Methylene Migration Research Articles

Forked Vein Structure W/WO3- x with Dual Active Sites in W and Oxygen Vacancies to Enhance Methylene Self-Coupling for Efficient Conversion of Methane to Ethylene.

The directional conversion of methane to ethylene is challenging due to the dissociation of the C─H bond and the self-coupling of methyl intermediates. Herein, a novel W/WO3- x catalyst with the fork vein structure consisting of an alternating arrangement of WO3- x and W is developed. Impressively, the catalyst achieves an unprecedented C2H4 yield of 1822.73µmolg-1h-1, with a selectivity of 82.49%. The enhanced catalytic activity is ascribed to the multifunctional synergistic effect induced by oxygen vacancies and W sites in W/WO3- x. Oxygen vacancies provide abundant coordination of unsaturation sites, which promotes the adsorption and activation of CH4, thus reducing the dissociation energy barrier of the C─H bond. The CH2 coupling barrier on the metal W surface is significantly lower compared to WO3, so CH2 can migrate to the W site for coupling. Importantly, the W/WO3- x with high periodicity provides multiple ordered local microelectric fields, and CH2 intermediates with dipole moments undergo orientation polarization and displacement polarization driven by the electric field, thus enabling CH2 migration. This work opens a new avenue for the structural design and modulation of photocatalysts, and provides new perspectives on the migration of methylene between multiple active sites.

Chiral Counteranion-Directed Catalytic Asymmetric Methylene Migration Reaction of Ene-Aldimines.

A catalytic asymmetric methylene migration reaction of ene-aldimines directed by chiral counteranions is developed, with the optimal catalyst identified as phenanthryl-substituted (R)-BINOL phosphate. Control experiments and density functional theory computations reveal the importance of the 2-hydroxy group of the ene-aldimine and attractive (e.g., OH···O, CH···O, CH···π, and π···π) interactions for high enantioselectivity (up to 74% ee). The results contribute to the design of asymmetric catalysis for the rearrangement of highly reactive iminium intermediates.

Photochemical Reactivity of 1,6-Methano[10]annulene.

1,6-Methano[10]annulene solutions in cyclohexane have been subjected to continuous and pulsed UV irradiation. Photolysis occurs in both cases, giving naphthalene as a minor and major product, respectively. The wavelength dependence of the reaction in solution indicates that the photochemical process occurs, exciting 1,6-methano[10]annulene in the second and third singlet electronic excited states. The reaction kinetics has been determined under pulsed irradiation. From the time dependence of concentrations, along with support of density functional theory calculations and early published data, two mechanisms are proposed for naphthalene production. Reaction steps such as direct migration of the bridging methylene of 1,6-methano[10]annulene to cyclohexane and 1,6-methano[10]annulene isomerization to benzotropilidene have been identified. The calculated energy diagrams relative to the ground and lowest excited states allow one to relate these steps to processes such as electrocyclic closure and sigmatropic shift. The norcaradienic form of 1,6-methano[10]annulene results in the critical species for methylene migration and the sigmatropic [1,5] shift. The present results and those arising from photolysis in the gas phase are good examples of the photochemical reactivity of 1,6-methano[10]annulene.

ChemInform Abstract: Triflic Acid‐Promoted Cycloisomerization of 2‐Alkynylphenyl Isothiocyanates and Isocyanates: A Novel Synthetic Method for a Variety of Indole Derivatives.

AbstractThe TfOH‐mediated cycloisomerization of isocyanate (VII) leads to furoindole (VIII), whilst reactions of substrates (I), (III), and (IX) are accompanied by a methyl or methylene migration.

