N-Heterocyclic carbene-mediated organocatalytic transfer of tin onto aldehydes: New access to α-silyloxyalkylstannanes and γ- silyloxyallylstannanes

Article abstract of DOI:10.1002/adsc.200900816

A new, highly efficient and mild N-heterocyclic carbene (NHC)-mediated organocatalytic procedure for the transfer of tin from tributyl(trimethylsilyl) stannane (Bu₃SnSiMe₃) onto aldehydes for the preparation of α-silyloxyalkylstannanes and γ-silyloxyallylstannanes has been developed.

Full text of DOI:10.1002/adsc.200900816

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DOI: 10.1002/adsc.200900816
N-Heterocyclic Carbene-Mediated Organocatalytic Transfer of
Tin onto Aldehydes: New Access to a-Silyloxyalkylstannanes and
g-Silyloxyallylstannanes
Romain Blanc,a Laurent Commeiras,a,* and Jean-Luc Parraina,*
a
Institut des Sciences Molꢀculaires de Marseille iSm2, UMR CNRS 6263, ꢀquipe STeRꢀO, Aix-Marseille Universitꢀ,
Campus Scientifique de Saint Jꢀrꢁme, service 532, 13397 Marseille cedex 20, France
Fax : (+33)-(0)491-288-861; phone : (+33)-(0)491-289-126; e-mail: laurent.commeiras@univ-cezanne.fr or
jl.parrain@univ-cezanne.fr
Received: November 24, 2009; Revised: January 22, 2010; Published online: March 4, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900816.
Abstract: A new, highly efficient and mild N-heter-
ocyclic carbene (NHC)-mediated organocatalytic
procedure for the transfer of tin from tributyl(tri-
methylsilyl)stannane (Bu3SnSiMe3) onto aldehydes
for the preparation of a-silyloxyalkylstannanes and
g-silyloxyallylstannanes has been developed.
catalyzed addition of Bu3SnSiMe3 to carbonyl com-
pounds[3] has also proved to be a mild and convenient
method, compared to the previously described ones
which are not always compatible with base-sensitive
substrates. Concerning enantiomerically-enriched a-
alkoxystannanes, several strategies are known. To the
best of our knowledge, the first reports involved the
asymmetric reduction of acylstannanes[4] and the cata-
lytic asymmetric addition of ethyl(tri-n-butylstannyl)-
zinc to aldehydes.[5] Recently, N-heterocyclic carbenes
(NHCs) have become a massive success in organic
and organocatalytic chemistry. Their use as ligands of
several metals has also been impressively demonstrat-
ed. Compared to a1-d1 or a3-d3 Umpolung type reac-
Keywords: N-heterocyclic carbenes; organocataly-
sis; a-silyloxyalkylstannanes; g-silyloxyallylstan-
nanes
Since the pioneering work of W. C. Still on the syn- tions, carbene-catalyzed 1,2-additions of nucleophiles
thesis of protected a-alkoxystannanes[1] 1, and their to carbonyl compounds (aldehydes and ketones) were
use as configurationally stable a-alkoxyorganolithium not studied so far. As nucleophile source, only cyano-
reagents,[2] methods for the preparation of 1, in race- and trifluoromethyl anions were described.[6]
mic and enantioenriched form, have been widely de-
Herein, we report the first organocatalytic, neutral
veloped (Scheme 1). The most common preparation 1,2-addition of Bu3SnSiMe3 to aldehydes, mediated by
of racemic 1 is the addition of lithium or magnesium- NHCs for the synthesis of a-alkoxystannanes
stannyl anions to carbonyl compounds. The Bu4NCN- (Scheme 1). It is important to note that the NHC
probably acts as an organocatalyst in the transfer of
metalloid and not as a ligand. To the best of our
knowledge, this is the first time that a carbene plays
such a role.
