Thioacetalization of aldehydes and ketones catalyzed by hexabromoacetone

Article abstract of DOI:10.1080/10426507.2017.1321646

Protection of p-anisaldehydewith 1,3-propanedithiol under UV irradiation without a catalyst resulted in 87% yield of 1,3-dithiane in 20 min. Addition of hexabromoacetone further reduced the reaction time and UV irradiation also accelerated the formation o

Full text of DOI:10.1080/10426507.2017.1321646

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Thioacetalization Of Aldehydes And Ketones
Catalyzed By Hexabromoacetone
Kittichai Chaiseeda & Warinthorn Chavasiri
To cite this article: Kittichai Chaiseeda & Warinthorn Chavasiri (2017): Thioacetalization Of
Aldehydes And Ketones Catalyzed By Hexabromoacetone, Phosphorus, Sulfur, and Silicon and the
Related Elements, DOI: 10.1080/10426507.2017.1321646
To link to this article: http://dx.doi.org/10.1080/10426507.2017.1321646
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Date: 03 May 2017, At: 04:51

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THIOACETALIZATION OF ALDEHYDES AND KETONES CATALYZED BY
HEXABROMOACETONE
Kittichai Chaiseeda, and Warinthorn Chavasiri*
Natural Products Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn
University, Bangkok 10330, Thailand.
*E-mail: warinthorn.c@chula.ac.th
Abstract
Protection of p-anisaldehyde with 1,3-propanedithiol under UV irradiation without a catalyst
resulted in 87% yield of 1,3-dithiane in 20 min. Addition of hexabromoacetone further reduced
the reaction time and UV irradiation also accelerated the formation of dithianes, yielding 95% of
1,3-dithiane in only 1 min. Aromatic aldehydes or ketones with electron-donating substituents at
the ortho or para position increased the yield of the corresponding dithianes, while electron-
withdrawing group decelerated the reaction. Aldehydes were easier to protect than ketones, and
aliphatic ketones were also more readily protected than aromatic ketones. The protection was
highly selective towards an aldehyde compared to a ketone.
Graphical Abstract
Keywords
hexabromoacetone, dithioacetal, dithiane, dithiol, aldehyde, ketone
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INTRODUCTION
Several methods are available for the protection of carbonyl compounds such as
aldehydes and ketones. One of these methods is to convert them to thioacetals.1 Two drawbacks
of this method are first, that the reagents used for protection have a foul smell and second, that
the method occasionally produces by products that are difficult to remove during deprotection.
However, 1,3-dithiane and 1,3-dithiolane are exceptionally stable under acidic conditions
compared to the dioxolane or 1,3-dioxane groups.1 In addition, the dithianes or dithiolanes
formed from aldehydes can be deprotonated by n-BuLi to produce an anion that can react with
various electrophiles, a very useful method for synthesizing more complex molecules, also
known as the Corey-Seebach Reaction.2 Recently, 2-aryl-1,3-dithianes have also been used for
palladium-catalyzed cross-coupling reaction with aryl bromides.3 Moreover, there are several
other reagents that can be used in place of 1,3-propanedithiol and 1,2-ethanedithiol, which will
4
help avoid the stench.1,
Catalysts that have been recently used include sulfonated
polyanthracene,5 graphene oxide,6 silica phenyl sulfonic acid,7 Brønsted acidic imidazolium
salts,8 dialkyltriazolium salts,9 Al(HSO4)3,10 I2/nanostructured pyrophosphate,11 tungstate sulfuric
acid,12 InF3,13 and MCM-41-supported mercapto ruthenium complex.14 Additionally, there are
also other halides or halogenating compounds that have been used such as tetrabutylammonium
tribromide (TBATB),15 molecular iodine,16 molecular iodine supported on neutral alumina,17 and
N-bromosuccinimide.18
We have been interested in halogenating compounds and have reported their application
in various roles.19 We herein report the use of hexabromoacetone (HBA) as a catalyst for
thioacetalization of aldehydes and ketones with 1,3-propanedithiol.
