Tetrahedron Letters
Environmentally benign deprotection of dithioacetals using 30%
hydrogen peroxide catalyzed by Fe(acac)3 and sodium iodide
⇑
Masayuki Kirihara , Satoshi Suzuki, Yuki Ishizuka, Kento Yamazaki, Ryoji Matsushima, Takaya Suzuki,
Toshiaki Iwai
Department of Materials and Life Science, Shizuoka Institute of Science and Technology, 2200-2 Toyosawa, Fukuroi, Shizuoka 437-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of dithioacetals with 30% hydrogen peroxide in the presence of catalytic amounts of iron(III)
Received 13 May 2013
Revised 19 July 2013
Accepted 26 July 2013
Available online 2 August 2013
acetylacetonate and sodium iodide efficiently produced the corresponding carbonyl compounds in high
yields.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Deprotection of dithioacetals
Tantalum(V) chloride
Iodide ion
Hydrogen peroxide
Dithioacetals are widely used as carbonyl protecting groups1 or
desired carbonyl compounds, the catalysts (TaCl5 and NbCl5) are
expensive. To remedy this limitation, in this Letter, much more
inexpensive iron(III) acetylacetonate [Fe(acac)3]–NaI is shown to
effectively catalyze the deprotection of dithioacetals with 30%
H2O2 to produce carbonyl compounds (Scheme 1).
First, a dithioacetal was prepared for use as the substrate from
2-acetonaphthone and 1,3-propanedithiol according to the litera-
ture,9 and various iron catalysts were investigated (Table 1). The
substrate was treated with 0.1 equiv of the iron compound,
0.2 equiv of sodium iodide, and 4.0 equiv of 30% H2O2 (Table 1).
All of the iron compounds tested, except for ferrocene, exhibited
catalytic activity, and the desired 2-acetonaphthone was obtained
as acyl anion equivalents2 in organic synthesis. However deprotec-
1,3
tion of dithioacetals to regenerate the carbonyl functionality
is
not particularly easy because dithioacetals are quite stable under
a variety of reaction conditions including acid or base catalyzed
hydrolysis. Although many procedures for dedithioacetalyzation
have been developed, most of them have serious disadvantages.
The most reliable and frequently used deprotection procedures in-
volve using a stoichiometric amount or an excess amount of heavy
metals such as mercury.4 These procedures need to treat highly
toxic and expensive reagents, and produce undesirable wastes.
Halonium ion sources, such as N-bromosuccinimide, are also uti-
lized to deprotect dithioacetals,5 however, these methods have
drawbacks, such as the necessity of an excess amount of reagents,
and the requirement of expensive silver salts in the case of olefinic
compounds.
Environmentally-benign methods for dithioacetal-deprotection
mediated by hydrogen peroxide (H2O2) have been developed re-
cently by several research groups, in which nontoxic water is the
sole waste product of these reactions.5
We have been exploring the use of these reactions for organic
syntheses,6 and have found that dithioacetal deprotection can be
achieved smoothly by reaction with 30% H2O2 catalyzed by tanta-
lum(V) chloride (TaCl5)–sodium iodide (NaI)7,8 or niobium(V) chlo-
ride (NbCl5)–NaI.7 Although, this procedure efficiently provides the
Previous Work
NbCl5 (0.02 eq.), NaI (0.1 eq.),
30% H2O2 (4 eq.)
AcOEt-H2O = 1 : 1, r.t.
O
S
R
S
R'
R
R'
TaCl5 (0.1 eq.), NaI (0.1 eq.),
30% H2O2 (20 eq.)
quant~49%
AcOEt-H2O = 1 : 1, r.t.
This Work
Fe(acac)3 (0.1 eq.), NaI (1.0 eq.),
30% H2O2 (4.0 eq.)
O
S
R
S
R'
R
R'
AcOEt-H2O = 1 : 1, r.t.
99~78%
⇑
Corresponding author. Tel.: +81 538 45 0166; fax: +81 538 45 0110.
Scheme 1.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.