N,B-bidentate boryl ligand-supported iridium catalyst for efficient functional-group-directed C-H borylation

Article abstract of DOI:10.1021/jacs.6b11867

Convenient silylborane precursors for introducing N,B-bidentate boryl ligands onto transition metals were designed, prepared, and employed in ready formation of irdium(IIl) complexes via Si-B oxidative addition. A practical, efficient catalytic ortho-borylation reaction of arenes with a broad range of directing groups was developed using an in situ generated catalyst from the silylborane preligand 3c and [IrCl(COD)]₂.

Full text of DOI:10.1021/jacs.6b11867

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Communication
N,B-Bidentate Boryl Ligand-Supported Iridium Catalyst
for Efficient Functional Group-Directed C-H Borylation
Guanghui Wang, Li Liu, Hong Wang, You-Song Ding, Jing Zhou, Shuai Mao, and Pengfei Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b11867 • Publication Date (Web): 19 Dec 2016
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N,B-Bidentate Boryl Ligand-Supported Iridium Catalyst for Ef-
ficient Functional Group-Directed C-H Borylation
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Guanghui Wang,† Li Liu,† Hong Wang,† YouꢀSong Ding,† Jing Zhou,† Shuai Mao,§ Pengfei Li*†‡
†Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, 710054, China
‡State Key Laboratory of ElementoꢀOrganic Chemistry, Nankai University, Tianjin, 300071, China
§Department of Medicinal Chemistry, School of Pharmacy, Xi’an Jiaotong University, Xi’an, 710061, China
Supporting Information Placeholder
ABSTRACT: Convenient silylborane precursors for introducing
Scheme 1. Design of N,B-Bidentate Ligands for Directed
N,Bꢀbidentate boryl ligands onto transitionꢀmetals were designed,
Borylation
prepared and employed in ready formation of irdium(III)
complexes via SiꢀB oxidative addition. A practical, efficient
catalytic orthoꢀborylation reaction of arenes with a broad range of
directing groups was developed using in situ generated catalyst
from the silylborane preligand 3c and [IrCl(COD)]2.
Boryl ligands, formally with a negatively charged sp2ꢀhybridized
boron as the coordinating atom, are isoelectronic with carbene
ligands.1 In sharp contrast to the broad applications of carbene
ligands, boryl ligands have been rarely used as supporting ligands
in transitionꢀmetal catalysis. The utilization of metalꢀboryl cataꢀ
lysts have been challenging because 1) methods for introducing
boryls onto metals in a convenient and reliable way are limited,
and 2) the resulting BꢀM bonds are highly reactive, often leading
to loss of the boryl groups in following steps.
In order to overcome the problems, XꢀBꢀX type pincer boryl
ligands with wellꢀdefined rigid framework have been reported.2
Organoboron compounds are versatile intermediates in synꢀ
thetic chemistry and have been extensively used in syntheses of
drugs, agrochemicals and organic materials.6 To facilitate the
preparation of arylboron compounds, catalytic CꢀH borylation
reactions have received significant research interests and found
broad applications.7 In particular, several approaches for catalytic
ortho CꢀH borylation have been developed using catalysts based
on Rh, Ir, Pd, Ru etc.8 Among them, two iridiumꢀbased catalytic
systems have been effective for substrates with a broad range of
directing groups. Sawamura’s heterogeneous catalysts featured
silicaꢀsupported monophosphine ligands.8bꢀ8d Smith, Maleczka
and Krska used novel P,Siꢀ or N,Siꢀanionic ligand to support a
homogenous iridium catalyst.8p Mechanistically, both systems
might have created electronꢀrich irdium intermediates containing
two vacant coordination sites, one for the directing group and the
other for C–H cleavage (Scheme 1b).9 Inspired by these works
and our previous results,5 we envisioned that an N,Bꢀ bidentate
ligand might be employed for selective ortho borylation. We hyꢀ
pothesized that silylboranes of generic structure PL-2 (Scheme 1c)
might be suitable precursors of N,Bꢀligands due to three considerꢀ
ations. First, the SiꢀB bond may undergo oxidative addition with a
Indeed, efficient catalytic reactions have been reported based on
PꢀBꢀP ligandsꢀsupported transitionꢀmetal catalysts.3 In compariꢀ
son, bidentate boryl ligands may have more flexible coordination
sphere and still good stability as supporting ligands.4 However,
the application of bidentate boryls in catalysis had no precedence
prior to our recent work.5 We prepared a symmetric pyridineꢀ
tethered tetraaminodiborane(4) compound and used it as a conꢀ
venient precursor (PL-1, Scheme 1a) to simultaneously introduce
two N,Bꢀligands onto iridium via BꢀB oxidative addition. The in
situ generated catalyst by heating the precursor together with
[Ir(OMe)(cod)]2 was shown to be highly effective in borylation of
various (hetero)arenes. The regioselectivity was mainly governed
by steric hindrance.
