Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions.

Aryl-substituted pyridine(diimine) iron complexes promote the catalytic [2 + 2] cycloadditions of alkenes and dienes to form vinylcyclobutanes as well as the oligomerization of butadiene to generate divinyl(oligocyclobutane), a microstructure of poly(butadiene) that is chemically recyclable. A systematic study on a series of iron butadiene complexes as well as their ruthenium congeners has provided insights into the essential features of the catalyst that promotes these cycloaddition reactions. Structural and computational studies on iron butadiene complexes identified that the structural rigidity of the tridentate pincer enables rare s-trans diene coordination.

Ligand Substitution and Electronic Structure Studies of Bis(phosphine)Cobalt Cyclooctadiene Precatalysts for Alkene Hydrogenation.

Diene self-exchange reactions of the 17-electron, formally cobalt(0) cyclooctadienyl precatalyst, (,)-(DuPhos)Co(COD) ( , (,)-DuPhos = 1,2-bis((2,5)-2,5-diisopropylphospholano)benzene, COD = 1,5-cyclooctadiene) were studied using natural abundance and deuterated 1,5-cyclooctadiene. Exchange of free and coordinated diene was observed at ambient temperature in benzene- solution and kinetic studies support a dissociative process. Both neutral and the 16-electron, cationic cobalt(I) complex, [(,)-(DuPhos)Co(COD)][BAr ] (BAr = B[(3,5-(CF))CH]) underwent instantaneous displacement of the 1,5-cyclooctadiene ligand by carbon monoxide and generated the corresponding carbonyl derivatives.

Iron-catalysed synthesis and chemical recycling of telechelic 1,3-enchained oligocyclobutanes.

Closed-loop recycling offers the opportunity to mitigate plastic waste through reversible polymer construction and deconstruction. Although examples of chemical recycling of polymers are known, few have been applied to materials derived from abundant commodity olefinic monomers, which are the building blocks of ubiquitous plastic resins. Here we describe a [2+2] cycloaddition/oligomerization of 1,3-butadiene to yield a previously unrealized telechelic microstructure of (1,n'-divinyl)oligocyclobutane.

Bis(dialkylphosphino)ferrocene-Ligated Nickel(II) Precatalysts for Suzuki-Miyaura Reactions of Aryl Carbonates.

Aryl carbonates, a common protecting group in synthetic organic chemistry, are potentially valuable electrophiles in cross-coupling reactions. Here, after performing a thorough evaluation of different precatalysts, we demonstrate that (dcypf)Ni(2-ethylphenyl)(Br) (dcypf = 1,1-bis-(dicyclohexylphosphino)ferrocene) is an efficient precatalyst for Suzuki-Miyaura reactions using a variety of aryl carbonates as substrates. Mechanistic studies indicate that (dcypf)Ni(2-ethylphenyl)(Br), which contains a bidentate phosphine that binds in a trans geometry, is an effective precatalyst for these reactions for two reasons: (i) it rapidly forms the Ni(O) active species and (ii) it minimizes comproportionation reactions between the Ni(O) active species and both the unactivated Ni(II) precatalyst and on-cycle Ni(II) complexes to form catalytically inactive Ni(I) species.

Modifications to the Aryl Group of dppf-Ligated Ni ‑Aryl Precatalysts: Impact on Speciation and Catalytic Activity in Suzuki-Miyaura Coupling Reactions.

There is currently significant interest in the development of efficient nickel precatalysts for cross-coupling. In this work, 14 nickel(II) precatalysts of the form (dppf)Ni(aryl)(X) (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) were synthesized. In particular, both the electronic and steric properties of the aryl group were modified to understand how this affects precatalyst activation.

Synthesis and Reactivity of Paramagnetic Nickel Polypyridyl Complexes Relevant to C(sp )-C(sp )Coupling Reactions.

