Α-hydrogen Migration Research Articles

Structural and mechanistic investigation of a cationic hydrogen-substituted ruthenium silylene catalyst for alkene hydrosilation

The cationic ruthenium silylene complex [Cp*(iPr3P)Ru(H)2(SiHMes)][CB11H6Br6], a catalyst for olefin hydrosilations with primary silanes, was isolated and characterized by X-ray crystallography. Relatively strong interactions between the silylene Si atom and Ru–H hydride ligands appear to reflect a highly electrophilic silicon center. The mechanism of olefin hydrosilation was examined by kinetics measurements and other experiments to provide the first experimentally determined mechanism for the catalytic cycle. This mechanism involves a fast, initial addition of the Si–H bond of the silylene complex to the olefin. Subsequent elimination of the product silane produces an unsaturated intermediate, which can be reversibly trapped by olefin or intercepted by the silane substrate. The latter reaction pathway involves activation of the reactant silane by Si–H oxidative addition and α-hydrogen migration to regenerate the key silylene intermediate.

ChemInform Abstract: Carbene Formation, Hydrogen Migration, and Fluorescence in the Excited States of Dialkyldiazirines.

Photolysis of 3,3-dimethyldiazirine produces an electronically excited singlet state which decays by at least three competitive pathways: (1) fluorescence, (2) nitrogen extrusion and carbene formation, and (3) α-hydrogen migration and propylene formation concurrent with nitrogen extrusion. The temperature dependence of the fluorescence intensity was determined

A Hydrogen-Substituted Osmium Stannylene Complex: Isomerization to a Metallostannylene Complex via an Unusual α-Hydrogen Migration from Tin to Osmium

An osmium complex bearing a terminal hydrogen-substituted stannylene ligand, Cp*((i)Pr(3)P)(H)Os=SnH(trip) (1) (trip = 2,4,6-triisopropylphenyl), has been prepared by stannylene extrusion, and the complex has been structurally characterized. Complex 1 coordinates Lewis bases and activates the O-H bonds of water and methanol. Most interestingly, 1 converts to the metallostannylene complex Cp*((i)Pr(3)P)(H)(2)OsSn(trip) (2) thermally or photochemically by what appears to be a radical process.

Reaction of a Tungsten Alkylidyne Complex with a Chelating Diphosphine. α-Hydrogen Migration in the Intermediates and Formation of an Alkyl Alkylidene Alkylidyne Complex

The reaction of W(CH2SiMe3)3(≡CSiMe3) (1) with DMPE (DMPE = Me2PCH2CH2PMe2) gives an adduct W(CH2SiMe3)3(≡CSiMe3)(DMPE-P) (4a), which undergoes a rarely observed reversible transformation to its bis-alkylidene tautomer W(CH2SiMe3)2(═CHSiMe3)2(DMPE-P) (4b) through α-H migration. The DMPE ligands in both 4a and 4b contain a dangling P atom (P). Thermodynamic and kinetic studies of the 4a ⇌ 4b exchange show that it is slightly endothermic with ΔH° = 5.1(1.1) kcal/mol and ΔS° = 24(4) eu; ΔH1⧧ = 7.5(0.9) kcal/mol, ΔS1⧧ = −51(2) eu for the 4a → 4b forward reaction; ΔH−1⧧ = 2.0(0.8) kcal/mol, ΔS−1⧧ = −76(1) eu for the 4b → 4a reverse reaction. Activation entropies are the major contributors to the activation barriers of the exchange: ΔG1⧧278K = 21.7(1.5) kcal/mol and ΔG−1⧧278K = 23.1(1.1) kcal/mol. The 4a ⇌ 4b exchange at 283 K is faster than that of the previously reported PMe3 analogs W(CH2SiMe3)3(≡CSiMe3)(PMe3) (2a) ⇌ W(CH2SiMe3)2(═CHSiMe3)2(PMe3) (2b).(1) The 4a ⇌ 4b mixture undergoes an α-H abstraction, yie...

