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During recent years, the catalytic transformation of CO2 using silicon‐hydrides as reductants has emerged as a promising methodology that allows the selective reduction of CO2 to the formate, formaldehyde, methoxide or methane level under mild reaction conditions. Moreover, some catalysts have been employed for the formylation and/or methylation of the N−H bonds of secondary and/or primary amines by their reaction with CO2 and hydrosilanes. This work summarizes the different catalytic systems that have shown to be efficient for the above‐mentioned reactions. Furthermore, a brief description of the reactions performance and the conditions employed in each case is included. From silicon valley to silicon hydrides: Catalytic reduction of CO2 with silicon‐hydrides could be considered as a promising and emerging chemical methodology. [ABSTRACT FROM AUTHOR]

The dehydrogenation of formic acid permits the production of hydrogen virtually free of carbon monoxide, which is a key requisite for its use in fuel cells. Moreover, HCOOH is a suitable hydrogen carrier, better in several crucial characteristics than other liquid organic hydrogen carriers, namely, low toxicity, high hydrogen content and recyclability (via CO2 hydrogenation). The most successfully employed catalysts are those based on Ir, Ru, Rh, and Fe metal centers. The catalytic cycles through which homogeneous metal complexes operate show a wide mechanistic diversity. The nature of catalytic cycle depends mainly on the nuclearity of the active species, the type of CO2 formation step (β‐hydride elimination, hydride abstraction, or outer‐sphere mechanism), and whether the rate‐limiting step relates to CO2 or H2 formation. Different outer‐sphere interactions have been proposed in the literature in order to explain the success of homogeneous catalyst, which seems to be common ground for most of the catalysts so far reported. [ABSTRACT FROM AUTHOR]

The hydrosilylation of CO2 with different silanes such as HSiEt3, HSiMe2Ph, HSiMePh2, HSiMe(OSiMe3)2, and HSi(OSiMe3)3 in the presence of catalytic ammounts of the iridium(III) complex [Ir(H)(CF3CO2)(NSiN*)(coe)] ( 1; NSiN*= fac-bis-(4-methylpyridine-2-yloxy); coe= cis-cyclooctene) has been comparatively studied. The activity of the hydrosilylation catalytic system based on 1 depends on the nature of the reducing agent, where HSiMe(OSiMe3)2 has proven to be the most active. The aforementioned reactions were found to be highly selective toward the formation of the corresponding silylformate. It has been found that using 1 as catalyst precursor above 328 K decreases the activity through a thermally competitive mechanistic pathway. Indeed, the reduction of the ancillary trifluoroacetate ligand to give the corresponding silylether CF3CH2OSiR3 has been observed. Moreover, mechanistic studies for the 1-catalyzed CO2-hydrosilylation reaction based on experimental and theoretical studies suggest that 1 prefers an inner-sphere mechanism for the CO2 reduction, whereas the closely related [Ir(H)(CF3SO3)(NSiN)(coe)] catalyst, bearing a triflate instead of trifluoroacetate ligand, follows an outer-sphere mechanism. [ABSTRACT FROM AUTHOR]

The IrIII complexes 4 and 5 bearing bis-NHC ligands (NHC = N-heterocyclic carbene) composed of one classical NR,NR NHC and one N,NR NHC donor were prepared by the reaction of the azolium/azole compounds 2I and 3Br, respectively, with [{Cp*IrCl(µ-Cl)}2] (Cp*=η5-C5Me5) in the presence of NaOAc as base. Most likely, the salts 2I and 3Br were first selectively deprotonated at the C2 position of the disubstituted (NR,NR) diazaheterocycle to generate an NHC donor, which then coordinated to the IrIII center. Subsequently, NaOAc promoted C-H bond activation at the pendant imidazole moiety of the intermediate IrIII mono-NHC complexes led to the formation of the six-membered iridacycles 4 and 5, which bear a chelating, doubly C-metalated C(NHC)^C(NHC′) bis-NHC ligand. The IrIII complexes 4 and 5 were tested as precatalysts for the reduction of imines with molecular hydrogen. Moderate to good activity was observed at a catalyst loading of 5 mol-% and an H2 pressure of 3 bar in MeOH. [ABSTRACT FROM AUTHOR]

