A New Paradigm in Enantioselective Cobalt Catalysis: Cationic Cobalt(I) Catalysts for Heterodimerization, Cycloaddition, and Hydrofunctionalization Reactions of Olefins

One of the major challenges facing organic synthesis in the 21st century is the utilization of abundantly available feedstock chemicals for fine chemical synthesis. Regio- and enantioselective union of easily accessible 1,3-dienes and other feedstocks like ethylene, alkyl acrylates, and aldehydes can provide valuable building blocks adorned with latent functionalities for further synthetic elaboration. Through an approach that relies on mechanistic insights and systematic examination of ligand and counterion effects, we developed an efficient cobalt-based catalytic system [(P∼P)CoX2/Me3Al] (P∼P = bisphosphine) to effect the first enantioselective heterodimerization of several types of 1,3-dienes with ethylene. In addition to simple cyclic and acyclic dienes, siloxy-1,3-dienes participate in this reaction, giving highly functionalized, nearly enantiopure silyl enolates, which can be used for subsequent C-C and C-X bond-forming reactions. As our understanding of the mechanism of this reaction improved, our attention was drawn to more challenging partners like alkyl acrylates (one of the largest volume feedstocks) as the olefin partners instead of ethylene. Prompted by the intrinsic limitations of using aluminum alkyls as the activators for this reaction, we explored the fundamental chemistry of the lesser known (P∼P)Co(I)X species and discovered that in the presence of halide sequestering agents, such as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBARF) or (C6F5)3B, certain chiral bisphosphine complexes are superb catalysts for regio- and enantioselective heterodimerization of 1,3-dienes and alkyl acrylates. We have since found that these cationic Co(I) catalysts, most conveniently prepared in situ by reduction of the corresponding cobalt(II) halide complexes by zinc in the presence of NaBARF, promote enantioselective [2 + 2]-cycloaddition between alkynes and an astonishing variety of alkenyl derivatives to give highly functionalized cyclobutenes. In reactions between 1,3-enynes and ethylene, the [2 + 2]-cycloaddition between the alkyne and ethylene is followed by a 1,4-addition of ethylene in a tandem fashion to give nearly enantiopure cyclobutanes with an all-carbon quaternary center, giving a set of molecules that maps well into many medicinally relevant compounds. In another application, we find that the cationic Co(I)-catalysts promote highly selective hydroacylation and 1,2-hydroboration of prochiral 1,3-dienes. Further, we find that a cationic Co(I)-catalyst promotes cycloisomerization followed by hydroalkenylation of 1,6-enynes to produce highly functionalized carbo- and heterocyclic compounds. Surprisingly the regioselectivity of the alkene addition depends on whether it is a simple alkene or an acrylate, and the acrylate addition produces an uncommon Z-adduct. This Account will provide a summary of the enabling basic discoveries and the attendant developments that led to the unique cationic Co(I)-complexes as catalysts for disparate C-C and C-B bond-forming reactions. It is our hope that this Account will stimulate further work with these highly versatile catalysts which are derived from an earth-abundant metal.

Catalytic Enantioselective Addition of Alkylzirconium Reagents to Aliphatic Aldehydes

A catalytic methodology for the enantioselective addition of alkylzirconium reagents to aliphatic aldehydes is reported here. The versatile and readily accessible chiral Ph-BINMOL ligand, in the presence of Ti(OⁱPr)₄ and a zinc salt, facilitates the reaction, which proceeds under mild conditions and is compatible with functionalized nucleophiles. The alkylzirconium reagents are conveniently generated in situ by hydrozirconation of alkenes with the Schwartz reagent. This work is a continuation of our previous work on aromatic aldehydes.

Cyclic Bis-alkylidene Complexes of Titanium and Zirconium: Synthesis, Characterization, and Reaction

Transition-metal alkylidenes have exhibited wide applications in organometallic chemistry and synthetic organic chemistry, however, cyclic Schrock-carbene-like bis-alkylidenes of group 4 metals with a four-electron donor from an alkylidene have not been reported. Herein, the synthesis and characterization of five-membered cyclic bis-alkylidenes of titanium (4 a,b) and zirconium (5 a,b) are reported, as the first well-defined group 4 metallacyclopentatrienes, by two-electron reduction of their corresponding titana- and zirconacyclopentadienes. DFT analyses of 4 a show a four-electron donor (σ-donation and π-donation) from an alkylidene carbon to the metal center. The reaction of 4 a with N,N'-diisopropylcarbodiimide (DIC) leads to the [2+2]-cycloaddition product 6. Compound 4 a reacted with CO, affording the oxycyclopentadienyl titanium complex 7. These reactivities demonstrate the multiple metal-carbon bond character. The reactions of 4 a or 5 a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine-coordinated zirconacyclopentadiene 9, respectively, which exhibit two-electron reductive ability.