Benzaldehyde was chosen as a benchmark sub-
strate, for the catalyst NHC screening. The reaction
was initially performed with 1.0 equivalent of benzal-
dehyde, 1.05 equivalents of Bu3SnSiMe3, 5 mol% of
NHC catalyst generated from 5 mol% of the corre-
sponding imidazolium salt A and 5 mol% of t-BuOK
in THF at 0.25M for 1 hour (Table 1). Due to the sen-
sitivity of 1a, the rate of conversion was monitored by
1H NMR analysis and was based up on the transfor-
mation of substrate to the corresponding sensitive a-
Scheme 1. Synthesis of protected a-alkoxystannanes 1.
siloxyalkoxystannanes.[7] The reaction proceeded
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Table 1. Optimization studies on NHCs.
KHMDS). Interestingly, the nature of the counter
anion exhibited a strong influence on the reaction
yield (C vs. D). The commercially available NHCs G,
H and I (entries 7–9) led to similar or better success
than the corresponding in situ prepared NHCs. Even
if H gave a lower conversion than [F+KHMDS]
(98% vs. 99% respectively), for reasons of practicabil-
ity H was used for further work. Next, the influence
of the nature of solvent was investigated. Both polar
(Et2O, THF) and apolar aprotic solvents (n-hexane,
benzene, toluene, dichloromethane) were tested.
Whatever the nature of the solvent, the reaction pro-
ceeded smoothly with excellent conversions rates (95
to 99%). Best results were observed with Et2O and
CH2Cl2 with 99% of conversion. Based on previous
studies of addition of nucleophilic carbenes onto alde-
hydes,[6] two possible reaction pathways for the stan-
nylsilylation reaction may be envisaged (Scheme 2).
The first one involves an active zwitterionic inter-
mediate L obtained by nucleophilic attack of NHC to
a carbonyl carbon, whereas the second mechanism
would involve a reactive pentavalent silicon complex
K, which would transfer a tributyltin group to the car-
bonyl compound to form a-siloxytributylstannane.
Even though the formation of an NHC-aldehyde
adduct has already been demonstrated by ORTEP[8]
Entry
Catalyst
Conversion (%)[7]
1
2
3
4
5
6
7
8
9
A+t-BuOK (or KHMDS)
B+t-BuOK (or KHMDS)
C+t-BuOK (or KHMDS)
D+t-BuOK (or KHMDS)
E+t-BuOK (or KHMDS)
F+t-BuOK (or KHMDS)
G
85 (96)
52 (80)
85 (96)
45 (71)
75 (68)
87 (99)
95
1
as well as H NMR and 13C NMR monitoring studies
1
of a 1/1 mixture of Bu3Sn-TMS/NHC. H NMR spec-
troscopy showed a new TMS peak at d=0.11 ppm
(Bu3Sn-TMS peak at d=0.29 ppm). Similar results
were observed in the 13C NMR spectrum. Chemical
shifts of TMS peak (Dd=0.44 ppm) and SnBu3 peaks
were observed. Taken together, these data infer a re-
action between Bu3Sn-TMS and NHC H leading the
H
I
98
70
smoothly and rapidly to give 1a with a good conver- formation of pentavalent silicon complex K.
sion (85%, entry 1). With an optimized protocol [Et2O, H (5 mol%),
To optimise the yield, several NHC salts were 1 h, [0.25M]), the scope and limitations of the organo-
tested and the generation of the NHC was performed catalytic 1,2 addition of Bu3SnSiMe3 to a variety of ar-
with two different bases: t-BuOK and KHMDS (en- omatic aldehydes, were investigated (Table 2, en-
tries 1 to 9). In all the cases, a-silyloxystannane 1 was tries 1–16). In general, the methodology was tolerant
formed with moderate (entries 2 and 4 with t-BuOK) to a variety of electron-donating substituents (en-
to excellent conversions (entries 1, 2 and 6 with tries 1–10). Substrates were converted into the corre-
Scheme 2. Possible reaction mechanisms.
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Adv. Synth. Catal. 2010, 352, 661 – 666

N-Heterocyclic Carbene-Mediated Organocatalytic Transfer of Tin onto Aldehydes
Table 2. Organocatalytic 1,2 addition of Bu3SnSiMe3 to aro-
matic aldehydes.
