Search

Reaction mechanisms of hydrosilylation of ketone and alkene with tertiary silane using the Wilkinson-type catalyst were theoretically investigated on the basis of density functional calculations using ωB97XD functional. Previously proposed three mechanisms, the Chalk-Harrod (CH) mechanism, the modified Chalk-Harrod (mCH) mechanism, and the outer-sphere mechanism were examined. Besides, we also found two mechanisms, the alternative CH (aCH) mechanism and the double hydride (DH) mechanism. In the aCH mechanism, a four-coordinate rhodium hydride complex formed through the elimination of R

Studies on the deactivation mechanism of the reaction of N,N-dialkylamides with TMDS catalyzed by Vaska’s complex, IrCl(CO)(PPh3)2 (1a), triggered the discovery of highly active Ir-PPh3 catalysts: photochemically activated 1a and thermally activated IrCl(PPh3)3 (8). Both catalysts showed excellent activity toward the selective conversion of a variety of N,N-dialkyl-, N-alkyl-N-aryl-, and N,N-diarylamides to the corresponding enamines with low catalyst loadings. The 14-electron species “ClIr(PPh3)2”, which is stabilized by solvents or reactants in the actual catalytic reactions, could be involved in the catalysis, which produces “HIr(PPh3)2” and “SiIr(PPh3)2” (Si = Me2HSiOMe2Si−) species in the catalytic cycle. An in situ generation method for the “ClIrL2” species was established by simply mixing [IrCl(η4-COD)]2 with PPh3 or other phosphorus ligands, which realized the facile large-scale syntheses of enamines.

Palladium-catalyzed reactions of dodecamethylcyclohexasilane [(SiMe2)6] (1) with alkynes led to efficient preparation of 1,1,4,4-tetramethyl-1,4-disilacyclohexadienes (3). The reactions were best catalyzed by Pd(0) species generated from Pd2(dba)3·CHCl3 and 1-isocyanoadamantane (AdNC). Terminal and internal alkynes bearing aryl and alkyl substituents could be used as substrates, and the reaction allowed gram-scale preparation of 3. A dimethylsilylene (Me2Si:) species, generated by activation of Si–Si bonds in 1 by Pd(0) species, was involved in the reaction mechanism. The DFT calculations suggest that oxidative addition of Si–Si bonds in 1 to Pd(CNAd)2 species is followed by extrusion of a Me2Si═Pd(CNAd) intermediate. Reaction of the resulting palladium-coordinated silylene with an alkyne forms a silacyclopropene, which dimerizes to give 3. The extrusion is accompanied by ring contraction of 1 to generate (SiMe2)5, which also contributes to formation of 3 and (SiMe2)4 by the Pd(0)-catalyzed reaction with ...

There are several reports on the synthesis of alkylamines, but most of the reported methods are not suitable for the synthesis of hindered amines. In this research, we found that a copper catalyst is effective for the formation of congested C-N bonds at room temperature. Control experiments revealed that a copper amide is a key intermediate. Moreover, when a chiral amine was used, a quaternary carbon stereogenic center was created with good selectivity.

Nanosheet compounds Pd11 (SiiPr)2 (SiiPr2 )4 (CNtBu)10 (1) and Pd11 (SiiPr)2 (SiiPr2 )4 (CNMes)10 (2), containing two Pd7 (SiiPr)(SiiPr2 )2 (CNR)4 plates (R=tBu or Mes) connected with three common Pd atoms, were investigated with DFT method. All Pd atoms are somewhat positively charged and the electron density is accumulated between the Pd and Si atoms, indicating that a charge transfer (CT) occurs from the Pd to the Si atoms of the SiMe2 and SiMe groups. Negative regions of the Laplacian of the electron density were found between the Pd and Si atoms. A model of a seven-coordinated Si species, that is, Pd5 (Pd-SiMe), is predicted to be a stable pentagonal bipyramidal molecule. Five Pd atoms in the equatorial plane form bonding overlaps with two 3p orbitals of the Si atom. This is a new type of hypervalency. The Ge analogues have geometry and an electronic structure similar to those of the Si compounds. But their formation energies are smaller than those of the Si analogues. The use of the element Si is crucial to synthesize these nanoplate compounds.

