Experimental and theoretical study of the 2-alkoxyethylidene rearrangement

The rearrangement of 2-ethoxyethylidene, generated photochemically from a nonnitrogenous precursor, leads to ethyl vinyl ether. Although this product could result, in principle, from a 1,2-hydrogen shift and/or a 1,2-ethoxy shift in the carbene, a deuterium labeling study indicates an essentially exclusive preference for hydrogen migration. The experimental results are in agreement with CCSD and W1BD calculations for the simpler 2-methoxyethylidene system that show a prohibitively large barrier for the methoxy shift and a negligible barrier for the hydride shift.     

Mechanism of 1,2-shift through p -complex transition state

A systematic calculation of the potential curves or surfaces for 1,2-shift has been realized by using MNDO or other models in MOPAC programs. By referring to the previous authors' viewpoints, the 1,2-shift can be divided into two categories. 1,2-electron-deficient shift is that the electronic configuration of the atom which accepts the migrating group is a cation or an electron- deficient atom, and 1,2-anion shift is the one that the accepted atom of the migration group is a negative ion. In terms of the experimental facts and the calculation of the potential surfaces, in electron-deficient shift such as Beckmann or Baeyer-Villiger rearrangement, the migration occurs through a transition complex formed between the p -bond and the cation or electron-deficient migrating group, but in anion shift such as Wittig or Stevens rearrangement, the electron pair in p -orbit excites at first to p * orbit, and then the migration occurs through the new formed complex between the anion migration group and the vacant p orbit. The above mechanisms explain reasonably the intramolecular properties, the configuration retentions of the migration group, and the corresponding migratory aptitudes of the two type 1,2-shifts. The partial and less important free radical reaction of 1,2-anion shift has been explained by the p -complex mechanism too.     

DFT study of the mechanisms of in water Au(I)-catalyzed tandem [3,3]-rearrangement/Nazarov reaction/[1,2]-hydrogen shift of enynyl acetates: a proton-transport catalysis strategy in the water-catalyzed [1,2]-hydrogen shift

A computational study with the Becke3LYP density functional was carried out to elucidate the mechanisms of Au(I)-catalyzed reactions of enynyl acetates involving tandem [3,3]-rearrangement, Nazarov reaction, and [1,2]-hydrogen shift. Calculations indicate that the [3,3]-rearrangement is a two-step process with activation free energies below 10 kcal/mol for both steps. The following Nazarov-type 4pi electrocyclic ring-closure reaction of a Au-containing dienyl cation is also easy with an activation free energy of 3.2 kcal/mol in CH2Cl2. The final step in the catalytic cycle is a [1,2]-hydride shift, and this step is the rate-limiting step (with a computed activation free energy of 20.2 kcal/mol) when dry CH2Cl2 is used as the solvent. When this tandem reaction was conducted in wet CH2Cl2, the [1,2]-hydride shift step in dry solution turned to a very efficient water-catalyzed [1,2]-hydrogen shift mechanism with an activation free energy of 16.4 kcal/mol. Because of this, the tandem reaction of enynyl acetates was found to be faster in wet CH2Cl2 as compared to the reaction in dry CH2Cl2. Calculations show that a water-catalyzed [1,2]-hydrogen shift adopts a proton-transport catalysis strategy, in which the acetoxy group in the substrate is critical because it acts as either a proton acceptor when one water molecule is involved in catalysis or a proton-relay stabilizer when a water cluster is involved in catalysis. Water is found to act as a proton shuttle in the proton-transport catalysis strategy. Theoretical discovery of the role of the acetoxy group in the water-catalyzed [1,2]-hydrogen shift process suggests that a transition metal-catalyzed reaction involving a similar hydrogen shift step can be accelerated in water or on water with only a marginal effect, unless a proton-accepting group such as an acetoxy group, which can form a hydrogen bond network with water, is present around this reaction's active site.     

Electrophilic fluorination of aromatic compounds with NF type reagents: kinetic isotope effects and mechanism

H/D Isotope effects in fluorination of aromatic compounds with NF type reagents have been studied to reveal the reaction mechanism. The results obtained are consistent with a polar S_EAr mechanism. Small deuterium isotope effects (k_H/k_D = 0.86-0.99) show that decomposition of a Wheland-type intermediate is not rate determining. The first example of a 1,2-hydrogen shift accompanying electrophilic fluorination of arenes has been observed in the fluorination of 1,3,5-trideuterobenzene.     

