Zuowei Xie

Copper-Catalyzed Highly Enantioselective Cyclopentannulation of Indoles with Donor–Acceptor Cyclopropanes

A highly diastereo- and enantioselective BOX/Cu(II)-catalyzed C2,C3-cyclopentannulation of indoles with donor-acceptor cyclopropanes has been developed on the basis of asymmetric formal [3 + 2] cycloaddition of indoles. This reaction provides rapid and facile access to a series of enantioenriched cyclopenta-fused indoline products and can be further extended to the construction of tetracyclic pyrroloindolines. The synthetic potential of the reaction was demonstrated in a four-step synthesis of the core structure of borreverine.

Advances in the chemistry of metallacarboranes of f-block elements

Abstract The chemistry of metallacarboranes of f-block elements has experienced considerable growth in the past 10 years. The recent discovery of many novel complexes, interesting reactivity patterns and new bonding mode in this field has made this chemistry very attractive, diverse and unique. This review summarizes the development in this research field since the successful preparation of the first metallacarborane of f-block elements. Achievements, problems and perspectives are discussed in this article.

Transition Metal−Carboryne Complexes: Synthesis, Bonding, and Reactivity

The construction and transformation of metal-carbon (M-C) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M-C bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal-carbon (M-C(cage)) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M-C(cage) bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal-carboryne complexes. Our work has shown that the M-C(cage) bonds in transition metal-carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M-C bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the nontraditional M-C(cage) bonds, we prepared a series of group 4 and group 10 transition metal-carboryne complexes (where carboryne is 1,2-dehydro-o-carborane), because the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M-C(cage) bonds with electrophiles. Structural and theoretical studies on metal-carboryne complexes suggest that the bonding interaction between the metal atom and the carboryne unit is best described as a resonance hybrid of the M-C σ and M-C π bonds, similar to that observed in metal-benzyne complexes. The nickel-carboryne complex (η(2)-C(2)B(10)H(10))Ni(PPh(3))(2) can (i) undergo regioselective [2 + 2 + 2] cycloaddition reactions with 2 equiv of alkyne to afford benzocarboranes, (ii) react with 1 equiv of alkene to generate alkenylcarborane coupling products, and (iii) also undergo a three-component [2 + 2 + 2] cyclotrimerization with 1 equiv of activated alkene and 1 equiv of alkyne to give dihydrobenzocarboranes. The reaction of carboryne with alkynes is also catalyzed by Ni species. Subsequently, a Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,3-dehydro-o-carborane with 2 equiv of alkyne was developed, leading to the efficient formation of C,B-substituted benzocarboranes in a single process. In contrast, the zirconium-carboryne species, generated in situ from Cp(2)Zr(μ-Cl)(μ-C(2)B(10)H(10))Li(OEt(2))(2), reacts with only 1 equiv of alkyne or polar unsaturated organic substrates (such as carbodiimides, nitriles, and azides) to give monoinsertion metallacycles, even in the presence of excess substrates. The resultant five-membered zirconacyclopentenes, incorporating a carboranyl unit, are an important class of intermediates for the synthesis of a variety of functionalized carboranes. Transmetalation of zirconacyclopentenes with other metals, such as Ni and Cu, was also found to be a very useful tool for various chemical transformations. Studies of metal-carboryne complexes remain a relatively young research area, particularly in comparison to the rich literature of metal-benzyne complexes. Other transition metal-carborynes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further that effort by providing easy access to a wide range of functionalized carborane derivatives.

Asymmetric Nazarov Reaction Catalyzed by Chiral Tris(oxazoline)/Copper(II)†

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. 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. Over several decades, great efforts have been devoted to the exploitation of the Nazarov reaction, further highlighting its synthetic utility, as demonstrated by its increasing use in total syntheses. 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. Of those catalytic asymmetric reactions developed, 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).

Iridium Catalyzed Regioselective Cage Boron Alkenylation of o-Carboranes via Direct Cage B–H Activation

Iridium catalyzed alkyne hydroboration with o-carborane cage B-H has been achieved, leading to the formation of a series of 4-B-alkenylated-o-carborane derivatives in high yields with excellent regioselectivity via direct B-H bond activation. In this reaction the carboxy group is used as a traceless directing group, which is removed during a one-pot process. After the confirmation of a key intermediate, a possible mechanism is proposed, involving a tandem sequence of Ir-mediated B-H activation, alkyne insertion, protonation, and decarboxylation.

Nickel-catalyzed regioselective [2+2+2] cycloaddition of carboryne with alkynes.

Transition metal−benzyne complexes have found many applications in organic synthesis, mechanistic studies, and the synthesis of functional materials. In sharp contrast, the chemistry of transition metal−carboryne complexes, especially late transition metal complexes, is virtually unknown. This communication reports a novel nickel-mediated regioselective [2 + 2 + 2] cycloaddition reaction of carboryne with alkynes via the Ni−carboryne intermediate (η2-C2B10H10)Ni(PPh3)2. Because of the bulkiness of the carborane moiety, a high regioselectivity was achieved in the reactions involving unsymmetrical alkynes. This work furnishes a novel method for the preparation of highly substituted benzocarboranes which are difficult to obtain by other methods.

[O−NSR]TiCl3‐Catalyzed Copolymerization of Ethylene with Functionalized Olefins

Much attention has been paid to the copolymerization of ethylene with polar alkenes catalyzed by transition-metal complexes, because it provides easy and low-cost access to functional polyethylenes (PEs) with unique properties. Since the active species in olefin polymerization are metal cations in most cases, the polar alkene monomer will poison and deactivate the catalytic species under the polymerization conditions (Scheme 1). Two strategies, either the use of late-

Nickel-mediated three-component cycloaddition reaction of carboryne, alkenes, and alkynes.

Transition metal mediated multicomponent cross-coupling reactions are a powerful strategy to assemble complex molecules from very simple precursors in a single operation. This Communication reports a nickel-mediated three-component cycloaddition reaction of carboryne with alkenes and alkynes giving dihydrobenzocarborane derivatives with excellent chemo- and regioselectivity. A reaction mechanism involving sequential alkene and alkyne insertion, followed by reductive elimination, is proposed. The key intermediate of nickelacyclopentane was isolated and confirmed by single-crystal X-ray analyses. This work furnishes a new method for the preparation of substituted dihydrobenzocarboranes that are difficult to obtain by other methods.

Ti-amide Catalyzed Synthesis of Cyclic Guanidines from Di-/Triamines and Carbodiimides

A titanacarborane monoamide catalyzed, one-step synthesis of mono/bicyclic guanidines from commercially available di/triamines and carbodiimides is reported. The reaction mechanism is also proposed.

Palladium-Catalyzed Regioselective Intramolecular Coupling of o-Carborane with Aromatics via Direct Cage B–H Activation

Palladium-catalyzed intramolecular coupling of o-carborane with aromatics via direct cage B-H bond activation has been achieved, leading to the synthesis of a series of o-carborane-functionalized aromatics in high yields with excellent regioselectivity. In addition, the site selectivity can also be tuned by the substituents on cage carbon atom.