Synthesis of some ketones via nano-nickel oxide catalyzed acylation of arylzinc reagents; strategy involving the use of mixed (methyl)(aryl)zincs

Article abstract of DOI:10.3906/kim-1712-28

Nano-NiO catalyzed acylation of mixed (methyl)(aryl)zincs with aromatic acyl halides in THF at room temperature provides a new facile route for aryl–aroyl coupling. Among NiCl2 .L2 and NiCl2 .L (L = monodentate and bidentate phosphine ligand) catalysts, the lower catalyst loading of NiCl2 (dppf) may seem attractive; however, nano-NiO, being the lowest cost catalyst, is more favorable for aroylation of (methyl)(aryl)zincs. This procedure also provides a supplement to Cu and Pd catalyzed acylation of diorganozincs.

Full text of DOI:10.3906/kim-1712-28

Turk J Chem
Turkish Journal of Chemistry
(2018) 42: 759 – 767
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http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1712-28
Research Article
Synthesis of some ketones via nano-nickel oxide catalyzed acylation of arylzinc
reagents; strategy involving the use of mixed (methyl)(aryl)zincs
∗
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Ozgen OMUR PEKEL , Ender ERDIK, Melike KALKAN
¨
¨
˙
Department of Chemistry, Faculty of Science, Ankara University, Ankara, Turkey
Received: 12.12.2017
•
Accepted/Published Online: 21.02.2018
•
Final Version: 01.06.2018
Abstract: Nano-NiO catalyzed acylation of mixed (methyl)(aryl)zincs with aromatic acyl halides in THF at room
temperature provides a new facile route for aryl–aroyl coupling. Among NiCl2 .L2 and NiCl2 .L (L = monodentate
and bidentate phosphine ligand) catalysts, the lower catalyst loading of NiCl2 (dppf) may seem attractive; however,
nano-NiO, being the lowest cost catalyst, is more favorable for aroylation of (methyl)(aryl)zincs. This procedure also
provides a supplement to Cu and Pd catalyzed acylation of diorganozincs.
Key words: Mixed diorganozincs, acylation, nickel catalyst, group selectivity
1. Introduction
Aromatic ketones have attracted much attention because of their various synthetic and biological applications.
Ketones are prepared by using either direct coupling of acyl chlorides with organometallic compounds1−3 or
using Friedel–Crafts acylation.4 Chemoselective conversion of ketones to functional groups1,2,4 has advanced
rapidly in recent years leading to synthesis of pharmaceutical and material science products. They are also
important for forming chemoselective stable linkages with functionalized fragments in vitro and in vivo.5−7
In recent years, a large number of useful methodologies have been developed for the acylation of mostly
Grignard,8−15 organocopper,16−18 and -zinc19−21 compounds. Organolithium,22 -bor,23−25 -aluminium,26
-
tin,27−29 -bismuth,30,31 and -manganese32 compounds have been also reported to furnish ketones with acyl
halides.
In the acylation process, generally transition metal catalysis and/or organic catalysis is used. Grignard
reagents, RMgX, have some disadvantages such as formation of tert-alcohols as side products. However,
diorganozinc, R2 Zn, and monoorganozinc, RZnX, reagents, due to their high tolerance to many functional
groups and high efficiency toward many electrophiles, are now widely used in organometallic synthesis. In the
acylation of organozinc reagents, initially the use of Cu and Pd catalysis was reported.12,33 However, the high
cost of Pd catalysts34,35 led to advances in the use of Co,36,37 Fe,36 and Ni38,39 as catalysts in the acylation
of RZnX34,35,37,39 and R2 Zn36,38 reagents.
