Catalytic Hydroalkylation of Allenes

Metal-Hydride C-C Cross-Coupling of Alkenes Through a Double Outer-Sphere Mechanism

This Synopsis covers recent reports of metal-catalyzed alkene functionalizations that likely involve iterative outer-sphere reactions in which the substrate reacts directly with a metal ligand instead of with the metal center itself. Traditional metal hydride-catalyzed alkene functionalizations involve this latter pathway whereby the alkene forms part of the metal ligand sphere (i.e. an inner-sphere reaction). In contrast, alkenes do not ligate the metal in so-called outer-sphere reactions and instead react with a metal ligand. These transformations have proved crucial for the synthesis of high fraction sp3 (Fsp3) targets, especially in hindered fragment couplings of relevance to natural product space.

Recent Advances in the Synthesis of 3,n-Fused Tricyclic Indole Skeletons via Palladium-Catalyzed Domino Reactions

3,n-fused (n = 4-7) tricyclic indoles are pervasive motifs, embedded in a variety of biologically active molecules and natural products. Thus, numerous catalytic methods have been developed for the synthesis of these skeletons over the past few decades. In particular, palladium-catalyzed transformations have received much attention in recent years. This review summarizes recent developments in the synthesis of these tricyclic indoles with palladium-catalyzed domino reactions and their applications in the total synthesis of representative natural products.

Copper Hydride-Catalyzed Enantioselective Olefin Hydromethylation

The enantioselective installation of a methyl group onto a small molecule can result in the significant modification of its biological properties. While hydroalkylation of olefins represents an attractive approach to introduce alkyl substituents, asymmetric hydromethylation protocols are often hampered by the incompatibility of highly reactive methylating reagents and a lack of general applicability. Herein, we report an asymmetric olefin hydromethylation protocol enabled by CuH catalysis. This approach leverages methyl tosylate as a methyl source compatible with the reducing base-containing reaction environment, while a catalytic amount of iodide ion transforms the methyl tosylate in situ into the active reactant, methyl iodide, to promote the hydromethylation. This method tolerates a wide range of functional groups, heterocycles, and pharmaceutically relevant frameworks. Density functional theory studies suggest that after the stereoselective hydrocupration, the methylation step is stereoretentive, taking place through an SN2-type oxidative addition mechanism with methyl iodide followed by a reductive elimination.

Copper-Catalyzed Reductive Ireland-Claisen Rearrangements of Propargylic Acrylates and Allylic Allenoates

The copper-catalyzed reductive Ireland-Claisen rearrangement of propargylic acrylates led to 3,4-allenoic acids. The use of silanes or pinacolborane as stoichiometric reducing agents and triethylphosphite as a ligand facilitated the divergent and complementary selectivity for the synthesis of diastereomeric anti- and syn-rearranged products, respectively. Copper-catalyzed reductive Ireland-Claisen rearrangement of allylic 2,3-allenoates proceeded effectively only when pinacolborane was used as a reductant to generate various 1,5-dienes in excellent yields and with good diastereoselectivities in some cases. Mechanistic studies showed that the silyl and boron enolates, rather than the copper enolate, underwent a stereospecific rearrangement via a chairlike transition state to afford the corresponding Claisen rearrangement products.

Cu-catalyzed coupling of vinylidene cyclopropanes with allyl and allenyl boronates

The development of Cu-catalyzed coupling of vinylidene cyclopropanes with allyl or allenyl boronates is reported. The reaction forms a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes with concurrent opening of the cyclopropane ring. In addition, the resulting Cu-enolate intermediate can be intercepted by external electrophiles.

Generation of Axially Chiral Fluoroallenes through a Copper-Catalyzed Enantioselective β-Fluoride Elimination

Herein we report the copper-catalyzed silylation of propargylic difluorides to generate axially chiral, tetrasubstituted monofluoroallenes in both good yields (27 examples >80%) and enantioselectivities (82-98% ee). Compared to previously reported synthetic routes to axially chiral allenes (ACAs) from prochiral substrates, a mechanistically distinct reaction has been developed: the enantiodiscrimination between enantiotopic fluorides to set an axial stereocenter. DFT calculations and vibrational circular dichroism (VCD) suggest that β-fluoride elimination from an alkenyl copper intermediate likely proceeds through a syn-β-fluoride elimination pathway rather than an anti-elimination pathway. The effects of the C1-symmetric Josiphos-derived ligand on reactivity and enantioselectivity were investigated. Not only does this report showcase that alkenyl copper species (like their alkyl counterparts) can undergo β-fluoride elimination, but this elimination can be achieved in an enantioselective fashion.

Copper-Catalyzed Borylative Methylation of Alkyl Iodides with CO as the C1 Source: Advantaged by Faster Reaction of CuH over CuBpin

CuH and CuBpin are versatile catalysts and intermediates in organic chemistry. However, studies that involve both CuH and CuBpin in the same reaction is still rarely reported due to their high reactivity. Now, a study on CuH- and CuBpin-catalyzed borylative methylation of alkyl iodides with CO as the C1 source is reported. Various one carbon prolongated alkyl boranes (RCH₂ Bpin and RCH(Bpin)₂ ) were produced in moderate to good yields from the corresponding alkyl iodides (RI). In this cooperative system, CuH reacts with alkyl iodide faster than CuBpin.

