Synthesis of acylsilanes by copper(I)-catalyzed addition of silicon nucleophiles onto acid derivatives

Article abstract of DOI:10.1002/adsc.201300621

The transition metal-catalyzed transfer of silicon nucleophiles onto various electrophiles has recently gained considerable attention, due to the now readily available silicon pro-nucleophiles such as silylboronates. Our interest lies in the addition of such species to acid derivatives for the generation of acylsilanes. We report herein an efficient method to synthesize these compounds, starting from easy-to-form anhydrides, with very good yields. Copyright

Full text of DOI:10.1002/adsc.201300621

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DOI: 10.1002/adsc.201300621
Synthesis of Acylsilanes by Copper(I)-Catalyzed Addition of
Silicon Nucleophiles onto Acid Derivatives
Virginie Cirriez,a Corentin Rasson,a and Olivier Rianta,*
a
Institute of Condensed Matter and Nanosciences, Molecules, Solids and Reactivity (IMCN/MOST) – Universitꢀ
catholique de Louvain, Place Louis Pasteur 1, bte L4.01.02, 1348 Louvain-la-Neuve, Belgium
Fax : (+32)-10-474-168; e-mail: Olivier.riant@uclouvain.be
Received: July 16, 2013; Published online: November 4, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300621.
Abstract: The transition metal-catalyzed transfer of
silicon nucleophiles onto various electrophiles has
recently gained considerable attention, due to the
now readily available silicon pro-nucleophiles such
as silylboronates. Our interest lies in the addition of
such species to acid derivatives for the generation
of acylsilanes. We report herein an efficient method
to synthesize these compounds, starting from easy-
to-form anhydrides, with very good yields.
the oxidation of gem-borylsilylalkylcoppers.[16] A con-
venient and general synthesis of acylsilanes from the
corresponding morpholine amides and silyllithium
species was also reported by the Scheidt group.[1,17]
However, most of the standard procedures for access-
ing these compounds have some limitations such as
low overall efficiency, a reliance on stoichiometric
quantities of a transition metal or even more incon-
veniently the need to be prepared in several steps.
Thus, there is still a need for designing new straight-
forward and versatile methods for the synthesis of
acylsilanes.
Keywords: anhydrides; borosilanes; copper; homo-
geneous catalysis; nucleophilic addition
To the best of our knowledge, there is only one ex-
ample of an acylation reaction with nucleophilic cop-
per(I) species. Indeed, Ball and co-workers described
the addition of well identified arylcopper compounds
Acylsilanes [RCOSi(R’)3] are compounds in which (NHCCuAryl) to acetyl chloride.[18] However, to the
a silane fragment is directly attached to a carbonyl best of our knowledge, there are still no examples of
group resulting in unique chemical properties. The a catalytic acylation reaction of copper(I) nucleophile
use of acylsilanes in organic synthesis has increased species in the literature and we wish to report here
significantly over the last few years due to the discov- our own contribution leading to a useful and straight-
ery of valuable new transformations and some im- forward method to access acylsilanes. Indeed, based
provements in the synthetic methods leading to acylsi- on our previous work in the area of transition metal-
lanes.[1] Such compounds are versatile intermediates catalyzed transfer of silicon nucleophiles,[19] we envis-
in organic synthesis,[2,3] and are used mainly as umpo- aged a new strategy for the preparation of acylsilanes
lung reagents for the preparation of carbonyl deriva- (Scheme 1) through acylation of a copper(I)-silyl in-
tives, employing the Brook rearrangements.[4] New termediate. Starting from type 1 acid derivatives in
types of catalysts have also been developed with the presence of Suginomeꢁs reagent[20] 2 and a copper
regard to the activation of acylsilanes in 1,2-benzoin- catalyst, the process should lead to the corresponding
type additions and 1,4-Stetter reactions, e.g., thiazoli- acylsilanes 3.
um salts,[5] cyanides[6] and metallophosphites.[7] More
Our investigation started with a screening of differ-
recently, new applications of acylsilanes have been es- ent acid derivatives 1a–j (Table 1). We decided to use
tablished with the addition of enolates,[8] Pd-catalyzed
coupling reactions,[9] synthesis of substituted g-lac-
tams,[10] or photochemical transformations.[11]
Many synthetic routes have been designed for the
preparation of acylsilanes, including the hydrobora-
tion–oxidation of alkynylsilanes,[12] the use of di-
thianes[13] or benzotriazoles[14] as intermediates, the
Ni-catalyzed reactions of a-hydroxyallylsilanes[15] and Scheme 1. New synthetic strategy towards acylsilanes.
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Table 1. Screening of different acid derivatives.
Table 2. Optimization.
Entry
Additive (equiv.)
Equiv. of 2
Yield[a] [%]
1
2
3
4
5
6
7
–
–
–
1.2
1.5
1.8
1.2
1.2
1.2
1.2
1.2
1.5
50
68
65
8
0
2
52
54
93[c]
MeOH (1)
MeOH (4)
KO-t-Bu (1)
CsF (1)
TBAT (1)
TBAT (1)
8
9[b]
Yield[a] [%]
[a]
1
Entry
Acid derivatives
Yield determined by H NMR with hexamethylbenzene
as internal standard.
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
5
5
0
0
6
1
1
11
30
27
[b]
[c]
Addition of 2 over 10 min.
Isolated yield.
strate and 2 as pronucleophile with catalyst 4 in tolu-
ene at room temperature. In a first series of experi-
ments, we have shown that increasing the amount of 2
to 1.5 equivalents gave better yields (Table 2, entry 2).
Addition of methanol or potassium tert-butoxide
had a negative effect on this reaction (Table 2, en-
tries 4–6). However, addition of a fluoride source
proved to be much more interesting. Indeed, the acyl-
silane was obtained in 54% yield with the addition of
10
1j
[a]
1
