TiCl4-catalyzed synthesis of peroxyacetals from aldehydes

Article abstract of DOI:10.1139/v11-158

Peroxyacetals 2a-2j were prepared by TiCl₄-promoted nucleophilic addition of both tert-butyl hydroperoxide (TBHP) and an alcohol to the corresponding aldehyde. The reaction works well with a variety of aldehydes, but not with ketones. The magnitude of the equilibrium constant for hemiacetal formation plays an important role; a large constant enables high conversion to peroxyacetal.

Full text of DOI:10.1139/v11-158

321
TiCl4-catalyzed synthesis of peroxyacetals from
aldehydes
Chen-Yu Tsai, Lu-An Chen, and Kuangsen Sung
Abstract: Peroxyacetals 2a–2j were prepared by TiCl4-promoted nucleophilic addition of both tert-butyl hydroperoxide
(TBHP) and an alcohol to the corresponding aldehyde. The reaction works well with a variety of aldehydes, but not with ke-
tones. The magnitude of the equilibrium constant for hemiacetal formation plays an important role; a large constant enables
high conversion to peroxyacetal.
Key words: peroxyaceta , aldehyde, titanium chloride, peroxide.
Résumé : On a préparé les peroxyacétals 2a–2j par addition nucléophile catalysée par le TiCl4 simultanée de l’hydroperoxyde
de tert-butyle et d’un alcool sur l’aldéhyde correspondant. La réaction fonctionne bien avec un grand nombre d’aldéhydes,
mais elle ne donne pas de bons résultats avec les cétones. La valeur de la constante d’équilibre de formation de
l’hémiacétal joue un rôle important; avec une constante dont la valeur est élevée, on obtient un degré élevé de conver-
sion en peroxyacétal.
Mots‐clés : peroxyacétal, aldéhyde, chlorure de titane, peroxyde.
[Traduit par la Rédaction]
thesized by the reaction of acetals or ketals with alkyl
hydroperoxides in the presence of protic or Lewis acids
(BF3·OEt2).5 Recently, a newly developed methodology for
monoperoxyacetals or bisperoxyacetals used iodine as a cata-
lyst to peroxidize carbonyl compounds and, in this method,
both ketones and aldehydes have the possibility of being con-
verted into bisperoxyacetals or ketals or monoperoxyacetals
or ketals.6 In this paper, we will introduce a novel synthetic
method that uses TiCl4 as a catalyst and selectively converts
an aldehyde into monoperoxyacetals. For this methodology, it
is unnecessary to convert aldehydes into acetals or vinyl
ethers before the preparation of peroxyacetals. In addition,
this method for peroxyacetal synthesis works only for alde-
hydes, not for ketones, and that is quite different from ketal
synthesis. To figure out what causes the difference, we pro-
pose a reaction mechanism for the TiCl4-promoted peroxy-
acetal synthesis. A detailed analysis will lead us to the
answer.
Introduction
The number of reported peroxide natural products contin-
ues to increase and some organic peroxides have been dis-
covered to have antimalarial properties,1 therefore, peroxide
synthesis has been studied for a long time and is of continu-
ing interest for chemists.2–4,5,6 In last two decades, peroxyace-
tals have been used as substrates in the development of new
methodology for peroxide synthesis. For example, neighbor-
ing iodo, alkoxy, acetoxy, and silyl groups impart useful lev-
els of diastereoselection in the Lewis acid-mediated allylation
of peroxyacetals, generating chiral peroxides.2 Peroxyacetals
undergo reaction with allyltrimethylsilane, silyl enol ethers,
and silyl ketene acetals in the presence of Lewis acids, such
as TiCl4, SnCl4, and trimethylsilyl triflate, to afford homo-
allyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, re-
spectively.3 Acid hydrolysis of peroxyacetals may generate
alkyl hydroperoxides.4 Hence, the synthesis of peroxyacetals
has become interesting as well.
