Bis-TADDOLs as bifunctional organocatalysts for trifluoromethylation of aldehydes and ring-opening of cyclohexene oxide with p-toluidine

Article abstract of DOI:10.1016/j.tetasy.2010.05.030

A set of bis-TADDOLs, in which two TADDOL moieties are connected by different linkers, have been prepared. A great dependence of the catalytic activities of the bis-TADDOLs on the distance between the TADDOL moieties was found by the addition of trifluoromethyltrimethylsilane to benzaldehyde and ring-opening of cyclohexene oxide with or without alkali metal salts added.

Full text of DOI:10.1016/j.tetasy.2010.05.030

Tetrahedron: Asymmetry 21 (2010) 1793–1796
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Bis-TADDOLs as bifunctional organocatalysts for trifluoromethylation
of aldehydes and ring-opening of cyclohexene oxide with p-toluidine
a
a
a
b
Yuri N. Belokon a, , Zalina T. Gugkaeva , Victor I. Maleev , Margarita A. Moskalenko , Michael North ,
*
Alan T. Tsaloev a
a Institute of Russian Academy of Sciences, A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences (INEOS RAS), 28, Vavilova Str.,
119991 Moscow, Russia
b School of Chemistry and University Research Centre in Catalysis and Intensified Processing, Bedson Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A set of bis-TADDOLs, in which two TADDOL moieties are connected by different linkers, have been pre-
pared. A great dependence of the catalytic activities of the bis-TADDOLs on the distance between the
TADDOL moieties was found by the addition of trifluoromethyltrimethylsilane to benzaldehyde and
ring-opening of cyclohexene oxide with or without alkali metal salts added.
Received 22 April 2010
Accepted 14 May 2010
Available online 21 June 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Dedicated to Professor Henri Kagan on the
occasion of his 80th birthday
1. Introduction
to the spatially favourable arrangement of several acidic groups
capable of activating substrates. The use of cyclohexanedione-
Asymmetric organocatalysis with chiral organic Brønsted acids
and bases is an important field of investigation in modern asym-
metric synthesis.1–3 This approach is very attractive due to its con-
ceptual and experimental simplicity, the absence of hazardous
heavy metal salts in reaction mixtures and the extremely high effi-
ciency of the catalysis.4 The use of chiral Brønsted acids as catalysts
for asymmetric C–C bond-forming reactions has recently become a
particularly popular and productive field of research.5a–h This class
of catalysts includes, amongst other chiral polyfunctional Brønsted
based bis-TADDOLs11,12 in photochemical reactions has been docu-
mented.13 However, these compounds were used in equimolar
amounts with respect to the substrates.
Enhanced basicity
Ph
Ph
Nu
O
O
O
H
H
acids,
a,a, ,
a0 a0-tetraaryl-1,3-dioxolane-4,5-dimethanols [(R,R)-
TADDOLs],6a–d in which the acidity of one hydroxy group increases
due to the presence of the other hydroxy group as a result of intra-
molecular hydrogen bonding, as shown for dioxolane 1 in Figure 1.
This type of catalyst activated by Brønsted acids is called Brønsted
acid-assisted Brønsted acid catalysts.7 Asymmetric Diels–Alder
reactions,8 Mukaiyama aldol reactions6c and the TADDOL-pro-
moted condensations of nitroso compounds with enamines9 pro-
vide examples of this class of catalysis.
O
E
Ph
Ph
1
Enhanced acidity
Figure 1. Effect of intramolecular hydrogen bonding on the reactivity of TADDOLs.
Recently, we reported the synthesis of a set of bis-(R,R)-TADD-
OLs based on phthalaldehyde, differing in the distances between
their pairs of hydroxy groups.14 The catalytic activities of the bis-
(R,R)-TADDOLs were studied by the addition of TMSCN to benzal-
dehyde and ring-opening of cyclohexene oxide by TMSCN, leading
to the formation of hydroxynitriles. The catalytic activities of the
chiral tetraols were found to depend crucially on the distances be-
tween the TADDOL moieties of the catalysts. Only a few examples
of organic catalysts for the addition of TMSCN to benzaldehyde are
known, and these involve the use of bases or carbenes.15
Evidently, an increase in the acidity of one hydroxy group leads
to an increase in the basicity of the other hydroxy group, for which
the hydrogen atom is involved in hydrogen bonding.6c,7 Hence,
TADDOLs would be expected to act as chiral bifunctional catalysts
by activating both the electrophilic and nucleophilic components
of the reaction, as shown by the hypothetical transition state
(Fig. 1).10
Presumably, the presence of two (R,R)-TADDOL fragments in a
single molecule can increase the efficiency of such catalysts due
Herein we report some novel applications of the catalytic sys-
tem for the ring-opening of cyclohexene oxide. Further modifica-
tion of the catalytic system with alkali metal salts resulted in
very efficient catalysts for epoxide ring-opening by p-toluidine,
* Corresponding author. Tel.: +7 499 135 6356; fax: +7 499 135 5085.
