Thermal Cyclodehydrogenations To Form 6-Membered Rings:  Cyclizations of [5]Helicenes

Abstract

Flash vacuum pyrolysis (FVP) of [5]helicene induces a thermal cyclodehydrogenation to form benzo[ghi]perylene. Evidence is presented that supports an electrocyclization-rearomatization mechanism and is inconsistent with mechanistic alternatives involving the intermediacy of aryl radicals or carbenes in the helicene fjord region.

Helicene-Based Chiral Auxiliaries and Chirogenesis

“…However, any fused ring which is not present in the ortho-fused manner is not included in the n counting. For example, structures 7 [17], 8 [18], and 9 [19] are all [5]helicenes, but not [6]helicenes, despite the presence of six aromatic rings in total ( Figure 2). The functionalized helicenes such as 10 [20], 11 [21], and 12 [22] (Figure 2) which have a heteroatom as a substituent on the aromatic ring, as in the case of amino, hydroxyl (phenolic or alcoholic), thiol, or phosphorus-containing groups, are still carbohelicenes but not heterohelicenes.…”

Domino Diels–Alder Cycloadditions of Arynes: New Approach to Elusive Perylene Derivatives

“…This is a remarkable outcome since harsh conditions such as strong oxidants, prolonged UV irradiation, or flash vacuum pyrolysis (FVP) are normally required to perform the cyclodehydrogenation of pentahelicene. [15] Therefore, the three benzene rings fused to the pentahelicene moiety in compound 1 c are the crucial structural motif for this spontaneous cyclodehydrogenation to afford poly-A C H T U N G T R E N N U N G arene 9.…”

Synthesis and Structures of Multifunctionalized Helicenes and Dehydrohelicenes: An Efficient Route to Construct Cyan Fluorescent Molecules

“…Recently, Xue and Scott reported a thermal cyclodehydogenation of [5]heliA C H T U N G T R E N N U N G cene by flash vacuum pyrolysis at 1000 8C, but under the same conditions, a significant portion of benzo [5]helicene stayed unchanged. [16] Herein, we report 1) a convenient and efficient synthesis of a dimethoxy-substituted benzo [5]helicene and a benzo [7]helicene, 2) direct bromination of the benzo [5]helicene and synthesis of a series of multifunctionalized [5]helicenes by the Suzuki-Miyaura cross-coupling reactions, and 3) the further oxidative cyclodehydrogenation of the [5]helicene derivatives to afford a series of multifunctionalized dehydro [5]helicene (or naphthoperylene) derivatives with the cyan fluorescent properties and high quantum yields.…”