Diels–Alder Reactions of 9‑Ferrocenyl- and 9,10-Diferrocenylanthracene: Steric Control of 9,10- versus 1,4-Cycloaddition

Cycloadditions
of benzynes, N-methyl- or N-phenylmaleimide,
dimethyl acetylenedicarboxylate, and benzoquinone to 9-ferrocenylanthracene
(1) and 9,10-diferrocenylanthracene (2)
are described. Benzyne and 3-fluorobenzyne add to 1 and 2 to form the corresponding 9-ferrocenyl- or 9,10-diferrocenyltriptycenes;
in contrast, bulkier benzynes such as 3-trifluoromethylbenzyne preferentially
add to 2 across C1 and C4 to form
6,11-diferrocenyl-5,12-etheno-5,12-dihydrotetracenes. The maleimides
undergo Diels–Alder reactions with 1 to form the
9-ferrocenylbarrelenes 10 and 11, but cycloaddition
to 2 occurs not only across C9 and C10 to form the barrelene 13 but also across C1 and C4 to yield ethenoanthracenes as both endo and exo
adducts, 14 and 15, respectively. Likewise,
DMAD forms 11,12-dicarbomethoxy-9-ferrocenylbarrelene (16) with 1 but reacts with 2 by addition
across C1 and C4 to form 17. Hydrolysis
of 16 cleaves only the ester distant from the ferrocenyl
group. Treatment of the Diels–Alder adduct 20 of
9-ferrocenylanthracene (1) and benzoquinone with base
and RX (R = Me, Et, CH2OMe, CH2CO2Me) yields the corresponding 1,4-dialkoxy-9-ferrocenyltriptycenes 22–25. The factors influencing the regiochemistry
of the cycloadditions are discussed, and several examples of each
reaction type have been characterized by X-ray crystallography.