First Stable and Persistent 1,3-Bisketene and Trisketene

First Stable and Persistent 1,3-Bisketene and Trisketen

Theoretical Study of the Reactivity of Ketene with Free Radicals

The structures and energies for the addition of free radicals R• (R = H, CH3, OH, F, SiH3, Cl) to CH2CO to give the radicals RCH2ĊO, ĊH2(CO)R, CH2ĊOR have been calculated by ab initio and B3LYP-DFT methods, and the latter method gives good agreement with available experimental energies. Product radicals ĊH2C(O)R for groups R which possess electron lone pairs are stabilized and have predominant spin density on carbon, and this is attributed to conjugation of the carbonyl group in the product with substituents OH, F, and Cl at the α-position. Additions of H and SiH3 have lower barriers to form the more stable product RCH2ĊO, which for the latter is favored due to hyperconjugative stabilization by the β-SiH3. For CH3 attack at both carbons is competitive, while for OH, F, and Cl, the barriers are low for attack at either carbon, although attack at Cα gives much more stable products. Initial complexes between ketene and the CH3, OH, SiH3, and Cl radicals are detected, and for Cl using B3LYP this species has the structure of a π-complex with the CC double bond that is stabilized by 16.2 kcal/mol relative to the reactants and forms ĊH2C(O)Cl with a barrier of 2.8 kcal/mol. For F no barriers for addition to either carbon were found, but for B3LYP there is a barrier of 27.6 kcal/mol for conversion of FCH2ĊO to ĊH2C(O)F, which is more stable by 19.1 kcal/mol. The corresponding rearrangement of ClCH2ĊO has a barrier of 4.6 kcal/mol, and the predicted preference for initial attack at Cβ to give the less stable product agrees with experiment

First Stable and Persistent 1,3-Bisketene and Trisketene

First Stable and Persistent 1,3-Bisketene and Trisketen

Amination of Ketenes: Kinetic and Mechanistic Studies

The rate constants for reaction of PhMe2SiCHCO (6) with amines to form amides in CH3CN are best fitted with a mixed second- and third-order dependence on [amine], in stark contrast to previous studies of Ph2CCO and other reactive ketenes in which only a first-order dependence on [amine] was observed in H2O or in CH3CN. Derived third-order rate constants for 6 depend on the amine basicity, with a 1.7 × 107 greater reactivity for n-BuNH2 compared to CF3CH2NH2. These kinetic results are consistent with recently reported theoretical studies for reaction of CH2CO with NH3. For 6 the relative reactivity k(n-BuNH2)/k(H2O) is estimated to be 1013 in CH3CN. The crowded ketene t-Bu2CCO (10) is enormously deactivated toward amination and reacts in neat n-BuNH2 with rates 1012 and 2 × 105 times slower than those for t-BuCHCO and t-BuC(i-Pr)CO (11), respectively. The observed rate constants for 11 also show a higher than first-order dependence on [n-BuNH2]. The absence of higher order kinetic terms in [amine] for more reactive ketenes is attributed to irreversibility of addition of an initial amine to the ketene, while with more stable ketenes the initial step is reversible and later steps involving additional amine molecules are kinetically significant. The general acid CF3CH2NH3+ catalyses the addition of CF3CH2NH2 to 6 in a process independent of [CF3CH2NH2]. The reactivity of 6 with n-BuNH2 is 370 times greater in CH3CN compared to isooctane, a result attributed to the polar nature of the transition state and possible catalysis of the addition by CH3CN

Amination of Ketene: A Theoretical Study

The reactions of NH3, (NH3)2, and (H3N·H2O) with ketene have been studied by ab initio calculations. Attack by the dimer (NH3)2 or by (H3N·H2O) through six-membered cyclic transition states is found to be favored, with a preference of 12.8 kcal/mol for initial addition of (NH3)2 to the CO bond of the ketene giving the enol amide, as compared to initial addition to the CC bond to give the more stable amide directly. These results are in contrast to previous theoretical studies for the reaction of monomeric NH3 with ketene, in which direct addition to the CC bond was reported to be favored, and for the reaction of ketene with (H2O)2, in which addition to the CO bond is calculated to be only 1.9 kcal/mol more favorable than addition to the CC bond. The barrier via 10ts for the NH3-catalyzed rearrangement of the enol amide/ NH3 complex 6 to the amide is 16.5 kcal/mol above that of 6, consistent with the experimental observation of an intermediate in ketene amination, and the barrier for reversion of 6 to the reactant complex 5a is 4.0 kcal/mol lower than that for amide formation via 10ts, suggesting the initial amination may be reversible. These results demonstrate that previous theoretical calculations of ketene amination used inadequate levels of theory. The structure of the zwitterion CH2C(O-)NMe3+ (14) from reaction of ketene with NMe3 has also been calculated, and in the gas phase this reaction is calculated to be exothermic by only 0.6 kcal/mol

