Trimethylsilyloxide-Catalysed Peterson Olefinations with 2,2-Bis(trimethylsilyl)-1,3-dithiane

Article abstract of DOI:10.1002/ejoc.201501185

The synthesis of the reagent 2,2-bis(trimethylsilyl)-1,3-dithiane was directly achieved in an excellent yield from 1,3-dithiane in one step. The bench-stable reagent can be utilized for Peterson olefinations using either TMSOK/Bu₄NCl or fluorid

Full text of DOI:10.1002/ejoc.201501185

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201501185
Trimethylsilyloxide-Catalysed Peterson Olefinations with
2,2-Bis(trimethylsilyl)-1,3-dithiane
Atul Manvar[a] and Donal F. O’Shea*[a]
Keywords: 2,2-Bis(trimethylsilyl)-1,3-dithiane / Bench-stable reagents / Synthetic methods / Olefination / Ketene
dithioacetals
The synthesis of the reagent 2,2-bis(trimethylsilyl)-1,3-dithi-
ane was directly achieved in an excellent yield from 1,3-di-
thiane in one step. The bench-stable reagent can be utilized
for Peterson olefinations using either TMSOK/Bu4NCl or
fluoride ion as promoter of α-silyl carbanion formation,
thereby generating ketene dithioacetals, which are versatile
intermediates in organic synthesis. Olefination reactions
were successfully achieved with aromatic, heteroaromatic,
aliphatic aldehydes and ketones.
[bis(phenylthio)methyl]boronic ester 6 (Route D) is a less
preferable choice owing to the laborious synthetic operation
and limitation of bench stability[10] The use of the Takai
alkenation employing 1,3-dithian-2-one 7 in the presence of
dibromoalkane 8, zinc and TiCl4 are scarce (Route E).[11]
Introduction
Since the seminal reports by Corey and Seebach on tem-
porary reversal of polarity of carbonyl groups (umpolung),
2-lithio-1,3-dithiane derivatives have been widely exploited
as sulfur-stabilized acyl anion equivalents.[1] 1,3-Dithiane-
derived compounds have had substantial impact on natural
product synthesis and in the construction of myriads of het-
erocycles.[2] The synthetic potential of ketene dithioacetals
1 is also broad, because the double bond is susceptible to
both electrophilic and nucleophilic transformations due to
the stabilizing effect of the adjacent sulfur atoms.[3–5] Poly-
functionalised ketene dithioacetals were first prepared from
carbon disulfide and substrates bearing active methylene
groups followed by alkylation of the sulfur atoms.[6] Subse-
quent approaches developed for the synthesis of cyclic
ketene dithioacetals 1 use various classical olefination reac-
tions to form the C–C double bond from carbonyl groups
and suitably substituted 1,3-dithiane or 1,3-dithiolane carb-
anions (Scheme 1). For example, Wittig or Horner–Em-
mons olefinations utilising the ylides 2 or 3 have been re-
ported, though the synthesis of these ylides requires numer-
ous synthetic operations and produce high-molecular-
weight by-products (Route A).[7] Similar issues occur with
the use of (2-diphenylphosphinoyl)- or (2-diphenylthio-
Scheme 1. Routes for the synthesis of ketene dithioacetals 1.
phosphinoyl)dithianes
4 (analogous to Horner–Wittig
ylides), and their use is rare (Route B).[8] Peterson ole-
finations have proved successful with α-silyl carbanion 5 be-
ing generated by deprotonation using strong base (e.g.
BuLi) at low temperature, followed by treatment with a
carbonyl compound to yield 1 (Route C).[9] The utility of
Of this collection of olefination approaches, the Peterson
olefination provides superior atom economy over the phos-
phorus reagents as it produces low-molecular-weight vola-
tile by-products.[12] Despite these significant merits, the Pe-
terson olefination remains under-utilized presumably due to
the additional synthetic efforts required for its use. For ex-
ample, in the literature, the α-silyl carbanion 10 is typically
generated by the deprotonation of 9 using strong base, e.g.