Table 3. Organocatalytic 1,2 addition of Bu3Sn-SiMe3 to ali-
phatic aldehydes.
Entry
R
Conversion[7] [%]
1
1
2
3
4
5
6
7
8
Ph
98
98
90
98
0
100
96
98
90
94
0
0
0
98
98
96
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
4-Me-C6H4
4-MeO-C6H4
4-F-C6H4
4-Br-C6H4
2-Me-C6H4
2-MeO-C6H4
2-F-C6H4
2,4-(MeO)2-C6H3
3,4,5-(MeO)3-C6H2
4-NO2-C6H4
4-F3C-C6H4
4-CO2Me-C6H4
3-furyl
Entry
R
Conv.[7] [%]
1
2
1
2
3
CH2Ph
91
96
1q (80) 2q (20)
1r (96) 2r (4)
1s (99) 2s (1)
CH
2A
CH
2A
9
4
85
1t (93) 2t (7)
10
11
12
13
14
15
16
obtained with the organocatalytic, neutral 1,2-addition
of Bu3SnSiMe3 to aromatic and aliphatic aldehydes,
we next investigated the addition onto a,b-unsaturat-
ed aldehydes for a new approach to g-siloxyallyltribu-
tylstannanes. To the best of our knowledge
Bu3SnSiMe3 was not used so far for the synthesis of g-
2-furyl
2-thionyl
sponding a-silyloxystannanes with good to excellent siloxyallyltributylstannanes or 3-tributylstannylated
conversions (90 to 100%).
carbonyl products.[12] g-Siloxyallyltributylstannanes
ortho-Substitution (entries 6–9) did not influence were previously prepared using the conjugate addition
the reaction and similar or better conversions were of or Lipshutzꢃs reagent
R3SnLi[13]
observed. While ortho- and para-fluorobenzaldehyde [Bu3Sn(Bu)CuCNLi2].[14]
(entries 4 and 8) afforded a good reactivity, para-bro- Starting from 2-butylacrolein (Table 4) as model
mobenzaldehyde (entries 5) failed to give the corre- substrate, the same optimized conditions used for ali-
sponding stannyl adducts. phatic and aromatic aldehydes (Et2O, H, 1 hour,
The limits of the reaction were reached when the [0.25M]), gave, with a total conversion, an 84/16 mix-
benzaldehyde was substituted with electron-withdraw- ture of 1,4- and 1,2-addition, respectively (entry 1). To
ing substituents (entries 11–13) where no product was improve the chemoselectivity, the influence of solvent
formed from 1,2 addition of Bu3SnSiMe3. The reac- was evaluated (entries 2–6). The results showed that
tion can be also performed with heteroaromatic alde- all solvents allowed predominantly the formation of
hydes such as furancarboxaldehyde or thiophenecar- the 1,4-addition adduct 5a with excellent conversion.
boxaldehyde (entries 14–16) with excellent conver- THF was the best, with a conversion of 99% and a
sions. We next turned our attention towards aliphatic ratio of 99/1 in favor of 1,4-addition. Allylstannane 5a
aldehydes (Table 3, entries 1–4). All reactions proceed was obtained as a 100/4 mixture of E and Z diastereo-
smoothly to give predominantly the corresponding a- mers, respectively. It is worthy of note that up to 10%
1
alkoxystannanes 1. During this reaction, the strong of aldehyde 6a was observed in the crude H NMR
basicity of NHC carbenes[9,10] was also expressed with spectrum. Further optimization studies on NHC
the formation of up to 20% of an E and Z mixture of showed that G (entry 7) proceeded smoothly to com-
1
silyl enol 2 detected in the H NMR (entry 1) spec- pletion without observing 1,2-addition. Interestingly,
trum of the drude reaction mixture along with the for- the diastereoselectivity (E/Z; 12/100, entry 8) of the
mation of tributyltin hydride. An aldehyde having a addition can be reversed by using the NHC I. The in-
chiral center (entry 4) led to a 1/1 mixture of diaste- fluence of the nature of the substituent borne by the
reomers 1t. Finally, starting from 6-oxoheptanal silicon atom was also evaluated (Table 4, entries 10–
(entry 3), a total chemoselectivity of the addition onto 12) to obtain less sensitive allylstannanes.