E-mail address: yubel@ineos.ac.ru (Y.N. Belokon).
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.05.030

1794
Y. N. Belokon et al. / Tetrahedron: Asymmetry 21 (2010) 1793–1796
producing hydroxyamines and highly effective catalysts for triflu-
oromethylation of aldehydes.
Asymmetric epoxide ring-opening catalysed by chiral Lewis
acids16,17 has been studied in detail. However, to our knowledge,
there are no examples of the asymmetric catalysis of epoxide
ring-opening promoted by chiral Brønsted acids activated by weak
Lewis acids.
2. Results and discussion
Activation of TADDOLs by the introduction of tetraphenylborate
salts of alkali metal ions (Li+, Na+ and K+) into the reaction mixture
(Scheme 1) resulted in an increase of TADDOL activity associated
with the Lewis acid-assisted Brønsted acid catalysis principle.7 To
suppress the spontaneous reaction catalysed by the salts them-
selves (Table 1, entry 1), a fivefold excess of bis-TADDOLs and
10-fold excess of mono-TADDOL relative to the amount of alkali
metal salt were used in the experiments to secure almost complete
complex formation.
Mono-(R,R)-TADDOL 1 and bis-(R,R)-TADDOLs 2–5 (see Fig. 2)
were synthesised as described previously.14
Ph
O
O
O
O
Ph
Ph
Ph
Ph
O
Ph
Ph
Ph
Ph
O
OH HO
HO
OH
Table 1
Ph
HO
Ph
OH
Cyclohexene oxide ring-opening with toluidine catalysed by a series of bis-TADDOL/
Ph
a
1
Ph
MBPh4 systems in CH2Cl2
2
Entry
TADDOL
Yield (eeb), % with co-catalyst
NaBPh4 LiBPh4
80
Ph
Ph
HO
Ph
Ph
OH
O
O
1
2
3
4
5
6
7
—
1
2
37
80
80
O
O
70
90
3
88
98
99
90
3
OH
Ph
HO
Ph
4a
4b
5
97 (25b)
Ph
Ph
99 (40b)
100
100
Ar
Ar
Ar
Ar
a
Reaction conditions: room temperature, CH2Cl2 0.5 mL, cyclohexene oxide
O
O
O
0.5 mmol, p-toluidine 0.5 mmol, dry MBPh4 0.5 mol %, TADDOLs (5 mol % in case of
bis-TADDOLs 2–5 and 10 mol % in case of mono-TADDOL 1), 72 h under Ar.
Enantiomeric analysis by HPLC on a Chiralcel OD-H column, the configuration is
(S,S) in both cases.
HO
OH
b
HO
Ar
OH
Ar
O
4a Ar = Ph
4b Ar = 2-Naphthyl
Ar
Ar
The cyclohexene oxide ring-opening with toluidine (Scheme 1)
was not catalysed by the mono and bis-TADDOLs and/or KBPh4.
However, the reaction was catalysed by NaBPh4 and LiBPh4 alone
and their 1:10 mixtures with TADDOLs (Table 1, entries 1–7). Thus,
the usual Lewis acidity order Li > Na > K was followed by the cata-
lysts. The observation of some asymmetric induction in the case of
LiBPh4/4a (entries 5 and 6) indicated that most (if not all) of the Li
cations were involved in complexation with TADDOLs. The very
similar activities of 1 and other bis-TADDOLs (entries 2 and 3–7)
reflect a monofunctional character of the catalysis with Lewis
acid-assisted Brønsted acid activation of cyclohexene oxide, being
its predominant function.
Ph
Ph
OH
Ph
Ph
HO
HO
Ph
O
O
O
O
5
OH
Ph
Ph
Ph
Figure 2. Mono-(R,R)-TADDOL 1 and bis-(R,R)-TADDOLs. 2–5.
2.1. Epoxide ring-opening with p-toluidine
2.2. Addition of trifluoromethyltrimethylsilane to aldehydes
Epoxide ring-opening with p-toluidine would be expected to be
promoted by the set of (R,R)-TADDOLs acting as Brønsted acids.
