Alkene hydrosilylation, the addition of a silicon hydride (Si-H) across a carbon-carbon double bond, is one of the largest-scale industrial applications of homogeneous catalysis and is used in the ...commercial production of numerous consumer goods. For decades, precious metals, principally compounds of platinum and rhodium, have been used as catalysts for this reaction class. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. Here, we report that well-characterized molecular iron coordination compounds promote the selective anti-Markovnikov addition of sterically hindered, tertiary silanes to alkenes under mild conditions. These Earth-abundant base-metal catalysts, coordinated by optimized bis(imino) pyridine ligands, show promise for industrial application.
The aryl-substituted bis(imino)pyridine cobalt methyl complex, (MesPDI)CoCH3 (MesPDI = 2,6-(2,4,6-Me3C6H2-NCMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to ...selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C–H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.
Iron dialkyl complexes, N3Fe(CH2SiMe3)2, with three different classes of tridentate, nitrogen-based “N3” ligands, aryl-substituted bis(imino)pyridines, terpyridine, and pyridine bis(oxazoline), have ...been synthesized and evaluated in the catalytic hydrosilylation of olefins with tertiary silanes. The 2,2′:6′,2″-terpyridine (terpy) complex, (terpy)Fe(CH2SiMe3)2, was prepared either via alkylation of (terpy)FeCl2 with LiCH2SiMe3 or by pyridine displacement from (pyridine)2Fe(CH2SiMe3)2 by free terpyridine. The aryl-substituted bis(imino)pyridine compounds, (RPDI)Fe(CH2SiMe3)2 (RPDI = 2,6-(2,6-R2-C6H3NCMe)2C5H3N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2- i Pr substituent (2‑iPrPDI)Fe(CH2SiMe3)2 (2‑iPrPDI = 2,6-(2- i Pr-C6H4NCMe)2C5H3N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me3SiO)2MeSiH and (EtO)3SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et3SiH. The most hindered member of the series, ( iPrPDI)Fe(CH2SiMe3)2, and the pyridine bis(oxazoline) iron compound, (R,R)-( iPrPybox)Fe(CH2SiMe3)2 ( iPrPybox = 2,6-bisisopropyl-2-oxazolin-2-ylpyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes studied. By contrast, the terpyridine precursor, (terpy)Fe(CH2SiMe3)2, reached >95% conversion at 60 °C with Et3SiH and (Me3SiO)2MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH2SiMe3)2, demonstrating functional group compatibility unique to this compound that is absent from bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been evaluated by a combination of X-ray diffraction, magnetochemistry, Mössbauer spectroscopy, and density functional theory calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to chelate radical anions.
Aryl-substituted bis(imino)pyridine iron dinitrogen complexes are active for the hydrosilylation of 1,2,4-trivinylcyclohexane with tertiary alkoxy silanes, a process used in the manufacture of low ...rolling resistance tires. The iron compounds exhibit unprecedented selectivity for the monohydrosilylation of the desired 4-alkene that far exceeds results obtained with commercially used platinum compounds.
The ionic Pd(C7H8C(O)R)(Ar-BIAN)X (R = Me, Et, iPr, Ph; X = Cl, Br, I; Ar = p-An, p-FC6H4, p-BrC6H4, p-Tol, Ph, o,o‘-Me2C6H3, o,o‘-iPr2C6H3) complexes (1b−12b), bearing the bidentate nitrogen ligand ...bis(arylimino)acenaphthene (Ar-BIAN), have been synthesized via reaction of the corresponding neutral acylpalladium complexes Pd(C(O)R)X(Ar-BIAN) (1a−12a) with norbornadiene (nbd). For the first time, an extensive kinetic study of this migratory alkene insertion into acyl−palladium bonds of neutral complexes containing α-diimine ligands has been carried out. It has been found that under pseudo-first-order circumstances these reactions follow the rate law k obsd = k 1 + k 2nbd, which shows that these reactions proceed via a pathway independent of alkene concentration (k 1 pathway) and a pathway dependent on alkene concentration (k 2 pathway). The dramatic decrease of the rate constants k 1 and k 2 upon increasing the steric bulk of the BIAN ligand and the large negative entropy of activation and low enthalpy of activation for both pathways indicate that the k 1 and k 2 pathways are closely related and involve associative processes. From the influence of solvent, X and C(O)R ligand, steric and electronic properties of the BIAN ligand, the presence of free halide and free BIAN, and the parameters of activation, mechanisms have been proposed for both pathways. The k 1 pathway may proceed via a rate-determining solvent-assisted halide or nitrogen dissociation, followed by alkene association and migratory insertion, while the k 2 pathway may occur via a rate-determining migratory alkene insertion in a contact ion pair intermediate. This species may be formed via alkene association followed by either halide or nitrogen dissociation.
