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Metal-Mediated Diboration of Alkynes with [2]Borametalloarenophanes under Stoichiometric Homogeneous and Heterogeneous Conditions.

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Communications
Catalytic Diboration
DOI: 10.1002/anie.200603130
Metal-Mediated Diboration of Alkynes with
[2]Borametalloarenophanes under
Stoichiometric, Homogeneous, and
Heterogeneous Conditions**
Holger Braunschweig,* Thomas Kupfer, Matthias Lutz,
Krzysztof Radacki, Fabian Seeler, and Rainer Sigritz
Dedicated to Professor Manfred Christl
on the occasion of his 65th birthday
Boron compounds are among the most useful reagents for the
functionalization of unsaturated organic substrates,[1] and
many borylated organic species thus obtained have become of
[*] Prof. H. Braunschweig, T. Kupfer, Dr. M. Lutz, Dr. K. Radacki,
F. Seeler, R. Sigritz
Institut f-r Anorganische Chemie
Bayerische Julius-Maximilians-Universit7t W-rzburg
Am Hubland, 97074 W-rzburg (Germany)
Fax: (+ 49) 931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[**] This work was supported by the DFG. T.K. thanks the FCI for a Ph.D.
stipend. We are grateful to the BASF AG for a generous donation of
chemicals.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
8048
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 8048 –8051
Angewandte
Chemie
major importance for various applications, for example, in
organic synthesis or materials science.[2] Over the past
15 years a broad variety of transition-metal-catalyzed protocols have been developed for the preparation of specific
organoboranes,[3] and detailed knowledge on boryl and
bis(boryl) complexes, which commonly occur as key intermediates in such homogeneously catalyzed transformations,[4]
has helped to establish them as standard procedures. While
hydroboration is probably the best-known of these reactions,
the metal-mediated diboration, first reported in 1993,[5] is
likewise a versatile synthetic method facilitating the addition
of a diborane(4) to organic substrates such as alkynes,
alkenes, dienes, or a,b-unsaturated ketones.[3f,g] The mechanism for this homogeneous catalysis is well-understood and
involves the oxidative addition of a diborane(4) to a lowvalent metal center, most commonly Pt0, with formation of a
cis-bis(boryl) species.[4–6]
Detailed knowledge about catalytically active species
allows for the development of “tailor-made” catalysts with
specific highly selective properties, which is generally recognized as the major advantage of homogeneous catalysis.[7]
Nevertheless, the proportion of heterogeneous protocols in
industrially applied catalytic processes still amounts to about
80 %, since heterogeneous catalysis facilitates the separation
of catalyst and products and, in addition, the handling and
lifetime of heterogeneous catalysts are usually advantageous.[8]
In the course of our recent studies on [2]borametalloarenophanes,[9] we observed a pronounced reactivity of the
bridging B B bond toward the oxidative addition to
Pt0 centers. We anticipated that the moderately strained,
however thermally stable [2]borametalloarenophanes could
serve as facile diborane(4) precursors for the diboration of
alkynes. Herein, we report on the metal-mediated insertion of
alkynes into the B B bridge of Cr- and Fe-based [2]borametalloarenophanes. This diboration was achieved under homogeneous and heterogeneous conditions and represents, to the
best of our knowledge, the first instance of a heterogeneously
catalyzed diboration of an unsaturated organic substrate.
The readily accessible [2]borametalloarenophanes 1 and 2
were prepared according to known procedures[9c, 10] and
treated with stoichiometric amounts of [Pt(PEt3)3] to yield
the [3]boraplatinacomplexes 3 and 4 (Scheme 1).
