Novel parallel reaction between a [1,5] sigmatropic alkylthio shift and a [1,5] sigmatropic hydrogen shift observed in a 2H-azepine ring

Although heating 2-methoxy-2H-azepine results in a [1,5] sigmatropic hydrogen shift, heating 2-propylthio-2H-azepine results in not only a [1,5] sigmatropic hydrogen shift but also a [1,5] sigmatropic propylthio shift. Kinetic measurements reveal that migratory aptitudes increase in the order of MeO < H, PrS. These [1,5] sigmatropic shifts are discussed on the basis of ab initio DFT calculations. [reaction: see text].     

Re‐examining the Mechanisms of Competing Pericyclic Reactions of 1,3,7‐Octatriene

DFT (both B3LYP and M06‐2X), CASSCF, and CASPT2 calculations were used to investigate competing [3, 3] and [3, 5] sigmatropic shifts and intramolecular [4+2] cycloaddition of 1,3,7‐octatriene. In accord with previous results on 1,5‐hexadiene, CASSCF calculations found both stepwise and concerted pathways for the [3, 3] rearrangement. For the competing [3, 5] sigmatropic rearrangement, CASSCF and CASPT2 calculations revealed three stepwise pathways with similar barriers. UB3LYP and UM06‐2X calculations predicted a different potential energy landscape: no stepwise [3, 3] pathway, only two competing [3, 5] sigmatropic shifts, and an intramolecular Diels–Alder cycloaddition/homolytic ring‐opening pathway. Significant lowering of barriers for all rearrangements was predicted for some 1,3,7‐octatrienes with substituents at the 4‐ and 7‐positions.     

Re-examining the Mechanisms of Competing Pericyclic Reactions of 1,3,7-Octatriene

DFT (both B3LYP and M06-2X), CASSCF, and CASPT2 calculations were used to investigate competing [3,?3] and [3,?5] sigmatropic shifts and intramolecular [4+2] cycloaddition of 1,3,7-octatriene. In accord with previous results on 1,5-hexadiene, CASSCF calculations found both stepwise and concerted pathways for the [3,?3] rearrangement. For the competing [3,?5] sigmatropic rearrangement, CASSCF and CASPT2 calculations revealed three stepwise pathways with similar barriers. UB3LYP and UM06-2X calculations predicted a different potential energy landscape: no stepwise [3,?3] pathway, only two competing [3,?5] sigmatropic shifts, and an intramolecular Diels-Alder cycloaddition/homolytic ring-opening pathway. Significant lowering of barriers for all rearrangements was predicted for some 1,3,7-octatrienes with substituents at the 4- and 7-positions.     

Helicoid Shiftamers for the Transport of π‐Clumps and Charges

Hypothetical helical organic polymers with localized electrons, charges, and/or atoms are described, in which these localized substructures can move along the backbones of the polymers via sigmatropic H shifts.     

The thermal rearrangement of endo-7-methyl-exo-7-vinylbicyclo[3.2.0]Hept-2-ene

Gas-phase thermolysis of the title compound in the temperature range of 150°–166°C yields predominantly cis-6-methyl-4,7,8,9-tetrahydroindene by tandem [1,3]–[3,3) sigmatropic shifts: however, we cannot exclude some direct conversion by an alternative [1,3] sigmatropic migration.     

Macroexpansion methodology: an efficient eight unit ring expansion

An efficient eight unit ring expansion method which is postulated to proceed by either a [5,5] or consecutive [3,3] sigmatropic shifts is described.     

Stereospecificity and stereoselectivity in the [3,2] sigmatropic rearrangement of open chain sulphonium ylides

The [3,2]sigmatropic rearrangement of sulphonium ylides derived from allyl sulphides with 1,2-related chiral centres was carried out with excellent stereochemical control. 1,4-Related chiral centres across an E double bond are set up stereospecifically by this reaction and good 4,5 diastereoselectivity is observed.     

Sigmatropic Reactions in Carbanions. I. The 5, 6-dihydro-2H-pyran-2-ide cyclopropyl-enolate rearrangement

Lithiation of 5,6-dihydro-2H-pyran ( 7 ) gives a mixture of dihydropyranyllithiums 14 and 2 . 14 rearranges to 2 , and 2 in turn undergoes a [1,4] sigmatropic shift to give the lithium cyclopropyl-enolate 3 . Lithiation of nerol oxide 6 gives the lithio derivative 24 , which likewise undergoes [1,4] shifts to give cyclopropyl-enolates 28 and 29 .     

