Catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines

Article abstract of DOI:10.1080/10426500490426683

A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either primary or secondary amine-borane adducts has been developed using late transition metal complexes as precatalysts. The dehydrocoupling of Me₂NH·BH

Full text of DOI:10.1080/10426500490426683

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Phosphorus, Sulfur, and Silicon and the Related
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CATALYTIC DEHYDROCOUPLING OF AMINE-BORANE
ADDUCTS TO FORM AMINOBORANES AND BORAZINES
a
a
a
a
Cory A. Jaska , Karen Temple , Alan J. Lough & Ian Manners
a
University of Toronto , Toronto, Canada
Published online: 12 Aug 2010.
To cite this article: Cory A. Jaska , Karen Temple , Alan J. Lough & Ian Manners (2004) CATALYTIC DEHYDROCOUPLING OF
AMINE-BORANE ADDUCTS TO FORM AMINOBORANES AND BORAZINES, Phosphorus, Sulfur, and Silicon and the Related Elements,
179:4-5, 733-736, DOI: 10.1080/10426500490426683
To link to this article: http://dx.doi.org/10.1080/10426500490426683
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Phosphorus, Sulfur, and Silicon, 179:733–736, 2004
Copyright ꢀC Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490426683
CATALYTIC DEHYDROCOUPLING
OF AMINE-BORANE ADDUCTS TO FORM
AMINOBORANES AND BORAZINES
Cory A. Jaska, Karen Temple, Alan J. Lough, and Ian Manners
University of Toronto, Toronto, Canada
(
Received August 17, 2003; accepted October 3, 2003)
A mild, catalytic dehydrocoupling route to aminoboranes and borazine
derivatives from either primary or secondary amine-borane adducts
has been developed using late transition metal complexes as precata-
lysts. The dehydrocoupling of Me NH·BH was found to be catalyzed by
2
3
◦
0
(
.5 mol% [Rh(1,5-cod)(µ-Cl)] in solution at 25 C to give [Me N BH ]
2 2 2 2
1) quantitatively after ca. 8 h. This new catalytic method was ex-
ꢁ
tended to other secondary adducts RR NH·BH which afforded the
3
dimeric [(1,4-C H )N BH ] (2) and [PhCH (Me)N BH ] (3) or
4
8
2 2
2
2 2
i
the monomeric aminoborane Pr N BH (4) under mild condi-
2
2
tions. The catalytic dehydrocoupling of NH ·BH , MeNH ·BH , and
3
3
2
3
◦
PhNH ·BH at 45 C affords the borazine derivatives [RN BH] (5:
2
3
3
R = H; 6: R = Me; 7:R = Ph). TEM analysis of the contents of the re-
action solution for the [Rh(1,5-cod)(µ-Cl)] catalyzed dehydrocoupling
2
of Me NH·BH together with Hg poisoning experiments suggested a
2
3
soluble heterogeneous catalyst involving Rh(0) nanoclusters.
Currently, there is much interest in the development of catalytic
methods towards the formation of new bonds involving the p-block
1
elements. For example, the catalytic dehydrocoupling of silanes, ger-
manes and stannanes are now well established routes toward high
molecular weight polysilanes,2 polygermanes, and polystannanes.
3
4
There are now numerous examples of both catalytic homo- and hetero-
dehydrocoupling bond forming reactions. Recently, we reported the first
5
examples of the transition metal-catalyzed formation of P B bonds.
◦
The dehydrocoupling of phosphine-borane adducts at 60–130 C in the
presence of a range of precatalysts (e.g., Rh complexes) was found
I
to provide a new route to phosphinoborane rings, chains, and
Address correspondence to Ian Manners, Department of Chemistry, University of
Toronto, 80 St. George Street, Toronto, M5S 3H6 Ontario, Canada. E-mail: imanners@
chem.utoronto.ca
7
33

