Metal‐Free and Pd^II‐Promoted [2+3] Cycloadditions of a Cyclic Nitrone to Phthalonitriles: Syntheses of Oxadiazolines as well as Phthalamide–Pd^II and Dihydropyrrolyl‐iminoisoindolinone–Pd^II Complexes with High Catalytic Activity in Suzuki–Miyaura Cross‐Coupling Reactions

Abstract

The previously unknown reactions between phthalonitriles, 1,2-(CN)2(C6)R1R2R3R4 1 (1 a, R1=R2=R3=R4=H; 1 b, R1=R2=R4=H, R3=CH3; 1 c, R1=R4=H, R2=R3=Cl; 1 d, R1=R2=R3=R4=Cl; 1 e, R1=R2=R3=R4=F), and a cyclic nitrone, -O+N==CHCH2CH2CMe2 2, proceed under heating in a sealed tube to give phthalimides 3, 2-oxadiazolyl-benzonitriles 4 or ortho-bis(oxadiazolyl)tetrafluorobenzene 4 e'. In the presence of palladium(II) chloride, phthalonitriles 1 react with 2 at room temperature, to give bis(pyrrolidin-2-ylidene)phthalamide PdII complexes 5 via metal-promoted rupture of the N--O bond of the oxadiazoline ring. The ketoimine ligands thus generated can be liberated from the metal by displacement with a diphosphine. Although the first [2+3] cycloaddition of 2 to 1 can occur in the absence of the metal to give the mono-cycloadducts 4, the second [2+3] coupling at the still-unreacted cyano group requires its activation by coordination to PdII, affording complexes 6 containing two ligated oxadiazolyl-benzonitriles. These ligands undergo either i) further cycloaddition with 2 to afford ultimately (upon rearrangement) the bis(pyrrolidinylidene)phthalamide complexes 5 or ii) N--O bond cleavage in the oxadiazoline ring with intramolecular attack of the imine nitrogen on the cyano carbon and bridging to a second PdII center to afford dimeric palladium(II) complexes 7, with chloride bridges, that bear a dihydropyrrolyl-iminoisoindolinone, a new type of ligand.The compounds were characterized by IR, 1H, and 13C NMR spectroscopy, ESI MS or FAB+ MS, elemental analyses and, in the case of 4 c, 5 a, 5 c, and 7 c, also by X-ray diffraction analysis. Complexes 5 a and 7 c show high catalytic activity for the Suzuki-Miyaura cross-coupling reaction of bromobenzene and phenylboronic acid and give biphenyl in high yields with turnover frequencies (TOFs) of up to 9.0x10(5) h(-1).

Oxadiazoline and Ketoimine Palladium(II) Complexes as Highly Efficient Catalysts for Suzuki–Miyaura Cross‐Coupling Reactions in Supercritical Carbon Dioxide

“…After 72 h, the 13 C NMR spectrum of the mixture reveals signals of the methyl and methylene carbons of the pyrrolidine rings in the 22.6-37.4 ppm range, the quaternary car- The resonance at 165.8 ppm is characteristic [3] of the N=C of the oxadiazoline ring, while the signals at 170.1 and 174.5 ppm are assigned to benzamide (NHC=O) and ketoimine (NC=O), respectively. [2][3][4] Interestingly, refluxing the mixture of 5, 6 and 7 in acetone for 3 d leads to the formation of the novel fused tricyclic ketoimine complex 8 as the exclusive product (Scheme 1, reaction c). The formation of 8 is believed to proceed firstly via N À O bond cleavage of the oxadiazoline ring in 5 to afford the derived keto-A C H T U N G T R E N N U N G imine 6 and the mixed unsymmetric pyrrolylbenz-A C H T U N G T R E N N U N G amide-ketoimine 7.…”

“…Procedure for Batch Suzuki-Miyaura CrossCouplings in Supercritical CO 2 Aryl halide (1.0 mmol), boronic acid (1.1 mmol), base (2.0 mmol), 1 mL of a freshly prepared (see below) metha-…”

“…[7] For instance, Holmes et al reported a Suzuki-Miyaura reaction in scCO 2 , using a polyurea-encapsulated PdA C H T U N G T R E N N U N G (OAc) 2 catalyst in the presence of tetra-n-butylam-monium salts [8] (a small quantity of MeOH proved to have a dramatic influence in the reaction yields). [8b] Despite the recyclability of such systems, the very high Pd catalyst loadings (typical substrate-to-catalyst molar ratios of ca.…”

“…[1] Very recently, we reported that Pd complexes containing (pyrrolidin-2-ylidene)phthalamide, [2] (dihydropyrrolyl)imino-A C H T U N G T R E N N U N G isoindolinone, [2] 2,3-dihydro-[1.2.4]oxadiazole (D 4 -1,2,4-oxadiazoline) [3] and (pyrrolidin-2-ylidene)picolinA C H T U N G T R E N N U N G amide [4] ligands show remarkably high catalytic activities towards the Suzuki-Miyaura reaction under thermal and microwave conditions. The current impetus for the development of ecofriendly chemical processes includes attempts at the replacement of organic solvents by alternative reaction media, [5] and supercritical carbon dioxide (scCO 2 ) is an attractive solvent in view of its mild critical data and environmentally benign (non-toxic and non-flammable, but greenhouse effect) character, [6] having also the advantage of an easy separation.…”

Azide Chemistry – An Inorganic Perspective, Part II[‡][3+2]‐Cycloaddition Reactions of Metal Azides and Related Systems

“…pyrroline N-oxide) have been studied. [582,583] The reaction of nitrile complexes with imidazoline N-oxides created "a novel type of heterocycles 137, which do not exist without a metal center". [584] www.zaac.wiley-vch.…”