Synthesis and Liquid Crystalline Self‐Assembly of Concave Diindoles with a Hydropentalene Core

Abstract

Fischer indole reaction of Weiss diketone with 4‐bromophenylhydrazine provided the 2,8‐dibromo‐hexahydropentaleno[2,1‐b:5,4‐b’]diindole key intermediate, which was converted to the target compounds by N‐protection/Suzuki cross‐coupling. Variation of protecting groups, mesogenic units, and their alkoxy substitution gave calamitic diindole mesogens. Both N‐protection and alkoxy chain influenced the mesomorphic properties of phenyl diindoles. Among the differently N‐protected derivatives only ethylcarbamate‐protected ones formed enantiotropic mesophases. Mesophase range and type were controlled by the chain lengths: chains≤C12 gave nematic (N) phases, diindoles with side chains≥C14 formed additional lower temperature smectic (SmA) phases. Irrespective of the chain lengths 4′‐alkyloxybiphenyl diindoles formed N and SmA phases upon first heating but tended to decomposition below their clearing points. X‐ray crystal structure analysis of bis(4′‐decyloxy)biphenyl diindole reveals that in the solid‐state the folding angle of the hydropentalene core caused an almost perpendicular orientation of the two indole/biphenyl parts with respect to each other.

Organocatalytic Tandem Synthesis of Chiral Hexacyclic Bisindoles from Propargylic 3-Methyleneindoles with 2-Indolylmethanols

“…To show the practical efficacy of the methodology, the reaction could be scaled up to 1 mmol, and the corresponding chiral product 3a was obtained in 82% yield with 92% ee (Scheme a). Moreover, a gram scale reaction was successfully performed to synthesize the racemic product 3a using diphenyl phosphate as a catalyst, highlighting the potential application value of this method in liquid crystalline indole materials (Scheme b) . Furthermore, the two indole N–H groups of 3a could be selectively or simultaneously protected with the Boc group in good yield without a loss of enantiomeric purity (Scheme c).…”

“…Bisindole alkaloids, in particular, have been a subject of significant attention due to their highly potent biological properties in comparison to monoindole alkaloids . On the other hand, pentaleno bisindole compounds have great potential in the field of materials chemistry, especially as liquid crystalline indole materials . However, to date, the methods of constructing the pentaleno bisindole skeletons are rather limited, which was prepared by a 2-fold Fischer indole cyclization process that demands the use of stoichiometric amounts of Lewis acid .…”

“…2 On the other hand, pentaleno bisindole compounds have great potential in the field of materials chemistry, especially as liquid crystalline indole materials. 3 However, to date, the methods of constructing the pentaleno bisindole skeletons are rather limited, which was prepared by a 2-fold Fischer indole cyclization process that demands the use of stoichiometric amounts of Lewis acid. 4 Therefore, it has become an important task to design energy-efficient and "green" catalytic methods that employ easily accessible substrates to synthesize these complex polycyclic bisindole structures.…”

“…Moreover, a gram scale reaction was successfully performed to synthesize the racemic product 3a using diphenyl phosphate as a catalyst, highlighting the potential application value of this method in liquid crystalline indole materials (Scheme 5b). 3 Furthermore, the two indole N−H groups of 3a could be selectively or simultaneously protected with the Boc group in good yield without a loss of enantiomeric purity (Scheme 5c). To our excitement, the bisindole product 3k was efficiently converted into heptacyclic compound 8 through an N-alkylation reaction, without any erosion of enantioselectivity (Scheme 5d).…”