Copper(I)-catalyzed asymmetric 1,6-conjugate allylation

Recent Progress of Substituted Allylboron Compounds in Catalytic Asymmetric Allylation Reactions

The catalytic asymmetric allylation reaction involving allylboron species has emerged as a powerful tool to access highly stereoselective allylation products. In the early years, most of the researches focused on the reaction of unsubstituted allylboronates with ketones or imines. With the synthesis of complex substituted allylboronates, allylboronic acids and allyltrifluoroborates, the type of reactions and the variety of substrates are greatly expanded. Therefore, this review article will emphasize on the aspect of regio- and stereoselectivity when substituted allylboron species involving and their application on the construction of versatile organic building blocks, drugs and natural products.

Enantio- and Z-Selective δ-Hydroarylation of Aryl-Substituted 1,3-Dienes via Rh-Catalyzed Conjugate Addition

Metal-catalyzed enantioselective conjugate arylations of electron-poor alkenes are highly selective processes for C(sp2)-C(sp3) bond formation. δ-Selective hydroarylations of electron-poor 1,3-dienes are less well developed and reactions that deliver high enantioselectivity while giving single alkene isomer products are elusive. Here we report the Rh-catalyzed δ-arylation of aryl-substituted 1,3-dienes that gives nearly exclusive Z-1,4-addition products (generally with >95 : 5 positional and geometrical selectivity). This remote functionalization provides access to chiral diarylated alkenes from readily available precursors poised for further functionalization, including in the synthesis of bioactive molecules. Mechanistic studies suggest that protonolysis of a Rh-allyl intermediate generated by diene insertion into a Rh-aryl is the turnover limiting step and occurs by an inner-sphere proton transfer pathway.

Chiral π-Cu(ii)-catalyzed site-, exo/endo-, and enantioselective dearomative (3 + 2) cycloadditions of isoquinolinium ylides with enamides, dienamides, and a trienamide

Here, we report a highly effective dearomative (3 + 2) cycloaddition reaction between isoquinolinium ylides and α,β-enamides, α,β-γ,δ-dienamides, or an α,β-γ,δ-ε,ζ-trienamide, which is catalyzed by a chiral π-Cu(ii) complex (1-10 mol%) and proceeds in a site-selective, exo/endo-selective, and enantioselective manner. The (3 + 2) cycloaddition involving the α,β-enamides proceeds with high exo-selectivity and enantioselectivity. This method is applicable to various substrates including α-substituted, α,β-disubstituted, or β,β-disubstituted α,β-enamides, which are compounds with an intrinsically low reactivity. This method provides synthetic access to pyrroloisoquinoline derivatives with up to three chiral carbon centers, including those featuring fluorine and trifluoromethyl groups, as well as quaternary carbon centers. The (3 + 2) cycloaddition involving α,β-γ,δ-dienamides proceeds with high γ,δ-site-selectivity and enantioselectivity, whereby the exo/endo-selectivity depends on the substrates and ligands. Remarkably, the (3 + 2) cycloaddition of δ-phenyl-α,β-γ,δ-dienamide proceeds with high α,β-site-selectivity, exo-selectivity, and enantioselectivity. In a manner similar to the reaction with the α,β-γ,δ-dienamides, α,β-γ,δ-ε,ζ-trienamide furnishes a (3 + 2) cycloadduct with good ε,ζ-site-selectivity, endo-selectivity, and enantioselectivity.

Copper-Catalyzed Enantioselective 1,2-Allylation of Azadienes with Allylboronates

Catalytic asymmetric 1,2-allylation of aurone-derived azadienes is very difficult to achieve due to the driving force for aromatization and the greater steric hindrance of 1,2-addition compared with 1,4-addition. By taking advantage of the ability of nitrogen ligated metal complexes, we successfully demonstrated the first example of copper-catalyzed 1,2-allylation of azadienes with allylboronates for the highly enantioselective synthesis of homoallylic amines. Meanwhile, the enantioenriched 1,4-addition products could also be obtained through a subsequent 3,3-sigmatropic rearrangement of the 1,2-addition products. Extensive DFT calculations were carried out to elucidate the origins of high regioselectivity (1,2- vs 1,4-) and enantioselectivity.

