Application of rhenium complexes to organic synthesis has progressed rapidly in 1990's. In this review are summarized the recent advances in synthetic reactions catalyzed or promoted by rhenium complexes. In the first half were described the reactions with high valent rhenium-oxo species. Methyltrioxorhenium catalytically promotes the oxidation of olefins, aromatic rings, and hetero atoms with hydrogen peroxide. The combined use of tetrabutylammonium perrhenate and a Brønsted acid catalyzes the 1, 3-rearrangement of allylic alcohols, the Beckmann rearrangement, and the cyclization of benzylacetone oxime derivatives. In the latter half, low valent rhenium-catalyzed reactions were summarized. A chiral rhenium cationic compound readily forms the complexes with olefins and carbonyl compounds in high enantiofacial selectivity. The asymmetric hydrocyanation of aldehydes and the alkylation of quinolines or isoquinolines are developed by diastereoselective addition of nucleophiles to these chiral complexes. Bromopentacarbonylrhenium acts as an efficient Lewis acid, realizing the catalytic Friedel-Crafts acylation of arenes with acyl chloride.
Fluorine has remarkable electronic effects and leaving-group ability as fluoride ion. Paying particular attention to the use of these properties in organic synthesis, we discuss the preparation of gemdifluoroolefins and their synthetic applications. gem-Difluoroolefins with a wide range of substituents are synthesized from commercially available 2, 2, 2-trifluoroethyl p-toluenesulfonate (1), wherein the two processes are included : (i) a boron-mediated alkylation leading to gem-difluorovinylboranes 4, and (ii) a variety of their direct functionalizations via gem-difluorovinylcoppers 5. This methodology provides a useful synthetic route to unsymmetrically disubstituted difluoroolefins via the introduction of difluorovinylidene unit (CF2=C). The obtained difluoroolefins readily undergo addition-elimination, cycloaddition-elimination, and cationic cyclization-elimination processes leading to new synthetic reactions. Regiocontrolled syntheses of fluorinated heterocycles, hetero Diels-Alder reactions, fluorine directed Nazarov cyclizations, Friedel-Crafts cyclizations, and tandem cyclizations in their combination are achieved to construct highly functionalized and/or fused ring systems. Throughout these reactions, fluorine substituent functions as an activator of the substrates and a controller over the reaction pathways.
Some synthetic electroorganic reactions promoted by chemically reactive electrodes (CRE) have recently been exploited and CRE is one of the most attractive new techniques in electroorganic chemistry (EOC). Altohugh the definition of CRE is not always clear at present, the role of electrode in CRE method is quite different from that in the conventional EOC. Namely, the role of electrode in the conventional EOC is generally just a donor or acceptor of an electron and it is not involved in the organic reaction as a reagent. On the other hand, in CRE method, some electrode mterials made of chemically reactive materials such as Mg, Al, and Zn are involved in the reaction as reagents and promote unique reactions which are not attained under the usual conditions of EOC. In this article some synthetic reactions using CRE are described, and the main contents of the article are as follows : 1) Electroreuction of aliphatic ester and amide and related C-C bond forming reactions, 2) Si-Si and Ge -Ge bonds formation reactions and their application to the synthesis of ploysilane and polygermane high polymers, 3) Coupling of dienes and styrenes with aliphatic esters, 4) Reduction of aromatic compounds, 5) Generation of highly reactive metals (Electrogenerated metals).
Unstable intermediates were generated in situ by electrochemical oxidation of phenolic compounds to give varied cycloadducts with dienes or alkenes by using hydrophobic fields in the reaction system. In an aqueous medium, Diels-Alder reactions of quinones and dienes were markedly prometed in the presence of sodium dodecylsulfate (SDS). Electrochemical generation of unstable quinones and subsequent cycloaddition reactions with varied dienes were mediated by SDS micelle. These electrolytic reactions were also accomplished in a conductive lithium perchlorate/nitromethane system in which Diels-Alder reactions were highly promoted. In this non-aqueous reaction system, electrochemical generation and cycloaddition reactions of ο-quinone methides and unactivated alkenes were also successful to give chroman and spirochroman skeletons which have been proven to be difficult to yield in usual methods. The electrolysis in the lithium perchlorate/nitromethane system was further applied to the proposed biomimetic cycloaddition of ο-quinone methides and terpenes to give natural euglobal skeletons.
Pentacoordinate oxaphosphetanes, oxasiletanides, oxagermetanides, oxastannetanides, and tetracoordinate oxaboretanides were successfully synthesized as stable intermediates of the Wittig reaction, the Peterson reaction, Peterson-type reactions, and boron-Wittig reaction, respctively Tetracoordinate and pentacoordinate oxathietanes, tetracoordinate oxaselenetanes and pentacoordinate azaphosphetidines were also synthesized as their sulfur and selenium-analogs, and aza-analog of oxaphosphetanes. The X-ray crystallographic analyses of Si, Ge, P, S, and Se compounds indicated that they have a distorted trigonal bipyramidal structure with two apical oxygen atoms, but those of B, Sn, and another type of Ge compounds demonstrated that they have a very distorted tetrahedral boron, a nearly square pyramidal tin, and a tetrahedral germanium strongly interacting with an oxido anion, respectively. Expectedly, on heating all compounds containing group 13, 14, and 15 elements underwent the Wittig-type reaction to give the corresponding olefins. Although the thermolysis of oxachalcogenetanes with a tetracoordinate and pentacoordinate S and Se centers gave no olefin, it was suggested that the former compounds are intermediates of the Corey-Chaykovsky reaction of sulfur ylides.