Synthesis of acylsilanes via oxidative hydrolysis of 2-silyl-1,3-dithianes mediated by N-bromosuccinimide

Article abstract of DOI:10.1016/S0022-328X(00)00181-9

The oxidative method for the hydrolysis of 1,3-dithianes was applied to 2-silyl-1,3-dithianes using N-bromosuccinimide providing acylsilanes with good yields under a short reaction period. The oxidation of aroylsilanes to carboxylic acid was prevented by

Full text of DOI:10.1016/S0022-328X(00)00181-9

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Journal of Organometallic Chemistry 603 (2000) 220–224
Synthesis of acylsilanes via oxidative hydrolysis of
2-silyl-1,3-dithianes mediated by N-bromosuccinimide
Amauri F. Patroc´ınio, Paulo J.S. Moran *
Instituto de Qu´ımica, Uni6ersidade Estadual de Campinas, C.P. 6154-13083-970 Campinas, SP Brazil
Received 7 December 1999; accepted 4 February 2000
Abstract
The oxidative method for the hydrolysis of 1,3-dithianes was applied to 2-silyl-1,3-dithianes using N-bromosuccinimide
providing acylsilanes with good yields under a short reaction period. The oxidation of aroylsilanes to carboxylic acid was
prevented by the addition of bases and the lowering of the reaction temperature. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Aroylsilanes; Oxidation; 1,3-Dithianes; N-Bromosuccinimide
1. Introduction
of a dithiane moiety [7], but for 2-silyl-1,3-dithianes the
mercuric chloride hydrolysis appears to be the most
useful. We have extensively prepared acylsilanes
through the dithiane route in our work [8] and found
problems with the unmasking of the carbonyl. We have
noted that the hydrolysis using mercuric chloride needs
a long period to complete the reaction and, in some
cases, after 12 h at room temperature there was also
starting material and the corresponding aldehyde (re-
sulting from the SiꢀCO cleavage) as a by-product.
Moreover, mercuric chloride is poisonous and experi-
ments involving this compound should be handled with
extreme caution.
The aim of this work is to search for other method-
ologies to the carbonyl regeneration on 2-silyl-1,3-dithi-
anes. We found the oxidative hydrolysis with
N-bromosuccinimide, which is presented in this paper,
as a good alternative to obtain acylsilanes in a shorter
reaction time. To our knowledge, the excellent method
presented by Corey and Erickson [9] for general dithi-
anes was not applicable to 2-silyl-dithianes, possibly
due to the oxidative conditions. The oxidation poten-
tials of acylsilanes (1.45 V) are lower than ketones
(\2.5 V) [10], therefore acylsilanes are more likely to
suffer oxidation to a carboxylic acid mediated by per-
oxide [11], ozone [12], Fe+3, or nitric acid [13a]. The
mechanism proposed for the oxidative hydrolysis with
N-bromosuccinimide probably occurs through an at-
tack at sulfur by the active bromine (generated in situ)
Acylsilanes are compounds that exhibit unique chem-
ical properties and are frequently applied in several
synthetic methodologies such as Reformatsky reactions
[1], intermolecular cyclization [2], stereocontrol in nu-
cleophilic addition [3], photo-oxidations [4] and many
other general reactions [5]. A variety of methods for the
synthesis of acylsilanes are found in the literature and
the dithiane route [6] (Scheme 1) appears to be the
principal method. In contrast to other synthetic meth-
ods, the 1,3-dithiane route enables one to synthesize
acylsilanes with a great variety of silyl and R groups
such as alkyl, aryl and a,b-unsaturated acylsilanes.
2. Results and discussion
The aldehyde conversion to the corresponding 1,3-
dithiane 2 and subsequent silylation to 4 through the
masked acyl anion 3, generally occurs in excellent yields
(Table 1). However, the regeneration of the carbonyl
group (4“5) is the crucial step in this method because
the CꢀSi bond in acylsilanes is relatively weak and can
be cleaved under the hydrolysis conditions. There are
many efficient methods that can be applied for removal
* Corresponding author. Fax: +55-19-7883023.
E-mail address: moran@iqm.unicamp.br (P.J.S. Moran)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00181-9