Skeletal Wagner-Meerwein rearrangement of perhydro-3a,6;4,5-diepoxyisoindoles

An investigation of a skeletal Wagner-Meerwein rearrangement of variously substituted or quinoline-annulated 3a,6;4,5-diepoxyisoindol-1-ones is reported. Optimum reaction conditions (Ac₂O, BF₃·OEt₂, rt) were discovered for the formation of the target 4,6-epoxycyclopenta[c]pyridines in 40-80% yields. It was shown that the direction of the sigmatropic rearrangement of 3a,6;4,5-diepoxyisoindol-1-ones depended dramatically on the carboxyl group position (exo-/endo-) in the oxabicyclo[2.2.1]heptane moiety. The spatial structure of previously unknown 7,9-epoxycyclopenta[4,5]pyrido[1,2-a]quinolines derived from Wagner-Meerwein rearrangement of 2,11b-epoxyoxireno[6,7]isoindolo[2,1-a]quinolines was established based on the X-ray analysis data. The skeletal rearrangement proceeded regio- and stereospecifically in all the cases examined due to the absence of the epimerization of the carbon atoms adjacent to the carbocation centres.     

A cornucopia of cycloadducts: theoretical predictions of the mechanisms and products of the reactions of cyclopentadiene with cycloheptatriene

The potential cycloaddition reactions between cyclopentadiene and cycloheptatriene have been explored theoretically. B3LYP/6-31G was used to locate the transition states, intermediates, and products for concerted pathways and stepwise pathways passing through diradical intermediates. Interconversions of various cycloadducts through sigmatropic shifts were also explored. CASPT2/6-31G single point calculations were employed to obtain independent activation energy estimates. MM3 was also used to compute reaction energetics. Several bispericyclic cycloadditions in which two cycloadducts are linked by a sigmatropic shift have been identified. B3LYP predicts, in line with frontier molecular orbital predictions, that the [6+4] cycloaddition is the favored concerted pathway, but an alternative [4+2] pathway is very close in energy. By contrast, CASPT2 predicts that a [4+2] cycloaddition is the preferred pathway. B3LYP predicts that the lowest energy path to many of the cycloadducts will involve diradical intermediates, whereas CASPT2 predicts that each of the products of orbital symmetry allowed reactions will be reached most readily by closed shell processes-concerted cycloadditions and sigmatropic shift rearrangements of cycloadducts.     

Thermal conversion of 15,9-triynes : (2+2+2]cycloadditions or [3.3]sigmatropic shifts?

The gas phase pyrolyses of variously labeled 1,5,9-decatriynes (1) and 1,5,9-cyclododecatriynes (2) were investigated to determine possible modes of thermal isomerizations. Conditions iocluded temperatures in the range 400–600°, pressures of 40–10^-4 Torr,and contact times of ca 1 ms to to 15s.The labeling patterns in 1 and 2 were chosen such as to be able to distinguish direct intramolecular [2 +2+2]cycloadditions of the alkyna units to form an aromatic ring (perbaps with subsequent rearrangements), and [3.3]sigmatropic shifts of the 1,5-diyne moieties. Methods for synthesizing the isotopically (particularly ¹³C) labeled triynes were devised and unplemented. The route to 5,6-¹³C₂-1,5,9-decatriyne (1c) made use of a new procedure for the synthesis of symmetrically diaubstituted alkynes involing coupling between two equivalents of an alkl copper reagent and diiodoacetylene-¹³C₂. The synthesis of 1,10-¹³C₂-1,5,9-cyclododecatriyne (2b) was accomplished starting with K¹³CN, elaboration to labeled diethylsuccinate, a crucial bis-Wittig condensation to labeled l,5,9-cyclododecatriene 10, and bromination-dehydrobromination of the latter (NaOH-ethylene glycol). Products from the pyrolysis of unlabeled 1a included [1,2:4,5] dicyclobutabenzene, naphthalene, and 3,4-dimethylidene-1-(but-3-ynyl)cyclobutene. Pyrolysis of 1b gave 3,6-dideuterio[l,2:4,5]dicyclobutabenzene and partially deuterated naphthalene, that of 1c produced 1,2-¹³C₂-[1,2,:4,5]dicyclobutabenzene and 9,10-¹³C₂-naphthalene. While the pyrolysis of 2a resulted in hexamethylidenecyclohexane (hexaradialene), 2b furnished 1,4-¹³C₂-hexaradialene. The results rule out the occurrence of [2+2+2]cycloadditions of the alkyne units, but are consistent with the intervention of a series of [3.3]sigmatropic shifts which connect starting materials with products.     

Methyllithium-promoted Wittig rearrangements of alpha-alkoxysilanes

[formula: see text] The Wittig rearrangements of alpha-alkoxysilanes, promoted by the action of methyllithium were studied. Depending on both the substrate and reaction conditions employed, [2,3]-, [1,2]-, or [1,4]-Wittig rearrangements can be realized. These rearrangements were shown to be initiated by either Si/Li exchange or deprotonation alpha to the silane. Furthermore the sigmatropic shifts can often be followed by other synthetically useful in situ chemical events.     