A variety of [M(L)2(L′)2{κ C, C′-bis(NHC)}]BF4 complexes (M = Rh or Ir; L = CH3CN or wingtip group; L′ = I- or CF3COO-; NHC=N-heterocyclic carbene) have been tested as pre-catalysts for the transfer hydrogenation of ketones and imines. The conversions and TOF's obtained are closely related to the nature of the ligand system and metal centre, more strongly coordinating wingtip groups yielding more active and recyclable catalysts. Theoretical calculations at the DFT level support a classic stepwise metal-hydride pathway against the concerted Meerwein-Ponndorf-Verley (MPV) mechanism. The calculated catalytic cycle involves a series of ligand rearrangements due to the high trans effect of the carbene and hydrido ligands, which are more stable when situated in mutual cis positions. The reaction profiles obtained for the complexes featuring an iodide or a trifluoroacetate in one of the apical positions agree well with the relative activity observed for both catalysts. [ABSTRACT FROM AUTHOR]

The copper(I) complexes [Cu(X){2,6-diisopropylphenyl-NHC-(CH2)3Si(O iPr)3}] (X=Cl ( 2 a); I ( 2 b), NHC=N-heterocyclic carbene) have been synthesized and characterized. Furthermore, the structure of 2 b has been confirmed by X-ray diffraction studies. Complex 2 a has been successfully anchored in MCM-41 to afford 2-MCM-41. The activity of both the homogeneous, 2 a, and heterogeneous, 2-MCM-41, catalysts in acetophenone hydrosilylation with HSiEt3 and [3+2] cycloaddition of benzyl azide and phenylacetylene has been investigated. The heterogeneous catalyst exhibits catalytic activity for the cycloaddition reaction though, unexpectedly, shows no catalytic activity for hydrosilylation. [ABSTRACT FROM AUTHOR]

Metallaheteroboranes are versatile compounds that can be conveniently modified and eventually tailored by ligand modification at either the metal centre or the boron vertices. Recently, we have discovered that protonation of some rhodathiaboranes affords cationic clusters with interesting reaction chemistry. In order to tune the reactivity of some of these polyhedral boron-based compounds, we have prepared air-stable orange [1,1-(η2-dppe)-3-(NC5H5)-closo-1,2-RhSB9H8] (2) by the treatment of the known hydridorhodathiaborane [8,8,8-(H)(PPh3)2-9-(NC5H5)-nido-8,7-RhSB9H9] (1) with dppe. The new 11-vertex rhodathiaborane, 2, reacts readily with triflic acid (TfOH) in CH2Cl2 to give orange cationic [8,8-(η2-dppe)-9-(NC5H5)-nido-8,7-RhSB9H9]+ (3). VT NMR experiments have allowed the characterization of a structural closo↑nido tautomerism, which involves hapticity changes in the ligation of the {SB9H9-(NC5H5)} moiety to the {Rh(dppe)} fragment, with the proton moving between the Rh(1)–B(3) and the B(9)–B(10) edges of the closo- and nido-isomers, respectively. The proton enhances the stereochemical non-rigidity and Lewis acidity of 3versus the neutral 2. This modification of the chemical and structural basis permits the efficient heterolytic splitting of the H–H bond, leading to the formation of new hydridorhodathiaborane isomers [8,8,8-(H)(η2-dppe)-μ-8,9-(H)-9-(NC5H5)-nido-8,7-RhSB9H10]+ (4) that are in equilibrium with the reactants, H2 and 3. [ABSTRACT FROM AUTHOR]

