“…The fact that 42 compounds comprising three completely different classes of ketone react through such similar solid-state geometries argues very strongly that these data may be taken as being generally indicative of successful γ-hydrogen atom abstraction . The data obviously do not provide exact information on the geometric situation in the excited state, where bond lengths and angles can differ somewhat from those in the ground state, but there is every reason to believe that the structure−reactivity relationships developed above are approximately correct and can now be used in a predictive sense.…”
The Norrish/Yang type II photochemistry of 16 ketones having the basic cis-4-tert-butyl-1benzoylcyclohexane or 2-benzoyladamantane structure has been investigated in the solid state and solution. In both media, ketones bearing methyl substituents R to the benzoyl group undergo stereoselective Yang photocyclization to afford endo-arylcyclobutanols. Quantum yield and quenching studies in solution show that the reactions are efficient triplet-mediated processes. Asymmetric induction studies were carried out by providing the reactants with carboxylic acid substituents to which "ionic chiral auxiliaries" were attached through salt formation with optically active amines. Irradiation of the salts (17 in total) in solution gave racemic cyclobutanols, but in the crystalline state, moderate to near-quantitative enantiomeric excesses were obtained. Single crystal X-ray diffraction studies were successfully performed on 10 neutral ketones and four salts. This allowed the reactive γ-hydrogen atoms to be identified and the distance and angular parameters associated with their abstraction to be tabulated. For the 14 compounds whose crystal structures were determined, the average value of d, the CdO‚‚‚H abstraction distance, was 2.61 ( 0.07 Å, and the values of ω (the γ-hydrogen out-of-plane angle), ∆ (the CdO‚‚‚H γ angle) and θ (the C-H γ ‚‚‚O angle) were 53 ( 11, 84 ( 7, and 115 ( 2°, respectively. In a similar manner, the geometric parameters associated with the ring closure reactions of the intermediate 1,4-hydroxy biradicals were estimated from the crystallographic data. This indicates that the biradicals are 66.5 ( 9.8 and 32.7 ( 3.2°out of alignment for cleavage, but that they are well oriented for cyclization, with radical separations (D) of 3.08 ( 0.09 Å. For one of the salts in the adamantane series, the solid-state photoreaction was shown to be topotactic; that is, a single crystal of the reactant salt was transformed quantitatively into a single crystal of the corresponding cyclobutanol salt. Since the absolute configuration of the ionic chiral auxiliary in this case was known, this permitted the absolute steric course of the reaction to be mapped by X-ray crystallography and the abstracted γ-hydrogen to be identified unequivocally. In addition, the crystal structure of a partially reacted crystal containing 60% product and 40% reactant was successfully determined. This showed that the two compounds in the mixed crystal have nearly identical shapes and orientations, thus accounting for the single crystal nature of the process. Finally, it was found that ketones lacking methyl substituents R to the benzoyl group are either photochemically unreactive or undergo Norrish type II cleavage. Possible reasons for this difference in behavior are presented and discussed.
“…The fact that 42 compounds comprising three completely different classes of ketone react through such similar solid-state geometries argues very strongly that these data may be taken as being generally indicative of successful γ-hydrogen atom abstraction . The data obviously do not provide exact information on the geometric situation in the excited state, where bond lengths and angles can differ somewhat from those in the ground state, but there is every reason to believe that the structure−reactivity relationships developed above are approximately correct and can now be used in a predictive sense.…”
The Norrish/Yang type II photochemistry of 16 ketones having the basic cis-4-tert-butyl-1benzoylcyclohexane or 2-benzoyladamantane structure has been investigated in the solid state and solution. In both media, ketones bearing methyl substituents R to the benzoyl group undergo stereoselective Yang photocyclization to afford endo-arylcyclobutanols. Quantum yield and quenching studies in solution show that the reactions are efficient triplet-mediated processes. Asymmetric induction studies were carried out by providing the reactants with carboxylic acid substituents to which "ionic chiral auxiliaries" were attached through salt formation with optically active amines. Irradiation of the salts (17 in total) in solution gave racemic cyclobutanols, but in the crystalline state, moderate to near-quantitative enantiomeric excesses were