Allylic Alkylation Research Articles

Hydroxy-directed iridium-catalyzed enantioselective formal β-C(sp2)-H allylic alkylation of α,β-unsaturated carbonyls.

Hydroxy-directed iridium-catalyzed enantioselective formal β-C(sp2)-H allylic alkylation of kojic acid and structurally related α,β-unsaturated carbonyl compounds is developed. This reaction, catalyzed by an Ir(i)/(P,olefin) complex, utilizes the nucleophilic character of α-hydroxy α,β-unsaturated carbonyls, to introduce an allyl group at its β-position in a branched-selective manner in good to excellent yield with uniformly high enantioselectivity (up to >99.9 : 0.1 er). To the best of our knowledge, this report represents the first example of the use of kojic acid in a transition metal catalyzed highly enantioselective transformation.

Pd-Catalyzed Formal [3 + 3] Heteroannulation of Allylic gem-Diacetates: Synthesis of Chromene-Based Natural Products and Exploration of Photochromic Properties

Palladium-catalyzed allylic alkylation reactions of allylic gem-diacetates are rarely explored. This work unveils an unusual chemical reactivity pattern of allylic gem-diacetates and establishes them as new prototypes to synthesize complex benzo[f]chromene systems. Under the reaction conditions, the diacetates behave as 1,3-dicationic equivalents and undergo a [3 + 3] heteroannulation with 2-naphthols (and meta-substituted phenols) to produce novel polycyclic chromenes possessing spiro-, tri-, and tetrasubstituted carbon centers. The versatility of the method is demonstrated in the synthesis of several chromene-based bioactive natural products. Further, interesting photochromic properties of the new classes of benzo[f]chromenes are also discovered.

ProPhenol Derived Ligands to Simultaneously Coordinate a Main-Group Metal and a Transition Metal: Application to a Zn-Cu Catalyzed Reaction.

A new bifunctional ligand bearing chiral N-heterocyclic carbene (NHC) and prolinol moieties is presented. Utilizing the designed ligand, an in situ formed Cu/Zn hetero-bimetallic complex unlocks the asymmetric allylic alkylation reactions of allyl phosphates with zinc keto-homoenolates, leading to the formation of various γ-vinyl ketones with good regio- and enantio-selectivity. DF sT calculation supports that the chelation of allyl phosphates with catalyst promotes the SN 2' addition and the ligand-substrate steric interactions account for the stereoselective outcome.

The Preparation and Resolution of Novel Axially Chiral Pyrazine‐Containing P,N Ligands for Asymmetric Catalysis and Their Application in Palladium‐Catalysed Allylic Substitution

AbstractThe design and synthesis of two novel pyrazine‐containing ligands, 3,6‐diisopropyl‐Pyrazinap and Np−Cy‐Pyrazinap, has been described. Their synthesis was accomplished in six and nine steps, respectively, in which the key steps were the formation of the biaryl linkage through a Suzuki cross‐coupling reaction and the formation of the aryl to phosphorus bond employing nickel catalysis. Both ligands were resolved through the formation of diastereomeric palladacycles and in the case of the isolated diastereomer formed from 3,6‐diisopropyl‐Pyrazinap, an X‐ray crystal structure showed the axial chirality to possess (S)‐configuration. The two enantiopure ligands were applied to the palladium‐catalysed allylic alkylation of 1,3‐diphenylprop‐2‐enyl acetate with dimethyl malonate and afforded conversions of up to 100 % and 99 % with enantioselectivities of up to 92 % ee and 78 % ee, respectively. A series of 1,1,3‐triphenylallyl and 1,3‐diphenylallyl palladium complexes were prepared and their solution structures and dynamic behaviour elucidated by 2D‐COSY and NOESY NMR spectroscopic experiments.