Methylidyne Complexes Prepared by Reactions of Laser-Ablated Os Atoms with Methane, Methyl Halides, and Ethane: Observation of the C—H and Os—H Stretching Absorptions

Simple Os carbyne complexes (HC≡OsH3, HC≡OsH2F, HC≡OsH2Cl, HC≡OsH2Br, and CH3C≡OsH3) are produced in reactions of laser-ablated Os atoms with small alkanes and methyl halides via oxidative C−H insertion and α-hydrogen migration. The diagnostic methylidyne C—H stretching absorptions of HC≡OsH3 and its halide derivatives are observed about 200 cm−1 above the precursor C—H stretching bands, and the frequencies are consistent with s character in hybridization in the C—H bond. The methylidyne complexes all have computed Cs structures, and particularly the HC≡OsH3 and CH3C≡OsH3 complexes have two equivalent shorter and one longer Os—H bonds, parallel to the case of Re in similar complexes. The strong unique Os—H stretching absorption at an exceptionally low frequency is traced to the stretching mode of the long Os—H bond and antibonding character in the doubly occupied HOMO.

Ab initio quantum mechanical investigation of the reaction mechanisms for the formation of from o -, m -, and p -chlorotoluene radical cations

The reaction pathways to (tolyl, benzyl, and tropylium) from o-, m-, and p-chlorotoluene radical cations have been studied at the SCF level of theory using the DZ and DZP basis sets. The equilibrium structures and relative energies for the local minima and transition states have been fully optimized and the harmonic vibrational frequencies have been evaluated to differentiate the local minimum and the transition state. The lowest barrier rearrangement pathway is the formation of the benzyl cation for all three isomers, corroborating the recent experimental findings of Kim and Shin. This benzyl channel involves [1,3] α-hydrogen migration at the entrance and the activation energy is calculated to be 40.3, 39.7, and 42.2 kcal mol–1 for the o-, m-, and p-isomer, respectively. The other competing rearrangement pathway involves [1,2] α-hydrogen migration at the entrance, which leads to the tropylium ion, and the activation barrier is predicted to be 55.5, 54.2, and 56.62 kcal mol–1 for the o-, m-, and p-isomer, respectively. The highest barrier process involves direct C–Cl bond cleavage that yields the tolyl cation with a 0 K dissociation energy of 61.6, 64.6, and 68.92 kcal mol–1 for the o-, m-, and p-isomer, respectively.

Synthetic Development and Chemical Reactivity of Transition‐Metal Silylene Complexes

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Methane and methyl halide activation by Sc atoms: Infrared spectra and DFT calculations for the CH 3–ScX and CH 2–ScHX complexes

Reactions of laser-ablated scandium atoms with methane and methyl halides have been done during condensation in excess argon, and matrix infrared spectra were recorded for the products. Scandium is as effective as titanium in producing insertion products (CH 3–ScX, X = H, F, Cl, Br) and higher oxidation-state methylidene complexes (CH 2–ScHX) following α-hydrogen migration. However, unlike the Group 4–6 metal complexes, the Group 3 methylidene complexes do not show agostic distortion, and the computed C–Sc bond lengths of the methylidene complexes are slightly shorter than those for the insertion products. This shows that carbon–transition metal bond lengths in the double bond range are needed to support the agostic interaction. The computed spin densities are consistent with weak C(p)–Sc(d) π-bonding in the methylidene complexes.

Synthetic Development and Chemical Reactivity of Transition-Metal Silylene Complexes

A variety of transition-metal complexes with terminal silylene ligands have become available in recent years, because of the discovery of several preparative methods. In particular, three general synthetic routes to these complexes have emerged, on the basis of anionic group abstraction, coordination of a free silylene, and alpha-hydrogen migration. The direct transformation of organosilanes to silylene ligands at a metal center (silylene extrusion) has also been observed, and this has further spurred the exploration of silylenes as ligands. This Account describes the synthetic development of silylene ligands in our laboratory and resulting investigations of stoichiometric and catalytic chemistry for these species.

Formation of CH3TiX, CH2TiHX, and (CH3)2TiX2 by Reaction of Methyl Chloride and Bromide with Laser-Ablated Titanium Atoms: Photoreversible α-Hydrogen Migration

The simple methylidene (CH2=TiHX) and Grignard-type (CH3TiX) complexes are produced by reaction of methyl chloride and bromide with laser-ablated Ti atoms and isolated in a solid Ar matrix, and they form a persistent photoreversible system via alpha-hydrogen migration between the carbon and titanium atoms. The Grignard-type product is transformed to the methylidene complex upon UV (240 nm < lambda < 380 nm) irradiation and vice versa with visible (lambda > 530 nm) irradiation. More stable dimethyl dihalide complexes [(CH3)2TiX2] are also identified, whose relative concentration increases upon annealing and at high methyl halide concentration. The reaction products are identified with three different groups of absorptions on the basis of the behaviors upon broadband photolysis and annealing, and the vibrational characteristics are in a good agreement with DFT computation results.