To utilize bidentate boryl ligands in other reactions, however,
a method for selectively introducing a single N,Bꢀligand would be
needed.4g In this communication, we describe our efforts in design
and synthesis of a new type of readily accessible, air insensitive
and structurally tunable precursors of N,Bꢀbidentate ligands. Apꢀ
plying these preligands in catalysis culminated in a broadꢀscope
iridiumꢀcatalyzed functional groupꢀdirected CꢀH borylation reacꢀ
tion.
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lowꢀvalent transition metal. Second, the redundant silyl group
The chlorine ligand in iridium precursor also showed significant
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2
3
4
5
6
7
8
may be selectively removed by either reductive elimination or
ligand exchange.10 Third, the sterically demanding silyl group
might suppress introduction of two N,Bꢀligands on the metal,
while hydroboranes or diboranes have been known to form double
X,Bꢀligated complexes.4g,5
positive effect over the methoxy counterpart (entries 5 and 6 vs 4).
Finally, preligand 3c that incorporates 6ꢀmethoxy group on the
pyridine moiety, in combination with [IrCl(cod)]2 further imꢀ
proved both reactivity and selectivity. The orthoꢀborylation prodꢀ
ucts were formed in high conversion and yields, and excellent
selectivty (o/(m+p) > 99:1) (entry 7). [Ir(OMe)(cod)]2 again, when
combined with 3c, led to inferior results (entry 8).
Scheme 2. Preparation of Preligand 3 and Their Iridium
Complexes
Table 1. Catalyst Screening for ortho Borylationa
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Entry
Precatalyst
Conv.
(%)b
89
7
4
58
22
43
84
Yield
(%)b
Ratio of
o/(m+p)b
5 : 95
—
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2
3
4
5
6
7
8
3a/[Ir(OMe)(cod)]2
3a/[IrCl(cod)]2
complex 4a
3b/[Ir(OMe)(cod)]2
3b/ [IrCl(cod)]2
complex 4b
4 (6a)
—
—
—
36 (6a)+4 (6a‘)
19 (6a)+1 (6a‘)
35 (6a)+3 (6a‘)
70 (6a)+13 (6a‘)
7 (6a)
69 : 31
91 : 9
91 : 9
> 99 : 1
35 : 65
3c/[IrCl(cod)]2
3c/[Ir(OMe)(cod]2
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aReaction conditions: methyl benzoate 5a (0.5 mmol), B2pin2 (0.5 mmol.),
[Ir(X)(cod)]2 (0.005 mmol), preligand (0.01 mmol) or complex (0.01 mmol)
in 1.0 mL of n-hexane, 80 oC, 8 h. bConversions, yields and ratio were based
on 1H NMR analyses of the crude products with 5a as the limiting reagent.
Figure 1. Molecular structures of 4a and 4b (50% thermal ellipꢀ
soids; hydrogen atoms were omitted for clarity)
Encouraged by the preliminary results, we explored the subꢀ
strate scope of alkyl arenoates (Table 2). Under the above estabꢀ
lished conditions, the catalyst 3c/[IrCl(cod)]2 demonstrated generꢀ
ally high activity and excellent orthoꢀselectivity for a wide range
of substituted arenoates. The ratio of monoborylation vs diborylaꢀ
tion could be improved when bulkier ester groups are used (6aꢀ
6d). Thus, tertꢀbutyl benzoate cleanly afforded monoborylation
product 6d in good yield and no diborylation was observed. With
this method, borylated arenoates with various substituents were
readily prepared (6eꢀ6p). When isopropyl ester of adapalene was
used, a separable mixture of α and β monoꢀborylation products
was obtained in 87% yield (6q). Moreover, heterocyclic comꢀ
The syntheses of silylborane preligands and their iridium
complexes are shown in Scheme 2. Treatment of Nꢀ(2ꢀpyridyl)ꢀ
phenylenediamine (1a) with 1.3 equiv. of the known silylbornae
211 in toluene gave a new silylborane 3a (11B NMR: 32.1 ppm) in
78% isolated yield. To our delight, when a solution of 3a and 0.5
equiv. of [IrCl(cod)]2 in nꢀhexane was heated at 70 °C, complex
4a (11B NMR: 38.6 ppm) was quantitatively formed based on
NMR spectroscopic analysis and isolated in 85% yield. A single
crystal of 4a suitable for Xꢀray analysis was obtained from
CH2Cl2/nꢀhexane solution. Using the same method, we have preꢀ
pared silylboranes 3b (92%, 11B NMR: 35.1 ppm) and 3c (91%,
11B NMR: 33.7 ppm) featuring a 1,8ꢀnaphthyldiamine backbone.
