Regiospecific [2 + 2] photocyclodimerization of trans-4-styrylpyridines templated by cucurbit[8]uril

Abstract

Addition of HCl accelerated the photocyclodimerization of trans-4-styrylpyridine 1a in methanol and increased the yield of syn-head-to-tail (syn-HT) dimer 2a through the effect of cation-π interactions between the pyridinium ion of one molecule and the phenyl group of the other. We examined the photoirradiation products of derivatives of 1a having alkyl substituents on the phenyl group (1b-1f). The effect of the alkyl substituent on product distribution was rather limited for the photoreaction in MeOH solutions. However, the substituents had a distinct effect on the product distribution for the photoreaction of the inclusion complexes of hydrochloride salts of trans-4-styrylpyridines with cucurbit[8]uril in aqueous solutions. Introducing an alkyl group at the 2- or 3-position of the phenyl group completely shifted the major product from the syn-HT dimer to the syn-head-to-head (syn-HH) dimer. By adjusting the balance of host-guest interactions and cation-π interactions between guest molecules through systematic changes in the substituents on the phenyl ring of trans-4-styrylpyridine, we could change the orientation of the reactant molecules in the host cavity, resulting in a change of the major regioisomer of the photocyclodimerization products.

[2+2] Photodimerization of Stilbazoles Promoted by Oxalic Acid in Suspension

“…Theoretically,a nu ncontrolled reaction in solution would lead to am ultitudeo fp ossible stereoisomers due to the combination of two monomers in cis or trans configuration in different mannersh ead-head vs. head-tail and syn vs. anti addition. [1] Since the seminalw ork of Schmidt et al on solid-state [2+ +2] photocycloaddition reaction about 50 years ago, [2] different strategies [3] have been developed to achieve such ar eaction with reasonable level of reactivity and highs tereo-a nd regioselectivities, by using small organic molecules, [4,5] metal-organic frameworks, [6] coordinationw ith metal cations, [7] cation-p interactions throughs alt formation, [8] host-guest interactions, [9] and other. [10] All of them were based on the noncovalent rigidification of the relative conformation between two proximal molecules in confined media, especially in crystalline states, to achievet he favorable geometry for the photocycloaddition.…”

Stereoselective Solid‐State Synthesis of Substituted Cyclobutanes Assisted by Pseudorotaxane‐like MOFs

“…The physical state of the precursor alkene is one factor that can impact the reaction outcome . For example, photoirradiation of a dilute solution of trans ‐4‐styrylpyridine (4‐spy) affords only the cis ‐isomer because of an accompanying trans ‐to‐ cis photoisomerization reaction . In contrast, photodimerization of an asymmetrical, substituted 4‐spy in a concentrated solution generally produces a mixture of stereo‐ and regio‐isomeric cyclobutanes, the formation of which is governed by Woodward–Hoffmann rules, electronic and steric effects, and the thermodynamic stability of the diradical intermediates .…”

Stereoselective Solid‐State Synthesis of Substituted Cyclobutanes Assisted by Pseudorotaxane‐like MOFs

“…[19] With respect to the reaction of olefins,t he preorganization prior to UV irradiation provided by small molecule H-bonding templates such as resorcinol (and various substituted resorcinols) allows asymmetrical olefins to be arranged in ah ead-to-head [1b,20] or head-to-tail [21] orientation. Other templates such as cucurbit[8]ril [3,22] can also facilitate the regiospecific formation of cyclobutanes. This effect is enhanced by adding hydrochloric acid.…”

“…[1] Thep hysical state of the precursor alkene is one factor that can impact the reaction outcome. [3] In contrast, photodimerization of an asymmetrical, substituted 4-spy in aconcentrated solution generally produces amixture of stereo-and regio-isomeric cyclobutanes,the formation of which is governed by Woodward-Hoffmann rules,e lectronic and steric effects,a nd the thermodynamic stability of the diradical intermediates. [3] In contrast, photodimerization of an asymmetrical, substituted 4-spy in aconcentrated solution generally produces amixture of stereo-and regio-isomeric cyclobutanes,the formation of which is governed by Woodward-Hoffmann rules,e lectronic and steric effects,a nd the thermodynamic stability of the diradical intermediates.…”

“…[2] Forexample,photoirradiation of adilute solution of trans-4-styrylpyridine (4-spy) affords only the cis-isomer because of an accompanying trans-to-cis photoisomerization reaction. [3] In contrast, photodimerization of an asymmetrical, substituted 4-spy in aconcentrated solution generally produces amixture of stereo-and regio-isomeric cyclobutanes,the formation of which is governed by Woodward-Hoffmann rules,e lectronic and steric effects,a nd the thermodynamic stability of the diradical intermediates. [4] Inspired by nature, which exploits tailored microenvironments within enzymes to promote avariety of biological reactions with high regio-and stereo-selectivity,i ts hould be possible to obtain unique stereoselectivity and quantitative yields by preorganizing the reactants within an appropriately structured environment.…”