A.F. Patroc´ınio, P.J.S. Moran / Journal of Organometallic Chemistry 603 (2000) 220–224
221
to afford halo-sulfides [14] and the subsequent forma-
tion of disulfide derivatives [7]. Thus, when the product
is an acylsilane (properly aroylsilanes) a subsequent
reaction of oxidation to the corresponding carboxylic
acid may take place (Scheme 2). We observed in our
laboratory, that the bromine (in excess) promotes this
undesirable acylsilane oxidation [13b].
Although alkanoylsilanes could be obtained without
problem due to their resistance to oxidation (entries 1
and 2, Table 2), some 2-aryl-2-silyl-dithianes lead to a
great amount of the corresponding carboxylic acid. In
the latter case, when electron-withdrawing groups are
attached to the aromatic ring, the carboxylic acid was
obtained rather than acylsilanes (entries 10 and 12). In
some cases, we have partially circumvented this prob-
lem by the addition of base to stop the formation of
excess of bromine through HBr neutralisation. The
lowering of the reaction temperature either inhibits the
oxidation of the aroylsilanes allowing for better yields
of compounds 5g–h at −23°C using acetonitrile in-
Scheme 1.
Scheme 2.
Table 1
Yields obtained in preparation of 1,3-dithianes and 2-silyl-1,3-dithianes
Entry
Aldehyde 1
1,3-Dithiane (2) (%) a
2-Silyl-1,3-dithiane (4) (%) a
1
2
3
4
5
6
7
8
1a (R=PhCH2)
1b (R=Ph2CH)
1c (R=Ph)
1d (R=2-MeOC6H4)
1e (R=3,4-(MeO)2C6H3)
1f (R=3,4-(OCH2O)C6H3)
1g (R=4-ClC6H4)
1h (R=3-CF3C6H4)
80
96
94
93
90
89
95
88
69
76
82
90
85
82
85
92
a Purified by crystallization from hexane, except for 4h (soluble).

222
A.F. Patroc´ınio, P.J.S. Moran / Journal of Organometallic Chemistry 603 (2000) 220–224
Table 2
Hydrolysis of 2-silyl-1,3-dithianes
Entry
2-Silyl-1,3-dithianes (4)
Acylsilanes 5
NBS
HgCl2
Base
Reaction time (min)
Yield (%) c
Yield (%) e
1
2
3
4
5
6
7
8
4a (R=PhCH2)
4b (R=Ph2CH)
4c (R=Ph)
–
–
–
–
40
40
40
40
40
30
30
30
40
30
30
30
30
88 a
80 a
74 b
70
65
4d (R=2–OMeC6H4)
77 a
Et3N
–
Et3N
Ba(OH)2
Imidazole
–
Ba(OH)2
–
Ba(OH)2
90 a
4e (R=3,4-(OMe)2C6H3
90 a
75
95 a
96 a
9
4f (R=3,4-(OCH2O)C6H3)
4g (R=4-ClC6H4)
70 a
76
25
10
11
12
13
27 b
40 b,d
7 a, 20 b
44 b,d
4h (R=3-CF3C6H4)
30
a Reaction temperature at 0°C.
b Reaction temperature at −23°C.
c Total dithiane conversion.
d Acetonitrile was used as solvent.
e 8–12 h at r.t.
stead of acetone as the solvent. Table 2 shows the
hydrolysis yields using mercuric chloride and NBS. It is
evident that besides the shorter reaction time, the hy-
drolysis of 2-aryl-2-silyl-dithianes using NBS increases,
in some cases, the reaction yields in comparison with
the hydrolysis using mercuric chloride.
3.1. General procedure for the preparation of 1,3-dithi-
anes 2 [6]
BF3·OEt2 (80.0 mmol) was added slowly to a stirred
solution of aldehyde (20.0 mmol), 1,3-propanedithiol
(20.5 mmol) in CH2Cl2 (50.0 ml) and molecular sieves
,
(4 A) (12–15 g) at 0°C. The solution was stirred for 2
h at 0°C and warmed to room temperature and was
stirred for another 15 h. After that, aqueous sodium
bicarbonate was added, the product was extracted with
CHCl3 (3×40 ml), the organic phase dried with
MgSO4 and the solvent was evaporated under reduced
pressure. The product was purified by crystallization in
hexane.
3. Experimental
Melting points were measured on a Microquimica
MQAFP-301 apparatus and are uncorrected. IR spec-
tra were recorded on a Perkin–Elmer 1600 FT or
Bomen MB series spectrophotometer. NMR spectra
were recorded on a Bruker AC 300P, Varian Gemini
300 or Inova-500 spectrometer, with CDCl3 as solvent
and CHCl3 as internal standard. Elemental analyses
were measured on a Perkin–Elmer 2400 CHN. Chro-
matography columns were performed with Silica gel-60.
Mass spectra were obtained on a Shimadzu CG/MS-
QP5000. Commercially available chemicals and solvents
were used without further purification. The 1,3-dithi-
anes 2c–g, 2-silyl-1,3-dithianes 4c–g and acylsilanes
5c–g were described previously (Ref. [8] and references
cited therein). Also, the dithiane 2a and the acylsilane
5a were described in Refs. [5c] and [9], respectively.
3.2. General procedure for the preparation of
2-trimethylsilyl-1,3-dithianes 4 [6]
Under Ar atmosphere, n-BuLi (21.0 mmol in hexane
solution) was added cautiously to a stirred solution of
2-aryl-1,3-dithiane (20.0 mmol) in THF (50 ml) at
−23°C. After 1 h, the solution was warmed to 0°C and
then trimethylchlorosilane (25.0 mmol) was added. The
reaction mixture was stirred for another 1 h at 0°C.
After that water was added and then the product was

