Enantioselective Synthesis of Chiral Indane Derivatives by Rhodium-Catalyzed Addition of Arylboron Reagents to Substituted Indenes

Article abstract of DOI:10.1021/acs.orglett.0c03651

Rhodium-catalyzed asymmetric addition of arylboron reagents to indene derivatives proceeded to give 2-arylindanes in good yields with high enantioselectivity. Deuterium-labeling experiments indicated that the present reaction involved a 1,4-Rh shift from an initially formed benzylrhodium to an arylrhodium intermediate before protonation leading to the corresponding addition product. The asymmetric addition was also successful for acenaphthylene, which has a similar skeleton to indene, where it was found that the benzylrhodium intermediate underwent direct protonation without the 1,4-Rh shift.

Full text of DOI:10.1021/acs.orglett.0c03651

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Letter
Enantioselective Synthesis of Chiral Indane Derivatives by Rhodium-
Catalyzed Addition of Arylboron Reagents to Substituted Indenes
Moeko Umeda, Hikaru Noguchi, and Takahiro Nishimura*
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ABSTRACT: Rhodium-catalyzed asymmetric addition of arylboron
reagents to indene derivatives proceeded to give 2-arylindanes in good
yields with high enantioselectivity. Deuterium-labeling experiments
indicated that the present reaction involved a 1,4-Rh shift from an initially
formed benzylrhodium to an arylrhodium intermediate before protonation
leading to the corresponding addition product. The asymmetric addition
was also successful for acenaphthylene, which has a similar skeleton to
indene, where it was found that the benzylrhodium intermediate underwent
direct protonation without the 1,4-Rh shift.
Scheme 1. Key Intermediates in This Work
hodium-catalyzed asymmetric addition reaction of
Rarylboron reagents to electron-deficient C−C unsatu-
rated bonds has gained an established position in
enantioselective C−C bond formation reactions.1 In 1998,
Hayashi, Miyaura, and co-workers reported the first
enantioselective conjugate addition of arylboronic acids to
enones.2 The reaction involves transmetalation of the
arylboronic acid with the rhodium complex to give an
arylrhodium species, and subsequent alkene insertion
generates oxa-π-allyl species, which readily undergoes
hydrolysis to form a formal hydroarylation product and to
regenerate the Rh catalyst.3 Following the report, a large
number of successful examples of the rhodium-catalyzed
asymmetric addition to electron-deficient alkenes have
appeared. In contrast, the enantioselective addition to alkenes
without electron-deficient substituents has been under-
developed because of undesired β-hydrogen elimination
from an alkylrhodium intermediates. Lautens and co-workers
reported that the Rh-catalyzed addition−elimination reaction
of styrene proceeded to give trans-stilbene, whereas the
addition reaction was successful for 2- or 4-vinylpyridine.4
Lam and co-workers disclosed that introducing an electron-
withdrawing NO2 group at the para position of the styrene
derivatives enabled the asymmetric addition suppressing β-
hydrogen elimination.5 There have been a few reports on the
asymmetric addition to alkenes without particular electron-
withdrawing substituents.6−9 Recently, Wang7 and we8
independently reported the asymmetric addition of arylbor-
onic acids to 2H-chromene derivatives. Here we report
rhodium-catalyzed asymmetric addition of arylboron reagents
to indene derivatives giving 2-arylindanes10 in good yields
with high enantioselectivity (Scheme 1).11 To the best of our
knowledge, there have been no reports on the catalytic
enantioselective addition of arylmetal reagents to indene
derivatives. The reaction involves a 1,4-Rh shift from an
initially formed benzylrhodium to an arylrhodium intermedi-
ate before protonation leading to the corresponding addition
product. The asymmetric addition is also successful for
acenaphthylene, where the benzylrhodium intermediate
undergoes direct protonation without the 1,4-Rh shift.
After the reactivity of 1H-indene toward Rh-catalyzed
addition of arylboronic acids was investigated under several
reaction conditions (Table S1), conventional chiral ligands
were tested for the enantioselective addition to substituted
Received: November 3, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03651
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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indene derivatives (Table 1). Treatment of 7-phenyl-1H-
indene (1a) with 3.0 equiv of p-tolylboronic acid (2a) in the
presence of [RhCl(cod)]2 (5 mol % of Rh, cod = 1,5-
cyclooctadiene), (R)-binap12 (6 mol %), and an equivalent of
K3PO4 in 1,4-dioxane at 60 °C for 20 h gave the addition
product 3aa in 76% yield, with 95% ee (Table 1, entry 1).
