Double-Fold Ortho and Remote C-H Bond Activation/Borylation of BINOL: A Unified Strategy for Arylation of BINOL

Article abstract of DOI:10.1021/acs.orglett.9b02347

A double-fold ortho and remote C-H borylation of BINOL is described. The proposed mechanisms involved electrostatically and sterically directed ortho and remote C-H activation processes, respectively. While B₂eg₂ (eg = ethylene glyco

Full text of DOI:10.1021/acs.orglett.9b02347

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Double-Fold Ortho and Remote C−H Bond Activation/Borylation of
BINOL: A Unified Strategy for Arylation of BINOL
Ranjana Bisht,†,‡,§ Jagriti Chaturvedi,†,‡,§ Gajanan Pandey,‡ and Buddhadeb Chattopadhyay*,†
†Center of Bio-Medical Research, Division of Molecular Synthesis & Drug Discovery, SGPGIMS Campus, Raebareli Road, Lucknow
226014, Uttar Pradesh, India
‡BBAU, Department of Applied Chemistry, Vidya Vihar, Raebareli Road, Lucknow 226025, Uttar Pradesh, India
S
* Supporting Information
ABSTRACT: A double-fold ortho and remote C−H borylation of
BINOL is described. The proposed mechanisms involved electrostatically
and sterically directed ortho and remote C−H activation processes,
respectively. While B2eg2 (eg = ethylene glycolate) directs the C−H
activation at ortho positions, a combination of HBpin and B2pin2 activates
remote C−H bonds. The strategy was combined with Suzuki arylation as a
one-pot protocol for the rapid synthesis of BINOL derivatives with
retention of chirality.
hile numerous C−H bond functionalizations1 are now
Wavailable, C−H bond borylation has potential because of
Figure 2. Proposed working model for ortho and remote borylation.
the versatile synthetic transformations2 of B−C bonds. Usually,
a major challenge in C−H bond borylation of arenes is the site
selectivity.3 For the selective ortho-borylation of arenes, the
common strategies are the directed metalation4 and functional
group5 directed borylation. However, these are highly depend-
ent on what kind of functional groups are present on the
substrates. On the other hand, remote C−H borylations are still
difficult to realize but nevertheless practically beneficial.6
Recently, few strategies have been developed for the remote
C−H borylation using various noncovalent interactions.7
However, there is no general method available to achieve the
remote borylations, and every method has its own restriction.
Moreover, examples for the selective ortho and remote C−H
activation/borylation from the same substrate are almost rare,7a
although extremely desirable.
On the other hand, 3,3′-biaryl and 6,6′-biaryl BINOLs have
been recognized as the ideal backbones for chiral ligands that are
being tremendously utilized in asymmetric catalysis.8−10 There
Received: July 9, 2019
Figure 1. Previous works and new challenge.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02347
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
technique, which adds an extra step. Thus, these findings are
indicative of a lack of efficient methods for the preparation of
chiral 3,3′-biaryl BINOLs. Moreover, direct preparation of chiral
6,6′-biaryl BINOLs remains an unmet challenge due to the
difficulty of the remote C−H bond functionalization. There are
only a few reports for the preparation of chiral 6,6′-biaryl
BINOLs via electrophilic bromination and Fe-based catalyst by
Pappo and Toste (Figure 1B).16 Herein, we report the discovery
of double fold ortho and remote C−H arylation of chiral BINOL
using a C−H activation/borylation strategy directed by an
electrostatic interaction and a steric control (Figure 1C).
Recently, it has been demonstrated17 that ortho C−H
borylation of phenol is possible via an electrostatic interaction
between a partial negatively charged in situ generated OBpin/
OBeg group and a partial positively charged ligand of the Ir
catalyst. Thus, we hypothesized that there is a fair possibility for
the electrostatic interaction9 between the positively charged
ligand and the negatively charged OBeg intermediate, which
might selectively increase the acidity of both the ortho C−H
bonds of BINOL (Figure 2, TS-1), although BINOL has many
similar types of C−H bonds relative to the small phenol
substrate.