An iridiumIII complex 4b based on 3b was also obtained in good
isolated yield. In 4a and 4b, the central iridiumIII atom was surꢀ
rounded by a N,Bꢀbidentate boryl, a silyl, a chloride and a 1,5ꢀ
cyclooctadiene (cod) ligand in a distorted octahedral framework.
The SiꢀIrꢀCl angles are smaller than 90o (88.9o in 4a and 85.1o in
4b), suggesting potentially facile SiꢀCl reductive elimination.
Unfortunately, the reaction of 3c with [IrCl(cod)]2 produced a
complicated mixture based on NMR spectroscopy.
pounds such as 2ꢀmethoxycarbonylbenzofuran (6r) or
benzothiophene (6s) could be borylated at the sterically hindered
3ꢀposition.
ꢀ
Next we evaluated arene substrates with different directing
groups (Table 3). Aryl acetates are readily available phenol derivꢀ
atives and challenging substrates in previous catalytic system.8d
Herein under standard conditions aryl acetates could be orthoꢀ
borylated in good efficiency probably via a sixꢀmembered iridaꢀ
cycle intermediate (8aꢀ8d). The oꢀboryl acetates were deacetylatꢀ
ed upon silica gel chromatography and the corresponding phenols
(8a’, 8b’) could be obtained in good yields. Aryl carbamates were
also viable substrates (8eꢀ8k). Compared to N,Nꢀ
dimethylcarbamate (8e, 47%), N,Nꢀdiethylcarbamate (8f, 78%)
showed higher reactivity. N,Nꢀdimethylbenzamide was moderateꢀ
ly reactive under the standard conditions (8l, 47%), but naphthaꢀ
leneꢀderived amides gave higher yields of the desired products
(8m, 8n). The competition between different directing groups
could lead to chemoselective borylation. Thus, aryl benzoates
were selectively borylated on the carbonyl side instead of the
phenol side (8o, 8p). Interestingly, borylation selectively (10:1)
took place at ortho position to the ester group instead of the amide
group (8q), contrasting the more prevailing directing effect obꢀ
We then sought to test iridiumꢀcatalyzed ortho CꢀH borylation
using methyl benzoate (5a) as the substrate and B2pin2 (1.0 equiv.)
as the borylating reagent. The effects of preligands, iridium preꢀ
cursors and preformed IrꢀB complexes were studied and selected
results are shown in Table 1. The combination of ligands and
metal salts was crucial. A high conversion (89%) was observed
when preligand 3a and [Ir(OMe)(cod)]2 were used as the catalyst.
However, the regioselectivity was poor and only trace amount of
orthoꢀborylated product was formed (entry 1). The combination of
3a with [IrCl(cod)]2 or preformed complex 4a showed low activiꢀ
ty in this reaction (entries 2 and 3). In comparison, preligand 3b
containing a 1,8ꢀnaphthyldiamine backbone displayed dramaticalꢀ
ly improved selectivity favoring orthoꢀborylation (entries 4ꢀ6).
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Journal of the American Chemical Society
served by Smith et al.8p Nitrogenꢀbased directing groups were also
effective in this system.8g Substrates containing a pyridine moiety
reacted at 40 oC and afforded monoꢀborylation products in moderꢀ
ate isolated yields (8rꢀ8t). Borylation of N,Nꢀ
dimethylbenzylamine using excess B2pin2 generated 8u in excelꢀ
lent yield. Oxime ethers and hydrazones derived from aldehydes
or a ketone all gave high yields of the corresponding borylation
products (8vꢀ8ad).