BuLi (–25 °C to –78 °C), in THF, which subsequently is
[a] Department of Pharmaceutical and Medicinal Chemistry,
Royal College of Surgeons in Ireland
123 St. Stephen’s Green, Dublin 2, Ireland
http://www.rcsi.ie/index.jsp?p=265&n=808&a=3810
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201501185.
treated with carbonyl compound 12 to produce
1
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A. Manvar, D. F. O’Shea
SHORT COMMUNICATION
(Scheme 2).[9] It has been previously reported that bis(tri-
With an efficient route to 11 in hand, optimization of the
methylsilane) reagents can readily participate in olefination olefination conditions was conducted using benzaldehyde
reactions with suitable Lewis base activation; though this as model electrophile. Encouragingly, using 1 equiv. of CsF
approach has not been adopted for ketene dithioacetal syn- in DMF at room temperature for 15 h gave ketene di-
thesis.[13] The use of bis(trimethylsilane) compounds as Pe- thioacetal 1a in a 73% yield (Table 1, Entry 1). Examina-
terson olefination reagents offers an advantage in that it tion of 20 mol-% tetrabulylammonium fluoride (TBAF)
removes the need to pre-form the carbanion species as its and tetrabutylammonium triphenyldifluorosilicate (TBAT)
equivalent is generated in situ in the presence of the alde- as alternative fluoride activators in THF at room tempera-
hyde (Scheme 2). As such, the merit of using reagent 11 in ture gave lower but satisfactory yields of 1a (Table 1, En-
olefination reactions was investigated.
tries 2, 3). A solvent-effect study was carried out using
20 mol-% TBAT as activator in toluene, DCM and aceto-
nitrile at room temperature; however, each gave a poorer
yield than THF (Entries 4–6). Increasing the reaction tem-
perature (10 mol-% TBAT) in either THF or toluene gave
improved yields when compared to room temperature, and
1a was obtained in 72% and 83% yields, respectively, and
in a shorter reaction time of 3 h (Table 1, Entries 7, 8). The
use of Me3SiO–/Bu4N+ as a general activation reagent of
organotrimethylsilanes for addition reactions has been es-
tablished, though its ability to activate geminal bis(silanes)
for Peterson olefinations has not been previously re-
ported.[15] For comparison with fluoride, TMSOK/Bu4NCl
(10 mol-%) was used as activator in toluene at 80 °C for 3 h
and found to give 1a in a comparable 81% yield (Entry 9).
Notably, under ambient conditions TMSOK/Bu4NCl
(10 mol-%) gave 79% isolated yield of the olefin 1a. Collec-
tively, from these results TBAT and TMSOK/Bu4NCl
(10 mol-%) in toluene at 80 °C were chosen as conditions to
explore the substrate scope and generality of this method.
Scheme 2. Ketene dithioacetals by Peterson olefination.
In the current paper we report on an efficient route for
the synthesis of reagent 11, its optimal C–Si bond activation
with trimethylsilyloxide or fluoride for the olefination of
carbonyl compounds.
Table 1. Reaction optimization.[a]
Results and Discussion
At the outset of this work, the synthesis of reagent 2,2-
bis(trimethylsilyl)-1,3-dithiane (11) was accomplished by
two related approaches. Deprotonation of 1,3-dithiane (13)
with BuLi (1.2 equiv.) in THF at 0 °C and treatment with
TMSCl gave 2-(trimethylsilyl)-1,3-dithiane (9) in 89% iso-
lated yields. Repeating this reaction sequence with 9 af-
forded 11 in 87% yield. Remarkably, this synthesis could be
streamlined into a one-pot strategy from dithiane 13 using
2.2 equiv. of BuLi in THF at 0 °C (15 min), with subsequent
chlorotrimethylsilane quench giving an excellent 90% yield
of product 11 (Scheme 3).[14] Presumably, this works by first
formation of 9 with rapid in situ deprotonation and conver-
sion into 11. Compound 11 can be considered a bench-
stable surrogate of 10 requiring a Lewis base promoter to
generate its α-silyl carbanion equivalent.
