Photoinduced C-Br homolysis of 2-bromobenzophenones and Pschorr ring closure of 2-aroylaryl radicals to fluorenones

A variety of diversely substituted 2-aroylaryl radicals, generated by photoinduced homolysis of 2-bromoarylketones, is shown to undergo Pschorr cyclization to yield fluorenones in moderate to excellent yields. The photochemical results illustrate that the substituents in the two phenyl rings of the 2-bromobenzophenone skeleton exert a dramatic influence on the reactivity of the derived 2-aroylaryl radicals. The disubstitution by methoxy groups in the radical ring renders the aryl sigma-radical highly electrophilic and unreactive for hydrogen abstraction and cyclization. On the other hand, the substituents in the non-radical ring that strongly stabilize the hydrofluorenyl pi-radical, formed subsequent to the attack of the 2-aroylaryl radical on the non-radical ring, promote cyclization to furnish fluorenones in excellent isolated yields.     

Temperature-controlled enhancement of oxygen uptake from water using oxygen carrier solution

Development of an artificial gill, for the uptake of oxygen from water to air, requires an increase in oxygen transfer rate. In the present study, oxygen transfer rate was enhanced using a washed red blood cell suspension as a thermo-responsive oxygen carrier solution, which changes oxygen affinity with temperature. Oxygen dissolved in water first combined with the oxygen carrier solution at a low temperature using a membrane module. The oxygen carrier solution was then heated to release oxygen into the air using a second membrane module. The water flow rate required to sustain a human being at rest was greatly reduced by heating the oxygen carrier solution due to increase in the limit of the oxygen partial pressure of water of which can be transferred, compared with when oxygen was transferred directly from water. The required membrane surface area is 225 m², sufficient for the development of a compact artificial gill.     

A facile oxidation and oxygen insertion of the cyclopentadiene ring by molecular oxygen in solution

[reaction: see text] Oxidation of substituted cyclopentadienes by molecular oxygen afforded the corresponding pyrylium cations in acidic solution, whereby an oxygen atom was inserted into the cyclopentadiene ring. This novel one-step reaction proceeds in a different yield depending on substitution patterns of the cyclopentadiene. A possible reaction pathway and formation mechanism of the pyrylium cation are proposed and discussed.     

The Walden cycle revisited: a computational study of competitive ring closure to alpha- and beta-lactones

The text-book Walden cycle which interconverts the stereochemical configurations of chlorosuccinic and malic acids involves a beta-lactone intermediate in preference to an alpha-lactone intermediate because the O(nuc) C Cl angle in the transition structure for the former (174 degrees) is more favourable than that for the latter (139 degrees), as determined by PCM(epsilon = 78.4)/B3LYP/6-31+G* calculations; the smaller ring-strain energy of the beta-lactone contributes little to the reactivity difference.     

Photoinduced oxygen dynamics in lyophilized hemoglobin

Reversible laser induced deoxygenation in the lyophilized phase of hemoglobin is demonstrated by means of resonant Raman scattering, luminescence, and optical transmission. Specific Raman modes, which are both sensitive to the spin states of Fe(II) in the hemes and resonant in the visible, are monitored as a function of time to evaluate the effect of the illuminating laser. These modes act as in-situ markers of the oxygen content of the protein. The reversible photoinduced deoxygenation can be observed through both the Raman spin-markers and the optical transmission experiments. In the former, reversible changes in the intensities of specific Raman modes are observed, while in the latter, the oscillator strength of the two main absorptions of oxyhemoglobin in the visible are seen to vary accordingly. The luminescence in lyophilized hemoglobin is found to have at least two different contributions, (i) a resonant component with the Raman modes and; (ii) a nonresonant contribution, which increases at high input laser powers and eventually masks the Raman signals. The nonresonant contribution is the luminescence of the photoproduct achieved by thermal denaturation of the protein and remains standing as a permanent nonreversible damage in the illuminated spot. Semiempirical electronic calculations of the wavefunction and total energy of the iron porphyrin reveal the underlying physical origin of the laser induced deoxygenation process in the hemes and are also presented.     

Photoinduced chemiluminescence in salicylaldehyde hydrazone solution

Simultaneously with the chemiluminescent reaction of salicylaldehyde hydrazone (G) oxidation by hypochlorite, the competing chemiluminescent reaction of G with the hydroxide ion occurs in alkaline media. The former reaction is accompanied by green chemiluminescence, whereas the latter, with blue chemiluminescence. The estimation of the rate constant for the bimolecular reaction G + OH^– gives the value 2×10⁵ l mol^–1 s^–1. The necessary condition for appearance of chemiluminescence upon interaction of G with OH^– is the preliminary illumination of the solution of G with light at a wavelength of 400–540 nm, which is accompanied by the formation of an acoplanar quinonoid isomer of G interacting with the hydroxide ion. The height of the energetic barrier for transition of G molecules from the illuminated to the dark state is 18 kcal mol^–1.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 129–134.Original Russian Text Copyright © 2005 by Tsaplev.This revised version was published online in April 2005 with a corrected cover date.     

