Flash Photolytic Generation and Study of Ynolate Ions and the Corresponding Ketenes in Aqueous Solution

Heterocycles from cyclopropenones

Great attention has been paid to cyclopropenones as they are present in many natural sources.

Mechanism of the cyclopropenone decarbonylation reaction. A density functional theory and transient spectroscopy study

The density functional theory analysis predicts that the thermal decarbonylation of cyclopropenones proceeds by the sequential and regioselective cleavage of both single bonds in a three-membered ring. The initial ring-opening process results in the formation of a reactive zwitterionic intermediate 6, which is separated from the free alkyne and carbon monoxide by a very low energy barrier. Femtosecond pump-probe transient absorption spectroscopy experiments showed that light-induced decarbonylation is also a stepwise process but apparently proceeds on the excited-state surface. The lifetime of the intermediate in the photodecarbonylation reaction is very short and is dependent on substitution and solvent polarity. Thus, bis-p-anisyl-substituted species decays with tau = 0.6 ps, bis-alpha-naphthyl-substituted intermediate has a lifetime of tau = 11 ps, while the bis(2-methoxy-1-naphthyl)-substituted analogue survives for 83 ps in chloroform and for 168 ps in argon-saturated methanol. The loss of carbon monoxide from these intermediates results in the formation of corresponding acetylenes in an electronically ground state. The addition of triplet quenchers does not affect the dynamics or outcome of the reaction.

Fragmentation pathways of negative ions produced by electrospray ionization of acyclic dicarboxylic acids and derivatives

Fragmentation pathways have been studied on the monoanions formed during electrospray ionization of a wide range of aliphatic dicarboxylic acids and their monoesters. All negative ion spectra were obtained from alcoholic or aqueous methanolic solutions without buffers or adjustment of pH, using either a Finnigan LCQ ion trap or a VG-Micromass Quattro triple quadrupole mass spectrometer. Fragmentation pathways were studied using collision-induced dissociation and isotopic-labelling techniques. Two primary fragmentation pathways of the dicarboxylic acid monoanions were observed, namely decarboxylation of the non-ionized carboxyl group and loss of water from this group. The fragmentations were strongly dependent on the chain lengths of the diacids. In the case of a monoester anion, loss of a molecule of alcohol paralleled the loss of water from the diacid monoanion. Losses of water or alcohol were shown to lead to formation of reactive ynolate anions (HOOC(CH2)xC≡CO–, x = 3–9), which in the ion trap spectrometer engaged in complex ion – molecule reactions consistent with the chemistry of these anions. For the longer chains (x > 6), the interactions between the ionized and non-ionized carboxyl groups led to readily formed ion–neutral complexes, which are described as a neutral molecule (ROH, R = H or alkyl) held by a pair of molecular tweezers.Key words: ESI-MS/MS on negative ions, fragmentation pathways of acyclic carboxylic acid monoanions, ion–molecule reactions in an ion trap mass spectrometer, hydrogen–deuterium exchange in a gas-phase anion–neutral complex.

The 2-oxo-3,3,5,5-tetramethylcyclopentanecarboxylic acid keto–enol system in aqueous solution — Generation of the enol by flash photolytic Wolff rearrangement of 2-diazo-4,4,6,6-tetramethylcyclohexane-1,3-dione followed by hydration of the acylketene thus formed

Flash photolysis of 2-diazo-4,4,6,6-tetramethylcyclohexane-1,3-dione in aqueous solution produced 2-oxo-3,3,5,5-tetramethylcyclopentylideneketene, which underwent hydration to the enol of 2-oxo-3,3,5,5-tetramethylcyclo pentanecarboxylic acid; the enol then isomerized to the keto form of the acid. Rates of hydration of the ketene and rates of ketonization of the enol were measured in perchloric acid, sodium hydroxide, and buffer solutions, and rate profiles were constructed. Rates of enolization of 2-oxo-3,3,5,5-tetramethylcyclopentanecarboyxlic acid were also measured, using bromine to scavenge the enol as it formed, and rates of enolization and ketonization were then combined to give the keto–enol equilibrium constant pKE = 1.65. This and other results are discussed in comparison with the behavior of the unmethylated 2-oxocyclopentanecarboxylic acid system.Key words: flash photolysis, photo-Wolff reaction, ketene hydration, enolization, ketonization, keto–enol equilibria, β-oxocarboxylic acids.