Nickel-Catalyzed Enantioselective Allylic Alkylation of Phosphinates

Key words nickel catalysis - phosphinates - allylic substitution - enantioselectivity

Palladium-Catalyzed Decarboxylative Asymmetric Allylic Alkylation of Thietane 1,1-Dioxides.

A palladium-catalyzed decarboxylative asymmetric allylic alkylation of thietane 1,1-dioxides via linear enolate intermediates from racemic starting materials has been developed. This process installs an α-sulfonyl tetrasubstituted stereogenic center with high enantioselectivity. The potential to transform the alkylated products to novel types of enantioenriched spirocycles for medicinal chemistry applications has also been demonstrated.

A Dipolar Cyclization/Fragmentation Strategy for the Catalytic Asymmetric Synthesis of Chiral Eight-Membered Lactams

A Dipolar Cyclization/Fragmentation Strategy for the Catalytic Asymmetric Synthesis of Chiral Eight-Membered Lactams

Ternary Catalysis Enabled Three-Component Asymmetric Allylic Alkylation as a Concise Track to Chiral α,α-Disubstituted Ketones.

Multicomponent reactions that involve interception of onium ylides through Aldol, Mannich, and Michael addition with corresponding bench-stable acceptors have demonstrated broad applications in synthetic chemistry. However, because of the high reactivity and transient survival of these in situ generated intermediates, the substitution-type interception process, especially the asymmetric catalytic version, remains hitherto unknown. Herein, a three-component asymmetric allylation of α-diazo carbonyl compounds with alcohols and allyl carbonates is disclosed by employing a ternary cooperative catalysis of achiral Pd-complex, Rh2(OAc)4, and chiral phosphoric acid CPA. This method represents the first example of three-component asymmetric allylic alkylation through an SN1-type trapping process, which involves a convergent assembly of two active intermediates, Pd-allyl species, and enol derived from onium ylides, providing an expeditious access to chiral α,α-disubstituted ketones in good to high yields with high to excellent enantioselectivity. Combined experimental and computational studies have shed light on the mechanism of this novel three-component reaction, including the critical role of Xantphos ligand and the origin of enantioselectivity.

Catalyst-Controlled Regiodivergent Synthesis of α/β-Dipeptide Derivatives via N-Allylic Alkylation of O-Alkyl Hydroxamates with MBH Carbonates.

A controllable and regiodivergent N-allylation reaction involving readily available O-alkyl hydroxamates derived from natural α-amino acids has been developed, allowing regiospecific access to α/β-dipeptides containing α-unsaturated β-amino acids moieties in moderate to good yields. The regioselectivity could be conveniently switched by alternation of the catalysts and solvents.

Copper-Catalyzed Asymmetric Allylic Alkylation of Homoenolates Derived from Cyclopropanols

Copper-Catalyzed Asymmetric Allylic Alkylation of Homoenolates Derived from Cyclopropanols

Enantioselective Tertiary Electrophile (Hetero)Benzylation: Pd‐Catalyzed Substitution of Isoprene Monoxide with Arylacetates**

The enantioselective generation of quaternary carbon centers remains challenging but is of growing importance for the preparation of functional molecules. Metal catalyzed allylic alkylations of tertiary electrophiles can provide access to these substructures but remain generally incompatible with organometallic benzyl nucleophiles. Here we demonstrate that electron-deficient arylacetates can serve as benzyl nucleophile surrogates to generate enantioenriched acyclic molecules containing a quaternary carbon center via a two-step substitution-decarboxylation process using isoprene monoxide. Products are often obtained in >90 % ee using a commercially available catalyst. An array of electron-withdrawing functional groups on the arylacetate moiety are tolerated. The lactone generated by the initial substitution reaction can be used in further stereoselective transformations to prepare molecules with acyclic vicinal quaternary stereocenters.