Energy of N-H-bond dissociation in phenothiazines and diphenylamines

Energies of the dissociation of N-H-bonds (D _N-H) in 3 phenothiazines, phenoxazine, phenoselenoazine, and 9 diphenylamines (AmH) are determined. The D _N-H values are calculated from kinetic data by means of intersecting parabolas. The rate constants of the following types of reaction are used in calculations: RO ₂ ^· + Am _i H, R^· + Am _i H, Am _i ^· + PhMe₂CH, and Am _i ^· + ROOH. As a rule, the results obtained for the reactions of different types are in good agreement with each other and with the results obtained using other methods.     

Mechanistic and synthetic aspects of macrolide ring closure

Kinetic studies have substantiated the idea of a “double-activation” mechanism for macrolide ring closure from relatively electron-rich 2-pyridyl thiolesters while a change of mechanism is observed for electron-poor 2-pyridyl thiolesters. Mechanistic and synthetic aspects of the ion-assisted cyclization are also discussed.     

Synthesis of azaphenanthridines via anionic ring closure

A new and convergent synthesis of azaphenanthridines via an anionic ring closure is reported. Ortho-lithiation/in situ borylation of cyanopyridines produces the corresponding cyanopyridylboronic esters, which undergo a Suzuki-Miyaura cross-coupling to give the key intermediates. Addition of lithium morpholide produces the azaphenanthridines.     

Photochemical ring closure of muconic acid anhydride

2.5-Dimethyl muconic acid anhydride has been synthesized in three steps from catechol and transformed via a high yielding photochemical ring closure into cis-l,5-dimethyl-3-azabicyclo [3.2.0] heptane-2,4-dione.     

Furan ring opening—indole ring closure: pseudooxidative furan ring opening in the synthesis of indoles

A new approach to the synthesis of 4-(2-indolyl)-3-buten-2-ones is described based on condensation of 2-tosylaminobenzyl alcohols with N-tosylfurfurylamine. Pseudooxidative furan ring opening occurred via protolytic elimination of tosylamide from the side chain of the 2-tosylaminoaryl(5-tosylaminomethylfur-2-yl)methanes formed in situ.     

The use of sodium hydride in a ring closure

The use of sodium hydride for the synthesis of 1,5-methano-3-benzazocine derivatives, which involves the intramolecular acylation of the α-methylene group of tetralone by methylene isocyanate substituents, is discussed in detail.     

Rules for anionic and radical ring closure of alkynes

This work reexamined the stereoelectronic basis for the "favored attack trajectories" regarding the nucleophilic and radical cyclizations of alkynes. In contrast to the original Baldwin rules, the acute attack angle of a nucleophile leading to the proposed endo-dig preference for the formation of small cycles is less favorable stereoelectronically than the alternative obtuse trajectory leading to the formation of exo-dig products. For smaller cycles, this intrinsic stereoelectronic preference can be masked by the greater thermodynamic stability of the less strained endo-products. Unbiased comparison of competing cyclization attacks has been accomplished via dissection of the activation barrier into the intrinsic barrier and thermodynamic component via Marcus theory. Intrinsic barriers of thermoneutral reactions strongly favor exo-dig closures, in full accord with the greater magnitude of two-electron bond forming interactions for the obtuse trajectory. This analysis agrees very well with experimental observations of efficient 3-exo-dig and 4-exo-dig cyclizations predicted to be unfavorable by the Baldwin rules and with the calculated 3-exo-/4-endo-, 4-exo-/5-endo-, and 5-exo-/6-endo-dig selectivities in the cyclizations of carbon-, nitrogen-, and oxygen-centered nucleophiles. The generality of these predictions is confirmed by analogous trends for the related radical cyclizations where the stereoelectronically favorable exo-closures are also preferred kinetically, with a few exceptions where a large difference in product stability skews the intrinsic stereoelectronic trends.     

Pyrroles and pyrazolines from the ring closure of azines

Alkyl substituted pyrroles and pyrazolines are obtained in good yields by catalytic ring closure of azines, using nickel or cobalt (II) halides as catalysts. The specificity of the reaction is determined by the structure of the azines and a notable exception is discussed.     

Photoinduced activation of molecular oxygen by various porphyrins,bis-porphyrins,phthalocyanines, pyridinoporphyrazins,and their metal derivatives

Aerobic irradiation of tetraphenylporphyrins, phthalocyanines, tetra-t-butylphthalocyanines, tetracarboxylphthalocyanines, tetrapyridinoporphyrazins and some of their metal derivatives with visible light (λ > 420 nm) give singlet oxygen by energy transfer and oxygen superoxide by electron transfer, but some of their Cu, Zn of Pt derivatives are efficient quenchers for superoxide formation.     

Photoinduced oxygen adsorption of AgBr crystals at 77 K

Photoinduced adsorption (PIA) of oxygen molecules on AgBr films at 77 K has been experimentally observed and investigated. Lifetimes of PIA centers and desorption activation energies of O₂ molecules have been determined. The process is suggested to compete with the ionic step of AgBr photolysis.

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- () - AgBr 77 K. - O₂. , , - AgBr.