Dehydrogenative Coupling Reactions between a Monosubstituted Alkyne and Monosilane in the Presence of Lithium Aluminum Hydride

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Rhodium-Catalyzed Intermolecular Silylation of Csp -H by Silacyclobutanes

The signature reactivity of silacyclobutane (SCB) is their cycloaddition reactions with various π bonds. Recently, the first cases were disclosed where SCBs reacted with both C_sp2 -H and C_sp3 -H σ bonds in an intramolecular fashion. Herein, it is reported that SCB is also an efficient reagent for C_sp -H bond silylation. Thus, rhodium-catalyzed intermolecular reactions between SCBs and terminal alkynes produced a series of symmetrical and unsymmetrical tetraorganosilicons bearing a C_sp -Si functionality. Preliminary studies suggested that the reaction did not involve a cycloaddition pathway, but instead a direct activation of C_sp -H bonds.

Insights into LiAlH4 Catalyzed Imine Hydrogenation

Commercial LiAlH4 can be used in catalytic quantities in the hydrogenation of imines to amines with H2 . Combined experimental and theoretical investigations give deeper insight in the mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH4 is exchanged for Na or K. Exchanging Al for B or Ga also led to dramatically reduced activities. This indicates a heterobimetallic mechanism in which cooperation between Li and Al is crucial. Potential intermediates on the catalytic pathway have been isolated from reactions of MAlH4 (M=Li, Na, K) and different imines. Depending on the imine, double, triple or quadruple imine insertion has been observed. Prolonged reaction of LiAlH4 with PhC(H)=NtBu led to a side-reaction and gave the double insertion product LiAlH2 [N]2 ([N]=N(tBu)CH2 Ph) which at higher temperature reacts further by ortho-metallation of the Ph ring. A DFT study led to a number of conclusions. The most likely catalyst for hydrogenation of PhC(H)=NtBu with LiAlH4 is LiAlH2 [N]2 . Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal mol-1 (ΔH). The rate-determining step is hydrogenolysis of LiAlH[N]3 with H2 with a barrier of +29.2 kcal mol-1 . In agreement with experiment, replacing Li for Na (or K) and Al for B (or Ga) led to higher calculated barriers. Also, the AlH4 - anion showed very high barriers. Calculations support the experimentally observed effects of the imine substituents at C and N: the lowest barriers are calculated for imines with aryl-substituents at C and alkyl-substituents at N.

Dehydrogenative Coupling of Terminal Alkynes with O/N-Based Monohydrosilanes Catalyzed by Rare-Earth Metal Complexes

Newly synthesized rare-earth metal alkyl complexes bearing a tripyrrolyl ligand act as excellent precatalysts for the cross-dehydrogenative coupling between various terminal alkynes and O/N-based monohydrosilanes of HSi(OEt)₃/HSi(NMe₂)₃, leading to the formation of a variety of alkoxysilylalkyne and aminosilylalkyne derivatives in good to high yields. The precatalysts LRE(CH₂SiMe₃)(thf)₂ (RE = Y(1a), Er(1b), Yb(1c), L = 2,5-[(2-C₄H₃N)CPh₂]₂(C₄H₂NMe), thf = tetrahydrofuran) were easily prepared in high yields via the reactions of RE(CH₂SiMe₃)₃(thf)₂ with the proligand H₂L in a single step. Mechanistic studies reveal that treatment of 1 with phenylacetylene could generate the active catalytic species: dinuclear rare-earth metal alkynides (L(thf)_n[RE(μ-C≡CPh)]₂L) (RE = Y(5a), n = 1; Yb(5c), n = 0), which could react with HSi(OEt)₃ to produce the coupling product 4aa and the dinuclear rare-earth metal hydrides (L (thf)[RE(μ-H)]₂L) (RE = Y(6a); Yb(6c)). By contrast, prior treatment of 1c with HSi(OEt)₃ proceeds via cleavage of the Si-O bond to produce the dinuclear ytterbium alkoxide (LYb(μ-OEt))₂ 7c, which is inert in the dehydrogenative coupling reaction. The results of the mechanistic studies are consistent with the observation that the reaction is greatly influenced by the addition sequence of precatalyst/alkynes/silanes.

Cross-Dehydrogenative Alkynylation: A Powerful Tool for the Synthesis of Internal Alkynes

Alkynes are among the most fundamentally important organic compounds and are widely used in synthetic chemistry, biochemistry, and materials science. Thus, the development of an efficient and sustainable method for the preparation of alkynes has been a central concern in organic synthesis. Cross-dehydrogenative coupling utilizing E-H and Z-H bonds in two different molecules can avoid the need for prefunctionalization of starting materials and has become one of the most straightforward methods for the construction of E-Z chemical bonds. This Review summarizes recent progress in the preparation of internal alkynes by cross-dehydrogenative coupling with terminal alkynes.

LiAlH4 : From Stoichiometric Reduction to Imine Hydrogenation Catalysis

Imine-to-amine conversion with catalytic instead of stoichiometric quantities of LiAlH₄ is demonstrated (85 °C, catalyst loading≥2.5 mol %, pressure≥1 bar). The effects of temperature, pressure, solvent, and catalyst modifications, as well as the substrate scope are discussed. Experimental investigations and preliminary DFT calculations suggest that the catalytically active species is generated in situ: LiAlH₄ +Ph(H)C=NtBu→LiAlH₂ [N(tBu)CH₂ Ph]₂ . A cooperative mechanism in which Li and Al both play a prominent role is proposed.

Lithium diamidodihydridoaluminates: bimetallic cooperativity in catalytic hydroboration and metallation applications

Cooperativity between the Li and Al centres is implicated in catalytic hydroboration reactions of aldehydes and ketones with pinacolborane via heteroleptic lithium diamidodihydridoaluminates. In addition to implementing hydroalumination, these versatile heteroleptic ates can also perform as amido bases as illustrated with an acidic triazole.

Role of C, S, Se and P donor ligands in copper(i) mediated C–N and C–Si bond formation reactions

The first comparative study of C, S, Se and P donor ligands-supported copper(i) complexes for C–N and C–Si bond formation reactions are described. The syntheses and characterization of eight mononuclear copper(i) chalcogenone complexes, two polynuclear copper(i) chalcogenone complexes and one tetranuclear copper(i) phosphine complex are reported. All these new complexes were characterized by CHN analysis, FT-IR, UV-vis, multinuclear NMR and single crystal X-ray diffraction techniques. The single crystal X-ray structures of these complexes depict the existence of a wide range of coordination environments for the copper(i) center. This is the first comparative study of metal–phosphine, metal–NHC and metal–imidazolin-2-chalcogenones in C–N and C–Si bond formation reactions. Among all the catalysts, mononuclear copper(i) thione, mononuclear copper(i) N-heterocyclic carbene and tetranuclear copper(i) phosphine are exceedingly active towards the synthesis of 1,2,3-triazoles as well as for the cross-dehydrogenative coupling of alkynes with silanes. The cross-dehydrogenative coupling of terminal alkynes with silanes represents the first report of a catalytic process mediated by metal–imidazolin-2-chalcogenones.

The first comparative study of C, S, Se and P donor ligands-supported copper(i) complexes for C–N and C–Si bond formation reactions.

Unexpected catalytic activity of simple triethylborohydrides in the hydrosilylation of alkenes

Highly regioselective hydrosilylation of olefins with aryl- and alkoxysilanes has been developed using a simple sodium triethylborohydride.