Synthesis, Optical Properties, and Regioselective Functionalization of 4a-Aza-10a-boraphenanthrene

Abstract

4a-Aza-10a-boraphenanthrene has been synthesized in only four steps from commercially available materials with a remarkable overall yield of 62%. In contrast to other BN-isosteres of phenathrene, this isomer is weakly fluorescent, which has been explained by means of computational studies that found a low energy conical intersection for the nonradiative deactivation of the excited state. Moreover, a completely regioselective functionalization of 4a-aza-10a-boraphenanthrene at C-1 by reaction with activated electrophiles has been achieved.

Remarkable effect of alkynyl substituents on the fluorescence properties of a BN-phenanthrene

“…We are interested in evaluating the reactivity [24] and properties of 1a in greater detail as this could provide valuable information that leads to a better understanding of the behaviour of BN-aromatics. Interestingly, 1a turned out to be weakly fluorescent [23], in contrast to other BN-phenanthrene isomers described previously [25–26]. The presence of aryl or amino substituents at C1, which can be introduced via bromination and subsequent palladium-catalyzed cross coupling, does not have a significant impact on the fluorescence of these compounds (Fig.…”

“…We have recently designed an efficient synthesis for one of the simplest BN-PAHs, namely BN-phenanthrene 1a [23]. We are interested in evaluating the reactivity [24] and properties of 1a in greater detail as this could provide valuable information that leads to a better understanding of the behaviour of BN-aromatics.…”

“…The presence of aryl or amino substituents at C1, which can be introduced via bromination and subsequent palladium-catalyzed cross coupling, does not have a significant impact on the fluorescence of these compounds (Fig. 1) [23]. In order to gain a deeper understanding of the photophysical properties of the BN-phenanthrene core, we decided to evaluate the influence of substituents located in other positions.…”

Late-stage functionalization of BN-heterocycles

“…Vaquero synthesized the 4a,10a-BN-phenanthrene 97, and like its simpler analogue 9,10-BN-naphthalene, it is attacked by halogen electrophiles at the carbon position adjacent to boron (Scheme 43a). 45 Either a Cl or a Br can be installed in good to excellent yield.…”

“…Brominated 98b serves as the electrophile in three different cross coupling reactions. 45 Suzuki cross-coupling leads to arylated 107, Buchwald-Hartwig coupling furnishes aminesubstituted 108, and Sonogashira coupling installs an alkyne in product 109 (Scheme 46). The brominated BN-anthracenes 102b and 104 also undergo Suzuki cross-coupling with aryl-boronic acids to furnish 110 (Scheme 47a) 46 and 111 (Scheme 47b), respectively.…”