Photochemical studies (original) (raw)
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Chemical and photochemical behavior of nondissociating cyclopentadienone dimers
The Journal of Organic Chemistry, 1981
The nondissociating character and endo configuration of the substituted cyclopentadienone dimers 2c,d along with the novel "mixed dimers" 2e,f were unequivocally demonstrated. Irradiation of the dimers 2c,d brings about only reversible 1,3-rearrangement to llc,d of all the possible modes available to the system (Scheme IV). In contrast to that, the "mixed dimers" 2e,f undergo only internal cycloaddition to the dissymmetric cage products 6e,f. Stereoelectronic arguments are put forward to rationalize this behavior in a general framework of this class of compounds.
Tetrahedron, 1981
A&ISCI-The I,(dimethyl-. 1.2.~trimethyl and 12.4.7-letramelhyl subslilultd 3.4o.o'~bipbcnykne) cycbpcntodienoncdimcrs ((14 were prepared and found IO undergo phobchcmically or fkrr~//y a 1.3.reananfcmcnl IO [he ccntrosymmetrical diketons (II). Only the tetnmethyl derivative ((r) undergoes al room lempcnlure a hsl. depncrale [X3] Cope reamngemenl wilh AC;,, = Il.4 kcrl md. ' AII these dimers and rearrangement products appear aof IO dissociate IO their monomers. Inn react with dienophilcs IO give the adducts (12, 13). A slcpwiw mechanism involving diradicll inlermcdialcs (19) is invoked. The "mixed dimcr" (II) was also prepared and studied. The stcric and ckclronic cfftc~s determining OK behavtiur of these compounds are discussed. Photocbcmical studies-XX 3ol b&avj~w in cycbaddithn processes. No other features were "13' AI/or of OrpMk Componnh Buttcrworths, London I%6. lluofiomed here. '94. L. AquiIar. M. and A. Maoyarrez_ Boil. Inst. @im. Unb. 9. W. Jones. 1. Cheat Sot. Chtm. Commw 199 (Iws). Nor. Aufor. Ma. 16.40 (1964); Chcm. Abrrr. 0. 1693t (I964). T. I_. C. braoyoviu and J. E. Downcs. BP. 1.052.9Sl clvm "One Sb0dd recall PI his point. lhl "chhcd" non-dissociating Abstr. 64. WZ'le (I%?). dimcrs' consistently hikd to tive adducts of type 12 in heir '9. Fuchs and M. Pastemak. 1. Chum. SM. C&t. Comm. 537 reaction with malcic anhydride. This was adequarely discussed (19n). in Ref. I.
Journal of the American Chemical Society, 1971
We have recently reported on the variegated photochemical reaction modes of cyclopentadienonbdimers. While the parent, unsubstituted system has been more thoroughly i,2 investigated and found to follow three conversion paths of the t= I-ZZ +ZZZ+ZV,-substituted derivatives appeared 3 to undergo peculiar transformations according to whether they were reversibly dissociating or irreversible (stable) cycle-, pentadienone-dimers4. To gain insight into the mther subtle behavior of these two classes of compounds we
Journal of The American Chemical Society, 1977
Photochemical rearrangement of ketone 1 furnishes 3-5 by way of the diastereomeric biradicals 11 and 12. Through use of le and If, isotopic isomers of 1 specifically labeled in one or the other of the diastereotopic methyl groups, the contribution of 11 and 12 as reaction intermediates was investigated. Results summarized in Scheme 111 show that 92% of the rearrangement proceeds through 11 and only 8% through 12. Experiments with deuterium-labeled l d and with the methoxy-substituted ketone 2 indicate that 1 $hydrogen transfer (six-membered intermediate) is preferred, that 1,6-transfer is possible but slow, and that there is no evidence for 1,4-hydrogen transfer in these systems. Preparation of the various substrates and proofs of structure of products are described. 7 8 10 9 stereochemistry to 9 and 10 have been described previous-These rearrangements of 1 afford an opportunity to examine two different stereochemical problems. W e discuss first the question raised by bicyclic ketones 4 and 5, which differ only in the stereochemistry of the methyl group a t C(3). These are the products anticipated from collapse of one or the other of the diastereomeric biradicals 11 and 12, and the general ly.'O Journal of the American Chemical Society / 99:18 / August 31, 1977
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.
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, 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 Photochemistry and Photobiology A: Chemistry, 2004
A combined matrix isolation and molecular orbital study of the vibrational spectra and photochemistry of 4,6-dimethyl-␣-pyrone (DMAP) was undertaken. Two types of photoreactions: ring opening leading to conjugated ketene and valence isomerization to the Dewar form (1,5-dimethyl-2-oxa-3-oxobicyclo[2.2.0]hex-5-ene; DOOBH), occurred upon UV (λ > 315 nm) irradiation. The latter reaction was efficient, whereas aldehyde-ketene was produced only in little amounts. In addition to the IR spectroscopic study of DMAP, the full mid-IR spectrum of the photoproduced DOOBH is reported and interpreted. Observation of 1,3-dimethyl-cyclobutadiene (DMCB), created by shorter wavelength UV irradiation (λ > 235 nm) of DOOBH, is reported for the first time. In the matrices, DMCB forms a complex with CO 2 ; the structure and IR absorption features of this cage confined DMCB-CO 2 complex are also investigated.
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.
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.