Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

Abstract

Herein we report the synthesis of Cr imido complexes in bis(alkoxide) ligand environments and their nitrene transfer reactivity with isocyanides. The reaction of Cr 2 (OR) 4 (OR = OC t Bu 2 Ph) with bulky aryl or alkyl azide results in the formation of the trigonal-planar Cr(IV) mono(imido) complexes Cr(OR) 2 (NR 1 ), whereas less bulky aryl azides form the Cr(VI) bis(imido) complexes Cr(OR) 2 (NR 1 ) 2 . Cr(IV) mono-(imido) complexes undergo facile reaction with 1 equiv of 2,6-dimethylphenyl isocyanide (CNR 2 ) to form the corresponding carbodiimides R 1 NCNR 2 . In contrast, no reaction of Cr(OR) 2 (NR 1 ) 2 complexes with CNR 2 is observed. The reaction of Cr(OR) 2 (NR 1 ) with excess isocyanide leads to the isolation of the Cr(II) complex Cr(OR) 2 (CNR 2 ) 4 , along with the observation of the anticipated carbodimide product. Cr(OR) 2 (CNR 2 ) 4 , which can also be obtained by treating Cr 2 (OR) 4 with 4 equiv of isocyanide, reacts with azides N 3 R 1 (R 1 = adamantyl, mesityl) to produce the respective carbodiimides. Catalytic formation of carbodiimides R 1 NCNR 2 is observed from the mixtures of azides R 1 N 3 (R 1 = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl, adamantyl) and several different aryl isocyanides CNR 2 using 2.5 mol % of Cr 2 (OR) 4 . ■ INTRODUCTIONCarbodiimides are widely used organic compounds that hydrolyze readily and thus serve as excellent dehydration agents. Large-scale applications of carbodiimides include as stabilizers and antihydrolysis agents in polymers production, as coupling agents in peptide synthesis, and in various organic transformations. 1−6 Carbodiimides are synthesized by a plethora of transition-metal-catalyzed routes, 7 including oxidative coupling of amines with isocyanides 8 and coupling of two isocyanates R′NCO with loss of 1 equiv of CO 2 , 9 as well as by transition-metal-free stoichiometric routes such as the reaction of isocyanates with phosphinimines (aza-Wittig) 10 and by tinmediated addition of silylamines to isocyanates. 11 Coupling of an [NR] (nitrene) group (originating in organoazides) with isocyanide is among the most attractive methods to synthesize carbodiimides, as (1) a large variety of azides are easily accessible using simple synthetic procedures, (2) both symmetrical and asymmetrical carbodiimides can be synthesized, and (3) the only side product in this reaction is the environmentally friendly dinitrogen. Therefore, numerous efforts have been made to make carbodiimides via coupling of isocyanides and organoazides. Most of these coupling reactions take place at low-coordinate late-transition-metal catalysts stabilized by nitrogen, carbon, or phosphorus ligands, where the MNR functionality is suggested to serve as the reactive intermediate. 12−21 The combination of the soft, electron-rich late transition metals and the hard π-donating imido functionality make the imido functionality reactive and thus susceptible to transfer. In contrast, imido compounds of earlier transition metals are significantly more stable, because the higher oxidation stat...

Base Metal Catalyzed Isocyanide Insertions

“…Thec arbodiimides (157)c an be prepared by aC r-catalyzed nitrene transfer to isocyanides. [72] Theb ulkiness of both the azide and the isocyanide proved to be highly important. The use of sterically less demanding azides resulted in the formation of an unreactive,saturated Cr VI complex.…”

Base Metal Catalyzed Isocyanide Insertions

“…A reaction based on this principle is shown in Scheme . The carbodiimides ( 157 ) can be prepared by a Cr‐catalyzed nitrene transfer to isocyanides . The bulkiness of both the azide and the isocyanide proved to be highly important.…”

Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

“…Molecules 2020, 24, x 2 of 16 chromium(IV)-imido to transfer nitrene to olefins or isocyanides to form aziridines and carbodiimides, respectively [29][30][31]. C-C and C-H bond activation reactions featuring reactive chromium(IV)-imido were also reported [32].…”

“…In contrast, early transition metals (particularly chromium) typically exhibit less reactive nitrene functionalities due to the stability of multiple metal-nitrogen bonds [22][23][24][25][26][27][28]. However, recent studies demonstrated the ability of coordinatively unsaturated chromium(IV)-imido to transfer nitrene to olefins or isocyanides to form aziridines and carbodiimides, respectively [29][30][31]. C-C and C-H bond activation reactions featuring reactive chromium(IV)-imido were also reported [32].…”

“…C-C and C-H bond activation reactions featuring reactive chromium(IV)-imido were also reported [32]. As part of our ongoing investigation of nitrene-and carbene-transfer chemistry mediated by 3d metal complexes in bulky alkoxide ligand environments [33,34], we have reported the synthesis and reactions of chromium-imido complexes featuring two bulky monodentate alkoxides [OC t Bu 2 Ph] [31]. We have demonstrated that Cr IV (OC t Bu 2 Ph) 2 (NR') complexes, which were formed for bulky R' groups (R' = Mes, 2,6-Et 2 C 6 H 3 , 1-adamantyl), enabled catalytic transfer of the nitrene group [NR'] to isocyanides to form carbodiimides.…”