NHC-coordinated palladacycle catalyzed 1,2-addition of arylboronates to unactivated ketones

Article abstract of DOI:10.1080/00397911.2019.1594307

Palladium catalyzed intermolecular 1,2-addition of arylboronate to unactivated ketone was investigated. NHC-coordinated palladacycle 4c exhibited catalytic activity for the reactions and provided the corresponding tertiary alcohols and γ,γ-disubstituted γ-lactones in good to excellent yields.

Full text of DOI:10.1080/00397911.2019.1594307

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
NHC-coordinated palladacycle catalyzed 1,2-
addition of arylboronates to unactivated ketones
Ryo Akiyama, Mariko Sugaya, Hiraku Shinozaki & Tetsuya Yamamoto
To cite this article: Ryo Akiyama, Mariko Sugaya, Hiraku Shinozaki & Tetsuya Yamamoto (2019):
NHC-coordinated palladacycle catalyzed 1,2-addition of arylboronates to unactivated ketones,
Synthetic Communications, DOI: 10.1080/00397911.2019.1594307
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https://doi.org/10.1080/00397911.2019.1594307
NHC-coordinated palladacycle catalyzed 1,2-addition of
arylboronates to unactivated ketones
a
a
b
b
Ryo Akiyama , Mariko Sugaya , Hiraku Shinozaki , and Tetsuya Yamamoto
a
Department of Materials Science and Engineering, Graduate School of Engineering, Tokyo Denki
b
University, Adachi-ku, Tokyo, Japan; Department of Applied Chemistry, School of Engineering, Tokyo
Denki University, Adachi-ku, Tokyo, Japan
ABSTRACT
ARTICLE HISTORY
Palladium catalyzed intermolecular 1,2-addition of arylboronate to
unactivated ketone was investigated. NHC-coordinated palladacycle
Received 5 February 2019
4
c exhibited catalytic activity for the reactions and provided the cor-
KEYWORDS
responding tertiary alcohols and c,c-disubstituted c-lactones in good
to excellent yields.
1
,2-Addition; arylboronate;
palladacycle; palladium
catalysis; unacti-
vated ketone
GRAPHICAL ABSTRACT
Introduction
[
1]
[2–4]
[5]
[6]
[7]
In the past two decades, the transition metal such as Rh, Pd,
Ni, Ru, Pt,
catalyzed 1,2-addition of organoboron compounds to car-
bonyl compounds such as aldehydes or ketones have been developed and is the attract-
[
8]
[9]
[10]
[11]
Co, Cu, Fe
or Ir
[
12]
ive synthetic methods as an alternative to the Grignard reaction.
Because their
catalytic 1,2-addition using organoboron compounds as nucleophilic reagents is tolerant
to a more wide range of functional groups than using organomagnesium reagents. So
but the reaction with unactivated ketones is limited to Rh or Ni catalysts. Previously,
we and Hu independently reported phosphine coordinated organopalladium complexes
such as phosphapalladacycles are suitable catalysts for the 1,2-addition of arylboronic
[
3i]
acids to aldehydes and activated ketones.
Although N-heterocyclic carbene (NHC)
ligands show the electron-donation equal to or more effective than phosphine ligands
[
13]
for transition metal catalyst,
catalysis have not been almost examined.
sis of imidazoline-type NHC coordinated palladacycles and clarified their catalysis for
NHC coordinated palladacycles are rare and their
[
4,14]
Thus, we demonstrated the facile synthe-
CONTACT Tetsuya Yamamoto
t-yamamoto@mail.dendai.ac.jp
Department of Applied Chemistry, School of
Engineering, Tokyo Denki University, 5 Senju Asahi-Cho, Adachi-ku, Tokyo 120-8551, Japan.
ß 2019 Taylor & Francis Group, LLC