Are Organozirconium Reagents Applicable in Current Organic Synthesis?

The search for mild, user-friendly, easily accessible, and robust organometallic reagents is an important feature of organometallic chemistry. Ideally, new methodologies employing organometallics should be developed with respect to practical applications in syntheses of target compounds. In this short review, we investigate if organozirconium reagents can fulfill these criteria. Organozirconium compounds are typically generated via in situ hydrozirconation of alkenes or alkynes with the Schwartz reagent. Alkyl and alkenylzirconium reagents have proven to be convenient in conjugate additions, allylic substitutions, cross-coupling reactions, and additions to carbonyls or imines. Furthermore, the Schwartz reagent itself is a useful reducing agent for polar functional groups.

1 Introduction

2 Synthesis and Generation of the Schwartz Reagent

3 Structure and Properties of Cp2Zr(H)Cl

4 Reactivity of Organozirconium Reagents

4.1 Asymmetric Conjugate Addition

4.2 Asymmetric Allylic Alkylations

4.3 Desymmetrization Reactions

4.4 Cross-Coupling Reactions

4.5 1,2-Additions

5 Conclusions

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

Modular access to functionalized 5-8-5 fused ring systems via a photoinduced cycloisomerization reaction

A 5-8-5 carbocyclic ring system forms the core of over 30 distinct natural products. Several members of this family have gained attention for their diverse activity in cell culture. In these cases, biological function is mediated by the arrangement of substituents around a conserved 5-8-5 nucleus. Despite the potential applications of this privileged substructure in medicinal chemistry, modular strategies for its assembly are underdeveloped. Herein, we describe a cycloisomerization reaction that forms the 5-8-5 framework directly. This strategy uniquely allows access to gram quantities of this valuable scaffold in four steps.

Catalytic Enantioselective Addition of Organozirconium Reagents to Aldehydes

A catalytic enantioselective addition reaction of alkylzirconium species to aromatic aldehydes is reported. The reaction, facilitated by a chiral nonracemic diol ligand complex with Ti(OⁱPr)₄, proceeds under mild and convenient conditions, and no premade organometallic reagents are required since the alkylzirconium nucleophiles are generated in situ by hydrozirconation of alkenes with the Schwartz reagent. The methodology is compatible with functionalized nucleophiles and a broad range of aromatic aldehydes.

Asymmetric Catalysis with Ethylene. Synthesis of Functionalized Chiral Enolates

Trialkylsilyl enol ethers are versatile intermediates often used as enolate surrogates for the synthesis of carbonyl compounds. Yet there are no reports of broadly applicable, catalytic methods for the synthesis of chiral silyl enol ethers carrying latent functionalities useful for synthetic operations beyond the many possible reactions of the silyl enol ether moiety itself. Here we report a general procedure for highly catalytic (substrate:catalyst ratio up to 1000:1) and enantioselective (92% to 98% major enantiomer) synthesis of such compounds bearing a vinyl group at a chiral carbon at the β-position. The reactions, run under ambient conditions, use trialkylsiloxy-1,3-dienes and ethylene (1 atm) as precursors and readily available (bis-phosphine)-cobalt(II) complexes as catalysts. The silyl enolates can be readily converted into novel enantiopure vinyl triflates, a class of highly versatile cross-coupling reagents, enabling the syntheses of other enantiomerically pure, stereodefined trisubstituted alkene intermediates not easily accessible by current methods. Examples of Kumada, Stille, and Suzuki coupling reactions are illustrated.

Activation of Marginally Reactive Boron Enolates by MeLi for the Formation of Enol Phosphates and Synthesis of the Δ(9)-THC Intermediate

The addition of MeLi to boron enolates produced by the 1,4-addition of Ar2Cu(CN)Li2 to BF3·OEt2-activated enones was followed by the reaction with ClP(O)(OEt)2 to afford the corresponding enol phosphates in moderate to good yields. The scope of this method was examined with sterically hindered or electronically biased enones and/or reagents. This activation of boron enolates was successfully applied to the synthesis of the methyl ether of Δ(9)-tetrahydrocannabinol.

Strategy of total synthesis based on the use of Rh-catalyzed stereoselective 1,4-addition

In 1998, Hayashi and Miyaura reported the first asymmetric conjugate addition of aryl- and alkenyl-boronic acids to α,β-unsaturated ketones using chiral rhodium complexes as catalysts.