Mechanistic and exploratory organic photochemistry. LXXXVIII. Stereochemistry at the methane carbon in the di-.pi.-methane rearrangement (original) (raw)
Related papers
Journal of the Brazilian Chemical Society, 1999
A fotólise direta de 1,1-diciano-3-fenilbutene-1 (3-MDCN) foi pesquisada a temperatura ambiente em solventes de diferentes polaridades (hexano, diclorometano e acetonitrilo). Foram obtidos fotoprodutos originários dos processos di-π-metano e π-metano (migração de hidrogênio 1,2). As estruturas dos produtos foram determinadas por 1 H-NMR, GC/MS, IV e cromatografia. Os resultados das determinações dos rendimentos quânticos relativos e as análises cromatográficas de irradiações sequênciais evidenciaram que i) não ocorrem reações secundárias, até a altas conversões; ii) o rearranjo di-π-metano é mais afetado pelas variações de solvente que o rearranjo π-metano. Não foram observados produtos por fotosensitização com acetofenona ou acetona. A presença de mecanismo simultâneos e os efeitos de solvente foram considerados como evidência de excitações localizadas e deslocalizadas sobre a superfície de energia potencial. The direct photolysis of 1,1-dicyano-3-phenylbut-1-ene (3-MDCN) was investigated at room temperature in solvents of different polarities (hexane, dichloromethane and acetonitrile). Cyclopropanes arising from both the di-π-methane and π-methane (1,2-H migration) processes were obtained as photoproducts. The structures of the products were elucidated by 1 H-NMR, GC/MS, IR and chromatography. Relative quantum yield determination and GC analysis of sequential irradiations gave evidence that: i) no secondary reactions occur, even at high conversions; ii) the di-π-methane rearrangement is significantly more affected by the solvent variation than the π-methane reaction. Photosensitization with acetophenone or acetone did not yield any observable products. The existence of the simultaneous mechanisms and the observed effects were considered as evidence of a possible differentiation between localized and delocalized excitation on the excited state surface.
Journal of the American Chemical Society, 1974
The di-T-methane rearrangement of the 3,3,5-trimethyl-l -phenyl-l,4-hexadiene stereoisomers was studied in order to determine the stereochemical course of the reaction at C-1. cis-and trans-3,3,5-trimethyl-lphenyl-l,4-hexadiene were synthesized and subjected to direct and sensitized irradiations. It was observed that direct photolysis of the cis hexadiene reactant led to cis-3,3-dimethyl-2-(2 ',2 '-dimethylvinyl)-1-phenylcyclopropane as the major product. Similar photolysis of the trans hexadiene afforded the trans vinylcyclopropane. Additionally, a 2 + 2 cycloaddition product was obtained from both reactants and cis-trans isomerization was observed as a competing process. Quantum yields were determined for all processes. In the case of sensitized runs, it was found that the triplet of hexadiene reactants merely underwent cis-trans isomerization and gave no di-r-methane rearrangement. The results are explicable in terms of an excited singlet Mobius orbital array and are not understood merely in terms of least motion.
Journal of the American Chemical Society, 1978
The photochemical behavior of two chiral monocyclic enones, 4-methyl-4-phenyl-and 4-methyl-4-propyl-2-cyclohexenone. has been investigated. In both systems, it was found that the rearrangement to bicyclo[3.1 .O]hexan-2-ones (lumiketones) proceeds stereospecifically, with no loss in optical purity. The same conclusion is reached for the products of aryl migration in the former case, while the rearrangement to a 3-substituted 2-cyclopentenone in the latter case is at least 64% stereospecific. The results are consistent with synchronous mechanisms for these rearrangements describable in terms of orbital symmetry formalisms as [,2a + ,2,] and [,2, + .2,] cycloadditions, originating from triplet excited states of the reactants. Open diradical intermediates are clearly not involved in these reactants. This supports the contention of Woodward and Hoffmann that considerations of orbital symmetry can be used to predict the stereochemical course of reactions proceeding from excited states with unpaired electrons. The inefficiency in these reactions is found not to be due to reversion to starting material from diradical intermediates, but rather is a consequence of the shape of the T I and So potential surfaces in the region corresponding to twisting of the enone around the C=C bond.
1972
The photolyses of 1-bromo-2_phenylpropane, I, and 2-bromo-1-phenylpropane, II, in hexane yield a variety of disproportionation products, III-VIII, in addition to higher molecular weight material (see . The fact that disproportionation occurs, as well as, the fact that I forms chlorides when irradiated in the presence of CC141 suggests that a free radical intermediate is formed. As in the case of the irradiation of I in the presence of cc141, the observed extent of rearrangement was much greater upon irradiation with 254 nm than with 313 nm.
