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Angewandte
Communications
that palladium-catalyzed hydroboration of enynes supported
by a 1,4-azaborine-based phosphine ligand resulted in distinct
regio- and diastereoselectivity. In view of the importance of
arene–metal p-interactions in catalytic applications, our work
opens a pathway for the development of the 1,4-azaborine
motif as an arene substitute in the design of ligands for
homogenous catalysis. Investigations along this route are
currently underway in our laboratories.
Received: April 1, 2014
Published online: && &&, &&&&
Keywords: Azaborine · boron · BN heterocycle ·
.
coordination complex · phosphine
Figure 2. Electrostatic potential surfaces of 8g and its carbonaceous
analogue C8g computed at the 0.002 electron a.u. density
isocontour level (ꢀ100 to ꢀ50 kcalmolꢀ1) and illustrations of their
HOMOs.
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[2] For pioneering work, see: a) M. J. S. Dewar, V. P. Kubba, R.
Pettit, J. Chem. Soc. 1958, 3073 – 3076; b) M. J. S. Dewar, R.
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Marr, J. Am. Chem. Soc. 1962, 84, 3782.
catalyzed hydroboration of an enyne as a model reaction
(Scheme 6). The regio- and diastereoselectivity of this hydro-
boration reaction largely depends on the nature of the
phosphine ligand. The use of a monodentate tertiary phos-
phine leads to allenylborane formation, while the reaction in
[3] For recent work, see: a) A. J. Ashe III, X. Fang, Org. Lett. 2000,
2, 2089 – 2091; b) A. J. Ashe III, X. Fang, X. Fang, J. W. Kampf,
Organometallics 2001, 20, 5413 – 5418; c) A. N. Lamm, S.-Y. Liu,
Mol. BioSyst. 2009, 5, 1303 – 1305; d) A. J. V. Marwitz, S. P.
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46, 779 – 781; e) T. Taniguchi, S. Yamaguchi, Organometallics
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S.-Y. Liu, Angew. Chem. 2011, 123, 8307 – 8310; Angew. Chem.
Int. Ed. 2011, 50, 8157 – 8160; g) T. Hatakeyama, S. Hashimoto,
S. Seki, M. Nakamura, J. Am. Chem. Soc. 2011, 133, 18614 –
18617; h) A. J. V. Marwitz, A. N. Lamm, L. N. Zakharov, M.
Vasiliu, D. A. Dixon, S. Y. Liu, Chem. Sci. 2012, 3, 825 – 829;
i) D. H. Knack, J. L. Marshall, G. P. Harlow, A. Dudzik, M.
Szaleniec, S. Y. Liu, J. Heider, Angew. Chem. 2013, 125, 2660 –
2662; Angew. Chem. Int. Ed. 2013, 52, 2599 – 2601; j) J. S. Lu,
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Dixon, S.-Y. Liu, Angew. Chem. 2013, 125, 7675 – 7679; Angew.
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[5] 1,2-Azaborine, 1,3-azaborine, and 1,4-azaborine are abbrevia-
tions for 1,2-dihydro-1,2-azaborine, 1,3-dihydro-1,3-azaborine,
and 1,4-dihydro-1,4-azaborine, respectively.
[6] a) M. Kranz, F. Hampel, T. Clark, J. Chem. Soc. Chem. Commun.
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Scheme 6. Unique selectivity in Pd-catalyzed hydroboration of 2-
methyl-1-buten-3-yne using ligand 8h.
the presence of bidentate bisphosphine ligands yields pre-
dominantly cis-hydroboration of the alkyne.[32] Preliminary
results show that in the presence of 8h as the supporting
ligand,[33] the hydroboration reaction was completed in
30 minutes to afford the trans-hydroboration product 17 as
the major product. On the other hand, the carbonaceous
ligand 19 furnished allene 18 as the major product under
otherwise identical reaction conditions, which is consistent
with literature reports on monodentate phosphine ligands.[32]
Thus, ligand 8h provides unique selectivity for trans-hydro-
boration product 17 that has not been achieved with other
ligand architectures.
[7] H. Braunschweig, A. Damme, J. O. Jimenez-Halla, B. Pfaffinger,
K. Radacki, J. Wolf, Angew. Chem. 2012, 124, 10177 – 10180;
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Lamm, A. W. Baggett, L. N. Zakharov, S.-Y. Liu, J. Am. Chem.
Soc. 2013, 135, 12908 – 12913.
In summary, we have developed the first general synthesis
of boron-substituted monobenzofused 1,4-azaborines. As part
ꢀ
of our synthetic investigations, we discovered that the B C3
moiety in a monobenzofused 1,4-azaborine can uniquely act
as an h2-L-type ligand to transition-metal centers. As a dem-
onstration of the utility of the new ligand motif, we showed
4
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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