Iridium-catalysed desilylative acylation of 1-alkenylsilanes

Article abstract of DOI:10.1016/j.molcata.2016.10.035

We report the iridium-catalysed desilylative acylation of styryl and dienyl silanes by acid anhydrides to afford (E)-α,β-unsaturated ketones. The [{Ir(μ-Cl)(cod)}₂] catalyst is the first non-rhodium complex successfully applied for this type of

Full text of DOI:10.1016/j.molcata.2016.10.035

Accepted Manuscript
Title: Iridium-catalysed desilylative acylation of
1-alkenylsilanes
Author: Maciej Zaranek Maciej Skrodzki Justyna
˛
Szudkowska-Fratczak Maciej Dodot Ireneusz Kownacki
´
Bartosz Orwat Piotr Pawluc
PII:
S1381-1169(16)30453-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.035
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Journal of Molecular Catalysis A: Chemical
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23-10-2016
25-10-2016
Please cite this article as: Maciej Zaranek, Maciej Skrodzki, Justyna Szudkowska-
˛
´
Fratczak, Maciej Dodot, Ireneusz Kownacki, Bartosz Orwat, Piotr Pawluc, Iridium-
catalysed desilylative acylation of 1-alkenylsilanes, Journal of Molecular Catalysis A:
Chemical http://dx.doi.org/10.1016/j.molcata.2016.10.035
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Iridium-catalysed desilylative acylation of 1-alkenylsilanes
Maciej Zaranek,a Maciej Skrodzki,a Justyna Szudkowska-Frątczak,a Maciej Dodot,a
Ireneusz Kownacki,a,b Bartosz Orwat,a Piotr Pawluć a,b
*
a
Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań,
Poland
b
Centre for Advanced Technologies, Adam Mickiewicz University, Umultowska 89c, 61-614
Poznań, Poland
* piotrpaw@amu.edu.pl

GRAPHICAL ABSTRACT
Highlights:



The iridium-catalyseddesilylative acylation of vinylsilanes with acid anhydrides.
Synthesis of (E)-styryl and (E,E)-buta-1,3-dien-1-yl ketones
Mechanism proposal of the [{Ir(µ-Cl)(cod)}2]-catalysed acylation of vinylsilanes.

Abstract: We report the iridium-catalysed desilylative acylation of styryl and dienyl silanes
by acid anhydrides to afford (E)-α,β-unsaturated ketones. The [{Ir(µ-Cl)(cod)}2] catalyst is
the first non-rhodium complex successfully applied for this type of transformation.
Stoichiometric reaction of [{Ir(µ-Cl)(cod)}2] with (E)-trimethyl(4-chlorostyryl)silane was
carried out to gain insight into the reaction mechanism.
Keywords: iridium catalyst, desilylative acylation, anhydride, enone

Introduction
α,β-Unsaturated ketones are important reagents for organic synthesis and can be used in
many organic transformations.[1] Their synthesis via the classic Claisen-Schmidt
condensation as not particularly selective does not fit modern chemistry trends.[2] Some
catalytic attempts have been made to synthesise α,β-unsaturated ketones with allyl alcohols
oxidation,[3] Friedel-Crafts acylation with α,β-unsaturated acyl chlorides,[4,5] carbonylative
addition of boronic acids to terminal alkynes,[6] Rupe rearrangement of propargylic
alcohols,[7] and metathesis of vinyl ketones with terminal alkenes among them.[8] A variety
of chalcones have been synthesized by solid-phase-acid-catalysed condensation of
acetophenones with benzaldehydes.[9] On the other hand, visible-light-promoted
organocatalysis has been used to obtain linear and cyclic enones bearing both aromatic and
aliphatic substituents as well as enals.[10] Acylation of stereodefined vinylsilanes proceeds in
a regioselective manner to afford α,β-unsaturated ketones.[11] Desilylative acylation has been
first described by Fleming and Pearce, who used acyl chlorides and AlCl3 as Lewis acid
catalyst.[12] Narasaka et al. have been the first to report the use of acid anhydrides instead of
chlorides in a reaction catalysed by a rhodium complex [{Rh(µ-Cl)(CO)2}2].[13] The
mechanism of the catalytic acylation proceeds via transmetalation between the Rh(I) complex
and vinylsilane to afford vinylrhodium intermediate, followed by sequential oxidative
addition of acid anhydride and reductive elimination of α,β-unsaturated ketone with
regeneration of Rh(I) catalyst.[13] In our previous work, we have reported a versatile method
for the preparation of (E)-styryl ketones via a sequence of ruthenium-catalysed silylative
coupling of styrenes with trimethylvinylsilane and desilylative acylation of silylated products
in the presence of [{Rh(µ-Cl)(CO)2}2] – in the manner presented by Narasaka et al. Knowing
the advantages of silylative coupling – its substrate scope and selectivity,[14,15] we decided
to make it the starting point on a route to enones. In the subsequent paper we have extended