The appropriate design of a ligand (L) in IrCl(CO)(L)2 (4) realized the efficient synthesis of π-conjugated enamines possessing hole-transport properties. The iridium complex with electron-withdrawing phosphorus ligands catalyzed the hydrosilylation of amides to the corresponding silylhemiaminals, which were transformed to the enamines by heat or by treatment with acids. High catalytic efficiency (TON > 10,000) was achieved, which made it possible for the residual iridium in the enamine product to be below 20 ppb.

(COT)2Fe and the open ferrocenes (MPDE)2Fe (MPDE = η5-3-methylpentadienyl) and (DMPDE)2Fe (DMPDE = η5-2,4-dimethylpentadienyl) were found to function as catalyst precursors for the hydrosilylation of alkenes in the presence of auxiliary ligands. Screening trials determined that the optimal catalyst system was composed of (COT)2Fe and adamantyl isocyanide, allowing the selective hydrosilylation of styrene derivatives with trisubstituted hydrosiloxanes and a polydimethylsiloxane bearing Me2SiH moieties as the end groups. Under the appropriate conditions, the dehydrogenative silylation side reaction was completely suppressed, and the reaction TON exceeded 5000.

Well-defined iron complex-mediated catalytic C-3-selective silylation of indole derivatives and stoichiometric C–H bond functionalization of arenes were achieved using an iron dicarbonyl complex containing the disilaferracycle moiety [o-(SiMe2)2C6H4]Fe(CO)2[o-{η2-(H-SiMe2)2}C6H4] (1). Facile liberation of the η2-(H-Si) groups in 1 was the key to effective promotion of these reactions.

Environmentally friendly iron(II) catalysts for atom-transfer radical polymerization (ATRP) were synthesized by careful selection of the nitrogen substituents of N,N,N-trialkylated-1,4,9-triazacyclononane (R3 TACN) ligands. Two types of structures were confirmed by crystallography: "[(R3 TACN)FeX2 ]" complexes with relatively small R groups have ionic and dinuclear structures including a [(R3 TACN)Fe(μ-X)3 Fe(R3 TACN)](+) moiety, whereas those with more bulky R groups are neutral and mononuclear. The twelve [(R3 TACN)FeX2 ]n complexes that were synthesized were subjected to bulk ATRP of styrene, methyl methacrylate (MMA), and butyl acrylate (BA). Among the iron complexes examined, [{(cyclopentyl)3 TACN}FeBr2 ] (4 b) was the best catalyst for the well-controlled ATRP of all three monomers. This species allowed easy catalyst separation and recycling, a lowering of the catalyst concentration needed for the reaction, and the absence of additional reducing reagents. The lowest catalyst loading was accomplished in the ATRP of MMA with 4 b (59 ppm of Fe based on the charged monomer). Catalyst recycling in ATRP with low catalyst loadings was also successful. The ATRP of styrene with 4 b (117 ppm Fe atom) was followed by precipitation from methanol to give polystyrene that contained residual iron below the calculated detection limit (0.28 ppm). Mechanisms that involve equilibria between the multinuclear and mononuclear species were also examined.

A series of group 8 half-sandwich disilametallacycles, (η6-arene)MII(Me2SiC6H4SiMe2)L and (η6-arene)MIV(H)2(Me2SiC6H4SiMe2) (M = Fe, Ru) in the formal oxidation states of M(II) and M(IV) were synthesized and characterized. Both the M(II) and the M(IV) oxidation states were effectively stabilized by the disilametallacycle skeleton, and facile interconversion between (η6-arene)MII–dinitrogen, (η6-arene)MII–carbonyl, and (η6-arene)MIV–dihydride complexes bearing a disilaferracycle framework was accomplished. These M(II) and M(IV) complexes can easily generate coordinatively unsaturated 16e disilametallacycles, (η6-arene)MII(Me2SiC6H4SiMe2), by dissociation of L or H2, and stoichiometric and/or catalytic double silylation of alkenes and alkynes was realized thorough this 16e intermediate.

The mechanisms associated with the hydrogenation of alkenes catalyzed by the iron complex Fe(cis-CO)2{o-(SiMe2)2C6H4}2(H)2 (1) were investigated by DFT calculations. The complex 1 has a structure in which the iron center is bonded to four silicon atoms and two hydrides. Secondary Si···H···Si interactions were also observed. The exchange of a 1,2-bis(dimethylsilyl)benzene ligand with ethylene and hydrogen gives a disilaferracycle bearing η2-(CH2═CH2) and η2-H2 ligands. The catalytic cycle initiated from the disilaferracycle involves cleavage of a H–H linkage assisted by an Fe–Si bond to form Fe–H and η1-(H–Si) moieties (step 1), hydrogen migration from the Fe–H group to the η2-(CH2═CH2) ligand which accomplishes the insertion of ethylene into the Fe–H bond (step 2), and reaction of the resulting β-agostic ethyl moiety with the η2-(H–Si) group to form ethane on the iron atom (step 3). The octahedral geometry of 1 as well as the presence of π-acidic CO ligands and Fe–Si σ-bonds contributes to all of the cata...

