Ab initio study of the chair cope rearrangement of 1,5-hexadiene

Ab initio calculations up to the MP4SDQ/6-31 G* level are reported for the chair Cope rearrangement of 1,5-hexadiene. These are consistent with a recent AM1 study, which showed the reaction to take place via a biradicaloid intermediate.     

Stereo- and enantioselective polymerization of hexa-1,5-diene on heterogeneous Ziegler-Natta catalysts obtained with the use of optically active internal donors

Pure racemates and individual enantiomers of diethyl 2,3-diisopropylsuccinate ((+,−)-DIPS and (−)-DIPS) and 2,3-diisopropyl-1,4-dimethoxybutane ((+,−)-DPDMB and (+)-DPDMB) were prepared. The synthesized compounds and diethyl phthalate were used as internal donors for the preparation of heterogeneous Ziegler-Natta catalysts. Polymerization of hexa-1,5-diene was carried out on these catalysts. The obtained samples of poly(methylene-1,3-cyclopentane) were studied by ¹³C NMR spectroscopy and polarimetry. The induction of optical activity to polymer is observed with (−)-DIPS as the internal donor. The induction value makes up 15–20% from the value recorded in the case of optically active metallocene catalysts. The emergence of induction may be associated with the presence of close contact between the titanium atom and the donor molecule within the active site of catalyst, as well as with the fact that the donor molecule either deactivates a part of the stereospecific titanium centers or influences the structure of the titanium centers during their formation and development.     

Balancing dynamic and nondynamic correlation for diradical and aromatic transition states: a renormalized coupled-cluster study of the cope rearrangement of 1,5-hexadiene

Single-reference coupled-cluster calculations employing the completely renormalized CCSD(T) (CR-CCSD(T)) approach have been used to examine the mechanism of the Cope rearrangement of 1,5-hexadiene. In agreement with multireference perturbation theory, the CR-CCSD(T) method favors the concerted mechanism of the Cope rearrangement involving an aromatic transition state. The CCSD(T) approach, which is often regarded as the "gold standard" of electronic structure theory, seems to fail in this case, favoring pathways through diradical structures.     

Synthesis of aryl-containing isoprenoids from 1,5-dimethylcycloocta-1,5-diene

A procedure has been developed for the synthesis of Pro-Drone homo analogs from the ozonolysis products of 1,5-dimethylcycloocta-1,5-diene and 1,5-dimethylcyclooctene.     

NBO and NICS analysis of the allylic rearrangements (the Cope and 3-aza-Cope rearrangements) of hexa-1,5-diene and N-vinylprop-2-en-1-amine: A DFT study

In this work, ab initio density functional theory (DFT) calculations have been performed on the 3,3-sigmatropic rearrangements of hexa-1,5-diene (Cope) and N-vinylprop-2-en-1-amine (3-aza-Cope) in the gas phase. The barrier heights and heats of reactions calculated at the B3LYP/6-311G** level of theory were in good agreement with experimental data. Transition states optimized with B3LYP/6-311G** theory were used for calculating the nucleus independent chemical shift (NICS) and, a natural bond orbital (NBO) analysis was also performed at the same level of theory. Our results indicate that the aromaticities of the transition states are controlled by the out-of-plane component and that the chair-like transition state of the Cope rearrangement exhibits the strongest aromatic character. Analysis of donor-acceptor (bonding and anti-bonding) interactions of σ_3–4 → π*_1–2 suggests that the TS structure in the hexa-1,5-diene reaction (the Cope rearrangement) has more aromatic character than the N-vinylprop-2-en-1-amine reaction (the 3-aza-Cope rearrangement). The NBO results show that in the hexa-1,5-diene and N-vinylprop-2-en-1-amine rearrangements, activation energies are controlled by σ_3–4 → π*_1–2 and σ_3–4 → π*_1–2 resonance energies.      

Stereospecific cope rearrangement of the germacrene-type sesquiterpenes

Stable, conformations of several germacrene-type sesquiterpenes in solution are deduced with the aid of intramolecular nuclear Overhauser effects (NOE) in the NMR spectra.From these conformations, stereochemical aspects of the Cope rearrangement as well as the stereochemistry of the elementype rearrangement products can readily be understood.     

A homologous ester claisen rearrangement via the cope rearrangement of O-trimethylsilylcyanohydrins

The Cope rearrangement of 1,5-dienyl trimethylsilyl-cyanohydrins to α-trimethylsilyoxy-α,β-unsaturated nitriles and their cleavage to methyl δ-hexenoates is discussed.     

