Alkyl Elimination Research Articles

Pd0‐Catalyzed Asymmetric Carbonitratation Reaction Featuring an H‐Bonding‐Driven Alkyl−PdII−ONO2 Reductive Elimination

AbstractReductive elimination of alkyl−PdII−O is a synthetically useful yet underdeveloped elementary reaction. Here we report that the combination of an H‐bonding donor [PyH][BF4] and AgNO3 additive under toluene/H2O biphasic system can enable such elementary step to form alkyl nitrate. This results in the Pd0‐catalyzed asymmetric carbonitratations of (Z)‐1‐iodo‐1,6‐dienes with (R)‐BINAP as the chiral ligand, affording alkyl nitrates up to 96 % ee. Mechanistic studies disclose that the reaction consists of oxidative addition of Pd0 catalyst to vinyl iodide, anion ligand exchange between I− and NO3−, alkene insertion and SN2‐type alkyl−PdII−ONO2 reductive elimination. Evidences suggest that H‐bonding interaction of PyH⋅⋅⋅ONO2 can facilitate dissociation of O2NO− ligand from the alkyl−PdII−ONO2 species, thus enabling the challenging alkyl−PdII−ONO2 reductive elimination to be feasible.

Iron-Catalyzed Cross-Electrophile Coupling of Inert C–O Bonds with Alkyl Bromides

Iron-Catalyzed Cross-Electrophile Coupling of Inert C–O Bonds with Alkyl Bromides

Rare-Earth-Metal Complexes Bearing an Iminodibenzyl-PNP Pincer Ligand: Synthesis and Reactivity toward 3,4-Selective Polymerization of 1,3-Dienes.

A PNP-pincer ligand provides a versatile ligation framework, which is highly useful in organometallic chemistry and catalytic chemistry. In this work, by a de novo strategy, a simple and efficient synthetic pathway, has been developed to prepare the new iminodibenzyl-based PNP pincer proligand imin-RPNP(Li or H) (R = isopropyl, phenyl). By employing salt metathesis or direct alkyl elimination, we successfully synthesized a series of iminodibenzyl-PNP rare-earth-metal (Ln = Sc, Y, Dy, Ho, Er, Tm, Lu) complexes and characterized them by NMR and X-ray diffraction analyses. Upon addition of a borate and triisobutylaluminum (TIBA), the rare-earth-metal complexes 2-Y, 2-Dy, 2-Ho, 2-Er, and 2-Tm bearing the imin-PhPNP ligand exhibited unexpectedly high 3,4-selectivity (up to 95%) for the polymerization of 1,3-dienes (isoprene and myrcene); in particular, the chosen yttrium complex 2-Y promoted the 1,3-diene polymerization in a living manner. A computational study suggested that the sterically congested configuration around the metal center imposed by the imin-RPNP ligand might be the main reason for this unusual selectivity.

Nickel-Templated Replacement of Phosphine Substituents in a Tetradentate Bis(amido)bis(phosphine) Ligand.

The replacement of phosphine substituents in nickel-bound PNNP ligands is reported as an alternative method for preparing multidentate phosphine ligands with alkyl substituents. Treatment of the previously reported bis(phosphide) complex {K(THF)x}22Ph[PNNP]Ni (2) with 2 equiv of MeI, iPrI, and 1,3-dibromoethane formed alkyl-substituted complexes 2Ph,2Me[PNNP]Ni (3), 2Ph,2iPr[PNNP]Ni (4), and 2Ph,propylene[PNNP]Ni (5), respectively. The stereoselectivity (racemic vs meso) of these reactions can be controlled by varying the reaction temperature. The racemic mixtures of products with the new alkyl substituents in an anti configuration were favored at lower temperatures, whereas a larger proportion of meso compounds was acquired at higher temperatures. Further treatment of 3 with KH resulted in selective elimination of the remaining phenyl groups rather than the methyl substituents, affording bis(methylphosphide) complex {K(THF)x}22Me[PNNP]Ni (6). Subsequent treatment of 6 with additional MeI formed 4Me[PNNP]Ni (7), in which all four phenyl groups were replaced with methyl substituents. As a proof of concept, demetalation of the ligand from 7 was achieved using aqueous KCN to form a free dimethylphosphine-substituted ligand H24Me[PNNP] (8), and 8 was subsequently coordinated to a different metal, using PdCl2 to form 4Me[PNNP]Pd (9). Unlike the clean elimination of phenyl substituents from 3, the reactions of KH with 4 and 5 exhibited competitive elimination of both alkyl and phenyl substituents and/or attenuated reactivity.

