AM1 studies of photochemical reactions occurring from the S1 and the T1 states of some cyclopropyl ketones (original) (raw)
Related papers
Journal of the American Chemical Society, 1981
The photochemical behavior of a number of 3-(o-alkylphenyl)-substituted cyclopropenes which contain a benzylic hydrogen in the y-position of the side chain has been studied in mechanistic detail. The results obtained indicate that the products of the direct and triplet-sensitized photolysis are completely different. The singlet states of these systems react by u-bond cleavage of the ring to give products which are explicable in terms of the chemistry of vinyl carbenes. The triplet state, generated by sensitization techniques, undergoes hydrogen atom abstraction by a mechanism analogous to the well-known Norrish type I1 process of carbonyl compounds. Rate constants for hydrogen abstraction were obtained by plotting @,,/&J~ vs. trans-stilbene at a constant quencher to cyclopropene ratio. In contrast to the symmetrically substituted 1,2-diphenylcyclopropenes, the quantum efficiency of hydrogen abstraction of the 1,3 isomers was found to depend on the concentration of starting material. The primary deuterium isotope effect encountered with the symmetrical 1,2-diphenylcyclopropene systems is significantly larger than any previously reported value for hydrogen transfer to an excited state (kH/kD a. 20/1). A substantial tunnel effect is proposed to rationalize the results. In contrast to the results obtained with the symmetrical cyclopropenes, a much smaller effect on the quantum efficiency was observed with the unsymmetrical systems (kH/kD = 3.3/1). Of known photochemical processes, hydrogen abstraction has been surely one of the most intensively investigated reactions. Most studies have centered on the photochemistry of the carbonyl group. These include the photoreductionZ of ketones in solvents with abstractable hydrogens and the type I1 reaction of ketones possessing y-hydr~gens.~ The reactivity of the carbonyl group with respect to hydrogen abstraction depends dramatically on the configuration of the lowest lying triplet state.4 The reactivity of n,r* triplets approximates that of alkoxy radical^,^,^ whereas hydrogen abstraction by A,A* triplets is not observed or occurs a t significantly lower rate^.^-^ The higher unpaired electron density on oxygen appears responsible for the greater reactivity in the former state. In contrast to carbonyl compounds, examples of hydrogen abstraction in the direct and sensitized photolysis of olefins are less common. Nevertheless, a number of reports have appeared in the literature which show that the excited r,r* state of certain olefins have the ability to abstract hydr~gen.'"~ Thus, both (1) Photochemical Transformations of Small Ring Compounds.
Journal of the American Chemical Society, 1980
The photochemical reactions of several 3-allyl substituted diphenylcyclopropenes have been studied. Irradiation of 2-methyl-3-( l-methyl-2,3-diphenyI-2-cyclopropen-1 -yl)-I-propene produced products derived from both cyclopropenyl ring opening and side-chain fragmentation. The formation of a bicyclo[3.1 .O]hex-2-ene from the irradiation can be rationalized in terms of a vinylcarbene intermediate which undergoes internal cycloaddition. Successive introduction of methyl groups on the (Y or y carbon of the allyl side chain was found to dramatically increase the importance of side-chain cleavage relative to ring opening. Thus, irradiation of the 3-prenyl substituted system was found to give rearranged products corresponding to a 1,2-substituent shift. The results obtained from our studies reveal the following trends: ( I ) excited cyclopropene singlets give products consistent with ring cleavage or side-chain fragmentation, whereas triplet states undergo intramolecular [2 + 21 cycloaddition; (2) the 1,2-substituent shift only occurs with 1,2-diphenyl substituted cyclopropenes; (3) the rearrangement is completely regiospecific leading to unsymmetrically substituted cyclopropenes; (4) side-chain fragmentation competes with ring opening as a primary process when stable radicals are produced. A mechanism involving a dissociation-reassociation process is proposed to account for the formation of the rearranged cyclopropenes.
