B. Łe˛ska et al. / Tetrahedron Letters 47 (2006) 5673–5676
5675
substitution, reduction, alkylation, etc.) under solid–
liquid conditions in chlorobenzene as a low polarity
medium and acetonitrile as a polar aprotic solvent. A
comparison of the results of the catalytic activity of
B-podands 1–4 with the well known PTC catalysts:
crown ether DCH18C6 (5), PEG400Me2 (6) and Si-pod-
and (7), (Scheme 2), studied previously by us9–11 is
collected in Tables 2–4.
same SL-PTC conditions (NaBH4 (salt)–chlorobenz-
ene), the catalytic activity of boron polypodands 1–4
was much better than the classical PT catalysts
DCH18C6 (5), PEG400Me2 (6) and Si-polypodand (7)
(Table 3). We observed that the SL-PTC reduction
was about 10 times faster with all the B-podands than
for catalysts 5 and 6, and was twice as fast as the reac-
tion with 7.
We measured the typical SN2 nucleophilic substitution
reaction of n-octyl methanesulfonate or 1-bromooctane
with a series of alkali metal salts M+Yꢁ (M+ = Na+,
K+; Yꢁ = Iꢁ, Brꢁ) in a two phase solid–liquid system
(salt–chlorobenzene or salt–acetonitrile) in the presence
of a catalytic amount of catalysts 1–7. The data reported
in Table 2 show that under identical conditions, the
reaction times (at conversion = 90%) decrease in the fol-
lowing order 6 > 1 ꢂ 2 > 5 > 4 = 7 > 3. The best results
were obtained for the B-podands 3 and 4 possessing 13
and 11 oxygen atoms in each polyoxaalkyl chain, respec-
tively (Table 2).
We also studied the alkylation reaction of potassium
phthalimide with 1-bromooctane, performed in
a
C6H4(CO)2NK (salt)–acetonitrile two-phase system
in the presence of catalytic amounts of the ligands
3, 5–7. Hitherto, we have performed measurements
only for boron podand 3, but the results look prom-
ising. We obtained 90% reaction conversion three
times faster with 3 than with the other ligands
(Table 4).
In conclusion, the catalytic activity of boron podands in
the studied reactions increases with increasing number
of polyoxaethylene units in the polyoxaalkylchains and
depends on their complexing ability. The obtained data
show that the complexing ability of boron polypodands
is similar to or even higher than those of typical PTC
catalysts such as crown ethers, and much higher than
those of open chain ligands (PEGs). B-podands are also
more effective than the Si-podand studied by us before.9–11
Boron podands are a new class of phase transfer cata-
lysts that combine easy availability and good stability
with high catalytic activity being a valid alternative to
the more sophisticated crown ethers as catalysts in
solid–liquid phase-transfer reactions, particularly on a
large scale.
It is particularly interesting to note that in the reduction
reactions of benzaldehyde to benzyl alcohol under the
Table 3. The catalytic activity of B-podands 1, 3, 4, PHDCH18C6 (5),
PEG400Me2 (6) and Si-podand (7) in the reduction of benzaldehyde
under SL-PTC conditions
NaBH4; cat: 1; 3–7
C H CHO
C H CH OH
ð2Þ
!
6
5
6
5
2
PhCl; 25 ꢀC
Catalyst
Reaction time (min)a,b
—
1
2900.0
13.4
3
12.9
4
15.7
Acknowledgements
5
109.7
138.4
20.9
6
7
The authors thank the Polish Ministry of Education
and Science for financial support. R.P. wishes to
thank the Foundation for Polish Science for the
fellowship.
a Conversion = 90% (by HPLC).
b A chlorobenzene solution (10 ml) of substrate C6H5CHO (3.5 mM),
catalyst (0.35 mM) with one molar equivalent of NaBH4 as the solid
phase.
References and notes
Table 4. The catalytic activity of B-podands 3, 5 (DCH18C6), 6
(PEG400Me2) and Si-podand 7 in the N-alkylation of potassium
phthalimide under SL-PTC conditions
1. Ma˛kosza, M. Pure Appl. Chem. 1975, 43, 439–462.
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3rd ed.; VCH: New York, 1993.
O
O
N
O
´
3. Ma˛kosza, M.; Fedorynski, M. Catal. Rev.—Sci. Eng.
2003, 45, 321–367.
cat. 3, 5-7
-
n C8H17Br
NK
O
+
-
n C8H17
+
KBr
4. Montanari, F.; Quici, S.; Banfi, S. Phase Transfer Cata-
lysis. In Comprehensive Supramolecular Chemistry—
Supramolecular Technology; Reinhoudt, D. N., Ed.; Else-
vier: Oxford, UK, 1996; Vol. 10, p 389.
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lysis in Environmentally Benign Reaction Media. In
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o
CH CN
, 80
C
3
ð3Þ
Catalyst
Reaction time (min)a
—
3
1980.0
75.0
5
202.4
668.2
238.0
6. Gokel, G. W.; Murillo, O. Podands. In Comprehensive
6
Supramolecular
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7
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a Conversion = 90% (by HPLC).