Further considerations of the sources of the volatiles from pyrolysis of polystyrene

Formation of the radical precursor to trimer (T) during pyrolysis of polystyrene features a 1,5-hydrogen shift. However because 1,3-shift is so much slower, the sources of the less abundant dimer (D) and tetramer (Te) remain unclear. While we and others have proposed addition of small radicals to olefinic polymer end-groups as a route to oligomer precursor radicals, others recently suggested that such addition is also too slow and proposed a third alternative: 1,7-shift followed serially by 7,3-shift to give the precursor for D. Although considerable evidence suggests that 1,7-shift would be much slower than 1,5-shift, this alternate kinetic model assigned them as comparably rapid. We apply a computational method to predict initial product distributions based on estimated, and empirically varied, propagation rate constants for 1,x-shift, β-scission, hydrogen transfer, and addition radical steps. The addition mechanism successfully predicted the relative amount of D but systematically underestimated Te. This deficiency could be removed by empirical inclusion of a small amount of 1,7-shift, although the other literature evidence still causes this to remain a questionable hypothesis.     

Photochemistry of 1,2-dihydronaphthalene oxide: concurrent triplet and singlet processes via singlet excitation

The photochemistry of 1,2-dihydronaphthalene oxide (254 nm) was reexamined and indan was found to be a primary photoproduct, as well as the traditionally assumed secondary photoproduct. Quenching studies demonstrated that indan, as a primary photoproduct, is derived from a triplet pathway, competing with a singlet route, back to the ground state surface. CASSCF calculations strongly suggest that the triplet pathway consists of a dissociation of the oxirane moiety to give a triplet carbene and aldehyde, which via hydrogen abstraction-decarbonylation-ISC recloses to give indan. Conical intersections corresponding to the presumed 1,2-hydrogen shift and 1,2-alkyl shift to give 2-tetralone and 1-indancarbaldehyde, respectively, were located computationally.     

Mechanism of 1,2-shift through π-complex transition state

A systematic calculation of the potential curves or surfaces for 1,2-shift has been realized by using MNDO or other models in MOPAC programs. By referring to the previous authors' viewpoints, the 1,2-shift can be divided into two categories. 1,2-electron-deficient shift is that the electronic configuration of the atom which accepts the migrating group is a cation or an electron-deficient atom, and 1,2-anion shift is the one that the accepted atom of the migration group is a negative ion. In terms of the experimental facts and the calculation of the potential surfaces, in electron-deficient shift such as Beckmann or Baeyer-Villiger rearrangement, the migration occurs through a transition complex formed between the 7i-bond and the cation or electron-deficient migrating group, but in anion shift such as Wittig or Stevens rearrangement, the electron pair in it-orbit excites at first to π* orbit, and then the migration occurs through the new formed complex between the anion migration group and the vacant rc     

MASS SPECTRA OF SULFUR COMPOUNDS. ASSESSMENT OF 1, X (X=2,3,4,5) HYDROGEN TRANSFER IN DIALKYL SULFIDES AND DISULFIDES UNDER ELECTRON IMPACT. A NOVEL 1,5-H SHIFT

Abstract

The transfer of hydrogen atoms from the gamma carbon of dialkyl disulfides upon electron impact is presented and its occurrence is formally shown by the electron ionization mass spectrum of 1-(2′,2′-dideuteriocyclohexyl)-2,3-dithiapent-1,1′-ene (4b). Also, the spectrum of 1-(2′,2′-dideuteriocyclohexyl)-2-thiahex-1-ene (3b) is analyzed in terms of hydrogen/deuterium transfer, where it is absent. This result is compared with selected mass spectral data of eighteen other dialkylthianes. Evidence is put forth to indicate that dialkyl sulfides are prone to undergo only 1,3-H shift upon electron inpact, whereas in dithianes 1,3- and 1,5-Hydrogen transfer take place. The evidence collected suggests that neither 1,2- nor 1,4-hydrogen transfer occurs in both sulfur derivatives.     

