Rh-catalyzed ortho -selective C-H borylation of N-functionalized arenes with silica-supported bridgehead monophosphine ligands

Article abstract of DOI:10.1021/ja208364a

Supported phosphine-Rh systems, prepared in situ from silica-supported bridgehead monophosphines and [Rh(OH)(cod)]₂, have enabled ortho-selective C-H borylation for a range of arenes containing nitrogen-based directing groups. The regioselectivity was excellent with various N-directing groups, including saturated and unsaturated N-heterocycles, tert-aminoalkyl groups, and imine-type C-N double bonds. The reaction showed significant tolerance toward steric repulsion around the reacting C-H bond. This Rh catalysis complements the Ir-catalyzed ortho-borylation, which is effective for arenes with oxygen-based directing groups.

Full text of DOI:10.1021/ja208364a

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pubs.acs.org/JACS
Rh-Catalyzed Ortho-Selective CꢀH Borylation of N-Functionalized
Arenes with Silica-Supported Bridgehead Monophosphine Ligands
Soichiro Kawamorita, Tatsuya Miyazaki, Hirohisa Ohmiya, Tomohiro Iwai, and Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
S Supporting Information
b
ABSTRACT: Supported phosphine-Rh systems, prepared
in situ from silica-supported bridgehead monophosphines
and [Rh(OH)(cod)]2, have enabled ortho-selective CꢀH
borylation for a range of arenes containing nitrogen-based
directing groups. The regioselectivity was excellent with
various N-directing groups, including saturated and unsatu-
rated N-heterocycles, tert-aminoalkyl groups, and imine-
type CꢀN double bonds. The reaction showed significant
tolerance toward steric repulsion around the reacting CꢀH
bond. This Rh catalysis complements the Ir-catalyzed ortho-
borylation, which is effective for arenes with oxygen-based
directing groups.
rylboronic acids and their derivatives are versatile synthetic
A
intermediates.1 Among the routes to arylboronic acid deri-
vatives, direct CꢀH borylation of arenes is the most straightfor-
ward and attractive.2ꢀ4 In fact, Ir-catalyzed borylations of arenes
with bis(pinacolato)diboron or pinacolatoborane have found
wide applications in many fields.1 While the original Ir-catalyzed
borylations showed regioselectivities controlled by steric effects
that directed the borylation to positions distal to a substituent,3
Ir-catalyzed ortho-selective borylations have also been developed:
Hartwig and co-workers introduced a new ortho-directing group,
a dialkylhydrosilyl group tethered to an aromatic ring, for Ir
catalysis with a 4,40-di-tert-butyl-2,20-bipyridine (dtbpy) ligand;5
Ishiyama, Miyaura, and co-workers described ortho-borylation of
benzoate derivatives catalyzed by the [3,5-(CF3)2-C6H3]3P-Ir
system;6 and our group developed a silica-supported catalyst
system that allowed ortho-borylation of a wide range of arenes
with different directing groups, such as CO2R, CONMe2,
SO3Me, CH[O(CH2)3O], CH2OMOM, and OCONEt2, as well
as a Cl atom.7 Despite these efforts, nitrogen-based functional
groups such as N-heterocycles and imino groups have not been
used as directing groups.8
Figure 1. Ligand effects in Rh-catalyzed ortho-selective CꢀH boryla-
tion of 2-phenylpyridine (1a) with bis(pinacolato)diboron 2. Condi-
tions: 1a, 1.0 mmol; 2, 0.5 mmol; [Rh(OH)(cod)]2 (Rh, 0.0025 mmol);
ligand (P, dtbpy, 0.0025 mmol); THF, 1.0 mL; rt, 1 h. Yields based on 2
were determined by GC analysis.
