Metal-Pyridoxal Cooperativity in Nonenzymatic Transamination.

Coenzymes are involved in ≥30% of enzymatic reactions and likely predate enzymes, going back to prebiotic chemistry. However, they are considered poor organocatalysts, and thus their pre-enzymatic function remains unclear. Since metal ions are known to catalyze metabolic reactions in the absence of enzymes, here we explore the influence of metal ions on coenzyme catalysis under conditions relevant to the origin of life (20-75 °C, pH 5-7.5). Specifically, Fe or Al, the two most abundant metals in the Earth's crust, were found to exhibit substantial cooperative effects in transamination reactions catalyzed by pyridoxal (PL), a coenzyme scaffold used by roughly 4% of all enzymes. At 75 °C and 7.5 mol % loading of PL/metal ion, Fe3+-PL was found to be 90-fold faster at catalyzing transamination than PL alone and 174-fold faster than Fe3+ alone, whereas Al3+-PL was 85-fold faster than PL alone and 38-fold faster than Al3+ alone. Under milder conditions, reactions catalyzed by Al3+-PL were >1000 times faster than those catalyzed by PL alone. Pyridoxal phosphate (PLP) exhibited similar behavior to PL. Experimental and theoretical mechanistic studies indicate that the rate-determining step in the PL-metal-catalyzed transamination is different from metal-free and biological PL-based catalysis. Metal coordination to PL lowers the pKa of the PL-metal complex by several units and slows the hydrolysis of imine intermediates by up to 259-fold. Coenzymes, specifically pyridoxal derivatives, could have exhibited useful catalytic function even before enzymes.

Cyclic Diaryl λ3 -Bromanes as Original Aryne Precursors.

Despite the widespread application of hypervalent iodines, the corresponding λ3 -bromanes are less explored. Herein we report a general, safe, and high-yielding strategy to access cyclic diaryl λ3 -bromanes. These unique compounds feature reactivity that is appealing and complementary to that of λ3 -iodanes, generating arynes under mild reaction conditions and in the presence of a weak base. Accordingly, formal meta-selective transition-metal-free C-O and C-N couplings may be achieved. Mechanistic studies unambiguously support the aryne generation mechanism.

Enantioselective Copper-Catalyzed Borylative Cyclization for the Synthesis of Quinazolinones.

Quinazolinones are common substructures in molecules of medicinal importance. We report an enantioselective copper-catalyzed borylative cyclization for the assembly of privileged pyrroloquinazolinone motifs. The reaction proceeds with high enantio- and diastereocontrol, and can deliver products containing quaternary stereocenters. The utility of the products is demonstrated through further manipulations.

The Affinity of Some Lewis Bases for Hexafluoroisopropanol as a Reference Lewis Acid: An ITC/DFT Study.

To figure out the possible role of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) as well as to provide reference thermochemical data in solution, the formation of Lewis acid-base complexes between HFIP (Lewis acid) and a series of 8 different Lewis bases (3 sulfoxides, 3 Nsp2 pyridine derivatives, 1 aromatic amine, 1 cyclic aliphatic ether) was examined by isothermal titration calorimetry (ITC) experiments and static density functional theory augmented with Dispersion (DFT-D) calculations. Measured ITC association enthalpy values (ΔHa ) lie between -9.3 and -14 kcal mol-1 . Computations including a PCM implicit solvation model produced similar exothermicity of association of all studied systems compared to the ITC data with ΔHa values ranging from -8.5 to -12.7 kcal mol-1 . An additional set of calculations combining implicit and explicit solvation by chlorobenzene of the reactants, pointed out the relatively low interference of the solvent with the HFIP-base complexation: its main effect is to slightly enhance the Gibbs energy of the HFIP-Lewis base association. It is speculated that the interactions of bulk HFIP with Lewis bases therefore may significantly intervene in catalytic processes not only via the dynamic microstructuring of the medium but also more explicitly by affecting bonds' polarization at the Lewis bases.

Copper-Catalyzed Borylative Couplings with C-N Electrophiles.