Another reaction involving trimethylsilyl nucleophiles is the
addition of trifluoromethyltrimethylsilane (Prakash-Ruppert re-
agent) to aldehydes (see Scheme 2).18 The reaction is more difficult
to promote and usually, either strong base catalysis19 or a strong
Lewis acid/basic ligand combination20 is required for the trifluo-
romethylation of aldehydes.
O
O
H
O
H
M
TADDOL, 5 mol %,
MBPh4, 0.5 mol %
Me3SiCF3
O
[0.05 eq. 4 + 0.05 eq. KF] or
H
CH2Cl2; rt
+
4
0.01 eq. [ ]¯ K
Nu
RCHO
M= Li, Na
Nu = p-Toluidine
MeCN, RT
R
OSiMe3
CF3
R= Ph, 2-Naphthyl, 1-Naphthyl, 4-NO2-C6H4,
Me
HO
HN
Scheme 2. TADDOL/KF or potassium TADDOLate catalysed trifluoromethylation of
aldehydes.
It was therefore tempting to apply the bis-TADDOL/metal fluo-
ride (MF) system as a potential bifunctional catalyst. The complex-
ation of the metal fluoride by the hydroxyl groups might lead to MF
Scheme 1. TADDOL/MBPh4 catalysed ring-opening of cyclohexene oxide with
toluidine.

Y. N. Belokon et al. / Tetrahedron: Asymmetry 21 (2010) 1793–1796
1795
ion separation and activation of the salt, with the free fluoride ion
functioning as a base. Another TADDOL moiety with its OH group
manifold would act as a Brønsted acid activating the aldehyde.
No reaction of Me3SiCF3 with benzaldehyde was observed in
MeCN at room temperature in the presence of KF without any TAD-
DOL added. A 1:1 mixture of 4a and KF was catalytically inert in
THF, toluene and CH2Cl2. Mixtures of LiF or NaF with 4a were also
inactive. However, the mixture of KF and 4 was a good catalytic
system in MeCN (Scheme 2, Table 2). Even at a 1000:1 ratio of
benzaldehyde/4a, a small amount (2%) of the product was detected
in the reaction mixture after 1 h. A two and a half fold increase of
the amount of the catalyst led to 8% of the product after 5 min. A
further twofold increase in the amount of the catalyst led to an in-
crease of the yield to 67% (Table 2, run 1). Evidently, the associated
nature of the catalyst may explain such a greatly disproportional
increase of the catalytic activity.
kali metal salts has been developed and tested in a set of reactions
involving the addition of trimethylsilyl nucleophiles to aldehydes
and epoxide ring-opening reactions. The importance of simulta-
neous acidic and basic activation of both electrophiles and nucleo-
philes was studied and its dependence on the activity of the
electrophiles and nucleophilic reagents was established. Although
the control of enantioselection of the reaction is modest, future
modifications of the system based on this mechanistic understand-
ing may improve the stereochemical performance of the catalysts.
4. Experimental
4.1. 2-(p-Toluidino)cyclohexanol
A Schlenk flask was evacuated and filled with argon. A catalyst
(0.024 g, 0.025 mmol), dichloromethane (0.5 mL), cyclohexene
oxide (0.05 mL, 0.5 mmol) and p-methylaniline (0.05 g, 0.5 mmol)
were placed in the flask. The reaction mixture was stirred at
ꢁ20 °C for 72 h. To determine the yield of the product, the mixture
was evaporated and the residue was dissolved in CDCl3 and ana-
lysed by 1H NMR spectroscopy (no signals of byproducts were ob-
served). Then the product was purified by silica gel
chromatography using a 5:1 hexane/ethyl acetate mixture as the
eluent (Rf 0.8). 1H NMR: 7.03–7.01 (d, 2H, J = 6.1); 6.68–6.66 (d,
2H, J = 6.2); 3.39–3.33 (m, 1H); 3.14–3.08 (m, 1H); 2.26 (s, 3H);
2.15–2.11 (m, 2H); 1.81–1.70 (m, 2H); 1.46–1.29 (m, 3H); 1.10–
1.00 (m, 1H). The ee and configuration of the product was deter-
mined by HPLC (Chiralpak OD-H, n-hexane/iso-propanol = 9:1,
1 mL/min). tR (major, (S,S)-according to the lit. data21) = 9.82 min
and tR (minor, (R,R)-according to the lit. data21) = 17.48.
