Stoichiometric and homogeneous-catalytic diboration of the N=N double bond of azobenzene

Article abstract of DOI:10.1021/ja8006879

The diboration of the N=N double bond of azobenzene was achieved by reaction of the [3]diboraplatinametalloarenophanes derived from ferrocene, [Fe(η⁵-C₅H₄)B(NMe₂)Pt(PEt₃)₂B(NMe₂)

Full text of DOI:10.1021/ja8006879

Published on Web 03/08/2008
Stoichiometric and Homogeneous-Catalytic Diboration of the NdN Double
Bond of Azobenzene
Holger Braunschweig* and Thomas Kupfer
Institut fu¨r Anorganische Chemie, Julius-Maximilians-UniVersita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany
Received January 28, 2008; E-mail: h.braunschweig@mail.uni-wuerzburg.de
Hydroboration1 and diboration2 reactions are among the most
valuable transformations in organic chemistry and have enabled
Scheme 1. Stoichiometric Diboration of Azobenzene To Yield the
ansa-Bis(boryl)hydrazines 3 and 4
convenient access to a variety of useful reagents.3 However, the
selective diboration of unsaturated organic substrates that contain
elements other than carbon (i.e., substances other than alkenes,
alkynes, 1,3-dienes, and R,â-unsaturated ketones) still remains a
challenging area of research.2a For example, the diboration of the
NdN double bond of diimines to yield the well-known class of
bis(boryl)hydrazines has only been reported twice and only in the
case of species with a highly reactive B-B bond, namely the
1
materials (1: δ ) 3.03, 3.27; 2: δ ) 3.08, 3.31).7 The H NMR
dichlorodiboranes(4) R(Cl)B-B(Cl)R (R ) Mes, NMe2)4 and the
resonances of the η5- and η6-coordinated carbocyclic ligands show
splitting patterns, which are characteristic for substituted metal-
loarenophanes and strongly resemble those reported for the
analogous ansa-bis(boryl)alkenes of ferrocene and bis(benzene)-
chromium.7
t
azadiboriridine NB2 Bu3.5 During the past decade, it was impres-
sively demonstrated that the strained character of [n]metalloareno-
phanes is associated with an enhanced reactivity of either the bond
between the carbocyclic ligands and the bridging element(s), that
between the bridging elements, or those between the metal and one
of the carbocyclic ligands, which facilitated a very interesting
insertion and substitution chemistry.6 Recently, we have shown that
the molecular ring strain present in [2]borametalloarenophanes
derived from ferrocene and bis(benzene)chromium can be exploited
in the selective diboration of alkynes even in the absence of boron
bound substituents, commonly increasing the reactivity of diborane-
(4) precursors. The strained character of these species also offered
the possibility for an unprecedented heterogeneous-catalytic dibo-
ration reaction.7 Herein we report on the extension of this reactivity
to the diboration of the NdN double bond of azobenzene by
[n]metalloarenophanes of iron and chromium. This diboration
was accomplished both under stoichiometric and homogeneous
catalysis conditions and represents, to the best of our knowledge,
the first instance of a metal mediated diboration of a NdN double
bond, thus employing less reactive non-halogenated diborane(4)
precursors.
The diboration of the NdN double bond was achieved by
reaction of the readily accessible [3]diboraplatinametalloarenophanes
1 and 27 with azobenzene at elevated temperature. In a typical
experiment, a Schlenk tube equipped with a resealable Teflon valve
was charged with the ansa-complex and a 4-fold excess of
azobenzene in toluene and was heated to 120 °C (Scheme 1). The
progress of the reaction was monitored by multinuclear NMR
spectroscopy, and the data confirmed the gradual consumption of
the starting materials (1, 2) and the almost quantitative formation
(>90-95%) of the ansa-bis(boryl)hydrazines 3 and 4 over a period
of 24 h (3) and 48 h (4), respectively. After workup and
recrystallization from hexanes, the products (3, 4) were isolated as
analytically pure, red crystalline solids in yields of 52% and 54%.
The NMR spectroscopic data are in agreement with the antici-
pated constitution of 3 and 4 in solution and suggest the presence
of C2 symmetric species. Accordingly, two resonances are observed
for the aminomethyl groups (3: δ ) 2.49, 2.50; 4: δ ) 2.45, 2.46)
that feature a noticeable highfield shift in comparison to the starting
As expected for the substitution of a boron bound platinum center
by an additional nitrogen atom, the resonances of the boron nuclei
in the 11B NMR spectrum of 3 (δ ) 33.0) and 4 (δ ) 33.8) are
significantly shielded with respect to the corresponding signal of
the starting materials (3: δ ) 62.2; 4: δ ) 61.6).7 According to
