Diastereoselective Ullmann Coupling to Bishelicenes by Surface Topochemistry

Area of Science:

• Organic Chemistry
• Surface Science
• Supramolecular Chemistry

Background:

• Helicenes are chiral polycyclic aromatic hydrocarbons with unique structural and electronic properties.
• On-surface synthesis offers a powerful platform for constructing complex organic molecules with controlled architectures.
• Understanding self-assembly processes is crucial for designing functional nanomaterials.

Purpose of the Study:

• To investigate the self-assembly behavior of 9,9′-bisheptahelicene on the Au(111) surface.
• To compare the self-assembly outcome with on-surface synthesis of the same molecule.
• To elucidate the factors governing stereoselectivity in these processes.

Main Methods:

• Scanning tunneling microscopy (STM) was employed to study the self-assembly of 9,9′-bisheptahelicene on Au(111).
• On-surface synthesis via Ullmann coupling of 9-bromoheptahelicene was performed to obtain the target molecule.
• Analysis of STM images allowed for the determination of diastereomeric ratios.

Main Results:

• Self-assembly of 9,9′-bisheptahelicene on Au(111) resulted in a diastereomeric excess (de) of 50% for the (M, P)-meso-form.
• On-surface synthesis also yielded the same diastereomeric excess, indicating consistent stereochemical outcomes.
• The observed stereoselectivity was attributed to a topochemical effect.

Conclusions:

• Surface alignment of reactants and intermediates sterically favors the (M, P)-transition state during on-surface synthesis.
• Topochemical control is a key factor in achieving high stereoselectivity in surface-mediated reactions.
• This study provides insights into the rational design of chiral molecules with specific spatial arrangements on surfaces.