Bond Of Ketene Research Articles

Construction of Saturated Oxazolo[3,2-b][1,2]oxazines via Tandem [3+2]-Cycloaddition/[1,3]-Rearrangement of Cyclic Nitronates and Ketenes.

Reaction of six-membered cyclic nitronates with disubstituted ketenes affords hitherto unknown saturated oxazolo[3,2-b][1,2]oxazines possessing up to four contiguous stereogenic centers. The process involves a tandem of [3+2]-cycloaddition across the C═O bond of ketene, followed by a spontaneous [1,3]-rearrangement of transient vinylidene-substituted bicyclic nitrosoacetals. DFT calculations of the mechanism suggest that the [1,3]-O,C-shift proceeds through a recyclization of a biradical intermediate formed by an unusually mild homolytic cleavage of the N-O bond. The resulting products can be utilized as precursors of other fused 1,2-oxazines derivatives, in particular 1,2-oxazino-1,2,4-triazin-3-ones.

Ammonolysis of ketene as a potential source of acetamide in the troposphere: a quantum chemical investigation.

Quantum chemical calculations at the CCSD(T)/CBS//MP2/aug-cc-pVTZ levels of theory have been carried out to investigate a potential new source of acetamide in Earth's atmosphere through the ammonolysis of the simplest ketene. It was found that the reaction can occur via the addition of ammonia at either the C[double bond, length as m-dash]C or C[double bond, length as m-dash]O bond of ketene. The potential energy surface as well as calculated rate coefficients indicate that under tropospheric conditions, ammonolysis would occur almost exclusively via ammonia addition at the C[double bond, length as m-dash]O bond with negligible contribution from addition at the C[double bond, length as m-dash]C bond. The reaction of ketene with water has also been investigated in order to compare between hydrolysis and ammonolysis, as the former is known to be responsible for the formation of acetic acid. The rate coefficient for the formation of acetamide was found to be ∼106 to 109 times higher than that for the formation of acetic acid from the same ketene source in the troposphere. By means of the relative rate of ammonolysis with respect to hydrolysis, it was shown that acetamide formation would dominate over acetic acid formation at various altitudes in the troposphere.

Changing the regioselectivity and asynchronicity of the possible concerted [2+2] and [4+2] cycloadditions of ketene and halogenated ketenes with cyclopentadiene: a DFT survey

Two concerted Diels–Alder [4+2] and two concerted Staudinger [2+2] reactions between ketene and/or dihaloketenes with cyclopentadiene are examined at B3LYP/6-31+G* level. For the parent ketene, two cycloaddition modes including [2+2] across the CC and [4+2] across the CC bond of ketene are thermodynamically favorable (ΔE=−13.4 and −17.3 kcal/mol, respectively) while the two other including [2+2] across the CO and [4+2] across the CO bond of ketene are endoergic. The corresponding barrier heights change to a lesser amount and ranging from 20.8 to 35.1 kcal/mol. Halogenated ketenes significantly affect both the thermodynamics and kinetics of these reactions in a way that they increase the exoergicity and reduce the barrier heights result in the decrease of regioselectivity. Unsymmetrical bond formations in the structures of TSs indicate that all the reaction modes are concerted and synchronous but halogens increase the asynchronicity due to the electronic effects. Moreover, three transient halogenated cycloadducts obtained through the [4+2] cycloaddition across the CO bond are converted to some new products via a [3,3] sigmatropic rearrangement. The AIM pictures show that in the [4+2] cycloadditions four TS structures represent pericyclic reactions, while all the [2+2] reactions are classified as pseudopericyclic. FMO theory is applied to justify the findings. The reliability of B3LYP results is confirmed through comparison with the HF and MP2 methods.

Ketene-acetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

The [2 + 2] cycloaddition of monosubstituted acetylenes to ketene has been studied by ab initio(G2(MP2,SVP) and DFT (B3LYP/6-31Gd)) methods. The activation barrier decreases with increasing electron-donating ability of the acetylene substituent, and it can be roughly correlated with the energy of the acetylene HOMO. The addition to the C[double bond, length as m-dash]C bond of ketene (giving cyclobutenones) is preferred for the less electron-rich acetylenes, but for the most electron rich ones (X = NH(2) and NMe(2)) the addition to the C[double bond, length as m-dash]O bond (giving oxetes) becomes competitive, with activation barriers as low as ca. 45 (30) kJ mol(-1) for the two computational methods used. The cyclobutenones and oxetes can undergo ring opening to vinylketenes and acylallenes, respectively. Furthermore, the latter two compounds can interconvert by a 1,3-shift of the substituent X. The acylallenes become thermodynamically more stable than the vinylketenes for [small pi]-(lone pair) donating substituents X, and the 1,3-shift barrier also decreases, to ca. 130 kJ mol(-1) for X = NMe(2). In contrast, the 1,3-shifts of CH(3) and H have very high barriers.