Table 2
TADDOL/MF catalysed trifluoromethylation of aldehydesa
Entry
Time
Catalyst (mol %)
Aldehyde (R in RCHO)
Yield (eeb), %
1
2
3
4
5
6
7
8
9
5 min
1 h
1 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
4a/KF (0.5%)
4a/KF (2.5%)
4a/KF (5%)
1/KF (10%/5%)
2/KF (5%)
3/KF (5%)
5/KF (5%)
4a/KF (5%)
4a/KF (5%)
4a/KF (5%)
Ph
Ph
Ph
Ph
Ph
Ph
Ph
67/86c
73 (17)
100 (40)
28
83 (3)
80
12 (52)
100
100
2-Naphthyl
1-Naphthyl
4-NO2–C6H4–
10
100
a
Reaction conditions: room temperature, MeCN 0.5 mL, aldehyde (0.47 mmol),
TMSCF3 (0.92 mmol), catalyst was bis-TADDOLs/dry MF (1:1 ratio) and mono-
TADDOL 1/MF (2:1 ratio), vigorous stirring under Ar at 20 °C.
b
Mostly racemic products were obtained, ee of the exceptions indicated in
4.2. 2,2,2-Trifluoro-1-phenylethanol
parentheses after chemical yields values. The ee and (S)-configuration of product
were determined by HPLC (Chiralpak OJ-H).
c
The experiment was run at +40 °C.
To a mixture of benzaldehyde (0.05 mL, 0.47 mmol), catalyst
(22.4 mg, 0.0235 mmol) and KF (1.2 mg, 0.0235 mmol) in CH3CN
(0.5 mL) was added Me3SiCF3 (0.13 mL, 0.92 mmol) at room tem-
perature. After stirring for 1 h, the reaction was quenched with
water and the aqueous layer was extracted with ethyl acetate.
Increasing the reaction temperature to 40 °C led to an almost
quantitative yield of the product after 5 min (Table 2, run 1). The
reaction could be carried out at ꢀ15 °C but as in the other cases,
no asymmetric induction was observed. When the reaction was
carried out with 2.5 mol % of catalyst the product was obtained
in 73% yield and 17% ee (run 2). Further increase in the amount
of the catalyst to 5 mol % gave a quantitative yield of the product
with 40% ee (run 3). The data also indicate that several catalytic
species may participate in the reaction and their asymmetric
induction properties differ.
A 2:1 mixture of 1 and KF was a much less efficient catalyst, giv-
ing only 28% of the product after 24 h (entry 4). Other bis-TADDOLs
2, and 3, were less active than 4a, though better catalysts than 1
(entries 4 and 5). bis-TADDOL 5 was the least effective in the series,
although the product had the largest ee of 52% (entry 7). Other aro-
matic aldehydes were also trifluoromethylated smoothly under 4a/
KF catalysis conditions (entry 8–10).
The superiority of 4a over mono-TADDOL and other bis-TADD-
OLs seems to be a consequence of a cooperative interaction in a di-
meric form of 4a as discussed above. One set of (R,R)-TADDOL
moieties of the two molecules forms a KF complex supported by
a hydrogen bonded H–F array and O–K interaction, keeping the
structure together in solution, and the other two moieties, func-
tioning as catalytic duets, separated the K and F ions, activate both
aldehyde (OH–O hydrogen bond) and CF3SiMe3 (Si–F supervalent
interaction) and bring the two reactants together.
The solvent was removed in vacuo. Then HF (500 lL, prepared by
dilution of 49% aq HF with CH3CN) was added and the combined
solution was stirred at room temperature until all the TMS pro-
tected intermediate was converted to product. Water (15 mL)
was added and the aqueous layer was extracted with CH2Cl2
(3 ꢂ 10 mL). The combined organic layers were dried over anhy-
drous Na2SO4. Solvent was removed under vacuum to give the
crude product, which was purified by flash chromatography on sil-
ica gel (petroleum ether/ethyl acetate = 10:1). 1H NMR: 7.54–7.49
(m, 2H), 7.48–7.46 (m, 3H), 5.02 (q, 1H, J = 6.8), 3.4 (c, 1H). The
ee and configuration of product was determined by HPLC (Chir-
alpak OJ-H, n-hexane/iso-propanol = 95:5, 1 mL/min). tR (major,
(S)-stereoisomer corresponding to the lit. data22) = 21.01 min and
tR (minor, (R)-stereoisomer corresponding to the lit. data22) =
16.47.
Acknowledgement
The authors thank INTAS-05-1000008-7822 Grant for financial
support.
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