Investigations concerning the organolanthanide and group 3 metallocene-catalyzed cyclization–functionalization of nitrogen-containing dienes

Concept-Driven Chemoselective O/N-Derivatization of Prolinol: A Bee-Line Approach to Access Organocatalysts

An investigation into the sensitivity of reaction conditions to a highly utilized protocol has been reported, wherein the mono-Boc functionalization of prolinol could be controlled for the exclusive synthesis of either N-Boc, O-Boc, or oxazolidinone derivatives. Mechanistic investigation revealed that the elementary steps could possibly be controlled by (a) a requisite base to recognize the differently acidic sites (NH and OH) for the formation of the conjugate base, which reacts with the electrophile, and (b) the difference in nucleophilicity of the conjugate basic sites. Herein, a successful chemoselective functionalization of the nucleophilic sites of prolinol by employing a suitable base is reported. This has been achieved by exploiting the relative acidity difference of NH and OH along with the reversed nucleophilicity of the corresponding conjugate bases N^- and O^-. This protocol has also been used for the synthesis of several O-functionalized prolinol derived organocatalysts, few of which have been newly reported.

Poly(ε-caprolactones) Initiated by Chiral Compounds: A New Protocol to Support Organocatalysts

This work investigates the support of organocatalysts in polyesters, a class of polymers seldom used for this purpose. The proposal is to use the hydroxyl groups present in the structure of the chosen chiral compounds to promote the polymerization of ε-caprolactone, generating the support and anchoring the organocatalyst in a single step. A very simple method, with acid catalysis, was employed, that showed versatility in generating supported catalysts with different structures and functional groups and modulating the mass of the materials to generate specific solubility characteristics. In this way, the catalysts can be solubilized in some organic solvents, such as dichloromethane, but at the end of the reaction, they can be recovered in a heterogeneous way, through precipitation in more apolar solvents. The materials were applied as organocatalysts on an aldol addition test reaction and the product could be obtained in excellent yields and good stereoselectivity. The polymer did not show signs of degradation after the reaction, proving to be robust and suitable for use in catalysis; however, a recycling process appears to be necessary for its reuse.

1,5-Hydride-Shift-Triggered Cyclization for the Synthesis of Unsymmetric Julolidines

Directly accessible 8-substituted tetrahydroquinolines undergo 1,5-hydride-shift-triggered cyclization to provide difficult to access julolidine derivatives in yields of 21–98% under scandium(III) triflate catalysis. Additionally, the scope of the reaction, several follow-up transformations and a remarkable side process discovered during optimization of the conditions are highlighted.

Chiral Amphiphilic Secondary Amine-Porphyrin Hybrids for Aqueous Organocatalysis

Two chiral proline-derived amphiphilic 5-substituted-10,15,20-tris(4-sulfonatophenyl)porphyrins were prepared, and their pH-dependent supramolecular behavior was studied. In neutral aqueous solutions, the free-base form of the hybrids is highly soluble, allowing enamine-based organocatalysis to take place, whereas under acidic conditions, the porphyrinic protonated core of the hybrid leads to the formation of self-assembled structures, so that the hybrids flocculate and their catalytic activity is fully suppressed. The low degree of chirality transfer observed for aqueous Michael and aldol reactions strongly suggests that these reactions take place under true "in water" organocatalytic conditions. The highly insoluble catalyst aggregates can easily be separated from the reaction products by centrifugation of the acidic reaction mixtures, and after neutralization and desalting, the sodium salts of the sulfonated amine-porphyrin hybrids, retaining their full catalytic activity, can be recovered in high yield.

Diamidophosphites from β-hydroxyamides: readily assembled ligands for Pd-catalyzed asymmetric allylic substitution

Novel diamidophosphites based on β-hydroxyamides were prepared, and their individual and in situ formed complexes were tested in Pd-mediated allylations.

2-(Hydroxyimino)aldehydes: Photo- and Physicochemical Properties of a Versatile Functional Group for Monomer Design

In the context of our research on stimuli-responsive polymers bearing the 2-(hydroxyimino)aldehyde (HIA) group, we have explored the photochemical behavior and physicochemical properties of a number of HIAs. Interpretation of the experimental data is supported by quantum mechanical calculations. HIAs are expected to undergo photoisomerization, chelate metal ions, yield hydrogen-bonded dimers or oligomers, exhibit relatively low pK_a s, and form >C=NO^. radicals through OH hydrogen abstraction or oxidation of the oximate ion. Besides the well-established E/Z oxime photoisomerism, we observed a Norrish-Yang cyclization resulting in cyclobutanol oximes, to our knowledge not previously described in the literature. The acidity, bond dissociation enthalpies, and electrochemical properties of the HIAs are compared with literature data of simple oximes. The results are discussed in relation to the many potential applications for HIAs, with emphasis on the synthesis of novel HIA-containing responsive polymers.

A first study on copolymers of a methacrylate containing the 2-(hydroxyimino)aldehyde group and OEGMA. RAFT polymerization and assessment of thermal and photoresponsive polymer behavior

RAFT copolymerization of 4-[(hydroxyimino)aldehyde]butyl methacrylate with OEGMA₄₇₅ and first assessment of photoisomerization and thermal behavior of the copolymers in solution.

Complex N-heterocycle synthesis via iron-catalyzed, direct C-H bond amination

The manipulation of traditionally unreactive functional groups is of paramount importance in modern chemical synthesis. We have developed an iron-dipyrrinato catalyst that leverages the reactivity of iron-borne metal-ligand multiple bonds to promote the direct amination of aliphatic C-H bonds. Exposure of organic azides to the iron dipyrrinato catalyst furnishes saturated, cyclic amine products (N-heterocycles) bearing complex core-substitution patterns. This study highlights the development of C-H bond functionalization chemistry for the formation of saturated, cyclic amine products and should find broad application in the context of both pharmaceuticals and natural product synthesis.

Pd-catalyzed enantioselective aerobic oxidation of secondary alcohols: applications to the total synthesis of alkaloids

Enantioselective syntheses of the alkaloids (-)-aurantioclavine, (+)-amurensinine, (-)-lobeline, and (-)- and (+)-sedamine are described. The syntheses demonstrate the effectiveness of the Pd-catalyzed asymmetric oxidation of secondary alcohols in diverse contexts and the ability of this methodology to set the absolute configuration of multiple stereocenters in a single operation. The utility of an aryne C-C insertion reaction in accessing complex polycyclic frameworks is also described.

Stereoselective synthesis of imidazolidin-2-ones via Pd-catalyzed alkene carboamination. Scope and limitations

A method for the synthesis of imidazolidin-2-ones from N-allylureas and aryl or alkenyl bromides via Pd-catalyzed carboamination reactions is described. The N-allylurea precursors are prepared in one step from readily available allylic amines and isocyanates, and the Pd-catalyzed reactions effect the formation of a C-C bond, a C-N bond, and up to two stereocenters in a single step. Good diastereoselectivities are obtained for the conversion of substrates bearing allylic substituents to 4,5-disubstituted imidazolidin-2-ones, and excellent selectivity for the generation of products resulting from syn-addition across the alkene is observed when substrates derived from cyclic alkenes or E-1,2-disubstituted alkenes are employed. A brief discussion of reaction mechanism and product stereochemistry is presented.