A modular approach to a new class of phosphinohydrazones and their use in asymmetric allylic alkylation reactions

Modular Synthesis of Phosphino Hydrazones and Their Use as Ligands in a Palladium-Catalysed Cu-Free Sonogashira Cross-Coupling Reaction

Phosphino hydrazones represent a versatile class of nitrogen-containing phosphine ligands. Herein, we report a modular synthesis of phosphino hydrazone ligands by hydrazone condensation reaction of three different aryl hydrazines with 3-(diphenylphosphino)propanal (PCHO). Complexation reactions of these phosphino hydrazone ligands with palladium(II) and platinum(II) were investigated and the catalytic activity of the palladium(II) complexes was explored in a Cu-free Sonogashira cross-coupling reaction achieving yields up to 96 %. Additionally it was shown that the catalytically active species is homogeneous.

A supramolecular double-helix based on complementary phosphate-guanidinium pairing

A double-helical supramolecular structure was formed by self-assembly of 1,1'-binaphthyl-based bisguanidines and bisphosphoric acids. Interestingly the homochiral (S,S) + (S,S)-pair forms a left-handed double-helix, while the heterochiral (S,S) + (R,R)-pair forms a non-helical dimer.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

Improved Access to Chiral Tetranaphthoazepinium-Based Organocatalysts Using Aqueous Ammonia as Nitrogen Source

The class of 3,3'-diaryl substituted tetranaphthobisazepinium bromides has found wide application as highly efficient C₂-symmetrical phase-transfer catalysts (PTCs, Maruoka type catalysts). Unfortunately, the synthesis requires a large number of steps and hampers the build-up of catalyst libraries which are often desired for screening experiments. Here, we present a more economic strategy using dinaphthoazepine 7 as the common key intermediate. Only at this stage various aryl substituents are introduced, and only two individual steps are required to access target structures. This protocol was applied to synthesize ten tetranaphthobisazepinium compounds 1a-1j. Their efficiency as PTCs was tested in the asymmetric substitution of tert-butyl 2-((diphenylmethylene)amino)acetate. Enantioselectivities up to 92% have been observed with new catalysts.

Economy of Catalyst Synthesis-Convenient Access to Libraries of Di- and Tetranaphtho Azepinium Compounds

Efficient optimization procedures in chiral catalysis are usually linked to a straightforward strategy to access groups of structurally similar catalysts required for fine-tuning. The ease of building up such ligand libraries can be increased when the structure-modifying step (introduction of a substituent) is done at a later stage of the synthesis. This is demonstrated for the extended family of di- and tetranaphtho azepinium compounds, widely used as chiral phase transfer catalysts (PTC). Using 2,6-diiodo-4,5-dihydro-3H-dinaphtho[2,1-c:1',2'-e]azepine and 4,8-diiodo-6,7-dihydro-5H-dibenzo[c,e]azepine, respectively, as key intermediates, 18 spiro-azepinium compounds were synthesized in a total yield of 25-42% over 6-7 steps from 1,1'-binaphthyl-2,2'-dicarboxylic acid or diphenic acid, respectively. The replacement of iodo groups with aryl substituents was performed as the last or the penultimate step of the synthesis.

Recent advances in chiral imino-containing ligands for metal-catalyzed asymmetric transformations

In this review, the recent applications of a variety of chiral imino-containing ligands classified by different types of metal-catalyzed asymmetric reactions are summarized. The progress made in this area would encourage us to design and synthesize more novel chiral imino-containing ligands, and explore their applications in metal-catalyzed asymmetric transformations.

Development of Chiral Bis-hydrazone Ligands for the Enantioselective Cross-Coupling Reactions of Aryldimethylsilanolates

A palladium-catalyzed, enantioselective, aryl-aryl cross-coupling reaction using 1-naphthyldimethylsilanolates and chiral bis-hydrazone ligands has been developed. A family of glyoxal bis-hydrazone ligands containing various 2,5-diarylpyrrolidine groups was prepared to evaluate the influence of ligand structure on the rate and enantioselectivity of the cross-coupling. New synthetic routes to the 1-amino-2,5-diarylpyrrolidines were developed to enable the structure/reactivity-selectivity studies. Role reversal experiments of aryldimethylsilanolates and aryl bromides result in biaryl products with the same configuration and similar enantioselectivities implying that reductive elimination is the stereodetermining step. The origin of stereoselectivity is rationalized through computational modeling of diarylpalldium(II) complex which occurs through a conrotatory motion for the two aryl groups undergoing C-C bond formation.

Spectral properties of binaphthalene-coumarins interconnected through hydrazone linkage

Photophysical properties of new coumarin-3-carbaldehyde (dihydrodinaphtho[2,1-c:1',2'-e]azepin-N-yl)imines bearing dimethylamino and methoxy groups at position 7 of coumarin were investigated. Dimethylamino derivative exhibits different solvent polarity dependence of fluorescent characteristics for nonpolar, medium polar and highly polar solvents. This effect can be rationalized by diverse charge distribution in the singlet excited state due to its different stabilization by solvation in the solvents of particular group. While 2-fold higher values of Stokes shift were observed for methoxy derivative, its quantum yield of fluorescence is much lower due to high nonradiative decay rate constant of the excited state.