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

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 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

Nitroxyl Radical Addition to Pentafulvenones Forming Cyclopentadienyl Radicals: A Test for Cyclopentadienyl Radical Destabilization

Photochemical Wolff rearrangements in alkane solvents of the 6-diazo-2,4-cyclohexadienones 4 and 13−15 give pentafulvenone (1), 2,3-benzopentafulvenone (2), dibenzopentafulvenone (3), and 2,4-di-tert-butylpentafulvenone (16), as identified by conventional UV and IR spectroscopy. Reactions of these fulvenyl ketenes with tetramethylpiperidinyloxyl (TEMPO) proceed by addition of TEMPO to the carbonyl carbon forming delocalized radicals for 1 and 2 which add one or more further TEMPO molecules, while the initial radical products formed from 3 and 16 dimerize. The rate constants of these reactions compared to hydration rate constants for the same compounds show the benzannulated derivatives 2 and 3 fit a previous correlation k2(TEMPO) vs k((H2O), whereas for 1 and 16 there is evidence for inhibition of reactions with radicals. The deviations are consistent with an absence of aromatic stabilization of the cyclopentadienyl radicals from 1 and 16 that is compensated in the benzannulated derivatives

<i>N-</i>Pyrrolylketene: A Nonconjugated Heteroarylketene

N-Pyrrolylketene (5) is calculated to be destabilized and nonconjugated, with a preferred geometry with the pyrrolyl ring orthogonal to the ketenyl group. Ketene 5 is generated from N-pyrrolylacetic acid (7) with use of Mukaiyama’s reagent, and reacts with imines forming β-lactams 10, with a product ratio correlation of log(cis/trans) with σ+. Photolysis of N-diazoacetylpyrrole (14) in MeOH gives methyl N-pyrrolylacetate (15) from 5 and also ester 17, evidently by trapping of 2-(1-pyrrolylketene) (21), formed by a new vinylogous Wolff rearrangement

Hydration of Pyridylketenes: Formation of Acid Enol and Dihydropyridine (Eneaminone) Transients

2-, 3-, and 4-Pyridylketenes 4 formed in water by photochemical Wolff rearrangements using flash photolysis undergo rapid hydration forming transient intermediates observed by UV spectroscopy. 3-Pyridylketene (3-4) formed the acid enol intermediate 3-10 which was converted to the acid 3-11, and phenylketene gave similar behavior. 4-Pyridylketene (4-4) reacted with a similar initial rate constant of 5.0 × 104 s-1 for decay of an absorption at 275 nm, with concomitant formation of a strong absorption at 370 nm with the same rate constant. The intermediate absorbing at 370 nm decayed with a lifetime 2.4 × 103 fold longer than that of the ketene, and is identified as 4-(carboxymethylene)-1,4-dihydropyridine (4-13), resulting from conjugate 1,6-addition of H2O to 4-4. 2-Pyridylketene (2-4) underwent hydration with a similar rate constant of 1.1 × 104 s-1 forming a transient with a UV absorption with maxima at 310 and 380 nm that decayed with biexponetial kinetics, with rate constants slower than the rate of formation by factors of 5.2 and 110, respectively. These results are interpreted as indicating the presence of two species, namely Z- and E-2-(carboxymethylene)-1,2-dihydropyridines (2-13), resulting from conjugate 1,4-addition of H2O to 2-4. The identifications of the 1,2- and 1,4-(carboxymethylene)dihydropyridines 2- and 4-13 were confirmed by comparison of their UV spectra with those of the corresponding N-methyl derivatives. The amination of 2-pyridylketene in CH3CN was reinvestigated, and spectroscopic evidence, computational studies, and preparation of the N-methyl analogue demonstrated formation of the 1,2-dihydropyridine Z-2-8f as the long-lived intermediate

Amination of Bis(trimethylsilyl)-1,2-bisketene to Ketenyl Amides, Succinamides, and Polyamides: Preparative and Kinetic Studies

The reaction of the bisketene (Me3SiCCO)2 (1) with amines is facile and proceeds by two distinct steps forming first ketenylcarboxamides 3 and then succinamides 5. Successive reaction of 1 with two different amines gives mixed succinamides, while phenylhydrazine gives succinimide 7. The reactions of 1.8 equiv of 1 with 1,4-(H2NCH2)2C6H4 gives α,ω-bisketenyldiamide 13, while equivalent amounts of 1 and diamines gave polymeric amides. Mixed ester amides 8 are formed by sequential reaction of 1 with an alcohol, followed by an amine, or vice versa. Kinetic studies of the amination reaction of 1 with excess amines in CH3CN gave rate constants kobs for the formation of ketenylcarboxamides that were fit by the relationship kobs = ka[amine]2 + kb[amine]3. Further reaction of the n-butyl ketenylcarboxamide 3b with n-BuNH2 to give the succinamide 5b was first order in [n-BuNH2], while the further reaction of the CF3CH2 ketenylcarboxyamide 3c with CF3CH2NH2 to form 5c was fit by the equation kobs = kc[amine]2/(kd[amine] + 1). The reaction of 3b with CH3OH to form the ester amide 8a is strongly accelerated compared to CH3OH addition to the corresponding ketenyl ester and gives significant stereoselectivity for formation of erythro product, and both these effects, as well as the absence of higher order kinetic terms in the reaction of 3b with n-BuNH2, may arise from coordination by the carboxamido group to the nucleophile