Selenocysteine as a Latent Bioorthogonal Electrophilic Probe for Deubiquitylating Enzymes.

Deubiquitylating enzymes (DUBs) remove ubiquitin (Ub) from various cellular proteins and render eukaryotic ubiquitylation a dynamic process. The misregulation of protein ubiquitylation is associated with many human diseases, and there is an urgent need to identify specific DUBs associated with therapeutically relevant targets of Ub. We report the development of two facile selenocysteine-based strategies to generate the DUB probe dehydroalanine (Dha). Optimized oxidative or alkylative elimination of Se yielded Dha at the C-terminus of Ub. The high utility of alkylative elimination, which is compatible with multiple thiols in Ub targets, was demonstrated by generating a probe derived from the Ub ligase tripartite motif protein 25 (TRIM-25). Successful capture of the TRIM-25-associated DUB, ubiquitin-specific protease 15, demonstrated the versatility of our chemical strategy for identifying target-specific DUBs.

Markovnikov-Selective Hydrosilylation of Electron-Deficient Alkenes with Arylsilanes Catalyzed by Mono(phosphine)palladium(0)

Markovnikov-selective hydrosilylation of electron-deficient alkenes with HSiPh3 is catalyzed by a mono(phosphine)palladium(0) complex, Pd(η2:η2-C6H10O)(PMe3) (1a). The hydrosilylation of acrylonitrile with HSiPh3 at 30 °C proceeds to completion within 40 min in the presence of 5 mol % of 1a. The complex 1a also shows the catalytic activity for the hydrosilylation with mono- and diarylsilanes and monochlorosilane such as HSiPhMe2, HSiPh2Me, and HSiClMe2. The hydrosilylation using para-substituted styrenes clearly shows the electron-withdrawing substituent promoting the reaction. Mechanistic studies indicate that the reaction is proceeding by a Chalk–Harrod mechanism with the reductive elimination of an (alkyl)(silyl)palladium(II) intermediate being the rate-determining step.

Characterization of a B₁₂trafficking chaperone protein from caenorhabditis elegans.

The human B₁₂trafficking chaperone protein hCblC is responsible for escorted delivery and early processing of B₁₂in intracellular B12 metabolism. In this study, we characterized a putative B₁₂trafficking chaperone of Caenorhabditis elegans (cCblC), which shows 26% amino acid sequence identity with hCblC. cCblC was shown to bind B₁₂with a broad specificity for the upper axial ligand, as previously observed with other homologous proteins. In addition, cCblC catalyzed glutathione (GSH)-dependent elimination of alkyl and GSH upper axial ligands from alkylcobalamins and glutathionylcobalamin (GSCbl), respectively. Dealkylation of methylcobalamin (MeCbl) generated cob(II)alamin with S-methylglutathione. Cob(I)alamin was detected as the intermediate for cob(II)alamin generation, indicating that the reaction is a nucleophilic displacement using the thiolate of GSH. Deglutathionylation of GSCbl also generated cob(II)alamin, via cob(I)alamin intermediate, with glutathione disulfide, indicating the reaction is chemically analogous with dealkylation. Cob(II)alamin generated by dealkylation and deglutathionylation was bound to cCblC in the base-off state and stable under aerobic conditions, which would be favorable for subsequent enzyme cofactor synthesis. These results demonstrate that cCblC is a B₁₂trafficking chaperone of C. elegans catalyzing dealkylation and deglutathionylation via a nucleophilic displacement using the thiolate of GSH.