In addition, Rh catalysts have concomitant benzylic borylation
activity toward alkylarenes, which is predominant in some
cases.3b,4b,4c
Ligand effects for Rh-catalyzed borylation of 2-phenylpyridine
(1a) are summarized in Figure 1. With the immobilized catalyst
prepared in situ from Silica-SMAP (0.5 mol%) and [Rh(OH)-
(cod)]2 (0.5 mol% Rh) (P/Rh 1:1), which appeared to be the
most active system, reaction of 1a (1.0 mmol, 2 equiv relative
to 2) and bis(pinacolato)diboron (pinB-Bpin, 2, 0.5 mmol) in THF
proceeded smoothly at room temperature, and 2 was consumed
completely within 1 h to afford the corresponding ortho-boryla-
tion product 3a in 103% GC yield based on 2.13ꢀ16 Neither meta
nor para isomers were detected in the crude mixture. The
pyridine ring was not borylated, and no 20,60-diborylation
occurred. The yield in excess of 100% indicated that byproduct
pinB-H also functioned as a reagent, but its reactivity was much
lower than that of 2.17 To our surprise, ortho-borylation occurred
Herein, we report that immobilized phosphine-Rh systems
(P/Rh 1:1), prepared from silica-supported bridgehead mono-
phosphines (Silica-SMAP9,10 and Silica-TRIP,11,12 Figure 1) and
[Rh(OH)(cod)]2, have enabled ortho-selective CꢀH borylation
for a wide range of arenes containing different nitrogen-based
directing groups. Furthermore, the reaction showed significant
tolerance toward steric hindrance around the reacting CꢀH
bond. Although Rh-catalyzed arene borylations have been
described in the literature,4 they have not been widely applied
to organic synthesis because they need relatively harsh reac-
tion conditions compared with the Ir-dtbpy catalytic system.
Received: September 5, 2011
Published: November 02, 2011
r
2011 American Chemical Society
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Table 1. Silica-SMAP-Rh-Catalyzed Ortho-Selective CꢀH
Borylation of Arenes 1 Containing a Nitrogen-Based Func-
tional Group with Bis(pinacolato)diboron 2a
even in the absence of a phosphine ligand with exclusive
regioselectivity, but the yield was as low as 10% under otherwise
identical conditions.18 The supported triptycene-type bridge-
head triarylphosphine (Silica-TRIP hereafter) gave 3a in only
24% yield. The immobilized but unconstrained triphenylpho-
sphine-type ligand (Silica-TPP)19 was not effective (6% yield). In
contrast to these immobilized ligands, the corresponding homo-
geneous ligands, Ph-SMAP, L1, and PPh3, induced little or no
borylation activity, indicating the importance of phosphine
immobilization. Other homogeneous ligands with different steric
and electronic natures were also tested. Among those, only PMe3
and PCy3 showed some acceleration effects, as judged from the
product yields (21% and 35%, respectively), which are higher
than the yields obtained in the absence of a ligand.20 The
sterically demanding and electron-rich phosphine XPhos inhib-
ited the reaction, and no reaction occurred with PBu3, PtBu3, and
the N,N-chelating ligand dtbpy.
The Silica-SMAP-Rh catalyst system was applicable to various
derivatives of 1a with different substitution patterns. Furthermore, a
wide range of nitrogen-based functional groups appeared to be
suitable directing groups. The results are listed in Table 1. Substitu-
tion at the 30-position of 2-phenylpyridine with electron-donating
Me or MeO groups did not affect the reactivity of the CꢀH bond at
the para position (entries 1 and 2). In turn, an electron-withdrawing
CO2Me substituent slightly reduced the reactivity, but the reaction
proceeded smoothly at 40 °C (entry 3). A few features are to be
noted for the reactions of monosubstituted 2-phenylpyridines
1bꢀd: (1) borylation occurred predominantly (g95:5) at the less
hindered position ortho to the directing group; (2) the CO2Me
group did not deliver the boron atom to its ortho position, whereas it
was the strongest directing group in Ir-catalyzed ortho-borylation
with Silica-SMAP ligand; and (3) no benzylic borylation was
observed in the reaction of 1b despite using the Rh catalyst.3b,4b,4c
Remarkably, 30,50-dimethyl-2-phenylpyridine (1e) was a sui-
table substrate despite the considerable steric hindrance around
the reacting CꢀH bonds. The reaction proceeded smoothly
under mild heating conditions (Table 1, entry 4). In contrast,
almost no reaction occurred with PCy3 under the sameconditions
(data not shown). The borylation of benzo[h]quinoline (1f)
proceeded at room temperature in the same fashion (entry 5).