Diorganozincs, R2 Zn, which have higher reactivity toward many electrophiles compared to monoorganoz-
inc halides, RZnX, are not atom-economic due to their transfer of only one of the R groups. For this purpose,
mixed diorganozincs, R1 R2 Zn, in which R1 and R2 groups react with different transfer yields have been
developed.40−54 Recently, RR RT Zn reagents,55,56 which have a transferable RT group together with a resid-
∗Correspondence: oomur@science.ankara.edu.tr
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ual group, RR group, have been successfully used in 1,2-addition,55−58 1,4-addition,59−62 and C–C coupling
reactions.63−65
Our research group has a long standing interest in research of synthetic use of mixed diorganozincs.66−75
We have already investigated alkyl group transfer of (n-alkyl)(aryl)zincs reagents to aroyl chlorides and reported
successful transition metal free coupling in THF in the presence of tri-n-butylphosphine.66 We also found that
group selectivity also depends on the donor solvent and in the CuI catalyzed acylation aryl group transfer
takes place in THF:TMEDA (2:1). In this project, we planned to develop a new atom-economic protocol for
aryl–aroyl coupling of (n-alkyl)(aryl)zincs using low cost and easily available nickel catalysis.
Herein, we report the use of nano-NiO catalysis in the convenient and efficient aryl selective aroylation
of (n-alkyl)(aryl)zincs.
2. Results and discussion
Based on our previous experience with acylation of mixed diorganozincs,66,68,70,71 first we decided to screen the
suitability of nickel catalysts in the aryl–aroyl coupling of (n-alkyl)(aryl)zincs with benzoyl chloride. We chose
reaction of (n-Bu)(Ph)Zn 1ab with benzoyl chloride in THF as the model reaction (Scheme). As nickel catalysts,
we analyzed extensively used NiCl2 and NiCl2 .L2 (L = ligand) to find the efficient catalytic conditions on the
group selectivity of (n-Bu)(Ph)Zn 1ab (Table 1). As ligands, we chose monodentate phosphine ligands, L =
Ph3 P, (c-Hexyl)3 P and a bidentate phosphine ligand, L = dppf (1,2-bisdiphenylphosphino ferrocene), which
we recently found as active ligands in the NiCl2 .L2 catalyzed aryl–allyl coupling of (n-butyl)(aryl)zincs.69
We also investigated the catalytic activity of nano-NiO in the acylation reaction. For this purpose, we chose
nano-NiO (<50 nm) as the first example.
Scheme. Group selective coupling of (n-butyl)(phenyl)zinc 1ab with benzoyl chloride 2a in the presence of nickel
catalysts.
In the optimized catalyst concentrations, NiCl2 performed poorly (Table 1, entry 2). Addition of LiCl
resulted in a better yield of 49% with a n-Bu transfer:Ph transfer ratio of 16:84 (Table 1, entry 3). NiCl2
with Ph3 P and (c-Hexyl)3 P ligands gave coupling with 44% and 57% yields, respectively, and with low Ph
selectivity (Table 1, entries 4 and 5). However, gratifyingly, using NiCl2 (dppf) as a catalyst in the catalyst
loading of 1-0.5 mol % increased the yield to 68%–77% and seemed promising for Ph selectivity resulting in a
n-Bu transfer:Ph transfer ratio of 12:88 (Table 1, entries 6 and 7). Coupling in the presence of nano-NiO was
also quite successful (Table 1, entries 8–10). Optimized 10 mol % catalyst afforded 71% coupling and n-Bu
transfer:Ph transfer ratio of 8:92 (Table 1, entry 9). Addition of LiCl had no effect (Table 1, entry 10). In this
work, we did not investigate the effect of particle size of nano-NiO on coupling yield.
It seems that NiCl2 (dppf) and nano-NiO are the best nickel catalysts for Ph selective coupling of (n-
Bu)(Ph)Zn 1ab with benzoyl chloride 2. Before deciding between NiCl2 (dppf) or nano NiO as the most active
catalyst in the benzoyl coupling of (n-butyl)(phenyl)zinc 1ab, we compared the atom economic utility of 1ab
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Table 1. Screening nickel catalysts in the coupling reaction of (n-butyl)(phenyl)zinc 1ab with benzoyl chloride 2.
Optimization of the reaction parameters.a
b,c
d
Entry Nickel catalyst/additive
Total yield, %
Group selectivity 3a:3b
1
2
3
4
5
6
7
8
9
10
-
20
22
49
44
57
68
77
66
71
50:50
27:73
16:84
36:64
39:61
12:88
12:88
7:93
NiCl2 (20 mol %)
NiCl2 (20 mol %) / LiCl (1 equiv.)