Theoretical study on the optical activity of spiro[3.3]hepta-1,5-diene

The spectroscopic and chiroptical properties of the title compound 1 has been predicted and interpreted by a quantum mechanical approach using the CNDO-CI modification of Del Bene and Jaffé. The calculated rotational strength of the longest wavelength transitions according to the method of Hug and Wagnière suggests left-handed chirality for levorotatory 1. The assignment is in agreement with previous work. In addition, the same computational method has been employed for spiro[4.4]nona-1,6-diene (2) and spiro[4.4]nonatetraene (3).     

Synthesis and degenerate cope rearrangement of 2,6-diphenylbarbaralane

The oxygen-sensitive 2,6-diphenylbarbaralanes 15 and 16 are synthesized from bicyclo[3.3.1]nonane-2,6-dione(9) via dibromide 14 in four steps with 46 and 54% overall yield, respectively. Crystalline 2,6-diphenylbarbaralane (15) is yellow at room temperature and decolourizes reversibly on cooling. Accordingly, the UV spectrum of trans-decalin solutions of 15 changes reversibly in the temperature range 295–450 K. The bromodiphenylbarbaralane 16 is shown to exist as a single valence tautomer. The ¹³C chemical shifts of 16 serve as estimates of the analogous shifts in the non-exchanging valence tautomer of 2,6-diphenylbarbaralane (15) itself. The barrier for the degenerate Cope rearrangement of 15 in [D₂]dichloromethane has been determined from ¹³C-NMR line width measurements in the temperature range of 169–233 K to be ΔG3₂₀₀ = 21.6 kJ mol^-1 . Hence phenyl substituents at the termini of the allylic parts of barbaralane accelerate the degenerate Cope rearrangement somewhat more than cyano groups.     

Selective stabilization of transition state structures for cope rearrangements of semibullvalene and barbaralane through interactions with halogens

A new means-discovered through quantum chemical studies (B3LYP/6-31G(d))-of selectively stabilizing the delocalized forms of unsubstituted semibullvalene and barbaralane is described. Calculations indicate that complexation of delocalized semibullvalene and barbaralane Cope rearrangement transition structures by halogen-containing molecules (homo- and heteronuclear diatomics and Lewis acids) can reduce the barrier significantly, in many cases causing it to disappear entirely.     

Valence-bond isomer chemistry. Part 11 [1]. Thermal rearrangement of the Diels-Alder adduct of hexafluorobicyclo-[2.2.0]hexa-2,5-diene and 2,3-Dimethylbuta-1,3-diene: a [1,3] sigmatropic shift of fluorine

Flow pyrolysis of the Diels-Alder adduct of hexafluorobicyclo[2.2.0]hexa-2,5-diene and 2,3-dimethylbutadiene gives an unusual product, the Diels-Alder adduct of hexafluorobenzene and 2,3-dimethylbuta-1,3-diene, and thence via an exclusive [1,3] sigmatropic fluorine shift, its isomeric triene. Loss of hydrogen fluoride from the unusual Diels-Alder adduct readily affords 1,2,3,4-tetrafluoro-6,7-dimethylnaphthalene.     

Investigations on the question of multiple mechanisms in the cope rearrangement

The possible occurrence of the ionic Cope rearrangement, and other non-concerted mechanisms is discussed. The synthesis of 2 - (1 - ethyl - 1 - propenyl) -2- (3 - p - methoxyphenylallyl)malononitrile (1b) and its clean thermal 1,3 rearrangement to (1 - ethyl - 5 - p - methoxyphenyl - 2 - methyl - 4 - pentenylidene)malononitrile (4) are reported. This result contrasts with the rearrangement of 2 - (1,1 - dideuterioallyl) - 2 -(1 - ethyl - 1 - propenyl)malononitrile (1c) which isomerizes cleanly in a 3,3 rearrangement. Rearrangement of 2 - (1 - cyclohexenyl) - 2 - (3 - p - methoxyphenylallyl)malononitrile (11), however, leads sluggishly to [2 - (p - methoxy - α - vinylbenzyl)cyclohexylidene]malononitrile (19) (3,3 shift) and rearrangement of 2 - (1 - isopropyl - 2 - methyl - 1 - propenyl) - 2 -(3 - p - methoxyphenylallyl)malononitrile (12) leads, also slowly, to (1 - isopropyl - 5-p- methoxyphenyl - 2,2 - dimethyl - 4 - pentenylidene)malononitrile (14) (1,3 shift). Rearrangement of 1b in the presence of sodium borohydride allows interception of the proposed ionic intermediates and isolation of 2 - (1 - ethylpropylidene)malononitrile (5) and anethole (21c). Ion trapping experiments also gave positive results in the 3,3 rearrangement of 11. These results are discussed in terms of the ionic Cope rearrangement.     

