Short Articles
Bull. Chem. Soc. Jpn., 77, 1931–1932 (2004) 1931
should be noted that the current method is successfully applied
to the deprotection of a phosphite–borane for the first time.
The deprotection of phosphine–boranes 1 was carried out as
follows. The starting materials, phosphine–boranes 1, were
prepared by the reaction of readily available phosphines with
BH3(SMe2). A solution of phosphine–borane 1 in a protic sol-
A Novel Mild Deprotection Method
for Phosphine–Boranes
ꢀ;1
ꢁ
¨
Marc Schroder, Kyoko Nozaki,
vent was allowed to react with activated MS4A at 25–115 C
ꢀ
(depending on the substituents at phosphorus) for over 22 h.
After removal of MS4A by filtration, evaporation of solvents
in vacuo afforded the deprotected phosphine in its pure form.
The optimized conditions and results are summarized in
Table 1. The characteristic features are; (1) The existence of
molecular sieves was essential for the reaction. In its absence
a highly unselective product formation was observed. For ex-
ample, when entry 2 was run without molecular sieves, no
phosphine 2b but 3% of the corresponding phosphine oxide
and 75% of unidentified products were detected by
31P{1H} NMR spectroscopy. (2) The existence of a protic sol-
vent was also essential for the reaction. For example, the use of
only THF instead of THF/MeOH for the experiment of entry 1
resulted in 1.5% conversion of 1a to give the corresponding
phosphine and phosphine oxide in 1% and 0.5% yields, respec-
tively. (3) The deprotection proceeded without inversion of the
chirogenic phosphorus atom (entry 8),3 if the reaction temper-
and Tamejiro Hiyama
Department of Material Chemistry, Graduate School of
Engineering, Kyoto University, Katsura, Nishikyo-ku,
Kyoto 615-8510
1Department of Chemistry and Biotechnology, Graduate
School of Engineering, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-8656
Received March 22, 2004; E-mail: nozaki@chembio.t.u-
tokyo.ac.jp
Treatment of phosphine–boranes with molecular sieves
4A in a mixture of an ethereal solvent and an alcohol provid-
ꢀ
ed deprotected free phosphines in quantitative yields. The
phosphines can be obtained by a simple filtration/crystalliza-
tion procedure in most cases. It should be noted that the cur-
rent method is successfully applied to the deprotection of a
phosphite–borane for the first time.
ꢁ
ature did not exceed 70 C. (4) Functional groups like acetals
(entry 7) and hydroxy groups (entry 8) were tolerated by this
mild deprotection method. (5) By lowering the reaction tem-
perature to 25 ꢁC, deprotection of a phosphite–borane was suc-
cessfully achieved for the first time (entry 9). No substituent
scrambling was observed in this case, when the bulky t-BuOH
was used instead of methanol.
Because addition of alcohol is essential for the deprotection,
it seems that the reaction involves the formation of boron
alkoxide accompanied by the molecular hydrogen production.
Either or both of the Lewis acidic and basic sites of the
aluminum/silicon oxide of molecular sieves likely catalyzed
the process.
Phosphines are now widely used as ligands for transition
metal catalysts. They are often air-sensitive and get oxidized
easily, with a few exceptions such as triarylphosphines. The
complex formation of phosphines with borane,1 keeping the
lone pair of the phosphorus atom from further reactions such
as oxidation, is a powerful method for the protection of phos-
phines. The air-stability of phosphine–boranes facilitates the
purification process and makes them storable for a long time.
In addition, phosphine–boranes are easily accessible from
phosphine oxides or substituted chlorophosphines by the reac-
tion with borane, without isolating the intermediate phos-
phines. Thus, phosphine–boranes are known as one of the
key intermediates for the preparation of new phosphines for
catalysis.2,3
For the deprotection of phosphine–boranes to generate the
desired free phosphines, several methods have been developed.
The use of an excess amount of either secondary amines, such
as diethylamine or morpholine,3 or tertiary amines, such as
DABCO or triethylamine4 is one of the most common proce-
dures ꢁfor this purpose. Generally, treatment for a few hours
at 40 C in toluene completes the reaction. The P–B bond of
phosphinite–boranes was also quantitatively cleaved.5 Depro-
tection using strong protic acids, such as CH3SO3H,
CF3SO3H, and HBF4, is an alternative choice, especially in
the case of electron-rich phosphines.6
Experimental
Preparation of Phosphine–Boranes. The phosphine–boranes
1 except for 1h were prepared from commercially available phos-
phines according to the literature.7 Chiral phosphine 1h was pre-
pared from PhMe2P(BH3) according to a literature method.8
ꢀ
Activation of Molecular Sieves 4A. Molecular sieves (MS)
with a cavity of 4 A were heated up to 250 ꢁC under vacuum
ꢀ
for several minutes and then purged with argon. Subsequently,
the MS were ground up to homogeneously pulverize the catalyst.
Finally the powder was then heated again to 250 ꢁC under vacuum
for several minutes, flushed with argon and this procedure was re-
peated several times.
Deprotection of Phosphine–Boranes. Method A: In a com-
ꢀ
mon Schlenk tube was added to the activated molecular sieves 4A
(500 mg) a mixture of the ethereal solvent (THF or dioxane) (7
mL) and MeOH (3 mL). To this suspension was added the phos-
phine–boranes (0.3 mmol), and the mixture was stirred under ar-
gon at atmospheric pressure. Filtration through a pad of Celite un-
der argon, rinsing with THF and removal of the solvents in vacuo
provided pure phosphines, without further purification (phos-
phines 2a, 2c–f, 2i). Recrystallization was necessary to further pu-
rify the products: 2a and 2g from MeOH/AcOEt (5:95); 2i from
Here we report a novel and mild deprotection process of
phosphine–boranes; that is, the reaction of phosphine–boranes
ꢀ
with molecular sieves 4A (MS4A) in a mixture of an ethereal
solvent and an alcohol. The phosphines can be obtained by a
simple filtration/crystallization procedure in most cases. It
Published on the web October 9, 2004; DOI 10.1246/bcsj.77.1931