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hydrogen sources, as opposed to the use of hydrogen gas. The
versatility of IPrG–CuCl is further illustrated by performing
carboboration reaction with alkynes. Further mechanistic inves-
tigations of the semihydrogenation process and the development of
a more effective catalyst with broader scope are now the focus of
our ongoing efforts.
This work was supported by Taiwan National Science Council
(NSC: 101-2628-M-001-004-MY3) and Academia Sinica.
Scheme 6 Carboborylation process with alkynes. Reaction conditions:
a (0.25 mmol), MeI (4 equiv.), B2Pin2 (1.5 equiv.), IPrG–CuCl (10 mol%),
Cs2CO3 (1.5 equiv.) in DMF (2 ml) at 60 1C for 24 h.
Notes and references
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using commercially available boron reagents. Phenylalkynes with
alkyl fragments such as linear butyl (13a) or hindering t-butyl (14a)
as well as terminal alkyne (26a) are also compatible for this catalytic
reaction with complete syn stereoseletivity. Similarly, good yield
with high Z selectivity was observed for 2-(phenylethynyl)thiophene
(27a) and propargyl alcohol derivatives like (28a) and (22a) in
semihydrogenation reaction. Equally important, the alkyne
derivative bearing the ester moiety (18a) could also be reduced
without perturbing the carboethoxy functionality albeit in a
lower yield of 48%.
Based on the positive results of the semihydrogenation
process, we were curious if a new chemical transformation
can further be realized by exploiting the reaction of vinylcopper
intermediate X with other electrophiles like alkyl halide instead
of methanol to perform syn-carboboration of simple alkynes.7
To our delight, the methylboration of phenylacetylene afforded
the desired product 15d in high yield (72%) in the presence of
B2pin2, methyl iodide and Cs2CO3 base with catalytic amounts
of IPrG–CuCl (Scheme 6). Similarly, internal alkynes like 11a
and 1a could be effectively transformed into carboboration
adducts 11d and 1d with only one regiomer in a moderate
yield, respectively. Other aryl alkyne derivatives like 7a and 29a
are also suitable for this reaction, affording a good yield of the
final product.
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5 For recent examples of copper catalyzed borylation reaction, see:
(a) H. Yoshida, S. Kawashima, Y. Takemoto, K. Okada, J. Ohshita and
K. Takaki, Angew. Chem., Int. Ed., 2012, 51, 235–238; (b) Y. Sasaki,
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(d) H. R. Kim and J. Yun, Chem. Commun., 2011, 47, 2943–2945;
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In short, we have successfully prepared and isolated a new
kind of functional NHC and its copper complex IPrG–CuCl,
namely guanidine NHC. It is found to be a highly active catalyst
for hydroboration of various kinds of alkynes bearing active
functional groups with a good regioselectivity. More importantly,
we have also unraveled a highly efficient catalytic system for the
Z-selective semihydrogenation of alkynes using commercially
available diboron reagents and water or alcohol as viable
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´
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(e) A. L. Moure, R. Gomez Arrayas, D. J. Cardenas, I. Alonso and
J. C. Carretero, J. Am. Chem. Soc., 2012, 134, 7219–7222; ( f ) H. Jang,
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133, 7859–7871.
6 See ESI† for further detail.
7 We were inspired by the recent work on carboboration mediated by
´
Cu: R. Alfaro, A. Parra, J. Aleman, J. Luis, G. Ruano and M. Tortosa,
J. Am. Chem. Soc., 2012, 134, 15165–15168.
4346 | Chem. Commun., 2014, 50, 4344--4346
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