The rhodium(I) complexes [Rh(Cl)(COD)(R-NHC-(CH2)3Si(O iPr)3)] [COD=cyclooctadiene; R=2,6-diisopropylphenyl ( 1 a); n-butyl ( 1 b)] are effective catalyst precursors for the homogeneous hydrodechlorination of benzyl chloride using HSiEt3 as hydrogen source. This reaction is selective to the formation of toluene. However, in presence of a stoichiometric amount of potassium tert-butoxide (K tBuO) the formation of mixtures containing toluene together with 17-19 mol % of the CC homocoupling product, namely PhCH2CH2Ph, is observed. A mechanism proposal based on experimental insights and theoretical calculations at the DFT level that allows explanation of the experimental findings is included. Moreover, the heterogeneous catalytic system based on catalyst 1 a supported on MCM-41 has been demonstrated to be effective for the solvent-free hydrodechlorination of benzyl chloride using HSiEt3 and HSiMe(OSiMe3)2. [ABSTRACT FROM AUTHOR]

[Rh(μ-Cl)(H)2(IPr)]2 (IPr=1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) catalyzes the selective functionalization of 2-(2-thienyl)pyridine efficiently with a range of alkenes and internal alkynes. A catalytic cycle is proposed on the basis of the identification of key reaction intermediates and the study of their reactivity by NMR spectroscopy. Theoretical calculations at the DFT level support that the reaction proceeds by initial κ1 N coordination of 2-(2-thienyl)pyridine followed by the loss of H2 to afford the active catalyst. Subsequently, cyclometalation of 2-(2-thienyl)pyridine, coordination of the unsaturated substrate (alkyne or alkene) at the vacant position trans to the hydride, and reductive elimination of the thiophene moiety occur. Finally, cyclometalation of the thiophene moiety renders the hydride cis to the unsaturated substrate, which leads to migratory insertion into the RhH bond and subsequent reductive elimination of the functionalized product. [ABSTRACT FROM AUTHOR]

Breaking ties: Recent reports on iridium and ruthenium homogeneous hydrosilylation catalysis point to a heterolytic activation of SiH bonds promoted by the oxophilicity of the silicon atom and the Lewis acidity of the metal center. This unusual type of mechanism opens the door to a more widespread use of catalysts that operate by an ionic or concerted outer‐sphere mechanism in organic synthesis. [ABSTRACT FROM AUTHOR]

The iridium(III) complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN= fac-coordinated bis(pyridine-2-yloxy)methylsilyl, coe=cyclooctene) has been proven to be an effective catalyst precursor for hydrogen production from the hydrolysis of hydrosilanes at room temperature. The reaction performance depends both on the nature of the silane and the solvent. Interestingly, high turnover frequencies of around 105 h−1 were obtained by using Et2SiH2 or (Me2HSi)2O as hydrogen sources and THF as the solvent. Moreover a mechanistic insight into this Ir-catalyzed hydrogen generation process, based on both theoretical calculations and NMR spectroscopy, is reported. The overall catalytic cycle can be viewed as a two-stage process that involves water-promoted SiH bond activation followed by water splitting by a proton transfer. [ABSTRACT FROM AUTHOR]

The β- Z selectivity in the hydrosilylation of terminal alkynes has been hitherto explained by introduction of isomerisation steps in classical mechanisms. DFT calculations and experimental observations on the system [M(I)2{κ-C,C,O,O-(bis-NHC)}]BF4 (M=Ir ( 3 a), Rh ( 3 b); bis-NHC=methylenebis( N-2-methoxyethyl)imidazole-2-ylidene) support a new mechanism, alternative to classical postulations, based on an outer-sphere model. Heterolytic splitting of the silane molecule by the metal centre and acetone (solvent) affords a metal hydride and the oxocarbenium ion [R3SiO(CH3)2]+, which reacts with the corresponding alkyne in solution to give the silylation product [R3SiCHCR]+. Thus, acetone acts as a silane shuttle by transferring the silyl moiety from the silane to the alkyne. Finally, nucleophilic attack of the hydrido ligand over [R3SiCHCR]+ affords selectively the β-( Z)-vinylsilane. The β- Z selectivity is explained on the grounds of the steric interaction between the silyl moiety and the ligand system resulting from the geometry of the approach that leads to β-( E)-vinylsilanes. [ABSTRACT FROM AUTHOR]