Enantioselective Tertiary Electrophile (Hetero)Benzylation: Pd‐Catalyzed Substitution of Isoprene Monoxide with Arylacetates**

AbstractThe enantioselective generation of quaternary carbon centers remains challenging but is of growing importance for the preparation of functional molecules. Metal catalyzed allylic alkylations of tertiary electrophiles can provide access to these substructures but remain generally incompatible with organometallic benzyl nucleophiles. Here we demonstrate that electron‐deficient arylacetates can serve as benzyl nucleophile surrogates to generate enantioenriched acyclic molecules containing a quaternary carbon center via a two‐step substitution–decarboxylation process using isoprene monoxide. Products are often obtained in >90 % ee using a commercially available catalyst. An array of electron‐withdrawing functional groups on the arylacetate moiety are tolerated. The lactone generated by the initial substitution reaction can be used in further stereoselective transformations to prepare molecules with acyclic vicinal quaternary stereocenters.

Ni-Catalyzed Enantioselective Allylic Alkylation of H-Phosphinates.

The asymmetric synthesis of P-stereogenic phosphinates through allylic alkylation of H-phosphinates has been developed. With H-phosphinates and allylic acetates as the starting materials, a variety of allylic P-chiral phosphinates were accessed in high enantioselectivities of up to 92% ee and generally high yields. In addition, a further study demonstrated the applicability of this protocol, including the scale-up synthesis and facile transformation of chiral products from phosphinates to phosphine oxides with organolithium reagents under mild reaction conditions.

Inclusion of bidentate phosphine molecules in the supercage of FAU zeolite

Bidentate phosphines (Ph 2 P(CH 2 ) n PPh 2 ) with different alkyl chain lengths (n = 1–10) were included in the supercage (SC) of FAU-zeolite. Thermal treatment of the mixture of a bidentate phosphines and FAU-zeolite at 443 K resulted in the inclusion of the former into the SC, with up to 0.75 molecules per SC. The included molecules were strongly bound by FAU-zeolite, with desorption observed at higher temperatures compared to those of unloaded ones. The specific surface area of the FAU-zeolite decreased linearly with an increase in the number of phosphines included. This tendency was more emphasized in phosphines with n = 6–10 compared with those with n = 1–5. A characteristic shift in the C–C stretching band of the benzene ring was observed in the Fourier-transform infrared spectroscopy (FT-IR) spectrum, due to the strong interaction between bidentate phosphines and FAU-zeolite; the extent was more when using *BEA-loaded ones. Formation of the inclusion compounds between bidentate phosphines and FAU zeolites was observed with the molecular dynamics (MD) calculations. The bidentate phosphines included in the SC of the FAU-zeolite were reacted with PdCl 2 to form Pd complexes. The detailed analysis of the Pd complexes with Pd K-edge EXAFS revealed a characteristic decrease in the Pd–Cl bond distance with the use of complexes with n = 3 and 4, suggesting that the local structure around Pd was distorted through the interaction of the Pd complexes with the pores of the FAU-zeolite. The Pd complexes included in the SC were active for allylic alkylation between allyl methyl carbonate and ethyl acetoacetate. • Bidentate phosphines could be directly included in the supercage of FAU zeolite. • Formation of the inclusion compound was simulated by MD calculation. • Pd complexes with bidentate phosphines in SCs of FAU zeolite exhibited activity in the allylic alkylation.

Dynamic Kinetic Resolution of Flavonoids via Asymmetric Allylic Alkylation: Construction of Two Contiguous Stereogenic Centers on Nucleophiles

The extension of racemization strategies of dynamic kinetic resolution in organic synthesis is a longstanding challenge, especially racemizing two or more stereogenic centers simultaneously. Through the combination of a palladium-catalyzed asymmetric allylic alkylation and a base-promoted retro-oxa-Michael addition, a dynamic kinetic resolution of 2,3-disubstituted flavonoids was achieved with up to 99% enantioselectivities, and two contiguous stereocenters (including a quaternary stereogenic center) were constructed simultaneously on the nucleophile flavonoids. The key feature of the reaction was a base-promoted retro-oxa-Michael addition for fast racemization of two stereogenic centers on the nucleophiles, which can pave the way to developing asymmetric reactions of flavonoids through dynamic kinetic resolution.