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RESULTS AND DISCUSSION
p-Anisaldehyde was selected as a model compound for thioacetalization with 1,3-
propanedithiol. Various conditions were tested to find the optimal reaction conditions as well as
to observe the effect of UV irradiation and HBA (Table 1). Without UV irradiation and HBA,
there was no dithiane product formed after 20 min (entry 1). When the reaction was performed
under UV irradiation without HBA for 20 min, the amount of dithiane product was 87% (entry
2). According to a literature search, protection of carbonyl compounds using a thiol or dithiol
under UV irradiation has not been previously reported. When HBA was added at 2.5 mol%, 95%
of the product formed in only 1 min (entry 3). When the reaction time was increased to 20 min,
all p-anisaldehyde was converted to the desired product (entry 4). Using both UV irradiation and
HBA for 1 min (entry 5) did not improve the yield from the conditions without UV irradiation
(entry 3). Based on these results, protection of p-anisaldehyde with 1,3-propanedithiol was
possible under UV irradiation even without HBA. The addition of HBA significantly reduced the
reaction time and produced an excellent yield of the product, while using both UV irradiation
and HBA did not improve the yield. It should be noted that for the reaction under UV irradiation
without HBA, other aldehydes were successfully protected without a catalyst as well, but when
ketone substrates such as acetophenone were tested and there was no product. Therefore, this
condition could be applicable only with aldehyde substrates. The amount of HBA was reduced
from 2.5 to 1 and 0.5 mol% (Table S 1, Supplemental Materials) and the yields of the dithiane
product were still quite high in both cases. This shows that smaller amounts of HBA can be used
for protection of p-anisaldehyde with 1,3-propanedithiol.
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HBA was subsequently applied for the thioacetalization of various aldehydes and ketones
with 1,3-propanedithiol using 2.5 mol% of HBA as a catalyst in acetonitrile 1 mL (Table 2). For
benzaldehyde and 2-methoxybenzaldehyde, all of the substrate was converted within 1 min to the
corresponding dithiane product without UV irradiation (entries 1 and 2). Salicylaldehyde was
more difficult to protect. After 30 min under UV irradiation the yield was 93% (entry 3). Using
an aldehyde with an electron-withdrawing group at the ortho position such as 2-
nitrobenzaldehyde, the reaction without UV irradiation for 1 min gave only 4% of the product
(entry 4), indicating that an electron-withdrawing group impedes the reaction. Addition of UV
irradiation increased the yield to 10% (entry 5), and increasing the reaction time to 1 h increased
the yield to 93% (entry 6). Despite having an electron-donating group substituent at the para
position, using 4-diethylaminobenzaldehyde resulted in only a trace amount of dithiane product
after 1 min without UV irradiation (entry 7). Addition of UV irradiation (entry 8) or prolonging
the reaction (entry 9) did not help improve the yield substantially, and the yield was only 10%
after 1 h. This indicates that this method might not be compatible with an amine. Aliphatic
aldehydes were also protected rapidly as seen in the case of 2-ethylbutyraldehyde which was
complete within 1 min without UV irradiation (entry 10). Cyclic aliphatic ketones can also be
protected and the protection of cyclohexanone gave 100% yield of the corresponding dithiane
within 20 min under UV irradiation (entry 11). As mentioned above, the protection of ketones is
more difficult, especially using aromatic ketones. When the reaction was performed with
acetophenone with HBA and without UV irradiation only 1% of the dithiane was formed (entry
12). When UV irradiation was applied to the reaction, the yield increased to 6% (entry 13) and
when the reaction time was increased to 1 h, the yield was 74% (entry 14). Propiophenone also
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behaved similarly. In 1 h under UV irradiation, the reaction produced 73% of the dithiane
product (entry 15). Having an electron-donating group at the para position increased the yield as
seen in the case of 4’-methoxyacetophenone compared to acetophenone. For a 1 min reaction
time, the yield was 1% (entry 16), and adding UV irradiation increased the yield to 12% (entry
17), which was twice as much as acetophenone. After 1 h the yield was 80% (entry 18), which
was also higher than that of acetophenone. The reaction was also applied to α-tetralone and after
1 h under UV irradiation the yield was 89% (entry 19). Other thiols were also investigated, and
the protection of p-anisaldehyde with both thiophenol and 1-butanethiol produced 93% yield of
the corresponding dithioacetal after 20 min under UV irradiation (entries 20 and 21).
Since the protection of ketones with 1,3-propanedithiol by HBA is more difficult than
protection of aldehydes, various conditions were tested for protection of acetophenone (Table 3),
in order to improve the yield. As a reference, when the reaction was performed at room
temperature for 1 h without UV irradiation, the yield was only 66% (entry 1). Under UV
irradiation, the yield of dithiane increased to 74% (entry 2) and performing the reaction in
refluxing acetonitrile gave a 90% yield (entry 3). This result is better than literature results using
sulfonated polyanthracene (S-PAT), which gave only 75% yield in 7 h,5 or alumina sulfuric acid,
which produced 85% yield in 45 h.26 When the reaction mixture was sonicated for 1 h, the yield
was also relatively high at 88% (entry 4). These results indicate that various methods can be used
to improve the results.