Recent reports on homogeneous catalytic transformation of carbon dioxide by iridium complexes have prompted us to review the area. Progress on new iridium catalysts for carbon dioxide transformations should take into account the interaction of carbon dioxide with the iridium center, which seems to be governed by the oxidation state of iridium and the nature of the carbon dioxide molecule. Most examples of iridium catalyzed carbon dioxide reductions are based on IrIII centers. These reactions take place through outer-sphere mechanisms, by means of nucleophilic attack on the carbon atom. In all the reported systems, the nucleophile is always a hydrido ligand coordinated to a IrIII center. Future challenges on iridium catalyzed functionalization of carbon dioxide include the development of efficient electrophiles, compatible with the inclusion of appropriate nucleophiles, which would allow the preparation of value-added organic molecules using CO2 as C1 feedstock. [ABSTRACT FROM AUTHOR]

The preparation of 1-(3-triisopropoxysilylpropyl)-3-(2-methoxyethyl)-imidazolium bromide or chloride salts and their reaction with [Rh(COD)(μ-OMe)]2 (COD=1,5-cyclooctadiene) to afford the corresponding [Rh(COD)(NHC)X] (X=Br, Cl; NHC=1-(3-triisopropoxysilylpropyl)-3-(2-methoxyethyl)-2-ilydene-imidazol) species is described. These new compounds were used as catalyst precursors for acetophenone hydrosilylation. The higher activity of the rhodium-chlorido complex evidences a clear halide effect in the activation of the catalyst. Immobilization of the catalytic precursor [Rh(COD)(NHC)Cl] on mobile crystalline material 41 (MCM-41) allows for the preparation of the corresponding heterogeneous catalyst. Reduction of acetophenone to PhMeCH-O-SiMe(OSiMe3)2 by hydrosilylation with 1,1,1,3,5,5,5-heptamethyltrisiloxane is effectively catalyzed by both the homogeneous and the heterogeneous catalysts, in which the homogeneous system is the more active. Interestingly, the heterogeneous catalyst is reusable. Both homo- and heterogeneous catalysts are also effective for the copolymerization of terephthalaldehyde and 1,1,3,3,5,5-hexamethyltrisiloxane, which affords the corresponding poly(silyl ether). The catalyst yields the heterogeneous system polymers with higher molecular weights ( Mw=94 000 g mol−1) and a narrow molecular weight distribution (PDI=1.5-1.7). [ABSTRACT FROM AUTHOR]

Cationic rhodium(I) complexes with P,O-functionalised arylphosphine ligands are efficient catalysts for the regioselective anti-Markovnikov oxidative amination of styrene with piperidine. The mechanism of the catalytic reaction has been investigated by spectroscopic means under stoichiometric and catalytic conditions. In the presence of piperidine, the catalyst precursor [Rh{κ2-P,O-Ph2P(CH2)3OEt}2]+ ( 5) gave the piperidine complex [Rh{κ1-P-Ph2P(CH2)3OEt}2(HNC5H10)2]+ ( 8) that was transformed into the neutral amido-piperidine species [Rh{κ1-P-Ph2P(CH2)3OEt}2(NC5H10)(HNC5H10)] ( 9) under thermal conditions. NMR studies performed in the presence of styrene under catalytic conditions showed that 9 is a key species in the catalytic oxidative amination of styrene. Related cyclooctadiene-containing catalyst precursors [Rh(cod){κ1-P-Ph2P(CH2)3OEt}n]+ ( n=1, 2) also gave 9 under the same conditions. The proposed catalytic cycle has been established by a series of DFT calculations including the transition states of the key steps that have been identified and characterised. These studies have shown that, after elimination of the enamine, regeneration of catalytic active species takes place by direct transfer of the proton of a piperidine ligand to the alkyl group resulting from the insertion of styrene into the RhH bond and formation of ethylbenzene. Against the expectations, the formation of a dihydride intermediate by NH oxidative addition is a highly energy-demanding process. Catalyst 5 has also been applied for the oxidative amination of substituted vinylarenes with several secondary cyclic and acyclic amines. [ABSTRACT FROM AUTHOR]