this method by the use of (E)-1,2-bis(silyl)ethenes to obtain (E)-β-silyl-α,β-unsaturated
ketones.[16] The sequential Hiyama cross-coupling/Narasaka acylation of (E)-1,2-
bis(silyl)ethene has been also applied to the synthesis of α,β-unsaturated carbonyl
compounds.[16]
In the earlier studies, our effort has been put on extension of the use of rhodium
complexes. To the best of our knowledge, no report on other-metal-catalysed desilylative
acylation has been published. Moreover, the report of Wagner et al. might suggest, that only
[{Rh(µ-Cl)(CO)2}2] can be effectively used in the reaction, and no other complex of rhodium
offers this opportunity.[17]
Knowing the chemical similarity between rhodium and iridium, we have envisaged that the
acylation of vinylsilanes with acid anhydrides can successfully proceed with iridium(I)
complexes as catalysts. Given the relatively lower price and commercial availability of
iridium precursors, we chose several simple systems containing iridium to be tested in
desilylative acylation of styrylsilanes. Although iridium complexes are less frequently used
than their rhodium analogues, in some processes e. g. carbonylation, enantioselective
hydrogenation of C=N bonds and functionalisation of C(aryl)–H bonds, they may be more
effective.[18] In this communication we report a preliminary study of iridium(I)-promoted
acylation of styrylsilanes..
Results and discussion
We started the studies with optimisation of the reaction conditions. Catalytic screening
was applied in the model catalytic process of acylation of (E)-β-trimethylsilylstyrene with
acetic anhydride (Scheme 1). Selected monomeric and dimeric iridium(I) complexes were
tested in this process. In a typical procedure, the substrates (1:1 molar ratio) and catalyst (10
mol % corresponding to Ir) were dissolved in toluene (0.5 M concentration) and heated in a

Schlenk flask fitted with a plug valve at 120 °C for 24h under argon atmosphere. The
reactions were monitored by GC and GC-MS methods to measure conversion of the substrates
and products distribution. The GC-MS analysis of the reaction mixture confirmed the
formation of styryl methyl ketone as main product. Despite the use of dry toluene as a solvent,
small amounts of the competitive protodesilylation product, i.e. styrene, were detected in the
presence of all catalysts tested (Scheme 1). The results are summarised in Table 1.
These results indicate that Ir complex 2 exhibited the highest catalytic activity and
selectivity from among the tested group of iridium promoters. Noteworthy is the fact that
monomeric species did not lead to any conversion of (E)-β-trimethylsilylstyrene, whereas all
of the tested dimeric complexes showed the ability to catalyse desilylative acylation with at
least fairly good effects. The activity of the IrCl3/AgPF6 system 8 can be related to the ability
of AgPF6 to promote protodesilylation in the presence of water since iridium(III) chloride
exists as a hydrate. In the light of the preliminary results, we chose complex 2 for further
tests. Not only was it the most active one, but also the easiest to synthesise and handle,
requiring no special conditions of storage.
A series of catalytic trials were conducted to investigate the activity of complex 2 in
desilylative acylation. We chose (E)-silylstyrenes containing SiMe3 and SiMe2Ph groups,
known to provide better comparison between iridium and rhodium catalysts [13]. The
reactions performed aiming at optimisation of the conditions revealed that a reduction both in
the temperature and catalyst loading leads to a significant drop in the observed conversion. A
reaction between (E)-β-trimethylsilylstyrene and acetic anhydride in the presence of 2 mol%
of [{Ir(µ-Cl)(cod)}2] conducted at 120oC caused a decrease in styrylsilane conversion to 28%
after 24 h. No reaction occurred at 90°C and only a slight substrate conversion (10%) was

