Streamlined Synthesis of Polycyclic Conjugated Hydrocarbons Containing Cyclobutadienoids via C–H Activated Annulation and Aromatization

Abstract

The juxtaposition of fused cyclobutadienoid (CBD) with benzenoid creates intriguing alternating antiaromatic and aromatic conjugation. Synthetic accessibility of such molecules, however, has been challenging and limited in scope. We report a modular and streamlined synthetic strategy to access a large variety of polycyclic conjugated hydrocarbons with fused CBD. Synthesis was achieved through efficient palladium-catalyzed C-H activated annulation between abundant aryl bromides and oxanorbornenes, followed by aromatization under acidic conditions. The influence of four-membered ring was examined using spectroscopy, crystallography, and computation. This strategy will facilitate exploration on the chemical, structural, and electronic properties of such conjugated systems containing CBD.

Dinaphthobenzo[1,2:4,5]dicyclobutadiene: Antiaromatic and Orthogonally Tunable Electronics and Packing

“…[5] Thebest known class of CBD-containing PCHs are [N]phenylenes,wherein Nbenzenoids and N-1 CBDs are alternatively fused, which were developed by Vollhardt and co-workers. [6] Linear [N]phenylenes display high antiaromaticity,b ut even linear [3]phenylene is unstable [5b] and no mobility measurement has been reported for [N]phenylenes.I ntroducing CBD into acenes is attractive since acenes are prominent, widely explored organic semiconductors.I nt he past decade,t he groups of Swager, [5d] Bunz, [7] and Miao, [8] and our group [9] have reported linear acene analogues containing four-membered rings using different synthetic strategies.However,inall these cases,both crystal structures and calculations indicated strong bond localization exocyclic to the four-membered rings and diminishing antiaromaticity. [5d, 7b,9] To maximize the degree of antiaromaticity while still maintaining stability,w er eport an alternative design and synthesis of an ew family of dinaphthobenzo [1,2:4,5]dicyclobutadiene (DNBDC) [10] structures that feature strong local antiaromaticity of the CBD, independently tunable electronics and crystal packing,a nd good performance in organic field-effect transistors (OFETs).…”

Synthesis of Ladder Polymers: Developments, Challenges, and Opportunities

“…Unexpectedly,w ef ound that Pd-assistedo NBE ring-opening followed by b-oxygen elimination strongly competed with the CANAL pathway,w hen oNBEs rather than NBEs were used. [59] In order to suppress this side reaction pathway,w ei ntroduced two methyl groups to the bridge-headp ositionso fo NBE and used bromoarenes with no substituents ortho and meta to bromine. Gratifyingly,u nder the re-optimized conditions, we achieved high yields of the CANAL products, whichw ere then aromatized upon heating under acidic conditions.…”

“…Gratifyingly,u nder the re-optimized conditions, we achieved high yields of the CANAL products, whichw ere then aromatized upon heating under acidic conditions. [59] This facile two-steps ynthetic strategy has allowed us to synthesize a large number of extended ladder p-systems containing CBDs with av ariable degree of antiaromaticity as indicated by spectroscopic characterization and computation.…”

Synthesis of the Unknown Indeno[1,2‐a]fluorene Regioisomer: Crystallographic Characterization of Its Dianion

“…[1] Compared with the more commonly studied polycyclic aromatic hydrocarbons (PAHs), [2] the inherently low HOMO/ LUMO energy levels and small energy gaps of PA AHs make them ideal complements for acenes in materials applications. [3] Since 2010 our group [4] and others [5,6] have investigated ac lass of PA AHs based on the fully conjugated indenofluorene (IF) scaffold (1-5;F igure 1). Inclusion of the two five-membered rings within the conjugated skeleton has shown promise as am ethod to provide stability against oxidative and photolytic degradation.…”

“…[13] Thel arge differences in y values for the IF regioisomers can be rationalized by examining the number of Clar sextets [14] in the OS and CS resonance structures,a long with the orientation of the radicals in relation to the central benzene unit in the OS form. IFs with two Clar sextets in the CS form and three in the OS resonance structure exhibit only amodest gain in stability of one additional Clar sextet in the OS form, thus resulting in IFs with low diradical character (2,3,5). These three isomers also have either an ortho-or paraquinodimethane subunit at the central six-membered ring, thus permitting easy return to the CS form.…”