Table 2. Rh-Catalyzed Asymmetric Addition of Arylboron
Reagents 2 to 7-Phenylindene (1a)
a
Table 1. Rh-Catalyzed Asymmetric Addition of p-
a
Tolylboronic Acid (2a) to 7-Phenylindene (1a)
b
c
entry
Ar
X
yield (%)
ee (%)
d
1
2
3
4
5
6
7
8
9
Ph (2b−)
2
72 (3ab)
44 (3ac)
64 (3ad)
94 (3ae)
66 (3af)
67 (3ag)
56 (3ah)
79 (3ai)
64 (3aj)
92
92
91
91
97
92
93
93
92
4-PhC6H4 (2c′)
4-ClC6H4 (2d′)
4-CF3C6H4 (2e′)
4-MeOCOC6H4 (2f′)
4-NO2C6H4 (2g′)
3-MeC6H4 (2h′)
3-ClC6H4 (2i′)
3
3
3
3
5
5
1.5
5
2-naphthyl (2j′)
a
Reaction conditions: 1a (0.10 mmol), 2 (0.30 mmol), [RhCl(cod)]2
(5 mol % of Rh), (R)-binap (6 mol %), and tert-amyl alcohol, and
K3PO4 (0.10 mmol for entries 2−9) in 1,4-dioxane (0.2 mL) at 60 °C
b
c
entry
ligand
(R)-binap
yield (%)
ee (%)
b
c
for 20 h. Isolated yield. Determined by HPLC analysis with chiral
1
2
3
4
5
6
7
8
76
28
39
34
71
52
11
12
65
95
66
81
84
96
83
34
−
d
stationary phase columns. H2O was used instead of tert-amyl alcohol
(R)-xylbinap
(R)-tolbinap
(S)-segphos
(S)-difluorphos
(R)-DM-segphos
(R)-DTBM-segphos

without K3PO4.
Scheme 2. Rh-Catalyzed Asymmetric Addition of p-
a
Tolylboronic Acid to Indene Derivatives
d
9
(R)-binap
95
a
Reaction conditions: 1a (0.10 mmol), 2a (0.30 mmol), [RhCl-
(cod)]2 (5 mol % of Rh), ligand (6 mol %), and K3PO4 (0.10 mmol)
b
in 1,4-dioxane (0.2 mL) at 60 °C for 20 h. Isolated yield.
c
d
Determined by chiral HPLC analysis. [RhCl(coe)2]2 was used
instead of [RhCl(cod)]2.
Binap analogues, xylbinap and tolbinap (entries 2 and 3), and
segphos13 (entry 4) were less effective in terms of both yields
and enantioselectivities of 3aa. (S)-Difluorphos,14 which is
known as a relatively electron-deficient ligand, displayed the
comparable enantioselectivity of 96% ee with binap (entry 5).
The use of bulkier ligands, DM-segphos and DTBM-segphos,
did not improve the enantioselectivity (entries 6 and 7). The
reaction proceeded without adding the phosphine ligand
giving 3aa in 12% yield, indicating that the 1,5-cyclooctadiene
also worked as a ligand to influence the ee when the ligand
exchange is incomplete (entry 8). The reaction in the
presence of [RhCl(coe)2]2 (coe = cyclooctene) as a precursor
with (R)-binap displayed the same enantioselectivity as that
observed with [RhCl(cod)]2 (entry 9). The absolute
configuration of 3aa obtained by use of (R)-binap was
assigned to be S by analogy with (S)-3ad (Table 2, entry 3),
which was determined by X-ray crystallography.
The results obtained for the asymmetric addition of p-
tolylboronic acid (2a) to several indene derivatives 1 are
summarized in Scheme 2. The addition to indenes
substituted with several aryl groups at the 7-position
proceeded to give 3ba−ga in moderate to good yields
(55−75%), with the enantioselectivity ranging between 79%
and 97% ee. A slight decrease of the enantioselectivity (79%
a
Reaction conditions: 1 (0.10 mmol), 2a (0.30 mmol), [RhCl(cod)]2
(5 mol % of Rh), (R)-binap (6 mol %), and K3PO4 (0.10 mmol) in
1,4-dioxane (0.2 mL) at 60 °C for 20 h. [RhCl(cod)]2 (10 mol % of
Rh) and (R)-binap (12 mol %) were used. At 80 °C.
b
c
B
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ee) was observed in the addition to 3ea, which has an
electron-withdrawing CF3 group. Both m-chlorophenylindene
1f and o-chlorophenylindene 1g are also good substrates to
give 3fa and 3ga, respectively, with high enantioselectivity.