Diastereomeric geminate pairs of chiral bis(2-oxazoline) ruthenium complexes with bipyridyl-type N-heteroaromatics, Λ- and Δ-[Ru(L-L)(2)(iPr-biox)](2+) (iPr-biox=(4S,4'S)-4,4'-diisopropyl-2,2'-bis(2-oxazoline); L-L=2,2'-bipyridyl (bpy) for 1Λ and 1Δ, 4,4'-dimethyl-2,2'-bipyridyl (dmbpy) for 2Λ and 2Δ, and 1,10-phenanthroline (phen) for 3Λ and 3Δ), were separated as BF(4) and PF(6) salts and were subjected to the comparative studies of their stereochemical and photochemical characterization. DFT calculations of 1Λ and 1Δ electronic configurations for the lowest triplet excited state revealed that their MO-149 (HOMO) and MO-150 (lower SOMO) characters are interchanged between them and that the phosphorescence-emissive states are an admixture of a Ru-to-biox charge-transfer state and an intraligand excited state within the iPr-biox. Furthermore, photoluminescence properties of the two Λ,Δ-diastereomeric series are discussed with reference to [Ru(bpy)(3)](2+).

Three new disilaplatinacyclopentane complexes, (Me2S)Pt[(SiMe2)2C6H4]2 (1), (MeSCH2CH2CH2SMe)Pt[(SiMe2)2C6H4]2 (2), and (MeSCH2CH2CH2SMe)Pt[(SiMe2)2C6H4] (3), were prepared and characterized by the reaction of (PtMe2)2(μ-SMe2)2 with 1,2-bis(dimethylsilyl)benzene in the presence of dialkylsulfide ligands. Catalytic hydrosilane reduction of carboxamides with 1,2-bis(dimethylsilyl)benzene was investigated in the presence of these new complexes, and the possible involvement of disilaplatinacycle dihydride species, “Pt(Si)2(H)2(SR2)”, stabilized by dialkylsulfides in catalysis was discussed.

Reaction of 2 equiv of 1,2-bis(dimethylsilyl)benzene with [Fe(mesityl)2]2 in aromatic solvent under a nitrogen atmosphere afforded the dinuclear bis(silyl)iron complexes [(η6-arene)Fe(Me2SiC6H4SiMe2)]2(μ-η1:η1-N2) (1), which have an end-on-bound bridging dinitrogen ligand. The dinitrogen unit easily dissociated from the Fe center to generate the mononuclear (η6-arene)FeII(Si)2L or (η6-arene)FeIV(Si)2H2 complexes by the reactions with CO, PPh3, and H2.

A series of azanickellacyclopentene complexes having iodo, bromo, chloro, or triflate ligand on the Ni center were prepared, and are subjected to studies on ethylene polymerization catalysis. Activity of these mononuclear azanickellacyclopentene complexes was increased in the order, Ni–Cl

Three Ti−M2 (M2 = Ni, Pd, Pt) heterobimetallic complexes, [(η3-methallyl)M2(Ph2PNtBu)2TiCl2](OTf), were synthesized in which a TiIV←M2 interaction was suggested by crystallography and DFT calculati...

Reactions of Cp∗Ru(κ2-N(R) C(R′)NR) (1a; R = iPr, R′ = Me, 1b; R = tBu, R′ = Ph) with TCNE initially give dark green colored intermediary species, which are readily converted to brown colored “η2-C” coordination complexes, Cp∗Ru(κ2-N(R) C(R′)NR)(η2-TCNE) (3a; R = iPr, R′ = Me, 3b; R = tBu, R′ = Ph). These “η2-C” complexes are characterized by spectroscopy and crystallography. A stable ruthenium amidinate having a “κ1-N”-coordinated TCNE, Cp∗Ru(κ2-N(tBu) C(Mes)NtBu)(κ1(N)-TCNE) (2c), is synthesized by treatment of Cp∗Ru(κ2-N(tBu) C(Mes)NtBu) (1c) with TCNE, the structure of which is unequivocally confirmed by X-ray structure determination and the charge transfer nature is supported by ESR analysis. Close analogy in IR and UV–Vis spectroscopy of 2c with the dark green colored intermediary species formed from 1b suggests that this is “κ1-N” ruthenium amidinate, which is rearranged to the “η2-C” complex 3b.