NiCl2(Ph3P)2 (5 mol %)
NiCl2[(c-Hexyl)3P]2 (5 mol %)
NiCl2(dppf) (1 mol %)
NiCl2(dppf) (0.5 mol %)
Nano NiO (20 mol %)e
Nano NiO (10 mol %)e
8:92
Nano NiO (10 mol %)e / LiCl (1 equiv.) 72
14:86
a The reactions were carried out using 1 mmol of benzoyl chloride 2 and 2 mmol of (n-butyl)(phenyl)zinc 1ab. Mol %
amount of catalysts were indicated.
b The sum of GC yields of n-Bu coupling product 3a and Ph coupling product 3b.
c All the data are the average of at least two experiments.
d The ratio of GC yields of 3a and 3b.
e Aldrich (<50 nm).
in the presence of these catalysts (Table 2). For this purpose, we found the background yields of 1ab, i.e.
acylation yields of homo diorganozinc reagents, n-Bu2 Zn, 1a2 and Ph2 Zn, 1b2 . We also evaluated a methyl
group as residual group instead of a n-butyl group in the (n-alkyl)(phenyl)zinc 1 in order to find a higher yield
and phenyl selectivity of acylation. The background yields for uncatalyzed benzoylation of mixed diorganozincs
1ab were 90% for n-Bu2 Zn 1a2 (Table 2, entry 1) and 66% for Ph2 Zn 1b2 (Table 2, entry 2). We also found
the acylation yields of Ph2 Zn 1b2 in the presence of NiCl2 (dppf) and nano NiO (Table 2, entries 3 and 4). As
seen, the acylation of Ph group in (n-Bu)(Ph)Zn 1ab takes place with about the same yield as in the acylation
of Ph2 Zn 1b2 with catalysis of NiCl2 (dppf) (Table 2, entries 5 and 3) and also with catalysis of nano-NiO
(Table 2, entries 6 and 4). These results confirmed the atom-economic character of (n-Bu)(Ph)Zn 1ab in the
nickel catalyzed acylation with 2.
In addition, the data revealed that for aryl–aroyl coupling of either (n-Bu)(Ph)Zn 1ab or (Me)(Ph)Zn 1bc
could be used in NiCl2 (dppf) or NiO catalyzed reactions. With catalysis of 0.5 mol % NiCl2 (dppf), acylation
of (n-Bu)(Ph)Zn 1ab results in 77% yield and 88% transfer of Ph group, whereas acylation of (Me)(Ph)Zn
1bc gives 86% yield with complete Ph transfer (Table 2, entries 5 and 7). Nano-NiO catalyzed acylation of
(n-Bu)(Ph)Zn 1ab leads to 71% yield and 92% Ph transfer, while acylation of (Me)(Ph)Zn 1bc leads to 84%
yield and with complete Ph transfer (Table 2, entries 6 and 8). Among these catalysts, the lower catalyst
loading of NiCl2 (dppf) may seem attractive; however, nano-NiO, being the lowest cost catalyst, seemed more
favorable for aroylation of (methyl)(aryl)zincs. In addition, our detailed literature survey did not show use of
nano-NiO in the reactions of organozinc reagents so far.
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Table 2. Coupling of n-Bu2 Zn 1a2 , Ph2 Zn 1b2 , and (Me)(Ph)Zn 1bc with benzoyl chloride 2 in THF in the presence
of NiCl2 (dppf) and nano-NiO catalysts.a
b,c
d
Entry R1, R2, catalyst
Total yield, %
Group selectivity R1COPh:R2COPh
1
2
3
4
5
6
7
8
n-Bu, n-Bu
90
66
82
67
-
Ph, Ph
-
Ph, Ph, NiCl2(dppf) (0.5 mol %)
Ph, Ph, nano-NiO (10 mol %)f
n-Bu, Ph, NiCl2(dppf) (0.5 mol %)e 77
n-Bu, Ph, nano-NiO (10 mol %)e,f
Me, Ph, NiCl2(dppf) (0.5 mol %)
Me, Ph, nano-NiO (10 mol %)f
-
-
12:88
8:92
0:100
0:100
71
86
84
a The reactions were carried out using 1 mmol of benzoyl chloride 2 and 2 mmol of 1a2 , 1b2 , or 1bc. Mol % amount
of catalyst was indicated.
b The sum of GC yields of R1 COPh and R2 COPh.
c All the data are the average of at least two experiments.
d The ratio of GC yields of R1 COPh and R2 COPh.
e Taken from Table 1.
f Aldrich (<50 nm).
