Synthesis of Long Oxahelicenes by Polycyclization in a Flow Reactor

Abstract

A series of oxahelicenes composed of ortho/meta-annulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene.

Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal “Defect” and Helicene Strain for Cyclizations

“…Helicenes are helically chiral aromatic compounds, [1] where the helical distortion is imposed by intramolecular steric demands.The inherent twist in such extended p-systems leads to special optical and electronic properties. [2] Recently, considerable progress has been achieved in the synthesis and applications of various helicenes, [3] including heterohelicenes [4] and fused "multipole" helicenes. [5] Theh elical twist depends on the length of the helicenes,t he nature of the substituents,a nd the presence of defects in the backbone.…”

Symmetric, Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

“…To the best of our knowledge,t hese are the longest optically pure helicenes which have synthetically been prepared. [17,21] Heating (+ +)-25 at 200 8 8Ci nd iphenylether for 4h,n or acemization was observed (Supporting Information, Figure S2). It should be noted that all efforts towards extending the [13]helicene (25), using the presented method with 2-naphthol (20)o r3 -Scheme 1.…”

“…[15] Recently the transition-metal-mediated [2+ +2+ +2]cycloisomerization has been utilized for the synthesis of long helicenoids that have regions of linear annulation being part of the helical backbone,asin3,orthe inclusion of 2H-pyranes as in oxa [19]helicenoid (4). [17][18][19] Using furan formation as the annulating step allows for the convenient preparation of oxa[n]helicenes,such as 5 and 6. [20,21] Herein we describe how an annulative p extension (APEX) benzofuran scaffolding, between 3,6-dihydroxy-carbazoles (7)a nd 2-naphthols (8), allows us to construct long heterocyclic helicenes:d iazaoxa-[7]-, diazadioxa[10]-, and diazatrioxa [13]helicenes (9-11;F igure 1b).…”

“…In the work presented herein, the final [8]circulene forming oxidative furan formation is avoided by ap rotecting-group strategy.W eh ave developed two procedures to access diazaoxa [7]helicene 15 (Scheme 1b, Methods Aa nd B). In the first method, dihydroxycarbazole 13 was monoprotected to give 16,w hich upon subjection to chloranil and BF 3 ·OEt 2 ,a llowed for the preparation of the doubly protected diazaoxa [7]helicene (17). Thea lternate method (Scheme 1b,M ethod B) avoids the low-yielding mono-protection of 13.S ubjecting 3,6-dihydroxy-carbazoles (13)t ot he oxidative coupling conditions,t reatment solely with chloranil, 14 was isolated.…”

Symmetric, Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

“…Furthermore it has been used to prepare aza[ n ]helicenes such as 2 . Recently the transition‐metal‐mediated [2+2+2]‐cycloisomerization has been utilized for the synthesis of long helicenoids that have regions of linear annulation being part of the helical backbone, as in 3 , or the inclusion of 2H ‐pyranes as in oxa[19]helicenoid ( 4 ) . Using furan formation as the annulating step allows for the convenient preparation of oxa[ n ]helicenes, such as 5 and 6 .…”