Facile Route to Quadruply Annulated Borepins

A two-step
synthesis sequence furnishes quadruply annulated borepins
in high yields. The first step involves a nucleophilic substitution
reaction between aryl–BF3K salts (aryl = mesityl,
phenyl) and lithiated bromonapthalene derivatives LiNaphBr,R (HNaphBr,R = 8-bromonaphthalene (a), 5-bromoacenaphthene
(b), 5-bromoacenaphthylene (c)). In the second step, the resulting
heteroleptic triarylboranes aryl–B­(NaphBr,R)2 (3a–c) are subjected to
an intramolecular Ni-mediated Yamamoto reaction to close the seven-membered
rings and create the borepins 4a–c. Only in the case of 3b is the Yamamoto reaction accompanied
by a C–H activation reaction furnishing the 7-hydro-7-borabenzo­[de]­anthracene derivative 5. The product ratio 4b/5 can be influenced by control of the local
Ni(0) concentration. The borepins 4a–c are benchtop stable and highly soluble even in hexane. Compounds 4a–c undergo reversible one-electron reduction; 4c is also able to accept a second electron in a reversible
manner and already at moderate potential values (E1/2 = −1.49 V and −1.84 V (vs FcH/FcH+)). 4a, 4b, and 5 show
photoluminescence in the blue-green region of the spectrum, while 4c is nonfluorescent, which is likely attributable to an intramolecular
charge-transfer transition.