Copper-catalyzed borylative multicomponent reactions (MCRs) involving olefins and C-N electrophiles are a powerful tool to rapidly build up molecular complexity. The products from these reactions contain multiple functionalities, such as amino, cyano and boronate groups, that are ubiquitous in medicinal and process chemistry programs. Copper-catalyzed MCRs are particularly attractive because they use a relatively abundant and non-toxic catalyst to selectively deliver high-value products from simple feedstocks such as olefins. In this Minireview, we explore this rapidly emerging field and survey the borylative union of allenes, dienes, styrenes and other olefins, with imines, nitriles and related C-N electrophiles.

Enantio- and Diastereoselective Synthesis of Homopropargyl Amines by Copper-Catalyzed Coupling of Imines, 1,3-Enynes, and Diborons.

An efficient, enantio- and diastereoselective, copper-catalyzed coupling of imines, 1,3-enynes, and diborons is reported. The process shows broad substrate scope and delivers complex, chiral homopropargyl amines; useful building blocks on the way to biologically-relevant compounds. In particular, functionalized homopropargyl amines bearing up to three contiguous stereocenters can be prepared in a single step.

Copper-catalyzed functionalization of enynes.

The copper-catalyzed functionalization of enyne derivatives has recently emerged as a powerful approach in contemporary synthesis. Enynes are versatile and readily accessible substrates that can undergo a variety of reactions to yield densely functionalized, enantioenriched products. In this perspective, we review copper-catalyzed transformations of enynes, such as boro- and hydrofunctionalizations, copper-mediated radical difunctionalizations, and cyclizations. Particular attention is given to the regiodivergent functionalization of 1,3-enynes, and the current mechanistic understanding of such processes.

Two Stereoinduction Events in One C-H Activation Step: A Route towards Terphenyl Ligands with Two Atropisomeric Axes.

Herein we disclose the synthesis of original chiral scaffolds-ortho-orientated terphenyls presenting two atropisomeric Ar-Ar axes. These unusual structures were built up by using the C-H activation approach, and remarkably, both chiral axes were controlled with excellent stereoselectivity in a single transformation. During the reaction, not only does atroposelective functionalization of a biaryl precursor occur to establish one stereogenic axis, but an unprecedented atropo-stereoselective C-H arylation also takes place to generate the second stereogenic element. These enantiomerically pure ortho-terphenyls show an original tridimensional structure and thus constitute a unique foundation for building up a library of enantiomerically pure bidentate ligands, such as the new ligands S/N-Biax and diphosphine BiaxPhos.

1,1,1,3,3,3-Hexafluoroisopropanol as a Remarkable Medium for Atroposelective Sulfoxide-Directed Fujiwara-Moritani Reaction with Acrylates and Styrenes.

Axially chiral biaryls are ubiquitous structural motifs of biologically active molecules and privileged ligands for asymmetric catalysis. Their properties are due to their configurationally stable axis, and therefore, the control of their absolute configuration is essential. Efficient access to atropo-enantioenriched biaryl moieties through asymmetric direct C-H activation, by using enantiopure sulfoxide as both the directing group (DG) and chiral auxiliary, is reported. The stereoselective oxidative Heck reactions are performed in high yields and with excellent atropo-stereoselectivities. The pivotal role of 1,1,1,3,3,3-hexafluoropropanol (HFIP) solvent, which enables a drastic increase in yield and stereoselectivity of this transformation, is evidenced and investigated. Finally, the synthetic usefulness of the herein disclosed transformation is showcased because the traceless character of the sulfoxide DG allows straightforward conversions of the newly accessed, atropopure sulfoxide-biaryls into several differently substituted axially chiral scaffolds.

Synthesis of axially chiral biaryls through sulfoxide-directed asymmetric mild C-H activation and dynamic kinetic resolution.

A mild and robust direct C-H functionalization strategy has been applied to the synthesis of axially chiral biaryls. Such an efficient and stereoselective transformation occurs through an original dynamic kinetic resolution pathway enabling the conversion of diastereomeric mixtures of non-prefunctionalized substrates into atropisomerically pure, highly substituted biaryl scaffolds. The main feature of this transformation is the use of an enantiopure sulfoxide as both chiral auxiliary and traceless directing group. The potential of newly synthesized biaryls as valuable building blocks is further illustrated.