.
Angewandte
Communications
Shimizu, C. Nakamaki, K. Shimono, M. Schelper, T. Kurahashi,
[3] a) R. B. Coapes, F. E. S. Souza, R. L. Thomas, J. J. Hall, T. B.
Coapes, S. N. Edes, D. N. Coventry, F. E. S. Souza, R. L. Thomas,
[4] Diboration or hydroboration of alkynylborons allowed the
preparation of 1,1-diborylalkenes. See: a) K. Hyodo, M. Suet-
Eickhoff, J. Halama, S. Werner, J. Kahlert, H.-G. Stammler, B.
Shih, J. Zhou, J. H. Reibenspies, O. V. Ozerov, Angew. Chem. Int.
Ed. 2015, 54, 14003; Angew. Chem. 2015, 127, 14209.
[5] For reviews on diboration of unsaturated compounds, see:
Irvine, M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R. Rice,
E. G. Robins, W. R. Roper, G. R. Whittell, L. J. Wright, Chem.
[6] For vicinal diborations of alkynes under transition-metal-free
conditions, see: a) Y. Nagashima, K. Hirano, R. Takita, M.
[7] For base-catalyzed reactions of diboron reagents, see: a) A.
Bonet, C. Pubill-Ulldemolins, C. Bo, H. Gulyµs, E. Fernµndez,
7296; b) C. Pubill-Ulldemolins, A. Bonet, C. Bo, H. Gulyµs, E.
Fernµndez, Chem. Eur. J. 2012, 18, 1121; c) K.-S. Lee, A. R.
d) H. Wu, S. Radomkit, J. M. OꢀBrien, A. H. Hoveyda, J. Am.
See also Ref. [6b].
E/Z > 99:1) with the formation of a small amount of the
diphenylated product (10%). This stereoselectivity is prob-
ably because of the steric effect of the ester group. (Note that
no interaction exists between the B atoms and the ester
O atom in 3a (see above)). The second cross-coupling of 4a
with 4-bromoanisole produced isomerically pure trisubsti-
tuted alkene 5a in good yield (Z/E > 99:1). Copper-catalyzed
conjugate reduction of 3a with poly(methylhydrosiloxane)
(PMHS) afforded a functionalized geminal diborylalkane 6a
À
in quantitative yield with the two C B bonds remaining
untouched (Scheme 6b).[10]
In summary, we have developed a new method for the
synthesis of 1,1-diborylalkenes through a Brønsted base
catalyzed reaction between terminal alkynes and bis(pinaco-
lato)diboron. The procedure allows direct synthesis of
functionalized 1,1-diborylalkenes from various terminal
alkynes including propiolates, propiolamides, and 2-ethynyl-
azoles. The functionalized b,b-diborylacrylates and b,b-di-
borylacrylamides reported herein are difficult to obtain by
other methods (Scheme 1a–c).[1–4] Importantly, the two gemi-
nally installed boron substituents of the 1,1-diborylalkenes
could be differentiated and transformed in a stepwise manner,
showing the potential of the new 1,1-diborylalkenes as
versatile intermediates in organic synthesis.
Experimental Section
Gram-scale reaction (the reaction shown in Scheme 2 is representa-
tive): Bis(pinacolato)diboron (2; 3.81 g, 15 mmol) was placed in
a Schlenk flask containing a magnetic stirring bar. The flask was
evacuated and filled with argon. Acetonitrile (30 mL), ethyl propio-
late (1a; 1.47 g, 15 mmol), and LiOtBu (120 mg, 1.5 mmol) were
sequentially added to the flask. After 5 h stirring at 408C, the mixture
was filtered through a short plug of silica gel, which was then washed
with diethyl ether. The solvent was removed under reduced pressure
to give pure 3a (4.81 g, 13.7 mmol, 91% yield).
Acknowledgements
[9] The addition of Me3SiCl or aldehydes to a solution containing B
did not give the corresponding coupling products but gave 3a
after work-up. These results are incompatible with the formation
of a lithium allenolate intermediate (X) as a precursor to 3a.
This work was supported by the JSPS through a Grant-in-Aid
for Scientific Research B (No. 15H03803; to H.O.), and by
CREST, JST (to M.S.). Additionally, K.N. thanks the JSPS for
scholarship support.
Keywords: boration · Brønsted bases · diborylalkenes ·
organocatalysis · synthetic methods
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15859–15862
Angew. Chem. 2015, 127, 16085–16088
[1] a) T. Hata, H. Kitagawa, H. Masai, T. Kurahashi, M. Shimizu, T.
Received: October 2, 2015
Published online: November 13, 2015
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15859 –15862