aldehyde function occurred with an excellent conver-
sion (99%). Less than 1% of the corresponding silyl Bu3SnSiEt3 onto 2-butylacrolein allowed the forma-
enol species 2s was observed. tion of the corresponding g-siloxyallyltributylstannane
While the neutral 1,4-addition of Bu3Sn-TMS and
Synthesis of polypropionate natural products pos- with excellent conversion (entries 2 and 10), the reac-
sessing contiguous stereocenters often uses the aldol tion performed with more sterically hindered trialkyl-
condensation to join together two subunits. An alter- silyl moieties (such as TBS) was never complete
native of this methodology can be the utilization of (entry 11) and Bu3Sn-TBDPS failed to give the corre-
allylmetal species.[11] Based on our precedent results sponding allylstannane (entry 12).
Adv. Synth. Catal. 2010, 352, 661 – 666
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Romain Blanc et al.
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Table 4. Optimization studies on organocatalytic tin transfer to 2-butylacrolein.
Entry
NHC
Solvent
R
Conversion[7] [%]
5a [%] (E/Z)
6a [%]
7a [%]
1
2
3
4
5
6
7
8
H
H
H
H
H
H
G
I
Et2O
THF
Me3
Me3
Me3
Me3
Me3
Me3
Me3
Me3
Me3
Et3
t-BuMe2
t-BuPh2
100
99
99
99
99
99
98
97
95
100
62
0
84 (100/4)
88 (100/4)
90 (100/4)
73 (100/10)
74 (100/9)
73 (100/8)
98 (100/0)
68 (12/100)
90 (100/2)
96 (100/2)
45 (100/5)
–
0
10
6
6
6
6
0
29
5
4
16
1
3
20
20
20
0
0
0
0
0
CH2Cl2
n-hexane
benzene
toluene
THF
THF
THF
THF
THF
9
J
10
11
12
G
G
G
17
–
THF
–
Under the optimized reaction conditions {THF, G tions.[15] This procedure was exemplified starting from
(5 mol%), 1 h, [0.25M]}, a variety of a,b-unsaturated benzaldehyde, hexanal and thiophene 2-carbaldehyde
aldehydes were examined and results are collected in (Scheme 3).
Table 5. Whatever the nature of the substrate, the re-
Silylstannation was performed in THF[16] under
action occurred smoothly with excellent conversion 5 mol% of NHC H activation, then addition, at low
rates (89 to 99%). Without substitution at the b posi- temperature, of n-butyllithium afforded the corre-
tion to carbonyl function (entries 1 and 2), only the
product arising from 1,4 addition was observed with
excellent diastereoselectivity of the formed TMS-enol
while 3-substituted acrolein derivatives led to fair se-
lectivity (entries 3 and 4). Two substituents on the b
positions to the carbonyl function also increased the
ratio of 7 and a 57/43 mixture of 5g and 7g was ob-
tained starting from 3,3-dimethylacrolein. The limit of
the reaction was reached (no reaction) when a conju-
gated system, such as cinnamaldehyde, was used.
Finally, we investigated a “one-pot” synthesis of a-
hydroxysilanes from aldehydes, using a sequence in-
cluding NHC-catalyzed silylstannation reaction, trans-
Scheme 3. “One-pot” procedure for the synthesis of a-hy-
metallation and reverse Brook rearrangement reac- droxysilanes.
Table 5. Organocatalytic addition of Bu3Sn-SiMe3 to a,b-unsaturated aldehydes.