Stereospecific synthesis of aryltetraline lignan analogues using 1,6-bis(dipropylboryl)hexa-2,4-diene

A novel convenient two-stage synthesis of aryltetraline lignin analogues from aromatic aldehydes and diallyl diboron derivatives on the basis of intramolecular Friedel–Crafts reaction has been developed.     

Investigation of the enzymatic and nonenzymatic cope rearrangement of carbaprephenate to carbachorismate

The dimethyl esters of carbaprephenate and 4-epi-carbaprephenate were prepared by modification of published procedures. In methanol these compounds are converted quantitatively to isomeric 6-hydroxytricyclo[3.3.1.0(2,7)]non-3-en-1,3-dimethyl esters via a two-step sequence involving an initial Cope rearrangement, followed by intramolecular Diels-Alder reaction of the dimethyl carbachorismate or 4-epi-carbachorismate intermediates. Carbaprephenate and its epimer were obtained by alkaline hydrolysis of the corresponding dimethyl esters. These compounds, in contrast to their ester precursors, undergo spontaneous acid-catalyzed decarboxylation in aqueous solution. Only at high pH does the Cope rearrangement compete with decarboxylation. At pH 12 and 90 degrees C, carbaprephenate slowly rearranges to carbachorismate, which rapidly loses water to give 3-(2-carboxyallyl)benzoic acid as the major product. A small amount of the intramolecular Diels-Alder adduct derived from carbachorismate is also observed by NMR as a minor product. Carbaprephenate is not a substrate for the enzyme chorismate mutase from Bacillus subtilis (BsCM), nor does carbaprephenate inhibit the normal chorismate mutase activity of this enzyme, even when present in 200-fold excess over chorismate. Its low affinity for the enzyme-active site is presumably a consequence of placing a methylene group rather than an oxygen atom proximal to the essential cationic residue Arg90. Nevertheless, BsCM variants that lack this cation (R90G and R90A) do not accelerate the Cope rearrangement of carbaprephenate either, and a catalytic antibody 1F7, which exhibits modest chorismate mutase activity, is similarly inactive. Poor substrate binding and the relatively high barrier for the Cope compared to the Claisen rearrangement presumably account for the lack of detectable catalysis. Acceleration of this sigmatropic rearrangement apparently requires more than an active site that is complementary in shape to the reactive substrate conformer.     

Acid catalyzed cope rearrangements of 2-acyl-1,5-dienes

The Cope rearrangement of 1,5-dienes bearing acyl substituents in the 2-position of the diene system is strongly accelerated by protic and Lewis acids.     

Molecular structure of hexafluorobicyclo [2.2.0]hexa-2,5-diene (hexafluoro-dewar-benzene) in the vapour phase

Hexafluoro-Dewar-benzene has been studied by the electron-diffraction method. A model with C_2v symmetry gives excellent agreement between experimental and theoretical data. The structural parameters with error limits are (cf. Fig. 1): r(C₁-C₄)= 1.598 ±0.017 Å, r(C₁-C₂) = 1.505 ±0.005 Å, r(C₂-C₃) = 1.366 ± 0.015 Å, r(C₁-F₁) = 1.328±0.015 Å, r(C₂-F₂) = 1.319±0.007 Å, ∠F₁C₁C₄ = 118.7±0.7°, ∠F₂C₂C₃ = 133.6±0.7°, τ= 121.8±2.0°, and δ = -7.5±2.0°. Molecular orbital calculations by the CNDO/2 method gave τ = 119.8° and δ = ?4.2°.     

A reanalysis of the kinetic data on the boat pathway of the cope rearrangement of 1,5-hexadiene by comparing with quantum-chemical MINDO/3 results

An attempt has been made to match the availble kinetic data on the title process with the theoretical discovery that it is realized via an intermediate. A new fitting procedure is suggested which takes into account the presence of the intermediate.

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