2
R. AKIYAMA ET AL.
Scheme 1. Expansion of the substrate scope on the NHC-coordinated palladacycle catalyzed
,2-addition.
1
[
4]
1
,2-addition of arylboronic acids with aldehyde and activated hemiacetals. Herein, we
report the first examples of the Pd-catalyzed intermolecular 1,2-addition of arylboro-
nates to unactivated ketones using NHC coordinated palladacycles as catalyst
(Scheme 1). Additionally, we applied the catalytic system to the c,c-disubstituted c-lac-
tones synthesis.
Results and discussion
The optimization of reaction conditions of Pd-catalyzed 1,2-addition of 5,5-dimethyl-2-
phenyl-1,3,2-dioxaborinane 2a to benzophenone 1a is summarized in Table 1.
Phenylthio-substituted NHC coordinated complex 4c provided the corresponding alco-
hol in excellent yields (entry 1). The use of 4a and 4b instead of 4c resulted in lower
yields even 10 mol% catalyst loading (entries 2 and 3). Previously, we and Kuriyama
and coworkers independently reported the combination of imidazolinium salts 5 and
3
[
PdCl(g -allyl)] exhibited high activity for the addition of arylboronic acid to alde-
2
[
2]
hydes. However, these catalyst systems are not effective in this reaction (entries 4 and
). By the use of Cs CO , Na CO and K PO the corresponding product 3a was
5
2
3
2
3
3
4
obtained in less yields (entries 6–8). Phenylboronic acid afforded no product in this
addition (entry 9). The yield was declined in ether solvents such as 1,4-dioxane and 1,2-
dimethoxyethylene (entries 10 and 11).
The scope and limitation of the 4c-catalyzed 1,2-addition of phenylboronate 2a to
unactivated ketones are listed in Table 2. Acyclic type alkylarylketones such as 2-acetyl-
naphthalene 1b provided the corresponding alcohols in excellent yields under less than
1
mol% Pd catalyst loadings and reacted more easily than diarylketone 1a (entry 1).
Although acyclic type dialkylketones such as 4-phenyl-2-butanone 1c needed 3 mol%
catalyst loading in this phenylation, the reaction of cyclic type dialkylketones such as
cycloheptanone 1d, 1,4-dioxaspiro[4.5]decan-8-one 1e and tert-butyl 4-oxopiperidine-1-
carboxylate 1f proceeded smoothly in excellent yields (entries 3–6). Methyl 4-oxo-penta-
noate 1g as one of acyclic type dialkylketones needed 5 mol% catalyst loading in this
addition and was transformed to the corresponding c,c-disubstituted c-lactone 3g in

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Table 1. Optimization of Pd-catalyzed 1,2-addition of 5,5-dimethyl-2-phenyl-1,3,2-dioxaborinane 1a
to benzophenone 2a.
a
Entry
Catalyst
Base
Solvent
Yield (%)
1
2
3
4
5
6
7
8
2.5 mol% 4c (5 mol% Pd)
5 mol% 4a (10 mol% Pd)
K
K
K
K
K
Cs
Na
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
90
55
78
2
4
4
5 mol% 4b (10 mol% Pd)
3
5 mol% [PdCl(g -allyl)]
2
and 10 mol% 5a
and 10 mol% 5b
3
5 mol% [PdCl(g -allyl)]
2
2.5 mol% 4c (5 mol% Pd)
2.5 mol% 4c (5 mol% Pd)
2.5 mol% 4c (5 mol% Pd)
2.5 mol% 4c (5 mol% Pd)
2.5 mol% 4c (5 mol% Pd)
2.5 mol% 4c (5 mol% Pd)
2
CO
CO
PO
3
2
3
3
3
2
2
2
4
32
b
c
9
CO
3
CO
3
CO
3
NO
1
1
a
0
1,4-dioxane
1,2-dimethoxyethylene
60
70
1
Isolated yields.
b
Phenylboronic acid was used instead of 2a.
c
Not obtained.
moderate yield. In contrast, methyl 2-acetylbenzoate 1h was not reacted in this condi-
tion (entries 7 and 8).
The scope and limitation of the 4c-catalyzed arylation of tert-butyl 4-oxopiperidine-1-
carboxylate 1f and methyl 4-oxo-pentanoate 1g are summarized in Table 3. Electron-
rich arylboronic acid such as methoxyphenyl boronates were converted to the
corresponding products in excellent yields (entries 1 and 7). The reaction of 2-methyl-
phenylboronate 2c was a little difficult due to more sterically bulky and less electron-
rich than 2-methoxyphenylboronate 2b (entries 2 vs. 3). Weakly electron withdrawing
groups such as fluoro atom substituted arylboronate reacted smoothly to give the
desired alcohols in excellent yields under 1 mol% Pd catalyst loadings, while strongly
electron withdrawing group such as trifluoromethyl group substituted arylboronate 3n
required 6 mol% of catalyst (entries 4 and 5). Failed substrate was 4-bromophenylboro-
nate 2g afforded a complex mixture of products (entry 6).
Conclusion
In conclusion, we found the imidazoline-type NHC coordinated palladacycle 4c has
remarkable catalytic activity for the addition of arylboronates to unactivated ketones.

