Synthesis and Properties of Sila[n]helicenes via Dehydrogenative Silylation of C–H Bonds under Rhodium Catalysis

Abstract

Use of a rhodium catalyst with (R)-(S)-BPPFA ligand allows efficient synthesis of sila [n]helicenes via dehydrogenative silylation of CH bonds. By selecting the proper ligands, the current method provides the ability to prepare unsymmetrical sila [n]helicene derivatives without any oxidants. The resulting sila[6]helicene is a rare example of a five-membered ring-fused [6]helicene, which was isolated as a single pure enantiomer without substituents on the terminal benzene rings.Silicon-containing -conjugated molecules show promise as materials useful in the areas of electro-and photoluminescence. 1 The incorporated silicon atom has a significant impact on the energy levels of the frontier orbital, and that of the LUMO is effectively stabilized by the interaction between the low lying * orbital of two exocyclic -bonds of the silicon atom and the * orbital of the conjugated -system of the backbone. Catalytic dehydrogenative silylation of CH bonds is a straightforward, atom-efficient, and sustainable route toward these silicon-containing -conjugated molecules. 2-5 Since our first report in 2010 on the synthesis of silafluorenes, 3a rhodium-catalyzed dehydrogenative silylation has been developed for the synthesis of various silicon-containing -conjugated molecules, such as silicon-bridged p-terphenyl, 3a spirosilabifluorenes, 3b,3j benzosilolometallocenes, 3e-g dihydrobenzosilole, 3i and phenazasilines 3d (Figure 1). To further demonstrate the utility of this silylative cyclization, development of a robust catalyst system with improved reaction efficiency is highly desirable for the synthesis of more challenging siliconcontaining -systems. The present paper describes the synthesis of sila[n]helicenes (n = 4, 5 and 6) with uniquely merged -conjugated systems of helicene and silole. This study also demonstrates that the amino group-substituted (R)-(S)-BPPFA ((R)-N,N-dimethyl-1-[(S)-1',2-bis(diphenylphosphino)ferrocenyl]ethylamine) ligand is highly effective for the rhodiumcatalyzed dehydrogenative silylation of CH bonds.Helicene possesses a unique screw-shaped -system consisting of all ortho-annelated aromatic rings. 6 In addition to a unique function as an optoelectronic material, these compounds can be utilized as ligands and organocatalysts. 6c Despite their promising potential for functional materials, few [n]helicenes could allow customization of their properties, and dehydrogenative silylation with the activation of ubiquitous CH bonds is thought to allow a more straightforward approach. We previously reported that Wilkinson's catalyst, RhCl(PPh 3 ) 3 , is effective for the silylative cyclization of 2-(2-

Catalytic Enantioselective Transformations Involving C–H Bond Cleavage by Transition-Metal Complexes

“…In 2016, Murai, Takai, and their co-workers disclosed one additional example, as part of an unrelated mechanistic study, demonstrating that (R,S)-BPPFA also facilitated the reaction but provided a significantly lower level of enantiocontrol. 89 An intermolecular C−H functionalization approach to racemic fused ferrocenylpyridinones was disclosed by You and co-workers in 2016 (Scheme 23). 90 In this study, one example demonstrated that enantioinduction was possible via employment of chiral cyclopentadienyl Rh(I) complex Cat-1 (this ligand class was originally developed in the laboratory of Cramer; see section 3.1.1.1.2 for details regarding their inception).…”

Zirconium-Mediated Synthesis of Multi-substituted Dibenzosilole and Benzonaphthosilole Derivatives

“…Our report on the synthesis of 11 is unique, because to date, it is the only synthesized benzonaphthosilole derivative with substituents on the aromatic rings. The structure and synthesis strategy of 11 are rather different from those of the previous literature [17][18][19]. In contrast to the rhodium-catalyzed reaction, the synthesis strategy we designed uses readily available alkynes as the starting materials instead of dimethyl[2-(naphthalen-2-yl) phenyl]silane, which is difficult to prepare.…”

“…The synthesis of benzonaphthosilole (benzo[b]silafluorene) derivatives has seldom been reported although they are promising silole derivatives with extended π-conjugation based on dibenzosilole ( Fig. 1) [17][18][19], and no synthesis of benzonaphthosilole derivatives has been reported where alkyl, phenyl, and EWGs groups were substituted on the aromatic rings. Because substituents can effectively regulate molecular properties, lack of a synthesis method has limited the performance studies of benzonaphthosilole derivatives.…”

Consecutive HDDA and TDDA reactions of silicon-tethered tetraynes for the synthesis of dibenzosilole-fused polycyclic compounds and their unique reactivity

“…Substitution at the 6- and 15-positions did not affect their photophysical properties. The Stokes shifts were twice as much as those for simple sila[5]helicene,14 probably due to the fused siline ring. The torsion angle of the silahelicene moiety of 2a was 27.0°, which was significantly larger than that of the simple sila[5]helicene (17.6°) 14…”