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There is an increasing demand for the organoboron compounds in organic synthesis, since a variety of useful transformation reactions of organoboronic acid derivatives have been established. We have focused our attention to the synthesis of functionalized organoboronic acid derivatives by new catalytic reactions, in which the boryl group and the functional group are introduced concomitantly. Following are the catalytic borylation reactions such as carboboration and silaboration, which we have so far established.
There is an increasing demand for the organoboron compounds in organic synthesis, since a variety of useful transformation reactions of organoboronic acid derivatives have been established. We have focused our attention to the synthesis of functionalized organoboronic acid derivatives by new catalytic reactions, in which the boryl group and the functional group are introduced concomitantly in one step.
**Direct Carboborations: Cyanoboration and Alkynylboration
Particular efforts have been devoted to carboboration, in which a boryl group and organic group are introduced to unsaturated organic molecules in one step. As the first catalytic carboboration reaction, we established palladium-catalyzed cyanoboration of carbon-carbon triple bonds using cyanoboranes [Ref .1]. We then found alkynylboration, in which the C–B bonds of alkynylboranes add to carbon-carbon triple bonds in a '''cis'''-fashion [Ref. 2]. These carboboration reactions involve activation of the B–C bond in cyanoboranes and alkynylboranes, which is followed by insertion of alkynes into the transition metal–boron bonds. The products were found to be good synthetic intermediates for highly substituted or conjugated alkenes.
Particular efforts have been devoted to silaboration and carboboration, in which a boryl group and additional non-hydrogen groups are introduced simultaneously to unsaturated organic molecules.
**2.1. Catalytic Carboboration Reactions [#tdad7ddd]
No catalytic carboboration had been reported at the time in spite of the expectation that such reactions are highly useful for the synthesis of organoboron compounds. As the first catalytic carboboration reaction, we established palladium-catalyzed addition of cyanoboranes to carbon-carbon triple bonds (J. Am. Chem. Soc. 2003, Angew. Chem., Int. Ed. 2005). We then found alkynylboration, in which C–B bonds of alkynylboranes add to carbon-carbon triple bonds in a cis fashion (J. Am. Chem. Soc. 2006). These carboboration reactions involve activation of the B–C bond in cyanoboranes and alkynylboranes followed by insertion of alkynes into the transition metal–boron bonds.
#ref(Graphics for Reserch/cyanoboration80.jpg,center)
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#ref(Graphics for Reserch/alkynylboration80.jpg,center)
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**Transmetallative Three-Component Carboboration
We also established transmetallative carboboration in which chloroborane and organometallic reagents such as organostannane and organozirconium were used as a source of the boryl and organic groups, respectively [intermolecular Ref. 3; cyclizative Ref. 4]. Activation of the B–Cl bond, which has never been utilized in catalytic reactions, is crucially involved in the catalysis.
We also established carboboration in which chloroborane and organozirconium reagents were used as a source of the boryl and organic groups, respectively (Chem. Commun. 2008). In this three-component system, activation of the B–Cl bond, which has never been utilized in catalytic reactions, is crucially involved.
#ref(Graphics for Reserch/transmetallative three-component carboboration80.jpg,center)
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**Related Researches
-Intramolecular Cyanoboration
In relation to the three-component carboboration system, a two-component system in which the chloroborane moiety and the triple bonds are tethered by a B–O linkage was reported (J. Am. Chem. Soc. 2008). Interesting reversal of the addition mode was observed: when PMe3 was used as a ligand, cis-addition products were obtained exclusively, while trans-addition products were formed exclusively when PCy3, P(t-Bu)3, or PPh3 was utilized as a ligand. This tethered system has been extended to intramolecular carboboration of C=C bonds more recently (J. Am. Chem. Soc. 2011).
#ref(Graphics for Reserch/intramol. cyanoboration80.jpg,center)
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-Transmetallative Cyclizative Carboboration
#ref(Graphics for Reserch/transmetallative cyclizative carboboration80.jpg,center)
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**2.2. Catalytic Silaboration Reactions [#zf0b2419]
Although no catalytic reaction involving the activation of Si–B bond of silylboranes had been reported before our study (Chem. Commun. 1996), we established that the Si-B bond shows remarkable selectivity and reactivity in the presence of transition metal catalysts (Account: Bull. Chem. Soc. Jpn. 2009).
We could establish a variety of catalytic reactions involving (1) simple addition reactions (silaborations), (2) silaborative C–C coupling reactions, and (3) silaborative cleavage of C-C bonds in small rings. In addition, recent success in synthesizing silyl-functionalized silylboranes allowed us to find that (dialkylamino)diorganosilylboranes serves as a synthetic equivalent of “silylene” (J. Am. Chem. Soc. 2009).
