo-
adduct 3a (L8-Au: 0, 50, and 100% ee; 3a: 0, 90, and 94% ee,
respectively). This significant (+)-NLE clearly proved the
1
digold complex 5b (two steps, 76% yield). The H NMR
spectrum of 5b showed a single methoxy peak of 1a and two
aromatic peaks of Johnphos. The single set of peaks in a 1:2
ratio for the ynone and ligand was established by integration
and hence indicative of the overall molecular composition of
5b. The 31P NMR spectrum showed only a singlet peak and
upfield shifts of the phosphine atoms from δ = 63.6 ppm in 4b
C
DOI: 10.1021/acs.orglett.8b02263
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02263.
Experimental procedures and compound characteriza-
tion data (DOCX)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mikami.k.ab@m.titech.ac.jp.
ORCID
Koichi Mikami: 0000-0002-7093-2642
Notes
The authors declare no competing financial interest.
Figure 1. Positive nonlinear effect between the ee’s of the cycloadduct
3a and the catalyst (R)-L8-AuPF6.
ACKNOWLEDGMENTS
■
This research was supported by the Japan Science and
Technology Agency (JST) (ACT-C: Creation of Advanced
Catalytic Transformation for the Sustainable Manufacturing at
Low Energy, Low Environmental Load).
intervention of the gem-digold-activated terminal alkyne as a
triggering species.
The catalytic cycle was thus clarified (Scheme 5). The
cationic gold complex and ynone form gem-digold complex A
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DOI: 10.1021/acs.orglett.8b02263
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