A.F. Patroc´ınio, P.J.S. Moran / Journal of Organometallic Chemistry 603 (2000) 220–224
223
extracted with CHCl3 (3×40 ml). The organic phase
was dried, the solvent evaporated under reduced pres-
sure and the product purified by crystallization in
hexane.
2.38 (2H-eq, ddd, J 15.3, 4.1 and 3.3 Hz), 2.71 (2H-ax,
ddd, J 15.3, 11.1 and 3.3 Hz), 3.45 (s, 2H), 7.30–7.50
(3H, m) and 7.55 (2Har-ortho, dd, J 1.8 and 7.3 Hz). lc
(75 MHz, CDCl3) −3.0, 24.0, 24.1, 37.9, 45.0, 127.0,
128.2, 131.3 and 139.0. m/z 282 [M+ , 2%], 209 (20),
’
3.3. General procedure for hydrolysis of 2-silyldithianes
4
191 (84), 91 (38), and 73 (100). (Found C, 59.3; H, 7.3.
Anal Calc. C14H22S2Si: C, 59.6; H, 7.8%.)
The 2-silyldithiane 4 was slowly added to a stirred
mixture of N-bromosuccinimide (4–6 molar equiva-
lents) in an 80% aqueous acetone or acetonitrile (ꢀ25
ml) at 0°C or −23°C (see Table 2). During the addi-
tion of the dithiane, the pH was maintained nearly to
neutral by simultaneous addition of base (4–6 molar
equivalents of organic base or 2–3 molar equivalents of
barium hydroxide). After the total addition of dithiane,
the stirring of reaction mixture was maintained for an
additional 20–30 min at 0°C. The cold bath was re-
moved and sodium sulfide solution (ꢀ25 ml) was
rapidly added and after that the product was extracted
with ethyl acetate (3×25 ml). The combined organic
layers were washed with water (40 ml), dried with
MgSO4 and the solvent evaporated under reduced pres-
sure. The product was purified by filtration in a short
column (SiO2, 9:1 hexane–ethyl acetate).
3.7. 2-(Diphenylmethyl)-2-trimethylsilyl-1,3-dithiane
(4b)
Obtained as a crystalline solid, m.p. 126.0–128.0°C.
wmax (KBr, cm−1) 1490, 1424, 1233 and 836. lH (300
MHz, CDCl3) 0.01 (9H, s), 1.50–1.78 (2H, m), 2.05
(2H-ax, ddd, J 13.9, 11.1 and 3.3 Hz), 2.22 (2H-eq,
ddd, J 13.9, 4.5 and 3.6 Hz), 4.43 (1H, s), 7.20–7.38
(3H, m) and 7.94 (2Har-ortho, d, J 7.4 Hz). lc (75
MHz, CDCl3), −1.8, 23.0, 25.4, 39.4, 63.5, 126.9,
128.1, 130.7 and 142.8. m/z 358 [M+ none found], 167
’
(100), 165 (44) and 152 (30). (Found C, 66.6; H, 7.9.
Anal Calc. C20H26S2Si: C, 67.0; H, 7.3%.)
3.8. 2-(3-Trifluoromethylphenyl)-2-trimethylsilyl-
1,3-dithiane (4h)
3.4. 2-(Diphenylmethyl)-1,3-dithiane (2b)
Obtained as a yellow solid, m.p. 59–60°C. wmax (KBr,
cm−1) 1478, 1424, 1251 and 1324. lH (500 MHz,
CDCl3), 0.07 (9H, s), 1.90–2.09 (m, 2H), 2,46 (2H-eq,
ddd, J 15.0, 4,0 and 2.6 Hz), 2.72 (2H-ax, ddd, J 15.0,
12.1 and 2.7), 7.44–7.52 (2H, m), 8.12 (1H, d, J 8.3 Hz)