31P NMR spectroscopy, the reductive elimination of the [Pt(PEt3)2]
fragment is accompanied by the formation of the azobenzene
complex [(PhNdNPh)Pt(PEt3)2] (δ ) 5.90; JP,Pt ) 4067 Hz).8
1
To authenticate the formation of the bis-borylated product, the
molecular structure of 4 was determined by X-ray diffraction (Figure
1). As expected, the formation of a B-N-N-B bridge in 4 is
accompanied by the decrease of molecular ring strain in comparison
to the starting material 2, which is manifested in the almost coplanar
arrangement of the η6-coordinated ligands with a tilt-angle of
R ) 1.98(2)° and a deformation angle of δ ) 178.4° [2: R )
4.12(15)°; δ ) 176.6°].7 In agreement with the unstrained character,
the boron atoms are found to be almost ideally sp2-hybridized, thus
featuring a regular trigonal-planar geometry [119.1(2)-120.7(2)°;
B1, B2: Σ ) 360°]. The B-N bond lengths are in the typical range
found for the aminoboranes R2BNR′ and are indicative of mul-
2
tiple B-N bonding, whereby the distances to the NMe2-moieties
[both 1.402(3) Å] are significantly shorter than those to the nitrogens
of the hydrazine bridge [1.471(3) Å and 1.476(3) Å]. Hence, the
double bond character of the former appears to be much more
pronounced. The structural features of the hydrazine moiety, i.e.,
the N-N bond length [1.425(2) Å] and the distorted trigonal-planar
environment of the nitrogen atoms [114.7(2)-128.5(2)°; N1,
N2: Σ ) 360°] as well as the torsion angle B1-N3-N4-B2 )
-107.0(2)°, are comparable to those found in the related species
[-B(NMe2)N(Me)N(Me)-]29 or Mes2B(Ph)N-NH(Ph)10 and have
been discussed in detail by Power and No¨th.4,10
In order to develop a transition-metal-catalyzed protocol, the
[2]boraferrocenophane 511 and [Cr(η6-C6H5)B2(NMe2)2] (6)12 were
treated in subsequent experiments with azobenzene and catalytic
amounts of [Pt(PEt3)3]. The thermal lability of the chromium
9
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J. AM. CHEM. SOC. 2008, 130, 4242-4243
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C O M M U N I C A T I O N S
bis(boryl)hydrazine 3, however, only as a minor component <25%
and in combination with significant amounts of side and degradation
products. Moreover, the signals corresponding to 3 disappeared over
the extended course of the reaction and devolved into a new set of
signals [1H: δ ) 2.24 (NMe2); 11B: δ ) 32.1], whereby the identity
of this compound has not been clarified to date. It is noteworthy
that the same resonances were observed, when the reaction of 5
with azobenzene was performed in the presence of 10 mol %
Pd/C. In this case, only a static concentration of 3 was formed,
which was converted into the unknown compound within 10 d at
120 °C. The isolation and full characterization of this species, which
is hampered by the unselective nature of these transformations and
the significant amounts of side and degradation products, is the
purpose of current research efforts.
In this communication we provided our preliminary results on
the selective diboration of the NdN double bond of azobenzene
by [n]metalloarenophanes under stoichiometric and homogeneous
catalysis conditions to yield the ansa-bis(boryl)hydrazines 3 and
4. Due to the thermal lability of the chromium derivative 6, a
transition-metal-catalyzed diboration was only achieved with the
[2]boraferrocenophane 5.
Figure 1. Molecular structure of 4. Selected bond lengths (Å) and angles
(deg): Cr1-CPh between 2.126(2) and 2.158(2), C11-B1 1.585(3),
C21-B2 1.583(3), B1-N1 1.402(3), B2-N2 1.402(3), B1-N3 1.471(3),
B2-N4 1.476(3), N3-N4 1.425(2), Cr1-XPh1 1.603, Cr1-XPh2 1.611;
C11-B1-N3 119.8(2), C21-B2-N4 119.1(2), B1-N3-N4 117.2(2),
B2-N4-N3 116.9(2), C31-N3-N4 114.7(2), C41-N4-N3 114.7(2),
XPh1-Cr1-XPh2 178.4, B1-N3-N4-C41 72.6(2), B1-N3-N4-B2 -
107.0(2), B2-N4-N3-C31 71.9(2), C31-N3-N4-C41 -108.5(1),
XPh1-C11-C21-XPh2 -36.1, R 1.98(2) (XPh1 ) centroid of C11-C16,
XPh2 ) centroid of C21-C26).
Acknowledgment. This work was supported by the DFG. T.K.
thanks the FCI for a Ph.D. fellowship.
Scheme 2. Homogeneous-Catalytic Diboration of Azobenzene by
[2]Boraferrocenophane 5
Supporting Information Available: Details of the syntheses and
spectroscopic characterization of compounds 3 and 4, experimental
details on the X-ray diffraction of 4, and crystallographic data in CIF
format. This material is available free of charge via the Internet at http://
pubs.acs.org.
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1
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1
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1
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