Boronation of Alkynes
FULL PAPER
We were interested in determining whether the catalytic
activity of the copper-promoted borylation of alkynes is also
observed for other metals and particularly whether gold,
palladium, and platinum also promote the borylation reac-
tion. Therefore we performed the reaction of phenylacety-
lene (1) with diboronate 2 in the presence of a series of sup-
ported metal nanoparticles with different metals and sup-
ports. The results are given in Table 1 (see entries 9–16). It
can be seen that gold nanoparticles supported on magnesia,
titania, or ceria did not lead to the unsaturated organoboro-
nate. The low conversions of phenylacetylene observed at
long reaction times using gold as the catalyst is a result of
some oligomerization together with the formation of a very
minor amount of styrene. We noticed, however, that the
three gold catalysts were very different in terms of particle
Figure 8. Time/conversion plot for the diboronation of phenylacetylene
(
1) with diboronate 2 in the presence of 1.37 wt% Pt/MgO. (^) Disap-
pearance of 1; (
) yield of cinnamylboronate 3; (~) yield of styryldiboro-
&
nate 4. Reaction conditions: 0.25 mmol of phenylacetylene (1), 0.3 mmol
of diborane 2, 1 mL of toluene, 1608C, 1.5 bar argon, and 1% mol ratio
of Pt/substrate.
size distribution. Whereas in the Au/CeO sample prepared
2
[21]
by us and in the commercial Au/TiO (World Gold Coun-
2
0
cil, reference A catalyst) gold is present as small nanoparti-
cles with an average size of about 5 nm, in the case of Au/
MgO TEM images revealed that the average particle size
was 30 nm (see Figure 4). In gold catalysis it is a well-estab-
lished fact that catalytic activity decreases as the average
compound 3. As with [(Ph P) Pt ] in the homogeneous
3
4
phase, the diboronated product 4 is formed in the presence
of Pt/MgO in a stereoselective manner, the two boronate
groups being in a cis configuration. With platinum nanopar-
ticles as catalyst we also observed that in addition to MgO,
ceria is also a suitable support (Table 1, entry 16), giving es-
sentially the same results as when using MgO as the sup-
port.
[22,23]
particle size increases
and therefore, based exclusively
on this parameter, it can be expected that Au/MgO would
be the least active sample of the series.
In the case of Pd/MgO, a similar product distribution to
that found for the gold catalyst, that is, styrene and some
oligomers, was also observed but with an almost complete
conversion of the starting phenylacetylene (Table 1, entry 12
and footnote g). These results can be interpreted assuming
that gold and palladium do not interact with bis(pinacolato)-
diboron and that they act as oligomerization/hydrogenation
catalysts of phenylacetylene, particularly in the case of palla-
dium.
Scope of the catalytic borylation of alkynes: The scope of
the mono- (using copper as catalyst) and diboronation
(using platinum as catalyst) reactions was expanded by per-
forming the borylation of other aromatic and aliphatic, ter-
minal and internal alkynes. The results are summarized in
Table 2. As can be seen, high selectivity at high conversion
was attained in almost every case. Importantly, the same
features as those commented upon above for phenylacety-
lene were also observed for the other alkynes. Thus, in the
Totally different behavior was observed for the supported
platinum catalyst irrespective of the presence or absence of
triphenylphosphine (Table 1, entries 13–16). The most im-
portant feature of the use of platinum as catalyst is the for-
mation of the diboronate alkene 4 as the major product and
not the monoboronate compound 3. It has been reported in
the literature that tetrakis(triphenylphosphino)platinum(0)
complexes act as homogeneous catalysts for the diborona-
II
case of Cu /MgO, triphenylphosphine even in small amounts
was necessary to form the monoborylated product with high
selectivity, the reaction occurring with high regio- and ste-
reoselectivity. Also, as in the case of phenylacetylene, the re-
duced CuO/MgO sample exhibits far more catalytic activity
II
than the ambient-equilibrated Cu /MgO sample (Table 2,
entries 4, 5, 9, 10, 15, 16, 20, 21, and 26). As an example,
Figure 9 shows the time/conversion plot for the boronation
of diphenylacetylene (11) using CuO/MgO as the catalyst.
The results have been attributed to the presence of a residu-
[
24,25]
tion of alkynes.
In our case, we have reproduced the re-
(PPh ) ] as
ported diboronation of phenylacetylene using [PtAHCTUNGTRENNUNG
3
4
homogenous catalyst and observed the same reaction prod-
uct as when using Pt/MgO as the catalyst. Interestingly, the
0
al population of Cu species in the sample as observed by
1,1-diboronate isomer of compound 4 was undetectable al-
XPS.
though it had also been observed in the homogenous cata-
lytic reaction, which indicates the remarkable regioselectivi-
ty of Pt/MgO. However, we noticed that in the absence of
With Pt/MgO as the catalyst, the product formed was
almost exclusively the diborylated alkene, which was formed
even in the absence of triphenylphosphine. Again, the time/
conversion plots show that the diborylated compounds were
formed as primary products and not derived from mono-
borylated products. As an example, Figure 10 shows the
time/conversion plot for the diborylation of 4-octyne (18)
using Pt/MgO as the catalyst.
PPh the presence of very small, but detectable, amounts of
3
the monoboronated alkene 3 were also observed with Pt/
MgO or Pt/CeO2 as the catalyst (Table 1, entries 14–16).
The time/conversion plot obtained by using Pt/MgO as het-
erogeneous catalyst (Figure 8) shows that the diboronated
compound 4 is a primary reaction product and does not
arise from a sequential borylation of the monoboronated
The reaction appears to be general in scope for all the al-
kynes tested. For both MgO-supported catalysts, internal,
Chem. Eur. J. 2011, 17, 2467 – 2478
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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