Journal of the American Chemical Society, 1978
On irradiation cyclopentyl phenyl ketone has been found to rearrange principally t o l-phenyl-5-hexen-lone. Further irradiation affords l-phenyl-4-hexen-lone , l-phenyl-l-hydroxy-2-vinylcyclobutane, l-phenyl-3cyclohexen-1-01, a n d acetophenone. The photochemical transformations encountered in this work appear t o be best described by a transannular hydrogen abstraction followed by subsequent reactions of the diradical thus formed. T h e photorearrangement was shown t o proceed by way of a triplet n-T* state. In striking contrast to the extraordinarily slow a n d inefficient reaction in the cyclobutyl system, cyclopentyl phenyl ketone was found t o rearrange with high quantum efficiency and of the order of three powers of ten faster. pounds. XX.
The Journal of Organic Chemistry, 2001
The di-π-methane rearrangement with two π-groups bonded to a single carbon leading to π-substituted cyclopropanes is now well established. The present research had as its goal the exploration of molecular systems having three π-moieties attached to an sp 3-hybridized atom in a search for a tri-π-methane rearrangement. Indeed, it was found that such systems do rearrange photochemically to afford cyclopentenes. However, it was also established that vinylcyclopropanes ring-expand to cyclopentenes on direct irradiation. Since both three-ring and five-ring photoproducts often are found to be produced, it was important to establish that the observed photochemistry was really the result of a true single-step tri-π-methane rearrangement and not the consequence of two sequential rearrangements, first to form a vinyl cyclopropane which subsequently ring expanded to the cyclopentene. The general situation has three speciessA, B, and Cscorresponding to tri-π-methane reactant A, vinylcyclopropane photoproduct B, and cyclopentene photoproduct C. Three rate constants are involved, k 1 for A f B, k 2 for A f C, and k 3 for B f C. The kinetics were applied to two examples with provision to avoid differential light absorption; this utilized singlet sensitization. It was determined that direct formation of the cyclopentene photoproduct proceeds more rapidly than the ring-expansion route. In contrast to the di-π-methane rearrangement, the tri-π-methane reaction was found to be preferred by the singlet, while in these sterically congested systems, the triplet led to di-π-methane reactivity. Finally, a ground-state counterpart of the reaction was obtained.
Journal of the American Chemical Society, 1977
On irradiation cyclopentyl phenyl ketone has been found to rearrange principally t o l-phenyl-5-hexen-lone. Further irradiation affords l-phenyl-4-hexen-lone , l-phenyl-l-hydroxy-2-vinylcyclobutane, l-phenyl-3cyclohexen-1-01, a n d acetophenone. The photochemical transformations encountered in this work appear t o be best described by a transannular hydrogen abstraction followed by subsequent reactions of the diradical thus formed. T h e photorearrangement was shown t o proceed by way of a triplet n-T* state. In striking contrast to the extraordinarily slow a n d inefficient reaction in the cyclobutyl system, cyclopentyl phenyl ketone was found t o rearrange with high quantum efficiency and of the order of three powers of ten faster. pounds. XX.
Journal of the American Chemical Society, 2013
Flash vacuum pyrolysis studies of substituted 6acetoxy-2,4-cyclohexadienones (3 and 10) from 300 to 500°C provide strong experimental evidence that direct [3,5]sigmatropic rearrangements in these molecules are favored over the more familiar [3,3]-sigmatropic rearrangements. The preference holds when the results are extrapolated to 0.0% conversion, indicating that this is a concerted process. Pyrolysis of 6,6-diacetoxy-2-methyl-2,4-cyclohexadienone (9) at 350°C gives a modest yield of the initial [3,5]-sigmatropic rearrangement product, 2,6-diacetoxy-6-methyl-2,4-cyclohexadienone (11). Qualitative arguments and electronic structure theory calculations are in agreement that the lowest energy pathway for each [3,5]-sigmatropic rearrangement is via an allowed, concerted pseudopericyclic transition state. The crystal structures of compounds 3, 9, and 10 prefigure these transition states. The selectivity for the [3,5] products increases with an increasing temperature. This unexpected selectivity is explained by a concerted, intramolecular, and pseudopericyclic transition state (TS-5) that forms a tetrahedral interemediate (ortho-acid ester 4′), followed by similar ring openings to isomeric phenols, which shifts the equilibrium toward the phenols from the [3,5] (but not the [3,3]) products.