Borylated norbornadiene derivatives: Synthesis and application in Pd-catalyzed Suzuki–Miyaura coupling reactions

Abstract

The photochromic norbornadiene/quadricyclane system is among the most promising candidates for molecular solar thermal (MOST) energy storage. As in this context there is still the need for new tailor-made derivatives, borylated norbornadienes were synthesized that may be used as versatile building blocks. Thus, the 4,4,5,5-tetramethyl-2-(bicyclo[2.2.1]heptadien-2-yl)-1,3,2-dioxaborolane was prepared and shown to be a suitable substrate for Pd-catalyzed Suzuki–Miyaura coupling reactions with selected haloarenes. It was demonstrated exemplarily that the novel monosubstituted 2-(1-naphthyl)norbornadiene, that is accessible through this route, is transformed to the corresponding quadricyclane upon irradiation, whereas the back reaction can be accomplished by thermal treatment.

Monoaryl‐Substituted Norbornadiene Photoswitches as Molecular Solar Thermal Energy Storage Compounds: Synthesis and Investigation

“…The target norbornadiene derivatives were readily available with Suzuki-Miyaura coupling reactions of the key substrate 1 e to give the products 1 g, 1 h, 1 and 1 k in good yields, which are comparable to the ones reported earlier. [20] As the only exception, derivative 1 l was isolated only in 33 % yield, which is most likely caused by the apparent instability of this compound at ambient conditions. Overall, this reaction is a straightforward, variable and reliable key step for the synthesis of 2-arylsubstituted norbornadiene derivatives.…”

“…The known derivatives 1 f and 1 i [20] as well as the new derivatives 1 g, 1 h, 1 j-1 l were synthesized by Suzuki-Miyaura coupling reactions of the borononorbornadiene 1 e with selected haloarenes 3 f-3 l in moderate to good yields, ranging from 55 % (1 j) to 72 % (1 g) (Scheme 2). [20] As the only exception, 1 l was obtained with a yield of just 33 %. The novel compounds 1 g, 1 h, 1 j-1 l were identified and characterized by NMR spectroscopy ( 1 H, 13 C, COSY, HSQC, HMBC), melting point and elemental analysis.…”

“…Supporting Information Figure S1). [20] For each compound, the absorption properties were essentially the same in acetonitrile, ethanol, chloroform, n-hexane, ethyl acetate and toluene. However, within the series of derivatives the shift and shape of the absorption bands varied significantly with the substitution pattern.…”

“…[19] To fill this gap with additional experimental data, we have made available an efficient synthetic route to monosubstituted arylnorbornadienes by Suzuki-Miyaura coupling reaction of a 2-borononorbornadiene substrate. [20] As this method enables a straightforward modular approach to this class of compounds with varying aryl substituents, we employed it in the present study to synthesize a small series of derivatives that may increase the π system of the norbornadiene chromophore and, in turn, lead to a red-shifted absorption. Specifically, we synthesized norbornadienes with naphthyl, anthracenyl or donor-acceptor-phenyl substituents and examined their suitability as MOST systems.…”

“…At the same time, theoretical calculations have shown that mono‐substituted derivatives may provide a reasonable balance between reaction efficiency, absorption properties, and storage density [19] . To fill this gap with additional experimental data, we have made available an efficient synthetic route to monosubstituted arylnorbornadienes by Suzuki‐Miyaura coupling reaction of a 2‐borononorbornadiene substrate [20] . As this method enables a straightforward modular approach to this class of compounds with varying aryl substituents, we employed it in the present study to synthesize a small series of derivatives that may increase the π system of the norbornadiene chromophore and, in turn, lead to a red‐shifted absorption.…”