Inorganometallics (Transition Metal-Metalloid Complexes) and Catalysis

While the formation and breaking of transition metal (TM)–carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)–metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM–E and TM–H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid–hydrogen and metalloid–metalloid bonds to unsaturated compounds; activation and functionalization of C–H bonds and C–X bonds with hydrometalloids and bismetalloids; activation and functionalization of C–H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

Borylation and Stannylation Reactions with Tuning of Lewis Acidity

A B(dan) moiety (dan=naphthalene-1,8-diaminato) of diminished boron-Lewis acidity has efficiently been installed into organic frameworks by three-component carboboration of alkenes under copper catalysis, where a Cu-B(dan) species, generated by chemoselective σ-bond metathesis between a copper catalyst and an unsymmetrical diboron [(pin)B-B(dan)], acts as a key intermediate. The Cu-B(dan) species has also turned out to serve as a B(dan) nucleophile to afford various dan-substituted organoboranes via borylative substitution of carbon electrophiles. Furthermore, borylation reactions with another Lewis acidity-diminished boron unit, B(aam) (aam=anthranilamidato), have become feasible by use of (pin)B-B(aam) or H-B(aam). The resulting dan/aam-substituted organoboranes have been demonstrated to undergo direct cross-coupling without prior acidic deprotection, regardless of their diminished boron-Lewis acidity. Synthesis of diverse organostannanes based upon copper-catalyzed carbostannylation and borylstannylation, in which Lewis acidity increment of a tin center facilitates the progress in some cases, have also been described.

Illuminating Stannylation

We have developed photoboosted stannylation reactions of terminal alkynes (linear-selective hydrostannylation) and fluoroarenes (defluorostannylation), in which the stannyl anion is photoexcited to an excited triplet (T₁) stannyl diradical species. This unprecedented T₁-stannyl diradical species shows completely different reactivity and selectivity from those of stannyl anions and stannyl radicals. This methodology is operationally simple, has broad functional group tolerance, and proceeds in high yield without the need for any catalyst.

Iron-Catalyzed Regiodivergent Hydrostannation of Alkynes: Intermediacy of Fe(IV)-H versus Fe(II)-Vinylidene

We report an iron system, Cp*Fe(1,2-R₂PC₆H₄X), which controls the Markovnikov and anti-Markovnikov hydrostannation of alkynes by tuning the ionic metal-heteroatom bonds (Fe-X) reactivity. The sequential addition of nBu₃SnH to the iron-amido catalyst (1, X = HN^-, R = Ph) affords a distannyl Fe(IV)-H species responsible for syn-addition of the Sn-H bond across the C≡C bond to produce branched α-vinylstannanes. Activation of the C(sp)-H bond of alkynes by an iron-aryloxide catalyst (2, X = O^-, R = Cy) affords an iron(II) vinylidene intermediate, allowing for gem-addition of the Sn-H to the terminal-carbon producing β-vinylstannanes. These catalytic reactions exhibit excellent regioselectivity and broad functional group compatibility and enable the large-scale synthesis of diverse vinylstannanes. Many new reactions have been established based on such a synthetic Fe-X platform to demonstrate that the initial step of the catalysis is conveniently controlled by the activation of either the tin hydride or the alkyne substrate.

Silylboranes as Powerful Tools in Organic Synthesis: Stereo- and Regioselective Reactions with 1,n-Enynes

Bismetalated alkenes, accessible by element–element addition to alkynes, are valuable building blocks in organic synthesis, providing wide opportunities for divergent synthesis. Silaboration of alkynes with a pendant olefinic group, catalyzed by group 10 metal complexes, and subsequent transformation of the silicon and boron functional groups give access to densely functionalized 1,3-dienes and 1,3,5-trienes with defined stereo- and regiochemistry, 1,2-dienes, and carbocyclic and heterocyclic products.

1 Introduction

2 Background

3 Reactions with 1,3-Enynes

4 Cyclization 1,6-Enynes

5 Cyclization 1,7-Enynes

6 Cyclization of 1,n-Enynes (n > 7)

7 Cyclization of Dienynes and Enediynes

8 Cyclization of 1,6-Diynes

9 Conclusions

Synthesis of Aryl Trimethylstannane via BF3·OEt2-Mediated Cross-Coupling of Hexaalkyl Distannane Reagent with Aryl Triazene at Room Temperature

BF₃·OEt₂-mediated cross-coupling of (SnMe₃)₂ with aryl triazene offers a new strategy for the synthesis of aryl stannane. A variety of synthetically useful aryl trimethylstannanes were produced in moderate to good yields with this metal-free approach. One-pot sequential Stille cross-coupling with different aryl bromides provides a short entry to both symmetrical and unsymmetrical biaryl compounds.

Copper-catalyzed direct borylation of alkyl, alkenyl and aryl halides with B(dan)

Copper-catalyzed reaction of organic halides with an unsymmetrical diboron, (pin)B–B(dan), which leads to the direct formation of masked organoboron compounds, is reported.

Copper Catalysis for Synthesizing Main-Group Organometallics Containing B, Sn or Si

A copper complex has proven to be a potent catalyst for forming a C-B bond via diborylation of arynes and alkynes, affording vic-diborylarenes and vic-diborylalkenes with high efficiency. A boryl-substituted organocopper species, which is intermediately generated in the diborylation, has been found to be captured by a tin or a carbon electrophile, leading to three-component borylstannylation or carboboration, in which C-B and C-Sn (or C) bonds are constructed simultaneously. Furthermore, reducing the Lewis acidity of the boron center with 1,8-diaminonaphthalene decisively alters the regiochemical behavior of the borylcopper species, enabling the installation of a boryl moiety to occur at an internal carbon of terminal alkynes in borylstannylation and protoboration. Copper catalysis for C-Sn and C-Si bond-forming processes via distannylation, hydrostannylation and silylstannylation, as well as silver catalysis for a C-B bond-forming reaction, is also described.

Synthesis of aryl trimethylstannanes from aryl halides: an efficient photochemical method

An efficient transition-metal-free photochemical method featuring excellent functional group tolerance, mild reaction conditions and short reaction times has been discovered and developed for the synthesis of (hetero)aryl trimethylstannanes from (hetero)aryl halides.