Chemistry Reference
In-Depth Information
comparative properties of a series of reagents:
[R
f
n
(CH
2
)
m
]
3
SnH and [R
f
n
(CH
2
)
m
]Me
2
SnH (
n
= 4, 6, 8,
10;
m
= 2, 3). The measured partition coefficients of
these tin compounds in FC-72/CH
3
CN and FC-
72/C
6
H
6
have established clearly that the fluorine
content of a molecule is not the only factor of the
fluorous phase preference, but it is also strongly
influenced by the primary structure. By introducing
one more methylene unit into the (R
f
6
CH
2
CH
2
)
3
SnH
reagent, e.g. (R
f
6
CH
2
CH
2
CH
2
)
3
SnH, the fluorine
content decreases only slightly (from 64% to 62%)
and the partition coefficient diminishes drastically
from 160 to 6.4. On the other hand, sufficient
partitioning can be achieved for fluorous synthetic
purposes if three perfluorinated units are attached to
the tin reagent. The presence of three perfluorode-
cylethyl chains causes extremely high fluorophilic-
ity, but the applicability of this reagent—due to its
extremely large molecular weight—is strongly
limited because it is practically insoluble in organic
solvents and is not very soluble in fluorous liquids.
All things considered, the original (R
f
6
CH
2
CH
2
)
3
SnH
fluorous reagent seems to be the best alternative
owing to its optimal solubility and partitioning
behaviour. In this study it was concluded also that
the length of the insulator (-(CH
2
)
n
-) is a very sen-
sitive factor for the reactivity of this tin series due to
the strong electron-withdrawing effect of the per-
fluorinated units. Reagents such as (R
f
6
CH
2
CH
2
)
3
SnH
bearing three fluorous chains and short ethylene
spacers can be used in radical tin hydride chemistry,
but they do not work well in radical allylation pro-
cedures or ionic reactions. The propylene-spaced
derivatives are much more effective for these types
of transformations.
Another obvious application of the fluorous tin
strategy is the development of highly fluorinated aryl
tin reagents for Stille coupling reactions [103-105].
Fig. 22.13
The first application of the fluorous organotin
hydride reagent in a model parallel synthetic procedure.
of the latter one is that a range of traditional reac-
tions can be conducted only under homogeneous
liquid-phase conditions.
The reactivity of the (R
f
6
CH
2
CH
2
)
3
SnH reagent was
compared with the traditional tributyltin hydride
reagent Bu
3
SnH in a study during the evaluation of
a catalytic fluorous system for radical carbonylation
and hydroxymethylation [106]. In the case of
the carbonylation reactions of organic halides, the
hydrogen-donating ability of different tin hydride
reactants was compared by examining the formyl-
ation/reduction ratio under identical experimen-
tal conditions (concentration of reactants, pressure
of CO, etc.). The results have shown that
(R
f
6
CH
2
CH
2
)
3
SnH could be characterised by a smaller
ratio, indicating enhanced reducing power compared
with the classical Bu
3
SnH reagent. Therefore, in a
successful formylation reaction a higher CO pressure
and/or lower concentration of the (R
f
6
CH
2
CH
2
)
3
SnH
would be required to obtain results identical to those
with tributyltin hydride. A similar tendency in
the reactivity order was observed in a cyclisation-
formylation reaction (Fig. 22.14). In this paper
[106], a procedure for the transformation of organic
bromides into the corresponding hydroxymethyl
derivatives is described using a catalytic amount of
(R
f
6
CH
2
CH
2
)
3
SnH reagent in a mixed solvent (BTF/
t
-
BuOH) and NaCNBH
3
reducing agent, similar to the
previously discussed dehalogenation method [62],
and the alcohol products were purified again by
three-phase liquid-liquid extraction (in 42-81%
yields).
Recently, a general work [109] on fluorous tin
hydride chemistry described the preparation and