We recently found that pyridine derivative underwent dearomatizing silaboration in the presence of a palladium catalyst to give dihydropyridine derivatives under mild reaction conditions (J. Am. Chem. Soc. 2011). This reaction has been extended to our finding on the Rh-catalyzed dearomatizing hydroboration of pyridine derivatives (J. Am. Chem. Soc. 2012).
Typical catalytic reactions with silylborane reagents we have so far developed are shown below.
-Silaboration
#ref(Graphics for Reserch/silaboration_s80.jpg,center)
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-Silaborative Coupling Reaction
#ref(Graphics for Reserch/silaborative C-C coupling80.jpg,center)
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-Intramolecular Silaboration
#ref(Graphics for Reserch/intramolecular silaboration80.jpg,center)
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-Catalytic Asymmetric Silaboration
#ref(Graphics for Reserch/asymmetric silaboration of allene80.jpg,center)
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#ref(Graphics for Reserch/asymmetric silaboration of MCP80.jpg,center)
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**References
**References [#l66a6fdd]
-Ref. 1
-- Palladium- and Nickel-Catalyzed Intramolecular Cyanoboration of Alkynes M. Suginome, A. Yamamoto, M. Murakami, '''J. Am. Chem. Soc.''' ''2003'', 125, 6358-6359, [[10.1021/ja0349195>http://dx.doi.org/10.1021/ja0349195]]
-- Palladium-Catalyzed Addition of Cyanoboranes to Alkynes Leading to Regio- and Stereoselective Synthesis of β-Boryl-α,β-unsaturated Nitriles M. Suginome, A. Yamamoto, M. Murakami, '''Angew. Chem., Int. Ed.''' ''2005'', 44, 2380-2382, [[10.1002/anie.200462961>http://dx.doi.org/10.1002/anie.200462961]]
-- Intramolecular Cyanoboration of Alkynes via Activation of Boron-Cyanide Bonds by Transition Metal Catalysts M. Suginome, A. Yamamoto, M. Murakami, '''J. Organomet. Chem.''', ''2005'', 690, 5300–5308, [[10.1016/j.jorganchem.2005.05.005>http://dx.doi.org/10.1016/j.jorganchem.2005.05.005]]
-- Reactions of Cyanoboranes with a Palladium–PMeSUB{3}; Complex: Mechanism for the Catalytic Cyanoboration of Alkynes M. Suginome, A. Yamamoto, T. Sasaki, and M. Murakami, '''Organometallics''', ''2006'', 25, 2911–2913, [[10.1021/om060246y>http://dx.doi.org/10.1021/om060246y]]
-- Synthetic Application of Intramolecular Cyanoboration on the Basis of Removal and Conversion of a Tethering Group by Palladium-Catalyzed Retro-allylation T. Ohmura, T. Awano, M. Suginome, H. Yorimitsu, K. Oshima, '''Synlett''' ''2008'', 423-427, [[10.1055/s-2008-1032075>http://dx.doi.org/10.1055/s-2008-1032075]]
-Ref. 2
--Nickel-Catalyzed Addition of Alkynylboranes to Alkynes M. Suginome, M. Shirakura, A. Yamamoto, '''J. Am. Chem. Soc.''' ''2006'', 128, 14438-14439, [[10.1021/ja064970j>http://dx.doi.org/10.1021/ja064970j]]
-Ref. 3
--Palladium-Catalyzed Carboboration of Alkynes Using Chloroborane and Organozirconium Reagents M. Daini, M. Suginome, '''Chem. Commun.''' ''2008'', 5224-5226, [[10.1039/b809433k>http://dx.doi.org/10.1039/b809433k]]
-Ref. 4
--Nickel-Catalyzed trans-Alkynylboration of Alkynes via Activation of a Boron-Chlorine Bond A. Yamamoto, M. Suginome, '''J. Am. Chem. Soc.''' ''2005'', 127, 15706-15707, [[10.1021/ja055396z>http://dx.doi.org/10.1021/ja055396z]]
--Palladium-Catalyzed trans- and cis-Carboboration of Alkynes Tethered to Chloroborane with Organozirconium Reagents: Ligand-Dependent Complementary Stereoselectivity M. Daini, A. Yamamoto, M. Suginome, '''J. Am. Chem. Soc.''' ''2008'', 130, 2918-2919, [[10.1021/ja711160h>http://dx.doi.org/10.1021/ja711160h]]