Hydration of Phenylketene Revisited: A Counter-Intuitive Result

In previous work (Can. J. Chem. 1987, 65, 1719-1723 and J. Am. Chem. Soc. 1995, 117, 9165-9171), flash photolysis of diazoacetophenone or phenylhydroxycyclopropenone in aqueous solution was found to produce phenylketene as a short-lived transient species with absorbance at lambda congruent with 260 nm, which decayed with single-exponential kinetics. It has now been discovered that, in the acidity region [H(+)] = 0.000 01 to 0.06 M, this decay is preceded by a faster absorbance rise, and that the overall change conforms well to a double exponential rate law. Analysis of the new data produces rate profiles whose general shapes, as well as the numerical values of their constituent rate constants, plus the form of buffer catalysis, indicate that this newly discovered absorbance rise represents ketonization of phenylacetic acid enol, and that the subsequent absorbance decay represents addition of water to phenylketene. The chemistry of the system, however, requires ketene hydration to precede enol ketonization in a time sequence opposite from that of the absorbance changes. This seemingly counter-intuitive result is nevertheless consistent with the rate law that governs the time evolution of the central species in a two-step rise and decay, such as that observed here.

Highly Efficient Photochemical Generation of a Triple Bond: Synthesis, Properties, and Photodecarbonylation of Cyclopropenones

UV irradiation of alkyl-, aryl-, and heteroatom-substituted cyclopropenones results in the loss of carbon monoxide and the formation of quantitative yields of corresponding alkynes. The quantum yield of the photochemical decarbonylation reaction ranges from 20% to 30% for alkyl-substituted cyclopropenones to above 70% for the diphenyl- and dinaphthylcyclorpopenones. Rapid formation (<5 ns) and then a somewhat slower decay (ca. 40 ns) of an intermediate in this reaction was observed by using laser flash photolysis. The DFT calculations allowed us to identify this intermediate as a zwitterionic species formed by a cleavage of one of the carbon-carbon bonds of the cyclopropenone ring. The latter then rapidly loses carbon monoxide to produce the ultimate acetylenic product. Despite their high photoreactivity, cyclopropenones were found to be thermally stable compounds with the exception of hydroxy- and methoxy-substituted cyclopropenones. The latter undergo rapid solvolysis in hydroxylic solvents even at room temperature. The application of this reaction to the in situ generation of the enediyne structure was illustrated by the photochemical preparation of benzannulated enediyne 12.

Generation of 6-methylene-2,4-cyclohexadienylidene ketene by flash photolysis of benzocyclobutenone in aqueous solution and study of the reactions of this ketene in that medium

Flash photolysis of benzocyclobutenone in aqueous solution produced a transient species with a microsecond lifetime whose rates of decay were measured in perchloric acid, sodium hydroxide, and buffer solutions over the acidity range [H+] 1 × 10–13 – 100 M. This produced a rate profile, isotope effects, and buffer behaviour typical of ketene reactions, and that, together with product identification, served to identify this transient as 6-methylene-2,4-cyclohexadienylidene ketene, formed by electrocyclic opening of the four-membered ring of benzocyclobutenone. Comparison of rates of reaction of this ketene with those of its saturated analog, pentamethyleneketene, produced some expected as well as some unexpected results. Key words: cyclobutenone chemistry, electrocyclic ring opening, ketene hydration, rate profile, solvent isotope effects.