Exclusive formation of alpha-methyleneoxetanes in ketene-alkene cycloadditions. Evidence for intervention of both an alpha-methyleneoxetane and the subsequent 1,4-zwitterion.

This paper describes a new mechanistic feature for the Staudinger ketene-alkene cycloaddition reactions to give cyclobutanones. Low-temperature NMR (13C, 19F, and 1H) monitoring of a reaction between bis(trifluoromethyl)ketene (1) and ethyl vinyl ether (2) has shown that the Staudinger reaction proceeds to form initially and exclusively an alpha-methyleneoxetane (3) by [2 + 2](C=O) cycloaddition across the ketene C=O bond. The initial intermediate 3 undergoes ring cleavage to produce a 1,4-zwitterion (4), which is converted to the final [2 + 2](C=C)-type product, cyclobutanone (5). The key intermediate 3 has been isolated in its pure form and was found to be converted to the final products 5 on warming, via the 1,4-zwitterion 4. The alpha-methyleneoxetane 3 is so reactive that it reacts with methanol rapidly even at -80 degrees C via solvolysis to afford an adduct 7. The ion 4 derived from the pure isolated oxetane 3 was intercepted with acetone by a 1,4-dipolar cycloaddition to give a 1,3-dioxane 8. An open-chain alpha,beta-enone (6) has been also obtained from 3. We conclude that the (1 + 2) reaction proceeds in a new three-step mechanism; formation of an alpha-methyleneoxetane 3, a [2 + 2]-type cycloadduct across the C=O bond of ketene, followed by ring cleavage to give the zwitterion 4 and by recombination to form the final product, cyclobutanone 5. The zwitterion 4 is not equilibrating with reactants 1 and 2 but comes from the alpha-methyleneoxetane 3. Exclusive formation of another oxetane 12 has been observed in a reaction between diphenylketene (9) and methyl isopropenyl ether (11). The selectivity of initial formation of cyclobutanone or oxetane has been generalized with aid of frontier-orbital theory and ab initio calculations.

Contrasting mechanism of the hydration of carbon suboxide and ketene. A theoretical study

The protonation and hydration of carbon suboxide (O=C=C=C=O) were studied by ab initio molecular orbital methods. While the geometries of the stationary points were optimized using MP2/6-31G(d,p) calculations, relative energies were estimated using QCISD(T)/6-31G(d,p) and 6-311aaG(d,p)a ZPE. The behaviour of carbon suboxide was compared with that of carbon dioxide and ketene. The protonation at the b-carbon is consistently favoured over that at the oxygen; the proton affinities (PA) are estimated to be PA(C3O2) = 775 15 and PA(H2CCO) = 820 10 kJ mol ˇ1 (experimental: 817 3k J mol ˇ 1 ). The PAs at oxygen amount to 654, 641 and 542 kJ mol ˇ1 (experimental: 548 kJ mol ˇ1 ) for C3O2 ,H 2CCO and CO2, respectively. Using the approach of one and two water molecules to model the hydration reaction, the calculated results consistently show that the addition of water across the C=O bond of ketene, giving a 1,1-ethenediol intermediate, is favoured over the C=C addition giving directly a carboxylic acid. A reverse situation occurs in carbon suboxide. In the latter, the energy barrier of the C=C addition is about 31 kJ mol ˇ1 smaller than that of C=O addition. The C=C addition in C3O2 is inherently favoured owing to a smaller energetic cost for the molecular distortion at the transition state, and a higher thermodynamic stability of the acid product. Molecular deformation of carbon suboxide is in fact a fairly facile process. A similar trend was observed for the addition of H2, HF and HCl on C3O2. In all three cases, the C=C addition is favoured, HCl having the lowest energy barrier amongst them. These preferential reaction mechanisms could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks. Copyright © 2000 John Wiley & Sons, Ltd.