Styryl-substituted indene 1h also participated in the reaction
to give 3ha in 71% yield with 98% ee. The reaction of 3i
having a cyclopropyl group proceeded to give 3ia with 97%
ee. In contrast, the reaction of indene 1j substituted with a
less bulky bromo atom at the 7-position displayed a lower
enantioselectivity of 57% ee. The addition to indene 3k
substituted at the 6-position gave the addition product 3ka in
59% yield with 87% ee. The same product as 3ka with an
opposite absolute configuration (R) (3la) with 77% ee can be
obtained by the reaction of 3l, which has a p-chlorophenyl
group at the 5-position, using (R)-binap as a ligand.
the reaction of an equimolar mixture of 1k and 1l (eq 5),
indicating that the substituents far from the olefin moiety do
not influence the reactivity. It follows that the same face
selectivity of the olefin toward 1k and 1l furnishes both
enantiomers equally. The result also implies that the olefin
isomerization of the indene should be avoided to achieve the
high enantioselectivity.
The present catalytic system can also be applied to the
asymmetric addition to acenaphthylene as one of the indene
derivatives (Scheme 3), which has not been used as an
Scheme 3. Rh-Catalyzed Asymmetric Addition of
a
Arylboron Compounds to Acenaphthylene
Table 2 summarizes the results obtained for the addition of
several arylboron reagents 2 to 7-phenyl-1H-indene (1a). The
use of arylboron reagents, such as phenylborate (2b−)15 in
the presence of H2O (entry 1) and arylboronates (2′) with
tert-amyl alcohol (entries 2−9), brought about higher yields
than the corresponding arylboronic acids. Several aryl groups
are introduced into 1a giving the corresponding 2-arylindanes
3 in moderate to good yields with high enantioselectivity.
Unfortunately, however, the reaction of ortho-substituted aryl
groups, such as o-tolyl, o-chlorophenyl, and o-methoxyphenyl,
failed to give the corresponding arylation products.
The alkene moiety of indene 1a easily isomerized into 1a′
in the presence of K3PO4 (eq 1). The olefin isomerization,
however, was suppressed by p-tolylboronic acid (2a) even in
the presence of the base (eq 2), probably because the basic
character was suppressed by the formation of a borate anion.
In addition, the chlororhodium catalyst did not isomerize the
olefin (eq 3). The reaction of a 1:1 mixture of 1a and 1a′
gave 3aa in 45% yield, whose ee was 88% (S) (eq 4). The
result indicates that the reactivity of 1a is much higher than
1a′ in consideration of the relatively high enantioselectivity.
a
Reaction conditions: 4a (0.20 mmol), 2 (0.50 mmol), [Rh(OH)-
(cod)]2 (5 mol % of Rh), and (S)-difluorphos (6 mol %) in 1,4-
b
dioxane (0.4 mL) at 60 °C for 6 h. Arylboroxine (2′′) (2.5 equiv of
B) and tert-amyl alcohol (3.0 equiv) were used instead of the
c
corresponding arylboronic acids. Arylboroxine (2′′) (2.5 equiv of B)
and water (5.0 equiv) were used instead of the corresponding
arylboronic acids.
acceptor for Rh-catalyzed addition of arylboron reagents, to
the best of our knowledge. After the optimization of the
reaction conditions for the asymmetric addition (Table S2),
the catalytic system composed of [Rh(OH)(cod)]2 and (S)-
difluorphos was found to efficiently promote the reaction
with high enantioselectivity. Thus, treatment of acenaph-
thylene (4a) with 2.5 equiv of p-tolylboronic acid (2a) in the
presence of [Rh(OH)(cod)]2 (5 mol % of Rh) and (S)-
difluorphos (6 mol %) in 1,4-dioxane at 60 °C for 6 h gave
the addition product 5aa in 99% yield with 98% ee. The
The low reactivity of 1a′ might be due to the steric
hindrance of the phenyl group located close to the olefin
moiety. In sharp contrast, the ee of the product was lost in
C
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scope of arylboronic acids is fairly broad, and a variety of aryl
groups substituted with both electron-donating and -with-
drawing groups were introduced into 4a, thus giving the
corresponding addition products 5 in high yield with high
enantioselectivity (90−99% ee). 2-Thiopheneboronic acid
(2t) and alkenylboronic acid 2u also participated in the
reaction to give addition products 5at and 5au in 47% and
55% yields, respectively, with high enantioselectivity. The
reaction of acenaphthylene 5b substituted with two tert-butyl
groups also gave addition product 5ba in 78% yield with 99%
ee. The absolute configuration of the (S)-5am was
determined to be S by X-ray crystallographic analysis.
