Advances in addition-cure phenolic resins

A new homogeneous iridium catalyst gives hydrogenation of quinolines under unprecedentedly mild conditions-as low as 1 atm of H(2) and 25 °C. We report air- and moisture-stable iridium(I) NHC catalyst precursors that are active for reduction of a wide variety of quinolines having functionalities at the 2-, 6-, and 8- positions. A combined experimental and theoretical study has elucidated the mechanism of this reaction. DFT studies on a model Ir complex show that a conventional inner-sphere mechanism is disfavored relative to an unusual stepwise outer-sphere mechanism involving sequential proton and hydride transfer. All intermediates in this proposed mechanism have been isolated or spectroscopically characterized, including two new iridium(III) hydrides and a notable cationic iridium(III) dihydrogen dihydride complex. DFT calculations on full systems establish the coordination geometry of these iridium hydrides, while stoichiometric and catalytic experiments with the isolated complexes provide evidence for the mechanistic proposal. The proposed mechanism explains why the catalytic reaction is slower for unhindered substrates and why small changes in the ligand set drastically alter catalyst activity.

Iridium-catalyzed hydrogenation of N-heterocyclic compounds under mild conditions by an outer-sphere pathway.

Many (>170) exceptions to the ‘even-electron rule from the literature (unrecognized in most cases) and from the authors’ Laboratory are presented and discussed. The large number of violations suggests that although the generalization may be a helpful guide for the explanation of mass spectral behaviour of many organic compounds, the term ‘rule’ seems to be unjustified. THE RULE The ‘even-electron rule’ is an oft-quoted generalization important for the understanding and prediction of mass spectral behaviour of organic compounds. ’-’ It states that odd-electron cations (such as molecular ions or fragments formed by rearrangements) may eliminate either a radical or an even-electron neutral species, but even-electron ions (such as protonated molecules or fragments formed by a single bond cleavage) will not usually lose a radical to form an oddelectron cation. It can be summarized in the following scheme:

The ‘even‐electron rule’

Quasi-particles with fractional charge and statistics, as well as modified Coulomb interactions, exist in a two-dimensional electron system in the fractional quantum Hall (FQH) regime. Theoretical models of the FQH state at filling fraction \(v={^{5}}/{_{2}}\) make the further prediction that the wave function can encode the interchange of two quasi-particles, making this state relevant for topological quantum computing. We show that bias-dependent tunneling across a narrow constriction at \(v={^{5}}/{_{2}}\) exhibits temperature scaling and, from fits to the theoretical scaling form, extract values for the effective charge and the interaction parameter of the quasi-particles. Ranges of values obtained are consistent with those predicted by certain models of the \({^{5}}/{_{2}}\) state.

http://science.sciencemag.org/content/sci/320/5878/899.full.pdf

Quasi-particle properties from tunneling in the v = 5/2 fractional quantum Hall state.

Graphene provides a rich platform to study many-body effects, owing to its massless chiral charge carriers and the fourfold degeneracy arising from their spin and valley degrees of freedom. We use a scanning single-electron transistor to measure the local electronic compressibility of suspended graphene, and we observed an unusual pattern of incompressible fractional quantum Hall states that follows the standard composite fermion sequence between filling factors ν = 0 and 1 but involves only even-numerator fractions between ν = 1 and 2. We further investigated this surprising hierarchy by extracting the corresponding energy gaps as a function of the magnetic field. The sequence and relative strengths of the fractional quantum Hall states provide insight into the interplay between electronic correlations and the inherent symmetries of graphene.

/pdf/unconventional-sequence-of-fractional-quantum-hall-states-in-2byymimi6b.pdf

Unconventional sequence of fractional quantum Hall states in suspended graphene.

2, 6-Dimethylphenyl propargyl ether (10) and its derivatives 12–15 rearrange thermally to 1, 5-dimethyl-6-methylene-tricyclo [3.2.1.02,7]oct-3-en-8-one (9) and related compounds 16–19. The ethers undergo first an aromatic [3, 3]-sigmatropic rearrangement to ortho-allenyldienones 11, which then undergo ring closure to the tricyclic products by an electrocyclic reaction.