Fragmentation mechanisms in mass spectrometry of Chemical Weapons Convention related spiro alkylphosphonates and alkyldioxaphosphinane oxides

The availability of mass spectra and interpretation skills are essential for unambiguous identification of the Chemical Weapons Convention (CWC)-related compounds. This paper examines electron ionization (EI) and electrospray ionization (ESI) mass spectral fragmentation routes of spiro alkylphosphonates I and 5,5-bis(chloromethyl)-2-alkyl-1,3,2-dioxaphosphinane 2-oxides II as compounds which are covered under CWC schedule 2.B.4. Mass spectrometric studies revealed some fragmentation pathways, such as elimination of alkyl(oxo)phosphane oxide (RPO2), chlorine, chloromethylene, alkene, HCl, and H2O, α-cleavage and McLafferty-type and hydrogen rearrangements. The proposed fragmentation processes include some new fragmentation patterns, such as isomerization of cations to stabilized carbocations and oxocarbocations, and elimination of formaldehyde and alkoxy through concerted retro [2+2] cycloaddition reaction and 1,2 PO alkyl shift. Structures of fragments were confirmed using EI-MS and MS/MS analysis of the deuterated analogs. The results will make a contribution to the Organization for the Prohibition of Chemical Weapons (OPCW) Central Analytical Database (OCAD) which may be used for the detection and identification of CWC-related compounds during on-site inspection and OPCW proficiency tests.

A new anilido-imine compound containing o-OMe-anilinyl derived from an unexpected adduct: Synthesis, crystal structure and its coordination capability

A new compound, ortho-C6H4F[CH(NHC6H4OMe-2)2], 1, was obtained with ortho-flurobenzaldehyde and 2-methoxyaniline as the starting materials. Compound 1 was readily converted into ortho-C6H4(2-OMeC6H4)(CHNC6H4OMe-2) 2 after treatment with 1 equiv of n-BuLi. Treatment of compound 2 with 1.5 equiv of ZnEt2 afforded the trinuclear zinc complex 3 by alkyl elimination and alkylation of the imino group of the ligand. The molecular structures of two new organic compounds and of the trinuclear zinc complex were determined by single-crystal X-ray diffraction. The dianionic ONNO tetradentate ligands derived from compound 2 coordinate to zinc ions in four to five coordination modes, forming distorted tetrahedral and trigonal–bipyramidal geometry around three metal centers.

ChemInform Abstract: A One‐Pot Stereoselective Synthesis of Electron‐Deficient 4‐Substituted (E,E)‐1‐Arylsulfonylbuta‐1,3‐dienes and Their Chemoselective [3 + 2] Cycloaddition with Azomethine Ylides — A Simple Synthesis of 1,3,4‐Trisubstituted Pyrrolidines and Pyrroles.

AbstractSubstrates (I) undergo alkylative elimination upon treatment with bromides (II) to afford (E,E)‐1,3‐dienes (III).

Oxidation by Oxygen and Sulfur of Tin(IV) Derivatives Containing a Redox‐Active o‐Amidophenolate Ligand

Oxidation of tin(IV) o-amidophenolate complexes [Sn(ap)Ph(2)] (1) and [Sn(ap)Et(2)(thf)] (2) (ap=dianion of 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzoquinone (ImQ)) with molecular oxygen and sulfur in toluene solutions was investigated. The reaction of oxygen with 1 at room temperature forms a paramagnetic derivative [Sn(isq)(2)Ph(2)] (3) (isq=radical anion of ImQ) and diphenyltin(IV) oxide [{Ph(2)SnO}(n)]. Interaction of 1 with sulfur gives another monophenyl-substituted paramagnetic tin(IV) complex, [Sn(ap)(isq)Ph] (4), and the sulfide, [Ph(3)Sn](2)S. The oxidation of 2 with oxygen and with sulfur proceeds through the derivative [Sn(isq)(2)Et(2)] (7), which undergoes alkyl elimination to give two new tin(IV) compounds, [Sn(ap)(isq)Et] (5) and [Sn(ap)(EtImQ)Et] (6) (EtImQ=2,4-di-tert-butyl-6-(2,6-diisopropylphenylimino)-3-ethylcyclohexa-1,4-dienolate ligand), respectively, along with the corresponding alkyltin(IV) oxide and sulfide. Complexes 3-5 and 7 were studied by EPR spectroscopy. The structures of 3, 4 and 6 were investigated by X-ray analysis.

