Chemistry Reference
In-Depth Information
N
N
Et
Et
i-Pr
i-Pr
N
N
N
N
Et
Et
N
N
TEA
DABCO
DBN
DBU
DIPEA
(18.83)
(23.89)
(24.34)
(18.29)
NMe
2
NMe
2
NMe
2
N
NH
N
Me
Me
2
N
NMe
2
N
N
TMG
proton sponge
pyridine
(12.59)
MTBD
DMAP
(18.62)
(23.3)
(25.49)
(17.95)
Figure 5.1
Organic bases for organic synthesis
concept was used for the construction of a family of extremely strong noncharged organic
bases, P1-P7 phosphazene bases (iminophosphoranes), which are much stronger bases
than the well known diazabicycloundecene (DBU) or triazabicyclodecene (TBD) bases
[3] (Figure 5.2).
The hydrolytically stable phosphazene bases, especially
t
Bu-P4, were found to be useful
in organic synthesis. Besides the highly enhanced basicity, phosphazene bases combine:
high solubility in nonpolar organic solvents; easy handling and easier workup through
cleaner reactions; low sensitivity to moisture and oxygen; and the possibility of operating at
lower temperature and high selectivity. There are many examples which demonstrate the
superiority of the phosphazene bases and polymer-supported phosphazene reagents over
common inorganic and organic nitrogen bases in organic synthesis.
A simple high yield two-step protocol for the preparation of P1 is known. Reaction of
phosphorus pentachloride with dimethylamine affords the peralkylated cation as tetra-
fluoroborate, which is then demethylated with thiolate. The high basicity of triaminoimi-
nophosphoranes requires drastic conditions [4] (Scheme 5.1).
To further enhance basicity by the same formal homologation, replacement of the
dimethylamino group by modified, potentially stronger electron-donating groups is
considered to allow a tuning of the basicity. The nucleophilicity of the bases should be
effectively controlled by proper choice of the alkyl group on the basic centre. The synthesis
for P2, P3, P4 phosphazenes takes advantage of the P1 building block and HBF
4
salts are
usually obtained in high yields by replacement of the chlorine atom on the phosphorus by
the iminophosphorane. The most convenient method for the liberation of P4 phosphazene
was found to be the reaction of the hydrogen tetrafluoroborate (HBF
4
) salt with potassium
amide (KNH
2
) in liquid ammonia and separation of the base from inorganic material by
extraction with hexane [5] (Scheme 5.2).
A density functional theory (B3LYP/6-311
G**), ab initio (HF/3-21G*) and semi-
empirical (PM3) study of intrinsic basicities, protonation energies or protonation enthalpies
of phosphazene bases has been reported. The study shows that the organic superbases can
reach the basicity level of the strongest inorganic superbases, such as alkali metal
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