A new insight on the gas phase retro-Diels–Alder reaction of bicyclic compounds: density functional theory calculations

The mechanisms of the gas-phase thermal decomposition of bicyclo[2.2.1]heptadiene and 3,7,7-trimethylbicyclo[2.2.1]hept-2-ene were examined by density functional theory calculations with the hybrid functionals: B3LYP, CAM-B3LYP, MPW1PW91, and PBEPBE. Reasonable agreements were found between theoretical and experimental values with the B3LYP hybrid functional. Three molecular concerted pathways for bicyclo[2.2.1]heptadiene decomposition are proposed. The retro-Diels–Alder (retro-DA) pathway yields cyclopentadiene and acetylene through a nearly synchronous transition state structure (Sy = 0.97). The other two reaction channels are stepwise with a common step with the formation of the intermediate bicyclo[4.1.0] heptadiene. This reaction is dominated by C–C bond breaking leading to the methylene migration by an early transition state in the reaction coordinate (Sy = 0.91). The rearrangements of the latter intermediate producing toluene were also studied. The retro-DA elimination of 3,7,7-trimethylbicyclo[2.2.1]hept-2-ene gives 1,5,5-trimethyl-cyclopenta-1,3-diene in a less synchronous process (Sy = 0.77). This fact may be due to the electronic effects of the methyl substituent. The latter product is unstable and undergoes methyl migrations to give a more stable isomer 1,2,3-trimethylcyclopenta-1,3-diene. The stepwise mechanism for the retro-DA reaction through a biradical intermediate appears to be unfavourable because the barrier is bigger than that for the concerted reaction.

Methylene migration and coupling on a non-reducible metal oxide: The reaction of dichloromethane on stoichiometric α-Cr2O3(0001)

The reaction of CH2Cl2 over the nearly-stoichiometric α-Cr2O3(0001) surface produces gas phase ethylene, methane and surface chlorine adatoms. The reaction is initiated by the decomposition of CH2Cl2 into surface methylene and chlorine. Photoemission indicates that surface cations are the preferred binding sites for both methylene and chlorine adatoms. Two reaction channels are observed for methylene coupling to ethylene in temperature-programmed desorption (TPD). A desorption-limited, low-temperature route is attributed to two methylenes bound at a single site. The majority of ethylene is produced by a reaction-limited process involving surface migration (diffusion) of methylene as the rate-limiting step. DFT calculations indicate the surface diffusion mechanism is mediated by surface oxygen anions. The source of hydrogen for methane formation is adsorbed background water. Chlorine adatoms produced by the dissociation of CH2Cl2 deactivate the surface by simple site-blocking of surface Cr3+ sites. A comparison of experiment and theory shows that DFT provides a better description of the surface chemistry of the carbene intermediate than DFT+U using reported parameters for a best representation of the bulk electronic properties of α-Cr2O3.

Physical Constraints on Sesquiterpene Diversity Arising from Cyclization of the Eudesm-5-yl Carbocation

The biogenic origins of complex cyclic terpenes derive from the interplay of enzymes and the intrinsic reactivity of carbocation species at major branch-points along intramolecular cyclization pathways to ultimately determine the distribution of terpene skeletal types in nature. Solanaceous plants biosynthesize chemical defense compounds, largely derived from the eremophilane and spirovetivane-type sesquiterpenes. These hydrocarbon skeletons share a common biogenic origin, stemming from alternative Wagner-Meerwein rearrangements of the eudesm-5-yl carbocation during the cyclization of farnesyl pyrophosphate (FPP) catalyzed by sesquiterpene synthases. While the spirojatamane skeleton shares the same carbocation intermediate, this class of sesquiterpenes has not been reported in the Solanaceae and is exceedingly rare in nature. To investigate the physical basis for alternative rearrangements of the eudesm-5-yl carbocation, we carried out quantum mechanics (QM) analyses to calculate the allowable conformations, energies, and transition states linking conformers of the eudesm-5-yl carbocation to the eremophilene, spirovetivane, and spirojatamane skeletons. Additionally, we conducted parallel investigations on simplified decalin carbocation systems to examine the contribution of ring substituents to allowable conformations and rearrangement pathways. Our study reveals that ring substituents expand the conformational space accessible to the eudesm-5-yl carbocation while sterically blocking rearrangements in certain contexts. From our analysis, we define a conformational threshold for each possible rearrangement based on dihedral angles describing transition state geometry. Further, our calculations indicate that methylene migration rearrangements leading to spiro compounds are thermodynamically dominant in the eudesm-5-yl and simpler decalin carabocation systems. Interestingly, the theoretical abundance of sesquiterpene skeletal types arising from the intrinsic reactivity of the eudesm-5-yl carbocation stands in sharp contrast to their currently known natural abundance. The implications of these results for the catalytic tragectories catalyzed by sesquiterpene synthases are discussed.