The 2-oxocyclohexanecarboxylic acid keto-enol system in aqueous solution

Flash photolysis of 2-diazocycloheptane-1,3-dione or 2,2-dimethyl-5,6,7,8-tetrahydrobenzo-4H-1,3-dioxin-4-one in aqueous solution produced 2-oxocyclohexylideneketene, which underwent hydration to the enol of 2-oxocyclohexanecarboxylic acid, and the enol then isomerized to the keto form of the acid. Isomerization of the enol to keto forms was also observed using solid enol, a substance heretofore commonly believed to be the keto acid. Rates of ketonization were measured in perchloric acid, sodium hydroxide, and buffer solutions, and a ketonization rate profile was constructed. Rates of enolization of the keto acid were also measured using bromine to scavenge the enol as it formed. Rates of enolization and ketonization were then combined to provide the keto-enol equilibrium constant pK(E) = 1.27. This and some of the other results obtained are different from the corresponding quantities for the 2-oxocyclopentanecarboxylic acid keto-enol system. These differences are discussed.

Aromaticity and antiaromaticity in fulvenes, ketocyclopolyenes, fulvenones, and diazocyclopolyenes

The structures, energies, natural charges, and magnetic properties of 3-, 5-, 7-, and 9-membered cyclic polyenes 1-4, respectively, with exocyclic methylene, keto, ketenyl, and diazo substituents (a-d, respectively) were computed at the B3LYP/6-311G+ **//B3LYP/6-311+G** level to elucidate their aromatic and antiaromatic properties. The corresponding conjugated cyclic cations le and 3e were also studied. The criteria used are isomerization energies (ISE), magnetic susceptibility exaltations (lambda), aromatic stabilization energies (ASE), nucleus independent chemical shifts (NICS), and bond length alternation (deltaR). Planar C2v structures were found to be the lowest energy minima with the exceptions of diazocyclopropene (1d), cycloheptafulvenone (3c), diazocycloheptatriene (3d), and all of the cyclononatetraene derivatives (4). The fulvenes (1a-4a) have modest aromatic or antiaromatic character, and are used as standards for comparison. By these criteria the ketenylidene and diazo cyclopropenes and cycloheptatrienes 1,3-c,d and oxo cyclopentadiene and cyclononatetraene 2,4b are antiaromatic, while the 5- and 9-ring ketenyl and diazo compounds and 3- and 7-ring ketones are aromatic. The degree of aromatic/antiaromatic character decreases with ring size. The consistent agreement with Hückel rule predictions for all the criteria shows their utility for the evaluation of the elusive properties of aromaticity and antiaromaticity.

Carbon-to-carbon identity proton transfer from allene, ketene, ketenimine, and thioketene to their respective conjugate anions in the gas phase. An ab initio study

Gas-phase acidities of CH2=C=X (X = CH2, NH, O, and S) and barriers for the identity proton transfers (X=C=CH2 + HC triple bond C-X- right harpoon over left harpoon -X-C triple bond CH + CH2=C=X) as well as geometries and charge distributions of CH2=C=X, HC triple bond C-X- and the transition states of the proton transfer were determined by ab initio methods at the MP2/6-311+G(d,p)//MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The acidities were also calculated at the CCSD(T)/6-311+G(2df,p) level. A major objective of this study was to examine how the enhanced unsaturation of CH2=C=X compared to that of CH3CH=X may affect acidities, transition state imbalances, and intrinsic barriers of the identity proton transfer. The results show that the acidities are all higher while the barriers are lower than for the corresponding CH3CH=X series. The transition states are all imbalanced but less so than for the reactions of CH3CH=X.

