A Scalable and Chromatography-Free Synthesis of N,N-Bis(9,9-dimethyl-9H-fluoren-2-yl)-3′,3′,4′,7′-tetramethyl-2′,3′-dihydrospiro[fluorene-9,1′-indene]-2-amine, a new Hole Transport Material for Organic Solar Cells

The title triarylamine N,N-bis(9,9-dimethyl-9H-fluoren-2-yl)-3′,3′,4′,7′-tetramethyl-2′,3′-dihydrospiro[fluorene-9,1′-indene]-2-amine is a new hole transport material for organic solar cells. After investigating different discovery approaches (Schemes 1 + 2), a multi gram-scale synthetic sequence was developed (Scheme 4). The key intermediate 2-bromo-9-(2,5-dimethylphenyl)-9H-fluorene was accessible from 2-bromo-9-fluorenone by either the sequence Grignard reaction, Et₃SiH/BF₃ reduction or by direct arylation of the corresponding 2-bromo-9-fluorenol. Alkylation at C(9) of 2-bromo-9-(2,5-dimethylphenyl)-9H-fluorene with methallyl chloride and cyclization by an intramolecular Friedel-Crafts alkylation led to the key building block 2-bromo-3′,3′,4′,7′-tetramethyl-2′,3′-dihydrospiro[fluorene-9,1′-indene] (Scheme 4). A Buchwald-Hartwig coupling was employed (Scheme 3 + 4) for the assembly of the final triarylamines. The developed gram-scale synthesis of the title compound is scalable and chromatography-free with an overall yield >25 % over 5 steps.