obtained. Single crystal X-ray diffraction studies were successfully performed on 10 neutral ketones and four salts. This allowed the reactive γ-hydrogen atoms to be identified and the distance and angular parameters associated with their abstraction to be tabulated. For the 14 compounds whose crystal structures were determined, the average value of d, the CdO‚‚‚H abstraction distance, was 2.61 ( 0.07 Å, and the values of ω (the γ-hydrogen out-of-plane angle), ∆ (the CdO‚‚‚H γ angle) and θ (the C-H γ ‚‚‚O angle) were 53 ( 11, 84 ( 7, and 115 ( 2°, respectively. In a similar manner, the geometric parameters associated with the ring closure reactions of the intermediate 1,4-hydroxy biradicals were estimated from the crystallographic data. This indicates that the biradicals are 66.5 ( 9.8 and 32.7 ( 3.2°out of alignment for cleavage, but that they are well oriented for cyclization, with radical separations (D) of 3.08 ( 0.09 Å. For one of the salts in the adamantane series, the solid-state photoreaction was shown to be topotactic; that is, a single crystal of the reactant salt was transformed quantitatively into a single crystal of the corresponding cyclobutanol salt. Since the absolute configuration of the ionic chiral auxiliary in this case was known, this permitted the absolute steric course of the reaction to be mapped by X-ray crystallography and the abstracted γ-hydrogen to be identified unequivocally. In addition, the crystal structure of a partially reacted crystal containing 60% product and 40% reactant was successfully determined. This showed that the two compounds in the mixed crystal have nearly identical shapes and orientations, thus accounting for the single crystal nature of the process. Finally, it was found that ketones lacking methyl substituents R to the benzoyl group are either photochemically unreactive or undergo Norrish type II cleavage. Possible reasons for this difference in behavior are presented and discussed.
“…Several photochemical approaches starting with suitably substituted N-alkylphthalimides and silylalkyl analogues, involving a hydroxyazetidine intermediate that opens to give the expanded benzazepinedione ring, have been developed. [18Ϫ23] These photoprocesses follow various pathways that include Norrish type II or sequential SET desilylation reactions followed by Yang cyclization, [24Ϫ27] ω-hydrogen abstraction/elimination, [28,29] single-electron transfer (SET) together with proton transfer [30,31] and SET followed by decarboxylative photocyclization. [32] A more convergent approach is provided by the intermolecular photocycloaddition of alkenes to the C(O)ϪN bond of N-methylphthalimide (NMP).…”
The excited state of phthalimide anion adds to cyclic, acyclic and aryl-conjugated alkenes in an efficient and regioselective manner to form [2]benzazepine-1,5-diones, substituted at positions 3 and/or 4. The reaction is independent of the ionization potential of the alkene. This process contrasts with the related reactions of N-methylphthalimide or phthalimide, which are limited by the requirement that the participating
“…These processes are accompanied by low-yielding formation of the benzazepinedione 29, which results from a complex multiphoton pathway, initiated by competitive R-deprotonation of the proximal cation radical 30 and terminated by a Norrish type II photocleavage reaction (Scheme 9). 14 Previously, we described the results of a preliminary study of the photocyclization reactions of the phthalimido-R-silyl-polyethers 31 (n ) 1-5). 15 Like their thiaanalogues, these substrates also produce the corresponding crown ethers 32 upon irradiation in MeOH (Scheme 10).…”
mentioning
confidence: 99%
“…In a similar manner, the phthalimido-polythioethers 25 − 26 undergo efficient photocyclization when irradiated in MeOH solutions (Scheme ). These processes are accompanied by low-yielding formation of the benzazepinedione 29 , which results from a complex multiphoton pathway, initiated by competitive α-deprotonation of the proximal cation radical 30 and terminated by a Norrish type II photocleavage reaction (Scheme ) 8 9 …”
Irradiation of phthalimides which contain N-linked omega-trimethylsilylmethyl-substituted polyether, polythioether, and polysulfonamide chains results in efficient production of the corresponding macrocyclic polyether, polythioether, and polysulfonamide products. These photocyclization reactions follow sequential single electron transfer (SET)-desilylation pathways. Only in the cases of phthalimides, bearing mixed ether-thioether N-substituents, do these excited-state cyclization reactions proceed with lower degrees of regioselectivity. This is a result of competitive desilylation and alpha-to-sulfur deprotonation reactions of the zwitterionic diradical intermediates formed by initial SET.
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