P₂O₅-Promoted Cyclization of Di[aryl(hetaryl)methyl] Malonic Acids as a Pathway to Fused Spiro[4.4]nonane-1,6-Diones

Abstract

Conventional spiro-linked conjugated materials are attractive for organic optoelectronic applications due to the unique combination of their optical and electronic properties. However, spiro-linked conjugated materials with conjugation extension directed along the main axis of the molecule are still only rare examples among the vast number of spiro-linked conjugated materials. Herein, the synthesis, leading to π-extended spiro-linked conjugated materialsspiro[4.4]­nonane-1,6-diones and spiro­[5.5]­undecane-1,7-dioneshas been developed and optimized. The proposed design concept starts from readily available malonic esters and contains several steps: double alkylation of malonic ester with bromomethylaryl­(hetaryl)­s; conversion of a malonic ester into the corresponding malonic acid; electrophilic spirocyclization of the latter into the annulated spiro[4.4]­nonane-1,6-dione or spiro­[5.5]­undecane-1,7-dione in the presence of phosphorus pentoxide. On the basis of these insights, the developed method yielded spiro-linked conjugated cores fused with benzene, thiophene, and naphthalene, decorated with active halogen atoms. The structures of the synthesized spirocycles were determined by single-crystal X-ray diffraction analysis. Benzene fused spiro[4.4]­nonane-1,6-dione decorated with bromine atoms was transformed into V-shape phenylene-thiophene co-oligomer type spirodimers via Stille coupling. The spiro-bis­(4-n-dodecylphenyl)-2,2′-bithiophene derivative possessed high photoluminescence properties in both solution and solid state with a photoluminescence quantum yield (PL QY) of 38%.

Construction of Annulated Spiro[4.4]-nonane-diones via the Tandem [4 + 2]-Cycloaddition/Aromatization Reaction

“…While investigating the versatile synthetic pathways to spirocyclic π-conjugated systems, we first focused on the easiest route that involves the spirocycle construction at the final step via P 2 O 5 -promoted spirocyclization of the 2,2′substituted malonic acids 12 (Scheme 1, a). Although the approach has shown good to excellent yields of various spirocycles, including thiophene-containing ones, it is limited in terms of the range of substrates and functional group tolerance.…”

“…However, the synthesized halogen-containing 2,2′-spirobi[indene]-diones can be further modified via crosscoupling reactions, yielding perspective molecules for optoelectronic applications. 12 Another approach exploits basepromoted counterion-mediated enantioselective intramolecular enolate C-acylation 18 (Scheme 1, c). Although it is a powerful method of asymmetric spirocyclization, this approach neces-sitates the use of activated esters.…”

“…Nonetheless, construction of a rigid 2,2′-spirobi­[indene]-dione skeleton is still challenging, and a number of approaches to its synthesis have been developed (Scheme ). The first straightforward protocols involved the Friedel–Crafts acylation strategy, starting either with 2,2′-substituted malonic acids − and their acyl chlorides ,, (Scheme , a ) or with α-( ortho -carboxybenzyl) substituted acetic acids − (Scheme , b ). Both these approaches require harsh reaction conditions and are unsuitable for Lewis acid-sensitive functionalities.…”

“…Both these approaches require harsh reaction conditions and are unsuitable for Lewis acid-sensitive functionalities. However, the synthesized halogen-containing 2,2′-spirobi­[indene]-diones can be further modified via cross-coupling reactions, yielding perspective molecules for optoelectronic applications . Another approach exploits base-promoted counterion-mediated enantioselective intramolecular enolate C-acylation (Scheme , c ).…”