Radical Chemistry of Iminepyridine Ligands

Abstract

The reactivity of metal fragments MeLi(THF), Me2Mg, Me2Zn and Me3Al with a variety of imine/pyridine ligands was studied by DFT methods. Ligands having at least two such groups in conjugation very effectively accept an electron from a metal–carbon bonding orbital and thus assist\ud alkyl dissociation: the M–Me dissociation free energy ΔGd decreases by about 45 kcal/mol for the Li, Mg and Zn fragments, and by 60–70 kcal/mol for Me3Al. Paths for transfer of an alkyl group to the ligands were also explored. Paths for transfer to ligand carbon atoms were found to have high\ud barriers, and such transfers are instead proposed to proceed via initial M–Me dissociation. Direct alkyl transfer to ligand nitrogen atoms is somewhat easier, and for the most electropositive metal Li such a transfer might compete with radical chemistry. For Dimpy, the most stable products are the C3\ud and C4 alkylation ones. However, the observed regioselectivity of alkyl transfer (often to Npy or C2) does not correlate with product stability and is likely to be a complicated function of radical stability, steric factors and unpaired electron density distribution. The results illustrate that Dimpy and related ligands should be considered strong, reversible oxidants

Dimethyl-Aluminium Complexes Bearing Naphthyl-Substituted Pyridine-Alkylamides as Pro-Initiators for the Efficient ROP of ε-Caprolactone

“…0.2 Å is observed. The five-membered chelate rings display some variation in the degree of puckering between structures which is best exemplified by the C(7)-C( [45][46][47][48][49][50][51][52], and indeed computational studies suggest a radical pathway for the transformation [44]. The closest comparators to 1 are [2-{CMe2N(2,6-iPr2C6H3)}-6-(R)C5H3N]AlMe2 (R = i-Pr, t-Bu) which display similar bond parameters [45].…”

“…We have been attracted by the intriguing properties displayed by pyridylimine-containing ligands including their redox activity [43,44] and their capacity to undergo nucleophilic attack on the imine carbon [45][46][47][48][49][50] and the pyridine ring [51]. Indeed, the thermally-induced migration of an Al-alkyl to the imino carbon of a coordinated N,N-pyridylimine represents a powerful tool for transforming a pyridylimine into a pyridyl-alkylamide (and pyridyl-alkylamine on hydrolysis [45,52]).…”

Synthesis of two novel pyridine annulated pyrrolidine nitroxides

“…This fused heteroaromatic ring could confer some advantages to the skeleton such as rigidity and resistance towards ring opening reactions (Gryn'ova et al, 2012;Hansen and Blinco, 2018) and resistance towards the alteration of functions of biomolecules when they act as spin labels (Kalai et al, 2000). If the fused heteroaromatic ring is pyridine, it would further impart good σ donor capabilities as a monodentate ligand (Budzelaar, 2012). Therefore, this paper focuses on developing a novel unsubstituted heteroaromatic nitroxide with a fused pyridine ring.…”

Nucleophilic Additions to Coordinated 1,10‐Phenanthroline: Intramolecular, Intermolecular, Reversible, and Irreversible

“…In the reactions of bipy, phen and other conjugated diimines with main group metal nucleophiles (MY), following the initial formation of a diimine-MY adduct, a single electron transfer from MY to a low energy, empty molecular orbital of the diimine weakens the M-Y bond, facilitating its homolysis to generate a Y· radical, which eventually adds to the ring. [18] For the reactions of MY with transition metal complexes in which bipy and phen form robust, non-labile chelates, [19] such a low-energy [20] radical pathway is not available. The mechanism of most dearomatizations of TM-bonded pyridine rings (including the few examples involving bipy or phen) consists of an initial nucleophilic attack to the metal followed by an intramolecular migration of the nucleophile to the ring [5] reminiscent of the main group chemistry mentioned above.…”

“…formationo fadiimine-MY adduct, as ingle electron transfer from MY to al ow energy,e mpty molecularo rbitalo ft he diimine weakenst he MÀYb ond, facilitating its homolysis to generate aY C radical, which eventually adds to the ring. [18] For the reactions of MY with transition metal complexes in whichb ipy and phen form robust, non-labile chelates, [19] such al owenergy [20] radicalpathway is not available.…”