The hydrothiolation of unsaturated carbon-carbon bonds is a practical and atom-economical approach for the incorporation of sulfur into organic frameworks. In recent years, we have witnessed the development of a range of transition-metal-based catalytic systems for the control of the regio- and stereoselectivity. In this Minireview we highlight the mechanistic background behind this transformation so as to help the design of more specific and active organometallic hydrothiolation catalysts. [ABSTRACT FROM AUTHOR]

A series of cationic square-planar rhodium(I) complexes of type [Rh(cod){Ar2P(CH2) nZ}]+, [Rh(cod){Ar2P(CH2) nZ}2]+, and [Rh{Ar2P(CH2) nZ}2]+, which contained diverse functionalized hemilabile phosphine ligands of type Ar2P(CH2) nZ ( n=1-3; Z=OMe, OEt, O nBu, NMe2, SMe), were synthesized and spectroscopically characterized. The crystal structures of representative compounds were determined by X-ray diffraction. Most complexes were active catalysts for the anti-Markovnikov oxidative amination of styrene with piperidine to produce ( E)-1-styrylpiperidine. Catalyst screening showed a remarkable relationship between the hemilabile ligand, the precatalyst structure, and the catalytic activity. The more-efficient catalysts were those that had arylphosphine ligands with a 2-alkoxyethyl- or 3-alkoxypropyl hemilabile moiety, Ar2P(CH2) nOR ( n=2, 3; R=Me, Et, nBu). This study has revealed the outstanding catalytic activity of bis-phosphine complexes [Rh{(4-R-C6H4)2P(CH2)3OEt}2][PF6] (R=H, Me, OMe), with unprecedented turnover frequencies of up to 80 h−1 (R=Me) and excellent enamine selectivity (96 %). [ABSTRACT FROM AUTHOR]

Hot off the press: Developments and promotions at ChemCatChem. [ABSTRACT FROM AUTHOR]

Graphical abstract This work compiles and discusses the great variety of non-classical transition-metal-catalyzed mechanisms hitherto reported for the reduction of organic molecules with hydrosilanes. Highlights • Non-classical mechanisms for hydrosilane-driven reductions are discussed herein. • Non-classical mechanisms often involve the heterolytic activation of the Si–H bond. • Heterolytic Si–H bond activation requires a Lewis acid metal and a Lewis base. • Non-classical mechanism that need no heterolytic Si–H activation are also discussed. Abstract This article reviews the most recent advances on the study of non-classical mechanisms for the reduction of organic substrates with hydrosilanes catalyzed by transition metals. A wide variety of catalytic cycles that go beyond the classical steps described for Ojima, Chalk-Harrod and modified Chalk-Harrod mechanisms, as representative examples, have been proposed in recent years. In this review, these alternative mechanistic proposals have been analyzed and classified according to the type of substrate, focusing on the reduction/hydrosilylation of carbonyl compounds (ketones and aldehydes), carbon dioxide, silylesters, amides, N -heterocycles, alkyl halides, nitriles, alkenes and alkynes. In spite of the broad diversity of non-classical reaction mechanisms hitherto reported, the catalytic cycles described for each substrate have been arranged in different categories according to their characteristics. The epigraph dedicated to the first type of substrate (carbonyl compounds) comprises most of the mechanisms described in this review for the reduction of polar bonds, which, to some extent, show a relationship to those proposed for non-polar bonds (alkenes and alkynes). Remarkable types of reaction mechanisms that will be dealt with in this work are: ionic mechanisms, ligand-assisted mechanisms, nonhydride mechanisms, NHC-Cu-H mediated mechanisms, and silylene-mediated mechanisms. [ABSTRACT FROM AUTHOR]