Hydration of Pyridylketenes: Formation of Acid Enol and Dihydropyridine (Eneaminone) Transients

2-, 3-, and 4-Pyridylketenes 4 formed in water by photochemical Wolff rearrangements using flash photolysis undergo rapid hydration forming transient intermediates observed by UV spectroscopy. 3-Pyridylketene (3-4) formed the acid enol intermediate 3-10 which was converted to the acid 3-11, and phenylketene gave similar behavior. 4-Pyridylketene (4-4) reacted with a similar initial rate constant of 5.0 × 104 s-1 for decay of an absorption at 275 nm, with concomitant formation of a strong absorption at 370 nm with the same rate constant. The intermediate absorbing at 370 nm decayed with a lifetime 2.4 × 103 fold longer than that of the ketene, and is identified as 4-(carboxymethylene)-1,4-dihydropyridine (4-13), resulting from conjugate 1,6-addition of H2O to 4-4. 2-Pyridylketene (2-4) underwent hydration with a similar rate constant of 1.1 × 104 s-1 forming a transient with a UV absorption with maxima at 310 and 380 nm that decayed with biexponetial kinetics, with rate constants slower than the rate of formation by factors of 5.2 and 110, respectively. These results are interpreted as indicating the presence of two species, namely Z- and E-2-(carboxymethylene)-1,2-dihydropyridines (2-13), resulting from conjugate 1,4-addition of H2O to 2-4. The identifications of the 1,2- and 1,4-(carboxymethylene)dihydropyridines 2- and 4-13 were confirmed by comparison of their UV spectra with those of the corresponding N-methyl derivatives. The amination of 2-pyridylketene in CH3CN was reinvestigated, and spectroscopic evidence, computational studies, and preparation of the N-methyl analogue demonstrated formation of the 1,2-dihydropyridine Z-2-8f as the long-lived intermediate

Direct Detection of Wiberg's Silene (1,1-Dimethyl-2,2-bis(trimethylsilyl)silene) and Absolute Rate Constants for Its Reactions in Solution

Photolysis of (pentamethyldisilanyl)(trimethylsilyl)diazomethane (6a) and the analogous ketene derivative (6b) in hydrocarbon solution in the presence of aliphatic alcohols affords alkoxysilanes from trapping of four isomeric silene reactive intermediates, in yields of ca. 78%, 6%, 9%, and 8%, respectively. Nanosecond laser flash photolysis (193 or 248 nm) of 6a,b allows detection of two transient species, the most prominent of which has been assigned as 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (3; λmax ∼265 nm). The second species exhibits λmax = 310 nm and much lower reactivity toward alcohols and is consistent with any of E-1,2-dimethyl-1,2-bis(trimethylsilyl)silene (E-9), the corresponding Z-isomer (Z-9), or 1,2,2-trimethyl-2-pentamethyldisilanylsilene (8g). The four silenes are the products expected from the various possible 1,2-migrations in the carbene intermediate formed by photoextrusion of nitrogen or carbon monoxide from the precursors. Absolute rate constants for reaction of 3 and 9 (8g) with methanol, methanol-Od, 2-propanol, cyclohexanol, tert-butyl alcohol, tert-butylamine, and acetic acid have been determined in hexane solution at 24 °C. In all cases but one, the ratios of the rate constants (relative to tert-butyl alcohol) for reaction of silene 3 compare favorably with the relative rate data reported previously for this silene in diethyl ether solution at 100 °C. The rate constant for addition of methanol to 3 in hexane at 25 °C correlates with those of a series of C-substituted 1,1-dimethylsilenes and indicates this silene to be the most highly electrophilic derivative known. The rates exhibit a bell-shaped dependence on temperature over the 0−60 °C range and a maximum at 24 °C, consistent with a stepwise mechanism for reaction with methanol. The transient UV spectra and reactivity observed in THF solution at 24 °C indicate that 3 complexes strongly with the ether solvent, resulting in 103- to 104-fold reductions in its reactivity toward aliphatic alcohols compared to hexane solution. Smaller (ca. 20-fold) reductions in reactivity are observed for the minor silene product(s) under the same conditions

<i>N-</i>Pyrrolylketene: A Nonconjugated Heteroarylketene

N-Pyrrolylketene (5) is calculated to be destabilized and nonconjugated, with a preferred geometry with the pyrrolyl ring orthogonal to the ketenyl group. Ketene 5 is generated from N-pyrrolylacetic acid (7) with use of Mukaiyama’s reagent, and reacts with imines forming β-lactams 10, with a product ratio correlation of log(cis/trans) with σ+. Photolysis of N-diazoacetylpyrrole (14) in MeOH gives methyl N-pyrrolylacetate (15) from 5 and also ester 17, evidently by trapping of 2-(1-pyrrolylketene) (21), formed by a new vinylogous Wolff rearrangement