Yield determined by H NMR with hexamethylbenzene
as internal standard.
2 mol% of the copper catalyst CuF
ACHTUNGTRENNUNG
in THF at room temperature with 1.2 equivalents of
N
ACHTUNGTRENNUNG
silylborane 2. Acyl fluoride 1a and acyl chloride 1b a slow addition of silylborane 2 as well as the use of
1
gave only trace amounts (about 5% by H NMR) of carefully dried toluene, play a significant role in the
the desired product 3a (Table 1, entries 1 and 2). reaction. Indeed addition of 2 over 10 min with the
Other commonly known activated acid derivatives previously optimal conditions gave the acylsilane 3a
1c–g gave similar results (Table 1, entries 3–7). The in 93% isolated yield after 6 h (Table 2, entry 9).
best yield was obtained using benzoic anhydride 1i
A tentative mechanism can be proposed for this
which gave the desired acylsilane 3a in 30% yield process (Scheme 2). It begins with the formation of
À
(Table 1, entry 9). Surprisingly, unsymmetrical anhy- the active species Cu Si (I) by activation of the silyl-
drides[21] 1h and 1j gave lower yields (Table 1, en- borate with the copper fluoride complex. Coordina-
tries 8 and 10). To establish suitable reaction condi- tion of I to II allows the formation of intermediate
tions, we then optimized the solvent, the reaction IV. Elimination of the leaving group gives the desired
À
temperature and the copper source, thus concluding acylsilane V and Cu O carboxylate species VI. We
that running the reaction in toluene at room tempera- postulate that the role of the fluoride is to activate
ture with 2 mol% of catalyst 4 was the optimal result. the silylboronate by coordination to the boron atom
Under these conditions, the desired product was ob- to form the active intermediate VII which will facili-
tained in 50% yield (Table 2, entry 1).
We then screened different additives and reaction by reaction with VI to regenerate the active species I.
conditions as shown in Table 2. These experiments With the new optimal conditions in hand, it was
were performed using anhydride 1i as a model sub- then possible to study the scope of this reaction. First
tate the transfer of the silyl group on the copper atom
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Synthesis of Acylsilanes by Copper(I)-Catalyzed Addition of Silicon Nucleophiles
Scheme 2. Proposed catalytic cycle.
of all, the desired anhydrides have been prepared (Table 2, entry 9), the reaction gave satisfactory re-
starting from the corresponding acyl chlorides with sults for all types of substituents. The reaction tolerat-
moderate to good yields.[22] Our methodology was ed quite well both electron-deficient (Table 3, en-
then applied to these substrates and results are shown tries 3, 4, 6, 7 and 9) and electron-rich (Table 3, en-
in Table 3. Under the previously defined conditions tries 2, 5 and 8) aromatic systems. The para- or meta-
position of the substituent does not influence the de-
sired reactivity. Satisfyingly, aliphatic substituents also
Table 3. Scope of the reaction.
underwent the desired reaction and the corresponding
acylsilane was isolated in high yield (Table 3,
entry 11). However, the reaction is less efficient with
electron-rich heteroaromatic ring such as furan and
thiophene (Table 3, entries 12 and 13).
Finally we decided to carry out preliminary tests
that might lead to the development of a “one-pot”
procedure of the aforementioned reaction starting di-
rectly from the corresponding acid (Scheme 3). This
strategy required first the activation of the carboxylic
acid before applying our previously optimized meth-
odology. After some trials, we found that activation of
the carboxylic acid with dicyclohexylcarbodiimide in
toluene at 08C for 1 hour gave the best result. The in-
termediate was directly engaged in the second reac-
tion and we obtained the desired product in 51%
yield. This result is very interesting as it provides an
Entry
R
Product
Yield[a] [%]
1
2
3
4
5
6
7
8
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
93
52
76
82
98
89
85
>99
67
68
82
46
47
4-(MeO)C6H4
4-(Cl)C6H4
4-(Br)C6H4
4-(Me)C6H4
4-(F)C6H4
3-(F)C6H4
3-(Me)C6H4
3-(Br)C6H4
2-naphthyl
cyclohexyl
2-thiophene
2-furoyl
9
10
11
12
13
3j
3k
3l
3m
[a]
Isolated yield.
Scheme 3. Development of a “one-pot” procedure.
Adv. Synth. Catal. 2013, 355, 3137 – 3140
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Virginie Cirriez et al.
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easy access to acylsilanes directly from carboxylic
acids under mild conditions.
In conclusion, we have developed a new strategy
for the synthesis of acylsilanes by using symmetrical
carboxylic anhydrides that can be accessed from com-
mercially available acyl chlorides. Good to excellent
yields were obtained with various substrates. This
methodology is mild, practical and does not require
strongly basic precursors. Moreover, we have also
started to develop a “one-pot” reaction procedure
which allows us to access acylsilanes directly from
commercially available carboxylic acids.
Experimental Section
General Procedure for the Synthesis of Acylsilanes
A flame-dried Schlenk flask was loaded with the catalyst
CuFACHTUNGTRENNUNG(PPh3)3·2MeOH (0.02 equiv.), TBAT (1 equiv.) and an-
hydride (1 equiv.). The vessel was then sealed and, after 3
vacuum/argon cycles, toluene was added (2 mL). After dis-
solution of solids, Me2PhSi-Bpin (1.5 equiv,) was added
dropwise over 10 min and the mixture was stirred at room
temperature for 6 h. The solution was filtered over a pad of
flash silica gel eluting with Et2O and then concentrated
under reduced pressure. Hexamethylbenzene was added to
1
the crude product to determine the yield by H NMR. The
product was finally purified by flash chromatography on
silica gel. For further details see the Supporting Informa-
tion.
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