The selectivity of aldehyde and ketone was further investigated (Table 4). Using 1 mmol
of 1,3-propanedithiol and 1 mmol each of p-anisaldehyde and acetophenone, only the dithiane of
p-anisaldehyde formed (entry 1). When two aldehydes, p-anisaldehyde and benzaldehyde, and
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acetophenone were used, the yields of the dithiane of p-anisaldehyde and benzaldehyde were
67% and 33%, respectively, while the dithiane of acetophenone did not form (entry 2). Using
only aldehydes, p-anisaldehyde and benzaldehyde, the yields were similar to entry 2 with 66%
and 34% yield of the dithiane of p-anisaldehyde and benzaldehyde formed, respectively. The
results indicate that the reaction is highly selective towards aldehyde when there are an
aldehyde(s) and ketone. Furthermore, the reaction is more selective toward an aldehyde with an
electron-donating group.
To investigate the reaction mechanism, a radical scavenger (2,2,6,6-tetramethylpiperidin-
1-yl)oxyl (TEMPO) was added to the reaction of p-anisaldehyde with 1,3-propanedithiol (Table
5). After 20 min under UV irradiation, only 1% of the dithiane formed (entry 1) and when HBA
was added the amount of the dithiane increased to 2%. However, this is still considered to be
very low. These results illustrate that radical intermediates were generated in the reaction and the
reaction was inhibited by TEMPO. The mechanism is, therefore, proposed as shown in Scheme
1.
CONCLUSION
The formation of dithiane of aldehyde under UV irradiation without any catalyst is
advantageous in terms of reaction preparation and purification. Addition of HBA greatly reduced
the reaction time for the formation of dithiane. In most cases, the yields were very high in a very
short period of time using very small amount of HBA. For aldehydes, when HBA was used, UV
irradiation was generally not necessary, but it helped accelerate the formation of dithiane for
more difficult substrates. Having electron-donating substituents at the ortho and para position
increased the yield of the corresponding dithiane, while the electron-withdrawing group
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decelerated the reaction. Aldehydes were easier to protect than ketones, which could be a benefit
when selectivity is required for the protection of aldehydes. Aliphatic ketones were also more
readily protected than aromatic ketones. The protection was highly selective towards aldehyde
compared to ketone and towards an aldehyde with an electron-donating group compared to a
compound without such group. HBA is now commercially available and with the fast and
efficient protections reported herein, using HBA should be a good alternative for other methods.
EXPERIMENTAL
General Information
Chemicals and solvents were purchased from standard suppliers and were used without
1
13
further purification. HBA was obtained from Sigma-Aldrich. H and C NMR spectra were
recorded on a Bruker Ultrashield 300 NMR spectrometer, a Bruker Ultrashield 400 Plus NMR
spectrometer, or a Varian Mercury NMR spectrometer with an Oxford YH400 magnet. GC
analysis was performed using a Varian CP-3800 Gas Chromatograph equipped with SGE BP1
column. The home-made UV reactor consisted of eight UV lamps (6 W, 254 nm, Sylvania G6W
T5) and a fan for ventilation (Figure S 1-S 2, Supplemental Materials). A magnetic stirrer was
placed underneath the UV reactor for stirring the reaction. The reactions under UV irradiation
were performed in custom-made quartz tubes for maximum UV transmission (Figure S3,
Supplemental Materials). The tubes were placed in the middle of the UV reactor at
approximately 7 cm from the nearest UV lamp. Sonication was performed using an Elma
Transsonic T460/H ultrasonic unit.
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Typical Experimental Procedure
Representative procedure for thioacetalization of carbonyl compounds with
1,3-propanedithiol
To a 25-mL quartz tube was added op-anisaldehyde (1 mmol, 136 mg), HBA (0.025
mmol, 13 mg), of 1,3-propanedithiol (1.1 mmol, 119 mg), and CH3CN (1 mL). The reaction
mixture was stirred under UV irradiation for 1 min. To quantify the product using GC, 1 mmol
(154 mg) of biphenyl was added as an internal standard. Sample was diluted with CH3CN and
injected into GC equipped with SGE BP1 column. To verify the product, column
chromatography with 2% ethyl acetate in hexane was performed to separate the desired product
and 1H and 13C NMR were compared to literature.6, 14, 20-25
2-Ethyl-2-phenyl-1,3-dithiane, 2j
Colorless oil, 1H-NMR (300 MHz, CDCl3) δ: 7.90 (d, J = 7.2 Hz, 2H), 7.37 (t, J = 7.7 Hz,
2H), 7.25 (t, J = 7.4 Hz, 1H), 2.66-2.70 (m, 4H), 2.05 (q, J = 7.4 Hz, 2H), 1.89-1.97 (m, 2H),
0.81 (t, J = 7.5 Hz, 3H); 13C-NMR (75 MHz, CDCl3) δ: 141.6 (C), 129.0 (CH x 2), 128.4 (CH x
2), 126.8 (CH), 59.7 (C), 38.1 (CH2), 27.6 (CH2 x 2), 25.4 (CH2), 8.6 (CH3).
