Redox-active nitrogen donor ligands have exhibited broad utility in stabilizing transition metal complexes in unusual formal oxidation states and enabling multielectron redox reactions. In this report, we extend these principles to dinuclear complexes using a naphthyridine-diimine (NDI) framework. Treatment of ((i-Pr)NDI) with Ni(COD)2 (2.0 equiv) yields a Ni(I)-Ni(I) complex in which the two metal centers form a single bond and the ((i-Pr)NDI) ligand is doubly reduced. A homologous series of ((i-Pr)NDI)Ni2 complexes in five oxidation states were synthesized and structurally characterized. Across this series, the ligand ranges from a neutral state in the most oxidized member to a dianionic state in the most reduced. The interplay between metal- and ligand-centered redox activity is interrogated using a variety of experimental techniques in combination with density functional theory models.

Cycloaddition reactions provide direct and convergent routes to cycloalkanes, making them valuable targets for the development of synthetic methods. Whereas six-membered rings are readily accessible from Diels-Alder reactions, cycloadditions that generate five-membered rings are comparatively limited in scope. Here, we report that dinickel complexes catalyze [4 + 1]-cycloaddition reactions of 1,3-dienes. The C1partner is a vinylidene equivalent generated from the reductive activation of a 1,1-dichloroalkene in the presence of stoichiometric zinc. Intermolecular and intramolecular variants of the reaction are described, and high levels of asymmetric induction are achieved in the intramolecular cycloadditions using aC2-symmetric chiral ligand that stabilizes a metal-metal bond.

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The frequent occurrence of five-membered carbocycles in natural products and other biologically active compounds has provided a major impetus for the development of efficient methods for their construction.[1] Towards this end, the Nazarov cyclization, a stereospecific 4p electrocyclization that converts divinyl ketones into cyclopentenones through a conrotatory cyclization, has distinguished itself as a powerful tool for the synthesis of such compounds.[2] Over several decades, great efforts have been devoted to the exploitation of the Nazarov reaction, [3] further highlighting its synthetic utility, as demonstrated by its increasing use in total syntheses.[4] However, the asymmetric Nazarov reaction with a catalytic amount of chiral source was not reported until the end of 2003, probably because of the complex mechanism of the reaction.[5] Of those catalytic asymmetric reactions developed, [5] few examples have been reported that give good to excellent enantiocontrol in good yields, except for divinyl ketones that belong to type A or B (Scheme 1).Trauner and Liang reported the asymmetric Nazarov reactions of type A divinyl ketones, catalyzed by 10 mol % of indane-pybox/Sc(OTf) 3 2 a (pybox = 2,6-bis(2-oxazolin-2-yl)pyridine) with ee values of 72-97 % in good yields.[5f] Aggarwal and Belfield developed the first asymmetric Nazarov cyclization of type B ketones, promoted by a stoichiometric amount of pybox/Cu 2+ complex 2 b, with enantioselectivities of 44-88 %.[5h] Recently, Rueping et al. reported the first example of organocatalytic asymmetric Narazov reactions with high enantioselectivities and moderate diastereoselectivities [5a, d] using polarized alkoxy divinyl bketoesters (type C) that were elegant substrates in this transformation.[3f, m, n] To date, no highly enantioselective version has been reported.We found that both pybox-derived complexes 2 a and 2 b, which were excellent catalysts or promoters in the enantioselective Nazarov reaction of type A and B substrates, [5f,h] were not efficient for type C substrates. For example, when alkoxy divinyl b-ketoester 1 a underwent cyclization in the presence of indane-pybox/Sc(OTf) 3 2 a or iPr-Pybox/CuBr 2 / AgSbF 6 2 b, the desired product 3 a was formed as the sole product with 33 % ee in 5 % yield and 27 % ee in 94 % yield, respectively (Scheme 2). Compared with the planar pybox, we envisioned that a pendant group on the box ligand [6] may be better able to tune the three-dimensional conformation of the catalyst-substrate complex, and subsequently improve the enantiocontrol at the newly generated stereogenic centers (Figure 1). Therefore, several tris(oxazoline)s [7] were synthesized and were found to be excellent catalysts for the asymmetric Nazarov cyclization of alkoxy divinyl b-ketoesters, affording ee values of up to 98 %. Herein, we report our preliminary results with this catalyst system. Tris(oxazoline)s (toxs) 4 a, 4 b, and 5 c-d (Figure 2) are readily available from alkylation of their corresponding bis(oxazoline)s.[7h] As isopropyltris(oxazo...