The major synthetic route to monoperoxyacetals or bisper-
oxyacetals is through the ozonolysis of alkenes or enol
ethers, followed by alkylation or silylation of the resulting
hydroperoxyacetals.1c,4,7 The drawback of this general syn-
thetic method is that the selectivity of ozonolysis is poor and
cannot be used for substrates containing ozone-sensitive
groups. Bisperoxyacetals and (or) ketals can be synthesized
by another method, which involves the reaction of acetals or
ketals with H2O2 in the presence of protic or Lewis acids,
such as BF3·OEt2, HCl, AcOH, and tungstic acid, followed
by alkylation or silylation of the resulting dihydroperoxy-
acetals.8 Bisperoxyacetals or ketals can also be directly syn-
Results and discussion
In this paper, we found that reaction of an aldehyde with
1 equiv of alcohol and 1.7 equiv of TBHP in the presence of
a catalytic amount of TiCl4 (0.1 equiv) in CDCl3 solvent gen-
erated a peroxyacetal. To understand the scope of this reac-
tion, we tested various aldehydes and alcohols for this type
of reaction, and the results are shown in Table 1. First, we
used an aromatic aldehyde (benzaldehyde) as an aldehyde
substrate to react with a primary alcohol (n-BuOH), secondary
alcohol (i-PrOH), benzyl alcohol, or tertiary alcohol (t-BuOH)
for the peroxyacetal synthesis. What we found was that they
Received 23 August 2011. Accepted 19 October 2011. Published at www.nrcresearchpress.com/cjc on 9 March 2012.
C.-Y. Tsai, L.-A. Chen, and K. Sung. Department of Chemistry, National Cheng Kung University, Tainan, Taiwan.
Corresponding author: K. Sung (kssung@mail.ncku.edu.tw)
Can. J. Chem. 90: 321–325 (2012)
doi:10.1139/V11-158
Published by NRC Research Press

322
Can. J. Chem. Vol. 90, 2012
Table 1. TiCl4-promoted synthesis of the peroxyacetals 2.
O(t-Bu)
O
O
t-BuOOH
+
R''OH
cat. TiCl4
R'
H
R'
OR"
2
1
Compound
R″
n-Bu
i-Pr
PhCH2
t-Bu
Ph(CH2)3
Ph(CH2)3
n-Bu
n-Bu
n-Bu
Product
2a
2b
2c
2d
2e
2f
2g
2h
2i
Yield (%)
1a (R′ = Ph)
1a (R′ = Ph)
1a (R′ = Ph)
1a (R′ = Ph)
1e (R′ = i-Pr)
88
84
76
58
84
82
89
94
80
79
1f (R′ = n-Pr)
1g (R′ = p-NO2–Ph)
1h (R′ = m-NO2–Ph)
1i (R′ = p-Cl–Ph)
1j (R′ = p-MeO–Ph)
n-Bu
2j
Note: Reaction conditions: at room temperature over 1.5 days.
all generated the desired product in good yield, but a steri-
cally hindered alcohol such as t-BuOH may result in a
lower yield.
Afterwards, we used a primary alcohol (Ph(CH2)3OH) as a
nucleophile to react with two types of aliphatic aldehydes (1e
and 1f). These two aliphatic aldehydes have different degrees
of steric hindrance adjacent to their respective carbonyl
groups, but they both worked well and similarly in the peroxy-
acetal synthesis.
with a dehydration agent. This is quite different from ketal
synthesis and that inspired us to analyze the reaction mecha-
nism for the TiCl4-promoted peroxyacetal synthesis.
Advantages of the TiCl4-promoted peroxyacetal synthesis
are as follows: (i) The method can be carried out at room
temperature; (ii) Most peroxyacetal syntheses do not need a
dehydrating agent and still achieve high yields, except for al-
dehyde substrates with highly electron-donating substituents.
For aldehyde substrates with highly electron-donating sub-
stituents, one can still obtain peroxyacetals in high yield as
long as a dried solvent and dehydrating agent are used; (iii)
The peroxyacetal synthesis is selective for aldehyde sub-
strates in the presence of ketone groups; (iv) It is unnecessary
to convert aldehydes into acetals or vinyl ethers before prep-
aration of peroxyacetals.
The mechanism for the TiCl4-promoted peroxyacetal syn-
thesis from an aldehyde substrate is assumed to be similar to
that for acid-catalyzed acetal synthesis from an aldehyde sub-
strate. Acid-catalyzed acetal synthesis from an aldehyde sub-
strate usually involves two parts: (i) formation of hemiacetal
from aldehyde and (ii) formation of acetal from hemiacetal.
Then, the mechanism for the TiCl4-promoted peroxyacetal
synthesis from an aldehyde substrate is assumed to involve
the same two parts (shown in Scheme 1). Chemists paid
more attention to each of the two parts of the mechanism
separately rather than the whole integrated mechanism.9 In
this paper, we will not only investigate each part of the
mechanism, but also analyze the integrated mechanism as a
whole.