Unsaturated five-membered N-heterocycles featuring a metal-
coordinating sp2 nitrogen atom, such as imidazole, oxazoline, and
pyrazole, were also suitable directing groups, as shown in Table 1,
entries 6ꢀ9. Reaction of 1-methyl-2-phenylimidazole (1g) pro-
ceeded smoothly at room temperature (entry 6). Reaction of
2-phenyl-5,5-dimethyloxazoline (1h) proceeded cleanly at
40 °C, but the borylation product was prone to hydrolyze into
amide 3h0.21 A pyrazole ring was also a suitable directing group,
as shown in the successful conversion of 1i, despite having
potentially reactive heteroaromatic CꢀH bonds (entry 8).
Disubstitution at the 3- and 5-positions of the pyrazole resulted
in a significant decrease in reactivity, but borylation did occur at
higher temperature (85 °C) (entry 9). Combined with the results
from the substituted oxazoline derivative 1h, successful conver-
sion of the substituted pyrazole 1j is indicative of a significant
tolerance of the Silica-SMAP-Rh system toward sterically de-
manding directing groups.
a Conditions: 1, 1.0 mmol; 2, 0.5 mmol; [Rh(OH)(cod)]2 (Rh, 0.0025
mmol); Silica-SMAP (P, 0.0025 mmol); THF, 1.0 mL. b NMR yield
(isolated yield in parentheses). c Yield for a mixture of isomers: C60/C20
46:1 (entry 1) and 17:1 (entry 2). d 1.0 equiv of 1 was used. e [Rh-
(OMe)(cod)]2 and hexane were used in place of [Rh(OH)(cod)]2
and THF. f Yield of 3h determined by NMR. The borylation product was
isolated as amide 3h0 (isolated yield in parentheses). g Bis-ortho-boryla-
tion product was detected in the crude mixture: entry 7, 13%; entry 8,
9%; entry 9, 6%; entry 10, 4%; entry 15, 4%. h Crude product was a
mixture of 3l (83%), a pyrrazole ring borylation product (22%), and a
The pronounced efficacy of the Silica-SMAP-Rh system
enabled ortho-borylation of arenes with more flexible directing
groups with a distal nitrogen atom. In particular, 2-benzylpyridine (1k)
and 1-(3-methylbenzyl)pyrazole (1l) underwent borylation at room
temperature and 40 °C, respectively (Table 1, entries 10 and 11).
i
diborylation product (13%). Yield of 3o determined by NMR. The
borylation product was isolated as hydrochloride salt 3o0.
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Table 2. Rh-Catalyzed Ortho-Selective CꢀH Borylation of
Scheme 1. Complementary Use of Silica-SMAP-Rh and
Silica-SMAP-Ir Systems for Regioselective Borylation of
Bifunctional Arenes
Arenes 1 Containing an Imine-Type Functional Groupa
a Conditions: 1, 1.0 mmol; 2, 0.5 mmol; [Rh(OH)(cod)]2 (Rh, 0.0025
mmol); ligand (P, 0.0025 mmol); THF, 1.0 mL. b NMR yield (isolated
yield in parentheses). c Low yield of 3r is due to 2,6-diborylation (19%
GC yield) and substantial CdN reduction of 3r. d Diborylation was
detected in the crude mixture (19%). e 2,6-Diborylation and CdN
reduction of 3r were observed (1% and 5%, respectively).