Entry
R1/R2/R3
Conversion[7] [%]
5 [%] (% E/Z)
6 [%]
7 [%]
1
2
3
4
5
6
i-Pr/H/H
Bn/H/H
H/Pr/H
Me/Et/H
Bn/CH2Bn/H
H/Me/Me
99
5b 97 (100/0)
5c 95 (100/0)
5d 60 (100/6)
5e 76 (100/1)
5f 87 (100/2)
5g 57 (100/5)
6b: 2
6c: 4
6d: 1
6e: 2
6f: 2
6g: 0
7b: 0
7c: 1
7d: 39
7e: 21
7f: 0
100
100
99
89
100
7g: 43
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N-Heterocyclic Carbene-Mediated Organocatalytic Transfer of Tin onto Aldehydes
(258C). The crude stannyl silyl ether was dissolved in 10 mL
sponding a-hydroxysilanes 8 in, respectively, 65, 56
and 35% yields.
of THF and cooled to À788C. A hexane solution (2.5M) of
n-butyllithium (1 equiv.) was added dropwise via syringe.
The reaction mixture was then allowed to stir at À788C for
15 min and quenched by the rapid addition of water. After
warming to room temperature the reaction mixture was ex-
tracted with petroleum ether. The combined organic phase
were washed with aqueous saturated sodium chloride solu-
tion, and dried over anhydrous Na2SO4. The crude product
was purified by flash chromatography on silica gel using
100% petroleum ether to first elute the tetraalkylstannane
by-product followed by a gradient elution using 9/1 to 8/2
petroleum ether/ether system.
In summary, we have developed a highly efficient
organocatalytic procedure for the transfer of tin onto
aldehydes. a-Silyloxyalkylstannanes and g-silyloxyal-
lylstannanes can be efficiently prepared from
Bu3SnSiMe3 and N-heterocyclic carbenes. This is the
first time that a carbene acts as an organocatalyst in
the transfer of a metalloid and not as a ligand.
Experimental Section
Phenyl-trimethylsilanyl-methanol
(8a):
1H NMR
(300 MHz, C6D6): d=0.00 (s, 9H, 3ꢄCH3), 1.37 (d, J=
2.8 Hz, 1H, OH), 4.18 (d, J=2.8 Hz, 1H, CH), 7.01–7.22 (m,
5H, 5ꢄCHAr); 13C NMR (75 MHz, C6D6): d=À4.0 (3ꢄ
CH3), 70.3 (CH), 125.3 (2ꢄCHAr), 125.9 (CHAr), 128.3 (2ꢄ
CHAr), 145.0 (CAr).
General Procedure for the Synthesis of a-
Silyloxyalkylstannanes 1a–t
In an oven-dried tube were placed in Et2O (2 mL), TMS-
SnBu3 (0.525 mmol, 1.05 equiv.), aromatic compound
(0.5 mmol, 1 equiv.) followed by catalyst (0.025 mmol,
5 mol%). The reaction mixture was stirred at room tempera-
ture for 1 h (258C). The solvent was then removed under re-
duced pressure and the residue was purified by flash chro-
matography on silica gel.
Acknowledgements
Trimethyl-(phenyltributylstannanylmethoxy)-silane (1a):
1H NMR (300 MHz, C6D6): d=0.11 (s, 9H, 3ꢄCH3), 0.83–
1.07 (m, 15H, 3ꢄCH2 and 3ꢄCH3), 1.23–1.66 (m, 12H, 6ꢄ
CH2), 5.29 (s, 2JSn,H =28 Hz, 1H, CH), 6.93–7.00 (m, 1H,
CHAr), 7.13–7.30 (m, 4H, 4ꢄCHAr); 13C NMR (75 MHz,
C6D6): d=0.1 (3ꢄCH3), 9.6 (3ꢄCH2, 1JSn,C =302 Hz), 14.0
R.B. thanks La Region PACA for financial support. John E.
Moses is also gratefully acknowledged for the fruitful discus-
sion and for the careful reading of this manuscript.
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2
3
1
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Adv. Synth. Catal. 2010, 352, 661 – 666
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
665

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success.
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