Hydride Is Not a Spectator Ligand in the Formation of Hydrido Vinylidene from Terminal Alkyne and Ruthenium and Osmium Hydrides: Mechanistic Differences

RuHX(H2)L2 (L = PtBu2Me; X = Cl, I) and OsH3ClL2 (L = PiPr3) react (time of mixing) with terminal alkynes in a 1:2 stoichiometry to give MHX(CCHR)L2 and RCHCH2. The reactions of RuHX(H2)L2 with PhC⋮CD lead to RuDX(CCHPh)L2 as the only isotopomers. This result is understood in terms of an insertion of the alkyne in the Ru−H bond to make a vinyl, followed by an α-hydrogen migration, giving the hydrido/vinylidene. This reaction path has been studied by ab initio (B3LYP) calculations with location of the minima and transition states. In agreement with isotopic labeling experiments, it is shown that the path starting with insertion into the Ru−H bond is more favorable than a 1,2-migration within the coordinated alkyne ligand. The favored reaction is calculated to be exothermic from the starting compounds to intermediates and to products with the energies of all transition states well below that of the separated reactants. The mechanism involves the formation of a 14-electron η1-vinyl intermediate which, after ...

Osmium Converts Terminal Olefins to Carbynes: α-Hydrogen Migration Redox Isomers with Reversed Stability for Ruthenium and for Osmium

Os(H)2Cl2L2 (L = PiPr3) reacts with propylene or styrene to give equimolar amounts of OsHCl2(CCH2R)L2 and the hydrogenated olefin; these molecules are isomeric with the ruthenium carbenes RuCl2[C(H)CH2R]L2, yet these distinct redox alternatives are thermodynamically preferred. Ab initio (B3LYP) calculations show that the unsaturated five-coordinate MCl2(CHMe)(PH3)2 is more stable than the saturated hexacoordinate MCl2H(CMe)(PH3)2 for M = Ru, while the two species are almost isoenergetic in the case of Os; computationally, it is found that the osmium hydrido carbyne involves a large activation energy (27.2 kcal/mol) to transform unimolecularly to its carbene isomer.

Synthesis of Fischer Carbene Complexes of Iridium by C−H Bond Activation of Methyl and Cyclic Ethers: Evidence for Reversible α-Hydrogen Migration

We report here a mild and versatile route to Fischer carbene complexes of iridium via the activation of C-H bonds of methyl and cyclic ethers, along with our preliminary studies of this rare family of carbene complexes. Theoretical studies suggest that {alpha}-hydrogen migrations can be kinetically favorable if a coordinatively unsaturated species can be accessed. Thus, the lability of the triflate ligand presumably facilitates this process. Further evidence for the rapidity, as well as reversibility, of this rearrangement was obtained by NMR analysis. 20 refs.

Carbene formation, hydrogen migration, and fluorescence in the excited states of dialkyldiazirines

Photolysis of 3,3-dimethyldiazirine produces an electronically excited singlet state which decays by at least three competitive pathways: (1) fluorescence, (2) nitrogen extrusion and carbene formation, and (3) α-hydrogen migration and propylene formation concurrent with nitrogen extrusion. The temperature dependence of the fluorescence intensity was determined

The direct and photosensitized decomposition of alkyl azides

The products of photolysis of n-butyl, n-amyl, 4-heptyl, 2-phenylethyl, 3-phenylpropyl, 4-phenylbutyl, phenylmethyl, 1-phenylbutyl, 1,1-diphenylethyl, and cyclohexyl azides were determined. Reactions were carried out to 5 % completion in 0·10 molar benzene solution. The primary products of reaction are nitrogen and imines derived from hydrogen, alkyl or aryl migration to nitrogen. Corrected for the statistical factor the migratory order is H:R, 5:1; φ:H, 1:1; φ:R, 1:1. For cyclohexyl aride only α-hydrogen migration occurs. The rate of nitrogen evolution was essentially the same for all the azides studied. The product distribution is unaffected by the presence of oxygen. Efficient photosensitization was achieved using chrysene and an appropriate filter system. The product ratios were unchanged in the photosensitized decomposition compared to the direct photolysis. The results of this study are explained in terms of very shortlived singlet and triplet electronically excited azides. Evidence is summarized in favor of a closely synchronous migration of the α-substituent with the departure of molecular nitrogen from the excited azide. Conformational factors operative in the ground states of the alkyl azides may determine the migratory ratios and this possibility is discussed and evaluated.