Journal of the American Chemical Society, 1975
ment. Irradiation of exo-2-chlorobicyclo[2.2.2]octen-3-one (l)g in methanol at 300 nm gave endo-7-carbomethoxy-A2-norcarene (5)' O in 55 % yield as the only major product. l1 Similarly, photolysis of endo-2-chlorobenzobicyclo[2.2.2]octadien-3-one (3)9 gave the A2m4norcaradiene rearrangement product 612 in 60 % yield servative through the consequences of configuration l -~ It accounts for a variety of thermal structural isomerizations of cyclopropanes in a unified and theoretically plausible way. It does not provide a mechanism for the thermal epimerization of cyclopropanes, a reaction shown by many unconstrained cyclopropanes which is most simply treated as an independent competitive process. 17, l8 Acknowledgment.
J Am Chem Soc, 1978
The photochemical rearrangement of several 3-phenyl-and 3-vinyl-substituted cyclopropenes to indenes and 1,3 cyclopentadienes has been studied. The rearrangements were shown to be derived from the K,K* singlet state since triplet sensitization led to no reaction or else resulted in a 2 + 2 dimerization reaction. When an unsymmetrical cyclopropene such as 1,3-diphenyl-2-methyl-3-benzylcyclopropene (9) was used, a mixture of l-methyl-2phenyl-3-benzylindene (10; 80%) and l-phenyl-2-methyl-3-benzylindene (1 1; 20%) was obtained. Irradiation of 1,3-diphenyl-2-methyl-3-vinylcyclopropene (32) produced a mixture of 3-methyl-1,2-diphenyl-(27), l-methyl-2,3 diphenyl-(28), and 2-methyl-1,3-diphenyl-1,3-cyclopentadiene (33) as well as l-methyl-2-phenyl-3-vinylinderie (34) in good yield. The major products formed correspond to cleavage of the cyclopropene bond attached to the methyl group. Two fundamentally different mechanisms seem plausible. One path involves cyclopropene ring opening to give a 1,3-diradical intermediate which subsequently decays to a vinylcarbene. An alternate path involves T-K* bridging in the excited singlet state to give a bicyclo[2.1.O]pentane diradical, which then undergoes a subsequent fragmentation. Some support for the carbene mechanism was obtained from the irradiation of l-phenyl-2,3,3-trimethylcyclopropene (52). The distribution of methoxy ethers obtained from the irradiation of this compound In methanol also corresponds to cleavage of the cyclopropene bond attached to the methyl group.
J Am Chem Soc, 1977
Although the photochemical behavior of the carbonyl group has received much scrutiny in the past d e~a d e ,~.~ relatively little is known about the photochemistry of the small amount of enol tautomer which exists in equilibrium with the keto In an earlier reportS dealing with the photorearrangement of 4-phenyl-3-chromanone (1) to 4-phenyldihydrocoumarin (2),6 evidence was presented which demonstrated that the enol content can be an overriding factor in determining the quantum efficiency of a photoreaction. As part of our continuing studies dealing with enol photoc h e r n i~t r y ,~~~ we have undertaken an investigation of the excited state behavior of the 4-carbomethoxy-3-chromanone (3) system. W e now report that in extending our studies to this system, we have discovered an unusual solvent effect which controls the product distribution. In addition, we have uncovered a n unprecedented photodecarbonylation reaction which can best be explained by invoking the intermediacy of a transient cyclopropanone.
J Am Chem Soc, 1979
Although the photochemical behavior of the carbonyl group has received much scrutiny in the past d e~a d e ,~.~ relatively little is known about the photochemistry of the small amount of enol tautomer which exists in equilibrium with the keto In an earlier reportS dealing with the photorearrangement of 4-phenyl-3-chromanone (1) to 4-phenyldihydrocoumarin (2),6 evidence was presented which demonstrated that the enol content can be an overriding factor in determining the quantum efficiency of a photoreaction. As part of our continuing studies dealing with enol photoc h e r n i~t r y ,~~~ we have undertaken an investigation of the excited state behavior of the 4-carbomethoxy-3-chromanone (3) system. W e now report that in extending our studies to this system, we have discovered an unusual solvent effect which controls the product distribution. In addition, we have uncovered a n unprecedented photodecarbonylation reaction which can best be explained by invoking the intermediacy of a transient cyclopropanone.