derivative 1t and the cyclic acetal 1u by protecting the aldehyde
moiety of methyl 4-formylbenzoate in different ways. Borylation
of 1t with Silica-SMAP-Rh occurred at the position ortho to the
nitrogen-based directing group with exclusive regioselectivity to
afford the boronate 3t. Subsequent deprotection furnished the
3-borylated 4-formylbenzoate 4t. On the other hand, Ir-catalyzed
borylation of benzoate 1u with the acetal moiety occurred at the
position ortho to the ester substituent, again with complete
regioselectivity. The resulting arylboronate 3u was transformed
into the 2-borylated 4-formylbenzoate 4u. Regioselective synth-
esis of these isomeric arylboronates would be difficult with other
methods. This regioselectivity was enabled by the complemen-
tary catalytic natures of the supported Rh and Ir catalyst systems
combined with their broad substrate scopes.24
Although elucidating the mechanism of the supported phos-
phine-Rh catalysis demands further studies such as surface
analyses and kinetic studies, the critical importance of phosphine
immobilization strongly suggests that a rhodium complex that
binds only one phosphorus atom provides a vacant coordination
site for the nitrogen atom of the directing group. We previously
established that each P center of Silica-SMAP independently
binds to a Rh center to form a mono(phosphine)-Rh complex
selectively; i.e., two or more P centers are not capable of binding
to the same Rh center.9a,b In fact, the catalytic activity of Silica-
SMAP for borylation of 1a was maintained even when Silica-
SMAP was 2 equiv relative to Rh, while addition of 1 equiv of Ph-
SMAP to the Silica-SMAP-Rh (P/Rh 1:1) system severely
inhibited borylation (23% yield).
The superiority of the constrained phosphines, Silica-SMAP
and Silica-TRIP, over unconstrained PPh3-type immobilized
ligand Silica-TPP suggests that reactions of a Silica-TPP-Rh
complex encounter significant steric hindrance on the silica
surface. This assumption is reasonable because the degree of
freedom is higher in Silica-TPP than in the constrained ligands.
To shed light on the origins of the differences in the ligand effects
of Silica-SMAP and Silica-TRIP, more structureꢀactivity rela-
tionship studies must be performed.
In summary, supported bridgehead phosphine-Rh systems have
enabled ortho-selective CꢀH borylation of a range of arenes contain-
ing nitrogen-based directing groups.25ꢀ27 The regioselectivity was
excellent with various N-directing groups, and the reaction showed
significant tolerance toward steric hindrance around the reacting
CꢀH bond and in the directing groups. This Rh catalysis comple-
ments the substrate scope of the Ir-catalyzed ortho-borylation, which
is effective for arenes with oxygen-based directing groups.
The reaction of 1k with PCy3 afforded only a trace amount of 3k
(data not shown). It should be noted that these types of direct-
ing groups have rarely been employed in catalytic CꢀH
functionalizations.22
The scope of ortho-borylation has further been extended to
reaction of arenes with directing groups based on an sp3 nitrogen
atom (Table 1, entries 12ꢀ15). This type of directing group has
rarely been used in catalytic CꢀH functionalizations.23 Specifi-
cally, N,N-dimethylbenzylamine (1m,n) and N-benzylpyrrolidine
(1o) derivatives as well as 1,3-dimethyl-2-phenylimidazolidine
(1p) underwent borylation under mild conditions. Importantly,
the imidazolidine in 1p can function as a protecting group for
aldehydes (vide infra).
We also examined various imine-type functional groups for
their suitability as directing groups. It appeared in the initial
experiments that concomitant CdN reduction with pinB-H was
problematic, but the undesired reaction can be prevented by the
choice of N-substituents and/or ligands. The results are sum-
marized in Table 2. Specifically, N-mesityl acetophenone imine
(1q) was cleanly converted with Silica-SMAP ligand into the
corresponding arylboronate (3q, entry 1). Interestingly, we
found that borylation of imine 1q was more efficiently promoted
by the Rh complex with the triptycene-type ligand, Silica-TRIP,
than by the Silica-SMAP complex (entry 2). The soluble
phosphine PCy3 was also useful for this transformation, but with
lower catalytic efficacy (entry 3).
