Xphos ligand and platinum catalysts: A versatile catalyst for the synthesis of functionalized β-(E)-vinylsilanes from terminal alkynes

β-(Z)-Selective alkyne hydrosilylation by a N,O-functionalized NHC-based rhodium(I) catalyst

Neutral and cationic cyclooctadiene rhodium(I) complexes with a lutidine-derived polydentate ligand having NHC and methoxy side-donor functions, [RhBr(cod)(κC-tBuImCH2PyCH2OMe)] and [Rh(cod)(κ2C,N-tBuImCH2PyCH2OMe)]PF6, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)2(κ2C,N-tBuImCH2PyCH2OMe)]PF6 whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)2(κC-tBuImCH2PyCH2OMe)] and [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMe2Ph with a marked selectivity towards the β-(Z)-vinylsilane product. Catalyst [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)] showed a superior catalytic performance, in terms of both activity and selectivity, and has been applied to the hydrosilylation of a range of 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMe2Ph, HSiMePh2, HSiPh3 and HSiEt3, showing excellent β-(Z) selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the syn-addition vinylsilane products. The β-(Z) selectivity of these catalysts contrasts with that of related rhodium(I) catalysts based on 2-picolyl-functionalised NHC ligands, which were reported to be β-(E) selective. An energy barrier ΔG‡ of 19.8 ± 2.0 kcal mol-1 (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMe2Ph. DFT studies suggest that the methoxy-methyl group is unlikely to be involved in the activation of hydrosilane, and then hydrosilane activation is likely to proceed via a classical Si-H oxidative addition.

Hydrosilylation of Terminal Alkynes Catalyzed by an Air-Stable Manganese-NHC Complex

In recent years, catalysis with base metal manganese has received a significant amount of interest. Catalysis with manganese complexes having N-heterocyclic carbenes (NHCs) is relatively underdeveloped in comparison to the extensively investigated manganese catalysts possessing pincer ligands (particularly phosphine-based ligands). Herein, we describe the synthesis of two imidazolium salts decorated with picolyl arms (L₁ and L₂) as NHC precursors. Facile coordination of L₁ and L₂ with MnBr(CO)₅ in the presence of a base resulted in the formation manganese(I)-NHC complexes (1 and 2) as an air-stable solid in good isolated yield. Single-crystal X-ray analysis revealed the structure of the cationic complexes [Mn(CO)₃(NHC)][PF₆] with tridentate N,C,N binding of the NHC ligand in a facile fashion. Along with a few known manganese(I) complexes, these Mn(I)-NHC complexes 1 and 2 were tested for the hydrosilylation of terminal alkynes. Complex 1 was proved to be an effective catalyst for the hydrosilylation of terminal alkynes with good selectivity toward the less thermodynamically stable β-(Z)-vinylsilanes. This method provided good regioselectivity (anti-Markovnikov addition) and stereoselectivity (β-(Z)-product). Experimental evidence suggested that the present hydrosilylation pathway involved an organometallic mechanism with manganese(I)-silyl species as a possible reactive intermediate.

Iridium(i) complexes bearing hemilabile coumarin-functionalised N-heterocyclic carbene ligands with application as alkyne hydrosilylation catalysts

A set of iridium(i) complexes of formula IrCl(κC,η²-I^RCou^R')(cod) or IrCl(κC, η²-BzI^RCou^R')(cod) (cod = 1,5-cyclooctadiene; Cou = coumarin; I = imidazolin-2-carbene; BzI = benzimidazolin-2-carbene) have beeen prepared from the corresponding azolium salt and [Ir(μ-OMe)(cod)]₂ in THF at room temperature. The crystalline structures of 4b and 5b show a distorted trigonal bipyramidal configuration in the solid state with a coordinated coumarin moiety. In contrast, an equilibrium between this pentacoordinated structure and the related square planar isomer is observed in solution as a consequence of the hemilability of the pyrone ring. Characterization of both species by NMR was achieved at the low and high temperature limits, respectively. In addition, the thermodynamic parameters of the equilibrium, ΔH_R and ΔS_R, were obtained by VT ¹H NMR spectroscopy and fall in the range 22-33 kJ mol^-1 and 72-113 J mol^-1 K^-1, respectively. Carbonylation of IrCl(κC,η²-BzI^TolCou^7,8-Me₂)(cod) resulted in the formation of a bis-CO derivative showing no hemilabile behaviour. The newly synthesised complexes efficiently catalyze the hydrosilylation of alkynes at room temperature with a preference for the β-(Z) vinylsilane isomer.

