Notes
Organometallics, Vol. 26, No. 17, 2007 4351
Scheme 2
two possible diastereomers is observed, so we estimate a
diastomeric excess (de) higher than 95% for this reaction. The
1HNMR spectrum of a solution of 2 in CDCl3 did not change
after 1 week; thus epimerization to the other diastereomer did
not take place. The 13C{1H} NMR spectrum of 2 reveals that
the orthometalation has ocurred. A signal at 144.3 ppm is due
to the metalated phenyl carbon, while the resonance corre-
sponding to the carbene-carbon appears at 159 ppm, in the
region of previously reported Cp*Ir(NHC) complexes.5,6,8
The molecular structure of 2 was determined by means of
X-diffraction studies. Figure 1 shows the molecular diagram of
2 and the more relevant bond distances and angles.
The crystal structure of 2 confirms that only one enantiomer
is present in the crystal. The absolute configuration about the
metal center is S, which together with the configuration of the
two chiral carbon centers reveals that the (SIr,SC,SC)-2 diaste-
reomer was the only one obtained. The structure reveals that
the orthometalation of one of the phenyl substituents of the
carbene ligand has occurred, with the formation of a six-
membered iridacycle in a distorted boat conformation. The Cp*
ring and a chlorine ligand complete the coordination sphere
about the Ir center. The chelate bite angle is 87.7°, similar to
that shown for other cyclometalated complexes reported by us.5,6
The Ir-Ccarbene distance of 1.990 Å lies in the expected range
for other similar complexes.5,8 The Ir-C bond distance for the
cyclometalated phenyl ring is 2.043 Å.
Figure 1. Molecular diagram of compound (SIr,SC,SC)-2. Hydrogen
atoms (except at chiral centers) and crystallization solvent (tet-
rahydrofuran) have been omitted for clarity. Selected bond distances
(Å) and angles (deg): Ir(1)-C(1) 1.990(11), Ir(1)-Cl(1) 2.412(3),
Ir(1)-C(7) 2.043(10), Ir(1)-Cp*centroid 1.85, C(1)-Ir(1)-C(7) 83.8-
(4), C(1)-Ir(1)-Cl(1) 89.4(3), C(7)-Ir(1)-Cl(1) 87.7(3), Cp*centroid
-
Ir(1)-C(1) 133.1, Cp*centroid-Ir(1)-C(7) 126.9, Cp*centroid -Ir(1)-
Cl(1) 121.9.
Scheme 3
We have been recently interested in the catalytic B-addition
across unsaturated carbon-carbon bonds, to provide easy access
to organoborane compounds, in particular, the catalytic hy-
droboration9 and diboration10 reactions. While many efforts have
been focused on metal-phosphine complexes, we recently
described a series of NHC-based metal complexes that cleanly
performed B-H and B-B addition to alkenes.10 The potential
use of Ir-NHC complexes in this type of reaction is very
attractive, because this metal has offered significant benefits in
terms of increased stability of metal intermediates11 toward
catalytic hydroboration12 and C-H borylation.13 To the best of
our knowledge there is no previously reported work on the
catalytic diboration reaction with Ir complexes. The presence
of the stereogenic center at the metal in (SIr,SC,SC)-2, provides
additional interest since we can evaluate the asymmetric
induction of the reaction products of the catalytic reaction.
Compound (SIr,SC,SC)-2 was initially tested in the diboration
of styrene (Scheme 3), but it did not lead to the formation of
any diboronate esters when bis(catecholato)diboron, (B2cat2),
was added to a solution of the catalyst (5 mol %) and styrene
in THF under argon (Table 1, entry 1). Addition of AgBF4 for
the in situ generation of cationic species did not provide any
catalytic performance toward the desired product (Table 1, entry
2). However, addition of NaOAc and an excess of diboron
reagent afforded an almost quantitative conversion of the
styrene, providing an extraordinarily high chemoselectivity on
the diborated product, with values up to 99.2% (Table 1, entry
3). Other neutral and cationic Ir(I) complexes modified with
(S)-Quinap were not active under such conditions (Table 1,
entries 4 and 5), despite the effectiveness of their Rh analogue
complexes.14 The specific effectiveness of the NHC ligands
toward the high chemoselectivities on the diboronate product
has been shown in the comparative diboration reaction catalyzed
by [Cp*Ir(µ-Cl)Cl]2 and Cp*Ir(IMe)(Cl)2 (IMe ) 1,3-dimethyl-
imidazolylidene) (Table 1, entries 6 and 7). Also, high chemo-
selectivities were observed for the diboration of p(F)PhCHd
CH2 and p(MeO)PhCHdCH2 with complex 2 (Table 1, entries
8 and 9), although conversions dropped significantly for these
substituted vinylarenes. We also studied the catalytic B-B
addition to the aliphatic 1-alkene substrates vinylcyclohexane
and 3,3-dimethyl-1-butene (Table 1, entry 10 and 11). In both
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Yamaguchi, R. Organometallics 2006, 25, 826. Vogt, M.; Pons, V.;
Heinekey, D. M. Organometallics 2005, 24, 1832.
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