Only in the case of the γ-methylpropargyl ether 13, the ortho-dienone 11 is further rearranged in low yield to the para-butynylphenol 20, but the tricyclic ketone 17 is again the main product.



New data show that the known thermal cyclisation of aryl propargyl ethers to chromenes (e. g. 4 8) involves a preliminary [3, 3]-sigmatropic rearrangement.

Sigmatropische Umlagerungen von Aryl‐propargyläthern; Synthese von 1, 5‐Dimethyl‐6‐methylen‐tricyclo[3, 2, 1, 02,7]‐oct‐3‐en‐8‐on‐Derivaten. Vorläufige Mitteilung

The rearrangement of 10-allyl-2-oxo-Δ1(9), 3-hexahydronaphthalene (12) catalysed by trifluoroacetic acid and other Bronsted acids yielded almost exclusively the [3s, 3s]-products, 1- and 3-allyl-5,6,7,8-tetrahydro-2-naphthol (16 and 15, respectively). The rearrangement of 12 with trifluoroacetic anhydride or acetic anhydride/sulfuric acid, yields, besides 15 and 16, appreciable amounts of the [1s, 2s]-rearrangement product, 4-allyl-5,6,7,8-tetrahydro-2-naphthol (14) (table 1).



The CF3COOH catalysed dienone-phenol-rearrangement of 6-ally-5,6-dimethyl-cyclohexa-2,4-dien-l-one (11) in hexane at 0° yields4-allyl-2,3-dimethyl-phenol (19). Rearrangement of d3-ll containing a specifically deuteriated allyl group proves that the formation of d3-19 occurs via a [3s, 3s]-sigmatropic reaction. On the other hand, treatment of 11 with (CF3CO)2O at 0° in hexane gives (after saponification) 4-allyl-, 5-allyl- and 6-allyl-2,3-dimethyl-phenol (19, 20 and 21, respectivcly). This reaction occurs via an acyloxybenzenium-ion intermediate. The reactions performed with d3-11 demonstrate that the formation of d3-19 occurs both by a direct [3s, 3s]-shift and by a twofold [1s, 2s]-shift, respectively. d3-20 is formcd by a [3s, 4s]-sigmatropic reaction. d3-21 is obtained with about 95% inversion of the carbon skeleton of the allyl group. Thus d3-21 is mainly formed by a [1s, 2s]- followed by a [3s, 4s]-sigmatropic rearrangement.



6-Allyl-6-niethyl-cyclohexa-2,4-dien-1-one (4) yields with CF3COOH in hexane 4-allyl-2-methyl-phenol (5), whereas with (CF3CO)2O in hexane 5, 3-allyl- and 5-allyl-2-methyl-phenol (24 and 25, respectively) are formed in comparable amounts. As a minor product 6-allyl-2-methyl-phenol (26) was observed.



Based on these observations, the concept of charge-induced, e.g. schemes 2 and 3, and charge-controlled, e.g. scheme 7, sigmatropic reactions, has been elaborated. In the former, the charge serves only to accelerate appreciably thermal orbital-symmetry allowed reactions, whereas in the latter, the charge determines the course of the transformations according to the Woodward-Hoffmann rules.



Especially in acetylating systems, allylcyclohexdienones undergo charge-induced and charge-controlled reactions simultaneously.

Säurekatalysierte Dienon‐Phenol‐Umlagerungen von Allylcyclohexadienonen; ladungsinduzierte und ladungskontrollierte sigmatropische Reaktionen

7-Chloro-2-chloromethyl-benzofuran (13) and 3, 8-dichloro-2 H-1-benzopyran (12) are the main products from the thermal rearrangement (230–260°) of 2, 6-dichlorophenyl propargyl ether (7). Compounds 17, 18 and 19 are also formed, but in much smaller amounts (scheme 2 and table 1). However, in the case of the bromo-compounds 8 and 9 the rearrangement products are the benzofuran derivatives 21 and 22, containing one bromine atom less per molecule (scheme 4).