Group 11 Borataalkene Complexes: Models for Alkene Activation

A series of linear late transition metal (M=Cu, Ag, Au and Zn) complexes featuring a side-on [B=C]- containing ligand have been isolated and characterised. The [B=C]- moiety is isoelectronic with the C=C system of an alkene. Comparison across the series shows that in the solid-state, deviation between the η2 and η1 coordination mode occurs. A related zinc complex containing two [B=C]- ligands was prepared as a further point of comparison for the η1 coordination mode. The bonding in these new complexes has been interrogated by computational techniques (QTAIM, NBO, ETS-NOCV) and rationalised in terms of the Dewar-Chatt-Duncanson model. The combined structural and computational data provide unique insight into catalytically relevant linear d10 complexes of Cu, Ag and Au. Slippage is proposed to play a key role in catalytic reactions of alkenes through disruption and polarisation of the π-system. Through the preparation and analysis of a consistent series of group 11 complexes, we show that variation of the metal can impact the coordination mode and hence substrate activation.

Copper-Catalyzed Hydroalumination of Allenes with Diisobutylaluminum Hydride: Synthesis of Allylic Ketones with α-Quaternary Centers via Tandem Allylation/Oppenauer Oxidation

An efficient and straightforward approach to allylaluminum reagent synthesis through Cu-catalyzed hydroalumination of readily accessible allenes with diisobutylaluminum hydride is described. The N-heterocyclic carbene-based copper complex promotes hydride addition to various functionalized allenes under mild reaction conditions. The catalytic reaction is applied to a highly selective one-pot synthesis of allylic ketones with α-tertiary and α-quaternary centers through tandem nucleophilic addition of in situ-generated allylaluminums to aldehydes/Oppenauer oxidation.

Catalytic enantioselective allene-anhydride approach to β,γ-unsaturated enones bearing an α-all-carbon-quarternary center

A protocol of highly regio- and enantioselective copper-catalyzed hydroacylation of the non-terminal C[double bond, length as m-dash]C bond in 1,1-disubstituted terminal allenes with anhydrides has been developed. Both aromatic and aliphatic carboxylic anhydrides are applicable to the efficient construction of all carbon quarternary centers connected with a versatile C[double bond, length as m-dash]C bond and a useful ketone functionality. The synthetic potentials of the enantioenriched products have also been demonstrated. Density functional theory (DFT) calculations were performed to explain the steric outcome of the products: the hydroacylation proceeds through a six-membered transition state and the ligand-substrate steric interactions account for the observed enantioselectivity although the chiral ligand is far away from the to-be-genetated chiral center.

C–C and C–X coupling reactions of unactivated alkyl electrophiles using copper catalysis

Copper catalysts enable cross-coupling reactions of unactivated alkyl electrophiles to generate C–C and C–X bonds.

Cu-catalyzed C-C bond formation of vinylidene cyclopropanes with carbon nucleophiles

The development of Cu-catalyzed addition of carbon nucleophiles to vinylidene cyclopropanes was reported. The reactions with 1,1-bisborylmethane provided homopropargylic boronate products by forming a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes. Alkynyl boronates are also suitable nucleophile precursors in reactions with vinylidene cyclopropanes, and skipped diynes were obtained in high yields. In addition, the Cu-enolate generated from the initial addition of nucleophilic copper species to vinylidene cyclopropanes can be intercepted by an external electrophile. As such, vinylidene cyclopropane serves as a linchpin to connect a nucleophile and an electrophile by forming two carbon-carbon bonds sequentially.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Enantioselective and site-specific copper-catalyzed reductive allyl-allyl cross-coupling of allenes

A copper-catalyzed asymmetric reductive allyl-allyl cross-coupling reaction of allenes with allylic phosphates wherein allenes were used as allylmetal surrogates has been achieved for the first time. This protocol provides an efficient and straightforward route to optically active 1,5-dienes in a highly enantioselective and site-specific fashion. Furthermore, all-carbon quaternary stereogenic centers could also be constructed with this protocol. The versatility of the products is demonstrated through a diverse array of further transformations of the enantioenriched 1,5-dienes.

Cu–NHC azide complex: synthesis and reactivity

The first example of a NHC-based copper azide complex is reported. Its reactivity was investigated with various reagents affording 8 new complexes.

A Regio- and Enantioselective CuH-Catalyzed Ketone Allylation with Terminal Allenes

We report a method for the highly enantioselective CuH-catalyzed allylation of ketones that employs terminal allenes as allylmetal surrogates. Ketones and allenes bearing diverse and sensitive functional groups are efficiently coupled with high stereoselectivity and exclusive branched regioselectivity. In stoichiometric experiments, each elementary step of the proposed hydrocupration-addition-metathesis mechanism can be followed by NMR spectroscopy.