Keywords Catalysis · C–H bond functionalization · Dehydrogenative silylation ·
Hydrosilylation · Iridium
1 Introduction
In the past few decades, the synthesis of organosilanes by catalytic methods has
undergone an outstanding development propelled by the potential use of these
molecules as materials and convenient building blocks in organic synthesis. The
preparation of compounds that contain C–Si bonds by means of catalytic processes is
a sustainable alternative to conventional stoichiometric reactions. In addition, the
development of new catalysts and synthetic methodologies might open the door to
organosilanes hitherto unattainable by current procedures.
Organosilicon compounds are interesting intermediates for the preparation of
more intricate molecular architectures because of their stability, low toxicity, availability, and versatile reactivity [1–7]. C–Si bonds can be converted into a great
variety of new bonds that include C–C bonds (Hiyama-Denmark coupling [8]), C–O
bonds (Tamao-Fleming oxidation [9]), C–N bonds (amination [10]), or C–halogen
bonds (halogenation [11, 12]). Moreover, organosilicon compounds have found
application, for example, as OLEDs (organic light-emitting diodes) or as conducting
polymers [13–20]. On the other hand, organic molecules that contain Si–heteroatom
bonds can be prepared by hydrosilane-mediated reductions. The most synthetically
relevant catalytic process for the formation of these molecules is the hydrosilylation
of ketones or amines. The dehydrogenative silylation of alcohols or amines and the
reduction of ethers, esters, or amides with hydrosilanes are also interesting reactions
that give rise to, for example, N-, O-, and S-silylated compounds. A related
hydrosilane-mediated reduction is the dehalogenation of organohalides using
hydrosilanes as reducing agent; however, in this case, it is the hydrogen atom of
the hydrosilane – not the silicon – the one that is transferred to the substrate.
Homogeneous iridium catalysts have shown excellent activities in the synthesis
of fine chemicals as well as in large-scale industrial process [21]. Regarding the
former, organometallic iridium complexes have found remarkable success as catalysts in the hydrogenation of unsaturated bonds. Outstanding results have been
reported on the asymmetric hydrogenation of imines and alkenes, with the catalysts
developed by Pfaltz and co-workers being the most prominent examples [22–
24]. Moreover, Crabtree’s catalyst, i.e., [Ir(cod)(PCy 3 )(py)]PF 6 (cod ¼ 1,5cyclooctadiene), is among the most active catalysts for the challenging hydrogenation of encumbered olefins [25, 26]. Large-scale industrial processes also have
prominent examples such as the Cativa™ process for the manufacture of acetic
acid, by which [Ir(CO) 2 I 2 ]
À catalyzes the carbonylation of methanol [27], or the
synthesis of (S)-metolachlor, which requires the asymmetric hydrogenation of a
C¼N bond – a crucial reaction catalyzed by an Ir-xyliphos complex [28–31].
In this chapter, we intend to compile the most relevant iridium-catalyzed homogeneous processes for the silylation of organic molecules hitherto reported.
Undoubtedly, the sheer number of hydrosilane-mediated reductions so far disclosed
makes it impossible to deal with all of them in this revision. For this reason, we will
228
M. Iglesias and L. A. Oro
Hydrosilylation · Iridium
1 Introduction
In the past few decades, the synthesis of organosilanes by catalytic methods has
undergone an outstanding development propelled by the potential use of these
molecules as materials and convenient building blocks in organic synthesis. The
preparation of compounds that contain C–Si bonds by means of catalytic processes is
a sustainable alternative to conventional stoichiometric reactions. In addition, the
development of new catalysts and synthetic methodologies might open the door to
organosilanes hitherto unattainable by current procedures.
Organosilicon compounds are interesting intermediates for the preparation of
more intricate molecular architectures because of their stability, low toxicity, availability, and versatile reactivity [1–7]. C–Si bonds can be converted into a great
variety of new bonds that include C–C bonds (Hiyama-Denmark coupling [8]), C–O
bonds (Tamao-Fleming oxidation [9]), C–N bonds (amination [10]), or C–halogen
bonds (halogenation [11, 12]). Moreover, organosilicon compounds have found
application, for example, as OLEDs (organic light-emitting diodes) or as conducting
polymers [13–20]. On the other hand, organic molecules that contain Si–heteroatom
bonds can be prepared by hydrosilane-mediated reductions. The most synthetically
relevant catalytic process for the formation of these molecules is the hydrosilylation
of ketones or amines. The dehydrogenative silylation of alcohols or amines and the
reduction of ethers, esters, or amides with hydrosilanes are also interesting reactions
that give rise to, for example, N-, O-, and S-silylated compounds. A related
hydrosilane-mediated reduction is the dehalogenation of organohalides using
hydrosilanes as reducing agent; however, in this case, it is the hydrogen atom of
the hydrosilane – not the silicon – the one that is transferred to the substrate.
Homogeneous iridium catalysts have shown excellent activities in the synthesis
of fine chemicals as well as in large-scale industrial process [21]. Regarding the
former, organometallic iridium complexes have found remarkable success as catalysts in the hydrogenation of unsaturated bonds. Outstanding results have been
reported on the asymmetric hydrogenation of imines and alkenes, with the catalysts
developed by Pfaltz and co-workers being the most prominent examples [22–
24]. Moreover, Crabtree’s catalyst, i.e., [Ir(cod)(PCy 3 )(py)]PF 6 (cod ¼ 1,5cyclooctadiene), is among the most active catalysts for the challenging hydrogenation of encumbered olefins [25, 26]. Large-scale industrial processes also have
prominent examples such as the Cativa™ process for the manufacture of acetic
acid, by which [Ir(CO) 2 I 2 ]
À catalyzes the carbonylation of methanol [27], or the
synthesis of (S)-metolachlor, which requires the asymmetric hydrogenation of a
C¼N bond – a crucial reaction catalyzed by an Ir-xyliphos complex [28–31].
In this chapter, we intend to compile the most relevant iridium-catalyzed homogeneous processes for the silylation of organic molecules hitherto reported.
Undoubtedly, the sheer number of hydrosilane-mediated reductions so far disclosed
makes it impossible to deal with all of them in this revision. For this reason, we will
228
M. Iglesias and L. A. Oro