Mechanistic and exploratory organic photochemistry. 119. Cyclopropene photochemical studies. 3
The Journal of Organic Chemistry, 1979
Ph 'P h 6 cyclopropene (2) proved to be the product of a walk rearrangement of the 1-methylcyclopropene (1) and conversely, and thus the photochemistry of this isomer (i.e., 2) was of interest, too. Results Synthesis of Photoreactants and Potential Photoproducts, A convenient synthesis of both desired photochemical reactants, 1 and 2, was found in the reaction of isobutenylmagnesium bromide. with 1,2-diphenyl-3methylcyclopropenium fluoborate as depicted in Chart I. Additionally, syntheses were devised for those cyclopentadienes expected as potential products from the photochemistry of the 1-methylcyclopropene 1 based on the structural change shown in eq 1. These preparations, too, are given in Chart I. Finally, the previous studies had led to indene products as well, and Chart I also contains details of synthesis of this type of product. Exploratory Photolyses. High-conversion irradiations were run by using a 450-W immersion apparatus while Chart 11. Photochemical Transformations4
The Transfer and Conversion of Electronic Energy in Some ‘Model’ Photochemical Systems
Photochemistry and Photobiology, 1965
Recent studies of the effects of molecular structure and reaction environment on the mechanism of primary photochemical processes involving transfer and conversion of electronic energy in relatively 'simple' organic molecules are presented and discussed. A quantitative i.r. spectroscopic method for studying intramolecular and intermolecular photoprocesses of U.V. irradiated substrates dispersed in solid alkali halide matrices at room temperature is described. Current data for the substrates ortho-nitrobenzaldehyde, anthracene and benzophenonebenzhydrol are presented. A series of 'model' ketones containing cyclopropyl groups have been synthesized and while their absorption spectra are similar, the efficiency of vapor-phase photodissociation into radicals is shown to be strongly dependent on molecular structure. Butyrophenone and a series of ring substituted derivatives have been photolyzed in the liquid phase using the quantum yield of the photoelimination of ethylene (Type I1 split) as a "probe" to determine the effect of substituents on the internal H atom abstracting power of the excited carbonyl chromophore. @ c~R~ is very sensitive to ring substitution, dropping from 0.24 in butyrophenone to 0.20, 0.058 and 0.00 in the p-CH3, p-OCH3 and p-NHz derivatives respectively, and to 0.00 in both ortho and para hydroxy derivatives. This effect is correlated with their absorption spectra in terms of the lowest states of these alkyl aryl ketones being s(n, n*) rather than 3(n, n*) in character. While several 'classic' photochemical reactions, unimolecular and bimolecular, proceed efficiently in solid KBr matrices giving the same product as in liquid systems, the 'model' cyclopropyl compounds and the alkyl aryl ketones did not undergo their usual intramolecular processes. Implications of this molecular environment effect are pointed out. IN ORDER to obtain a 'complete' understanding of the photochemistry or photobiology of a given system, ideally one has to elucidate the entire 'life history' of the photoprocess, starting with the act of absorption and concluding with a detailed analysis of the system in its final physical and chemical state. Clearly this is a monumental (and perhaps impossible) task even for simple organic molecules, much less for complex molecules of biological significance. However, despite the formidable challenges that face us, in recent years, great advances have been made in the understanding of photoprocesses. In part, these have been *Presented at the Rapporteur Session,
Theoretical investigation of the photochemical reaction mechanism of cyclopropenone decarbonylation
Molecular Physics, 2011
The gas-phase decomposition mechanism of the photochemical and thermal reaction of cyclopropenone leading to carbon monoxide and acetylene has been investigated theoretically. We employed the B3LYP, MP2, and CASSCF methods with the 6-311 + G** basis set to determine the pathways and the potential energy surface (PES) of this reaction. PES minima were characterized by the absence of any imaginary frequencies and compared with the transition states that contained single imaginary frequencies. The intrinsic reaction coordinate (IRC) method was used to find the minimum energy paths in which reactants and products were connected to the transition states. Activation barrier, thermodynamic, and IRC analyses were performed using the above three methods. Our computations indicated that the decomposition of cyclopropenone proceeds through a stepwise mechanism containing two transition states (TS1 and TS2) and an intermediate. The results show that TS1, the critical transition state, determines the rate of the cyclopropenone decomposition reaction. Therefore, we employed natural bond order (NBO) calculations to probe the structure of the intermediate. The calculations showed that the intermediate has resonance structures containing a carbene and a zwitterion. Our results are in good agreement with previous theoretical and experimental studies. © 2011 Taylor & Francis.