Copper(I)-Catalyzed Asymmetric Conjugate Addition of 1,4-Dienes to β-Substituted Alkenyl Azaarenes

Chiral azaarene compounds are extremely important due to their prevalence in pharmaceutical ingredients. Herein, an array of chiral molecules bearing azaaryl groups is synthesized in moderate-to-excellent yields with moderate-to-excellent Z/E ratios, high dr, and excellent enantioselectivity by a copper(I)-catalyzed asymmetric conjugate addition of 1,4-dienes to (E)-β-substituted alkenyl azaarenes. The reaction is carried out under mild proton-transfer conditions, which enjoys very high atom economy. Moreover, the reaction features a broad substrate scope on (E)-α,β-unsaturated azaarenes as various azaarenes are well tolerated, such as benzothiazole, thiazole, N-methyl-benzimidazole, benzoxazole, quinoline, isoquinoline, pyrimidine, pyrazine, and triazine. Interestingly, the reaction with (Z)-α,β-unsaturated azaarenes affords the same products in excellent results but with a reversed absolute configuration. DFT calculations indicate that the C-C bond-forming nucleophilic addition is a Z-/E- and enantio-selectivities-determining step and provides a rationale for the origin of selectivities. At last, the synthetic utilities of the product are showcased by several transformations, including olefin metathesis, [4 + 2] cyclization, [2 + 1] cyclization, and cleavage of the benzothiazole ring.

Copper-catalysed asymmetric reductive cross-coupling of prochiral alkenes

Asymmetric construction of C(sp3)-C(sp3) bond with good stereocontrol of the two connecting carbon centres retaining all carbon or hydrogen substituents is a challenging target in transition metal catalysis. Transition metal-catalysed reductive coupling of unsaturated π-substrates is considered as a potent tool to expediently develop the molecular complexity with high atom efficiency. However, such an asymmetric and intermolecular process has yet to be developed fully. Herein, we report an efficient strategy to reductively couple two prochiral conjugate alkenes using a copper-catalysed tandem protocol in the presence of diboron. Notably, this transformation incorporates a wide range of terminal and internal enynes as coupling partners and facilitates highly diastereo- and enantioselective synthesis of organoboron derivatives with multiple adjacent stereocentres in a single operation.

Chiral oxazolidines acting as transient hydroxyalkyl-functionalized N-heterocyclic carbenes: an efficient route to air stable copper and gold complexes for asymmetric catalysis

Optically pure oxazolidines were synthesized in nearly quantitative yields from chiral hydroxyalkyl-functionalized imidazolinium salts. Acting as transient chiral diamino N-heterocyclic carbenes (NHCs), these oxazolidines allowed the efficient formation of well-defined copper(i) and gold(i) hydroxyalkyl-NHC complexes, which could be isolated, for the first time, as air stable complexes after silica gel chromatography. Interestingly, X-ray analysis of gold complexes revealed that the hydroxyl-function is not chelated to the metal. Computational studies suggested that both cyclisation to produce oxazolidine and O–H bond elimination to form the transient carbene (prior to coordination) occur through a concerted mechanism. The novel chiral copper-catalysts, as well as oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation with high regio- and enantio-selectivities (up to 99% ee).

Well-defined chiral Cu(i) and Au(i) hydroxyalkyl NHC complexes were synthesized from oxazolidines. The copper-catalysts and oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation.

Photochemical Organocatalytic Regio- and Enantioselective Conjugate Addition of Allyl Groups to Enals

We report the first catalytic enantioselective conjugate addition of allyl groups to α,β‐unsaturated aldehydes. The chemistry exploits the visible‐light‐excitation of chiral iminium ions to activate allyl silanes towards the formation of allylic radicals, which are then intercepted stereoselectively. The underlying radical mechanism of this process overcomes the poor regio‐ and chemoselectivity that traditionally affects the conjugate allylation of enals proceeding via polar pathways. We also demonstrate how this organocatalytic strategy could selectively install a valuable prenyl fragment at the β‐carbon of enals.

