Comparison of Templating Abilities of Urea and Thioruea During Photodimerization of Bipyridylethyelene and Stilbazole Crystals

Abstract

Photodimerization of cocrystals of four bispyridylethylenes and two stilbazoles with urea as a template in the solid state has been investigated following our success with thiourea. Four investigated olefins photodimerized quantitatively to a single dimer in the crystalline state only. The reactivity of urea-olefin crystals is understood on the basis of their packing arrangements in the crystalline state. In reactive crystals the adjacent reactive molecules are within 4.2 Å and parallel, whereas the unreactive ones have their adjacent molecules are farther than 4.6Å and nonparallel. Thus, with the knowledge of crystal packing the reactivity of urea-olefin crystals is predictable on the basis of Schmidt's topochemical postulates. The templating property of urea, similar to thiourea, derives from its ability to form hydrogen bonds with itself and the guest olefins. Despite the similarities in molecular structures of urea and thiourea their subtle electronic properties, yet to be fully understood, affect the crystal packing and consequently their reactivity in the crystalline state. Further work is needed to fully exploit the templating properties of urea.

Supramolecular Photochemistry as a Potential Synthetic Tool: Photocycloaddition

“…Considering that none of these pyridyl olefins dimerized on its own in the solid state the success with thiourea and resorcinol as templates is remarkable. Urea is less able to preorganize olefins than thiourea . It is not obvious why thiourea is a better template than urea with these olefins.…”

“…Applying the above hydrogen-bonding concept but making use of a totally different template, namely, thiourea, Ramamurthy's group was able to preorganize 11 stilbazoles/dipyridylethylenes. 241,242 Eleven of the 14 investigated compounds that did not dimerize on their own did so in the presence of thiourea (Scheme 22). Of the two types of complexes thiourea formed [2:2] and [1:1], the former is preferred with dipyridyl and the Achieving similar packing patterns with closely similar molecules remains a challenging task in crystal engineering.…”

“…Urea is less able to preorganize olefins than thiourea. 241 It is not obvious why thiourea is a better template than urea with these olefins. We speculate that the better hydrogen-bonding ability of the oxygen atom than the sulfur atom results in intermolecular hydrogen bonding between urea molecules than between urea and the dipyridylethylenes and stilbazoles.…”

Template-Directed Photochemical [2 + 2] Cycloaddition in Crystalline Materials: A Useful Tool to Access Cyclobutane Derivatives

“…1,2-Bis­(4-pyridyl)­ethylene (4,4′-bpe, 1 , Scheme ) is one of the most frequently studied olefins in terms of the [2 + 2] photodimerization reaction. This unit has been shown to undergo photochemical cycloaddition using a range of templates, such as diols, thioureas, , and dinuclear iridium or rhodium complexes, − to produce rctt -tetrakis­(4-pyridyl)­cyclobutane (4,4′-tpcb, 2, Scheme ). This work has already been summarized in a number of reviews. − ,− ,− , Readers are advised to consult these articles for details. − For example, a dinuclear molecular clip template in the form of organometallic macrocycle facilitate a [2 + 2] photochemical [2 + 2] reaction between two coordinated bpe ligands in a single-crystal-to-single-crystal transformation (Figure ).…”

Supramolecular photochemistry concepts highlighted with select examples

“…We were able to successfully employ thiourea as a template for nine different pyridyl olefin compounds to yield a single regio-isomer with > 95 % yield and have recently published the results. [203] Photodimerization of coumarin in the solid state provides the most impressive product control achieved. Excitation of coumarin dissolved in organic solvents yields several dimers while dimerization does not occur in crystalline state due to unfavorable packing.…”