Baeyer-villiger oxidation and oxidative cascade reactions with aqueous hydrogen peroxide catalyzed by lipophilic Li[B(C6F5) 4] and Ca[B(C6F5)4]2

Article abstract of DOI:10.1002/anie.201204286

Efficient and selective: Two lipophilic catalysts were used for Baeyer-Villiger (BV) oxidations to give lactones in high yields (see scheme). Cascade reactions involving this BV oxidation were used to selectively obtain either unsaturated carboxylic acids or hydroxylactones in high yields from β-silyl cyclohexanones. Copyright

Full text of DOI:10.1002/anie.201204286

Angewandte
Chemie
DOI: 10.1002/anie.201204286
Synthetic Methods
Baeyer–Villiger Oxidation and Oxidative Cascade Reactions with
Aqueous Hydrogen Peroxide Catalyzed by Lipophilic Li[B(C6F5)4] and
Ca[B(C6F5)4]2**
Muhammet Uyanik, Daisuke Nakashima, and Kazuaki Ishihara*
The Baeyer–Villiger (BV) oxidation is one of the most
important transformations in organic synthesis, because
valuable lactones and esters can be obtained directly from
the corresponding ketones.[1] Classically, percarboxylic acids
(that is, meta-chloroperbenzoic acid (mCPBA), peracetic
acid, etc.) are used for BV oxidation.[1] However, the use of
percarboxylic acids results in the formation of one equivalent
of the corresponding carboxylic acid or its salt as waste.
Moreover, percarboxylic acids are expensive and/or shock
sensitive. Therefore, current research is focused on the
replacement of percarboxylic acids with more atom-economic
and milder oxidants like aqueous hydrogen peroxide.[1c,2]
There have been some successful examples of the BV
oxidation of ketones with aqueous hydrogen peroxide pro-
moted by Lewis acids (PtII, SnIV, ScIII, etc.),[3] Brønsted acids
(TsOH, etc.),[4] tin-containing zeolites,[5] hydrotalcites,[6] sele-
nides,[7] and flavin-based organocatalysts.[8] Despite the con-
siderable progress in this field, the development of a more
selective and efficient BV oxidation under mild reaction
conditions is still required.[1,9] We envisaged that water- and
oxidant-tolerant lipophilic acids might be good candidates as
catalysts of the BV oxidation with aqueous hydrogen perox-
ide.
Fluorinated tetraarylborates such as tetrakis[3,5-bis(tri-
fluoromethyl)phenyl]borate, [B{3,5-(CF3)2C6H3)4}]À, and tet-
rakis(pentafluorophenyl)borate, [B(C6F5)4]À, are usually used
as weakly coordinating counter anions for transition metal
catalysts to enhance their acidity and/or solubility.[10] How-
ever, there have only been a few studies on the use of Group 1
and Group 2 metal salts of these borates.[11] In 2000, Sonoda,
Mori and co-workers used dehydrated Li[B{3,5-(CF3)2C6H3}4]
as a Lewis acid catalyst for the Diels–Alder reaction.[11a] In the
same year, Mukaiyama and co-workers reported the use of
Li[B(C6F5)4] for the benzylation of aromatic compounds
and alcohols.[11b,c] Later, Liu and co-workers used
Na[B{3,5-(CF3)2C6H3}4]·2H2O for the polymerization of
vinyl ethers,[11d] the Mannich reaction,[11e] Friedel–Crafts
addition,[11f] and the hydrolysis of acetals.[11g] However, to
the best of our knowledge, these alkali or alkaline earth metal
tetraarylborates have never been used for oxidation reactions.
Fluorinated tetraarylborates are highly lipophilic,[10]
therefore we envisioned that their alkali or alkaline earth
metal salts might exhibit efficient catalytic activity for the BV
oxidation with aqueous hydrogen peroxide under biphasic
conditions. Thus, we expected these salts to promote both the
nucleophilic attack of hydrogen peroxide to the carbonyl
group and the subsequent rearrangement of a Criegee
intermediate through acid activation of the carbonyl group
in the organic phase or at the phase interface. In sharp
contrast, conventional hydrophilic Lewis or Brønsted acids
serve as catalysts in the aqueous phase or at the phase
interface, and consequently lead to not only BV oxidation to
give lactones, but also to the subsequent hydrolysis of
lactones. Furthermore, when unsaturated ketones were used
as starting materials in the presence of conventional acid
catalysts, epoxidation competed with the BVoxidation.[3,4] We
report herein that Lewis acidic and lipophilic alkali or
alkaline earth metal borates, such as Li[B(C6F5)4] or
Ca[B(C6F5)4]2, are extremely active catalysts for the selective
BVoxidation of various cycloalkanones with 30 wt% aqueous
hydrogen peroxide.
