Sara Ariel

Norrish type II reaction in the solid state: Involvement of a boatlike reactant conformation

Untersucht wird das Verhaltnis der Produkte (II)-(IV), das bei Bestrahlung der Acetophenone (I) einmal in kristallinem Zustand und zum anderen in verschiedenen Losungsmitteln, insbesondere MeCN, entsteht.

Reaction selectivity in solid state photochemistry

Abstract Steric compression, arising from intermolecular or intermolecular H...H non-bonded repulsive interactions generated along the reaction pathway, is used to explain abnomal solid state photoreactivity. Computer simulations of the motions involved in each case, with calculations of the resulting non-bonded steric compression energies were performed. Three systems were studied: ( A ) Enone photorearrangements, where the observed changes in the solid state photoreactivity of eight closely related α,β-unsaturated cyclohexenones are correlated with their crystal and molecular structures as determined by X-ray diffraction methods. ( B ) Failure of [2+2] photocycloadditilon of an α,β-unsaturated ketone when irradiated in the solid state despite an almost perfect crystal lattice alignment of the potentially reactive double bonds. ( C ) An unsymmetrically substituted ene-dione for which solution photoreactlon results in four products and solid state photoreaction yields only one.

Cyclopropanol formation via β-hydrogen atom abstraction: the five membered transition state analogue of the Norrish type II reaction

Solution phase photolysis of the Diels-Alder adduct 3 formed between o-quinodimethane and 2,3-dimethyl-1,4-naphthoquinone affords, via β-hydrogen atom abstraction and closure of the resulting 1,3-biradical, the cyclopropanol 4. The cyclopropanol itself undergoes secondary photolysis initiated by a novel ring opening process. Irradiation of crystals of adduct 3 affords no detectable photoproducts. The crystal and molecular structure of 3 reveals that cyclopropanol formation in the solid state would involve prohibitive non-bonded steric interactions between lattice neighbors.

A Study of the Norrish Type II Reaction in the Solid State1

Abstract The solid state photochemistry of six α-cycloalkyl-p-chloroacetophenone derivatives (cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, exo-2-norbornyl and 1-adamantyl) is reported. All six undergo smooth type II photochemistry in the crystalline phase. The cyclization-to-cleavage ratios and the cis-to-trans cyclobutanol ratios are tabulated for each ketone, and the results are compared with the corresponding data from the solution photolyses. In general, the solid state medium was found to exert a relatively small effect on the product ratios. The γ-hydrogen atom to carbonyl oxygen abstraction distances, as well as the angular relationships between these two atoms, were determined form the X-ray crystal structure data for each ketone. The data showed that (1) six atom abstraction geometries other than chairlike can be accommodated, (2) abstraction can occur over distances much longer than previously supposed (up to 3.10 A), and (3) there is no strict requirement that the hydrogen undergoing abstr...

High resolution 13C NMR spectroscopy of oxygenated hexalin, octalin and decalin derivatives in solution and the solid state: conformational analysis and crystal lattice effects

Abstract The high resolution, proton-decoupled 13 C NMR spectra of several oxygenated hexalin, octalin and decalin derivatives have been obtained in solution and in the solid state (cross polarization, magic angle spinning). The results are used to deduce both the static (solid state) and dynamic (solution) conformational properties of these systems, and it is demonstrated that the solid state spectra provide information on the number of independent molecules in the crystallographic asymmetric unit.

Reaction pathway in the photoconversion of a tetrahydroanthracenedione (I) to its solid-state photoproduct (II)

Etude des structures cristallines du tetramethyl-2,3,4a,9a tetrahydro-4a,9,9a,10 anthracenedione-1,4 et de son photoproduit tetramethyl-1,3,4,6 benzo [8,9] tricyclo [4.4.0.0 3,10 ] decene-8 dione-2,5

The importance of non-minimum energy reactant conformations in the norrish type II reaction

The Norrish type II γ-hydrogen atom abstraction rate constants in solution for the ketone homologous series (1)–(4) do not correlate with the abstraction geometries as determined by X-ray crystallography, but follow instead the well established bimolecular free radical hydrogen abstraction reactivity pattern cycloheptane > cyclopentane ∼cyclohexane > cyclobutane; this is interpreted as being due to rapid hydrogen atom abstraction from non-minimum energy reactant conformations.

Conformation-specific photochemistry in the solid state

Abstract The unsymmetrical ene-dione 1b exists in solution as a mixture of two conformational isomers, A and B, in rapid equilibrium. Irradiation of 1b in this medium leads to four products, two from each conformer. In the solid state, however, compound 1b adopts a single conformation (A), and photolysis of crystals of 1b leads to a single product. The formation of one rather than two photoproducts in this instance is attributed to a unique solid state steric effect, termed steric compression.

Crystal structure of a cyclization photoproduct of 1-(4-chlorophenyl)-2-cyclooctylethanone, and reaction pathway in Norrish type II cyclizations

The title compound is a minor solid-state photoproduct in the Norrish type II reaction of 1-(4-chlorophenyl)2-cyclooctylethanone. C16H21C10 , Mr= 264.79, triclinic, PI, a=14.335(3), b=15.122(3), c= 15.611 (2),/k, t~ = 76.90 (2), fl= 72.27 (1), 7= 68.00 (1) °, V = 2964 (I) A 3, Z = 8 (four molecules per asymmetric unit), D x = 1. 187 g cm -3, ,;[.(Cu Ka l) = 1.54053/~,/~ = 22.1 cm -~, F(000) = 1136, T= 295 K, R =0.069 for 2958 observed reflections. The four crystallographically independent molecules form a tetrameric hydrogen-bonding pattern. In each of the four molecules the cyclobutanol ring is trans-fused to the cyclooctane ring, and the hydroxyl group is cis to the nearest bridgehead H atom. All four cyclooctane rings exhibit disorder of the outer atoms. On the basis of the above structure, the major photoproduct has been established as the trans-fused-trans-OH isomer. Furthermore, from the known conformation of the reactant, the detailed reaction pathway of the cyclization reaction has been constructed. The pathway involves y-H abstraction by oxygen, followed by rotations about single bonds and angle bending, to allow overlap of radical p-orbitals and formation of a C-C bond. The motions required are minimum for the observed major trans-fused-trans-OH photoproduct, which can be formed in a topochemically controlled reaction.