The corresponding naphthyl propargyl ethers 10 and 11 can be rearranged much more easily (180°) to the halogeno-naphthofurans 24 and 26 respectively. In the case of the bromo-ether 11, 2-methyl-naphtho[2, 1-b]furan (25) is also formed (scheme 5).



If the propargylic hydrogen atoms in 7 and 11 are replaced by deuterium atoms, then after rearrangement the deuterium atoms in the products d-13 and d-26 are found in the β-positions to the oxygen atom of the furan ring (schemes 3 and 5).



It is suggested that initially a [3s, 3s]-sigmatropic rearrangement of the aryl propargyl ethers to the 6-allenyl-6-halogeno-cyclohexa-2, 4-dien-1-ones (e.g.a) occurs and that from these the isolated products are formed via radical pathways (scheme 6).



Under neutral conditions aryl propargyl ethers containing a free ortho-position give on heating benzopyran derivatives [2]. When this thermal reaction is carried out in sulfolane in the presence of powdered potassium carbonate, 2-methyl-benzofuran derivatives are formed (table 2). This leads to the possibility of preparing, depending on the conditions, either benzopyran or benzofuran derivatives by the Claisen rearrangement of aryl propargyl ethers. The mechanism for the formation of the benzofurans is given in scheme 9.

Thermische Umlagerungen von halogensubstituierten Aryl-propargyläthern†

N-Propargyl-1-naphthylamine (1), the corresponding N-butinylcompound 5 and N-propargyl-2-naphthylamine (2) - unlike N-propargyl-anilines - on heating to 250° are converted to a mixture of py-tetrahydro-benzoquinolines and benzoquinolines (scheme 1). The mechanism of these reactions which are induced by a [3s, 3 s] rearrangement of the propargyl group and include a [1,5s] H-shift is depicted in scheme 4. N-Methylation of 1 and 2 reduces for steric reasons the rate of the [1,5s] H-shifts; therefore ring closure of the allenylnaphthylamine intermediates to indols 13 and 14 is favoured (schemes 2 and 5).

Über thermische, eine Amino‐Claisen‐Umlagerung einschliessende. Reaktionen von N‐Propargyl‐naphthylaminen

The acid catalysed dienol-benzene rearrangement of methyl substituted o- and p-propargylcyclohexadienols (18–22, 34 and 35) was investigated. In the first step water is eliminated to yield the corresponding methyl propargyl benzonium ions (cf. scheme 6, a), which undergo [1s, 2s] sigmatropic rearrangements to give propargylbenzenes (28, 29, 30, 38) and [3s, 4s] sigmatropic rearrangements to give allenylbenzenes (24–27, 40) (cf. schemes 2, 3, 5, 6). [3s, 3s] sigmatropic rearrangements occur only to a small extend. In the rearrangement of 2-propargyl-2,4,6-trimethylcyclohexa-3,5-dien-1-ol (18) a [1s, 2s] sigmatropic methyl shift is observed (4%).

Janos Zsindely

Papers

Sigmatropische Umlagerungen von Aryl‐propargyläthern; Synthese von 1, 5‐Dimethyl‐6‐methylen‐tricyclo[3, 2, 1, 02,7]‐oct‐3‐en‐8‐on‐Derivaten. Vorläufige Mitteilung

Säurekatalysierte Dienon‐Phenol‐Umlagerungen von Allylcyclohexadienonen; ladungsinduzierte und ladungskontrollierte sigmatropische Reaktionen

Thermische Umlagerungen von halogensubstituierten Aryl-propargyläthern†

Über thermische, eine Amino‐Claisen‐Umlagerung einschliessende. Reaktionen von N‐Propargyl‐naphthylaminen

Die Dienol-Benzol-Umlagerung von Propargylcyclohexadienolen: aromatische [1,2]-, [3,3]- und [3,4]-sigmatropische Umlagerungen