Catalytic and Regioselective Ring Expansion of Arylcyclobutanones with Trimethylsilyldiazomethane. Ligand-Dependent Entry to β-Ketosilane or Enolsilane Adducts

Divergent reactivity is uncovered in the homologation of arylcyclobutanones with trimethylsilyldiazomethane. With Sc(OTf)(3) as catalyst, enolsilanes are obtained with a high preference for methylene migration. By contrast, Sc(hfac)(3) gives beta-ketosilanes with both regio- and diastereocontrol. Each adduct affords the cyclopentanone upon hydrolysis.

Influence of co-adsorbates on the methylene coupling reaction on Ag(1 1 1)

The influence of co-adsorbates on the methylene coupling reaction on Ag(1 1 1) was investigated by means of reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed reaction spectroscopy (TPRS). Generated by thermal decomposition of adsorbed CH 2I 2, CH 2(a) selectively self-couples to form ethylene. The desorption of ethylene resulted from methylene coupling reaction shows an interesting coverage-dependent, which on basis of the RAIRS and TPD results, is attributed to both the existence of co-adsorbates on the surface and the coverage-dependent C–I rupture of CH 2I 2(a). The reaction kinetics of methylene coupling reaction is also affected by the co-existence of CF 3I(a) on the surface. It is the migration and/or assembly of methylene on the surface, not the actual methylene coupling reaction, which is influenced by the co-adsorbates.

Silicon-guided rearrangement of 10-methyl-4,5-epoxydecalins. Methyl versus methylene migration

The Lewis acid-promoted rearrangement of two 10-methyl-4,5-epoxydecalins bearing a trimethylsilyl (TMS) group on C-1 or C-9 has been studied. Migration of the C-9 methylene group to C-5 is the major reaction pathway when the TMS and the oxirane groups are on the same ring while methyl migration results exclusively when they are on different rings.

Regioselective Functionalization. 7.1 Unexpected Preferences for Bridgehead Migration in Schmidt Rearrangement Syntheses of Novel 2,6-Diazabicyclo[3.2.x]alkan-3-ones (x = 1−3)

Regioselective syntheses of 2,6-diazabicyclo[3.2.x]alkan-3-ones (x = 1−3) 34 by insertion of nitrogen next to the bridgehead (BH) of 2-azabicyclo[2.2.x]alkanones 32 with hydroxylamine-O-sulfonic acid are described. The ketones 32 under Schmidt reaction conditions (HN3/H2SO4) afford major amounts of BH migrated lactams 34 but also, when x = 2 or 3, the methylene (M) migrated lactams, 3,6-diazabicycl[3.2.x]alkan-2-ones 37. The present N-insertion reactions favoring BH migration with azabicyclic ketones contrast markedly with reactions of the related carbocycles 1a,b, which give only methylene migrated lactams 2a,b with HN3/H2SO4. Schmidt reactions of 3-anti/syn-methyl- and 3-anti-phenyl-2-azabicyclo[2.2.2]octan-5-ones 17, 20, and 23 (64:36 ± 9, BH:M) follow the reaction pattern of the parent ketone 14, but the 3-syn-phenyl ketone 26 gives major (65%) methylene migration. The results offer insights into the BH vs M migration dichotomy for the Beckmann and Schmidt reactions of bridged bicyclic ketones.

Regioselective Functionalization. 6.1 Migratory Preferences in Hydroxylamine-O-sulfonic Acid and Schmidt Rearrangements of 7-Substituted Norcamphors

Hydroxylamine-O-sulfonic acid reacted with syn-7-X- and anti-7-Y-substituted norcamphor derivatives [X = H, OMe, Cl, Br, OTos; Y = H, COOMe, Cl, Br, Tos, COOMe(5-exo-Br)], to give solely bridgehead migrated 2-azalactams, except for minor amounts of methylene migrated 3-azalactams from norcamphor (1) and the syn-7-Br ketone 19. Schmidt reactions of the same ketones provided varying mixtures of methylene and bridgehead migrated lactams, except for norcamphor (1) and anti-7-Br ketone 31, which provided solely 3-azalactams. Significant ratios (>0.4) of bridgehead migration to cleavage products were observed in the Schmidt reactions only with 7-OTos ketones 22 and 24 with exo-5-bromo-anti-7-methoxycarbonyl ketone 37. The Schmidt rearrangements most likely involve iminodiazonium ion intermediates in light of the large amounts of cleavage observed relative to lactam formation and the insensitivity of methylene migration to the substituent size in the reactions of syn-7-substituted norcamphors.