Disilanylketenes and -bisketenes

Pyrolysis of Me5Si2C⋮COEt (5) and (SiMe2C⋮COEt)2 (6), prepared by reaction of EtOC⋮CLi with Me5Si2Cl and ClMe2SiSiMe2Cl, respectively, at 180 °C gave Me5Si2CHCO (1), and (SiMe2CHCO)2 (2), respectively, which are long lived and were completely characterized by spectroscopic means. The novel 1,6-bisketene (CH2SiMe2CHCO)2 (3) was prepared similarly. The UV spectra of 1−3 and the known (Me3Si)2CCO (4) show longer wavelength absorption for the disilanylketenes, attributed to a lowering of the π* orbital by interaction with the Si−Si σ* orbital, while shifts to shorter wavelength in Me3SiCHCO and 4 compared to alkylketenes are attributed to a raising of the π* level by the C−Si σ orbital. Isodesmic comparisons using ab initio molecular orbital calculated energies indicate that the Si2H5 substituent is essentially equal to the SiH3 group in ketene-stabilizing ability, while (SiH2CHCO)2 is only destabilized by 0.6 kcal/mol compared to SiH3CHCO. The rates of hydration of 1 and 2 exceed that of 3 by factors of 2−4, while 3 has a reactivity similar to that of Me3SiCHCO. By contrast theoretical and experimental studies of (Me3Si)2CCO (4) reveal that the effects of the two silyl groups on the stability and spectra are largely additive but that the second Me3Si group greatly lowers the reactivity in both neutral and acid catalyzed hydration. This ketene ranks with t-Bu2CCO among the least reactive known in hydration, a property attributed to steric inhibition to nucleophilic attack and ground-state stabilization of the ketene by the silyl substituents

A Doubly Destabilized Antiaromatic Cyclopentadienyl Cation: Solvolysis of a 5-Trifluoroacetoxy-5-heptafluoropropyl 1,3-Cyclopentadiene^1a

The 5-trifluoroacetoxy-5-heptafluoropropylcyclopentadiene 15 rearranges to the isomeric trifluoroacetate 16 with a rate constant 5 × 105 less than that for solvolysis of the corresponding 5-CH3 derivative 5. Labeling of 15 with 18O shows the rearrangement occurs by a [1,5]-sigmatropic rearrangement. Solvolysis of 16 occurs at a rate 4 times slower than its formation from 15 and leads to the extensively rearranged fulvene 18, implicating formation of the doubly destabilized cation 20. Carbocation formation from 15 is retarded by a factor of 1020 relative to the model 11, showing cumulative destabilizing effects due to formation of the antiaromatic cyclopentadienyl carbocation and electron withdrawal by the fluoroalkyl group

Ketenes in Soluble Polymer Bound Synthesis: Preparation of Succinamides and 4-Pyridones

Ketenes in Soluble Polymer Bound Synthesis:  Preparation of Succinamides and 4-Pyridone

Amination of Ketenes: Kinetic and Mechanistic Studies

The rate constants for reaction of PhMe2SiCHCO (6) with amines to form amides in CH3CN are best fitted with a mixed second- and third-order dependence on [amine], in stark contrast to previous studies of Ph2CCO and other reactive ketenes in which only a first-order dependence on [amine] was observed in H2O or in CH3CN. Derived third-order rate constants for 6 depend on the amine basicity, with a 1.7 × 107 greater reactivity for n-BuNH2 compared to CF3CH2NH2. These kinetic results are consistent with recently reported theoretical studies for reaction of CH2CO with NH3. For 6 the relative reactivity k(n-BuNH2)/k(H2O) is estimated to be 1013 in CH3CN. The crowded ketene t-Bu2CCO (10) is enormously deactivated toward amination and reacts in neat n-BuNH2 with rates 1012 and 2 × 105 times slower than those for t-BuCHCO and t-BuC(i-Pr)CO (11), respectively. The observed rate constants for 11 also show a higher than first-order dependence on [n-BuNH2]. The absence of higher order kinetic terms in [amine] for more reactive ketenes is attributed to irreversibility of addition of an initial amine to the ketene, while with more stable ketenes the initial step is reversible and later steps involving additional amine molecules are kinetically significant. The general acid CF3CH2NH3+ catalyses the addition of CF3CH2NH2 to 6 in a process independent of [CF3CH2NH2]. The reactivity of 6 with n-BuNH2 is 370 times greater in CH3CN compared to isooctane, a result attributed to the polar nature of the transition state and possible catalysis of the addition by CH3CN

Phenyldimethylsilyl-Substituted Ketenes and Bisketenes

The phenyldimethylsilyl-substituted monoketene PhMe2SiCHCO (1) and bisketene (PhMe2SiCCO)2 (3) have been prepared and compared to the corresponding Me3Si- and t-BuMe2Si-substituted species. The 13C, 17O, and 29Si NMR spectra fit the pattern shown by other silylketenes and provide no evidence for transmission of a substituent effect of the Ph group through the silicon to the ketenyl group, as has been proposed for PhMe2Si-substituted radicals. The UV spectrum of 1 does show a longer λ and greater ε than for t-BuMe2SiCHCO, and this may indicate some interaction of the phenyl group with the ketene chromophore, while the greater reactivity of 1 in hydration compared to t-BuMe2SiCHCO is ascribed to the inductive effect of the phenyl. The very similar ring-opening reactivity of the bis(phenyldimethylsilyl)cyclobutenedione (6) to form 3 compared to the bis(Me3Si) analogues also provides no evidence of a significant interaction of the phenyl with the ketene. A new type of stabilized 1,8-bisketene based on the arylbis(dimethylsilyl) grouping, namely, 1,4-bis(ketenyldimethylsilyl)benzene (12), has been prepared for the first time