Entry
Activator (mol-%)
Solvent
T
[°C]
t
[h]
Yield
[%]
1
2
3
4
5
6
7
8
9
[b]CsF
TBAF (20)
TBAT (20)
TBAT (20)
TBAT (20)
TBAT (20)
TBAT (10)
TBAT (10)
DMF
THF
THF
MeCN
DCM
PhMe
THF
PhMe
PhMe
PhMe
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
70
80
80
r.t.
15
15
15
15
15
15
3
3
3
3
73
60
63
51
31
49
72
83
81
79
TMSOK/Bu4NCl (10)
TMSOK/Bu4NCl (10)
10
[a] Conditions: 11 (1.0 equiv.), benzaldehyde (1.5 equiv.) and sol-
vent (2 mL). [b] 1.0 equiv. of CsF and MS (4 Å) used.
The substrate scope was first evaluated with a variety of
enolisable and non-enolisable aromatic and aliphatic alde-
hydes using fluoride as activator (Scheme 4, Conditions A);
ortho-, meta- and para-substituted aldehydes worked well in
the presence of TBAT activator delivering the correspond-
ing products 1b–g in 62–90% yields (Scheme 4, Condi-
tions A). Pleasingly, (E)-cinnamaldehyde was also tolerated
Scheme 3. Synthesis of 2,2-bis(trimethylsilyl)-1,3-dithiane (11).
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Peterson Olefinations with 2,2-Bis(trimethylsilyl)-1,3-dithiane
to give 83% yield of the unsaturated ketene dithioacetal 1h, were unsuccessful in the presence of 10 mol-% TBAT acti-
and sterically hindered mesitaldehyde and 2-naphth- vator. Though switching to the more forcing conditions of
aldehyde gave 1i and 1j in 76% and 85% yield, respectively. 1 equiv. of CsF/DMF was sucessful giving 63–78% yield of
Encouraged by these results, the participation of hetero- the corresponding ketene dithioacetals 1p–r.
cyclic aldehydes was also tested. Furfurals and N-Boc-sub-
To illustrate the practicality of trimethylsilyloxide-cata-
stituted indolecarbaldehyde reacted successfully under the lysed reactions, a side-by-side comparison with fluoride ac-
established conditions providing 62–85% yield of the prod- tivation was conducted for eight carbonyl substrates
ucts 1k–m. A representative of the metallocene class, ferro- (Scheme 4, Conditions B). Aromatic p-F- and p-MeO-sub-
cenecarbaldehyde also proved to be an effective substrate stituted aldehydes gave 84% and 81% yield of the corre-
for olefination with 81% yield of the product 1n obtained. sponding products 1d, g, which was comparable to those
Gratifyingly, an enolisable alicyclic aldehyde, cyclohexane- under fluoride conditions. Similar yields for ketene di-
carbaldehyde, gave a moderate yield of olefin 1o (45%), thioacetals 1h, i, j, k and n were obtained from cinnam-
though acetophenone failed to give the desired product. aldehyde, mesitaldehyde, 2-naphthyl-, furyl- and ferrocene-
The olefination of ketones, such as benzophenone, 4-meth- substituted aldehydes (Scheme 4, Conditions B). A repre-
oxybenzophenone and 2,2,2-trifluoromethylacetophenone, sentative ketone, benzophenone, could also be induced to
react with TMSOK/Bu4NCl, though in a 42% yield – some-
what lower than under CsF/DMF conditions. Taken to-
gether, these results prove that reagent 11 is suitable for Pe-
terson olefination with either trimethylsilyloxide or fluoride
as silicon activator for the conversion of a wide set of carb-
onyl substrates into their corresponding ketene dithioacet-
als.
To further asses TBAT and TMSOK/Bu4NCl as silicon
activators, a comparison of conversion over time for the
reaction of 11 with benzaldehyde was carried out. Using
either TBAT (10 mol-%) or TMSOK/Bu4NCl (10 mol-%) in
toluene at ambient temperature, the reaction progress was
1
monitored for the formation of product 1a using H NMR
spectroscopy. Interestingly, the reaction with Me3SiO–/
Bu4N+ activation gave more than 90% conversion within
30 min, while TBAT reached only approximately 20% con-
version within the same time (Figure 1). This result shows
that Me3SiO–/Bu4N+ is a superior activator of 11 for the
generation of its corresponding α-silyl carbanion.
