Cobalt(I)-Catalyzed Reductive Cyclization of Enynes and Diynes Using Hydrogen Gas as a Reductant

Reductive cyclization of enynes and diynes by using H2 gas as a reductant was realized, and the corresponding cyclized products were obtained in good yields without olefin isomerization and over-reduction of the products. By an experiment using D2 instead of H2, it was confirmed that H2 unambiguously operates as a reductant in this reaction. The protocol of the reaction is very economical and user-friendly, using air- and moisture-stable CoBr2·6H2O and 1 atm of pressure of H2

Rh(I)-Catalyzed Formal [6 + 2] Cycloaddition of 4-Allenals with Alkynes or Alkenes in a Tether

Rh(I)-catalyzed formal [6 + 2] cycloaddition of allenal 6 having an alkyne or alkene in a tether proceeded smoothly, giving 5–8- and 6–8-fused bicyclic ketone derivatives 7 in good to excellent yields. It was also found that cyclization of enantiomerically enriched (S)-6a (94% ee) gave cyclic ketone derivative (S)-7a in high yield with reasonable chirality transfer (86% ee). This result indicates that this cyclization proceeds through stereoselective formation of rhodacycle H′ followed by insertion of a multiple bond

Rh(I)-Catalyzed Formal [6 + 2] Cycloaddition of 4-Allenals with Alkynes or Alkenes in a Tether

Rh(I)-catalyzed formal [6 + 2] cycloaddition of allenal 6 having an alkyne or alkene in a tether proceeded smoothly, giving 5–8- and 6–8-fused bicyclic ketone derivatives 7 in good to excellent yields. It was also found that cyclization of enantiomerically enriched (S)-6a (94% ee) gave cyclic ketone derivative (S)-7a in high yield with reasonable chirality transfer (86% ee). This result indicates that this cyclization proceeds through stereoselective formation of rhodacycle H′ followed by insertion of a multiple bond

Novel Synthesis of Conjugated Dienes Attached to a Quaternary Carbon Center via Pd(0)-Catalyzed Deconjugative Allylation of Alkenylidenemalonates

Palladium(0)-catalyzed deconjugative allylation of alkenylidenemalonates and alkylidenemalonates was achieved for the first time. Reactions of dimethyl 2-((E)-but-2-enylidene)malonate with various allylic acetates using LHMDS as a base in DMF in the presence of Pd2dba3 (2.5 mol %) and PPh3 (10 mol %) proceeded at room temperature to give the corresponding α-allylation products in good yields in a regio- and stereoselective manner. This reaction can also be used for allylation of dimethyl ethylidenemalonate or dimethyl 2-((E)-pent-2-enylidene)malonate and give the desired α-allylation products in good yields

Novel Rhodium(I)-Catalyzed Cycloisomerization of 1,3-Diene with Alkene in a Tether: Competitive Reaction with [4 + 2] Cycloaddition

Rh(I)-catalyzed cycloisomerization of 1,3-dienes with alkenes in a tether proceeded smoothly, giving cyclopentene derivatives in good yields. It was interesting that the existence of a heteroatom between a 1,3-diene moiety and alkene in the tether affected the reaction course and that only a [4 + 2] cycloaddition product was produced

Intramolecular Cyclization of Ene-Imine Using Dibutylzirconocene

The reaction of ene-imine with Cp2ZrBu2 was carried out. When a crude imine, which was prepared from ene-aldehyde and primary amine in the presence of MgSO4, was treated with Cp2ZrBu2 at room temperature overnight, cyclopentane derivative having trans-substituents was obtained in high yield along with a small amount of cyclopentane derivative having cis-substituents. Presumably, cis-zirconacycle is a thermodynamic product. Reactions using various ene-imines were carried out. In the case of ene-imine prepared from ene-aldehyde and tBuNH2, only cyclopentane having cis-substituents was produced. In this reaction, chiral amine was used, and diastereoselective cyclization of ene-imine was carried out. As a result, cyclopentane derivative having cis-substituents was obtained in an optically active form after hydrogenolysis of the cyclized compound

Study on Transmetalation of a Nickelacycle with Organometallic Reagents

A transmetalation of the oxanickelacycle 7, generated from the 1,3-diene and tethered aldehyde in 6 via oxidative cycloaddition to a zerovalent nickel complex, with various organometallic reagents was investigated. It was found that the tandem reaction, i.e., cyclization of 6 followed by transmetalation of the resulting oxanickelacycle 7, proceeded smoothly using a catalytic amount of zerovalent nickel complex

Practical Synthesis of <i>N</i>-Boc- and <i>N</i>-Cbz-α-Amido Stannanes from α-Amido Sulfones Using TMSSnBu₃ and CsF

In the presence of TMSSnBu3 and CsF, stannylation of N-Boc- and N-Cbz-α-amido sulfones proceeded very well to afford the corresponding α-amido stannanes in moderate-to-high yields. This reaction tolerated α-aryl-, alkenyl-, and alkyl-substituted α-amido sulfones as well as substrates containing either an ester or cyano moiety, which might be reactive with lithium or magnesium stannides employed in conventional stannylation

Total Synthesis of (−)-Herbindoles A, B, and C via Transition-Metal-Catalyzed Intramolecular [2 + 2 + 2] Cyclization between Ynamide and Diynes

The total syntheses of (−)-herbindoles A, B, and C as naturally occurring forms were accomplished for the first time through transition-metal-catalyzed intramolecular [2 + 2 + 2] cyclization between ynamide and diynes. This strategy provided a highly efficient synthetic route to all three herbindoles from an identical indoline derivative as a common intermediate

Dramatic Influence on the Olefinic Geometry in the Nickel(0)-Catalyzed Coupling Reaction of 1,3-Dienes and Aldehydes Using N-Heterocyclic Carbene as a Ligand

The use of N-heterocyclic carbene 2 as a ligand in a nickel(0)-catalyzed coupling reaction of 1,3-diene and aldehyde was investigated. It was found that the Ni(0)−carbene complex showed a unique property in the reaction of 1,3-diene 4 and aldehyde 5 to give the coupling product Z-6, having a (Z)-olefin in high yield in a highly stereoselective manner. Since the same reaction using the Ni(0)−PPh3 complex gave E-6a in a stereoselective manner, this result indicates that the Ni(0)−carbene complex has a property different from that of the traditional Ni(0)−phosphine complex