We next applied the optimal conditions for nano-NiO (<50 nm) catalyzed phenyl–benzoyl coupling
using (Me)(Ph)Zn 1bc to further evaluate the scope of this reaction for synthetic transfer of aryl groups of
(methyl)(aryl)zincs to aroyl chlorides. The results are summarized in Table 3. The data are averages of at
least two independent experiments. The products were characterized using 1 H NMR analysis. We were pleased
to find that acylation of a variety of (methyl)(substituted phenyl)zincs (Table 3, entries 1–5) succeeded with
high yields. Good yields were also obtained in the reaction for the transfer of the phenyl group (Table 3,
entry 6) and 4-tolyl group (Table 3, entry 7) to 4-toluoyl chloride. However, despite all efforts, a low yield
was obtained in the acylation of (methyl)(3-cyanophenyl)zinc with benzoyl chloride (Table 3, entry 8). The
expected ketones were not observed in the acylation of (methyl)(4-bromophenyl)zinc (Table 3, entry 9) and
(methyl)(4-methoxycarbonylphenyl)zinc (Table 3, entry 10) and polymerization was obtained possibly due to
reactive substituents.
3. Conclusions
In conclusion, we have demonstrated that
(i) Room temperature acylation of mixed (methyl)(aryl)zincs with aroyl halides in the presence of inexpensive
nano-NiO catalysis provides an atom-economic and facile new route for aryl–aroyl coupling of diorganoz-
incs. This procedure is a supplement to Cu and Ni catalyzed acylation of Grignard reagents10−15 and
organozinc reagents.19−21,33−39
(ii) Nano-NiO seems to be a good catalyst for acylation of organozinc reagents. However, further studies are
underway to investigate the effect of particle size on the catalysts’ efficiency and on the mechanism of the
nano-NiO catalyzed acylation.
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Table 3. Nano-NiO catalyzed aryl–aroyl coupling of (methyl)(aryl)zincs 1 with aroyl chlorides 2.a
Entry MeRZnb
R1COCl
Product
Yield, % c
91
1
2
3
4
5
6
79
81
64
76
75
7
8
9
84
14
-
10
-
a The reactions were run with 2:1 molar ratio of 1:2. General reactions conditions: (Me)(Ar)Zn reagent (10 mmol) in
THF, aroyl chloride (5 mmol), nano-NiO (Aldrich, <50 nm, 0.5 mmol) were reacted at room temperature for 2 h (see
Experimental section).
b (Me)(Ar)Zn reagent was prepared in situ by reacting arylzinc chloride with MeMgCl in THF (see Experimental
section).
c Isolated yields.
