Overclocking Nitronyl Nitroxide Gold Derivatives in Cross‐Coupling Reactions

Abstract

Nitronyl nitroxides are functional building blocks in cutting-edge research fields, such as the design of molecular magnets, the development of redox and photoswitchable molecular systems and the creation of redox-active components for organic and hybrid batteries. The key importance of the nitronyl nitroxide function is to translate molecular-leveloptimized structures into nano-scale devices and new technologies. In spite of great importance, efficient and versatile synthetic approaches to these compounds still represent a challenge. Particularly, methods for the direct introduction of a nitronyl nitroxide moiety into aromatic systems possess many limitations. Here, we report gold derivatives of nitronyl nitroxide that can enter Pd(0)-catalysed cross-coupling reactions with various aryl bromides, affording the corresponding functionalized nitronyl nitroxides. Based on the high thermal stability and enhanced reactivity in catalytic transformation, a new reagent is suggested for the synthesis of radical systems via a universal cross-coupling approach.

A Nitronyl Nitroxide‐Substituted Benzotriazinyl Tetraradical**

“…of NNÀ AuÀ PPh 3 with the help of the widely used Pd(PPh 3 ) 4 or the rarely applied Pd[(tBu) 3 P] 2 (up to 30 mol %) forms a complex mixture of products with a small amount (< 5 %) of the isolated tetraradical BltÀ (NN) 3 . We also examined the recently proposed thermally stable reagent NNÀ AuÀ TTMPP [44] in the presence of Pd(PPh 3 ) 4 . Using this reagent, the BltÀ (NN) 3 tetraradical was obtained in acceptable yield (~60 %).…”

“…Our systematic search [42,43] led to the creation of a second generation of organogold nitronyl nitroxide derivatives NNÀ AuÀ XPhos, NNÀ AuÀ Me CgPPh and NNÀ AuÀ TTMPP (Figure 2), which have exceptionally high thermal stability and high reactivity in cross-coupling reactions even with diamagnetic aryl bromides. [44] The Blatter moiety is significantly more electron-rich compared to typical diamagnetic π-systems. That is why the cross-coupling reactions of the NNÀ AuÀ PPh 3 reagent with iodo derivatives of Blatter radicals turned out to be ineffective and gave low yields of the desired high-spin molecules.…”

“…The reason is that the Okada's reagent (NN−Au−PPh 3 ), used in the first‐generation catalytic system, has limited thermal stability and begins to decompose at temperatures of ~60–65 °C, at which cross‐coupling reactions occur. Our systematic search [42,43] led to the creation of a second generation of organogold nitronyl nitroxide derivatives NN−Au−XPhos, NN−Au− Me CgPPh and NN−Au−TTMPP (Figure 2), which have exceptionally high thermal stability and high reactivity in cross‐coupling reactions even with diamagnetic aryl bromides [44] …”

From Stable Radicals to Thermally Robust High-Spin Diradicals and Triradicals

“…An alternative approach is based on the low p K a of C2–H, which was mentioned in the preceding paragraph. This allows for C2-functionalization of nitronyl nitroxide by deprotonation with NaOH or LHMDS and then reaction with electrophilic reagents, such as nitrones, or cross-coupling with aryl iodide. − One of the recent advances involves the use of electron rich sterically encumbered phosphines as ligands in NN-Au or in Pd catalysts, which enables cross-couplings with aryl bromides or electron-rich open-shell aryl iodides. , This direct coupling method also can install skeletons of high-spin NN radicals, providing a powerful and flexible way to synthesize and increase the diversity of high-spin NN radicals. Based on these two approaches, high-spin radicals have been synthesized (Scheme ).…”

“…344−346 One of the recent advances involves the use of electron rich sterically encumbered phosphines as ligands in NN-Au or in Pd catalysts, which enables cross-couplings with aryl bromides or electron-rich open-shell aryl iodides. 149,350 This direct coupling method also can install skeletons of highspin NN radicals, providing a powerful and flexible way to synthesize and increase the diversity of high-spin NN radicals. Based on these two approaches, high-spin radicals have been synthesized (Scheme 1).…”