Alkenylzirconium compounds react with various organic halides in the presence of a palladium or nickel catalyst to give the corresponding cross-coupling products in high yields with high selectivities. Carbometallation of alkynes using alkylaluminums such as trimethylaluminum is catalyzed by zirconocene dichloride. The triisobutylaluminum/zirconocene dichloride system induces hydrometallation and subsequent carbometallation of enynes to afford exocyclic olefins. Addition of 2 equiv. of n-butyllithium to zirconocene dichloride conveniently produces a zirconocene equivalent, i.e., “Cp2Zr”, which can induce bicyclization of enynes or diynes. Reactions of alkenes and alkynes with the zirconocene give zirconocene-alkene and-alkyne complexes, respectively, whose structures have been determined by X-ray studies. Novel 1, 2-migration reactions on zirconium or other metals are also discussed.
Several chiral ligands were designed and synthesized which perform highly enantioselective asymmetric reactions of carbonyl compounds. Enantioselective reduction of ketones with lithium borohydride modified with N, N'-dibenzoylcystine (or N-benzoylcystein) and tent-butyl alcohol (or ethanol) provided optically active sec.-alcohols in up to 9293% e.e. Functionalized ketones such as α-, β- haloketones, α- and β-aminoketones, pyridyl ketones, and fl-ketoesters were also reduced enantioselectively to afford opticall active epoxides, oxetanes, aminoalcohols, pyridyl alcohols, and β-hydroxyesters in high e.e.'s. Chiral pyrrolidinylmethanols, N, N-dibutylnorephedrine (DBNE), chiral piperazine, and polymer bound N-alkylnorephedrine were efficient chiral catalysts of the enantioselective addition of dialkyl zincs to aldehydes. Alkylation of both aliphatic and aryl aldehydes afforded optically active sec-alcohols in high e.e.'s. Formylesters afforded optically active lactones. Enantioselective conjugate addition of dialkyl zincs to enones was catalyzed by chiral complex prepared from DBNE and Ni (acac) 2 or NiBr2.
Polyfluoroaromatic compounds are now widely recognized as the important intermediates of bioactive materials and information recording media, because some of their fluorine atoms can be easily substituted to another functional groups by nucleophilic substitution reaction. However, as the effective fluorination method was not found, polyfluoroaromatic compounds were not yet produced in commercial scale. Among polyfluoroaromatic compounds, pentafluorobenzoic acid (PFBA) is an important raw material for photosensitive agents and bactericidal agents related to quinolone carboxylic acids. In 1984, we established the full commercial scale technology to produce PFBA started from benzonitrile by 1) vapour phase chlorination to pentachlorobenzonitrile (PCBN), 2) halogen exchange reaction of PCBN to pentafluorobenzonitrile (PFBN), 3) hydrolysis of PFBN to PFBA. This technology can be widely applied to production of tetrafluorophthalic acids, polyfluoropyridines and other polyfluoroaromatic compounds. As many patents and technical papers related to application of polyfluoroaromatic compounds are published lately, our technology will be expected to extend to vaious fields in the near future.
Selective florination methods with tetrabutylammonium fluoride, hydrogen fluoride, and silicon tetrafluoride are described. The application of these methods to the synthesis of the bio-active compounds specifically fluorinated is also presented, in which the effects of the introduced fluorine are discussed.
Synthetically useful radical reactions mediated by Et3B are described. An addition of catalytic amount of Et3B to a solution of triphenylstannane, triphenylgermane, or benzenethiol in toluene promotes the effective formation of the corresponding triphenylstannyl, triphenylgermyl, or benzenethiyl radical, respectively. Topics are (1) Et3B induced radical addition of R3SnH to acetylenes and its application to cyclization reaction, (2) Et3B induced stereoselective radical addition of Ph3GeH to acetylenes and its application to isomerization of olefins, (3) Et3B induced radical addition of thiols to acetylenes, (4) A facile reduction of dithiocarbonates with n-Bu3SnH-Et3B, (5) Et3B induced hydrodehalogenation of organic halides by tin hydrides, and (6) Et3B-mediated Reformatsky type reaction and three component coupling reaction of alkyl iodides, methyl vinyl ketone, and carbonyl compounds.
The stereoselective hydrolysis of the long-chain substrate p-nitrophenyl n-dodecanoyl-D (L) -phenylalaninate (D (L) -S12) in surfactant aggregates has been found to be easily controlled by changing amino acid sequence in peptide catalysts and composition of the aggregates. First, the LLL-tripeptide N- (benzyloxycarbonyl) -L-phenylalanyl-L-histidyl-L-leucine (Z-PheHisLeu) was most efficient for the enhancement of enantioselectivity among all the the peptide catalysts in this study. Second, remarkably high enantioselectivity of D (L) -S12 with Z-PheHisLeu was obtained in coaggregates composed of vesicular and micellar surfactants. It is emphasized on the basis of circular dichroism experiments that a favorable fitting of the L-isomer substrate and the active tripeptide like “key and lock” should be very important to enhance the enantioselectivity, and subsequently, the adjusting of the hydrophobic microenvironment to the optimum fit of reactants by changing appropriately the composition of coaggregates would induce the highest enantioselectivity.
Multi-armed compounds (e.g. Hexapus molecules) have six long alkyl chains with terminal ionic groups, attached to a core benzene molecule, and behave like crown ethers, cryptands, or micelles. The number of the arm can be increased with a mathematical growth progression (e.g. 1→3→9→27) by using new synthetic methodology named cascade syntheses, which is a stepwise procedure to yield the multibranched macromolecules. This new type of compounds (cascade molecules) have a spherical polyfunctionalized surface over a lipophilic core and would be an ideal model compound of unimolecular micelles. This article deals with two types of cascade molecules : Arborols and Dendrimers.