Flash vacuum pyrolysis of methoxy-substituted lignin model compounds

The flash vacuum pyrolysis (FVP) of methoxy-substituted beta-O-4 lignin model compounds has been studied at 500 degrees C to provide mechanistic insight into the primary reaction pathways that occur under conditions of fast pyrolysis. FVP of PhCH(2)CH(2)OPh (PPE), a model of the dominant beta-O-4 linkage in lignin, proceeds by C-O and C-C cleavage, in a 37:1 ratio, to produce styrene plus phenol as the dominant products and minor amounts of toluene, bibenzyl, and benzaldehyde. From the deuterium isotope effect in the FVP of PhCD(2)CH(2)OPh, it was shown that C-O cleavage occurs by homolysis and by 1,2-elimination in a ratio of 1.4:1, respectively. Methoxy substituents enhance the homolysis of the beta-O-4 linkage, relative to PPE, in o-CH(3)O-C(6)H(4)OCH(2)CH(2)Ph (o-CH(3)O-PPE) and (o-CH(3)O)(2)-C(6)H(3)OCH(2)CH(2)Ph ((o-CH(3)O)(2)-PPE) by a factor of 7.4 and 21, respectively. The methoxy-substituted phenoxy radicals undergo a complex series of reactions, which are dominated by 1,5-, 1,6-, and 1,4-intramolecular hydrogen abstraction, rearrangement, and beta-scission reactions. In the FVP of o-CH(3)O-PPE, the dominant product, salicylaldehyde, forms from the methoxyphenoxy radical by a 1,5-hydrogen shift to form 2-hydroxyphenoxymethyl radical, 1,2-phenyl shift, and beta-scission of a hydrogen atom. The 2-hydroxyphenoxymethyl radical can also cleave to form formaldehyde and phenol in which the ratio of 1, 2-phenyl shift to beta-scission is ca. 4:1. In the FVP of o-CH(3)O-PPE and (o-CH(3)O)(2)-PPE, products (ca. 20 mol %) are also formed by C-O homolysis of the methoxy group. The resulting phenoxy radicals undergo 1,5- and 1,6-hydrogen shifts in a ratio of ca. 2:1 to the aliphatic or benzylic carbon, respectively, of the phenethyl chain. In the FVP of (o-CH(3)O)(2)-PPE, o-cresol was the dominant product. It was formed by decomposition of 2-hydroxy-3-hydroxymethylbenzaldehyde and 2-hydroxybenzyl alcohol, which are formed from a complex series of reactions from the 2, 6-dimethoxyphenoxy radical. The key step in this reaction sequence was the rapid 1,5-hydrogen shift from 2-hydroxy-3-methoxybenzyloxy radical to 2-hydroxymethyl-6-methoxyphenoxy radical before beta-scission of a hydrogen atom to give the substituted benzaldehyde. The 2-hydroxybenzyl alcohols rapidly decompose under the reaction conditions to o-benzoquinone methide and pick up hydrogen from the reactor walls to form o-cresol.     

Generation of NH-azomethine imine intermediates through the 1,2-hydrogen shift of hydrazones and their intermolecular cycloaddition reaction with olefinic dipolarophiles

The thermal 1,2-hydrogen shift of the hydrazone generates the NH-azomethine imine intermediate in the 4-oxo-4H-pyrido[1,2-a]pyrimidine-3-carbaldehyde system under mild conditions. Therein, the resulting NH-azomethine imine should be stabilized by forming an internal hydrogen bond with the carbonyl oxygen at the 4-position. Its smooth stereoselective intermolecular cycloaddition reaction with olefinic dipolarophiles giving pyrazolidine derivatives is discussed.     

Ruthenium-catalyzed aromatization of aromatic enynes via the 1,2-migration of halo and aryl groups: a new process involving electrocyclization and skeletal rearrangement

The halo and aryl substituents of the 1,2-disubstituted styryl group of aromatic enynes undergo a 1,2-shift in the aromatization reaction catalyzed by TpRuPPh3(CH3CN)2PF6 (10 mol %) in toluene (110 degrees C, 6-8 h). The aryl group shifts to the neighboring olefin carbon, and the iodo (or bromo) substituent migrates to the terminal alkyne carbon. The mechanisms of these two migrations have been elucidated by isotope labeling experiments. It indicates that the 1,2-aryl shift arises from 5-endo-dig electrocyclization of a ruthenium-vinylidene species, whereas the 1,2-iodo shift follows a 6-endo-dig pathway.     

Iron catalysis of grignard reductions. Mechanism of 1, 3-reductive eliminations from γ-propyl halides

The iron-catalyzed reduction of various 3-substituted propyl bromides by Grignard reagents affords propylene and cyclopropane. The reduction to propylene is particularly noteworthy since it formally represents a 1, 2-hydrogen shift. Two key intermediates have been identified in propylene formation, in which 3-methoxypropyl bromide is first catalytically reduced to the magnesium derivative by Grignard reagent. The iron-catalyzed β-elimination of the 3-methoxypropylmagnesium intermediate affords allyl methyl ether, which is then reduction cleaved to propylene. Extensive studies of deuterium labeling in the reactants, as well as in both intermediates, allow the course of the hydrogen shift to be followed unequivocally. The mechanism of iron catalysis is proposed in Scheme 2 and 3, representing the first and second stages of the reduction to propylene.     

