Angel Ugrinov

Ruthenium(II)/N‐Heterocyclic Carbene Catalyzed [3+2] Carbocyclization with Aromatic NH Ketimines and Internal Alkynes

A convenient and highly efficient synthesis of indenamines has been developed via ruthenium-catalyzed [3+2] carbocyclization under very mild conditions. A catalyst system of Ru(II) π-allyl precursor and N-heterocyclic carbene (NHC) ligand promotes facile coupling between aromatic N–H ketimines with internal alkynes at mild temperatures, without added oxidants or other metal salts, and in non-polar solvents. A proposed mechanism involves imine-directed activation of aromatic C–H bond, alkyne insertion, and carbocyclization by intramolecular imine insertion into Ru–alkenyl linkages.

Cluster-Assembled Materials: Toward Nanomaterials with Precise Control over Properties

One pathway toward nanomaterials with controllable band gaps is to assemble solids where atomic clusters serve as building blocks, since the electronic structures of clusters vary with size and composition. To study the role of organization in cluster assemblies, we synthesized multiple architectures incorporating As(7)(3-) clusters through control of the countercations. Optical measurements revealed that the band gaps vary from 1.1-2.1 eV, even though the assemblies are constructed from the identical cluster building block. Theoretical studies explain this variation as being a result of altering the lowest unoccupied molecular orbital levels by changing the countercations. Additional variations in the gap are made by covalently linking the clusters with species of varying electronegativity to alter the degree of charge transfer. These findings offer a general protocol for syntheses of nanoassemblies with tunable electronic properties.

Supramolecular photocatalysis: insights into cucurbit[8]uril catalyzed photodimerization of 6-methylcoumarin

Guest induced shape change of the cucurbit[8]uril cavity is likely rate limiting in the supramolecular photocatalytic cycle for CB8 mediated photodimerization of 6-methylcoumarin.

Enantiospecific Photochemical Norrish/Yang Type II Reaction of Nonbiaryl Atropchiral α-Oxoamides in Solution—Axial to Point Chirality Transfer

Alpha-oxoamides 1 with o-tert-butyl substitution on the N-phenyl moiety were found to be stable axially chiral atropisomers. These axially chiral alpha-oxoamides undergo enantiospecific photochemical gamma-Hydrogen abstraction in CHCl(3) to yield beta-lactams with high enantioselectivity (e.r. approximately 90:10) in solution. The extent of enantioselectivity was found to be dependent on the reaction temperature.

Tailoring Atropisomeric Maleimides for Stereospecific [2 + 2] Photocycloaddition—Photochemical and Photophysical Investigations Leading to Visible-Light Photocatalysis

Atropisomeric maleimides were synthesized and employed for stereospecific [2 + 2] photocycloaddition. Efficient reaction was observed under direct irradiation, triplet-sensitized UV irradiation, and non-metal catalyzed visible-light irradiation, leading to two regioisomeric (exo/endo) photoproducts with complete chemoselectivity (exclusive [2 + 2] photoproduct). High enantioselectivity (ee > 98%) and diastereoselectivity (dr > 99:1%) were observed under the employed reaction conditions and were largely dependent on the substituent on the maleimide double bond but minimally affected by the substituents on the alkenyl tether. On the basis of detailed photophysical studies, the triplet energies of the maleimides were estimated. The triplet lifetimes appeared to be relatively short at room temperature as a result of fast [2 + 2] photocycloaddition. For the visible-light mediated reaction, triplet energy transfer occurred with a rate constant close to the diffusion-limited value. The mechanism was established by generation of singlet oxygen from the excited maleimides. The high selectivity in the photoproduct upon reaction from the triplet excited state was rationalized on the basis of conformational factors as well as the type of diradical intermediate that was preferred during the photoreaction.

Methoxy-Directed Aryl-to-Aryl 1,3-Rhodium Migration

Through-space metal/hydrogen shift is an important strategy for transition-metal-catalyzed C-H bond activation. Here we describe the synthesis and characterization of a Rh(I) 2,6-dimethoxybenzoate complex that underwent stoichiometric rearrangement via a highly unusual 1,3-rhodium migration. This aryl-to-aryl 1,3-Rh/H shift was also demonstrated in a Rh(I)-catalyzed decarboxylative conjugate addition to form a C-C bond at a meta position instead of the ipso-carboxyl position. A deuterium-labeling study under the conditions of Rh(I)-catalyzed protodecarboxylation revealed the involvement of an ortho-methoxy group in a multistep pathway of consecutive sp(3) and sp(2) C-H bond activations.

Programmed Photodegradation of Polymeric/Oligomeric Materials Derived from Renewable Bioresources†

Renewable polymeric materials derived from biomass with built-in phototriggers were synthesized and evaluated for degradation under irradiation of UV light. Complete decomposition of the polymeric materials was observed with recovery of the monomer that was used to resynthesize the polymers.

Light-Induced Enantiospecific 4π Ring Closure of Axially Chiral 2-Pyridones: Enthalpic and Entropic Effects Promoted by H-Bonding

Nonbiaryl axially chiral 2-pyridones were synthesized and employed for light-induced electrocyclic 4π ring closure leading to bicyclo-β-lactam photoproducts in solution. The enantioselectivity in the photoproducts varied from 22 to 95% depending on the reaction temperature and the ability of the axially chiral chromophore to form intramolecular and/or intermolecular H-bonds with the solvent. On the basis of the differential activation parameters, entropic control of the enantiospecificity was observed for 2-pyridones lacking the ability to form H-bonds. Conversely, enthalpy played a significant role for 2-pyridones having the ability to form H-bonds.

Exploring Bis(cyclometalated) Ruthenium(II) Complexes as Active Catalyst Precursors: Room‐Temperature Alkene–Alkyne Coupling for 1,3‐Diene Synthesis

Described is the development of a new class of bis(cyclometalated) ruthenium(II) catalyst precursors for C-C coupling reactions between alkene and alkyne substrates. The complex [(cod)Ru(3-methallyl)2] reacts with benzophenone imine or benzophenone in a 1:2 ratio to form bis(cyclometalated) ruthenium(II) complexes (1). The imine-ligated complex 1 a promoted room-temperature coupling between acrylic esters and amides with internal alkynes to form 1,3-diene products. A proposed catalytic cycle involves C-C bond formation by oxidative cyclization, β-hydride elimination, and C-H bond reductive elimination. This Ru(II)/Ru(IV) pathway is consistent with the observed catalytic reactivity of 1 a for mild tail-to-tail methyl acrylate dimerization and for cyclobutene formation by [2+2] norbornene/alkyne cycloaddition.

Linear polyester synthesized from furfural-based monomer by photoreaction in sunlight

A novel linear polyester was synthesized from furfural-based monomer through solvent-free polymerization using sunlight. Furfural was first converted to 2-furanacrylic acid as a key photoreactive building block. The desired monomer was then prepared by linking two 2-furanacrylic acid molecules with 1,5-pentanediol, which can also be produced from furfural. An important photoreactive crystalline assembly of the monomer was characterized by powder and single crystal X-ray diffraction. The new linear polyester that contains 100% components from biomass-derived chemicals was synthesized by sunlight photoreaction of the pre-organized monomer in the solid state in 24 hours. The solvent-free sunlight photoreaction process was monitored by FT-IR and a key intermediate was confirmed by single crystal X-ray diffraction. Nanofiber of the linear polyester was observed under TEM. Similar to polyethylene terephthalate (PET) used in beverage bottles, the novel polyester contains alternating rigid and flexible moieties in the polymer chain.