The oxime methyl ether in 1r also worked as a directing group
for ortho-borylation, but CdN reduction was significant when
Silica-SMAP was used as a ligand (Table 2, entry 4). To our
delight, the side reaction could be completely inhibited by the
use of Silica-TRIP without changing the high ortho-borylation
activity (entry 5). The use of PCy3 resulted in a low yield of 3r
and concomitant reduction of the CdN bonds of 1r and 3r
(entry 6). Borylation of 2-acetylnaphthalene oxime ether (1s)
with Silica-TRIP ligand occurred regioselectively at the C3-
position (entry 7).
Scheme 1 illustrates the synthetic utility of the complementary
catalytic activities of Silica-SMAP-Rh and previously described
Silica-SMAP-Ir systems.7 Thus, we prepared the imidazolidine
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COMMUNICATION
’ ASSOCIATED CONTENT
5381–5383. (b) Hamasaka, G.; Kawamorita, S.; Ochida, A.; Akiyama, R.;
Hara, K.; Fukuoka, A.; Asakura, K.; Chun, W. J.; Ohmiya, H.; Sawamura, M.
Organometallics 2008, 27, 6495–6506. (c) Kawamorita, S.; Hamasaka, G.;
Ohmiya, H.; Hara, K.; Fukuoka, A.; Sawamura, M. Org. Lett. 2008, 10,
4697–4700. (d) Kawamorita, S.; Ohmiya, H.; Iwai, T.; Sawamura, M.
Angew. Chem., Int. Ed. 2011, 50, 8363–8366.
S
Supporting Information. Experimental details and char-
b
acterization data for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
(11) Synthesis and properties of Silica-TRIP: Miyazaki, T.; Kawamor-
ita, S.; Ohmiya, H.; Sawamura, M. Presented at the 91st Annual Meeting of
the Chemical Society of Japan, Kanagawa, March 26ꢀ29, 2011, 4C9-13.
Details will be reported elsewhere.
(12) Synthesis of 9-phospha-10-silatriptycene derivatives was re-
ported by Tsuji, Tamao, and co-workers, while compound L1 and the
immobilization have not yet been described: Tsuji, H.; Inoue, T.;
Kaneta, Y.; Sase, S.; Kawachi, A.; Tamao, K. Organometallics 2006, 25,
6142–6148.
(13) Other transition metal catalysts based on Ir, Pd, and Pt were not
effective under these conditions. See Supporting Information for the
screening of metal sources.
(14) Silica-SMAP-Rh catalyst was easily separated from the products by
filtration through Celite. Attempts to reuse the catalyst were unsuccessful.
(15) With 1 equiv of 1a, borylation under otherwise the same
conditions afforded 3a (97%) and 20,60-diborylation product (3%).
(16) After the reaction, 1 equiv of pinB-H was detected in the crude
mixture by 1H NMR.
(17) Borylation of 1a with pinB-H proceeded at 60 °C (12 h, 71%).
(18) Reaction under phosphine-free conditions was complete in
12 h to afford 3a in 106% GC yield. Thus, [Rh(OH)(cod)]2 itself is an
effective catalyst precursor for borylation of 1a. However, the substrate
scope of the phosphine-free Rh catalysis appeared to be quite narrow.
See Supporting Information for the reaction of various substrates with
the phosphine-free Rh catalyst system.
(19) See Supporting Information for the synthesis of Silica-TPP.
Details will be reported elsewhere.
(20) See Supporting Information for the reaction of various sub-
strates with the Rh-PCy3 catalyst system.
’ AUTHOR INFORMATION
Corresponding Author
sawamura@sci.hokudai.ac.jp
’ ACKNOWLEDGMENT
This work was supported by Grants-in-Aid for Scientific
Research on Innovative Areas “Organic Synthesis Based on
Reaction Integration” and a Global COE grant (Project No.
B01: Catalysis as the Basis for Innovation in Materials Science),
MEXT. S.K. thanks JSPS for scholarship support.
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