568
Inorg. Chem. 2001, 40, 568-570
[7,8-R′R′′-7,8-C2B9H12] and [Bu4N][7,9-R′R′′7,9-C2B9H12], re-
spectively.5,7,9 Nido structures have been used to synthesize more
soluble analogues;10 however, in tetrabutylammonium salt this
deboronation method converts o-carborane to nido-carborane,
which has extremely low hydrophilicity. The water solubility
of the compounds containing carborane cages is one of the most
important physical properties for potential BNCT (boron neutron
capture therapy) reagents.11 Furthermore, it is difficult to change
the countercation to other salts. Getman has recently reported
that potassium fluoride supported on alumina could deboronate
o-carborane in acetonitrile and the countercation (K+) in its nido
anion K[7,8-C2B9H12] could be converted to [P(C6H5)3CH3]+.12
But the isolation of K[7,8-C2B9H12] itself was not achieved
because of the concurrent presence of an unidentified hygro-
scopic solid that is formed from the reaction of the KF/alumina
reagent with refluxing wet acetonitrile.12 In seeking a mild and
facile degradation route of o-carborane derivatives to produce
water-soluble nido monoanions, we perceived that cesium
fluoride is a good fluoride ion source that can be used as a base
in various organic reactions and has higher solubility in methanol
and ethanol than any other alkali metal fluoride.13 Accordingly,
we pursued the use of cesium fluoride as a deboronation base.
Herein we report that the CsF/ethanol pair is a very effective
system for deboronating o-carborane derivatives to give the
corresponding cesium salt of nido-carboranes in high yield, and
it is mild enough to degrade o-carborane containing an ester
bond without attacking the susceptible functional group.
Facile and Mild Deboronation of o-Carboranes
Using Cesium Fluoride
Jeongsoo Yoo, Jeong-Wook Hwang, and
Youngkyu Do*
Department of Chemistry, School of Molecular
Science-BK21 and Center for Molecular Design and
Synthesis, Korea Advanced Institute of Science and
Technology, Taejon 305-701, South Korea
ReceiVed July 12, 2000
Introduction
The nido monoanion [7,8-C2B9H12]-, which was first obtained
by alcoholic base degradation of o-carborane1 and substituted
derivatives [7,8-R′R′′-7,8-C2B9H12]-, are very important starting
reagents for the syntheses of a wide range of metallacarborane
compounds with potential applications as novel catalysts2 and
radiochemical drugs3 as well as for the syntheses of several nido-
and closo-carboranes and heteroboranes.4 However, the deboro-
nation using the potassium hydroxide/methanol or ethanol route
is not suitable for carborane derivatives that contain functional
groups susceptible to the attack of a strong base or nucleophile.5
Although other deboronation reagents such as tertiary amines,
hydrazine, ammonia, piperidine, and pyrrolidine are also avail-
able,6 the recently found deboronation reagent, the fluoride
ion,5,7 is the most promising alternative because it is a very weak
nucleophile but effective as well as easy to handle (no offensive
odor).8 Tetrabutylammonium fluoride (TBAF), the first reported
fluoride ion source, readily degrades o- and m-carboranes under
mild conditions to the corresponding nido monoanions [Bu4N]-
Experimental Sections
1H and 13C NMR spectra were measured on a Bruker AVANCE
400 spectrometer operating at 400.13 and 100.61 MHz and were
referenced to the internal solvent peaks. 11B NMR spectra were obtained
on a Bruker AM 300 spectrometer operating at 96.29 MHz using a
coaxial tube containing MeCN-d3 for the purpose of locking. External
references for the chemical shifts are BF3‚Et2O ) 0.0. IR spectra were
recorded in KBr disks on a Bruker EQUINOX 55 FT-IR spectrometer.
Materials. o-Carborane, 1-methyl-o-carborane, and m-carborane,
purchased from KATCHEM Ltd., were purified by sublimation under
vacuum prior to use. Cesium fluoride, absolute ethanol, and other
reagents were purchased from Aldrich and used as received. µ-1,2-
Trimethylene-o-carborane (1) was synthesized by modifying the
published method,14 using 1-bromo-3-chloropropne instead of 1,3-
dibromopropane. The synthesis of 3,3′-(3-(1,2-dicarba-closo-dodecabo-
rane-1-yl)carbpropoxy)-2,2′-bipyridine (2) will be reported elsewhere.15
Methanol, acetonitrile, and THF were dried according to the published
procedures.16 Other solvents were of reagent grade and used without
further purification.
* To whom correspondence should be addressed. Phone: +82-42-869-
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10.1021/ic000768k CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/04/2001