Figure 1. Relative percentage formation of 1a using TBAT (circles)
or TMSOK/Bu4NCl (triangles).
A plausible mechanistic cycle of trimethylsilyloxide-cata-
lysed olefinations with 11 is depicted in Scheme 5. The mix-
ing of TMSOK and Bu4NCl results in a cation exchange
Scheme 4. Scope of aldehydes and ketones.[a] [a] Conditions: 11
producing Me3SiO–Bu4N+ and KCl, which has the effect of
(1.0 equiv.), aldehydes or ketones (1.5 equiv.), PhMe (2.0 mL).
increasing the nucleophilic character of Me3SiO–. It could
[b] 1.0 equiv. CsF, DMF (2.0 mL) and MS (4 Å) used. [c] 30 mol-
% TMSOK/Bu4NCl used.
be expected that Me3SiO– interaction with the silicon atom
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A. Manvar, D. F. O’Shea
SHORT COMMUNICATION
of a TMS substituent on 11 gives rise to a hypervalent sili-
con species resulting in the breaking of a carbon–silicon
bond with the generation of carbanion 10 and hexamethyl-
disiloxane (HMDS). Addition of 10 to the carbonyl com-
pound would form alkoxide 13, which upon elimination of
Me3SiO– produces product 1 and regenerates the catalyst
(Scheme 5). To confirm that the solvent HMDS (Me3SiO-
SiMe3) with low molecular boiling point was formed as the
final by-product, the reaction of 11 and benzaldehyde was
Conclusions
The one-pot synthesis of 2,2-bis(trimethylsilyl)-1,3-dithi-
ane (11) as a bench-stable Peterson olefination reagent was
achieved in excellent yield. The mild activation of 11 using
either trimethylsilyloxide or fluoride induced a Peterson
olefination sequence with aldehydes and ketones to deliver
the corresponding ketene dithioacetals in good to excellent
1
yields. H NMR analysis indicates that trimethylsilyloxide
acts as a catalyst for the reaction, and fluoride initiates the
reaction with trimethylsilyloxide contributing in an auto-
catalysis manner. Further studies of the use of geminal bis-
(trimethylsilane)s for olefination reactions is ongoing and
will be reported on in due course.
1
conducted in [D8]toluene with monitoring of the H NMR
chemical shifts between δ = –0.1 and 0.4 ppm (Figure 2). It
was seen that the sole TMS-containing by-product with a
resonance at δ = 0.10 ppm was Me3SiOSiMe3, which was
formed in the same ratio as the starting material 11 (TMS:
δ = 0.26 ppm) (Figure 2, Panels A, B and D).
Experimental Section
General Procedure for the Olefination of 2,2-Bis(trimethylsilyl)-1,3-
dithiane (11) with Carbonyl Compounds:
A solution of 11
(0.50 mmol) and aldehydes or ketones (0.75 mmol) in toluene
(2.0 mL) was treated with TMSOK and Bu4NCl (0.05 mmol) or
TBAT (0.05 mmol) under N2, and the resulting solution was heated
at 80 °C for 3 h. The reaction mixture was cooled to room temp.,
the solvent removed under reduced pressure, and 2 m HCl (15 mL)
added. The residue was extracted with diethyl ether (15ϫ 3 mL),
the organic layers were combined, washed with brine, dried with
Na2SO4 and concentrated to dryness. Purification by aluminum ox-
ide chromatography eluting with cyclohexane/ethyl acetate gave the
corresponding ketene dithioacetal.
Acknowledgments
Financial support from the Irish Research Council and the Science
Foundation Ireland is gratefully acknowledged.
Scheme 5. Trimethylsilyloxide-catalysed Peterson olefination.
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Received: September 11, 2015
Published Online: October 22, 2015
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