The Reagents
Preparation of Clay-Supported Iron(III) Nitrate (Clayfen)
Whereas these procedures worked out very safely in our hands, nitrates
are
potentially dangerous compounds, and appropriate caution is to be
applied
in each step. In particular, we urge to avoid confinement conditions,
and
we recommend to proceed to scaling-up only after appropriate safety
tests.
Clay-Supported Iron(III) Nitrate (Clayfen) [4, 9]
Iron(III) nitrate nonahydrate (22.5 g) is added to acetone (375 mL) in
a 1
litre, pear-shaped evaporating flask. The mixture is stirred vigorously
for
~ 5 min until complete dissolution of the crystals of hydrated
iron(III)
nitrate. The first-formed homogenous rusty brown solution turns after a
short time into a muddy, light brown suspension. K 10 clay (30 g) is
added
in small amounts and stirring is continued for another 5 min. The
solvent
is then removed from the resulting suspension under reduced pressure
(rotary evaporator) on a water bath at 50 C. After 30 min. the dry
solid
crust adhering to the walls of the flask is flaked off and crushed with
a
spatula, and rotary evaporator drying is resumed for another 30 min.
This
procedure yields K10 clay-supported iron(III) nitrate (Clayfen) as a
yellow, floury powder; yield: ~ 50 g.
Warnings:
1. Prolonged heating, or use of a bath temperature > 50 C may yield an
unstable reagent, which decomposes in a vigorous exothermic
reaction, with
emission of a large amount of nitrogen dioxide fumes. This
decomposition
generally takes place within 15 min after the end of an incorrect
preparation of Clayfen.
2. Preparation of Clayfen includes flaking off of the dry solid crust
formed in the evaporator. The physical state of the reagent at this
step
is crucial: if this removal is performed while the solid is still a
little muddy, it will aggregate in spheres that remain wet inside.
and
the resulting reagent will display little or no activity, or will
decompose as described above. This difficulty will be best avoided
by
strict observance of the experimental conditions described
(quantities,
flask volume and shape).
3. Clayfen, spontaneously, slowly loses nitrous fumes, and should never
he
stored in a closed flask.
Stability of Clay-Supported Iron(III) Nitrate (Clayfen)
Left in air contact at room temperature, Clayfen retains its activity
for
only a few hours. It may be stored for a few days and retains its
activity
when it is covered with n-pentane immediately after preparation; the
hydrocarbon is evaporated prior to use. It is a good practice to use
only
freshly prepared reagent: it loses about 40 % of its reactivity upon
standing exposed to air for 4 h, or under n-pentane for 24 h.
The thermal stability of Clayfen was investigated by differential
calorimetric analysis. Above 59°C, it decomposes with an enthalpy
release
of ~ 20 cal/g. This is a first order process, with calculated halflives of
53 min at 69 C (in boiling n-hexane, for instance) or of 14 min at 80 C
(the temperature of boiling benzene). In our experience the latter is
the
upper temperature limit for the practical use of Clayfen.
Clay-Supported Copper(II)Nitrate (Claycop)
This reagent, also based on a salt forming covalent bidentate complexes
upon dehydrations [10,11], shows no loss of reactivity in the
applications
described hereafter, even after standing in an open powder box for one
month. In our hands, it never underwent any spontaneous decomposition.
Clay-supported Copper(II) Nitrate (Claycop) [12]
Clay-supported copper(II) nitrate (Claycop) is prepared in a process
similar to the preparation of Clayfen, by adding K10 clay (30 g) to a
solution of copper(II) nitrate trihydrate (20 g) in acetone (375 ml).
The
resulting suspension is placed in a rotary vacuum evaporator and the
solvent is eliminated under reduced pressure (water jet aspirator) on a
water bath at 50 C. After 30 min. the dry solid crust adhering to the
walls
of the flask is flaked off with a spatula, and vaporation is resumed
for
another 30 min in the same conditions, giving Claycop as a light blue,
free-flowing powder; yield: ~ 50 g.
For examples of applications of Clayfen, Claycop and K10 see references
13,14,15.
References:
1.
J A Ballantine in Chp 4. p100-103. In "Bologh, Laszlo, Organic
Chemistry Using Clays", 1993, Springer-Verlag.
2. From "Clay-Supported Copper(II) and Iron(III) Nitrates: Novel
Multi-Purpose Reagents for Organic Synthesis. A Cornelis, P Laszlo.
Synthesis 1985, 909-918.
3. Cornelis, Laszlo & Pennetreau, Clay Miner 1983, 18, 437; CA 1984,
100, 102362.
4. Cornelis, Laszlo & Pennetreau, Bull. Soc. Chim. Belg. 1984, 93, 961
5. B. Theng, The Chemistry of Clay-Organic Reactions, Wiley NY, 1974.
6. McKillop & Young, Synthesis 1979, 401, 481.
7. Theng, Dev. Sedimentol. 1982, 35, 197; CA 1982, 97, 197524;
8. Lagaly, Phil. Trans. R. Soc. Lond. A 1984, 311, 315
9. Cornelis & Laszlo, Synthesis 1980, 849.
10. Addison. Prog. Inorg. Chem. 1967, 8, 195.
11. Addison, Logan, Wallwork, Garner. Q. Rev. Chem. Soc. 1971, 25, 289.
12. Bologh, Hermecz, Meszaros, Laszlo. Helv. Chim. Acta 1984, 67, 2270.
13. Selective Nitration of Styrenes with Clayfen and Clayan: Solvent````free Synthesis of beta-Nitrostyrenes. Rajender S. Varma, Kannan P.
Naicker and Per J. Liesen. Tetrahedron Letters 39 (1998) 3977-3980
.
14. Varma, R.S.; Dahiya, R. Tetrahedron Lett. 1997,38. 2043;
15. Sodium Borohydride on Wet Clay: Solvent-free Reductive Amination of
Carbonyl Compounds Using Microwaves. Rajender S. Varma and Rajender
Dahiya. Tetrahedron 54, 6293-8 (1998)
16. Varma & Dahiya. Tetr. Lett. 39 (1998), 1307-8.