Convergent Synthesis of Fluorene Derivatives by a Rhodium‐Catalyzed Stitching Reaction/Alkene Isomerization Sequence

Abstract

A convergent synthetic method for the synthesis of fluorene derivatives has been developed by devising a rhodium‐catalyzed stitching reaction/alkene isomerization sequence. The reactions proceed smoothly under mild conditions for a variety of substrate combinations, and extended π‐conjugation systems are also readily accessible by utilizing this synthetic method. Optical properties of the obtained fluorene derivatives have also been examined.

Intermolecular Three-Component Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Remote Nucleophilic Substitution Sequence

“…In this context, we have recently reported an intermolecular synthesis of fluorene derivatives by employing a sequence of rhodium-catalyzed stitching reaction followed by alkene isomerization (Scheme a) . This method realized unprecedented bond connections toward the formation of a fluorene skeleton in a convergent manner to give a variety of 11-monosubstituted 11 H -benzo­[ a ]­fluorenes and their analogues.…”

“…Iodomethane (210 μL, 3.30 mmol) was added dropwise to a stirring suspension of Mg turnings (88.2 mg, 3.63 mmol) in Et 2 O (2.0 mL), and the mixture was stirred for 20 min at room temperature. This was then added dropwise with the aid of additional Et 2 O (1.0 mL) to a solution of 1-methyl-2-(1-pentynyl)-1H-indole-3-carbaldehyde 15 (495 mg, 2.20 mmol) in Et 2 O (12 mL) at 0 °C, and the mixture was stirred for 6 h at 0 °C. The reaction was quenched with saturated NH 4 Claq, and this was extracted with Et 2 O.…”

“…In this context, we have recently reported an intermolecular synthesis of fluorene derivatives by employing a sequence of rhodium-catalyzed stitching reaction followed by alkene isomerization (Scheme 1a). 15 This method realized unprecedented bond connections toward the formation of a fluorene skeleton in a convergent manner to give a variety of 11monosubstituted 11H-benzo [a]fluorenes and their analogues. Based on this synthetic approach, we imagined that it could be further extended to the synthesis of more substituted (benzo)fluorenes by combining the stitching reaction of 2alkynylarylboronates and 2-alkynylaryl ketones/aldehydes under rhodium catalysis with a subsequent aromatizationdriven remote nucleophilic substitution reaction.…”

Rhodium-Catalyzed Stitching Reactions for the Facile Synthesis of Extended π-Conjugated Compounds

“…By using this protocol, various 11 H -benzo[ a ]fluoren-5-ol derivatives could be efficiently synthesized under mild conditions (Scheme 10 ). 19 These novel fluorene derivatives were fluorescent molecules and the emission wavelength could be widely tuned from UV to NIR regions depending on the substituents (Figure 2 ).…”

“…By using this protocol, various 11H-benzo[a]fluoren-5-ol derivatives could be efficiently synthesized under mild conditions (Scheme 10). 19 These novel fluorene derivatives were fluorescent molecules and the emission wavelength could be widely tuned from UV to NIR regions depending on the substituents (Figure 2). After conducting the same stitching reaction as in Scheme 9a, a remote nucleophilic substitution with elimination of a hydroxy group, instead of alkene isomerization, could provide 11H-benzo[a]fluorenes having an additional substituent on the 5-membered ring (at position 11) (Scheme 9b).…”