A highly enantioselective synthesis of 1,12‐disubstituted [4]carbohelicenes is reported. The key step for the developed synthetic route is a Au‐catalyzed intramolecular alkyne hydroarylation, which is achieved with good to excellent regio‐ and enantioselectivity by employing TADDOL‐derived (TADDOL=α,α,α,α‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) α‐cationic phosphonites as ancillary ligands. Moreover, an appropriate design of the substrate makes the assembly of [4]helicenes of different substitution patterns possible, thus demonstrating the synthetic utility of the method. The absolute stereochemistry of the newly prepared structures was determined by X‐ray crystallography and characterization of their photophysical properties is also reported.
A series
of cationic phosphonites, all sharing a TADDOL skeleton
but decorated with different positively charged substituents at phosphorus,
were synthesized and tested as chiral ancillary ligands on the Au-catalyzed
intramolecular hydroarylation of appropriate diynes toward carbo[6]helicenes
with different substitution patterns. Our studies showed that the
Au complexes derived from phosphonites bearing 1,3-dimesityl-1,2,3-triazolium
and 1,4-dimesityl-1,2,4-triazolium substituents are the best precatalysts
for the desired cyclization in terms of regio- and enantioselectivity
of the products obtained. In contrast, all of our attempts to prepare
Au complexes from cationic phosphonites derived from CAACs failed,
and only ligand decomposition products could be isolated.
The synthesis of unprecedented BINOL‐derived cationic phosphonites is described. Through the use of these phosphanes as ancillary ligands in AuI catalysis, a highly regio‐ and enantioselective assembly of appropriately designed alkynes into 1‐(aryl)benzo[5]carbohelicenes is achieved. The modular synthesis of these ligands and the enhanced reactivity that they impart to AuI‐centers after coordination have been found to be the key features that allow an optimization of the reaction conditions until the desired benzo[5]helicenes are obtained with high yield and enantioselectivity.
A highly enantioselective synthesis of 5,13-disubstituted dibenzo[d,d']benzo [1,2-b:4,3-b']dithiophenes is reported. Key for the successful assembly of these helical architectures is the last two successive Au-catalyzed intramolecular alkyne hydroarylation events. Specifically, the second cyclization is the enantiodetermining step of the whole process and provides the desired helicenes with excellent ee values when a TADDOL-derived 1,2,3-(triazolium)phosphonite moiety (TADDOL: α,α,α', is employed as an ancillary ligand. The absolute stereochemistry of the newly prepared structures has been determined by X-ray crystallography to be P; the optical properties of these heterohelicenes are also reported. A three-step procedure was subsequently developed that allows the transformation of the initially obtained dithia-[5]helicenes into dithia [9]helicenes without erosion of the enantiopurity.
A highly enantioselective synthesis of 1,12‐disubstituted [4]carbohelicenes is reported. The key step for the developed synthetic route is a Au‐catalyzed intramolecular alkyne hydroarylation, which is achieved with good to excellent regio‐ and enantioselectivity by employing TADDOL‐derived (TADDOL=α,α,α,α‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) α‐cationic phosphonites as ancillary ligands. Moreover, an appropriate design of the substrate makes the assembly of [4]helicenes of different substitution patterns possible, thus demonstrating the synthetic utility of the method. The absolute stereochemistry of the newly prepared structures was determined by X‐ray crystallography and characterization of their photophysical properties is also reported.
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