Theoretical study on chlorine and hydrogen shift in cycloheptatriene and cyclopentadiene derivatives

The transition structures (TSs) for chlorine 1,7-shift and 1,5-shift in 1,7,7-trichlorocycloheptatriene (1) and those of chlorine 1,5-shifts in 1,5,5-trichlorocyclopentadiene (3) and 1,2,5-trichloro-1,3-pentadiene (5) derivatives have been located with density functional theory (DFT) at the Becke3LYP/6-311G [and Becke3LYP/6-311+G] level. The calculational results were compared with those for corresponding hydrogen shifts in nonsubstituted molecules (cycloheptatriene (2), cyclopentadiene (4), and 1,3-pentadiene (6)). The following points were clarified: (1) The activation energy (Delta E(++)) for chlorine 1,7-shift in 1 was evaluated to be only +50.1 [+49.2] kJ/mol, which is smaller than that (+69.9 [+68.3]) for a 1,5-shift, supporting the theory that the conversion between two equivalent A and A' proceeds through a TS for direct chlorine 1,7-shift (Figure 1), rather than through a TS for a 1,5-shift (Figure 2). (2) The considerable amount of charge separation between a migrating chlorine atom (Cl(m)) and a seven-membered ring (-0.53 and +0.47 for Merz-Singh-Kollman scheme) occurs in a chlorine 1,7-shift, which is in good contrast to the result that the migrating hydrogen atom (H(m)) for a 1,7-shift in cycloheptatriene (2) carries almost no charge (Figure 3). This large charge separation can stabilize the TS for the chlorine 1,7-shift pathway. (3) The Delta E(++) values for suprafacial hydrogen 1,7-shift in 2 are quite large (+288.0 [+284.8] kJ/mol), much larger than that (+166.8 [+167.0]) for a 1,5-shift in 4 which is orbital symmetrically allowed (Figure 3). The calculation suggests that the chlorine 1,7-shift in 1 occurs easily at room temperature (actually observed experimentally) by proceeding via concerted suprafacial 1,7-shift through the zwitterionic TS with the significant assistance of Coulomb interaction between charged fragments (negatively charged chlorine atom and positively charged tropylium ring), rather than via a suprafacial 1,5-sigmatropic pathway. Other cases studied in this paper showed usual results predicted by orbital symmetrical consideration.     

Separation of deuterium by IR multiphoton decomposition of chlorodifluoromethane. IR multiphoton absorption by and decomposition of a CF2DCl/CF2HCl mixture

The radical cations of formaldimine, methylamine, formaldehyde, methanol, diazene, hydrazine, nitroxyl, hydroxylamine and hydrogen peroxide, and of isomers derived formally from these systems by means of a 1,2-hydrogen shift have been studied using ab initio molecular orbital theory, including electron correlation. For the ions of formaldimine, methylamine and methanol, evidence is presented that the 1,2-hydrogen-shifted species lie lower in energy than the conventional isomers.     

New facets in the photochemistry and thermal reaction of 2,2-diphenylmethylenecyclopropane

Photochemistry and thermal reaction of 2,2-diphenylmethylenecyclopropane 1a have been reinvestigated to make their mechanistic refinement. A new finding in the photochemistry of 1a is the detection and isolation of cyclobutene 7. While fragmentation and 1,3-carbon (C) shift is responsible for previously reported photoproducts, 1,1-diphenylethylene 3 and diphenylmethylenecyclopropane 2a, respectively, a new pathway is required to explain the formation of cyclobutene 7. A mechanism involving the 1,2-C shift (Scheme 3, arrow b) followed by 1,2-hydrogen (H) shift is proposed. In the thermal reaction of 1a, on the other hand, a trimethylenemethane type of species is shown to be a common intermediate for degeneracy, rearrangement from 1a to 2a, and formation of indene 5. In the photochemistry of 1a, intervention of cyclobutylidene 8 is strongly supported by circumstantial evidences provided by steady-state and laser flash photolytic investigations. Further experiments designed to independently generate cyclobutylidene 8 showed the methylenecyclopropane/cyclobutene branching ratio (1a/7) to be in the range between 0 and 1.7, which is much lower than the value of 4.0 for parent cyclobutylidene. Nevertheless, the relative efficiency of 1,2-shift pathway to generate 8 is shown to be as high as 17–47% compared to 1,3-C shift to give 2a at the early photochemical stage of 1a.     

