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nanoparticles stabilized by chiral thiol ligands seem to be an
exo-norbornanediol and 1,2-indanediol[26] through the oxida-
tive workup of the bis(boronate)ester intermediates.
alternative catalytic system.[18] To this end, we chose l-
glutathione to stabilize gold NPs and followed the synthetic
process reported for l-glutathione–M NPs (M = Au, Pt).[19]
Thus HAuCl4 was treated with l-glutathione and the metal
was reduced to Au0 in the presence of NaBH4. Whereas the
1H NMR spectrum indicated that the particles were spectro-
scopically pure, the X-ray diffractogram revealed the pres-
ence of metallic Au0, and the TEM image with particle
size distribution mainly indicated a population of
particles about 2.8 Æ 1.4 nm in diameter. When we
tested the catalytic activity of the l-glutathione–gold
NPs in the diboration of styrene, we observed that the
conversion and chemoselectivity remained very high
(Table 2, entry 5) but that the enantioselectivity was
nil.
The high chemoselectivity described with the binap-
stabilized gold nanoparticles suggests that the catalytic
system could be involved in a base-mediated diboration,
and therefore transmetalation becomes the key step in the
mechanism instead of oxidative addition (Scheme 2). The
observation of the resulting by-product, IBcat, which is
We also found that gold nanoparticles were better
than other nanoparticles in the same group (Ag and Cu
nanoparticles) for the catalytic diboration of alkenes.
In fact when silver nanoparticles were synthesized
following the Yang method[20] and used to add B2cat2
catalytically to alkenes, conversions were moderate
(51% for styrene, 47% for indene, and 54% for trans-
b-methylstyrene) although the total chemoselectivity
towards the desired product remained the same.
Catalytic activity was lowest (26% for styrene) for
the copper nanoparticles synthesized following the
Zhong method.[21] This shows that the gold catalyst has
the advantages of higher activity, higher selectivity, and
milder reaction conditions.[22]
The scope of the diboration catalyzed by system 2
was extended to aliphatic alkenes such as vinylcyclo-
hexene and 3,3-dimethylbutene; the conversion was
complete and the chemoselectivity quantitative
(Figure 3), but no enantioselectivity was observed. The
Scheme 2. Proposed mechanism for the reaction of an alkene with B2Cat2
catalyzed by binap–gold nanoparticles generated in situ.
similar to the ClBcat observed by Baker et al.,[27] supports
À
À
diboration of internal alkenes led to the quantitative cis B
B addition to trans-b-methylstyrene and the selective forma-
tion of cis-bis(boryl)cyclic moieties for endocyclic alkenes
such as indene and norbornene (Figure 3). The diboration of
norbornene has been reported using base-free Pt0[23] and RhI
complexes,[24] which provided good yields and chemoselectiv-
ities but required long reaction times and higher temper-
atures. Only one report has described the diboration of indene
but with lower chemoselectivity (68%).[25] These approaches
open up new perspectives for synthethic pathways to 2-exo-3-
the B B heterolytic cleavage. A similar mechanism based on
a transmetalation sequence has been postulated for the first
catalytic diboration of alkenes with palladium complexes.[14]
The mechanism suggested for the gold nanoparticle mediated
diboration reaction gains strong support from the crossed-
[28]
addition experiment, in which B2cat2 and B2((4-Me)cat)2
were simultaneously added to styrene (Scheme 3). Although
the reactivity of B2((4-Me)cat)2 is lower than that of B2cat2
(23% of styrene converted to 1,2-bis(boronate)styrene), the
formation of crossdiborated product B (about 10%, detected
by GC–MS) in conjunction with A (about 60%) and C (about
30%) supported this hypothesis. It should be noted that if
diboration occurs by the oxidative addition of diborons to
Au0, product B should not be observed. In agreement with
this transmetalation pathway, similar trends were found by
Cheng et al. for the Pd0-mediated diboration of allenes[29] and
by Oshima et al. for the b-boration of esters and amides.[30]
Morken et al.[31] on the other hand, described some crossover
experiments that suggested that allene diboration could be
initiated by the oxidative addition of diboron to Pd0.
In summary, we have reported a new gold-mediated
diboration reaction, in which binap-stabilized gold(0) nano-
particles are responsible for the conversion of alkenes with
complete chemoselectivity to the 1,2-bis(boronate)esters. The
Figure 3. Scope of the diboration reaction catalyzed by binap-stabilized
gold nanoparticles generated in situ.
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Angew. Chem. Int. Ed. 2008, 47, 5194 –5197