Copper-catalyzed asymmetric conjugate addition of alkenyl- and alkylalanes to α,β-unsaturated lactams

Syntheses of Substituted α,β-Unsaturated δ-Lactams from N-Boc-2,4-dioxopiperidine

A method for the syntheses of substituted α,β-unsaturated δ-lactams (2) from the commercially available compound N-Boc-2,4-dioxopiperidine (1) has been developed. The α-substituents were introduced by a reductive Knoevenagel condensation reaction, and the β-substituents were installed by palladium-catalyzed cross coupling reactions. More than 20 diverse examples were prepared in 2-3 steps. The synthesis was operationally simple, user-friendly, and easy to scale up.

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.

Copper-catalysed alkylation of heterocyclic acceptors with organometallic reagents

Copper-catalysed asymmetric C–C bond-forming reactions using organometallic reagents have developed into a powerful tool for the synthesis of complex molecules with single or multiple stereogenic centres over the past decades. Among the various acceptors employed in such reactions, those with a heterocyclic core are of particular importance because of the frequent occurrence of heterocyclic scaffolds in the structures of chiral natural products and bioactive molecules. Hence, this review focuses on the progress made over the past 20 years for heterocyclic acceptors.

Coordinative Chain Transfer Polymerization of Butadiene with Functionalized Aluminum Reagents

Functionalized aluminum alkyls enable effective coordinative chain transfer polymerization with selective chain initiation by the functionalized alkyl. (ω-Aminoalkyl)diisobutylaluminum reagents (12 examples studied) obtained by hydroalumination of α-amino-ω-enes with diisobutylaluminum hydride promote the stereoselective catalytic chain growth of butadiene on aluminum in the presence of Nd(versatate)3 , Cp*2 Nd(allyl), or Cp*2 Gd(allyl) precatalysts and [PhNMe2 H+ ]/[B(C6 F5 )4 - ]. Carbazolyl- and indolylaluminum reagents result in efficient molecular weight control and chain initiation by the aminoalkyl rather than the isobutyl substituent bound to aluminum. As confirmed for (3-(9H-carbazol-9-yl)propyl)-initiated polybutadiene (PBD), for example, by deuterium quenching studies, polymer chain transfer by β-hydride transfer is negligible in comparison to back-transfer to aluminum.

Palladium-Catalyzed Carbonylative Coupling of Aryl Iodides with Alkenylaluminum Reagents

A highly reactive catalytic system for the carbonylative coupling of aryl iodides with alkenylaluminum reagents has been developed. Various β-substituted γ,δ-unsaturated ketones were produced under mild conditions in good to excellent yields even under ppm level of palladium catalyst. Notably, this also represents the first example on carbonylative transformation of alkenylaluminum compounds. Additionally, by the addition of zinc salt, the selectivity of the product can be modified.

N-Heterocyclic Carbene-Cu-Catalyzed Enantioselective Allenyl Conjugate Addition

A catalytic enantioselective conjugate addition with commercially available allenylboronic acid pinacol ester as nucleophile promoted by a chiral copper complex of N-heterocyclic carbene (NHC) is disclosed. This process constitutes an unprecedented instance of the conjugate addition that introduces an allenyl group into α,β-unsaturated carbonyl compounds, affording products that are otherwise difficult to access in up to 92% yield, >98% allenyl addition selectivity and 96:4 enantiomeric ratio. DFT calculations were performed to elucidate the origins of enantioselectivity.

Lewis Acid Enabled Copper-Catalyzed Asymmetric Synthesis of Chiral β-Substituted Amides

Here we report that readily available silyl- and boron-based Lewis acids in combination with chiral copper catalysts are able to overcome the reactivity issues of unactivated enamides, known as the least reactive carboxylic acid derivatives, toward alkylation with organomagnesium reagents. Allowing unequaled chemo-reactivity and stereocontrol in catalytic asymmetric conjugate addition to enamides, the method is distinguished by its unprecedented reaction scope, allowing even the most challenging and synthetically important methylations to be accomplished with good yields and excellent enantioselectivities. This catalytic protocol tolerates a broad temperature range (−78 °C to ambient) and scale up (10 g), while the chiral catalyst can be reused without affecting overall efficiency. Mechanistic studies revealed the fate of the Lewis acid in each elementary step of the copper-catalyzed conjugate addition of Grignard reagents to enamides, allowing us to identify the most likely catalytic cycle of the reaction.

Diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams

The conjugate addition reaction has been a useful tool in the formation of carbon-carbon bonds. The utility of this reaction has been demonstrated in the synthesis of many natural products, materials, and pharmacological agents. In the last three decades, there has been a significant increase in the development of asymmetric variants of this reaction. Unfortunately, conjugate addition reactions using α,β-unsaturated amides and lactams remain underdeveloped due to their inherently low reactivity. This review highlights the work that has been done on both diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams.

Diisobutylaluminum hydride promoted cyclization of o-(trimethylsilylethynyl)styrenes to indenes

The reaction of o-(trimethylsilylethynyl)styrenes with diisobutylaluminum hydride (DIBAL-H) provides 2-trimethylsilyl-1H-indenes efficiently. The cyclization mechanism involves regioselective hydroalumination of the alkynyl moiety, geometrical isomerization of the alkenylaluminums formed, and intramolecular carboalumination. With substrates bearing a 2-(trimethylsilyl)ethenyl group (R(1) = Me3Si, R(2) = R(3) = H), bis-silylated benzofulvenes are obtained upon treatment of the reaction mixture with an excess amount of benzaldehyde.