Catalytic Asymmetric Allylic Substitution with Copper(I) Homoenolates Generated from Cyclopropanols

By using copper(I) homoenolates as nucleophiles, which are generated through the ring-opening of 1-substituted cyclopropane-1-ols, a catalytic asymmetric allylic substitution with allyl phosphates is achieved in high to excellent yields with high enantioselectivity. Both 1-substituted cyclopropane-1-ols and allylic phosphates enjoy broad substrate scopes. Remarkably, various functional groups, such as ether, ester, tosylate, imide, alcohol, nitro, and carbamate are well tolerated. Moreover, the present method is nicely extended to the asymmetric construction of quaternary carbon centers. Some control experiments argue against a radical-based reaction mechanism and a catalytic cycle based on a two-electron process is proposed. Finally, the synthetic utilities of the product are showcased by means of the transformations of the terminal olefin group and the ketone group.

Bi(OAc)3/Chiral Phosphoric Acid-Catalyzed Enantioselective 1,2- and Formal 1,4-Allylation Reaction of β,γ-Unsaturated α-Ketoesters

A highly efficient asymmetric 1,2-allylation reaction of β,γ-unsaturated α-ketoesters was realized by using a Bi(OAc)₃/chiral phosphoric acid catalyst system under mild conditions. Meanwhile, using this combined strategy of enantioselective 1,2-allylation and subsequent anionic oxy-Cope rearrangement, the asymmetric formal 1,4-allylation reaction was achieved by a one-pot process. These reactions offer rapid access to an array of homoallylic tertiary alcohols and γ-allyl-α-ketoesters with good yields and excellent enantioselectivities. Density functional theory calculations were conducted to interpret the high enantioselectivity.

Diastereo-, Enantio-, and Z-Selective α,δ-Difunctionalization of Electron-Deficient Dienes Initiated by Rh-Catalyzed Conjugate Addition

Metal-catalyzed enantioselective conjugate additions are highly reliable methods for stereoselective synthesis; however, multicomponent reactions that are initiated by conjugate arylation of acyclic π-systems are rare. These reactions generally proceed with poor diastereoselectivity while requiring basic, moisture sensitive organometallic nucleophiles. Here, we show that Rh-catalysts supported by a tetrafluorobenzobarrelene ligand (Ph-tfb) enable the enantio-, diastereo-, and Z-selective α,δ-difunctionalization of electron-deficient 1,3-dienes with organoboronic acid nucleophiles and aldehyde electrophiles to generate Z-homoallylic alcohols with three stereocenters. The reaction accommodates diene substrates activated by ester, amide, ketone, or aromatic groups and can be used to couple aryl, alkenyl, or alkyl aldehydes. Diastereoselective functionalization of the Z-olefin unit in the addition products allows for the generation of compounds with five stereocenters in high dr and ee. Mechanistic studies suggest aldehyde allylrhodation is the rate-determining step, and unlike reactions of analogous Rh-enolates, the Rh-allyl species generated by δ-arylation undergoes aldehyde trapping rather than protonolysis, even when water is present as a cosolvent. These findings should have broader implications in the use of privileged metal-catalyzed conjugate addition reactions as entry points toward the preparation of acyclic molecules containing nonadjacent stereocenters.

Copper(I)-Catalyzed Asymmetric Conjugate 1,6-, 1,8-, and 1,10-Borylation

Catalytic asymmetric remote conjugate borylation is challenging as the control of regioselectivity is not trivial, the electrophilicity of remote sites is extenuated, and the remote asymmetric induction away from the carbonyl group is difficult. Herein, catalytic asymmetric conjugate 1,6-, 1,8- and 1,10-borylation was developed with excellent regioselectivity, which delivered α-chiral boronates in moderate to high yields with high enantioselectivity. The produced chiral boronate smoothly underwent oxidation, cross-coupling, and one-carbon homologation to give synthetically versatile chiral compounds in moderate yields with excellent stereoretention. Furthermore, a stereomechanistic analysis was conducted using DFT calculations, which provides insights into the origins of the regioselectivity. Finally, the present 1,6-borylation was successfully applied in an efficient one-pot asymmetric synthesis of (-)-7,8-dihydrokavain.