Conversions of Ruthenium(III) Alkyl Complexes to Ruthenium(II) through Ru−Calkyl Bond Homolysis

Single-electron oxidation of the Ru(II) complexes TpRu(L)(L‘)(R) (L = CO, L‘ = NCMe, and R = CH3 or CH2CH2Ph; L = L‘ = PMe3 and R = CH3) with AgOTf leads to alkyl elimination reactions that produce TpRu(L)(L‘)(OTf) and organic products that likely result from Ru−Calkyl bond homolysis. Density functional calculations on TpRu(CO)(NCMe)(Me) and its Ru(III) cation indicate that the Ru−CH3 homolytic bond dissociation enthalpy is substantially reduced (48.6 to 23.2 kcal/mol) upon oxidation.

C-Centred chiral metal alkyls, [MR*(Cl)(η-C5H5)2], rac- and meso-[ZrR*2(η-C5H5 )2] [R* = CH(SiMe3)C6H4 Me-o], and their one-electron reductions; reaction of Li[CH(SiMe3)2](Me2NCH2CH 2NMe2) with [MCl2(η-C5H5)2] (M = Zr or Hf )

The chloro(alkyl)metallocenes [MR*(Cl)(η-C 5 H 5 ) 2 ] [R* = - CH(SiMe 3 )C 6 H 4 Me-o; M = Zr 1 or Hf 2] have been prepared from [MCl 2 (η-C 5 H 5 ) 2 ] and LiR*(tmen) (tmen = N,N,N′,N ′-tetramethylethane-1,2-diamine); further alkylation occurred only for M = Zr, affording rac-3a and meso-3b, [ZrR* 2 (η-C 5 H 5 ) 2 ]. In contrast, reaction of 2 equivalents of Li[CH(SiMe 3 ) 2 ](tmen) with [ZrCl 2 (η-C 5 H 5 ) 2 ] yielded an alkyl elimination product possessing µ-η 1 ∶η 5 -C 5 H 4 2- ligands. Compound 1 was reversibly reduced (E 1/2 red = -1.72 V vs. saturated calomel electrode, SCE) whereas 2, 3a or 3b were irreversibly reduced (E red = -2.12, -2.08, -2.00 V respectively vs. SCE). Thermolysis of 3b in toluene gave 3a (80 °C) or ‘Zr(C 10 H 8 )’ (120 °C). Photolysis of 3a or 3b at 20 °C in tetrahydrofuran (thf) yielded a 1∶1 mixture of 3a and 3b; a d 1 intermediate has been unambiguously identified as [ZrR*(η-C 5 H 5 ) 2 ]. Reduction of 1 (Na–Hg), or 3a and 3b (Na[C 10 H 8 ]), in thf also gave [ZrR*(η-C 5 H 5 ) 2 ] but this slowly transformed into [ZrR*(η-C 5 H 5 ) 2 (thf)], and 1 with Na[C 10 H 8 ] and PPh 3 gave [ZrR*(η-C 5 H 5 ) 2 (PPh 3 )]. Reduction of 1, 3a or 3b, or photolysis of 3a or 3b, in the presence of PMe 3 gave [ZrR*(η-C 5 H 5 ) 2 (PMe 3 )]. Crystal structure determinations showed a slightly more crowded metal environment in 2 than in 1, consistent with the view that dialkylation in 2 is limited on steric grounds; M–σ-C 2.359(4) (1), 2.322(8) (2), metal–centroid 2.22 (1), 2.21 (2), M–Cl 2.444(1) (1), 2.418(3) (2) A. The structure of 3a confirms the rac assignment, M–σ-C 2.37 4 A, metal–centroid 2.23, 2.24 A.

Alkylative elimination of α,β-epoxy tosylhydrazones

Optically pure allyl alcohols have been prepared from tosylhydrazones derived from chiral epoxy aldehydes by alkylative elimination utilizing alkyl magnesium reagents.

ChemInform Abstract: Carbon‐Hydrogen and Carbon‐Carbon Bond Activation with Highly Electrophilic Transition Metal Complexes

Highly electron deficient scandocene derivatives of the types (q5-CsMe&ScR, {(q5-CsMe4)2SiMe2}ScR and {(q5-C5H3CMe3)2SiMe2}ScR catalyze the polymerization of ethylene, the head-to-tail dimerization of a olefins, the cyclization of a,w dienes to meth lene cycloalkanes, and the branching of 1,4 pentadienes to isoprenes. The mechanisms oythe individual steps have been studied. Key steps involve sequential and reversible olefin insertionlb H eliminationlb alkyl elimination, the last of which is particularly facile in these sys!ems. (((q5-C5Me4)Me2Si(qi-NCMe3)(PMe3)Sc(p-H))2, catalyzes the polymerization of a olefins. Evidence is presented in support of a well defined, one component catalyst system with all scandium centers functioning alike.

