1H‐Phosphindoles as Structural Units in the Synthesis of Chiral Helicenes

Abstract

Helical arrangements and the associated chirality have wellknown implications in a variety of areas ranging from natural biological systems to materials science. In particular orthofused polyaromatic systems with p-conjugated helical structures display unique physicochemical properties which have stimulated countless studies [1] oriented towards self-assembly and chiral recognition, [2] chiroptical devices, [3] molecular machines, [4] material chemistry, [5] organometallic and organocatalysis, [6, 7] as well as asymmetric synthesis. [8, 9] In the general context of the growing interest in helical derivatives, helicenes displaying phosphorus functions represent promising scaffolds. Trivalent phosphorus functions and the related metal complexes are easily designed to tune the electronic and physicochemical properties of helicenes, [10] as well as to offer a wide range of potential uses in organometallic chemistry and catalysis. It is therefore rather surprising to note that the field today is still in its infancy, first of all in terms of number of compounds as well as structural diversity. So far, most phosphorus derivatives having helical chirality display polyaromatic (or heteroaromatic) helical scaffolds with pendant phosphorus functions (phosphites, [2a, 6b, 11] trivalent phosphines and phosphine oxides, [6a, 12] phospholes, [10a,b] etc.). Very little is known about their properties and behaviors.The aim of this work is to expand the range of heterohelicenes to chiral helicenes where phosphorus is embedded within the helical framework itself, at the external edge of the fused ring sequence (Figure 1). Helical phosphines of this class would not only display peculiar electronic features resulting from extended p conjugation, but also take full advantage of the dissymmetric steric environment generated by the helical chirality at the external edge. Helicenes of this class are unknown to date, whereas phosphahelicenes, wherein phosphorus is embedded in the internal section of the helical framework, have been reported only recently by Tanaka and co-workers and Nozaki and co-workers. [13] We intend to disclose here: a) the first diastereo-and enantioselective access to [6]-and [7]helicenes in which the fused ring sequence ends with a phosphole unit; b) the structural and chiroptical properties of enantiomerically pure derivatives of this class; c) the enantiospecific self-assembly of columnar arrangements in the solid state; and d) the photochemical [2+2] annulations of phosphole-fused [6]helicenes into dimeric helical structures.When targeting helicenes where phosphorus is embedded within the helical framework itself, we envisioned 1Hphosphindole oxides as suitable building blocks and have adapted the classical helicene synthesis, based on the photochemical cyclization/dehydrogenation of diarylolefins, to these substrates. [14] Our starting materials are the olefins 5 which combine a 1H-phosphindole unit and a benzo[c]phenanthrene fragment. They have been prepared according to a known procedure, [15] by usi...

Heterohelicenes with Embedded P‐Chiral 1H‐Phosphindole or Dibenzophosphole Units: Diastereoselective Photochemical Synthesis and Structural Characterization

“…These two fragments were connected onto the same olefin group through two sequential palladium-catalyzed Suzuki reactions. The second coupling step, between compound 3 and phosphindole oxide triflate (R P )-4, [6] was performed in a THF/water mixture, in the presence of Cs 2 CO 3 , with 10 mol % [PdCl 2 A C H T U N G T R E N N U N G (SPhos) 2 ] as the catalyst (Scheme 2). [12] Then, compound 1 was submitted to a coupling reaction with bis-boronate 2 [13] in the presence of [PdA C H T U N G T R E N N U N G (PPh 3 ) 4 ] to afford the olefinic boronate 3 in 46 % yield.…”

“…[6] Its coupling with 3 afforded the stilbene derivative (R P )-5 which displayed an [a] D value of +35 (c = 0.4, CHCl 3 ). This epimer was obtained in its pure form by flash chromatography on silica gel of the corresponding epimeric mixture.…”

“…[2,3] Therefore, it is expected that unprecedented tools for both materials science and innovative catalytic applications might be developed in the future by taking advantage of such helicenes. [6] For their synthesis, we adapted the well-known photochemical oxidative cyclization reaction of diarylethenes [7] to phosphindole-substituted olefins, as shown in Scheme 1. So far, synthetic strategies have been reported by the groups of Tanaka and Nozaki, which make use of Rh I /H 8 -BINAP-catalyzed (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) enantioselective [2+2+2] cyclization [4] and palladium-promoted intramolecular P-arylation [5] reactions as the key steps, respectively.…”

“…The X-ray crystal structure of a gold chloride complex of a [6]helicene is reported. The helical phosphine oxides were characterized by X-ray diffraction.…”

“…able for the synthesis of P/N-bi-heterosubstituted dimeric helicenes, as well as for the syntheses of [6]-and [8]helicenes in high yields starting from dibenzophospholes as structural units. Finally, we demonstrate that both [6]-and [8]helicenes with trivalent phosphorus groups are effective ligands for transition-metal complexes. Finally, we demonstrate that both [6]-and [8]helicenes with trivalent phosphorus groups are effective ligands for transition-metal complexes.…”