Moreover, we anticipated that by altering the boron
functionality from small Beg to the larger Bpin, the ortho C−
H bonds of BINOL would be inaccessible (for steric reason),
enforcing the remote borylation (Figure 2, TS-2). Notably,
other remote C−H bonds might be competitive (C5 or C7), but
due to the combined steric and electronic factors of the relatively
larger OBpin groups they might facilitate the remote 6,6′-
borylation.16a,b,18
First, we focused on the double-fold ortho C−H borylation
with the (R)-BINOL. We studied the borylation of (R)-BINOL
using recently reported phenol’s ortho borylation conditions.17
Thus, (R)-BINOL, 2.5 equiv of B2eg2, and 2.5 equiv of Et3N
were heated in PhMe (Table 1, entry 1) and it was found that
phenol’s borylation conditions is unsuccessful, which afforded
only solvent borylation. To avoid the solvent borylation, we
Table 1. Optimization of the Double-Fold Ortho Borylation
boron source
(equiv)
entry
base (equiv)
solvent
yield (%) (2)
1
2
3
4
5
6
7
B2eg2 (2.5)
B2eg2 (2.5)
B2eg2 (2.5)
B2eg2 (2.5)
B2eg2(4.0)
HBpin (4.0)
B2pin2 (4.0)
Et3N (2.5)
Et3N (2.5)
Et3N (2.5)
Et3N (2.5)
Et3N (4.0)
Et3N (4.0)
PhMe
CyH
p-xylene
THF
THF
THF
solvent borylation
complex mixture
solvent borylation
b
53
b
79
complex mixture
complex mixture
Et3N (4.0)
THF
a
b
Scale of reaction 0.1 mmol. Isolated yields after transesterification
with pinacol.
are four different methods to prepare 3,3′-biaryl BINOLs, such
as (i) Snieckus’s five-step protocol,11 (ii) Jorgensen’s six-step
protocol,12 (iii) Fu’s three-step method,13 and (iv) Clark’s three-
step method.14 Importantly, the use of large amounts of air-
sensitive alkyl lithium salts, multistep reactions, and long
reaction time significantly limit the synthetic utility of the
Snieckus and Jorgensen methods. Likewise, employment of the
directing groups in C−H activation methods require extra steps
and generation of the stoichiometric waste from the attaching
and detaching that limits the wide applicability of Fu’s and
Clark’s method. More recently, Ye and co-workers reported an
efficient one step route to access varieties of 3,3′-biaryl BINOLs
(Figure 1A).15
However, the major problem of this method is the
racemization of the chiral BINOLs and the corresponding
arylated products under the employed reaction conditions. To
obtain the optically pure isomer it needs a chiral resolution
a
Table 2. Optimization of Double-Fold Ortho Arylation
b
entry
catalyst (mol %)
Pd(PPh3)4 (5)
Pd(PPh3)4 (5)
Pd(PPh3)4 (5)
Pd2dba3·CHCl3 (2)
PdCl2·dppf·CH2Cl2(2.5)
ligand (mol %)
base (equiv)
solvent
conv (%)
1
2
3
4
5
K2CO3 (2)
K2CO3 (2)
CsF (3)
K2CO3 (8)
Na2CO3(8)
PhMe/H2O (6/1)
DME/H2O (1/1)
THF/H2O (10/1)
THF/H2O (10/1)
nr
nr
nr
85
P(o-tolyl)3 (4)
c
dioxane/DME/H2O(1/1/1)
90 (79)
a
b
c
Scale of reaction: 0.1 mmol. NMR conversion. In parentheses, isolated yields.
B
DOI: 10.1021/acs.orglett.9b02347
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Letter
a
Scheme 1. Scope of Double-Fold Ortho Borylation/
Arylation
Table 3. Remote Double-Fold Borylation/Arylation
a
entry HBpin (equiv) B2pin2 (equiv) solvent
conv (%) (7)
1
2
3
4
5
6
4.0
THF
THF
PhMe
CyH
THF
THF
complex mixture
complex mixture
solvent borylation
multiple borylation
44
2.0
2.0
2.0
2.0
2.5
b
b
b
b
2.5
2.5
2.5
3.0
c
95 (87)
a
b
Scale of reaction 0.1 mmol scale; NMR conversion. In this case,
c
sequence of addition is important. For details, see the SI. Isolated
yields after 16 h are shown in parentheses.
relatively more stable Bpin product was isolated in 53% yield
(entry 4).