Table 3. Borylation with 3c/[IrCl(cod)]2 Catalysta
1
2
3
4
5
6
7
8
DG
DG
1-2 mol% [IrCl(cod)]2
3c
Bpin
2-4 mol%
1-2.5 equiv. B2pin2
R
8a 8ai
R
7
-
b
8a
X = H, , (72% ),
8e
X = H, R = Me, , 47%
8f
X = H, R = Et, , 78%
8g
X = 6-F, R = Et, , 91%
OCONR2
Bpin
OAc
8a'
, 68%
X = CF3, 8b, (94%b)
phenol
6
X
5
X
Bpin
Table 2. Scope of ortho-Borylation of Arenoatesa,b
8h
X = 6-Cl, R = Et, , 91%
8b'
phenol
, 88%
X = 5-CF3, R = Et, 8i, 70%
X = 4-Me, R = Et, 8j, 51%
8c
X = Me, , 83%
4
9
X = OMe, 8d, 75%
Me
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O
N
O
OCONEt2
Bpin
CONMe2
Bpin
CONMe2
Bpin
Bpin
Bpin
8k
8l
, 76%
, 47%
Bpin
8m, 68%
8n
, 91%
X
X
O
CO2Me
N
N
CONEt2
O
Bpin
Bpin
X
Bpin
c
8o
X = Me, , 89%
c
8s
X = C, , 58%
8r
8q
, 67%
, 67%
X = OMe, 8p, 78%
c
8t
X = O, , 68%
NMe2
NOMe
Bpin
X = H, Y = Bpin, 8v, 80%
8w
d
Bpin
Bpin
X = OMe, Y = Bpin,
, 94%
Y
8x
X = OMe, Y = H, , 65%
X = CF3, Y = H, 8y, 73%
X
8u
, 92%
OMe
R
N
N
R =
aSee SI for reaction conditions. bYields calculated with 5 as the limiting
Me
N
8ab
, di-, 90%
8ac
X = H,
X = 5-F,
X = 4-CF3,
reagent, NMR yields shown in parentheses; c5% 5-borylation product.
8z,
93%
Bpin
Bpin
, di-, 92%
, di-, 92%
Bpin
Bpin
N
O
R =
X
5
8ad
8aa,
81%
4
Furthermore, using 3c/[IrCl(cod)]2 as the catalyst in a prelimiꢀ
nary study, the sp3 CꢀH borylation of 2ꢀaminopyridine compounds
was achieved (Table 4).12 Nꢀadjacent primary and secondary alkyl
groups were selectively borylated under mild conditions, affordꢀ
ing the monoꢀborylation products in high yields (9aꢀ9c). Remarkꢀ
ably, the potentially more reactive phenyl group in 9a was not
borylated.
aSee SI for experimental details, isolated yields shown. bNMR yields. Reac-
c
tion temperature 40 oC. d2.7 equiv B2pin2 used
.
Table 4. C(sp3)-H Borylation using Preligand 3cab
Finally, to demonstrate the practicality of the borylation reacꢀ
tion in synthesis of orthoꢀfunctionalized arenes, 6e was prepared
in 90% isolated yield from methyl oꢀmethoxybenzoate (10 mmol)
using 0.35 mol% of [IrCl(cod)]2 and 0.7 mol% of 3c as the cataꢀ
lyst. Taking advantage of the versatility of CꢀB bond in chemical
transformations, a variety of orthoꢀfunctionalized benzoates could
be prepared in one step and good to excellent yields from 6e
without optimizations (see SI).
a
Reaction conditions: B2pin2 (1.0 equiv.), [IrCl(cod)]2 (2 mol%), 3c (4 mol%),
60 oC; bYields based on isolated materials and NMR yields shown in paren-
theses.
In conclusion, we have designed and prepared new pyridineꢀ
tethered silylboranes and demonstrated their utility as convenient
precursors for introducing a single N,Bꢀbidentate ligand onto
iridium via B−Si oxidative addition. The preligands are structuralꢀ
ly modifiable. Based on these preligands, we have developed a
highly effective and practical catalyst system for directed C(sp2)ꢀ
H and C(sp3)ꢀH borylation of a broad range of substrates.
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, spectral data of products and
Xꢀray crystallographic data for 4a and 4b. This material is availaꢀ
ble free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
lipengfei@mail.xjtu.edu.cn
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Notes
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The authors declare no competing financial interests.
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ACKNOWLEDGMENT
Financial supports are provided by NSFC (No. 21672168 and
21472146) and the Ministry of Science and Technology of PRC
(No. 2014CB548200). We are especially grateful to Prof. Yanꢀ
zhen Zheng for his help on Xꢀray crystallographic analyses.
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