Non-noble metal-catalysed carbonylative transformations

The main achievements on non-noble metal (Mn, Fe, Cu, Co, Ni) catalysed carbonylative transformations have been summarized and discussed.

Stereo-Selective Preparation of Teneraic Acid, trans-(2S,6S)-Piperidine-2,6-dicarboxylic Acid, via Anodic Oxidation and Cobalt-Catalyzed Carbonylation

Teneraic acid (piperidine-2,6-dicarboxylic acid) is a naturally occurring imino acid that comprises three stereoisomers due to its two asymmetric centers at C2 and C6. The configuration of natural teneraic acid is reported to correspond to trans-(2S,6S). However, a few studies are focused on the stereospecific synthesis of trans-(2S,6S)-teneraic acid. The present study investigates a convenient synthetic method that includes regiospecific anodic oxidation and stereospecific cobalt-catalyzed carbonylation to obtain trans-(2S,6S)-teneraic acid. Methyl (S)-N-benzoyl-α-methoxypipecolate, the key intermediate that displays a structure that corresponds to an intermediate (N-α-hydroxyalkyl amide) of intramolecular amidocarbonylation, was obtained via an anodic oxidation of methyl (S)-N-benzoylpipecolate. Subsequently, cobalt-catalyzed carbonylation converted the methyl (S)-N-benzoyl-α-methoxypipecolate to trans-(2S,6S)-N-benzoyl-teneraic acid dimethyl ester in good optical purity (>95% enantiomeric excess (ee)) and modest yield (63%). Finally, de-protection occurred via acidic hydrolysis to obtain trans-(2S,6S)-teneraic acid. The stereochemistry of synthesized teneraic acid was confirmed as corresponding to trans-(2S,6S) by comparing its physical properties with those of a cis-meso-isomer and those of a trans-(2S,6S)-isomer that were reported in previous studies.

Phosphabarrelenes as Ligands in Rhodium-Catalyzed Hydroformylation of Internal Alkenes Essentially Free of Alkene Isomerization

Despite significant research efforts in the past, one of the remaining problems to be solved in industrially important hydroformylation is the chemoselective low-pressure hydroformylation of internal alkenes. We report here on a new class of phosphabarrelene/rhodium catalysts 2 that display very high activity towards hydroformylation of internal alkenes with an unusually low tendency towards alkene isomerization. Preparation of new phosphabarrelene ligands, studies of their coordination properties, as well as results obtained in the rhodium-catalyzed hydroformylation of cyclic and acyclic internal alkenes are reported.

Recent advances in cyclohydrocarbonylation reactions

This article describes recent advances in the cyclohydrocarbonylation reactions catalyzed by transition-metal complexes and their applications in organic synthesis as a review covering the relevant literature up to the middle of 2004. The reactions are categorized into four types, i.e., intramolecular amidocarbonylation reactions, intramolecular aminocarbonylation reactions, cyclohydrocarbonylation reactions involving carbon–nucleophiles, and other cyclohydrocarbonylation reactions. Cyclohydrocarbonylation reactions provide efficient routes to various monocyclic, bicyclic, and polycyclic compounds as a one-step cascade process or a one-pot process. Reaction mechanisms for these cascade processes are discussed as needed for clarification. The heterocyclic and carbocyclic compounds, thus obtained, can be further transformed to specific targets. Examples of such applications are also discussed.Key words: catalysis, cyclohydrocarbonylation, hydroformylation, amidocarbonylation, cyclization, regioselectivity, aldehydes, regioselective, cascade, heterocycles, rhodium.

Phosphabarrelene–rhodium complexes as highly active catalysts for isomerization free hydroformylation of internal alkenes

A new class of phosphabarrelene-rhodium catalysts is described which allows for the first time hydroformylation of internal alkenes with very high activity and which proceeds essentially free of alkene isomerization.

Zinc-mediated chain extension of beta-keto phosphonates

A variety of beta-keto phosphonates can be converted to gamma-keto phosphonates through reaction with ethyl(iodomethyl)zinc. The presence of alpha-alkyl substituents, Lewis basic functionality, and modestly acidic NH-protons are accommodated in substrates of this reaction. Chain extension of beta-keto phosphonates that contained olefinic functionality proceeded more quickly than cyclopropanation; however, it was not possible to effect the chain extension to the exclusion of cyclopropane formation. A primary reason for this imperfect chemoselectivity appears to be the slow chain extension of beta-keto phosphonates. Nevertheless, the simplicity, the scope, and efficiency of this method serve to make it an attractive alternative to the established methods for gamma-keto phosphonate formation.

Asymmetric Hydroformylation of Heterocyclic Olefins Catalyzed by Chiral Phosphine-Phosphite-Rh(I) Complexes

Asymmetric hydroformylation of heterocyclic olefins catalyzed by phosphine-phosphite-Rh(I) complexes has been investigated. Hydroformylation of symmetrical heterocyclic olefins such as 2,5-dihydrofuran, 3-pyrroline derivatives, and 4,7-dihydro-1,3-dioxepin derivatives afforded the optically active aldehydes as single products in 64-76% ee. Unsymmetrical substrates such as 2,3-dihydrofuran and N-(tert-butoxycarbonyl)-2-pyrroline gave a mixture of regioisomers. From N-(tert-butoxycarbonyl)-2-pyrroline was obtained N-(tert-butoxycarbonyl)pyrrolidine-2-carbaldehyde in 97% ee. The hydroformylation products from 2,5-dihydrofuran and N-(tert-butoxycarbonyl)-3-pyrroline have the opposite configurations to those from 2,3-dihydrofuran and N-(tert-butoxycarbonyl)-2-pyrroline, respectively, with the same catalyst. The new phosphine-phosphite ligand (R,S)-3,3'-Me(2)-BINAPHOS [= (R)-2-(diphenylphosphino)-1,1'-binaphthalen-2'-yl (S)-3,3'-dimethyl-1,1'-binaphthalene-2,2'-diyl phosphite] was prepared and its hydridorhodium complex was characterized by NMR spectroscopy. Using (R,S)-3,3'-Me(2)-BINAPHOS as a ligand, the enantioselectivity was improved for some substrates. In addition, higher catalytic activity was observed with this ligand for most of the substrates employed.

Regiochemical Control and Suppression of Double Bond Isomerization in the Heck Arylation of 1-(Methoxycarbonyl)-2,5-dihydropyrrole

Arylation of 1-(methoxycarbonyl)-2,5-dihydropyrrole under standard Heck reaction conditions produces a mixture of compounds. The olefin undergoes two types of palladium-catalyzed reactions: (a) arylation to provide C-3 arylated derivatives and (b) competing double bond isomerization. Addition of silver carbonate and thallium acetate fully suppressed the isomerization, and good yields of C-3 substituted compounds were achieved after arylation with aryl halides. With regard to aryl triflates as arylating agents, addition of lithium chloride was necessary to promote the Heck reaction. This additive excluded the use of silver and thallium salts, but high regioselectivity and good yields could be obtained by employing tri-2-furylphosphine as ligand. Arylation was rendered both regioselective and enantioselective (58% ee) with 1-naphthyl triflate as substrate utilizing a (R)-BINAP/thallium acetate combination. The C-3 arylated enamides were converted further into the corresponding 3-arylpyrrolidines.