Remarkably, by repeating the same reaction using 4.0 equiv of
B2eg2 and 4.0 equiv of Et3N, we isolated the double-fold
relatively stable Bpin product in 79% after transesterification
(entry 5) along with some amount of the monoborylated
product. It should be mentioned that due to the rapid
protodeborylation, we were unable to measure the ratio of the
mono-/bis-borylated products. As expected, by performing the
borylations using either HBpin or B2pin2 we found that the
borylations were nonselective (entries 6 and 7).
Next, we examined the arylation of o,o-diborylated BINOL
(2) with 1-bromo-4-methylbenzene (3a) employing 5 mol % of
Pd(PPh3)4 catalyst and K2CO3 in PhMe/H2O at 80 °C (Table
2A, entry 1). However, no desired arylated product was
observed. Use of other bases and solvents were ineffective
(entries 2 and 3). Pleasingly, employment of Pd2dba3·CHCl3
and o-tolylphosphine gave 85% conversion of the product (entry
4). Performing the reaction in the presence of PdCl2·dppf·
CH2Cl2 and Na2CO3 in dioxane/DME afforded the product in
90% conversion (entry 5). Subsequently, we successfully
combined this two-step method in a one-pot method, which
afforded the desired product in 77% isolated yield with 99% ee
(Table 2B).
a
b
Scale of reaction 0.1 mmol; isolated yields. Reaction scale 5.0
c
mmol. Undetermined.
To show the synthetic usefulness of the protocol, a large-scale
(5 mmol) double-fold ortho C−H borylation/arylation of (R)-
BINOL was performed, which afforded the target 3,3′-biaryl
BINOL (4a) in 73% yield with 99% ee (Scheme 1, entry 1).
Notably, this one-pot preparation of highly important chiral
3,3′-biaryl BINOL (average cost ∼$1000/gram) using less
expensive (R)-BINOL ($1.0−4/gram) demonstrates that the
developed method would be a useful synthetic tool for chiral
3,3′-biaryl BINOL.
attempted the borylation in cyclohexane (CyH) or p-xylene;
however, in both cases, we did not observe our desired product
(entries 2 and 3). When the borylation was performed in THF,
no solvent borylation was detected and an appreciable amount
of the desired product was found by crude NMR analysis.
Notably, attempted isolation of the Beg-borylated product was
not successful due to the rapid proto-deborylation process.
Thus, when the borylation was completed, the eg (ethylene
glycolate) group was transesterified by treating the crude
reaction mixture with 2.5 equiv of pinacol in dry CHCl3, and the
Next, we explored the scope of the reaction (Scheme 1). For
example, use of electron-rich aryl bromide (3b) and electroni-
cally neutral bromobenzene (3c) afforded 79% (91% ee) and
70% (99% ee) yields, respectively. Electron-deficient aryl
C
DOI: 10.1021/acs.orglett.9b02347
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Letter
bromides such as 3d−3i were also appeared to be compatible
under the developed conditions affording good isolated yields
and ee (4d−4i). Moreover, the method successfully underwent
double-fold borylation/arylation with a number of electron-rich
and sterically demanding coupling partners with good yields and
ee (4j−4n). Notably, while preparation of chiral 3,3′-H8−
BINOLs19 is not straightforward and requires multiple synthetic
steps,20 we have demonstrated that our developed method could
be applied to achieve the synthesis of such compound with high
isolated yield (81%) and ee (98%, entry 6).
Scheme 2. Scope of Double-Fold Remote Borylation/
Arylation
a
Next, we focused on the remote 6,6′-double-fold C−H
activation and borylation of (R)-BINOL. Thus, a THF solution
of (R)-BINOL was treated with 4.0 equiv of HBpin, 5.0 mol % of
triethylamine, 1.5 mol % of Ir, and 3.0 mol % of dtbpy ligand
(Table 3, entry 1). Surprisingly, the outcome was a complex
mixture. When the boron source was changed from HBpin to
B2pin2 no improvement occurred. Next, we conducted the
borylation in two different solvents (PhMe and CyH). However,
what we observed is the solvent borylation for PhMe as the
solvent and multiple borylations for CyH as the solvent (entries
3 and 4). Interestingly, when the same reaction was performed in
THF solvent, we found 44% remote 6,6′-double-fold borylation
of the BINOL (entry 5) and some amount of monoborylated
product. Gratifyingly, by repeating the same reaction with an
increased amount of HBpin and B2pin2 for longer reaction time
(16 h), we found almost quantitative conversion into the desired
double-fold remote borylation product (along with trace
amount of other isomers), which was isolated in 87% pure
product (entry 6). After the double-fold remote 6,6′-
diborylation of chiral BINOL was applied, we applied our
developed arylation conditions (Table 2, entry 5), which
afforded the desired remote 6,6′-bis-arylated BINOL with high
yield (Table 3, bottom). In this context, it deserves mentioning
that HPLC experiment showed that during either double-fold
C−H activation/borylation or double-fold arylation, no
racemization occurred, which afforded excellent enantioselec-
tivity (95%).
