The Morita-Baylis-Hillman reaction, α-hydroxyalkylation of activated olefins, has attracted considerable research interest because of the synthetic utility of the densely functionalized product as well as the exquisite tandem Michael-aldol reaction process under nucleophilic catalysis. This review gives an overview on recent remarkable progress in the Morita-Baylis-Hillman reactions. Several other successful methods affording the Morita-Baylis-Hillman type adducts are also reviewed by focusing their imaginative strategies.
The asymmetric synthesis of 2-arylcyclohexene derivatives was realized via π-allylpalldium complex generated from 2-arylcyclohexenol derivatives, Pd (0), and (S) -BINAPO. Using this method, (-) -mesembrine, (-) -mesembrane, (+) -crinamine, (-) -haemanthidine and (+) -pretazetine were synthesized. Subsequently, we planed to synthesize indole alkaloids as a chiral form using this method. Thus, the novel procedure for the synthesis of indole skeleton was developed. Treatment of cyclohexenol derivative having the silyloxymethyl group at the 2-position with N-tosyl-ο-bromoaniline in the presence of Pd2dba3·CHCl3 and (S) -BINAPO gave cyclohexenylamine derivative with 84% ee in 76% yield. The siloxymethyl group was converted into the cyanomethyl group and the resulting compound was treated with Pd (OAc) 2 and PMe2Ph to give indole derivative. From this compound, we succeeded in the total synthesis of (-) -dehydrotubifoline and (-) -tubifoline.
Transition metal catalyzed cycloaddition and related carbon-carbon bond forming reactions are described. Zerovalent palladium species catalyze the cyclocoupling between two molecules of electron-deficient alkynes or 1, 6-diynes and alkenes or alkynes. Intramolecular [2+2+2] cycloaddition of dimethyl 5, 10-dioxa-tetradeca-2, 7, 12-triynediolate induces bis-annulation of 2, 5-dihydrofuran to give the corresponding phthalate derivative involving two 2, 5-dihydrofuran moieties. In stoichiometric reaction with Pd2 (dba) 3, the triyne furnishes a planar three-coordinated triyne complex of palladium. The cationic pentamethylcyclopentadienylruthenium (II) fragment, Cp*Ru+ (Cp*=η5-C5Me5) induces [4+4] cycloaddition of butadiene and isoprene. The pentamethylcyclopentadi-enylruthenium (II) chloride fragment, Cp*RuCl, is a highly efficient catalyst to induce [2+2+2] cycloaddition of 1, 6-diynes with several carbon-carbon and carbon-nitrogen multiple bonds to furnish bicyclic arenes, cyclohexadienes, pyridines, 2, 2'-dipyridyl derivatives, and pyridones fused by cyclopentane, 2, 5-dihydrofuran or pyrrolidine moieties. An indenylchlororuthenium (II) fragment induces unprecedented catalytic bis-cyclopropanation in the reaction of 1, 6-diynes with strained bicycloalkenes.
Redox reactions of a series of dicationic dyes (22+, 42+, 62+) having two triarylmethylenium chromophores were studied, which are interconvertible with colorless hexaarylethane derivatives (1, 3) or 9-membered cyclic peroxides (5), respectively, upon two-electron transfer. They constitute a new class of electrochromic systems endowed with high electrochemical bistability since the reversible bond making/breaking causes the drastic changes in the geometrical and electronic structures of the redox pairs. Bond dissociated cation radical 2+· is the stable intermediate for the interconversion between 1 and 22+, and the unprecedented tricolor chromic systems showing a hysteretic color change were constructed based on 1, 2+·, and 22+. Drastic change in CD spectrum was observed during the interconversion between chiral helicenes 3 and binaphthylic dications 42+, which is the successful demonstration of electrochiroptical response. Peroxides 5 were readily obtained by two-electron reduction of dications 62+ under air. Since 62+ were regenerated by the “oxidative deoxygenation” reaction of 5, these pairs can serve as a electrochemically controlled oxygen-storage system.
A series of the longest class of oligothiophenes extended at intervals of seven thiophene units from the 6-mer up to the 48-mer has been synthesized. It is estimated that the molecular length of the longest 48-mer reaches approximately 18.6 nm. The systematic studies of the oligothiophenes have provided valuable information on the optical and conductive properties of α-conjugated thiophene systems : an effective conjugation length extends to 20-30 thiophene units, and the active charge carrier species are both π-dimers and chain dimers. The efficient molecular-wire behaviors of oligothiophenes have been demonstrated by photoinduced electron and/or energy transfer characteristics of the fullerene-linked dyad and triad systems. The successful application of these systems to a photovoltaic molecular device is also described.
Polycondensation normally proceeds in a step-growth reaction manner to give polymers with a wide range of molecular weights. Chain-growth polycondensation (CGP) process like the synthetic process of natural polymeric materials such as polypeptides, DNA, RNA, cis-polyisoprene rubber, etc. has been developed to yield artificial condensation polymers having controlled molecular weights and low polydispersities. The requirement for CGP is the selective reaction of monomers with polymer end group without the reaction of monomers with each other. Two approaches to CGP are carried out : (1) the activation of propagating end group by different substituent effects on the reactive site between monomer and polymer, and (2) the prevention of reaction of monomers with each other in solid phase and successive reaction of monomers with polymer end group via phase transfer of monomers. Well-defined aromatic polyamides and polyethers with low polydispersities (Mw/Mn≤1.1) were produced in approach (1), whereas aliphatic polyesters with low polydispersities (Mw/Mn≤1.3) were obtained in approach (2).