We further explored electronic effects on this peroxyacetal
synthesis. We used various substituted benzaldehydes as an
aldehyde substrate to react with n-BuOH. The substituent in-
cludes p-NO2, m-NO2, p-Cl, and p-MeO, ranging from highly
electron-withdrawing to highly electron-donating. What we
found was that benzaldehyde with a highly electron-
withdrawing substituent gave the peroxyacetal very easily
and in high yield. Just like the synthesis of the other peroxy-
acetals mentioned above, they worked very well without us-
ing a dehydrating agent such as 4 Å molecular sieves.
However, when we carried out the peroxyacetal synthesis
for an aromatic aldehyde substrate with a strong electron-
donating substituent such as p-MeO, conversion to the per-
oxyacetal was very low (<10%). This low conversion was
significantly improved by using a very dry solvent and acti-
vated 4 Å molecular sieves.
In the TiCl4-promoted nucleophilic addition of both TBHP
and an alcohol to an aldehyde, we usually had acetal and bis-
peroxyacetal as by-products. When the ratio of the quantities
for aldehyde, alcohol, and TBHP was 1:1:1 in the peroxyace-
tal synthesis, we obtained little of the acetal by-product.
When the ratio was 1:1:2, we obtained little of the bisperoxy-
acetal. The best ratio for these substances was 1:1:1.7, where
the major product was peroxyacetal with negligible acetal
and bisperoxyacetal by-products.
For the second part of the mechanism, a hemiacetal inter-
mediate may eliminate the OH group with the assistance of
TiCl4 activation,10 forming a trigonal carbon and stabilized
carbocation 3. This ion will react very fast with an alcohol
or TBHP to yield an acetal or ketal. The carbocation 3 is bet-
ter stabilized by alkyl or aryl group than by H. If the forma-
tion of carbocation 3 is rate-limiting in the TiCl4-promoted
peroxyacetal synthesis, the activation energy for the formation
of peroxyketal is supposed to be smaller than that for the
formation of peroxyacetal. Therefore, we should have been
The limitation for the TiCl4-promoted peroxyacetal synthe-
sis is that the substrate cannot be a ketone. The ketone sub-
strates we tried included acetone, acetaphenone,
benzophenone, and benzil. This methodology did not work
out for all the ketones even under very dry conditions and
Published by NRC Research Press

Tsai et al.
323
Scheme 1. Proposed mechanism for TiCl4-promoted peroxyacetal synthesis.
TiCl4
O
TiCl4
O
R = H (aldehydes)
= alkyl or aryl (ketones)
R'
R
R'
R
1
TiCl4
R
K > 1 for aldehydes
K << 1 for ketones
O
OH
+
R''OH
O
R'
R'
R
R''
R
R
R
OH
TiCl4
O
O
O
R''
R''
R'
R'
R'
R''
+ H2O
3
R
R
O
O(t-Bu)
O
t-BuOOH
+
O
R''
R'
R'
R''
2
3
able to prepare peroxyketal with this method, but we could
not. This indicates that the second part of the mechanism
(formation of acetal from hemiacetal) does not play the
most important role in the TiCl4-promoted peroxyacetal syn-
thesis, but the first part of the mechanism (formation of
hemiacetal from aldehyde) does.
conversion levels. We solved the low-conversion problem by
removing the water by-product with 4 Å molecular sieves,
shifting the equilibrium position to the peroxyacetal product.
This is usually found in the acetal synthesis, where azeo-
tropic distillation is usually used as a way to remove the
water by-product. However, azeotropic distillation is not a
suitable way to remove water in the peroxyacetal synthesis
because heat may further degrade the peroxyacetal product.
Hence, the reason why the peroxyacetal synthesis does not
work with ketones is that the formation constants from the
ketones to the corresponding hemiketals in the first part of
the reaction are too small to generate a large enough concen-
tration of hemiketals for the second part of the reaction.