4
R. AKIYAMA ET AL.
Table 2. The scope and limitations of the 4c-catalyzed 1,2-addition of 2a to
unactivated ketones.
a
Entry
1
4c (mol%)
Ketone 1
Product 3
Yield (%)
0.5
3b
93
2
1.5
3c
93
3
4
0.5
3d
3e
70
97
0.05
5
0.05
2.5
3f
95
66
b
6
3g
b
c
7
2.5
3h
N.R.
a
Isolated yields.
The reactions were stirred for 24 h.
b
c
No reaction.
Further examinations of asymmetric catalytic reaction using chiral NHC palladium
complexes are underway in our laboratory.
Experimental
All reactions were carried out under an argon atmosphere. TLC was performed on alu-
minum silica gel 60 F254 (Merck) sheets, which were visualized by the quenching of

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Table 3. The scope and limitation of the 4c-catalyzed 1,2-addition of arylboronates to
1
f or 1g.
a
Entry
1
4c
Ketone 1
Arylboronate 2
Product 3
Yield (%)
(
mol%)
0.05
1e
3j
96
2
0.05
1e
3k
96
3
4
5
0.1
3
1e
1e
1e
3l
76
62
91
3m
3n
0.5
b
6
7
1.0
2.5
1e
1j
3o
3p
C.M.
96
a
Isolated yields.
Complex mixture.
b
UV fluorescence (254 nm). Column chromatography was conducted on silica gel (Cica,
1
13
19
6
0–210 mesh, spherical, neutral). H, C and F spectra were recorded on a Brucker
DPX-300 (300.13 MHz) or AVANCE III 400 spectrometer (400.13 MHz) at ambient
1
temperature. The chemical shifts of H were reported in delta (d) units, parts per mil-
13
lion (ppm) downfield from tetramethylsilane (0.0 ppm). Chemical shifts of C were
reported in delta (d) units, ppm relative to the center of the triplet at 77.0 ppm for
CDCl . Commercially available organic and inorganic compounds were used without
3

6
R. AKIYAMA ET AL.
[
4b]
[15]
[2a]
purification. Palladacycles (4a–c),
imidazolinium chloride 5a
and 5b
were pre-
pared according to the literature procedures.
General procedure for palladium catalyzed 1,2-addition of arylboronate 2
to ketone 1
Ketone 1 (0.5 mmol), arylboronate 2 (1.0 mmol), 4c (0.00025–0.0125 mmol/Pd
0
.0005–0.025 mmol) and K CO (207 mg, 1.5 mmol) were charged in a 10 mL screw-top
2 3
test tube sealed with a rubber septum. The test tube was evacuated and backfilled with
argon. This sequence was repeated five times. Then toluene (1.0 mL) was added via the
rubber septum with syringe. In an argon flow, the rubber septum was replaced with a
Teflon liner screw cap. The sealed test tube was placed into an oil bath preheated at
ꢀ
1
20 C. After the reaction mixture was stirred for 18 h, the reaction mixture was cooled
to room temperature. The obtained crude was purified by passing it through a silica gel
column with a hexane/ethyl acetate.
Triphenylmethanol (3a)
Benzophenone 1a (91.1 mg, 0.5 mmol), 5,5-dimethyl-2-phenyl-1,3,2-dioxaborinane 2a
(
190 mg, 1.0 mmol), 4c (13.9 mg, 0.0125 mmol [0.025 mmol Pd]) and K CO (207 mg,
2 3
1
.5 mmol) were used. The crude product was purified by column chromatography (silica
ꢀ
gel, hexane/EtOAc 4:1) to afford 3a (117 mg, 90%) as a white solid. mp 161–163 C [lit.
1
ꢀ
1
13
61–165 C]. H NMR (400 MHz, CDCl , ppm): d 7.33–7.24 (m, 15H), 2.81 (s, 1H).
C
3
NMR (100 MHz, CDCl , ppm) d 146.9, 127.9, 127.9, 127.3, 82.0.
3
Funding
This work was supported in part by the Research Institute for Science and Technology, Tokyo
Denki University Grant Number Q16E-02.
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