J. Sivaguru

From containers to catalysts: supramolecular catalysis within cucurbiturils.

Cucurbiturils are a family of molecular container compounds with superior molecular recognition properties. The use of cucurbiturils for supramolecular catalysis is highlighted in this concept. Both photochemical reactions as well as thermal transformations are reviewed with an eye towards tailoring substrates for supramolecular catalysis mediated by cucurbiturils.

Supramolecular Photochemistry as a Potential Synthetic Tool: Photocycloaddition

Photochemistry, bearing significant applications in natural and man-made events such as photosynthesis, vision, photolithography, photodynamic therapy, etc., is yet to become a common tool during the synthesis of small molecules in a laboratory. Among other rationale, the inability to influence photochemical reactions with temperature, solvent, additives, etc., dissuades chemists from employing light-initiated reactions as a routine synthetic tool. This review highlights how diverse, highly organized structures such as solvent-free crystals and water-soluble host-guest assemblies can be employed to control and manipulate photoreactions and thereby serve as an efficient tool for chemists, including those interested in synthesis. The efficacy of the media in modifying the excited-state behavior of organic molecules is illustrated with photocycloaddition in general and [2 + 2] photocycloaddition in particular, reactions widely employed in the synthesis of complex natural products as well as highly constrained molecules, as exemplars. The reaction media, highly pertinent in the context of green sustainable chemistry, include solvent-free crystals and solids such as silica, clay, and zeolite and water-soluble hosts that can solubilize and preorganize hydrophobic reactants in water. Since no other reagent would be more sustainable than light and no other medium greener than water, we believe that the supramolecular photochemistry expounded here has a momentous role as a synthetic tool in the future.

Enantioselective Organo‐Photocatalysis Mediated by Atropisomeric Thiourea Derivatives

Can photocatalysis be performed without electron or energy transfer? To address this, organo-photocatalysts that are based on atropisomeric thioureas and display lower excited-state energies than the reactive substrates have been developed. These photocatalysts were found to be efficient in promoting the [2+2] photocycloaddition of 4-alkenyl-substituted coumarins, which led to the corresponding products with high enantioselectivity (77-96% ee) at low catalyst loading (1-10 mol%). The photocatalytic cycle proceeds by energy sharing via the formation of both static and dynamic complexes (exciplex formation), which is aided by hydrogen bonding.

Manipulating Photochemical Reactivity of Coumarins within Cucurbituril Nanocavities

Coumarin derivatives that are either cationic (7-ammonium) or neutral (7-hydroxy, 7-methoxy, 6-methyl) form a 1:2 host-guest complex with cucurbit[8]uril (CB[8]). Direct irradiation of these coumarin@CB[8] complexes in water gives head-to-tail (HT) adduct as the major product. The nature of the functional group (polar or nonpolar) at the 6 or 7 position on the coumarin dictates the type of HT adduct ( syn- or anti-). It is postulated that the available free volume and the hydrophobic confined environment are responsible for the observed selectivity.

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.

Singlet Oxygen Mediated Oxidation of Olefins within Zeolites: Selectivity and Complexities

Abstract Thiazine dyes such as thionin, methylene blue and methylene green have been cation exchanged within monovalent cation exchanged Y zeolites. Depending on the water content, the dye molecules exist as either monomers (‘dry’) or dimers (‘wet’). The monomeric dye, upon excitation with visible light, generates singlet oxygen, which has been utilized to oxidize alkenes to hydroperoxides. In the case of trisubstituted alkenes, hydroperoxidation within zeolites occurs with a certain amount of regioselectivity. The oxidation within zeolites is accompanied by photodecomposition of the dye and the product hydroperoxides and acid catalyzed rearrangement of the alkenes. In order to understand the observed selectivity, ab initio and DFT calculations on model systems have been performed. The calculations confirm fairly strong cation-alkene binding as well as additional geometric and orbital distortions. Computed activation energies for hydrogen abstraction suggest a significant rate retardation due to metal coordination. At both the MP2 and B3LYP levels, formation of the tertiary hydroperoxide by hydrogen abstraction from the methyl group (4-position) of 2-methyl 2-butene is calculated to be favored by a small margin. Between the gem-dimethyl units, abstraction from the syn methyl group is favored slightly compared to the anti counterpart. These predictions are not compatible with the observed regioselectivities. Further experimental and theoretical studies are underway to understand the observed regioselective oxidation within zeolites.

Light-induced transfer of molecular chirality in solution: enantiospecific photocyclization of molecularly chiral acrylanilides.

Molecularly chiral o-tert-butylacrylanilides undergo enantiospecific 6pi-photocyclization to yield 3,4-dihydroquinolin-2-ones with very high enantioselectivity (>90%) in solution. The photocyclization results in the removal of the ortho tert-butyl substituent, presumably via a zwitterionic intermediate. Beta-substitution in the alkene is found to be critical for the transfer of molecular chirality (axial chirality) in the reactant to point chirality in the photoproduct(s).

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.

Photodimerization and complexation dynamics of coumarins in the presence of cucurbit[8]urils

Coumarin derivatives with non-polar substituents at the 6 or 7 position undergoes photodimerization in the presence of CB[8] in water to give the syn dimer as the major product. It is postulated that these neutral coumarins form dynamic complexes in the presence of CB[8] and the product selectivity is reflective of the type of complex, available volume in the CB[8] cavity and relative rate of photodimerization inside and outside the CB[8] cavity.