The oxidative addition of the boron–boron bond to the
Pt center leads to the expected[9a] significant low-field shift of
the 11B NMR resonances (3: d = 61.6 ppm; 4: d = 62.2 ppm)
compared to those of the starting materials (1: d = 44.4 ppm;
2: d = 46.3 ppm), and to the appearance of broad 31P NMR
resonances (3: d = 11.35 ppm; 4: d = 11.50 ppm), which are
flanked by characteristic 195Pt satellites (JP,Pt = 1127.6 Hz (3),
JP,Pt = 1154.6 Hz (4)) with peak widths at half height of about
50 Hz.
Single-crystal diffraction studies of 3 and 4 were conducted (Figure 1)[11] and reveal structural parameters which
resemble those of previously reported related complexes. The
incorporation of the {Pt(PEt3)2} fragment into the B B bond
leads to a significant release of the intramolecular strain, as
illustrated by the tilt angles a = 5.08 (3) and 4.28 (4) with
respect to the parent sandwich complexes (1: a = 12.88; 2: a =
Angew. Chem. Int. Ed. 2006, 45, 8048 –8051
Scheme 1. Oxidative cleavage of the boron–boron bond and formation
of ansa-bis(boryl)alkenes.
Figure 1. Structures of 3 (left) and 4 (right) in the solid state. Thermal ellipsoids
are shown at 50 % probability.
10.18),[9c] thus allowing the carbocyclic ligands in 3 and 4 to
adopt an almost parallel arrangement. The Pt centers display
distorted square-planar environments, and pertinent bond
lengths and angles lie within previously reported ranges (3:
Pt1 B1 2.116(3), Pt1 B2 2.109(3), Pt1 P1 2.3402(7), Pt1 P2
2.3322(7) C; B1-Pt1-B2 75.42(11), P1-Pt1-P2 100.75(3)8; 4:
Pt1 B1 2.109(7), Pt1 B2 2.108(8), Pt1 P1 2.3319(17),
Pt1 P2 2.3445(16) C; B1-Pt1-B2 76.1(3), P1-Pt1-P2
97.48(7)8).[4a, 9a, 12, 13]
To confirm the anticipated propensity of [2]borametalloarenophanes to undergo Pt-mediated insertion of alkynes
into the B B bond, the isolated species 3 and 4 were treated
with a 10-fold excess of propyne or 2-butyne to yield after
extraction and crystallization from hexanes the corresponding
novel ansa-bis(boryl)alkenes 5–8. In solution, the 1H NMR
spectra display characteristic resonances for the Me (5, 7) and
H substituents (6, 8) of the bridging -C(Me)=C(R)-moiety (5:
d = 1.91 ppm (6 H); 7: d = 1.96 ppm (6 H); 6: d = 2.14 (3 H),
6.44 ppm (1 H); 8: d = 2.18 (3 H), 6.48 ppm (1 H)). The
incorporation of the unsymmetrical propyne molecule furnishes four resonances for the aminomethyl groups in 6 and 8
in the 1H NMR spectra (6: d = 2.61, 2.65, 2.68, 2.69 ppm; 8:
d = 2.66, 2.67, 2.70, 2.74 ppm) while the detection of only two
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8049
Communications
Table 1: Comparison of the reaction conditions upon formation of the ansa-bis(boryl)alkenes 5–8.
corresponding resonances in 5 and
7 (5: d = 2.63, 2.68 ppm; 7: d = 2.65,
Stoichiometric[a]
Catalytic[b]
2.74 ppm) suggests C2 symmetry in
Catalyst
[Pt(PEt3)3]
Pt
Pd
solution for the latter compounds.
ansa
compound
3
4
1
2
1
2
1
2
The 11B NMR resonances for 5
mol % cat.[c]
5
5
6
6
6
6
(d = 40.6 ppm), 6 (d = 40.5 ppm),
mmol propyne
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
7 (d = 43.3 ppm), and 8 (d = 41.8,
mmol 2-butyne
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
–[d]
–[d]
80
80
100
100
100
100
T [8C][c]
43.1 ppm) resemble those of the
–[d]
–[d]
1
1
5
4
6
8
t [days] (CH3CCH)
starting compounds 1 (d =
t [days] (CH3CCCH3)
–[d]
–[d]
2
2
8
7
14
16
44.4 ppm) and 2 (d = 46.3 ppm).