A number of new transition metal catalyzed methods for the formation of C(sp )-C(sp ) bonds have recently been described. These reactions often utilize bidentate polypyridyl-ligated Ni catalysts, and paramagnetic Ni halide or aryl species are proposed in the catalytic cycles. However, there is little knowledge about complexes of this type.

Nickel(I) Aryl Species: Synthesis, Properties, and Catalytic Activity.

In this work, Ni(I) aryl species that are directly relevant to cross-coupling have been synthesized. Transmetalation of (dppf)NiX (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) with aryl Grignard reagents or aryl boronic acids in the presence of base produces Ni(I) aryl species of the form (dppf)Ni(Ar) (Ar = Ph, -tolyl, 2,6-xylyl, 2,4,6-mesityl, 2,4,6-PrCH). The stability of the Ni(I) aryl species is inversely correlated to the steric bulk on the aryl ligand.

Rapidly Activating Pd-Precatalyst for Suzuki-Miyaura and Buchwald-Hartwig Couplings of Aryl Esters.

Esters are valuable electrophiles for cross-coupling due to their ubiquity and ease of synthesis. However, harsh conditions are traditionally required for the effective cross-coupling of ester substrates. Utilizing a recently discovered precatalyst, Pd-catalyzed Suzuki-Miyaura and Buchwald-Hartwig reactions involving cleavage of the C(acyl)-O bond of aryl esters that proceed under mild conditions are reported.

Well-defined nickel and palladium precatalysts for cross-coupling.

Transition metal-catalysed cross-coupling is one of the most powerful synthetic methods and has led to vast improvements in the synthesis of pharmaceuticals, agrochemicals and precursors for materials chemistry. A major advance in cross-coupling over the past 20 years is the utilization of well-defined, bench-stable Pd and Ni precatalysts that do not require the addition of free ancillary ligand, which can hinder catalysis by occupying open coordination sites on the metal. The development of precatalysts has resulted in new reactions and expanded substrate scopes, enabling transformations under milder conditions and with lower catalyst loadings.

Mechanistic Study of an Improved Ni Precatalyst for Suzuki-Miyaura Reactions of Aryl Sulfamates: Understanding the Role of Ni(I) Species.

Nickel precatalysts are potentially a more sustainable alternative to traditional palladium precatalysts for the Suzuki-Miyaura coupling reaction. Currently, there is significant interest in Suzuki-Miyaura coupling reactions involving readily accessible phenolic derivatives such as aryl sulfamates, as the sulfamate moiety can act as a directing group for the prefunctionalization of the aromatic backbone of the electrophile prior to cross-coupling. By evaluating complexes in the Ni(0), (I), and (II) oxidation states we report a precatalyst, (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1'-bis(diphenylphosphino)ferrocene), for Suzuki-Miyaura coupling reactions involving aryl sulfamates and boronic acids, which operates at a significantly lower catalyst loading and at milder reaction conditions than other reported systems.

Pd-Catalyzed Suzuki-Miyaura and Hiyama-Denmark Couplings of Aryl Sulfamates.

Using a recently discovered precatalyst, the first Pd-catalyzed Suzuki-Miyaura reactions using aryl sulfamates that occur at room temperature are reported. In complementary work, it is demonstrated that a related precatalyst can facilitate the coupling of aryl silanolates, which are less toxic and reactive nucleophiles than boronic acids with aryl chlorides. By combining our results using modern electrophiles and nucleophiles, the first Hiyama-Denmark reactions using aryl sulfamates are reported.

Comparison of dppf-Supported Nickel Precatalysts for the Suzuki-Miyaura Reaction: The Observation and Activity of Nickel(I).

Ni-based precatalysts for the Suzuki-Miyaura reaction have potential chemical and economic advantages compared to commonly used Pd systems. Here, we compare Ni precatalysts for the Suzuki-Miyaura reaction supported by the dppf ligand in 3 oxidation states, 0, I and II. Surprisingly, at 80 °C they give similar catalytic activity, with all systems generating significant amounts of Ni(I) during the reaction.