Three rhodadisilacyclopentene complexes are synthesized by the reaction of 1,2-bis(dimethylsilyl)benzene with RhCl(PPh3)3 or Rh(H)(PPh3)4, and their contributions to the catalytic hydrosilylation of acetone are discussed. Treatment of the rhodium precursor RhCl(PPh3)3 or Rh(H)(PPh3)4 with 1,2-bis(dimethylsilyl)benzene affords an unstable Rh(V)-trihydride species having a rhodadisilacyclopentene skeleton, Rh(Me2SiC6H4SiMe2)(H)3(PPh3)2 (1), as a primary product, which is formed by double oxidative addition of 1,2-bis(dimethylsilyl)benzene to the rhodium center. The complex 1 eliminates H2 upon concentration to quantitatively form a Rh(III)-monohydride complex, Rh(SiMe2C6H4SiMe2)(H)(PPh3)2 (2). Further oxidative addition of 1,2-bis(dimethylsilyl)benzene to 2 gives a Rh(III)-trisilyl complex, Rh(Me2SiC6H4SiMe2)(η1-HSiMe2C6H4SiMe2)(PPh3) (3), in which there is an agostic interaction between the Si−H bond and the Rh(III) center. Elimination of H2 from 1 is reversible, and the most effective method for preparing...

A cobalt complex having bis-[C(trimethylsilyl)-N-arylimino]dimethylsilane, Me3Si(CNAr)SiMe2(CNAr)SiMe3 (Ar = 2,6-xylyl), as a new β-diimine ligand was synthesized, and its structure was compared with those of α- and β-diimine cobalt complexes. Ethylene polymerization catalyzed by these cobalt complexes and their in situ generated nickel homologues was investigated.

Triflate complexes of mono- and diruthenium amidinates, (η6-C6R6)Ru(κ1-OTf){η2-R′N C(R′′)NR′} (1: R = Me; 2: R = H) and (η5-C5Me5)Ru(μ-η2-iPrN C(Me)NiPr)Ru(κ1-OTf)(η5-C5R5) (3: R = Me; 4: R = H), are synthesized, and coordination behavior of the triflate anion to the coordinatively unsaturated ruthenium species is investigated by crystallography and variable temperature (VT) NMR spectroscopy (19F, 1H). The monoruthenium amidinate complexes have three-legged piano-stool structures in single crystals, which include a κ1-OTf ligand with the Ru–O bond of 2.15–2.20 A. In contrast, reversible dissociation of OTf is observed in variable temperature 1H NMR spectroscopy in liquid states; the activation energy for the dissociation and recombination of the OTf ligand is varied with the substituents on the arene and amidinate ligand in the corresponding ruthenium cation and the solvent used. A typical example of moderately coordinating ability of the OTf ligand is seen in 19F NMR spectra of (η6-C6Me6)Ru(κ1-OTf){η2-iPrN C(Me)NiPr} (1a) and (η6-C6H6)Ru(κ1-OTf){η2-iPrN C(Me)NiPr} (2a) in CD2Cl2 at the temperature range from −90 to 20 °C, in which the OTf anion is dissociated in 1a, whereas 2a has a relatively robust Ru–OTf bond. Combination of crystallography and VT NMR contributes to understanding the difference in coordination behavior of the OTf ligand between two diruthenium amidinates, (η5-C5Me5)Ru(μ-η2-iPrN C(Me)NiPr)Ru(κ1-OTf)(η5-C5Me5) (3) and (η5-C5Me5)Ru(μ-η2-iPrN C(Me)NiPr)Ru(κ1-OTf)(η5-C5H5) (4); the results suggest that the electron-donating and sterically demanding η5-C5Me5 helps for dissociation of the triflate ligand. Moderate coordinating ability of the triflate anion sometimes provides characteristic reactions of mono- and diruthenium amidinates which differ from the corresponding neutral halogeno-compounds or cationic coordinatively unsaturated homologues bearing fluorinated tetraarylborates; a typical example is given by inhibition of coordination of ethylene to the [(η6-C6H6)Ru{η2-tBuN C(Ph)NtBu}]+ species by the OTf ligand.

