Ethanol, acetonitrile, acetone etc. are miscible with water at any mixing ratio; however, the mixtures are not homogeneous in view of the molecular level owing to the hydrogen-bonding interaction between water molecules and the hydrophobic interaction between water and organic solvent molecules. Through these interactions, molecular clusters are easy to be formed in the aqueous organic solvents. The clustering structure of the aqueous organic solvents influences the microscopic environment of substrates in these solvents and controls various chemical reactions such as the hydrolysis of tert-butyl chloride (SN1 reaction). This review will focus on the solvent effects studied in terms of the molecular clusters which reflect the microscopic environment around the substrates. Furthermore, it will be described that the fluctuation of solvent molecules in the solvation cluster is another important factor to trigger a chemical reaction.
The enantioselective addition of trimethylsilyl cyanide to a variety of aldehydes proceeded by the aid of a catalyst prepared in situ from titanium tetraisopropoxide [Ti(O-i-Pr)4] and chiral Schiff bases and gave the corresponding cyanohydrins in high optical yield (up to 96% e.e.). A remarkable rate enhancement was brought about by the aid of the Schiff base to the titanium alkoxide mediated silylcyanation of aldehydes. This catalyst also promoted the highly enantioselective reaction of diketene with aldehydes to give the optically active 5-hydroxy-3-oxoesters, which can be easily converted to 6-substituted-4-hydroxy lactones. These lactones are known to be a very important component of inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase such as compacitn and mevinolin. The chemical structure of chiral Schiff base-titanium alkoxide complexes is discussed based on their 13C NMR spectra, field desorption (FD) mass spectra, and molecular weights.
The synthetic potential of 1-cyclobutenyl ketones (1) has been studied. First the ring expansion reactions of 1 to five-, six-, and eight-membered rings were examined. The reaction of 1 with 1, 3-dienes gave the Diels-Alder adducts (2), which were easily transformed to the hydrindanones by the treatment with ethylaluminum dichloride. The adducts (2) were also found to be good intermediates for the synthesis of 1-acetyl-1, 3, 5-cyclooctatrienes by the electrocyclic reaction. 1-Cyclobutenyl ketones (1) serve as diene components as well as dienophiles in Diels-Alder reaction : 1-acetyl-1-cyclohexenes were fomed when 1 were heated with various dienophiles at elevated temperatures. The alternative way to construct cyclohexenes using 1 as starting materials consists of the conjugate addition of 1-alkenylmagnesium bromides followed by the ethylaluminum dichloride-promoted ring enlargement reaction. Next the ring opening reactions of cyclobutanes derived from 1 were examined. 2- (Trimethylsilylmethyl) cyclobutyl ketones (3), which were prepared by the conjugate addition of trimethylsilylmethylmagnesium chloride to 1, were treated with a catalytic amount of triethylaluminum to give (Z) -6- (trimethylsiloxy) -1, 5-heptadienes stereoselectively.. Furthermore, the stereoselective addition of alkylmetals to 3 and the zinc bromide-catalyzed stereospecific ring opening reaction of the resulting alcohols or their methyl ethers gave (E) - and (Z) -1, 5-dienes with high stereoselectivity.. The highly stereoselective synthesis of various trisubstituted olefins using 2-phenylthiocyclobutyl ketones is also described.
Preparation, characterization, and synthetic application of hypercoordinate organosilicon compounds are described. Regiospecific and highly diastereoselective allylation of aldehydes, aldimines, α-hydroxy ketones, α-ketocarboxylicacids, β-hydroxy- and β-amino-α, β-enones, and 1, 2-diketones have been achieved by using pentacoordinate allylsilicates. Typically, reactions of stereohomogeneous (E) - and (Z) -crotyltrifluorosilanes with various aldehydes in the presence of CsF or a wide variety of hydroxy compounds and triethylamine in THE gave the corresponding anti- and syn-β-methylhomoallyl alcohols, respectively, in highly diastereoselective manner. The stereoselectivity and the results of ab initio MO calculations suggest that the present allylation proceeds via six-membered cyclic transition states including the coordination of carbonyl oxygen to the silicon of pregenerated pentacoordinate allylsilicates. Facile reduction of carbonyl compounds by using pentacoordinate hydridosilicates, structural characteristics of pentacoordinate silylsilicates, and oxidative Si-C bond cleavage of, alkyl- and aryltrifluorosilanes by trimethylamine-N-oxide are also described to present the unique feature of the organosilicates.
Three types of new heteroatom-stabilized carbanion equivalents have been developed : an α-keto vinyl anion equivalent (A), a vinyl anion equivalent (B), and an acetylide equivalent (C). These equivalents have a seleno, thio or sulfinyl group and a stannyl or silyl group at the α and β positions of the carbanion center, respectively, which work together as an olefin masking group. The equivalent A leads to a convenient synthesis of 2-alkyl-2-cycloalken-l-ones as well as an extremely shortstep-synthesis of the prostaglandin enone key intermediate. The procedure is composed of three consecutive reactions, starting with 2-seleno (thio) -2-cycloal-ken-1-ones, (i) conjugate addition of stannyl- or silyl-lithium, alkylation, and (iii) destannyl (silyl) -selenenylation. Optically, active allylic and propargylic alcohols can be efficiently prepared by the reaction of aldehydes with equivalents B and C derived from chiral β-silylethyl sulfoxides and β-silylvinyl sulfoxides, respectively.
A number of 1, 10-phenanthroline derivatives such as 2, 9-dialkyl phenanthroline derivatives, biscrown derivatives with crown ether moieties connected via amide linkages at the 2- and 9-positions, and oligomethylene bridged phenanthrolines which work as ionophores were synthesized. Their properties not only as sensing agents for ion-selective electrodes (ISEs) and fluorometry, but also as agents for selective ion transport were investigated. It was found that 2, 9-dibutyl-1, 10-phenanthroline is an excellent Li+-sensing agent for both ISEs and fluorometry. 1, 6-Bis (9-butyl-1, 10-phenanthrolin-2-yl) -hexane, which is one of the bridged bisphenanhtrolines, exhibits high selectivity toward Li+ among alkali and alkaline earth metal ions in liquid membrane system. Phenanthroline derivative with bis (benzo-15-crown-5) moieties works as a highly selective K+ carrier. The relationship between their structure and function is also discussed.