A quantitative study on the stereoselectivity of sigmatropic shift reactions in acyclic systems

The semiempirical MINDO/3 method has been used to study sigmatropic [1,3] shifts in propene, [1,5] shifts in pentadiene and [1,7] shifts in heptatriene, occurring in the suprafacial and antarafacial way. Hydrogen, fluorine and methyl (with retention or inversion of configuration) were taken as the shifting goups. For the [1,3] shifts some STO-3G and 4-31G calculations have also been performed. Good correspondence has been obtained with the stereoselectivity predictions of the Woodward-Hoffmann theory. The activation energy for the allowed reactions is 8–21 $\text{kcal}\text{mol}$ lower than the value for the forbidden modes. The shift of a F atom proceeds via an inversion-type mechanism.     

Thermal isomerizations of cis,anti,cis-tricyclo[7.4.0.0(2,8)]tridec-10-ene

cis,anti,cis-Tricyclo[7.4.0.0(2,8)]tridec-10-ene (13TCT) undergoes [1,3] sigmatropic rearrangements at 315 °C in the gas phase to the si product 1 and to the sr product 2 with si/sr = 2.1. The dominant thermal isomerization process, however, is epimerization at C8 to afford product 3. That stereomutation at C8 occurs 50% faster than the si and sr shifts combined.     

The [1, 3] O-to-C rearrangement: opportunities for stereoselective synthesis

The relay of stereochemistry of a breaking C-O bond into a forming C-C bond is well-known in the context of [3, 3] sigmatropic shifts; however, this useful strategy is less well-known in other types of molecular rearrangements. Though the first successful example of a [1, 3] O-to-C rearrangement was reported more than 100 years ago, this class of reactions has received less attention than its [3, 3] counterpart. This perspective analyzes the various methods used for the activation and [1, 3] rearrangement of vinyl ethers with an emphasis on mechanism and applications to stereoselective synthesis. We also highlight our own contributions to this area.     

Synthesis and Reactions of New Vinyl Isothiocyanates

Abstract

New vinyl isothiocyanates, which can be prepared from easily accessible thiocyanates by [3,3] sigmatropic shifts, show unusually high reactivity used for the synthesis of heterocyclic sulfur compounds.     

Geometric and electronic similarities between transition structures for electrocyclizations and sigmatropic hydrogen shifts

This paper reports new theoretical evidence that supports previous proposals concerning the similarity between transition structures for electrocyclizations and sigmatropic hydrogen shifts. This evidence was obtained using two recently proposed methodologies, namely the so-called generalized population analysis and the formalism of molecular quantum similarity indices. Analysis of multicenter bond indices shows that the transition structures for cationic [1,n] hydrogen shifts do indeed have three-center indices that are similar to those of other three-center carbocations. In addition, the close resemblance of the electronic structures of electrocyclic and sigmatropic transition structures that differ by only a proton is supported by the values of their quantum molecular similarity indices.     

Thermal 1,3-aroyl migration in 1 (α-aroyloxyarylideneamino) −1,2,3-triazoles

Kinetic studies on the thermal rearrangement of the 1-(α-aroyloxyarylideneamino)-4,5-di-methyl-1,2,3-triazoles (II) to the corresponding diaroylimides (III) have shown that the 1,3-aroyl migration from oxygen to nitrogen proceeds in a concerted way and the isomerization reaction can be described as an [1,3] sigmatropic aroyl shift.     

Chemistry of diazafulvenium methides in the synthesis of functionalized pyrazoles

The chemistry of diazafulvenium methides generated by the thermal extrusion of sulfur dioxide from 2,2-dioxo-1H,3H-pyrazolo[1,5-c] [1,3]thiazoles is described. The diazafulvenium methides unsubstituted at C-7 participate in [8 pi + 2 pi] cycloadditions giving pyrazolo-annulated heterocycles resulting from the addition across the 1,7-position. 1-Methyl-diazafulvenium methides and 7,7-dimethyl-diazafulvenium methides undergo intramolecular sigmatropic [1,8]H shifts giving vinyl-1H-pyrazoles.     

Dithioketals as precursors for [2,3] sigmatropic rearrangements. Synthesis of betweenanenes with vinylic heteroatoms.

Alkoxycarbonyl-stabilized ylides from $\text{gem}$-disulfides undergo [2,3]sigmatropic shifts and provided the first entry to betweenanenes with a heteroatom (sulfur) directly attached to the encapsulated olefinic carbon.