Massenspektrometrie und ihre Anwendung auf strukturelle und stereochemische Probleme CLXVII Langkettige Wasserstoffverschiebung in der Massenspektrometrie

The mass spectral fragmentation behavior of various α-substituted tetrahydrofuran derivatives and their deuterium labeled analogs has been examined. It was established that the ester and hydroxy derivatives undergo ring fragmentation associated with long range hydrogen transfer. The mass spectra of the deuterium labeled samples provided evidence that direct hydrogen transfer takes place from positions as far as thirteen or more carbon atoms away from the oxygen function in preference to the McLafferty rearrangement. Examination of the spectra of the deuterium labeled derivatives shed further light on the mechanism of the loss of water from the molecular ion and various fragment ions.     

Mechanism of meerwein-pondorf-verley type reductions

The reduction of benzophenone by lithium and chloromagnesium alkoxides has been studied as well as the transformation of certain lithium alkoxides to the corresponding ketones by electron transfer. Fluorenone was reduced by lithium sec-butoxide to the corresponding lithium ketyl to the extent of 65%. Lithium 9-fluoroenolate underwent in tetrahydrofuran a spontaneous transformation to lithium fluorenone ketyl. This process was interpreted as involving 1,2-hydrogen shift in an oxygen-centred radical. A mechanism for the Meerwein-Pondorf-Verley-type reductions is proposed, invoking single electron as well as 1,2-hydrogen shift steps.     

The bicyclo[2.2.2]octyl carbene system as a probe for migratory aptitudes of hydrogen to carbenic centers.

A series of tosylhydrazone derivatives of exo-6-substituted bicylo[2.2.2]octan-2-ones have been prepared. Thermal decomposition of the sodium salts of these tosylhydrazones gives carbene-derived products from 1,3-migration of either the C6 hydrogen (perturbed) or the C7 hydrogen (unperturbed), along with smaller amounts of alkenes derived from 1,2-hydrogen migration. The exo-6-substituent strongly activates 1,3-hydrogen migration in the case of SiMe(3) and weakly activates it in the case of CH(3) substitution. Thiomethoxy and carbomethoxy are weakly deactivating, while cyano and methoxy groups are strongly deactivating. B3LYP/6-31G* calculations on these substituted carbenes and transition states are in qualitative agreement with the ease of 1,3-hydrogen migration of perturbed vs unperturbed hydrogen. These experimental results and computational studies suggest carbene stabilization due to the exo-6-silyl group. They also suggest a reactant-like transition state for 1,3-hydrogen migration in which the inductive effect influences ease of migration. In the case of the exo-6-methoxy group, the inductive effect overwhelms any potential resonance-stabilizing effects.     

一硫代甲酸的异构化和单分子重排途径

本文对一硫代甲酸的各个异构体和单分子重排途径进行了半经验MNDO和在STO-3G,3-21G,3-21G~*基水平上的ab initio计算。不同计算的优化几何构型十分接近,硫羟甲酸的计算值同其已知的实验值吻合较好。在相对稳定性方面,反式异构体比顺式异构体稳定。在3-21G~*水平上计算的顺/反异构体能量差,对于硫羰甲酸为31.8KJ/mol,对于硫羟甲酸为7.7KJ/mol。反式-硫羰甲酸经1.3-分子内氢位移重排为反式硫羟甲酸的活化能是210.1KJ/mol。反式-硫羰甲酸和反式-硫羟甲酸经1.2-分子内氢位移重排为巯基羟基碳烯(∶CSHOH)的能量垒高度分别为409.2KJ/mol和415.8KJ/mol。计算结果表明,从能垒高度上看,硫羰甲酸异构化为硫羟甲酸经一个1.3-分子内氢位移途径较经两个连续的1.2-氢位移途径有利得多。     

Mechanistic investigation of enediyne-connected amino ester rearrangement. Theoretical rationale for the exclusive preference for 1,6- or 1,5-hydrogen atom transfer depending on the substrate. A potential route to chiral naphthoazepines

Memory of chirality (MOC) and deuterium-labeling studies were used to demonstrate that the cascade rearrangement of enediyne-connected amino esters 1a and 1b evolved through exclusive 1,5- or 1,6-hydrogen atom transfer, subsequent to 1,3-proton shift and Saito-Myers cyclization, depending on the structure of the starting material. These results were independently confirmed by DFT theoretical calculations performed on model monoradicals. These calculations clearly demonstrate that in the alanine series, 1,5-hydrogen shift is kinetically favored over 1,6-hydrogen shift because of its greater exergonicity. In the valine series, the bulk of the substituent at the nitrogen atom has a major influence on the fate of the reaction. N-Tosylation increases the barrier to 1,5-hydrogen shift to the benefit of 1,6-hydrogen shift. The ready availability of 1,6-hydrogen atom transfer was explored as a potential route for the enantioselective synthesis of naphthoazepines.