Bis(diisopropylphosphinomethyl)amine Nickel(II) and Nickel(0) Complexes: Coordination Chemistry, Reactivity, and Catalytic Decarbonylative C–H Arylation of Benzoxazole

Abstract

The facile one-step synthesis of five new bis(diisopropylphosphinomethyl)amine ligands RN(CH2DIP)2 (DIP = diisopropylphosphine, R = Me–, iPr–, PhCH2–, 2-ThCH2–, and 2-FuCH2−) based on the use of the air-stable phosphonium salt [DIP(CH2OH)2]Cl is presented. The phosphonium salt cleanly reacts with primary amines to afford amine-bridged bisphosphine ligands with variable backbone substitution in good yields. These DIP ligands are useful model systems for their chiral bisphospholane analogues. The coordination chemistry of neutral nickel(II) complexes [(RDIP)NiCl2], [(RDIP)Ni(CH3)2], [(iPrDIP)Ni(Cl)CH2Si(CH3)3], and [(RDIP)Ni(CH2Ph)2], as well as coordinatively unsaturated cationic nickel(II) complexes [(RDIP)Ni(THF)CH3]+BArF– and [(RDIP)NiCH2Ph]+BArF–, has been studied by spectroscopic and X-ray diffraction methods. The cationic methyl complexes reacted cleanly with 2-butyne and 1,1-dimethylallene, yielding allylic complexes [(RDIP)Ni(pmcb)]+BArF– and [(RDIP)Ni(tma)]+BArF–, respectively (pmcb: η3-1,2,3,4,4-pentamethylcyclobutenyl; tma: η3-2,3,3-trimethylallyl). Nickel(0) complexes [(BzDIP)Ni(trans-stilbene)] and [(RDIP)Ni(C2H4)] were synthesized in one step from the corresponding dichloro complexes and have been fully characterized and analyzed by X-ray diffraction methods. [(MeDIP)Ni(C2H4)] reacted with phenyl-2-thiophenecarboxylate, yielding [(MeDIP)Ni(OPh)(2-Th)] and [(MeDIP)Ni(CO)2] in a 2:1 ratio. Both complexes were also synthesized via alternative routes. The phenolato-thienyl complex represents an intermediate in the catalytic cycle of a recently reported decarbonylative arylation of azoles. Complexes [(RDIP)Ni(C2H4)] were shown to be active catalysts for this reaction.

Deep eutectic solvent-catalyzed arylation of benzoxazoles with aromatic aldehydes

“…Next, the scope of various benzoxazoles was also evaluated. As our expectation both 5-methylbenzoxazole and 5-chlorobenzoxazole generally underwent the arylation to give the corresponding products in very good yields ranging from 82 to 95% except the case of 4-uorobenzaldehyde and 4-hydroxybenzaldehyde whose resulting arylated products were only obtained in signicantly diminished yields of 70-75% (Table 3, entries [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. For the benzoxazole bearing a nitro substituent, the lower yields of arylation products with various aldehydes were recorded even though harsher conditions (higher temperature for prolonged reaction time) were employed ( Table 3, entries [31][32][33][34][35][36][37].…”

“… 21 Additionally, the requirement of high-cost palladium catalysts and other expensive additives also prevents the palladium-catalyzed cross-coupling procedure from being employed in large-scale synthesis. 22 Recently, the replacement of palladium by other transition metals, such as bis(diisopropylphosphinomethyl)amine nickel( ii ) and nickel(0) complexes, 23 metal–organic frameworks, 24,25 Cu/Fe system, 26 CeO 2 /Fe 3 O 4 , 27 CuCN(PPh 3 ) 2 , 28 and Ni(COD) 2 , 29 has been investigated. These direct arylation methods have received much attention due to their advantage of avoiding the use of stoichiometric amounts of expensive organometallic reagents or additives.…”

Methyl Complexes of the Transition Metals

“…In the latest years much attention has been focused on nickel organometallics both for catalytic applications and academic purposes, e. g. unusual oxidation states, low coordination numbers, stoichiometric activation of small molecules, bioinorganic model systems. Mononuclear square-planar Ni(II) complexes stabilized by bi-or tridentate ligands 100 A tetradentate trianionic ligand has been used by G.-H. Lee and collaborators in order to build up a model for the Methyl CoMreductase, which displays an unusual pentacoordinate environment for the metal in the +3 oxidation state (Scheme 16a). 103 The relevance of Ni(III) carbyl species supported by amido-pyridine N,N,N,N ligands in cross coupling reactions has also been established.…”

Benzothiazole Nickelation: An Obstacle to the Catalytic Arylation of Azoles by Cyclopentadienyl Nickel N-Heterocyclic Carbene Complexes

“…Complex 7 was also observed in the attempted couplings with the cationic complex 6 ( Figure S3), highlighting a general behaviour for these CpNi(NHC) species thereof. Interestingly, although C(2)-benzoxazolyl or benzothiazolyl complexes are often proposed as intermediates in C-H/C-X couplings between heteroarenes and aryl, alkenyl or alkyl electrophiles [5,10,12,14,16,17,19,20,22,23], they remain very rare. We are indeed aware of only four related PCN-Pd [39][40][41] and NNN-Ni [23] pincer species, which have been isolated with a benzothiazolyl ligand and demonstrated to be intermediates in similar couplings.…”

“…The recent trend to use 3d transition metal catalysts, that is driven by economic and environmental concerns, has led to a re-assessment of nickel's reactivity [1][2][3][4]. In particular, nickel catalysts have emerged as promising candidates for C-H bond functionalization [5][6][7][8], and nickel-catalyzed reactions between (hetero)arenes and aryl halides [9][10][11][12], phenol derivatives [13][14][15][16][17][18], aryl or alkenyl esters [19][20][21] or even alkyl halides [22,23] have been shown to be reliable atom-economical alternatives to palladium-catalyzed cross-couplings. In this context, we have recently reported that the unsaturated T-shaped nickelacycle 1, that bears a mono-anionic C,C-NHC-cyanoalkyl chelate (NHC = N-heterocyclic carbene), shows moderate activity for the coupling of benzothiazole with aryl iodides (Scheme 1) [24].…”