- Carboxy(vinyl)ketene intermediates in the thermolysis of methylthio- and methoxy-substituted Meldrum's acid derivatives
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Methylthio- and methoxy-substituted carboxy(vinyl)ketenes 10 and 16a have been identified by Ar matrix isolation FTIR spectroscopy following flash vacuum thermolysis (FVT) of Meldrum's acid derivatives 7 and 13a. Methylthio(methyl)methyleneketene 9 and alkoxy(methyl)methyleneketene 15 are formed concurrently at high FVT temperatures. The alkoxy(methyl)methyleneketenes 15 do not isomerise to alkoxy(vinyl)ketenes 18, which have been generated and identified in other reactions. Ethoxy(methyl)-methyleneketene 15b readily eliminates ethene in a retro-ene type reaction to produce acetylketene 22. Ketenes 15 react with alcohols and amines to produce 3-alkoxybutenoic acid derivatives 21.
- Bibas, Herve,Kappe, C. Oliver,Wong, Ming Wah,Wentrup, Curt
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p. 493 - 498
(2007/10/03)
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- The vinylketene-acylallene rearrangement: Theory and experiment
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Alkoxyviniyketenes 4 are generated by flash vacuum thermolysis (FVT) or photolysis of 3-alkoxycyclobutenones 3. The thermal interconversion of 4 and allene carboxylic acid esters 5 under FVT conditions is demonstrated by Ar matrix FTIR spectroscopy. In addition, ethoxyvinylketene 4b undergoes thermal elimination of ethene with formation of s-cis-and s-trans-acetylketene (8). An analogous aminovinylketene-to-allenecarboxymide conversion is observed on FVT of 3-dimethylaminocyclobutenone 3e. A facile 1,3-chlorine migration in 2,3-butadienoyl chloride (5d) is also reported. Consistent with the experimental observations, 1,3-methoxy, 1,3-chloro, and 1,3-dimethylamino migrations in vinylketene are calculated (G2(MP2,SVP) level) to have moderate barriers of 169, 157, and 129 kJmol-1, respectively, significantly less than the corresponding 1,3-H shift barrier (273 kJ mol-1). The stabilization of the four-center transition structures is rationalized in terms of the donor acceptor interaction between the lone pair electrons of the migrating donor substituent and the vacant central carbon p-orbital of the ketene LUMO. The predicted migratory aptitude in the series of substituted vinylketenes. R-C(=CH2)-CH=C=O, is in the order N(CH3)2>SCH3>SH>Cl>NH2>OCH3>OH>F>H>CH3, and correlates well with the electron-donating ability of the R group.
- Bibas, Herve,Wong, Ming Wah,Wentrup, Curt
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p. 237 - 248
(2007/10/03)
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