Carbon-hydrogen and carbon-carbon bond activation with highly electrophilic transition metal complexes

Abstract

The composition dependence of GaInAs grown by organometallic epitaxy

In this paper we describe a systematic study of the parameters affecting the composition of Ga 1− x In x As grown from trimethylgallium, triethylindium and arsine. Reactor pressure, reactant partial pressures, substrate temperature ans the susceptor slope were varied to determine the depletion of indium species as a function of these parameters. A two-step model is proposed, with the second step involving two parallel paths. In the first step, the triethylindium reacts with the arsine to form an adduct, which is transported to the hot zone of the reactor. There it dissociates to liberate triethylindium. Simultaneously, however, some adduct molecules decompose via alkyl elimination in the gass phase resulting in a loss of indium to the growth process. A bimolecular alkyl elimination reaction is proposed, consistent with the change in composition with reactor parameters. Conditions for compositionally uniform growth were determined, and uniform composition (within ±0.005) achieved over the entire 2.5 cm×1.5 cm face of the susceptor in a 50 mm ID reaction chamber.

ChemInform Abstract: Alkyl Elimination from Aldehydes on Zinc Oxide: Relevance to Allylic Oxidation Pathways

Abstract The reactions of acetaldehyde, propionaldehyde, and acrolein on the (0001)-Zn polar surface of zinc oxide were investigated using temperature-programmed desorption and X-ray photoelectron spectroscopy. Oxidation of these aldehydes on the (0001)-Zn surface proceeded via two competing pathways: nucleophilic addition of lattice oxygen at the carbonyl carbon followed by hydride elimination to form the corresponding surface carboxylate species, and nucleophilic attack of lattice oxygen at the carbonyl carbon followed by alkyl elimination to form surface formate species. There was a strong temperature dependence of the selectivity toward these two competing pathways; low temperatures favored alkyl elimination, while high temperatures favored hydride elimination. The decomposition of the higher carboxylates resulted in the formation of both unsaturated carbonyl compounds and carbon oxides. The possible implications of these results for allylic oxidation reactions on metal oxide catalysts are discussed.

Alkyl elimination from aldehydes on zinc oxide: Relevance to allylic oxidation pathways

The reactions of acetaldehyde, propionaldehyde, and acrolein on the (0001)-Zn polar surface of zinc oxide were investigated using temperature-programmed desorption and X-ray photoelectron spectroscopy. Oxidation of these aldehydes on the (0001)-Zn surface proceeded via two competing pathways: nucleophilic addition of lattice oxygen at the carbonyl carbon followed by hydride elimination to form the corresponding surface carboxylate species, and nucleophilic attack of lattice oxygen at the carbonyl carbon followed by alkyl elimination to form surface formate species. There was a strong temperature dependence of the selectivity toward these two competing pathways; low temperatures favored alkyl elimination, while high temperatures favored hydride elimination. The decomposition of the higher carboxylates resulted in the formation of both unsaturated carbonyl compounds and carbon oxides. The possible implications of these results for allylic oxidation reactions on metal oxide catalysts are discussed.

Mass spectrometric study of the pyrolysis of organometallic precursors usable in GaAs vapor phase epitaxy: II. The diarsine adducts [C1R 2GaAsEt 2] 2CH 2 ( R = Me, Et)

The thermal decomposition of adducts [C1R 2GaAsEt 2] 2CH 2 ( R = Me, Et) in helium and hydrogen atmosphere has been investigated by mass spectrometry. Using a simple analytical procedure, this paper reveals the importance of the alkyl elimination process in epitaxial growth. Hydrogen would favor homolytic fissions of M-Et bonds, whereas a β elimination would be dominant at higher temperatures. The first mechanism seems preferable in low temperature GaAs OMVPE to a β elimination which generates both hydride intermediates and a polycrystalline growth if the deposition temperature is not sufficiently high. Selection of useful new precursors requires thermolabile groups and must take in account these competitive alkyl elimination mechanisms.