- Boronic acid catalysis for mild and selective [3+2] dipolar cycloadditions to unsaturated carboxylic acids
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Herein, the concept of boronic acid catalysis (BAC) for the activation of unsaturated carboxylic acids is applied in several classic dipolar [3 + 2] cycloadditions involving azides, nitrile oxides, and nitrones as partners. These cycloadditions can be used to produce pharmaceutically interesting, small heterocyclic products, such as triazoles, isoxazoles, and isoxazolidines. These cycloadducts are formed directly and include a free carboxylic acid functionality that can be employed for fur-ther transformations, thereby avoiding prior masking or functionalization. In all cases, BAC provides faster reactions, under milder conditions, with much improved product yields and regioselectivities. In some instances, such as triazole formation from the reaction of azides with 2-alkynoic acids, catalysis with ort/io- nitrophenylboronic acid circumvents the undesirable product decarboxylation observed when using thermal activation. By using NMR spectroscopic studies, the boronic acid catalyst was shown to provide activation by a LUMO-lowering effect in the unsaturated carboxylic acid, likely via a monoacylated hemiboronic ester intermediate.
- Zheng, Hongchao,McDonald, Robert,Hall, Dennis G.
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experimental part
p. 5454 - 5460
(2010/09/15)
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- Polynuclear nonfused bis(1,3,4-oxadiazole)-containing systems
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Nonfused bis-1,3,4-oxadiazoles were synthesized by reaction of 5-substituted mono- and bis-tetrazoles with mono- and dicarboxylic acid chlorides. The results of kinetic studies showed that the transformation of tetrazoles into 1,3,4-oxadiazoles is accelerated by 1 to 2 orders of magnitude on addition of a catalytic amount of dimethylformamide, triethylamine, or pyridine. Pleiades Publishing, Inc., 2006.
- Vereshchagin,Petrov,Kizhnyaev,Pokatilov,Smirnov
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p. 1049 - 1055
(2007/10/03)
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- Photochemical fragmentation of unsubstituted tetrazole, 1,2,3-Triazole, and 1,2,4-triazole: First matrix-spectroscopic identification of nitrilimine HCNNH
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Equilibria between the tautomers of heterocyclic azoles like 1, 2, and 4 have been studied many times. We here show that the application of density functional methods in combination with matrix IR spectroscopy is a useful tool for determining which protomer is preferred in rare gas matrices and thus also can be assumed to dominate in the gas phase. Photolysis of tetrazole (4) in cryogenic matrices allows the IR-spectroscopic identification of a new CH2N2 isomer, the long sought-after nitrilimine HCNNH (6). Flash pyrolysis of 4 also yielded nitrilimine. Upon irradiation this species is converted to a second, previously unknown "isomer", an HCN/NH complex 13. Nitrilimine (6) can also be generated by photolyzing 1,2,3- (1) and 1,2,4-triazole (2) in Ar matrices. The complex photochemistry of all three heterocyclic precursor compounds was unveiled. Our findings were supported by isotopic substitution experiments and by high-level ab initio calculations. Moreover, the IR bands of iminocyanide HNCN were tentatively assigned. This radical has up to now not been observed in a matrix. VCH Verlagsgesellschaft mbH, 1996.
- Maier, Guenther,Eckwert, Juergen,Bothur, Axel,Reisenauer, Hans Peter,Schmidt, Christiane
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p. 1041 - 1053
(2007/10/03)
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- GAS-PHASE REACTIONS. 36. PYROLYSIS OF VINYL AZIDE.
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The gas-phase pyrolyses of vinyl azide as well as of 1H-1,2,3-triazole have been investigated by means of PE spectroscopy. In accordance with predictions from MNDO hypersurface studies, vinyl azide in its lowest thermal decomposition channel splits off nitrogen to yield predominantly 2H-azirine as identified by the PE spectrum of the cool-trapped compound, which at higher temperatures rearranges to the most stable C//2H//3N isomer, acetonitrile. The experimental observations and the quantum chemical calculations both indicate that among those reaction pathways, which can readily be characterized by a suitably selected pair of reaction coordinates from the 12-dimensional hyperspace for the C//2H//3N moiety, synchronous nitrogen extrusion-azirine ring formation is the energetically most favorable one.
- Bock,Dammel,Aygen
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p. 7681 - 7685
(2007/10/02)
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