A preliminary examination of the use of commercially
available Li[B(C6F5)4] (1 mol%) in the BV oxidation of
cyclopentanone (1a) with 1.1 equivalents of 30 wt% H2O2 in
dichloroethane (DCE) at 508C afforded d-valerolactone (2a)
in 94% yield (Table 1, entry 1).[12] In contrast, other conven-
tional lithium salts, that is, LiNTf2, LiBF4, and LiBPh4,
exhibited very poor catalytic activities under similar reaction
conditions (Table 1, entries 2–5). To our delight, the catalyst
loading of Li[B(C6F5)4] could be reduced to 0.1 mol%
without affecting the chemical yield (Table 1, entry 5). More-
over, the CaII salt, Ca[B(C6F5)4]2, which was easily prepared
by a cation exchange reaction from the corresponding LiI
salt,[13] showed higher catalytic activity (Table 1, entry 6).
It was expected that the hydrated metal complexes,
M[B(C6F5)4]m·(H2O)n might act as a Lewis acid assisted
Brønsted acid (LBA)[14] catalyst under aqueous reaction
conditions. After screening several Brønsted acids as alter-
native co-catalysts to water, we found that the reaction rate
was significantly accelerated with the addition of oxalic acid
(Table 1, entry 7).[13] Notably, no reaction occurred in the
presence of oxalic acid only (Table 1, entry 8). Therefore, an
[*] Dr. M. Uyanik, D. Nakashima, Prof. Dr. K. Ishihara
Graduate School of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan)
E-mail: ishihara@cc.nagoya-u.ac.jp
Homepage: http://www.nubio.nagoya-u.ac.jp/nubio4/index.htm
Prof. Dr. K. Ishihara
Japan Science and Technology Agency (JST), CREST (Japan)
[**] Financial support for this project was provided by JSPS.KAKENHI
(20245022, 22750087), NEDO, and the Global COE Program of
MEXT. We are grateful to the Tosoh Finechem Corporation for
providing Li[B(C6F5)4].
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204286.
in situ
generated
highly
reactive
LBA
species,
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Table 1: Investigation of the catalytic activities of LiI or CaII salts.[a]
Entry
Catalyst (mol%)
t [h]
2a,
Yield [%][b]
1
2
3
4
Li[B(C6F5)4]·2.5Et2O (1)
LiNTf2 (1)
LiBF4 (1)
8
24
24
24
22
6
94
28
10
<1
98
97
LiBPh4 (1)
5[c]
6[c]
7[c]
8
Li[B(C6F5)4]·2.5Et2O (0.1)
Ca[B(C6F5)4]2·11H2O (0.1)
Ca[B(C6F5)4]2·11H2O (0.1) + (CO2H)2 (1)
(CO2H)2 (1)
3
24
98 (94)[d]
<1
[a] Unless otherwise noted, a solution of 1a (1 mmol) and 30 wt% H2O2
in DCE was heated at 508C in the presence of catalyst. [b] Determined by
1H NMR analysis. [c] 20 mmol scale. [d] Yield of isolated product.
DCE=dichloroethane. Tf=trifluoromethanesulfonate.
M[B(C6F5)4]m·[(CO2H)2]n, might be the active catalyst when
oxalic acid is used as a co-catalyst.[13]
Importantly, lactone 2a was obtained in high yields when
less-polar solvents such as toluene, benzene, and dichloro-
ethane (DCE) were used, and the latter gave the best results.
In sharp contrast, the use of water-miscible organic solvents
such as acetonitrile, dimethyl formamide, ethanol, etc. led to
rather low conversion. Thus, these results suggest that the
biphasic system is essential for the efficiency of the present
BV oxidation.
Scheme 1. BV Oxidation of various cycloalkanones 1. Reaction condi-
tions and yields of isolated lactones 2 are shown. Yields of the minor
regioisomers 2’ are shown in parentheses. For details, see the
Supporting Information. [a] Reaction was performed in the absence of
oxalic acid. [b] CaII salt (2 mol%), oxalic acid (10 mol%), and 30 wt%
H2O2 (2 equiv) were used. Ac=Acetyl. TBS=tert-butyldimethylsilyl.