FUNDING
This work was supported by the 90th Year Chulalongkorn Scholarship and the Graduate
School, Chulalongkorn University, Thailand.
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Table 1 Effect of UV irradiation and HBA on thioacetalization of p-anisaldehyde with 1,3-
propanedithiol by HBA
Entry
HBA (mol%) UV Irradiation Time (min)
Remaining
Yield of Dithiane20
Aldehyde (%)a
(%)a
0
1
2
3
4
5
-
-
Yes
-
20
20
1
100
15
5
-
87
2.5
2.5
2.5
95
-
20
1
0
100
95
Yes
5
Reaction conditions: 1 mmol (136 mg) p-anisaldehyde, 1.1 mmol (119 mg) 1,3-propanedithiol, 1
mL CH3CN, rt.
aDetermined by GC and SGE BP1 column using biphenyl as an internal standard.
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Table 2 Thioacetalization of various carbonyl compounds with 1,3-propanedithiol by HBA
Entry
Reagent
Product
Time
(min)
Product
(%)a
1
2
3
2b21
1
1
100
100
93
1b
1c
1d
2c20
2d21
30b
4
5
6
7
8
9
2e21
1
1b
4
10
93
1
1e
1e
1e
1f
1f
1f
2e
2e
60b
1
2f22
2f
1b
2
60b
10
2f
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10
2g23
1
100
100
1g
11
2h20
20b
1h
12
2i20
2i
1
1
6
1i
13
1b
1i
14
60c
90
1i
2i
15
60b
73
1j
2j
16
2k24
2k
1
1
1k
17
1b
12
80
1k
18
60b
1k
2k
19
2l25
60b
89
1l
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20d
2m6
20b
93
93
1a
21e
2n14
20b
1a
Reaction conditions: 1 mmol substrate, 1.1 mmol 1,3-propanedithiol, 0.025 mmol (2.5 mol%)
HBA, 1 mL CH3CN, rt.
aDetermined by GC and SGE BP1 column using biphenyl as an internal standard.
bUnder UV irradiation.
cUnder refluxing CH3CN.
dThiophenol was used instead of 1,3-propanedithiol.
e1-Butanethiol was used instead of 1,3-propanedithiol.
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Table 3 Variation of reaction condition for protection of acetophenone with 1,3-propanedithiol
by HBA
Entry
Condition
RT, no UV
Yield of Dithiane (%)a
1
2
3
4
66
74
90
88
RT, UV irradiation
Reflux
Sonication
Reaction conditions: 1 mmol acetophenone, 1.1 mmol 1,3-propanedithiol, 0.025 mmol (2.5
mol%) HBA, 1 mL CH3CN, 1 h.
aDetermined by GC and SGE BP1 column using biphenyl as an internal standard.
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Table 4 Selectivity of aldehyde and ketone
Entry
1a (mmol) 1b (mmol)
1i (mmol)
2a (%)a
100
67
2b (%)a
2i (%)a
1
2
3
1
1
1
-
1
1
-
-
0
0
-
1
1
33
34
66
Reaction conditions: 1 mmol 1,3-propanedithiol, 0.025 mmol (2.5 mol%) HBA, 1 mL CH3CN,
20 min, rt, under UV irradiation.
aDetermined by GC and SGE BP1 column using biphenyl as an internal standard.
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Table 5 Addition of a radical scavenger TEMPO to the reaction
Entry
HBA (mol%)
Yield of Dithiane (%)a
1
2
-
1
2
2.5
Reaction conditions: 1 mmol (136 mg) p-anisaldehyde, 1.1 mmol (119 mg) 1,3-propanedithiol, 1
mmol (156 mg) (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 1 mL CH3CN, 20 min, rt, under UV
irradiation.
aDetermined by GC and SGE BP1 column using biphenyl as an internal standard.
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Scheme 1 Proposed mechanism
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