After establishing the double-fold remote borylation and
arylation, we subsequently became interested to develop this
two-step process in one-pot way without isolating the remote
bis-borylated intermediate (7). Pleasingly, this two-step
protocol was successfully accomplished in a one-pot method,
giving the desired product (8a) in 76% isolated yield with high
ee (95%) (Scheme 2, entry 8a). Moreover, to check the
synthetic utility of the method, a large-scale (10 mmol) double-
fold remote C−H borylation/arylation reaction of simple (R)-
BINOL was performed under the optimal conditions, which
gave the target 6,6′-biaryl BINOL (8a) in 71% yield without
compromising the enantioselectivity. Next, we investigated the
scope of the other coupling partners. All the reactions were
performed in a single flask without isolating the bis-borylated
product. We observed that the reactions are very general
affording good amount of the isolated yields and good to
excellent enantioselectivity with various coupling partners. For
example, electron-deficient, electron-rich, and electronically
neutral aryl bromides are highly suitable for the arylations.
Moreover, sterically demanding aryl bromides (8d, 8f, and 8j)
are also compatible, giving good yields. Importantly, all of the
compounds (8) are new except 8a, which was reported by Pappo
and co-workers with high yield with moderate ee. Notably, due
to difficulty in the chiral separation of products 8d, 8g, and 8j,
the ee was not determined.
a
b
Scale of reaction 0.1 mmol; isolated yields. 10 mmol scale.
c
Undetermined.
Scheme 3. Sequential Double-Fold Ortho and Remote
Borylation and Arylation
a
a
Conditions: (i) 2.0 mol % of Ir, 4.0 mol % of dtbpy, 4.0 equiv of
B2eg2, 4.0 equiv of Et3N, THF, 80 °C, 12 h; then 2.5 mol % of PdCl2·
dppf·CH2Cl2, 3.0 equiv of 3a, 8.0 equiv of Na2CO3, dioxane/DME/
H2O (1/1/1), 80 °C, 12 h; then short column filtration and dried; (ii)
3.0 equiv of HBpin, 5.0 mol % of Et3N, THF, 60 °C for 1 h, then 1.5
mol % of Ir, 3.0 mol % of dtbpy, 2.5 equiv of B2pin2, THF, 80 °C, 16
h; then 2.5 mol % of PdCl2·dppf·CH2Cl2, 3.0 equiv of 3a, 8.0 equiv of
Na2CO3, dioxane/DME/H2O (1/1/1), 80 °C, 12 h.
D
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M.; Hartmann, E.; Kuegel, W.; Parisienne-La Salle, J.-C.; Batsanov, A.
S.; Marder, T. B.; Snieckus, V. Chem. - Eur. J. 2010, 16, 8155.
(5) For recent reviews of functional-group-directed borylation, see:
(a) Ros, A.; Fernandez, R.; Lassaletta, J. M. Chem. Soc. Rev. 2014, 43,
3229. (b) Xu, L.; Wang, G.; Zhang, S.; Wang, H.; Wang, L.; Liu, L.; Jiao,
J.; Li, P. Tetrahedron 2017, 73, 7123.
(6) For an application of remote borylation towards total synthesis,
see: Feng, Y.; Holte, D.; Zoller, J.; Umemiya, S.; Simke, L. R.; Baran, P.
S. J. Am. Chem. Soc. 2015, 137, 10160.
Finally, further application of this method for 4-fold C−H
activation/borylation followed by arylation was tested sequen-
tially with (R)-BINOL. Thus, (R)-BINOL was subjected under
two different sets of conditions (for example, ortho and remote),
which afforded the complex tetra-arylated BINOL21 in good
yield (Scheme 3). This example showcases the efficiency of this
method for the development of new ligand and catalyst design in
organocatalysis for asymmetric synthesis.
