Sodium Tetraalkoxyborates by Milling of Aldehydes and Ketones with NaBH4
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Typical Example. Sodium Tetra(4-chlorobenzyloxy)borate (3a): M.p.
Ͼ300 °C. IR (dried KBr): ν = 2900, 1659, 1434, 1079, 956, 603,
˜
549, 438 cm–1; the KBr pellet when exposed to ambient air for 24 h
completely lost the bands of 3a and exhibited all of the known IR
bands of 4a with some minor additional bands and a significant
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1
band at 986–939 cm–1. H NMR ([D6]DMSO): δ = 7.54, 4.69 ppm
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(br., 4:2 ratio).
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Thermal Stability Tests of 3: About 50 mg of 3g, or 3δ in a 50-mL
evacuated flask, were heated to 150 °C for 1 h. IR analysis in dried
KBr indicated no change.
Isolation of Alcohols 4: Solid milling products 3a–ζ were hydrolyzed
with water. Insoluble solid alcohols were filtered off and dried. Sol-
uble and liquid alcohols were isolated by extraction (3 to 5 times)
from the water with ethyl ether in a small separating tube device
and dried with MgSO4·2H2O that was filtered off and washed with
ethyl ether for thermal reactivation. The solvent was recovered by
isothermal distillation in a closed system, and the residue was freed
from trace amounts of solvent by short-path distillation or subli-
mation. The purity of products 4 was confirmed by VPC analysis,
and their identity was confirmed by comparison of their spectro-
scopic data and melting points with those in the literature (Table 1).
The sodium borate content in the aqueous phases was ready for
recycling as Na2B4O7·10H2O by evaporation and recrystallization,
as no poisonous auxiliaries were used in the reduction process.
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Isolation of Methanol from 3s after Hydrolysis with Na2SO4·10H2O:
After the milling of paraformaldehyde (1s; 90 mg, 3.0 mmol) with
NaBH4 (2; 28.5 mg, 0.75 mmol) at 25 °C for 20 min to give 4s (m.p.
Ͼ300 °C), Na2SO4·10H2O (97 mg, 0.3 mmol) was added and mill-
ing was continued for 10 min. The hydrolytically liberated meth-
anol was distilled to a cold trap at 77 K by vacuum pumping di-
rectly from the milling beaker to give a yield of 96 mg (100%)
methanol. Similarly, most of the other alcohols in Table 1 might
be isolated by this vacuum distillation technique with appropriate
heating of the milling beaker if the use of extraction by an organic
solvent should be avoided, even though the recycling of
Na2B4O7·10H2O would not be facilitated by the addition of a dif-
ferent salt with crystal water. However, similar use of borax would
remove this problem.
G. Kaupp, “Prediction of Reactivity in Solid-State Chemistry”
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Test Run by Grinding of 1a with NaBH4: 4-Chlorobenzaldehyde (1a;
140 mg, 1.0 mmol) and NaBH4 (2; 38 mg, 1.0 mmol) were continu-
ously ground in an agate mortar with pestle for 30 min. The mix-
[20]
[21]
1
ture took up moisture from the air and a 36% H NMR spectro-
scopic yield of 4-chlorobenzyl alcohol (4a) was obtained after ex-
traction as above together with unreacted 1a (64%).
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We acknowledge the Research Council of the Iran University of
Science and Technology (IUST), Iran for partial financial support
of this work.
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www.eurjoc.org
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