and 8.21 (1H, broad s). lc (125 MHz, CDCl3), −4.1,
24.9, 25.1, 47.2, 122.1, 124.4 (q, J −271.5 Hz, CF3),
126.3, 128.8, 130.8 (q, J −32.2 Hz, CCF3), 132.9 and
Obtained as a crystalline solid, m.p. 114.0–116.5°C,
wmax (KBr, cm−1) 1599, 1480, 1440 and 1279. lH (300
MHz, CDCl3) 1.70–2.10 (m, 2H), 2.85–2.95 (4H, m),
4.15 (1H, d, J 10.2 Hz), 4.82 (1H, d, J 10.2 Hz) and
7.18–7.50 (10H, m). lc (75 MHz, CDCl3) 25.4, 30.7,
51.1, 56.9, 127.2, 128.3, 128.5 and 140.9. m/z 286 [M+
’
142.3. m/z 336 [M+ , 2%], 263 (8), 244 (39), 189 (28),
’
2%], 178 (36), 165 (60), and 119 (100). (Found: C, 70.7;
H, 5.8%. C17H18S2: Anal Calc.: C, 71.3; H, 6.3%.)
170 (19) and 73 (100). (Found C, 50.4; H, 5.5. Anal
Calc. C14H19F3S2Si: C, 50.0; H, 5.7%.)
3.5. 2-(3-Trifluoromethylphenyl)-1,3-dithiane (2h)
3.9. 2,2-Diphenylethanoyltrimethylsilane (5b)
Obtained as crystalline solid, m.p. 72.0–72.5°C. wmax
(KBr, cm−1) 1489, 1425 and 1331. lH (500 MHz,
CDCl3) 1.90–2.24 (2H, m), 2.94 (2H-eq, ddd, J 14.0,
4.7 and 3.4 Hz), 2.82 (2H-ax, ddd, J 14.0, 11.3 and 3.0
Hz); 4.60 (s, 1H), 7.48 (1H, dd, J 4.7, 4.7 Hz), 7.57 (1H,
d, J 4.7 Hz), 7.68 (1H, d, J 4.7 Hz) and 7,75 (1H, broad
s). lc (125 MHz, CDCl3) 24.8, 31.9, 31.9, 123.8 (q, J
−271.5, CF3), 124.8, 125.2, 129.2, 131.1 (q, J −32.0
Obtained as a yellow solid, m.p. 58–60°C. wmax (KBr,
cm−1) 1640, 1588, 1233 and 846. lH (300 MHz,
CDCl3), 0.07 (9H, s), 5.42 (1H, s) and 7.20–7.40 (10H,
m). lc (75 MHz, CDCl3), −2.3, 69.0, 127.4, 129.0,
130.0, 137.4 and 242.4. m/z 268 [M+ 1%], 253 (2), 167
’
(8), 101 (10) and 73 (100). (Found C, 75.5 and H, 6.9.
Anal Calc. C17H20OSi: C, 76.1; H, 7.5%.)
Hz, CCF3), 131.2 and 140.0. m/z 264 [M+ 20%] 190
’
(12), 189 (31), 145 (8), 74 (100). (Found: C, 49.6; H, 4.2.
C11H11F3S2: Anal Calc.: C, 50.0; H, 4.2%).
3.10. 3-Trifluoromethylbenzoyltrimethylsilane (5h)
3.6. 2-Benzyl-2-trimethylsilyl-1,3-dithiane (4a)
Obtained as yellow oil. wmax (KCl, film, cm−1) 1423,
1250, 1126 and 844. lH (500 MHz, CDCl3) 0.40 (9H, s)
and 7.60–8.20 (4H, m). lc (125 MHz, CDCl3), −2.0,
123.8 (q, J −271.5 Hz, CF3), 124.2, 129.2, 129.6,
130.9, 31.5 (q, J −32.0 Hz, CCF3), 140.2 and 235.3.
Obtained as a crystalline solid, m.p. 93.0–95.0°C.
wmax (KBr, cm−1) 1599, 1495, 1422, 1276 and 847. lH
(300 MHz, CDCl3) 0.10 (9H, s), 1.70–2.00 (2H, m),

224
A.F. Patroc´ınio, P.J.S. Moran / Journal of Organometallic Chemistry 603 (2000) 220–224
m/z 246 [M+ 2%], 231 (2), 154 (4), 145 (8), 126 (20)
and 73 (100). (Found C, 53.3; H, 6.0. Anal Calc.
C11H13F3OSi: C, 53.7; H, 5.3%.)
’
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