Theoretical Study of [2 + 1] cycloaddition of CO and CS to acetylenes forming cyclopropenones and cyclopropenethiones

The [2 + 1] cycloadditions of carbon monoxide and carbon monothioxide CX (X = O, S) to acetylenes (R1C triple bond CR2 with R1 = H, OH and R2 = CH3, OH, NH2, C6H5) have been studied at the B3LYP/6-311G(d,p) level. It has been shown that the reaction proceeds in two steps forming first an intermediate having the properties of both a carbene and a zwitterion followed by a ring closure leading to cyclopropenones or cyclopropenethiones. The solvent effect does not play an important role in the course of the cycloaddition. The estimation of the first vertical excitation energies by CIS and TD-B3LYP methods shows that the reactions likely take place in the ground state rather than in an excited state. All the studied cyclopropenones and cyclopropenethiones are aromatic as shown by their NICS values and confirmed by calculated and experimental NMR chemical shifts. Different reactivity criteria including HOMO coefficient, local softness, hardness, polarizability, and NICS are used to predict the site selectivity in all studied cases, and the NICS criterion seems to yield the best results among them.

Correlation of rates of uncatalyzed and hydroxide-ion catalyzed ketene hydration. A mechanistic application and solvent isotope effects on the uncatalyzed reaction

Rates of hydration of a number of ketenes were measured in neutral and basic solution using flash photolytic techniques, and rate constants for their uncatalyzed, kuc, and hydroxide-ion catalyzed, kHO, reactions were determined. These results, plus additional data from the literature, were found to provide the remarkably good correlation log kuc= -3.21 + 1.14 log kHO, which spans 10 orders of magnitude in reactivity and includes 31 ketenes. This good correlation implies that uncatalyzed and hydroxide-ion catalyzed ketene hydraton occur by similar reaction mechanisms, which for the hydroxide-ion catalyzed process is known to involve nucleophilic attack on the carbonyl carbon atom of the ketene. Rate constants for phenylhydroxyketene, on the other hand, do not fit this correlation, which suggests that the mechanistic assignment upon which these rate constants are based may not be correct. Solvent isotope effects on these uncatalyzed ketene hydrations are weak; most are less than kH/kD= 2. It is argued that these isotope effects are largely, if not entirely, secondary in nature and that they are consistent with both a reaction mechanism in which nucleophlic attack of a single water molecule on the ketene carbonyl carbon atom produces a zwitterionic intermediate and also a mechanism that avoids this intermediate by passing through a cyclic transition state involving several water molecules.Key words: ketene hydration, rate correlation, nucleophilic attack, solvent isotope effects, phenylhydroxyketene.

Addition of the nitroxyl radical TEMPO to 1-naphthylketene: formation of an unusual adduct

1-Naphthylketene (2), generated by thermal Wolff-rearrangement, is trapped in situ by 2,2,6,6-tetramethylpiridinyloxy radical (TEMPO, TO·) to form the adduct 1-naphthCH(OT)CO2T (4), whose structure is confirmed by an X-ray determination. The 1H NMR spectrum of 4 displays three CH3 groups with very high field chemical shifts (δ 0.10-0.47), and this is attributed to the location of these groups in the shielding region above the π system of the naphthyl ring. At -40°C, doubling of most of the 1H NMR signals occurs, and this is attributed to a freezing out of two conformations differing by rotation around the naphthyl—CH bond.Key words: ketene, TEMPO, restricted rotation, 1H NMR, conformational analysis, free radicals.

Article

Five ketenes, phenyl(ethyl)ketene, phenyl(methylthio)ketene, diphenylketene, pentafluorophenylketene, and 1-naphthylketene, were generated flash photolytically and solvent isotope effects (H2O vs. D2O) on their hydroxide-ion-catalyzed hydration in aqueous solution were determined. The values obtained are all weakly inverse and closely similar (kHO/kDO = 0.76-0.97), as expected for these fast, hydroxide-ion-consuming reactions, known to proceed by nucleophilic attack of hydroxide on the ketene carbonyl group. The characteristic magnitude of these isotope effects should prove useful in identifying new examples of this reaction.Key words: ketenes, flash photolysis, photo-Wolff reaction, solvent isotope effects on hydroxide ion consumption.