The first part of the mechanism (formation of the hemiace-
tal or hemiketal) is similar to that for aldehyde or ketone hy-
dration. Aldehyde hydration is usually thermodynamically
favorable with an equilibrium constant close to or greater
than 1, but ketone hydration is thermodynamically unfavora-
ble with an equilibrium constant much smaller than 1. There
are two pieces of evidence that support that the first part of
the mechanism (formation of hemiacetal from aldehyde)
plays the most important role in the TiCl4-promoted peroxy-
acetal synthesis. The first piece of evidence is that peroxyke-
tals cannot be prepared by this method. The possible
explanation is that the thermodynamically unfavorable forma-
tion of hemiketal makes the formation of peroxyketal impos-
sible in the TiCl4-promoted peroxyacetal synthesis. The
second piece of evidence is that the conversion to 2j in the
absence of a drying agent is much lower than the conversion
to 2g or 2h under the same conditions. The possible explan-
ation is that a strong electron-donating group (p-MeO) on 1j
and 2j makes the formation of hemiacetal less thermody-
namically favorable than a strong electron-withdrawing group
(NO2) and this causes the formation of peroxyacetal at low
Conclusion
TiCl4-promoted nucleophilic addition of both TBHP and
an alcohol to an aldehyde generated a peroxyacetal. This
methodology works selectively for aldehydes, but not for ke-
tones. Various alcohols and aldehydes can be used in this
peroxyacetal synthesis. To reduce the acetal and bisperoxy-
acetal by-products to a negligible amount, the best ratio of
the quantities for aldehyde, alcohol, and TBHP in the peroxy-
acetal synthesis is 1:1:1.7. The equilibrium constant for the
formation of hemiacetal plays the most important role in the
TiCl4-promoted peroxyacetal synthesis. The larger the forma-
tion constant of hemiacetal, the better conversion the peroxy-
acetal synthesis can reach.
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Can. J. Chem. Vol. 90, 2012
2.76 (t, J = 7.9, 2H), 1.98 (m, 2H), 1.69 (m, 2H), 1.49 (m,
2H), 1.29 (s, 9H), 0.99 (t, J = 7.3 Hz, 3H). 13C NMR
(75 MHz, CDCl3) d: 142.0, 128.4, 128.1, 125.6, 106.4, 80.0,
68.4, 34.4, 32.3, 31.5, 26.4, 18.2, 13.9. Anal. calcd for
C17H28O3: C 72.80, H 10.07; found: C 72.84, H 10.09.
Experimental section
General procedure for synthesis of peroxyacetals (2)
To aldehyde (1 mmol) and 4 Å molecular sieves (0.5 g) in
CHCl3 or CDCl3 (2 mL) at room temperature was added
TiCl4 (0.1 mmol, 10% in CHCl3 or CDCl3). The mixture
was stirred at room temperature for 1 h. Toluene solutions of
TBHP (1.7 mmol) and alcohol (1 mmol) were added into the
mixture, followed by stirring at room temperature for around
1–2 days. The reaction was monitored by H NMR spectrom-
etry until the reaction was complete. The product 2 was puri-
fied by column chromatography with silica gel as a stationary
phase and hexane/EtOAc as a mobile phase.
1-(tert-Butylperoxybutoxymethyl)-4-nitrobenzene (2g)
1
Colorless oil. H NMR (300 MHz, CDCl3) d: 8.21 (d, J =
8.1 Hz, 2H), 7.86 (d, J = 8.1 Hz, 2H), 5.85 (s, 1H), 4.05 (m,
1H), 3.71 (m, 1H), 1.65 (m, 2H), 1.46 (m, 2H), 1.25 (s, 9H),
0.93 (t, J = 7.2 Hz, 3H). 13C NMR (75 MHz, CDCl3) d:
148.0, 143.8, 127.9, 123.3, 104.2, 81.0, 68.8, 31.8, 26.4,
19.2, 13.8. Anal. calcd for C15H23NO5: C 60.57, H 7.80, N
4.71; found: C 60.54, H 7.81, N 4.70.
1
(tert-Butylperoxybutoxymethyl)benzene (2a)
1
Colorless oil. H NMR (300 MHz, CDCl3) d: 7.37–7.49
1-(tert-Butylperoxybutoxymethyl)-3-nitrobenzene (2h)
1
(m, 5H), 5.84 (s, 1H), 3.99 (dt, J = 9.6, 6.5 Hz, 1H), 3.67
(dt, J = 9.6, 6.7 Hz, 1H), 1.66 (m, 2H), 1.42 (m, 2H), 1.30
(s, 9H), 0.93 (t, J = 7.3 Hz, 3H). 13C NMR (75 MHz,
CDCl3) d: 136.9, 128.7, 128.1, 126.9, 106.0, 80.7, 69.3,
32.0, 26.5, 19.3, 13.8. The NMR data were in agreement
with those previously reported for this compound (ref. 11).
Anal. calcd for C15H24O3: C 71.38, H 9.59; found: C 71.41,
H 9.57.