observed after 24h, while heating the reactants at 100°C. The best results were obtained when
using 0.5 M solution of reagents and 5% of binuclear iridium catalyst 2.
In the above-optimised conditions, we investigated the scope of this reaction, using various p-
substituted (E)-silylstyrenes and selected acid anhydrides. These results are summarised in
Table 2.
The results show that catalyst 2 is active in desilylative acylation of (E)-silylstyrenes
with various acid anhydrides. Substituted (E)-silylstyrenes reacted successfully to give the
corresponding (E)-styryl ketones in good yields, irrespective of the substituent’s electronic
character (Table 2, entries 1-8). Aliphatic acid anhydrides can be employed for the iridium-
catalysed acylation of (E)-styrylsilanes to give corresponding (E)-styryl ketones with
moderate to high yields (Table 2, entries 1-11, 14-22). Reaction of (E)-styrylsilanes with
benzoic and methacrylic anhydride let to obtain a mixture of silyl ester (PhCO2SiR3 or
CH2=C(Me)CO2SiR3, respectively) and styrene (Table 2 entry 12-13, 23-24). It is worth
noting that the [{Ir(µ-Cl)(cod)}2]-catalysed acylation of (E)-styrylsilanes proceeded
efficiently using 1 equivalent of acid anhydride, in contrast to the rhodium catalysis which
needed 3 equivalents of substrate to achieve satisfactory conversion of silylstyrenes.[12,13]
The known products of acylation reactions were detected by a comparison of their GC-MS
with the chromatograms of ketones synthesised previously using rhodium catalyst[13] and
some of the products were isolated in order to check their structure (Table 2, entries 1-8, 19,
22). However, formation of styryl ketones is almost always accompanied by protodesilylation
of their silyl precursors under iridium catalysis (10-30%) and further investigation should be
performed to improve the selectivity of the reaction.

We also successfully extended our method for the synthesis of 4-phenyl substituted
(E,E)-buta-1,3-dien-1-yl ketones (Scheme 2). The reaction of 1-phenyl-4-(trimethylsilyl)-1,3-
butadiene (mixture of isomers (E,E)/(E,Z) = 93:7)[19] with 1 equiv. of acid anhydride in the
presence of iridium catalyst 2 (5 mol %) in dry toluene at 120°C for 24-48 h under argon
atmosphere allowed isolation of (E,E)-buta-1,3-dien-1-yl ketones as predominant products in
59-78 % yield (Table 3).
A preliminary mechanistic investigation was carried out. Heating a mixture of
complex 2 and 2 equivalents of (E)-trimethyl(4-chlorostyryl)silane in toluene-d8 led to
formation of chlorosilane and disiloxane, while no significant change was observed when the
mixture contained the catalyst and an acid anhydride. A deduction that comes along is that the
reaction studied runs according to the same mechanism as proposed by Narasaka et al for
[{Rh(µ-Cl)(CO)2}2] complex[13] (Scheme 3).
In the catalytic system created with vinylsilane derivative and appropriate carboxylic
acid anhydride, it can be expected that in the first step the initial complex [{Ir(µ-Cl)(cod)}2]
reacts with unsaturated silicon derivative (R3Si-CH=CH-Ar) yielding a corresponding
mononuclear Ir(I) species with π-bonded vinylsilane derivative. It is consistent with studies
conducted by Fougeroux, who reported formation of square-planar mononuclear Ir(I) species
with allyl derivatives π-bonded to the metallic centre after the reaction of the binuclear
iridium precursor [{Ir(µ-Cl)(cod)}2] with allyl derivatives such as allyl alcohol or allyl
ethers.[20] Then, in a further step, the previously formed square-planar Ir(I) species is
2
transformed into five-coordinated silyl-vinylene Ir(III) intermediate via Si-Csp bond
activation followed by evolution of chlorosilane and assumptive formation of vinyliridium
complex – the first key intermediate. According to Narasaka [13], should the latter act with an

acid anhydride via oxidative addition followed by reductive elimination of an α,β-unsaturated
ketone with creation of Ir(I) acetyl complex – the second key intermediate.
Experimental
Desilylative acylation of (E)-β-silylstyrenes
In a typical procedure, a previously evacuated and flame-dried Schlenk flask was charged
with 2 mL of dry toluene, 176 mg (1 mmol) of (E)-β-trimethylsilylstyrene, 96 μL (1 mmol) of
distilled acetic anhydride, and 33.6 mg (0.05 mmol) of the iridium complex 2, under
continuous flow of argon. 0.1 mL of decane was added as an internal standard. The reaction
vessel was closed tight, then stirred and heated at 120 °C for 24h. Once the specified time had
passed, the flask was cooled down to the room temperature, and the sample taken from the
mixture was analysed using GC-MS technique to determine the products distribution. Several
examples of reactions were selected to be isolated and subjected NMR analysis to determine
structure of the products. In such case, the reaction mixture was evaporated on a rotary
evaporator and column chromatography purification was carried out (silica gel, hexane –
diethyl ether 9:1 v/v used as eluent).
Desilylative acylation of (E)-trimethyl(4-chlorostyryl)silane – an NMR experiment
On the basis of Narasaka’s experiment,[13] dried and evacuated NMR tube with a Young
valve was charged with 10 mg (0.015 mmol) of the iridium complex 2, 6.3 mg of (E)-β-
trimethylsilyl-4-chlorostyrene, and 0.6 mL of dry toluene-d8. The 1H spectrum was measured,
and the tube was heated at 120 °C. Then the spectra were measured after 8 and 24 h of
heating.
Conclusions

