Intermolecular Hydroalkoxylation of Terminal Alkynes Catalyzed by a Dipyrrinato Rhodium(I) Complex with Unusual Selectivity

Nontraditional Synthesis of Disaccharides via Acyclic Vinylic Ether Intermediates: Catalytic C-O Cross-Coupling as the Enabling Link

We describe complementary methods for synthesizing acyclic vinylic ethers from two carbohydrate-derived synthons. We compare a nonstereoselective olefination approach with a stereoselective catalytic C-O cross-coupling method, preparing 1,2-disubstituted vinylic ethers with complexity on both sides of the ether linkage. Upon epoxidation/in situ oxacyclization of acyclic vinylic ethers, we synthesized disaccharides with α-d-galacto-, α-d-talo-, β-d-allo-, and α-d-altropyranoside stereochemistry, from d-lyxose and d-ribose precursors. Stereoselective CuI/CyDMEDA-catalyzed C-O cross-couplings offer considerable potential for broadly implementing this nontraditional strategy for glycoside synthesis.

Titanium-catalyzed highly stereoselective anti-Markovnikov intermolecular hydroalkoxylation of alkynes to prepare Z-enol ethers

Enol ethers are essential synthetic frameworks and widely applied in organic synthesis; however, high regio- and stereo-selective access to enol ethers remains challenging. Herein, we report a titanium-catalyzed stereospecific anti-Markovnikov hydroalkoxylation reaction of alkynes for the synthesis of Z-enol ethers with excellent functional group tolerance and yields. Mechanistic studies showed that the titanium coordinates with the alkyne and then an oxygen anion attacks the π-bond of the alkyne from the backside to provide a trans-oxygen metallation intermediate, which accounts for the high Z-stereoselectivity. Furthermore, Z-enol ethers could be applied as a kind of synthon for late-stage transformations and gram-scale synthesis, which demonstrates their potential value in organic synthesis.

Heterobimetallic complexes stabilized by the P2N2 macrocyclic ligand system: synthesis and reactivity of a rhodium-copper system that activates molecular hydrogen

The synthesis and reactivity of the bimetallic rhodium-copper complex, Rh(COE)[P₂N₂]Cu, which is stabilized by the P₂N₂ macrocycle, is reported. In the solid state, the rhodium and copper centers are on opposite sides of the macrocyclic ring with the Cu(I) in a linear environment and the Rh(I) in a square planar array. However, in solution a very symmetrical structure is suggested on the basis of the ¹H NMR data, which is consistent with at least two separate fluxional processes, rotation of the cyclooctene unit and movement of the Rh(I) unit between the two amido donors. Addition of H₂ to Rh(COE)[P₂N₂]Cu results in the formation of ([P₂N₂H]RhH(μ-H)₂Cu)₂via hydrogenation of the coordinated cyclooctene unit, oxidative addition of H₂ to the rhodium center and hydrogenolysis of the copper amido unit. Monitoring the reaction of H₂ by NMR spectroscopy indicated the formation of a number of intermediates which suggests hydrogenolysis of the copper amido linkage occurs to generate CuH in some form, along with Rh(COE)[P₂N₂H], which is converted to Rh(H)₂[P₂N₂H] by hydrogenation of the cyclooctene, which then recombines with the CuH present to generate the final product. Deuteration studies indicate that there is considerable H/D scrambling in the cyclooctane produced that we attribute to reversible beta-elimination, migratory insertion steps.

Forays into rhodium macrocyclic chemistry stabilized by a P2N2 donor set. Activation of dihydrogen and benzene