Insertion of the isocyanides 2,6-dimethylphenyl isocyanide (DIC), tert-butyl isocyanide (TIC) and tosylmethyl isocyanide (TosMIC) into the Pd−Me bond of complexes (N⌒N)Pd(Me)Cl (N⌒N = 2,2‘-bipyridine ...(bpy), 1,10-phenanthroline (phen), 2,2‘-bipyrimidine (bpm)) afforded quantitatively (N⌒N)Pd(C(NR)Me)Cl (R = 2,6-Me2C6H3, t-Bu, CH2Tosyl). The course of the reaction has been shown to proceed via the intermediates (N⌒N)Pd(CN−R)(Me)Cl (N⌒N = bpy, phen; R = 2,6-Me2C6H3, t-Bu, CH2Tosyl), which have been characterized at 250 K by NMR and IR spectroscopies and conductivity measurements. The mechanism of the insertion reaction involves substitution of the halide by the isocyanide followed by a rate-determining methyl migration to the precoordinated isocyanide. Kinetic measurements of the isocyanide insertion reaction, which provided the reaction rate constants of the isocyanide association (k 1), isocyanide dissociation (k - 1), and methyl migration reaction (k 2), have demonstrated that the migration rate of the methyl group to the precoordinated isocyanide increases with increasing electrophilicity of the isocyanide. Methyl migration in the intermediate complexes (N⌒N)Pd(CN−R)(Me)Cl (N⌒N = bpy, phen; R = 2,6-Me2C6H3, t-Bu, CH2Tosyl) also occurs in the solid state. The complexes (N⌒N)Pd(C(NR)Me)X (N⌒N = bpy, phen; R = t-Bu, CH2Tosyl; X = Cl, Br) show a fluxional behavior due to a site exchange of the nitrogen donor atoms. Spin saturation transfer 1H NMR experiments showed that the mechanism of this process involves Pd−N bond breaking and subsequent isomerization via a Y-shaped intermediate.
The insertion reactions of the allenes propadiene and 1,2-heptadiene in the Pd−C bond of complexes (N⌒N)Pd(R)X (N⌒N = 8-PQ, p-An-BIAN, i-Pr-DAB, p-An-DAB, i-Pr-PyCa; R = Me, C(O)Me, C(O)Ph, C(O)i-Pr; ...X = Cl, Br) have been investigated. An X-ray crystal structure determination of (8-PQ)Pd{(1−3-η)-2-methylpropenyl}Cl exhibited the unexpected monodentate coordination of the nitrogen ligand. The monodentate coordination in apolar solvents and bidentate coordination in polar solvents was demonstrated by means of NOE NMR experiments. Kinetic measurements revealed that the reactions are first order in the palladium concentration and occur via an allene concentration independent and dependent pathway. Reactions of complexes containing flexible bidentate nitrogen ligands were retarded by additional free bidentate nitrogen ligand indicating that initial dissociation of a nitrogen donor is an important step in the reaction. We have strong indications that the migration of the R group to the precoordinated allene is the rate-determining step. Instead of mass-law retardation by excess X- (X = Cl-, Br-), an enhancement of the reaction has been observed in case of the complexes (8-PQ)Pd(Me)Cl, (8-PQ)Pd(Me)Br, and (i-Pr-DAB)Pd(C(O)Me)Cl. Flexible bidentate nitrogen ligands greatly enhance the reaction, owing to the easy formation of an accessible site on the metal center. The insertion of allenes into the Pd−C bonds of complexes containing rigid bidentate nitrogen ligands probably proceeds via initial allene association followed by either halide or nitrogen dissociation and subsequent migration of the R group to the precoordinated allene.
The reactivity of Pd−carboimine complexes toward unsaturated hydrocarbon bonds has been studied. Insertion of norbornadiene and norbornene into the Pd−C bond of the neutral complexes ...(N⌒N)Pd(C(N-2,6-Me2C6H3)Me)X (X = Cl (1), Br (2), I (3); N⌒N = 2,2‘-bipyridine (bpy, a), 1,10-phenanthroline (phen, b)) afforded quantitatively the novel and stable complexes (N⌒N)Pd(C7H8C(NR)Me)X and (N⌒N)Pd(C7H10C(NR)Me)X (R = 2,6-Me2C6H3). Insertion reactions of the unstrained unsaturated hydrocarbons ethylene, propylene, 3-methyl-1,2-butadiene, and acetylene with the cationic complexes (N⌒N)Pd(C(NR)Me)X (N⌒N = bpy, phen; R = 2,6-Me2C6H3; X = BF4) provided the complexes (N⌒N)Pd(C2H4C(NR)Me)X, (N⌒N)Pd(C3H6C(NR)Me)X, (N⌒N)Pd(C5H8C(NR)Me)X, and (bpy)Pd(C2H2C(NR)Me)X. The remarkable stability of these products is caused by the strong coordination of the carboimine nitrogen to the palladium center. Reaction of 1a and 1b with HC⋮CCOOMe gave, instead of an insertion product, the Michael addition product (N⌒N)PdC(CH2)N(2,6-Me2C6H3)(CHCHCOOMe)Cl. Kinetic measurements carried out on the norbornadiene insertion reactions with 1a,b, 2a, and 3a revealed that the reactions are first order in the palladium concentration and occur via a norbornadiene concentration-independent and dependent pathway.
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