Interactions of Aziridines with Nickel Complexes:  Oxidative-Addition and Reductive-Elimination Reactions that Break and Make C−N Bonds

Abstract

Reaction of the N-tosylaziridines (p-CH(3)C(6)H(4)SO(2))NCH(2)CHR (1a, R = H; 1b, R = Me; 1c, R = n-Bu; 1d, R = i-Pr) with (bpy)Ni(cod) (2; bpy = 2,2'-bipyridine; cod = 1,5-cyclooctadiene) or (bpy)NiEt(2) (3) results in elimination of cod or butane from 2 and 3, respectively, and oxidative addition of an aziridine C-N bond to give the azametallacyclobutane complexes (bpy)Ni(NTosCHRCH(2)) (4a, R = H; 4b, R = Me; 4c, R = n-Bu; 4d, R = i-Pr) as maroon solids in 50-70% isolated yields. The structure of 4b exhibits a puckered four-membered azametallacycle containing a pyramidal nitrogen and with Ni-N(1) = 1.911(5) A; the tosyl group on N and the methyl substituent on the adjacent C are disposed in an anti conformation. The monodeuterated aziridine syn-(p-CH(3)C(6)H(4)SO(2))NCHDCH-n-Bu (1e) reacts with either 2 or 3 to give (bpy)Ni[NTosCH(n-Bu)CHD] (4e) in 60-65% yield, having an anti arrangement of the methine and methylene protons in the azametallacycle, and indicates that >95% inversion of stereochemistry has occurred at the methylene carbon during the oxidative-addition reaction. When the azametallacyclobutane complexes 4a-e are exposed to oxygen, oxidatively induced reductive elimination ensues, giving the free aziridines in 30-60% isolated yields. In the oxidation of 4e, the product aziridine is spectroscopically identical to its parent, 1e, indicating the elimination that forms the C-N bond also proceeds with inversion of stereochemistry (approximately 92% by (1)H NMR) at the methylene carbon.

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

“…4 However, the direct, thermal reductive elimination of an alkylamine from an isolated, low-valent metal amido complex is not well documented. 5 Several palladium-catalyzed reactions have been proposed to occur by reductive elimination from a palladium amido complex to form new C(sp 3 )−N bonds. Several C(sp 3 )−H amidation reactions with added oxidants have been reported.…”

“…5 Several palladium-catalyzed reactions have been proposed to occur by reductive elimination from a palladium amido complex to form new C(sp 3 )−N bonds. Several C(sp 3 )−H amidation reactions with added oxidants have been reported. 6−9 These reactions presumably generate a high-valent Pd intermediate that would be prone to C−N bond-forming reductive elimination.…”

“…6−9 These reactions presumably generate a high-valent Pd intermediate that would be prone to C−N bond-forming reductive elimination. In addition, several Pd-catalyzed reactions that form C(sp 3 )−N bonds without external oxidant have been reported, including the Pd-catalyzed formation of norbornylindoline 10,11 and an intermolecular amination of C− H bonds. 12 The species that undergoes the reductive elimination step has not been observed directly in these systems.…”

“…Recently, our group reported examples of C(sp 3 )−N and C(sp 3 )−O bond-forming reductive elimination of benzylamines and benzyl ethers from isolated benzylpalladium(II) amido and phenoxide complexes ligated by a chelating phosphine ligand, as shown in eq 1. 13,14 Kinetic and stereochemical studies implied that these reactions proceed by a stepwise pathway that involves dissociation of the diarylamido ligand, followed by nucleophilic attack of the amide onto a proposed η 3 -benzylpalladium intermediate.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…262 Selective palladium-catalysed oxidative coupling of analides with olefins via C-H activation at room temperature, as shown in Scheme 4, is a good example of a Heck-type coupling reaction that avoids stoichiometric quantities of salt waste. 263 Other reactions incorporating C-X bond cleavage include routes to fluorinated organic derivatives by nickel-mediated C-F activation of heteroaromatics, 264 nickelmediated C-F activation in the presence of a C-Cl bond, 265 temperature dependent C-H or C-F cleavage in platinum() arylamides, 266 C-C bond cleavage of aromatic nitriles using nickel(0) complexes, 267 C-N cleavage and formation in nickel-aziridine chemistry, 268 and thermal and photochemical C-Si bond activation of Me 3 SiCCPh by platinum(0). 269 N-Heterocyclic carbene complexes of group 10 metals continue to be developed and applied to a number of catalytic processes including the meta-cyclophane nickel() complex 31 that adopts a saddle-shape, 270 the first example of simple oxidative addition of an aryl chloride to a palladium(0) bis-N-heterocyclic carbene complex, 271 cationic and neutral palladium() methyl complexes, 272 palladium-catalysed Suzuki cross-coupling of aryl chlorides at room temperature, 273 chiral palladium() N-heterocyclic carbene-oxazoline complexes, 274 tridentate pincer bis-carbene ligands 275 and palladium() allyl complexes 276 for a range of C-C coupling reactions, palladium-catalysed telomerisation of 1,3-dienes and alcohols, 277 and nickel()-catalysed alkene dimerisation.…”