52
57
68
83
95
92
91
93
yield[e] [%](CH3CCH)
While both 6 and 8 comprise two
yield[e] [%](CH3CCCH3)
58
51
75
80
89
87
90
93
chemically nonequivalent boron
[a] For the stoichiometric experiments, 0.17 mmol of the corresponding [3]boraplatinacomplex was
nuclei, the expected two distinct
used. [b] For the catalytic experiments, 0.17 mmol of the corresponding ansa compound was employed.
resonances were only resolved in
[c] Conditions given for propyne and 2-butyne. [d] Reactions occurred spontaneously. [e] Yields of
[14]
the case of the latter. Owing to
isolated product after work up.
the quadrupolar momentum of the
neighboring boron nuclei, the
13
and the data confirmed a gradual consumption of the starting
C NMR signals of the olefinic bridge are broadened, but
materials and the formation of the aforementioned products
can be unequivocally determined in the two-dimensional
5–8. It should be noted that the NMR data indicated the
long-range 13C,1H NMR spectra (5: d = 143.8 ppm; 7:
absence of any soluble side or degradation products. Reaction
144.3 ppm; 6: 139.3, 156.0 ppm; 8: 139.1, 155.6 ppm).
times depended on the conditions (homogeneous or heteroFurthermore, a single-crystal X-ray diffraction study of 7
geneous) and the organic substrate, and were found to be in
was conducted (Figure 2).[11] The methyl groups of the
the range of one to two days in the case of [Pt(PEt3)3], and
bis(boryl)alkene subunit adopt a mutual cis arrangement,
four to eight days in the case of Pt metal. After work up and
recrystallization from pentanes, the pure products 5–8 were
isolated in very good yields of up to 95 %.
Corresponding experiments were conducted with commercially available Pd/charcoal under otherwise analogous
conditions. While reactions were found to take 6–16 days, and
hence, somewhat longer than in the case of Pt, the products
were isolated in very high yields, and their formation was,
according to multinuclear NMR spectroscopy, not accompanied by the formation of soluble side products. It should be
noted that control experiments were carried out, which
proved that the [2]boraarenophanes 1 and 2 underwent no
reaction with propyne and 2-butyne under analogous conditions in the absence of platinum and palladium.
Figure 2. Structure of 7 in the solid state. Thermal ellipsoids are
In conclusion, we provide in this article our results on the
shown at 50 % probability.
heterogeneous diboration of alkynes employing finely dispersed palladium or platinum. Relative to its lighter congener, platinum exhibits superior reactivity in the heterogewith torsion angles B1-C2-C3-B2 = 2.138, C11-B1-C2-C3 =
neous borylation of propyne and 2-butyne.
78.128, and C21-B2-C3-C2 = 72.948. Further notable metric
parameters include the B C bond lengths (1.587(2),
1.581(2) C) and the C C bond length (1.351(2) C), which
lie in the expected range for B C single and C C double
Experimental Section
All experimental details are contained in the Supporting Information.
bonds.[12d, 15] In contrast to the modest strain that is still present
in 3 and 4, the incorporation of the B C=C B moiety allows
Received: August 2, 2006
for an almost parallel alignment of the carbocyclic ligands in
Published online: November 8, 2006
7, as indicated by the tilt angle a = 2.03(9)8.
In subsequent experiments, the [2]borametalloarenoKeywords: boron · diboration · heterogeneous catalysis ·
phanes 1 and 2 were treated with propyne and 2-butyne,
palladium · platinum
respectively, in the presence of catalytic amounts of [Pt(PEt3)3] or a finely dispersed Group 10 metal (Table 1). In a
typical experiment, a sealable tube was charged with the
[1] Reviews: a) D. E. Kaufmann, D. S. Matteson, Science of Synansa complex, a 10-fold excess of the alkyne, and 5 mol % of
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Angew. Chem. Int. Ed. 2006, 45, 8048 –8051
Angewandte
Chemie
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Angew. Chem. Int. Ed. 2006, 45, 8048 –8051
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diboration, metali, heterogeneous, alkynes, homogeneous, stoichiometry, conditions, borametalloarenophanes, mediated
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