7
34
C. A. Jaska et al.
6
macromolecules. We have now extended this dehydrocoupling chem-
istry to amine-borane adducts, which form monomeric or dimeric
7
aminoboranes and borazine derivitives under mild conditions.
RESULTS AND DISCUSSION
Treatment of Me2NH·BH3 in toluene with a catalytic amount (0.5 mol%)
of [Rh(1,5-cod)(µ-Cl)]2 was found to result in the quantitative formation
◦
of [Me2N–BH2]2 (1) after 8 h at 25 C (Eq. 1). Raising the temperature
◦
to 45 C allowed the dehydrocoupling reaction to be performed as a neat
mixture of the catalyst and the molten adduct. This reduced the reaction
time to 2 h and allowed 1 to sublime directly out of the neat mixture in
high yields.
This catalytic dehydrocoupling strategy also can be generalized
to other secondary amine-borane adducts. For example, the cyclic
pyrrolidine-borane (1,4-C4H8)NH·BH3 and the unsymmetrically sub-
stituted PhCH2(Me)NH·BH3 were found to undergo catalytic dehy-
drocoupling to afford the cyclic dimers (1,4-C4H8)N BH2]2 (2) and
[
PhCH2(Me)N BH2]2 (3) respectively.
(1)
i
When Pr2NH·BH3 was treated with a Rh catalytst, dehydrocoupling
i
to form the monomeric species Pr2N BH2 (4) was observed (Eq. 2).
The formation of the monomeric 4 is likely due to the steric bulk of the
nitrogen substituents, which prevents dimerization.
(2)
Primary amine-boranes or NH3·BH3 also undergo catalytic dehydro-
coupling to afford borazine derivitives. For example, when ammonia-
borane was treated with a catalytic amount of [Rh(1,5-cod)(µ-Cl)]2
◦
in tetraglyme at 45 C, the formation of borazine 5 was observed

Catalytic Dehydrocoupling of Amine-Borane Adducts
735
after ca. 48–84 h (Eq. 3). However, attempts to isolate pure samples
of 5 by vacuum fractionation resulted in only 10% yield. It is sug-
gested that 5 undergoes further intermolecular dehydrocoupling to give
oligomeric and/or polymeric B N species. This may be a facile process,
◦
as the thermal (70 C) dehydrocoupling of borazine has been reported
ꢁ
to yield poly(borazylene) along with small amounts of diborazine (1,2 -
B3N3H5)2) and borazanaphthalene (B5N5H8).
8
(
When MeNH2·BH3 was treated with a catalytic amount of [Rh(1,5-
◦
cod)(µ-Cl)]2 in monoglyme at 45 C, the formation of the cyclic trimer
[
MeNH BH2]3 initially was detected by 11B NMR. This was followed
by further loss of hydrogen to give N-trimethylborazine [MeN BH]3
6). Isolation by vacuum fractionation yielded pure samples of 6 in
(
moderate yields (40%). Again, yields may be limited by undesirable
intermolecular dehydrocoupling. In the case of PhNH2·BH3, catalytic
dehydrocoupling to give N-triphenylborazine [PhN BH]3 (7) was ob-
served to occur rapidly in monoglyme in the presence of catalytic
◦
amounts of [Rh(1,5-cod)(µ-Cl)]2 at 25 C.
(3)
The observation that all catalytic trials involving Rh precatalysts
resulted in the formation of a black, opaque solution raised the problem
of whether the catalysis is homogeneous or heterogeneous. Heteroge-
neous catalysis involving Rh colloids was immediately considered. In
an attempt to more directly detect the colloidal species, a catalytic de-
hydrocoupling experiment involving Me2NH·BH3 was initiated and an
aliquot was removed from the reaction mixture after 6 h and evaporated
to dryness. Analysis by transmission electron microscope (TEM) of this
sample showed the presence of small (∼2 nm) Rh nanoclusters.
Monitoring the extent of reaction by 11B NMR initially indicated
the presence of an induction period in which no dehydrocoupling
occurs. During this induction period, the formation of colloidal metal
was not observed by visual inspection. Following this induction period,
colloidal metal was observed to form and the dehydrocoupling reaction
proceeded rapidly. After partial conversion of Me2NH·BH3 to 1, mercury
was added to the reaction mixture. Subsequently, a dramatic drop in
the dehydrocoupling rate was observed, with little further conversion

7
36
C. A. Jaska et al.
(
<5%) to 1 as the added Hg had poisoned the active catalytic species.
A dehydrocoupling trial initiated in the presence of Hg also showed no
conversion to 1 by 11B NMR. This verifies that the addition of mercury
inhibits the catalytic dehydrocoupling and suggests the operation of a
heterogeneous process involving soluble Rh(0) colloids as the true active
catalyst.
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