Synthesis and Structural and Optical Behavior of Dehydrohelicene-Containing Polycyclic Compounds

Abstract

Dehydrohelicene-based molecules stand out as highly promising scaffolds and captivating chiroptical materials, characterized by their unique chirality. Their quasi-helical π-conjugated molecular architecture, featuring successively ortho-annulated aromatic rings, endows them with remarkable thermal stability and optical properties. Over the past decade, diverse approaches have emerged for synthesizing these scaffolds, reinvigorating this field, with anticipated increased attention in the coming years. This review provides a comprehensive overview of the historical evolution of dehydrohelicene chemistry since the pioneering work of Zander and Franke in 1969 and highlights recent advancements in the synthesis of various molecules incorporating dehydrohelicene motifs. We elucidate the intriguing structural features and optical merits of these molecules, occasionally drawing comparisons with their helicene or circulene analogs to underscore the significance of the bond between the helical termini.

Keywords: Scholl reaction; chiroptical; circulenes; dehydrohelicene; helicenes; material-based applications; nanographene; photophysical; quasicirculene; synthesis.

Figure 1

Comprehensive overview of the pivotal accomplishments in the chemistry of carbo[n]dehydrohelicenes and hetero[n]dehydrohelicenes over last 50 years: (upper portion) hetero[n]helicenes, distinguished by their incorporation of sulfur, nitrogen, or oxygen atoms into the helical framework, adding an extra layer of complexity and functionality; (lower portion) carbo[n]dehydrohelicenes, characterized by a dehydrohelicene scaffold composed solely of carbon and hydrogen atoms. Blue color indicates the sigma bonds connecting the two helical termini of dehydrohelicenes.

Scheme 1

Synthesis of carbo[5]dehydrohelicene derivatives via Scholl reaction.

Scheme 2

Direct access to the unique architectures of amino benzo[ghi]perylene.

Scheme 3

Oxidative cyclo-rearrangement of carbo[n]helicenes into their corresponding carbo[5]dehydrohelicenes.

Scheme 4

Skeletal reconstruction of carbo[7]helicenes via a two-step oxidation protocol.

Scheme 5

Synthesis of various partially graphitized and fully graphitized spironanographene (dashed lines in compound 13 indicates that one of these bonds is not formed).

Scheme 6

Mechanochemical synthesis of functionalizable curved nanographenes.

Scheme 7

Photochemical synthesis of carbo[6]dehydrohelicene.

Scheme 8

Early synthesis of carbo[7]circulene and some carbo[6]dehydrohelicene derivatives.

Scheme 9

Preparation of a grossly warped nanographene with dehydrohelicene core.

Scheme 10

First report of synthesizing dehydrohelicene scaffold using the chloraluminate method.

Scheme 11

Dehydrogenation of heterohelicenes by a Scholl-type reaction.

Scheme 12

Cyclization of thiophene-based [7]helicene at the two bromine-substituted termini.

Scheme 13

Preparation of saddle-helix hybrid molecule with dehydrohelicene core moiety.

Scheme 14

Preparation of triaza[7]dehydrohelicene derivatives with low chiral stability.

Scheme 15

Preparation of modified triaza[7]dehydrohelicene derivatives with high chiral stability.

Scheme 16

Preparation of unique dehydro[7]helicene derivatives with three different heteroatoms.

Scheme 17

Facile synthesis of aza[7]dehydrohelicene 51 via intramolecular Scholl-type reaction.

Scheme 18

Sequential electrochemical synthesis of oxaza[7]dehydrohelicene: (upper) electrochemical one-pot process; (lower) stepwise enantioselective synthesis via chiral vanadium-catalyzed hetero-coupling and electrochemical oxidative transformations.

Scheme 19

Data-driven electrochemical synthesis of double oxaza[7]dehydrohelicene.