In conclusion, a double-fold ortho and remote C−H bond
activation and borylation of BINOL has been developed. While
ortho borylation is solely controlled by an electrostatically
directed mechanism, remote borylation is controlled by a steric
effect. The developed strategy was successfully combined with
cross-coupling as one-pot way for the rapid synthesis of a variety
of useful chiral 3,3′-disubstituted and 6,6′-disubstituted BINOL
derivatives, which overcomes the previous shortcomings.
(7) For noncovalent interaction in remote borylation, see: (a) Bisht,
R.; Chattopadhyay, B. J. Am. Chem. Soc. 2016, 138, 84. (b) Davis, H. J.;
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B. J. Am. Chem. Soc. 2017, 139, 7745. (f) Bisht, R.; Hoque, M. E.;
Chattopadhyay, B. Angew. Chem., Int. Ed. 2018, 57, 15762. (g) Haldar,
C.; Hoque, E.; Bisht, R.; Chattopadhyay, B. Tetrahedron Lett. 2018, 59,
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
(9) For selected reviews on chiral phosphoric acid, see: (a) Akiyama,
T. Chem. Rev. 2007, 107, 5744. (b) Terada, M. Chem. Commun. 2008,
4097. (c) Yu, J.; Shi, F.; Gong, L.-Z. Acc. Chem. Res. 2011, 44, 1156.
(d) Rueping, M.; Kuenkel, A.; Atodiresei, I. Chem. Soc. Rev. 2011, 40,
ACS Publications website at DOI: 10.1021/acs.orglett.9b02347.
1
Experimental details, spectral data, copies of H and
13CNMR spectra, and HPLC charts (PDF)
̌
́
4539. (e) Coric, I.; Vellalath, S.; Muller, S.; Cheng, X.; List, B. Top.
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Organomet. Chem. 2012, 44, 165. (f) Parmar, D.; Sugiono, E.; Raja, S.;
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AUTHOR INFORMATION
Corresponding Author
■
(10) (a) Nakashima, D.; Yamamoto, H. J. Am. Chem. Soc. 2006, 128,
9626. (b) Rueping, M.; Nachtsheim, B. J.; Koenigs, R. M.; Ieawsuwan,
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*E-mail: buddhadeb.c@cbmr.res.in, buddhachem12@gmail.
com.
ORCID
̈
Leconte, N.; Neudcŗ fl, J.-M.; Schafer, M. Eur. J. Org. Chem. 2010, 2010,
5165. (d) Treskow, M.; Neudorfl, J.; Giernoth, R. Eur. J. Org. Chem.
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2013, 52, 4312. (f) Hashimoto, T.; Maruoka, K. Chem. Rev. 2007, 107,
5656.
Buddhadeb Chattopadhyay: 0000-0001-8473-2695
Author Contributions
§R.B. and J.C. contributed equally.
(11) Cox, P. J.; Wang, W.; Snieckus, V. Tetrahedron Lett. 1992, 33,
2253.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(14) Ahmed, I.; Clark, D. A. Org. Lett. 2014, 16, 4332.
(15) Yang, J.-F.; Wang, R.-H.; Wang, Y.-X.; Yao, W.-W.; Liu, Q.-S.; Ye,
M. Angew. Chem., Int. Ed. 2016, 55, 14116.
(16) For early reports, see: (a) Hocke, H.; Uozumi, Y. Tetrahedron
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(17) Chattopadhyay, B.; Dannatt, J. E.; Andujar-De Sanctis, I. L.;
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(19) Chiral BINOL and H8−BINOL-based catalysts both are highly
important; H8−BINOL possesses many additional features over simple
BINOL such as, solubility, acidities, and racemization profiles as well as
geometries and bite angles. For selected reviews, see: (a) Akiyama, T.
Chem. Rev. 2007, 107, 5744. (b) Doyle, A. G.; Jacobsen, E. N. Chem.
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B.C. thanks DST-SERB, New Delhi, for a DST-SERB CRG
grant (CRG/2018/000133) and a Ramanujan Grant (SB/S2/
RJN-45/2013). R.B. thanks CSIR, New Delhi, for the SRF, and
J.C. thanks UGC for the SRF. We are thankful to the teams at the
NMR, HRMS and HPLC facilities of CBMR. We thank the
Director, CBMR, for research facilities.
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DOI: 10.1021/acs.orglett.9b02347
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