Abstract
A 1,3-diketone resin was developed as the basis for a selective
scavenger for hydrazines. In addition, it can be employed for the
selective removal of primary amines in the presence of secondary
amines which is of fundamental importance in the purification of
reductive alkylations. The resin’s specificity is based
on the sequestration of the hydrazine via their polymer-attached
pyrazoles and of the primary amines via their enamines.
Key words
amines - combinatorial chemistry - hydrazines - polymer supported reagent - scavenger resin
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12 Preparation of 2,4-pentandione resin 3: To a suspension of Merrifield polymer
(2.5 g, 4.3 mmol chloride/g, 2% DVB, 200-400
mesh) in dry DMSO (25 mL) was added sodium bicarbonate (9.24 g,
110 mmol) and gentle stirring was continued at 155 °C under
nitrogen for 20 h. Ice-cold water was added, the resin was filtered
and successively washed with water, methanol, dichloromethane, methanol,
and dried in high vacuum at 30 °C over P2O5.
The
dried resin (ca. 2.2 g) was suspended in dry dichloromethane (30
mL) and sodium iodide (8.24 g, 55 mmol) was added. Gentle stirring
was continued under nitrogen and trimethylchlorosilane (6.98 mL,
55 mmol) and acetylacetone (1.65 mL, 16 mmol) were added separately during
6 h via a syringe pump. The mixture was gently stirred overnight
at r.t. and then treated with water (50 mL). After 10 min the resin
was filtered and successively washed with methanol, water, methanol,
acetone, dichloromethane, methanol, and dried in high vacuum.
It
has to be pointed out that after thorough optimization we had to
notice the need for excessive washing of the resin before use, because
it tends to bind impurities including iodine. Resin 3 can
be stored at 4 °C and stays active for months. The commercial
Merrifield resin 5 as well as 1,3-diketone
resin 3 were analyzed by combustion analysis. Found
for 5: C 77.63, H 6.37, Cl 15.69 (equals
about 4.42 mmol/g loading); found for 3:
C 79.55, H 7.17, Cl 0.47 (equals about 3.37 mmol/g loading).
13 In addition, resin 3 is
also able to scavenge hydroxylamine such as the HCl salt of benzylhydroxylamine,
which was removed in CH2Cl2 at room temperature
within eight hours.
14 Typical scavenger procedure with 1
: To a solution
of ethylcarbazate (16 mg, 0.165 mmol) in dichloromethane
(1
mL) was added benzaldehyde resin 1 (150
mg, 3.0 mmol aldehyde/g) and stirred at r.t. until all
hydrazine has reacted. The procedure was monitored with TLC or GC.
After 72 h the suspension was filtered, the resin was washed with dichloromethane
(3 × 2 mL), and the combined filtrates were evaporated
in high vacuum. No residue remained in the flask.
Typical
scavenger procedure with 3: To a solution
of ethylcarbazate (17 mg, 0.165 mmol) in dichloromethane
(1
mL) was added 2,4-pentandione resin 3 (100
mg, 3.3 mmol diketone/g) and stirred at r.t. until all
hydrazine has reacted. The procedure was monitored with TLC or GC. After
3 h the suspension was filtered, the resin was washed with dichloromethane
(3 × 2 mL), and the combined filtrates were evaporated
under high vacuum. No residue was found in the flask.
15 Commonly, hydrazines were completely
scavenged within the given time. Occasionally, we observed a residue
after filtration and evaporation of the solvent which originated from
resin impurities.
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