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4. Experimental section
All reactions were carried out under nitrogen in well-dried glassware.76 All chemicals used were of reagent
grade or purified according to the literature procedures. GC analyses were carried out using a Thermo
Finnigan gas chromatograph equipped with a ZB-5 capillary column packed with phenylpolysiloxane using
the internal standard technique. For the preparation of (methyl)(aryl)zinc reagents (aryl: C6 H5 , 4-MeC6 H4 ,
3-MeC6 H4 , 4-MeOC6 H4 , 3-MeOC6 H4 , 4-BrC6 H4), arylzinc chlorides were reacted with methylmagnesium
chloride. Arylzinc chloride was prepared by addition of arylmagnesium bromide (10 mmol) to ZnCl2 (10 mmol)
in THF (10 ml) at –20 ◦ C and stirring at that temperature for 15 min. To freshly prepared arylzinc chloride (10
mmol), methylmagnesium chloride (10 mmol) was added and the mixture was stirred at –20 ◦ C for another 15
min. For the preparation of (methyl)(aryl)zinc reagents (aryl: 4-MeOOCC6 H4 , 3-NCC6 H4), arylmagnesium
bromides were reacted with methylzinc chloride. Arylmagnesium bromide (10 mmol) was prepared by addition
of isopropylmagnesium bromide (10.5 mmol) to a solution of aryl iodide (10 mmol) in THF (10 mL) at –
30 ◦ C and stirring at that temperature for 1 h to complete the I/Mg exchange. Arylmagnesium bromide
(10 mmol) solution was added to methylzinc chloride (10 mmol), which was freshly prepared by addition of
methylmagnesium chloride (10 mmol) to ZnCl2 (10 mmol) in THF (10 mL) at –20 ◦ C and the mixture was
stirred at that temperature for 15 min. The product ketones were identified by their melting points77 and 1 H
NMR spectra.78−80
4.1. Typical procedure for nano-NiO catalyzed coupling of (methyl)(aryl)zinc reagents with aroyl
chlorides: synthesis of 4-methylbenzophenone
To the freshly prepared mixed (methyl)(phenyl)zinc reagent (10 mmol), nano-NiO (Aldrich, <50 nm, 10 mol
%, 0.5 mmol, 0.0376 g) was added at –20 ◦ C followed by stirring at that temperature for 15 min. 4-Toluoyl
chloride (5 mmol) was then added at –20 ◦ C and the reaction mixture was stirred at room temperature for
2 h. The reaction mixture was hydrolyzed by addition of NH3 /NH4 Cl solution and subsequently extracted
with Et2 O. The combined ethereal solutions were washed with dilute HCl solution, dried, and concentrated by
rotary evaporation and subjected to silica gel column chromatography with hexane:EtOAc (10:1–5:1) as eluent
to give 4-methylbenzophenone. The products were characterized using 1 H NMR analysis:
4-Methylbenzophenone: 1 H NMR (400 MHz, CDCl3): δ 7.8–7.77 (d, 2H, J = 7.2 Hz), 7.74–7.71 (d,
2H, J = 8.0 Hz), 7.60–7.55 (t, 1H, J = 7.2 Hz), 7.50–7.45 (t, 2H,J = 7.6 Hz), 7.30–7.26 (d, 2H, J = 8.0 Hz),
2.44 (s, 3H).
Benzophenone: 1 H NMR (400 MHz, CDCl3): δ 7.82–7.79 (d, 4H, J = 7.6 Hz), 7.61–7.56 (t, 2H, J =
7.4 Hz), 7.51–7.46 (t, 2H, J = 8.0 Hz).
3-Methylbenzophenone: 1 H NMR (400 MHz, CDCl3): δ 7.80 (dd, 2H, J = 8 Hz, J = 1.2 Hz), 7.63
(s, 1H), 7.60–7.56 (m, 2H), 7.49–7.45 (t, 2H,J = 7.4 Hz), 7.40–7.33 (m, 2H), 2.41 (s, 3H).
4-Methoxybenzophenone: 1 H NMR (400 MHz, CDCl3): δ 7.83 (d, 2H, J = 9.2 Hz), 7.75 (d, 2H,
J = 6.8 Hz), 7.56 (t, 1H,J = 7.4 Hz), 7.47 (t, 2H,J = 7.4 Hz), 6.96 (d, 2H, J = 9.2 Hz), 3.88 (s, 3H).
3-Methoxybenzophenone: 1 H NMR (400 MHz, CDCl3): δ 7.82–7.79 (m, 2H), 7.59–7.55 (m, 1H),
7.48–7.44 (m, 2H), 7.38–7.32 (m, 3H), 7.13–7.10 (m, 2H), 3.83 (s, 3H).
4,4’-Dimethylbenzophenone: 1 H NMR (400 MHz, CDCl3): δ 7.71–7.69 (d, 4H, J = 8 Hz), 7.28–7.26
(d, 4H, J = 7.6 Hz), 2.44 (s, 6H).
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