A heterobimetallic Zr,Hf(μ-formaldehyde) complex from [(CH 2O)ZrCp 2]-dimer and hafnocene dichloride

The reaction of (η 2-formaldehyde)zirconocene dimer ( 3) with hafnocene dichloride ( 5) at room temperature gives the heterobimetallic complex [μ-(η 1-O: η 2-C,O-formaldehyde)(Cp 2ZrCl)(Cp 2HfCl)]. From thermochemical considerations it is suggested that methylene migration from zirconium to hafnium occurs during the reaction, favouring the regioisomer Cp 2(Cl)Zr- O-CH 2-H f(Cl)Cp 2 ( 7) under equilibrium conditions.

Facile conversion of organometallic halide complexes to halomethyl derivatives: synthesis, structure, and reactivity of the (.eta.5-C5R5)Cr(NO)2CH2X series (R = H, CH3; X = Cl, Br, I, OCH3, OCH2CH3, PPh3, CN, SO3C6H4CH3)

Treatment of (η 5 -C 5 R 5 )Cr(NO) 2 X with ethereal diazomethane in the presence of Cu powder provides access to the new (η 5 -C 5 R 5 )Cr(NO) 2 CH 2 X derivatives in 88-93% yield (R=H, CH 3 ; X=Cl, Br). In contrast to the chemistry of the isoelectronic/isostructural (η 5 -C 5 H 5 )Fe(CO) 2 CH 2 X systems, the CH 2 of the halomethyl ligand in the (η 5 -C 5 H 5 )Cr(NO) 2 CH 2 Cl complex undergoes a remarkably facile migration into a C-H bond of the coordinated η 5 -C 5 H 5 ring upon abstraction of the α-halide, giving the (η 5 -C 5 H 4 Me)Cr(NO) 2 + cation in >80% yield. Deuterium labeling shows the process to be intramolecular. Methylene migration is attributed to the extremely electrophilic nature of the Cr-alkylidene species generated upon halide abstraction. Intermolecular CH 2 transfer is only observed when η 5 -C 5 Me 5 derivatives are treated with Ag + in the presence of excess cyclohexene

ChemInform Abstract: Reactive Chromium Methylidene Cations: Intramolecular Migration of Methylene from Chromium into a C‐H Bond of the Cyclopentadienyl Ligand.

AbstractThe complexes (II) are produced in high yield (only one numerical value given) in a facile synthesis.

Reactive chromium methylidene cations: intramolecular migration of methylene from chromium into a C-H bond of the cyclopentadienyl ligand

Synthese avec un rendement eleve de complexes CpCr(NO) 2 -CH 2 X (X=Cl,Br). Etude de la reaction avec AgBF 4 dans CH 2 Cl 2 donnant (η-C 5 H 4 (CH 3 )Cr(NO) 2 + dans lequel le reste methylene s'est insere dans une liaison C−H du η-C 5 H 5

Unusual regiochemistry in a beckmann-like rearrangement of camphor. α-Camphidone via methylene migration.

Abstract Reaction of camphor with hydroxylamine-O-sulfonic acid affords the nitrogen insertion product α-camphidone by migration of the methylene group rather than the bridgehead. Since Beckmann rearrangements of trigonal oximes afford bridgehead cleavage products with camphor, an alternative synchronous rearrangement of a tetrahedral intermediate is proposed for this Beckmann-like reaction.

Selective Methyl and Methylene Migration in the Epoxy Derivatives Derived from 1-Abietic Acid

Selective Methyl and Methylene Migration in the Epoxy Derivatives Derived from 1-Abietic Acid

The Dienone-Phenol Rearrangement in the Benzo(c)phenanthrene Series. A Case of Concurrent Methyl and Methylene Migration

The Dienone-Phenol Rearrangement in the Benzo(c)phenanthrene Series. A Case of Concurrent Methyl and Methylene Migration