We next examined the substrate scope for the present BV
oxidation system under the optimized reaction conditions
(Scheme 1). Four-, five- and six-membered cycloalkanones
(1) were smoothly oxidized to the corresponding lactones (2)
in high yields. Notably, both Ca[B(C6F5)4]2 and Li[B(C6F5)4]
could be used as catalysts in most cases. However, the CaII salt
gave slightly better results than the LiI salt, particularly, for
the oxidation of five- and six-membered cycloalkanones.
Although the oxidation rate was accelerated in the
presence of oxalic acid, hydrolysis of the lactone products
was also promoted in some cases (2b and 2c). The BV
oxidation of unsymmetrical cycloalkanones gave the corre-
sponding lactones with normal regioselectivities (2d–h, and
2k–m).
Several functional groups such as terminal and internal
alkenyl (1c and 1k), hydroxy (1g, 1h, and 1l), acetoxy (1j),
silyloxy (1m), and halo (1l and 1m) groups were tolerated
under the present reaction conditions. In fact, these BV
oxidations gave the corresponding lactones 2 in high yields,
chemoselectively. Notably, the oxidation of hydroxysteroidal
ketones 1g and 1h gave the corresponding lactones 2g and
2h, respectively, in high yields as single regioisomers without
any other oxidation products. In contrast, the use of mCPBA
in place of H2O2 gave phenol or alcohol oxidation by-
products.[13] In addition, the oxidation of 1c and 1k with
mCPBA under standard reaction conditions gave the epox-
ides as the major product.[13] The use of TsOH[4a–c] as a catalyst
for the BV oxidation of 1c and 1k with hydrogen peroxide
gave a complex reaction mixture or very low conversions.[13]
The BV oxidation of (S)-2-(hydroxymethyl)cyclopenta-
none (1n) gave (S)-2n[15] without any racemization [Eq. (1)].
Interestingly, the BV oxidation of 4-hydroxycyclohexanones
1o and 1p gave the more-stable five-membered hydroxylac-
tones 2o and 2p through successive intramolecular transes-
terification [Eq. (2)].
When linear ketones or aldehydes were oxidized, the ester
products were also hydrolyzed simultaneously; the oxidation
of 1r gave the corresponding ester 2r (59% yield) together
with hexanoic acid (41% yield) [Eq. (3)]. Additionally, the
oxidation of 4-chlorobenzaldehyde (1s) gave the 4-chloro-
phenol in 80% yield after hydrolysis of the corresponding
formate [Eq. (4)].
2
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To demonstrate the practicality of the present BV
catalysis, we examined the oxidation of 2-adamantanone
(1t) on a 100 mmol (15 g) scale with only 0.01 mol% of CaII
salt catalyst [Eq. (5)]. The desired lactone 2t was obtained in
98% yield upon isolation (TON up to 9800). Notably, 2t is
used industrially as a starting material for the production of
polymers, drugs, etc.[16]
Scheme 2. a) Transformation of b-trimethylsilyl cyclohexanone 3a to
4a through a BV oxidation/b-elimination reaction sequence. b) Oxida-
tion with low catalyst loading (0.01 mol%). c) Control experiments:
Oxidation of 3a with mCPBA gave silyl lactone, which was easily
converted into 4a by b-elimination under our reaction conditions.
d) Cascade oxidative transformation of 3a into hydroxylactone 2n
through BV oxidation/b-elimination and subsequent epoxidation/cycli-
zation reaction sequences.
Next, we applied our new catalytic conditions to the
oxidation of b-silyl cyclohexanones. According to Hudrlik
et al.,[17] the BV oxidation of
b-silyl ketones with mCPBA is directed by the silyl group to
selectively give esters of b-hydroxysilanes, which are useful
for the synthesis of unsaturated esters or acids. Interestingly
and unexpectedly, the oxidation of b-trimethylsilyl cyclo-
hexanone (3a) with 1.1 equivalents of 30 wt% H2O2 in the
presence of Li[B(C6F5)4] (1 mol%) and oxalic acid (1 mol%)
at room temperature afforded 5-hexenoic acid (4a) in 96%
yield as the sole product (Scheme 2a). The expected lactone
5a, which was obtained by oxidation with mCPBA (Sche-
me 2c), was not detected. Notably, the oxidation rate was
significantly accelerated in the presence of a b-silyl group;
hence the reaction quickly occurred even at room temper-
ature (Scheme 2a versus Scheme 1, 2b). Moreover, the
catalyst loading of LiI salt could be reduced to 0.01 mol%
without affecting the chemical yield (Scheme 2b). In contrast,
the oxidation rate was not affected by the presence of a b-silyl
substituent when mCPBA was used.[17a] We believe unsatu-
rated acid 4a is generated through the BVoxidation of 3a and
the subsequent fast b-elimination of silyl lactone intermediate
5a. In fact, treatment of 5a under similar reaction conditions
in the absence of oxidant gave 4a quantitatively within a few
minutes (Scheme 2c). In sharp contrast, treatment with acid
(excess BF3·Et2O) or base (KOH, KH, etc.) under harsh
reaction conditions was needed for the b-elimination of silyl
lactones to give the corresponding unsaturated carboxylic
acids.[17] Thus, we have developed the first oxidative cascade
reaction of b-silyl cyclohexanones.
