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Small differences in molecular or solid-state structure can afford significant differences in properties. Here, a diene derivative, 1,3-bis((E)-2-bromostyryl)benzene (2Brm), is synthesized and crystallized into two unique solid-state forms, each exhibiting a different π–π stacking geometry, which imparts distinct reactivity and photoresponsivity. Upon exposure of the two solids to UV–Vis light, a [2 + 2] photocycloaddition occurs to afford regioisomeric products due to the difference in the stacking geometries of the dienes. From a single molecular precursor, we further demonstrate that under different wavelengths of light, the chemical functionality can be programmed into discrete and distinct products containing one, two, or three cyclobutane rings as well as oligomeric/polymeric products. Moreover, the two initial solid forms exhibit wavelength-dependent photomechanical behaviors (i.e., photosalience). This work demonstrates a rare, template-free, self-assembly-based strategy that enables access to a suite of discrete and oligomeric/polymeric products via regiocontrolled solid-state photocycloadditions and further presents potential routes toward the design of photoactuating materials. 

The coupling of carbon dioxide and ethylene to generate value added chemicals has been part of recent fundamental advances to improve sustainability in commercial chemical synthons. A formal zerovalent triphosphine ligated ruthenium complex, (tBuP(CH2CH2PEt2)2)Ru(κ-S-DMSO)(C2H4), was found to promote CO2 activation, affording products derived from both a 1:1 and 1:2 ethylene to CO2 coupling stoichiometry. The equimolecular coupling reaction selectively afforded a five-membered ruthenium lactone species, (tBuP(CH2CH2PEt2)2)Ru(κ-S-DMSO)(κ-C,κ-O-CH2CH2CO2), under low CO2 pressure. At higher CO2 pressure, the ruthenium lactone complex activated a second equivalent of CO2, yielding a dimeric methylmalonate ruthenium compound, [(tBuP(CH2CH2PEt2)2)Ru(μ2, κ1-O, κ2-O,O-O2CCHCH3CO2)]2. Both carbon dioxide activation products were characterized by X-ray diffraction. Preliminary mechanistic studies suggest that reversible β-H elimination is a key process in conversion between the two ruthenium carboxylate species. A rare formally zerovalent ruthenium coordination compound stabilized only by ethylene and DMSO ligands was also isolated and characterized. 

N-alkylation of 5-hydroxynicotinic acid esters with electrophiles containing diene functionality produces salts that undergo intramolecular (4+3) cycloaddition reactions upon heating in the presence of base. Initial studies used a three-carbon tether to join the pyridinium ion and diene, revealing some aspects of the inherent selectivity of the reaction with such substrates. Much more challenging was the synthesis of related species possessing only a two-carbon tether. Nevertheless, the cycloaddition of such compounds was successful, leading directly to the ABC ring system of the alkaloid daphnicyclidin A.