TETRAHEDRON
LETTERS
44
Pergamon
Tetrahedron Letters 44 (2003) 4149–4151
Silicon polypodands: powerful metal cation complexing agents
and solid–liquid phase-transfer catalysts of new generation
Angelamaria Maia,a,* Dario Landini,a Boguslawa Leskab and Grzegorz Schroederb
aIstituto di Scienze e Tecnologie Molecolari (ISTM) del CNR and Dipartimento di Chimica Organica e Industriale
dell’Universita`, Via C. Golgi 19, I-20133 Milano, Italy
bFaculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Received 30 January 2003; accepted 14 March 2003
Abstract—Silicon polypodands 5–7 are found to be powerful complexing agents of alkali metal salts in low polarity solvents and
very efficient catalysts in anion-promoted reactions under solid–liquid PTC conditions. The catalytic activity is comparable with
that of the cyclic polyether PHDB18C6 8. © 2003 Elsevier Science Ltd. All rights reserved.
In recent years, polypodands, open-chain ligands in
which several polyether chains are linked to the same
binding centre, have attracted increasing interest as
anion activators in homogeneous and heterogeneous
systems.1–6 They represent a valid alternative to the
cyclic analogues crown ethers and cryptands due to
their substantially lower cost, relative non-toxicity and
high efficiency.1–6 In addition, the possibility of regu-
lating their complex-forming properties by altering
their structure make them particularly suitable ligands
for phase-transfer catalyzed (PTC) processes.1–6 Here
we report that silicon polypodands 5–7, obtained in
high yields, by reacting trichloroethyl- (1) or
dichlorodimethylsilane (2) with the appropriate
polyethyleneglycol monomethylether 3 and 4 (Scheme
1),7 revealed powerful complexation of alkali metal
salts even in low polarity solvents and hence are very
efficient catalysts in a number of anion-promoted
reactions (nucleophilic substitution, reduction, alkyla-
tion, …) under solid–liquid (SL) PTC conditions.
Typical examples of complexation extent and catalytic
activity in PTC reactions are collected in Tables 1–3.
The data of Table 1 highlight the remarkable com-
plexing ability of polypodands 5–7, increasing in the
order 5<756. The complexation value (up to 4 mol
of salt per mol of ligand) mainly depends on the total
amount of donor atoms (oxygens) and, to a lesser
extent, on the number of polyether chains linked to
the central silicon atom. Catalytic activity of 5–7 par-
allels their complexing ability. Indeed, as reported in
Table 2 for typical SN2 reactions under SL–PTC con-
ditions, the shortest reaction times are always found
for polypodands 6 and 7 in line with their high com-
plexation values (Table 1). Comparison with classical
polyether ligands reveals that the catalytic efficiency
of polypodands 6 and 7 (Table 2) is comparable with
that of perhydrodibenzo-18-crown-6 PHDB18C6 (8),
whereas that of 5, with shorter chains and a lower
number of binding sites, is similar to that of open-
chain tris(polyoxaalkyl)amine TRIDENT (10), consid-
ered to be the catalyst of choice for many SL–PTC
reactions,8 or PEG400Me2 9. Analogously, 6 and 7
behave like crown ether 8 in alkylation reactions (2)
and (3) (Scheme 2), where PEG400Me2 9 requires
reaction times remarkably longer, close to those of
the uncatalyzed reaction (Table 3). In addition, the
SL–PTC reduction with NaBH4 of benzaldehyde to
the corresponding alcohol is three times faster with 6
and 7 than with crown 8.
Accordingly, the complexing ability of these silicon
podands is similar or even higher than that of the
cyclic polyether 8, and much higher than that of
open-chain ligands 9 and 10 (Table 1).
Keywords: polypodands; complexing agents; solid–liquid phase-trans-
fer catalysts; anion-promoted reactions.
Silicon podands 5–7 are found to be thermally stable,
almost up to the distillation temperature (250°C, 2
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00838-4