hydroxylactone 2n in 93% yield through epoxidation/cycli-
zation reactions (Scheme 2d). Notably, a higher loading of
oxalic acid (10 mol%) and a high temperature (708C) were
needed for the oxylactonization step. To the best of our
knowledge, this is the first example of the transition-metal-
free catalytic oxylactonization of unsaturated carboxylic acids
using hydrogen peroxide.
Under these optimized reaction conditions either unsatu-
rated acids 4 or hydroxylactones 6 could be obtained from the
corresponding b-silyl cyclohexanones 3, depending on the
amounts of hydrogen peroxide and oxalic acid used (Table 2).
Notably, the oxidation of 2-substituted b-silyl ketones 3 gave
either the corresponding unsaturated acids 4 or hydroxy-
lactones 6 as sole products in a stereospecific manner
(Table 2, entries 1 and 2). However, the oxidation of 3d
gave 6d as a diastereomeric mixture (Table 2, entry 3).
In summary, we have developed highly efficient and
selective Li[B(C6F5)4] or Ca[B(C6F5)4]2 catalyzed BV oxida-
tions of various cycloalkanones with 30 wt% aqueous hydro-
gen peroxide to give the corresponding lactones in high yield.
Additionally, we have found that the reaction rate is
accelerated with the use of oxalic acid as a co-catalyst.
Moreover, b-silyl cyclohexanones can be oxidized to either
the corresponding unsaturated carboxylic acids or hydroxy-
lactones in high yields through BV oxidation/b-elimination
reaction sequences and subsequent epoxidation/cyclization
reaction sequences, by controlling the amounts of H2O2 and
oxalic acid. These reactions proceed under mild conditions
and tolerate various functional groups.
Our catalytic system could also be used for the oxidative
cyclization of unsaturated acids.[18] After the conversion of
ketone 3a into unsaturated acid 4a, three additional equiv-
alents of H2O2 were added to the same flask to give
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X. Zhang, K. Ding, Angew. Chem. 2008, 120, 2882; Angew.
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Table 2: Oxidative cascade reactions of b-silyl cyclohexanones 3.
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6,[a] Yield [%]
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[12] We initially examined M[B{3,5-(CF3)2C6H3}4]n (M = Li, Na, Ca,
Sr) salts as catalysts for the BV oxidation under our reaction
conditions. Although they showed high catalytic activities
initially, the decomposition of the borate anion was observed
during oxidation reaction. In contrast, we confirmed that
M[B(C6F5)4]n (M = Li, Ca) salts were extremely stable under
our oxidation conditions. For the comparison of the stability of
[B{3,5-(CF3)2C6H3}4]À and [B(C6F5)4]À borate anions under
acidic conditions, see: M. Isshiki, H. Kobayashi, T. Sonoda,
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[13] For details, see the Supporting Information.
6d, 81 (ꢀ1.4:1 d.r.)
3e
4e, 90
6e, 80[b]
[a] 3 was treated with 1.1 equiv of H2O2 at room temperature until full
conversion, then the reaction mixture was heated at 708C in the presence
of an additional 3 equiv of H2O2. [b] After 3e was consumed, 8 equiv of
H2O2 were additionally added.
Received: June 2, 2012
Published online: && &&, &&&&
Keywords: alkali metals · alkaline earth metals · borates ·
lactones · oxidation
.
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4
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Synthetic Methods
M. Uyanik, D. Nakashima,
K. Ishihara*
&&&& — &&&&
Baeyer–Villiger Oxidation and Oxidative
Cascade Reactions with Aqueous
Hydrogen Peroxide Catalyzed by
Lipophilic Li[B(C6F5)4] and Ca[B(C6F5)4]2
Efficient and selective: Two lipophilic
catalysts were used for Baeyer–Villiger
(BV) oxidations to give lactones in high
yields (see scheme). Cascade reactions
involving this BV oxidation were used to
selectively obtain either unsaturated car-
boxylic acids or hydroxylactones in high
yields from b-silyl cyclohexanones.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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These are not the final page numbers!