Recent Synthetic Advances on the Use of Diazo Compounds Catalyzed by Metalloporphyrins

Diazo compounds are organic substances that are often used as precursors in organic synthesis like cyclization reactions, olefinations, cyclopropanations, cyclopropenations, rearrangements, and carbene or metallocarbene insertions into C-H, N-H, O-H, S-H, and Si-H bonds. Typically, reactions from diazo compounds are catalyzed by transition metals with various ligands that modulate the capacity and selectivity of the catalyst. These ligands can modify and enhance chemoselectivity in the substrate, regioselectivity and enantioselectivity by reflecting these preferences in the products. Porphyrins have been used as catalysts in several important reactions for organic synthesis and also in several medicinal applications. In the chemistry of diazo compounds, porphyrins are very efficient as catalysts when complexed with low-cost metals (e.g., Fe and Co) and, therefore, in recent years, this has been the subject of significant research. This review will summarize the advances in the studies involving the field of diazo compounds catalyzed by metalloporphyrins (M-Porph, M = Fe, Ru, Os, Co, Rh, Ir) in the last five years to provide a clear overview and possible opportunities for future applications. Also, at the end of this review, the properties of artificial metalloenzymes and hemoproteins as biocatalysts for a broad range of applications, namely those concerning carbene-transfer reactions, will be considered.

Carbene Radicals in Transition-Metal-Catalyzed Reactions

Discovered as organometallic curiosities in the 1970s, carbene radicals have become a staple in modern-day homogeneous catalysis. Carbene radicals exhibit nucleophilic radical-type reactivity orthogonal to classical electrophilic diamagnetic Fischer carbenes. Their successful catalytic application has led to the synthesis of a myriad of carbo- and heterocycles, ranging from simple cyclopropanes to more challenging eight-membered rings. The field has matured to employ densely functionalized chiral porphyrin-based platforms that exhibit high enantio-, regio-, and stereoselectivity. Thus far the focus has largely been on cobalt-based systems, but interest has been growing for the past few years to expand the application of carbene radicals to other transition metals. This Perspective covers the advances made since 2011 and gives an overview on the coordination chemistry, reactivity, and catalytic application of carbene radical species using transition metal complexes and catalysts.

A synthetic tactic to substitute axial ligands in sterically demanding Ru(II)porphyrinates

We report a synthetic strategy that allows for the preparation of sterically encumbered heteroleptic Ru(II)porphyrinates with the desired configuration of stable/inert and weak/labile axial ligands to direct reactions between substrates to exclusively occur at the sterically encumbered face. To demonstrate the method, we describe the synthesis of a strapped-Ru(II)porphyrinate bearing a stable/inert triphenylphosphine (PPh₃) bulky axial ligand coordinated exo to the central cavity and a weak/labile methanol molecule coordinated at the internal axial position. With this axial ligand configuration, the reported Ru(II)porphyrinate exclusively promotes carbene transfer reactions to olefins through the central cavity, which has been verified by the selective formation of cycloprane-linked [2]rotaxanes.

Self-Assembly of a Triazolylferrocenyl Dendrimer in Water Yields Nontraditional Intrinsic Green Fluorescent Vesosomes for Nanotheranostic Applications

The promising field of nanomedicine stimulates a continuous search for multifunctional nanotheranostic systems for imaging and drug delivery. Herein, we demonstrate that application of supramolecular chemistry's concepts in dendritic assemblies can enable the formation of advanced dendrimer-based nanotheranostic devices. A dendrimer bearing 81 triazolylferrocenyl terminal groups adopts a more compact shell-like structure in polar solvents with the ferrocenyl peripheral groups backfolding toward the hydrophobic dendrimer interior, while exposing the more polar triazole moieties as the dendritic shell. Akin to lipids, the compact dendritic structure self-assembles into uniform nanovesicles that in turn self-assemble into larger vesosomes in water. The vesosomes emit green nontraditional intrinsic fluorescence (NTIL), which is an emerging property as there are no classical fluorophores in the dendritic macromolecular structure. This work confirms the hypothesis that the NTIL emission is greatly enhanced by rigidification of the supramolecular assemblies containing heteroatomic subluminophores (HASLs) and by the presence of electron rich functional groups on the periphery of dendrimers. This work is the first one detecting NTIL in ferrocenyl-terminated dendrimers. Moreover, the vesosomes are stable in biological medium, are uptaken by cells, and show cytotoxic activity against cancer cells. Accordingly, the self-organization of these dendrimers into tertiary structures promotes the emergence of new properties enabling the same component, in this case, ferrocenyl group, to function as both antitumoral drug and fluorophore.

Sequence-Selective Decapeptide Synthesis by the Parallel Operation of Two Artificial Molecular Machines

We report on the preparation of a decapeptide through the parallel operation of two rotaxane-based molecular machines. The synthesis proceeds in four stages: (1) simultaneous operation of two molecular peptide synthesizers in the same reaction vessel; (2) selective residue activation of short-oligomer intermediates; (3) ligation; (4) product release. Key features of the machine design include the following: (a) selective transformation of a thioproline building block to a cysteine (once it has been incorporated into a hexapeptide intermediate by one molecular machine); (b) a macrocycle-peptide hydrazine linkage (as part of the second machine) to differentiate the intermediates and enable their directional ligation; and (c) incorporation of a Glu residue in the assembly module of one machine to enable release of the final product while simultaneously removing part of the assembly machinery from the product. The two molecular machines participate in the synthesis of a product that is beyond the capability of individual small-molecule machines, in a manner reminiscent of the ligation and post-translational modification of proteins in biology.

Functional Rotaxanes in Catalysis

Mechanically interlocked molecules (MIMs) have gained attention in the field of catalysis due to their unique molecular properties. Central to MIMs, rotaxanes are highly promising and attractive supramolecular catalysts due to their unique three-dimensional structures and the flexibility of their subcomponents. This Minireview discusses the use of rotaxanes in organocatalysis and transition-metal catalysis.

Ru(II)Porphyrinate-based molecular nanoreactor for carbene insertion reactions and quantitative formation of rotaxanes by active-metal-template syntheses

Selectivity in N–H and S–H carbene insertion reactions promoted by Ru(II)porphyrinates currently requires slow addition of the diazo precursor and large excess of the primary amine and thiol substrates in the reaction medium. Such conditions are necessary to avoid the undesirable carbene coupling and/or multiple carbene insertions. Here, the authors demonstrate that the synergy between the steric shielding provided by a Ru(II)porphyrinate-based macrocycle with a relatively small central cavity and the kinetic stabilization of otherwise labile coordinative bonds, warranted by formation of the mechanical bond, enables single carbene insertions to occur with quantitative efficiency and perfect selectivity even in the presence of a large excess of the diazo precursor and stoichiometric amounts of the primary amine and thiol substrates. As the Ru(II)porphyrinate-based macrocycle bears a confining nanospace and alters the product distribution of the carbene insertion reactions when compared to that of its acyclic version, the former therefore functions as a nanoreactor.

Selectivity in carbene insertion reactions promoted by Ru(II)porphyrinates is achieved only upon careful control of substrate stoichiometry. Here, the authors demonstrate that endotopic catalysis and formation of mechanical bonds enables carbene insertions to occur selectively and in quantitative yield regardless of substrate stoichiometry.