Treatment of a titanium phosphinoamide, (Ph2PNtBu)2TiCl2 (1), with [Cp∗RuCl]4 in THF at room temperature afforded a Ti Ru heterobimetallic complex, of which crystallographic study showed the molecular structure to be a six-membered dimetallacyclohexane, Cp∗Ru(μ-Cl)(Ph2PNtBu)2Ti O (3). A boat conformation of the dimetallacycle leads to effective linking of the two metal centers by two phosphinoamide ligands, and bridging of a chlorine atom over the Ti Ru axis. The Ti Ru heterobimetallics were synthesized by the reaction of 1 with Cp∗Ru(COD)Cl or Cp∗Ru(TMEDA)Cl, whereas no reaction occurred between 1 and Cp∗Ru(PCy3)Cl. A primary product of this reaction would be a trichloride, Cp∗Ru(μ-Cl)(Ph2PNtBu)2TiCl2 (2). In fact, careful treatment of 1 with [Cp∗RuCl]4 afforded 2 as the main product which was detected by NMR and ESI-MS spectra; 2 was converted to 3 in contact with 1 equivalent of water.

Reaction of [Cp*MCl 2 ] 2 ( 1a : M=Rh and 1b : M=Ir) or [(C 6 Me 6 )RuCl 2 ] 2 ( 1c ) with bis(diphenylphosphinomethyl)phenylphosphine (dpmp) in the presence of KPF 6 generated mono- or dinuclear complexes [Cp*RhCl(dpmp)](PF 6 ) ( 2a ), [(C 6 Me 6 )RuCl(dpmp)](PF 6 ) ( 5c ), [Cp*MCl 2 (dpmp)MClCp*](PF 6 ) ( 3a : M=Rh and 4b : M=Ir) or [(C 6 Me 6 )RuCl 2 (dpmp)RuCl(C 6 Me 6 )](PF 6 ) ( 6c ), depending on reaction conditions. These complexes have two chiral centers and the diastereomers were separated and characterized by spectrometry and X-ray analyses. A diastereomer 2a ( A ) was treated with AuCl(C 4 H 8 S), generating a hetero-tetranuclear complex [{Cp*RhCl 2 (dpmp)Au} 2 ](PF 6 ) 2 ( 7a ), whereas similar reactions of 5c gave dinuclear complex [(C 6 Me 6 )RuCl(dpmp)AuCl](PF 6 ) ( 8c ( A )).

Iron-promoted reduction reactions are described from the standpoint of the catalyst design. Catalysis of three well-defined precatalysts is the first topic; iron complexes having PNNP ligands, Shvo-type complexes, and pincer compounds are active towards catalytic hydrogenation and transfer hydrogenation of ketones. The reduction mechanisms are in line with the concept of Noyori's metal-ligand bifunctional catalysis, in which the ligand plays a key role in the hydrogenation of the carbonyl function. The redox-active ligands in iron-catalyzed hydrogenation and hydrosilylation of alkenes and alkynes provide other important aspects contributing to the catalyst design. In particular, redox properties of bis(imino)pyridine ligands assist the activation of H–H and H–Si bonds in molecular hydrogen and organohydrosilanes leading to efficient hydrogenation and hydrosilylation of alkenes. Reduction reactions mediated by iron compounds, which include catalytic hydrogenation, transfer hydrogenation, and hydrosilylation, are overviewed from organic synthesis, which is promoted by a mixture of iron salts and ligands, ill-defined catalyst precursors, and nanoparticles. From these, the possible catalyst design of iron is discussed. Keywords: hydrogenation; transfer hydrogenation; hydrosilylation; metal-ligand bifunctional catalysis; redox-active ligands; non-innocent ligands

A heptanuclear iron carbonyl cluster, [Fe(3)(CO)(11)(μ-H)](2)Fe(DMF)(4) (4), is found to be a highly efficient catalyst for the reduction of various carboxamides by 1,2-bis(dimethylsilyl)benzene (BDSB), which makes possible reducing the amount of the catalyst, shortening the reaction time, and lowering the reaction temperatures.

A new reaction mechanism for the iron-catalyzed cross-coupling reaction of ArMgX with alkyl halides using (TMEDA)FeAr(2) and (TMEDA)Fe(Ar)Br is proposed on the basis of the isolation and reaction of these organoiron intermediates.