Transition-Metal-Catalyzed Stereo- and Regioselective Hydrosilylation of Unsymmetrical Alkynes

Alkyne hydrosilylation is one of the most efficient methods for the synthesis of alkenyl silicon derivatives and has been a hot topic of research for decades. This short review summarizes the progress in transition-metal-catalyzed stereo- and regioselective hydrosilylation of unsymmetrical alkynes. Topics are discussed based on different types of alkynes and the selectivities.

1 Introduction

2 Terminal Alkyne Hydrosilylation

2.1 β-E Selectivity

2.2 β-Z Selectivity

2.3 α-selectivity

3 Internal Alkyne Hydrosilylation

3.1 Aryl–Alkyl Acetylenes

3.2 Alkyl–Alkyl Acetylenes

3.3 Internal Alkynes with Polarized Substituents

4 Summary and Outlook

Alkenyl-Functionalized Open-Cage Silsesquioxanes (RSiMe2O)3R'7Si7O9: A Novel Class of Building Nanoblocks

Trifunctional incompletely condensed polyhedral oligomeric silsesquioxanes (RSiMe₂O)₃R′₇Si₇O₉ (IC-POSSs) are considered as intriguing building nanoblocks dedicated to constructing highly advanced organic–inorganic molecules and polymers. Up to now, they have been mainly obtained via hydrosilylation of olefins, while the hydrosilylation of the C≡C bonds has not been studied at all, despite the enormous potential of this approach resulting from the possibility of introducing 3, 6, or even more functional groups into the IC-POSS structure. Therefore, in this work, we present a highly selective and efficient synthesis of the first example of tripodal alkenyl-functionalized IC-POSSs, obtained via platinum-catalyzed hydrosilylation of the terminal and internal alkynes, as well as symmetrically and nonsymmetrically 1,4-disubstituted buta-1,3-diynes with silsesquioxanes (HSiMe₂O)₃R′₇Si₇O₉ (R′ = i-C₄H₉ (1a), (H₃C)₃CH₂C(H₃C)HCH₂C (1b)). The resulting products are synthetic intermediates that contain C=C bonds and functional groups (e.g., OSiMe₃, SiR₃, Br, F, B(O(C(CH₃)₂)₂ (Bpin)), thienyl), which make them suitable for application in the synthesis of novel, complex, hybrid materials with unique properties.

The first example of the synthesis of alkenyl-functionalized open-cage silsesquioxanes (IC-POSS) via platinum-catalyzed hydrosilylation of C−C triple bonds in alkynes and buta-1,3-diynes is presented. The optimized synthetic procedure allowed for the selective and efficient synthesis of 20 new functional molecules capable of further modification by hydrosilylation, hydroboration, or other chemical processes.

Synthesis of bifunctional disiloxanes via subsequent hydrosilylation of alkenes and alkynes

The first protocol for the synthesis of unsymmetrical bifunctional 1,1,3,3-tetramethyldisiloxane derivatives via subsequent hydrosilylation of alkenes and alkynes is presented. The methodology described has vast functional group tolerance and is extremely efficient towards the formation of novel disiloxane-based building blocks.

Highly Efficient and Reusable Alkyne Hydrosilylation Catalysts Based on Rhodium Complexes Ligated by Imidazolium-Substituted Phosphine

Rhodium complexes ligated by imidazolium-substituted phosphine were used as catalysts in the hydrosilylation of alkynes (1-heptyne, 1-octyne, and phenylacetylene) with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) or triethylsilane (TES). In all cases, the above complexes showed higher activity and selectivity compared to their precursors ([Rh(PPh3)3Cl] and [{Rh(µ-Cl)(cod)}2]). In the reactions with aliphatic alkynes (both when HMTS and TES were used as hydrosilylating agents), β(Z) isomer was mainly formed, but, in the reaction of phenylacetylene with TES, the β(E) product was formed. The catalysts are very durable, stable in air and first and foremost insoluble in the reactants which facilitated their isolation and permitted their multiple use in subsequent catalytic runs. They make a very good alternative to the commonly used homogeneous catalysts.

Regiocontrol in the cobalt-catalyzed hydrosilylation of alkynes

A highly versatile cobalt-catalyzed hydrosilylations of alkynes is reported. Near-perfect regiocontrol/stereocontrol was induced by the choice of the ligand: bidentate phosphines afforded (E)-β-vinylsilanes; α-vinylsilanes formed with bipyridine ligands.

