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45.30.3.1.3.1 Variation 1: Using Organolithium Reagents

DOI: 10.1055/sos-SD-045-01426

Koo, S.Science of Synthesis, (2010451406.

Commercially available tropylium tetrafluoroborate (184) readily reacts with organolithium reagents to 7-alkyl- or 7-aryl-substituted cyclohepta-1,3,5-trienes. 7-Methylcyclohepta-1,3,5-triene can be obtained (83% yield after distillation) by the reaction with methyllithium in diethyl ether.[‌191‌] Various aryllithium reagents which can be prepared in a regioselective manner add to tropylium salt 184 to afford polycyclic aromatic compounds containing a cycloheptatrienyl moiety (Scheme 45).[‌192‌‌194‌] Selective monolithiation of 1,8-dibromonaphthalene with butyllithium provides (8-bromonaphthalen-1-yl)lithium (203), which reacts with 184 to give 1-bromo-8-(cyclohepta-2,4,6-trienyl)naphthalene (204) in 83% yield.[‌192‌] Deprotonation of 3-bromo-1-benzothiophene (205) with lithium diisopro­pylamide, followed by addition of 184 gives 3-bromo-2-(cyclohepta-2,4,6-trienyl)-1-benzothiophene (206) as expected,[‌193‌] whereas successive treatment of 2,3-dibromo-1-methyl-1H-indole (207) with tert-butyllithium and then with 184 abnormally generates 2-bromo-3-(cyclohepta-2,4,6-trienyl)-1-methyl-1H-indole (208).[‌194‌] It is plausible that the initially formed 3-bromo-2-lithio-1-methyl-1H-indole rearranges to 2-bromo-3-lithio-1-methyl-1H-indole before 184 is added.

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