Asymmetric synthesis from solid state photochemical asymmetric induction to catalytic asymmetric hydrogenation
Creators: name: Liu, Zhaoqing
Publisher: name: University of British Columbia
Year: 2009

description: A number of phosphine oxides were synthesized via different routes. Grignard displacement of menthyl phosphinates afforded different enantiopure monophosphine oxides. By coupling the monophosphine oxides, bisphosphine dioxides I were prepared. 1,2-Ethynediylbis(diphenylphosphine oxide) was reacted with anthracene and its derivatives to give Diels-Alder adducts ethenoanthracenes, and two of them were further reduced to ethanoanthracenes by magnesium in methanol. A new enantiopure bisphosphine ligand, anthraphos (II), was prepared. Dibenzobarrelene derivatives III form well defined crystalline complexes with triphenylphosphine oxide as well as with bisphosphine dioxides I. Irradiation of the complexes in the solid state led, via the di-π-methane rearrangement, to pairs of regioisomeric dibenzosemibullvalene photoproducts. The ratio in which these products are formed was found to vary markedly with the structure of the phosphine oxide. Low to moderate photoproduct enantioselectivities were observed in the complexes with enantiopure diphosphine dioxides. Five of the complexes studied had their crystal and molecular structures determined by X-ray crystallography. Correlations between the photochemical behavior of the complexes in the solid state and their crystal structures are discussed. The new host IV forms crystalline inclusion complexes with a wide variety of aliphatic solvent molecules. The molecular and crystal structures of these complexes were analyzed by X-ray crystallography. The crystal structures show that the complexes with ethanol, 1-propanol and 2-propanol have the chiral space group P2,2,2], where the absolute configuration of IV was assigned. The inclusion complexes can undergo the di- π-methane rearrangement to result in dibenzosemibullvalene derivative product V in solution and the solid state. In the case of the complexes with ethanol, 1-propanol and 2-propanol, the solid state photoreaction gave product V in more than 90% e.e.; this reaction was used for the asymmetric synthesis of bisphosphine ligand VI. The absolute configuration of dibenzosemibullvalene V was also determined. By correlating the absolute configuration of reactant IV with that of its photoproduct V, the key structural features responsible for the enantiospecific solid state photorearrangement were elucidated. Other ethenoanthracene phosphine oxide derivatives were also examined for inclusion complex formation as well as for solid state photochemical asymmetric induction. 9,10-Dihydro-9,10-ethenoanthracene-11-(diphenylphosphine oxide)-12-carboxylic acid and its ethyl ester were photolysed in different media including the solid state, and two regioisomers were obtained in each case. The medium-dependent regioselectivity was considered to result from the conformations in which the phosphinoyl group could be free or hydrogen bonded. Two different mechanisms were applied to rationalize the regioselectivity. The reactivity was correlated with the crystal structures of the starting material. An active asymmetric catalyst, [Rh(COD)(R,R-anthraphos)]BF₄, was prepared from enantiopure anthraphos (II). The molecular and crystal structure of this catalyst was analyzed by X-ray crystallography and compared with the known structures of chiraphos and norphos. Use of [Rh(COD)(R,R-anthraphos)]BF₄ to hydrogenate (Z)-α-acetamidocinnamic acid gave (S)-(+)-N-acetylphenylalanine in 90% e.e. Bisphosphine VI was also used for the same purpose, but gave lower enantioselectivity. Different ruthenium-anthraphos complexes were also synthesized by ligand exchange reactions in an attempt to use them as asymmetric catalysts in the hydrogenation of C=C, C=O and C=N bonds. [chemical compound diagrams]