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ABSTRACT: Foiled carbene structures comprising strong stabilizing interactions between the divalent carbon and the intramolecular double bond have been located by DFT calculations. These tetracyclic species bearing fused five-membered rings impeding intramolecular rearrangements are theoretically predicted to lie in a deep potential energy well. A suitable dibromocyclopropane precursor for this type of foiled carbene has been prepared in 12 steps. Its treatment with methyllithium led to the isolation of a product of a formal carbene dimerization, a bicyclopropylidene.
The Journal of Organic Chemistry 08/2011; 76(18):7491-6. · 4.45 Impact Factor
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ABSTRACT: Tetravinylbenzene 4 was prepared in nearly quantitative yield from commercially available tetrabromobenzene; the improved, one-step procedure now employs Suzuki-Miyaura cross-coupling conditions. Intermolecular cyclopropanation of 4 with dibromocarbene gave a series of gem-dibromide adducts. Intramolecular cyclopropanation of monoadduct 5, putatively by its methyllithium-generated cyclopropylidene(oid), produced compound 11, which features a highly distorted spiropentane having a C-C-C bond angle of 163.5°. The stability of the reported spiropentanes was investigated using DFT calculations.
The Journal of Organic Chemistry 10/2010; 75(21):7494-7. · 4.45 Impact Factor
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ABSTRACT: Two achiral diazirines 1a and 1b have been encapsulated in
the inherently chiral cavity of α-cyclodextrin (6-Cy), β-cyclodextrin
(7-Cy) and permethylated β-cyclodextrin (TRIMEB)
and photolyzed. Because of supramolecular photochirogenesis
the generated carbenes afford intramolecular C–H insertion
products not as a racemate but one enantiomer is slightly
favored. With 1a@(6-Cy)2 the ee of product 7a is doubled.
To the best of our knowledge for the first time for carbene
Introduction
It is often useful to perform a reaction in a constrained
system.[1] For example, zeolites are employed on a large
scale to force the isomerization of hydrocarbons in stereotype
patterns, amongst others.[2] Indeed, the use of a host
molecule as a reaction vessel opens up the possibility to
synthesize products that would be difficult to obtain in
solution[3] or to modify the course of a reaction.[4] This
technique can also be applied to the generation of reactive
molecules which might get bottleable if they are incarcerated
in a host molecule.[5] It is particularly meaningful to
perform carbene reactions in a molecular reactor because
of the high reactivity of the divalent carbon atom. In fact,
methylene has been described as the most indiscriminating
reagent in chemistry.[6] Even if all other carbenes are partly
stabilized and therefore more selective,[7] they still tend to
generate product mixtures. This is especially true for C–H
insertion reactions by which only stabilized carbenes like
the nucleophilic foiled carbenes[8] or the electrophilic dihalocarbenes[
9] give synthetically useful results. For more reactive
carbenes, encapsulation in a cavitand permits to reduce
the mobility of the intermediate, therefore lessening the
number of accessible bonds or lone-pairs that may act as
reaction partners and consequently, reducing the amount of
side-products. Moreover, the equilibrium conformation of a
guest molecule can also be modified.[10] This may permit to
[‡] Carbene Rearrangements, 77. Part 76: J.-L. Mieusset, A.
Schrems, M. Abraham, V. B. Arion, U. H. Brinker, Tetrahedron
2009, 65, 765–770.
[a] Lehrstuhl für Physikalisch-Organische Chemie und Strukturchemie,
Institut für Organische Chemie, Universität Wien,
Währinger Straße 38, 1090 Wien, Austria
Fax: +43-1-4277-52140
E-mail: udo.brinker@unive.ac.at
Eur. J. Org. Chem. 2009, 5907–5912 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5907
reactions, products are imprinted, though modestly, with
handedness derived from Cys. The reaction of 1b is highly
dependent on the molecular reactor used for inclusion.
Whereas 7-Cy is very efficient for favoring dimerization
through azine formation, the use of TRIMEB permits exclusive
formation of intramolecular insertion product 7b.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Annalen der Chemie und Pharmacie 10/2009; 2009(34):5907 - 5912. · 3.10 Impact Factor
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ABSTRACT: The molecule of the title compound, C(10)H(6)N(24), lies on a crystallographic inversion centre located in the middle of the benzene ring. Steric overcrowding by the bulky N(3) groups is avoided by the tendency of four azide entities to be arranged parallel to the benzene ring and the other four azide groups to be arranged alternately above and below the benzene plane in a skeletal C(i) symmetry. The compound is of interest for high-energy research and as a precursor for the synthesis of carbon nanotubes, nanospheres or high-nitrogen carbon nitrides with great potential for biological and technological applications.
Acta crystallographica. Section C, Crystal structure communications 06/2009; 65(Pt 5):o240-2. · 0.78 Impact Factor
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ABSTRACT: [reaction: see text] A reinvestigation of the thermolysis of 4,4-dibromotetracyclo[6.2.1.0(2,7).0(3,5)]undec-9-ene (2) affords diene 8 with a completely rearranged hydrocarbon skeleton via the isolable intermediate 4, along with cyclopentadiene and bromobenzene. DFT calculations show that the novel tandem retro-Diels-Alder-Diels-Alder reaction with role reversal is slightly less favored than the overall single-step Cope rearrangement.
Organic Letters 02/2007; 9(1):113-5. · 5.86 Impact Factor