The reaction of the dilithium diamido-diphosphine macrocycle, Li₂[N(SiMe₂CH₂P(Ph)CH₂SiMe₂)₂N] (Li₂[P₂N₂]) with [Rh(COD)Cl]₂ generates the dirhodium macrocyclic compound, [P₂N₂][Rh(COD)]₂ (where COD = η⁴-1,5-cyclooctadiene), wherein both rhodium-COD units are syn to each other and have square planar geometries. While this dirhodium derivative does react with H₂, no clean products could be isolated. Upon reaction of Li₂[P₂N₂] with [Rh(COE)₂Cl]₂ (where COE is η²-cyclooctene), the dilithium-dihodium derivative ([Rh(COE)][P₂N₂]Li)₂(dioxane) forms, which was characterized by single-crystal X-ray analysis and NMR spectroscopy. The cyclooctene derivative reacts with dihydrogen in benzene to generate the dilithium-dirhodium-dihydride complex ([Rh(H)₂][P₂N₂]Li)₂(dioxane); also formed is the dilithium-dirhodium-phenylhydride complex ([Rh(C₆H₅)H][P₂N₂]Li)₂(dioxane) via oxidative addition of a C-H bond of the solvent. The phenyl-hydride is eventually converted to the dihydride derivative via further reaction with H₂. This process is complicated by adventitious H₂O, which leads to the isolation of the amine-dihydride, Rh[P₂N₂H](H)₂; drying of the H₂ eliminates this side product. Nevertheless, careful addition of H₂O to ([Rh(COE)][P₂N₂]Li)₂(dioxane) results in protonation of one of the amido units and the formation of the rhodium-amine cyclooctene derivative, Rh[P₂N₂H](COE), which upon reaction with H₂ generates the aforementioned amine-dihydride, Rh[P₂N₂H](H)₂. The mechanism by which dihydrogen and C-H bonds of benzene are activated likely involves initial dissociation of cyclooctene from the 18-electron centers in ([Rh(COE)][P₂N₂]Li)₂(dioxane), followed by H-H and C-H bond activation. The ability of one of the amido units of the P₂N₂ macrocycle to be protonated is a potentially useful proton storage mechanism and is of interest in other bond activation processes.

DFT calculations bring insight to internal alkyne-to-vinylidene transformations at rhodium PNP- and PONOP-pincer complexes

Through DFT calculations, the equilibrium between Rh–alkyne and Rh–vinylidene species of PXNXP pincer ligated Rh cationic complexes is shown to be tuned by the P–Rh–P bite angle, which in turn is dictated by the nature of the X moiety of the pincer ligand.

Dipyrrin based metal complexes: reactivity and catalysis

Sometimes named half-porphyrins, bis-pyrrolic dipyrrin ligands endow their metal complexes with unique properties such as the potential to functionalize the heterocyclic backbone or the meso position and the ability to catalyze interesting chemical transformations. Thus, strategies towards the derivatization of or at the meso group and the use of dipyrrin metal complexes for the formation of a broad range of polypyrrolic derivatives such as 2,2'-bis-dipyrrins, nor-/hetero-corroles and porphynoids have been elaborated. Furthermore, the chelating ability of dipyrrins and the possibility of modifying their steric and electronic characteristics by functionalization can be exploited for the development of numerous complexes featuring appealing properties. Hence, C-H activation/amination, polymerization or oxidation reactions can be catalyzed by dipyrrin metal complexes and classical reagents such as Grignard species, Rh-based or Suzuki-Miyaura catalysts have been revisited by incorporation of dipyrrins in the coordination sphere of the metal cations. This contribution aims to review and illustrate all these aspects, highlighting the potential of these complexes for the design and synthesis of valuable organic compounds and metallo-organic architectures.

Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis

With their mechanistic novelty and various modalities of reactivity, transition metal unsaturated carbene (alkenylidene) complexes have emerged as versatile intermediates for new reaction discovery. In particular, the past decade has witnessed remarkable advances in the chemistry of metal vinylidenes and allenylidenes, leading to the evolution of a diverse array of new catalytic transformations that are mechanistically distinct from those developed in the previous two decades. This review aims to provide a survey of the recent achievements in the development of organic reactions that make use of transition metal alkenylidenes as catalytic intermediates and their applications to organic synthesis.

Silver-Catalyzed Olefination of Acetals and Ketals with Diazoesters to β-Alkoxyacrylates

The first silver-catalyzed reaction of acetals or ketals with diazoesters leading to trisubstituted or tetrasubstituted β-alkoxyacrylates is now reported. A broad range of acetals and ketals bearing different substituents is compatible with this protocol and thus provides an attractive approach for the synthesis of complex β-alkoxyacrylates. The power of this method was further demonstrated by the successful synthesis of picoxystrobin, which is one of the most popular agricultural fungicides commercialized by Dupont.

Deuteration and tautomeric reactivity of the 1-methyl functionality of free-base dipyrrins

Regioselective reactivity of the 1-methyl group of free-base dipyrrins is explored, including discussion of tautomerism to provide exocyclic alkenyl reactivity. Deuterium is installed so as to generate dipyrrins substituted with deuterated methyl groups. Furthermore, the 1-methyl group reacts to become involved in C-C bonds involving only sp³-hybridised carbon atoms. The isolation of an elusive framework featuring a dipyrrin substituted with a pyrrole in a non-vinylogous fashion is also reported. The use of asymmetric dipyrrins featuring an electron-withdrawing group on one of the pyrrolic units results in regioselective reaction of the alpha-methyl group distal to the electron-withdrawing group.