The number of late transition metal complexes bearing tridentate monoanionic silyl-based NSiN-type ligands has grown in the last few years. This review describes the synthetic methodologies that allow preparation of NSiN ligands precursors as well as the chemical and structural behavior of the transition metal complexes resulting from the reaction of these ligand precursors with different metallic starting materials. In addition, a description of the catalytic processes based on Rh- and Ir-NSiN catalysts so far reported is included. Moreover, an analysis of the chemistry of iridium complexes with different fac - κ 3 -( Si,N,N )-NSiN ligands allows to conclude that the nature of the silyl group along with the donor ability of the nitrogen atoms at the NSiN ligands influence on the reactivity and potential catalytic activity of the respective complexes. [ABSTRACT FROM AUTHOR]

Rhodium‐catalyzed polymerization goes heavyweightThe cover picture shows a neutral rhodium(I) N‐heterocyclic carbene hydrosilylation catalyst immobilized on the surface of MCM‐41. In their Full Paper on p. 1133 ff., F. J. Fernández‐Alvarez, L. A. Oro et al. describe how these rhodium‐containing materials catalyze the preparation of poly(silyl ether)s by hydrosilylation. These results show that the immobilized catalysts provide polymers with higher average molecular weight than those obtained by using the corresponding homogenous systems, which could be attributed to a confinement effect exerted by the channels of MCM‐41. [ABSTRACT FROM AUTHOR]

Abstract: The dinuclear gem-dithiolato bridged compounds [Rh2(μ-S2Cptn)(cod)2] (1) (CptnS2 2− =1,1-cyclopentanedithiolato), [Rh2(μ-S2Chxn)(cod)2] (2) (ChxnS2 2− =1,1-cyclohexanedithiolato), [Rh2(μ-S2CBn2)(cod)2] (3) (Bn2CS2 2− =1,3-diphenyl-2,2-dithiolatopropane) and [Rh2(μ-S2C i Pr2)(cod)2] (4) ( i Pr2CS2 2− =2,4-dimethyl-2,2-dithiolatopentane) dissolved in toluene in the presence of monodentate phosphine or phosphite P-donor ligands under carbon monoxide/hydrogen (1:1) atmosphere are efficient catalysts for the hydroformylation of oct-1-ene under mild conditions (6.8atm of CO/H2 and 80°C). The influence of the gem-dithiolato ligand, the P-donor co-catalyst and the P/Rh ratio on the catalytic activity and selectivity has been explored. Aldehyde selectivities higher than 95% and turnover frequencies up to 245h−1 have been obtained using P(OMe)3 as modifying ligand. Similar activity figures have been obtained using P(OPh)3 although the selectivities are lower. Regioselectivities toward linear aldehyde are in the range 75–85%. The performance of the catalytic systems [Rh2(μ-S2CR2)(CO)2(PPh3)2]/PPh3 has been found to be comparable to the systems [Rh2(μ-S2CR2)(cod)2] at the same P/Rh ratio. The system [Rh2(μ-S2CBn2)(cod)2] (3)/P(OPh)3 has been tested in the hydroformylation-isomerization of trans-oct-2-ene. Under optimized conditions up to 54% nonanal was obtained. Spectroscopic studies under pressure (HPNMR and HPIR) evidenced the formation of hydrido mononuclear species under catalytic conditions that are most probably responsible for the observed catalytic activity. [Copyright &y& Elsevier]

Abstract: In situ-generated N-heterocyclic carbene (NHC) palladium catalysts and isolated NHC-palladium complexes have been tested for the telomerization reaction of 1,3-butadiene with primary and secondary amines. Superior catalyst activity (TON up to 400.000) and selectivity are obtained. Applying optimized conditions a variety of octa-2,7-dienylamines were prepared in high yield and excellent selectivity. [Copyright &y& Elsevier]

The synthesis of arene–ruthenium(II) and C5Me5–rhodium(III) and –iridium(III) complexes of chiral arene-chromium-tricarbonyl-based P∧P and P∧N ligands is described. Three complexes were characterized in the solid state by X-ray structural analysis. The complexes were tested in the catalytic hydrogen transfer reactions as well as in the kinetic resolution of racemic alcohols, where some complexes showed good conversion, but low enantioselectivity. [Copyright &y& Elsevier]