Colorless oil. H NMR (300 MHz, CDCl3) d: 8.34 (s, 1H),
8.20 (d, J = 7.9 Hz, 1H), 7.83 (d, J = 7.9 Hz, 1H), 7.55 (t,
J = 7.9 Hz, 1H), 5.86 (s, 1H), 4.06 (m, 1H), 3.72 (m, 1H),
1.69 (m, 2H), 1.47 (m, 2H), 1.39 (s, 9H), 0.93 (t, J = 7.2 Hz,
3H). 13C NMR (75 MHz, CDCl3) d: 148.1, 139.2, 133.0,
129.0, 123.5, 122.2, 104.1, 81.0, 69.8, 31.8, 26.4, 19.2,
13.8. Anal. calcd for C15H23NO5: C 60.57, H 7.80, N 4.71;
found: C 60.56, H 7.84, N 4.72.
(tert-Butylperoxyisopropoxymethyl)benzene (2b)
Colorless oil. H NMR (300 MHz, CDCl3) d: 7.16–7.34
1-(tert-Butylperoxybutoxymethyl)-4-chlorobenzene (2i)
Colorless oil. H NMR (300 MHz, CDCl3) d: 7.42 (d, J =
1
1
(m, 5H), 5.73 (s, 1H), 4.04 (m, 1 H), 1.17 (d, J = 6.1 Hz,
3H), 1.12 (s, 9H), 1.05 (d, J = 6.1 Hz, 3H). 13C NMR
(75 MHz, CDCl3) d: 137.5, 128.6, 128.1, 126.9, 104.4, 80.6,
71.5, 26.5, 23.0, 22.5. Anal. calcd for C14H22O3: C 70.54, H
9.31; found: C 70.52, H 9.33.
8.2 Hz, 2H), 7.33 (d, J = 8.2 Hz, 2H), 5.78 (s, 1H), 3.95 (m,
1H), 3.67 (m, 1H), 1.66 (m, 2H), 1.43 (m, 2H), 1.27 (s, 9H),
0.94 (t, J = 7.2 Hz, 3H). 13C NMR (75 MHz, CDCl3) d:
135.4, 134.1, 128.3, 128.2, 105.1, 80.8. 69.4, 31.1, 26.5,
19.4, 13.8. Anal. calcd for C15H23ClO3: C 62.91, H 8.10;
found: C 62.89, H 8.12.
(Benzyloxy-tert-butylperoxymethyl)benzene (2c)
1
Colorless oil. H NMR (300 MHz, CDCl3) d: 7.27–7.52
1-(tert-Butylperoxybutoxymethyl)-4-methoxybenzene (2j)
1
(m, 5H), 5.97 (s, 1H), 5.02 (d, J = 12.3 Hz, 1H), 4.88 (d, J =
12.3 Hz, 1H), 1.31 (s, 9H). 13C NMR (75 MHz, CDCl3) d:
138.2, 136.5, 128.8, 128.3, 128.2, 127.7, 127.5, 126.9,
105.1, 81.0, 70.9, 26.5. Anal. calcd for C18H22O3: C 75.48,
H 7.75; found: C 75.50, H 7.72.
Colorless oil. H NMR (300 MHz, CDCl3) d: 7.41 (d, J =
8.4 Hz, 2H), 6.89 (d, J = 8.4 Hz, 2H), 5.78 (s, 1H), 3.93 (m,
1H), 3.81 (s, 3H), 3.65 (m, 1H), 1.65 (m, 2H), 1.44 (m, 2H),
1.28 (s, 9H), 0.94 (t, J = 0.72 Hz, 3H). 13C NMR (75 MHz,
CDCl3) d: 159.8, 128.1, 114.2, 113.5, 105.9, 80.5, 69.2, 55.2,
32.0, 26.5, 19.3, 13.8. Anal. calcd for C16H26O4: C 68.04, H
9.29; found: C 68.07, H 9.27.
(tert-Butoxy-tert-butylperoxymethyl)benzene (2d)
1
Colorless oil. H NMR (300 MHz, CDCl3) d: 7.32–7.49
(m, 5H), 5.96 (s, 1H), 1.30 (s, 9H), 1.26 (s, 9H). 13C NMR
(75 MHz, CDCl3) d: 138.8, 128.5, 128.0, 127.1, 100.4, 80.4,
74.7, 28.8, 26.6. Anal. calcd for C15H24O3: C 71.38, H 9.59;
found: C 71.36, H 9.63.
Acknowledgment
Financial support from the National Science Council of
Taiwan (NSC 98-2113-M-006-001-MY3 and NSC 100-
3113-E-024-001-CC2) is gratefully acknowledged.
(3-(1-tert-Butylperoxy-2-methylpropoxy)propyl)benzene
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