Scheme 4. Protonation Steps via Key Intermediates
Deuterium-labeling experiments provided mechanistic in-
sight into the protonation step of the present reactions (eqs
6−8). In the reaction of indene 1a with phenylboroxine 2b′
in the presence of D2O, where PhB(OD)2 is generated in
situ, deuterium incorporation was observed at the 2′-position
of the phenyl group (eq 6). The reaction using penta-
deuteriophenylboronic acid (2b-d5) generated in situ gave the
addition product, where a transfer of deuterium from the
phenyl group to a benzylic position of the indane occurred
(eq 7). These results indicate that the reaction involves a 1,4-
Rh16 shift before protonation. On the other hand, in the
reaction of acenaphthylene (4a) with phenylboroxine 2b′ in
the presence of D2O, H/D exchange was observed at the
benzylic position (eq 8). It was also confirmed that the
deuterium on 5ab-D was anti to the phenyl group. The result
indicates that an organorhodium species undergoes direct
protonation with inversion of stereochemistry at the rhodium
to give the addition product without the 1,4-Rh shift.17
acenaphthylene (4a) involves direct protonation of the
benzylrhodium intermediate as shown in Scheme 4b. The
π-benzylrhodium intermediate D, which may be stabilized not
to receive the 1,4-Rh shift, undergoes the protonation at the
benzylic position from the opposite side of rhodium to give
5aa.
In summary, we have developed Rh-catalyzed asymmetric
addition of arylboron reagents to indene derivatives giving 2-
arylindanes in good yields with high enantioselectivity. High
enantioselectivity was also observed for the addition to
acenaphthylene, which has a similar skeleton to indene.
Mechanistic investigations indicate that the present Rh-
catalyzed addition of arylboronic acids to indene derivatives
involves a 1,4-Rh shift before protonation of the benzylrho-
dium intermediate to release the product and the active Rh
species. In contrast, in the reaction of acenaphthylene, direct
protonation of the organorhodium species, which may have a
π-benzyl form, occurs without the 1,4-Rh shift to give the
addition product. Further studies on the asymmetric addition
to indene derivatives having a trisubstituted alkene moiety are
underway.
ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03651.
Experimental procedures and compound character-
ization (PDF)
Accession Codes
CCDC 2031738−2031739 contain 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 Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
Based on the results of the deuterium-labeling experiments
and previous studies,3,16 insertion and protonation steps of
the present reactions are proposed as illustrated in Scheme 4.
p-Tolylrhodium species A, generated by transmetalation
between p-tolylboronic acid (2a) and Rh, reacts with indene
1a to give benzylrhodium intermediate B, which undergoes
the 1,4-Rh shift to form arylrhodium C. Protonation of C
then gives 3aa and regenerates a Rh(OH) species. In contrast
to the protonation step for indene 1a, the reaction of
AUTHOR INFORMATION
Corresponding Author
■
Takahiro Nishimura − Department of Chemistry, Graduate
School of Science, Osaka City University, Osaka 558-8585,
D
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Japan; orcid.org/0000-0002-7032-8613;
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Letter
Asymmetric Hydroarylation. Angew. Chem., Int. Ed. 2019, 58, 5343−
5347.
(8) Umeda, M.; Sakamoto, K.; Nagai, T.; Nagamoto, M.; Ebe, Y.;
Nishimura, T. Rhodium-catalyzed asymmetric addition of arylbor-
onic acids to 2H-chromenes leading to 3-arylchromane derivatives.
Chem. Commun. 2019, 55, 11876−11879.
Email: tnishi@sci.osaka-cu.ac.jp
Authors
Moeko Umeda − Department of Chemistry, Graduate School
of Science, Osaka City University, Osaka 558-8585, Japan
Hikaru Noguchi − Department of Chemistry, Graduate
School of Science, Osaka City University, Osaka 558-8585,
Japan
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant No.
JP19H02721.
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