In summary, the first iridium-catalysed protocol for efficient desilylative acylation of
substituted vinylsilanes with acid anhydrides has been reported. Di-µ-chlorobis(1,5-
cyclooctadiene)diiridium(I) proved to be an efficient catalyst for desilylative acylation of
substituted (E)-styrylsilanes and (E,E)-1-phenyl-4-(trimethylsilyl)-1,3-butadiene to yield (E)-
styryl ketones and (E,E)-4-phenylbuta-1,3-dien-1-yl ketones, respectively. The catalytic
activity of the iridium catalyst [{Ir(µ-Cl)(cod)}2] in the acylation of substituted vinylsilanes
by acid ahydrides was similar to that of the earlier reported rhodium catalyst [{Rh(µ-
Cl)(CO)2}2] [13], however, the reaction over the Ir complex required higher temperature (120
°C) than that needed for Rh catalysis activity (90 °C). Although, under optimum conditions
the Ir complex promoted competitive protodesilylation of styrylsilanes, the isolation yields of
acylation products (58-85%) were slightly better in most cases than those obtained with Rh
catalyst (18-84%). Moreover, in the reactions performed over Rh catalysis, the competitive
decarbonylation, isomerisation and polymerisation were also observed. The iridium catalyst
[{Ir(µ-Cl)(cod)}2] seems to be also more effective for acylation of silylated dienes.
Preliminary mechanistic studies indicate that iridium catalysis proceeds according to the
transmetalation mechanism, analogous to that reported previously for rhodium(I) complexes.
Acknowledgment
Financial support from the National Science Centre (Poland), Grant No. Opus
2011/03/B/ST5/01034 is acknowledged.

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Scheme 1. Reaction of (E)-β-trimethylsilylstyrene with acetic anhydride in the presence of
iridium catalysts
Scheme 2. Synthesis of 4-phenyl-(E,E)-buta-1,3-dien-1-yl ketones with the use of complex 2
Scheme 3. Proposed mechanism of desilylative acylation of silylstyrenes by acid anhydride using
complex 2

Table 1. The results of catalyst screening in the desilylative acylation of (E)-β-
trimethylsilylstyrene with acetic anhydride.
Styrylsilane
conversion, % a
64
DA/PD ratio
Entry Catalyst
1
2
[{Ir(µ-Cl)(CO)2}n] 1
85:15
90:10
[{Ir(µ-Cl)(cod)}2] 2
[{Ir(µ-Cl)(coe)2}2] 3
[Ir(coe)2(µ-OMe)}2] 4
90
88
67
90:10
80:20
3
4
5
6
7
8
[IrCl(CO)(PPh3)2] 5
IrCl3 6
0
-
0
-
IrCl3 / 2 eq PPh3 7
IrCl3 / AgPF6 8
0
-
25
20:80
aMeasured by GC. Reaction conditions: toluene, 24h, 120 °C

Table 2. Results of iridium-catalysed acylation of (E)-styrylsilanes.
Entry
R1
R2
R3
Styrylsilane
Conversion [%]a
(isolated yield)
90 (72)
76 (65)
78 (59)
99 (85)
98 (58)
94 (76)
81 (67)
88 (62)
79
DA/PD [%]a
1
H
CH3
CH3
90:10
90:10
90:10
90:10
90:10
90:10
90:10
90:10
80:20
90:10
100:0
80:20b
90:10c
85:15
90:10
90:10
70:30
80:20
80:20
90:10
70:30
80:20
85:15b
90:10c
2
CH3
Cl
3
4
Br
H
5
C2H5
C3H7
6
CH3
Cl
7
8
Br
Br
H
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
i-C3H7
97
CH3
H
99
-C(CH3)=CH2 58
H
C6H5
CH3
99
H
C6H5
98
CH3
Cl
63
83
H
C2H5
97
CH3
Cl
93
99 (70)
85
H
i-C3H7
CH3
Cl
92
91 (61)
H
-C(CH3)=CH2 99
C6H5 66
H
Reaction conditions: [silylstyrene]:[acid anhydride]:[2] = 1:1:0.05, toluene (0.5 M), 24h, 120 C;
aMeasured by GC. b [CH2=C(Me)CO2SiR3]:[styrene] ratio. c [PhCO2SiR3]:[styrene] ratio.

Table 3. Results of iridium-catalysed acylation of 1-phenyl-4-(trimethylsilyl)-1,3-butadiene
Entry
R
Time [h]
24
Isolated yield [%]
EE/EZa
90:10
90:10
85:15
DA/PD
9:1
1
2
3
CH3
C2H5
C6H5
78
67
59
24
8:2
48
8:2
a
Reaction conditions: [silyldiene]:[acid anhydride]:[2] = 1:1:0.05, toluene (0.5 M), 120 C; Measured
by 1H NMR.