Hydrofunctionalizations of unsaturated hydrocarbons are key strategies for the synthesis of functionalized building blocks. Here, we report highly versatile cobalt-catalyzed hydrosilylations of alkynes that operate with minute amounts of the inexpensive, bench-stable pre-catalyst Co(OAc)₂·4H₂O under mild conditions (0.1–1 mol%, THF, r.t., 1 h). Near-perfect regiocontrol/stereocontrol was induced by the choice of the ligand: bidentate phosphines afforded (E)-β-vinylsilanes; α-vinylsilanes formed with bipyridine ligands.

Selective Hydrosilylation of Alkynes with Octaspherosilicate (HSiMe2 O)8 Si8 O12

Comprehensive studies on platinum-catalyzed hydrosilylation of a wide range of terminal and internal alkynes with spherosilicate (HSiMe2 O)8 Si8 O12 (1 a) were performed. The influence of the reaction parameters and the types of reagents and catalysts on the efficiency of the process, which enabled the creation of a versatile and selective method to synthesize olefin octafunctionalized octaspherosilicates, was studied in detail. Within this work, twenty novel 1,2-(E)-disubstituted and 1,1,2-(E)-trisubstituted alkenyl-octaspherosilicates (3 a-m, 6 n-t) were selectively obtained with high yields, and fully characterized (1 H, 13 C, 29 Si NMR, FTIR, MALDI TOF or TOF MS ES+ analysis). Moreover, the molecular structure of the compound (Me3 Si(H)C=C(H)SiMe2 O)8 Si8 O12 (3 a) was determined by X-ray crystallography for the first time. The developed procedures are the first that allow selective hydrosilylation of terminal silyl, germyl, aryl, and alkyl alkynes with 1 a, as well as the direct introduction of sixteen functional groups into the 1 a structure by the hydrosilylation of internal alkynes. This method constituted a powerful tool for the synthesis of hyperbranched compounds with a Si-O based cubic core. The resulting products, owing to their unique structure and physicochemical properties, are considered novel, multifunctional, hybrid, and nanometric building blocks, intended for the synthesis of star-shaped molecules or macromolecules, as well as nanofillers and polymer modifiers. In the presented syntheses, commercially available reagents and catalysts were used, so these methods can be easily repeated, rapidly scaled up, and widely applied.

PtO2/PTSA system catalyzed regioselective hydration of internal arylalkynes bearing electron withdrawing groups

A highly efficient PtO₂/PTSA catalyst system for the hydration of a wide array of alkynes was developed. This method proved to be compatible with a large range of functional groups and the ketone products were obtained in high yields. The scope of this methodology was also extended to the synthesis of 3-aryl-isochromenones, -indoles and -benzofurans.

A highly efficient PtO₂/PTSA catalyst system for the hydration of a wide array of alkynes was developed.

Pincer cobalt complex-catalyzed Z-selective hydrosilylation of terminal alkynes

Co-Catalyzed Z-selective hydrosilylation of terminal alkynes produces (Z)-β-vinylsilanes, which are further converted to (Z)-disubstituted alkenes by Pd-catalyzed cross couplings.

Enantioselective synthesis of α-aminosilanes by copper-catalyzed hydroamination of vinylsilanes

The synthesis of α-aminosilanes by a highly enantio- and regioselective copper-catalyzed hydroamination of vinylsilanes is reported. The system employs Cu-DTBM-SEGPHOS as the catalyst, diethoxymethylsilane as the stoichiometric reductant, and O-benzoylhydroxylamines as the electrophilic nitrogen source. This hydroamination reaction is compatible with differentially substituted vinylsilanes, thus providing access to amino acid mimics and other valuable chiral organosilicon compounds.

Development of a general, sequential, ring-closing metathesis/intramolecular cross-coupling reaction for the synthesis of polyunsaturated macrolactones

A general strategy for the construction of macrocyclic lactones containing conjugated Z,Z-1,3-diene subunits is described. The centerpiece of the strategy is a sequential ring-closing metathesis (RCM) that forms an unsaturated siloxane ring, followed by an intramolecular cross-coupling reaction with a pendant alkenyl iodide. A highly modular assembly of the various precursors allowed the preparation of unsaturated macrolactones containing 11-, 12-, 13-, and 14-membered rings. Although the RCM process proceeded uneventfully, the intramolecular cross-coupling required extensive optimization of palladium source, solvent, fluoride source, and particularly fluoride hydration level. Under the optimal conditions (including syringe pump high dilution), the macrolactones were produced in 53-78% yield as single stereoisomers. A benzo-fused 12-membered-ring macrolactone containing an E,Z-1,3-diene unit was also prepared by the same general strategy. The E-2-styryl iodide was prepared by a novel Heck reaction of an aryl nonaflate with vinyltrimethylsilane followed by iododesilylation with ICl.