Enantioselective Palladium-Catalyzed Decarboxylative Dearomative Asymmetric Allylic Alkylation of Benzofurans: Diversity-Oriented Synthesis of Flavaglines

AbstractWith the introduction of new Trost-type bisphosphine ligands bearing a chiral cycloalkane framework, the highly efficient and enantioselective palladium-catalyzed decarboxylative dearomative asymmetric allylic alkylation (AAA) of benzofurans was achieved. This enabled a diversity-oriented synthesis (DOS) of previously unreachable flavaglines, which features two diversification stages. A new avenue for developing flavagline-based drugs was thus established.1 Introduction2 The Dearomative Asymmetric Allylic Alkylation of Benzofurans3 Synthesis of Flavaglines4 Conclusion and Outlook

Chiral P*,S-bidentate diamidophosphites with 1,2-thioether alcohol–based exocyclic substituents in asymmetric Pd-catalyzed reactions

Chiral P*,S-diamidophosphites with a 1,3,2-diazaphospholidine core and exocyclic 1,2-hydroxyl-thioether fragments were prepared. These asymmetric inducers provided up to 86% ee in the Pd-catalyzed allylic substitution of (E)-1,3-diphenylallyl acetate with dimethyl malonate and pyrrolidine, and up to 76% in the Pd-mediated allylic alkylation of cinnamyl acetate with ethyl 2-oxocyclohexane-1-carboxylate.

Insights into Organoamine-Catalyzed Asymmetric Synthesis of Axially Chiral Allenoates Using Morita-Baylis-Hillman Carbonates and Trisubstituted Allenoates: Mechanism and Origin of Stereoselectivity.

A computational study was performed to explore the possible mechanisms of β-isocinchonine-catalyzed asymmetric C(sp2)-H allylation of trisubstituted allenoates using Morita-Baylis-Hillman (MBH) carbonates for synthesis of axially chiral tetrasubstituted allenoates. The calculated results indicate that the most energetically favorable pathway includes (1) nucleophilic attack on MBH carbonate by β-isocinchonine, (2) BocO- dissociation, (3) stereoselective formation of the C-C bond, and (4) regeneration of the catalyst. By tracking the orbital overlap/interaction changes, the half shoulder-to-head orbital overlap mode can be smoothly switched to a head-to-head orbital overlap mode for the key C-C σ bond formation, which is also identified as the stereoselectivity-determining process. Further distortion/interaction, noncovalent interaction (NCI), and atom-in-molecule (AIM) analyses demonstrate that C-H···O and C-H···π interactions should be key for controlling the axial and central chirality. This work would be useful for rational design of organocatalytic allylic alkylation reactions for synthesis of axially chiral compounds in the future.

Enantioselective Construction of Axially Chiral Azepine-Containing P,N-Ligands from l-Alanine.

A family of axially chiral azepine-containing seven-membered cyclic P,N-ligands (named Indole-azepinap) have been prepared by using l-alanine as an original chirality source. The direct chromatographic separation of two diastereomeric phosphine oxides on silica gel enabled these ligands to be easy available, allowing further structural and electronic modifications. Preliminary application of these Indole-azepinaps has been demonstrated in a Pd-catalyzed asymmetric allylic alkylation with high yields and moderate enantioselectivities.

Direct C(sp3)-H allylation of 2-alkylpyridines with Morita-Baylis-Hillman carbonates via a tandem nucleophilic substitution/aza-Cope rearrangement.

A base- and catalyst-free C(sp3)–H allylic alkylation of 2-alkylpyridines with Morita–Baylis–Hillman (MBH) carbonates is described. A plausible mechanism of the reaction might involve a tandem SN2’ type nucleophilic substitution followed by an aza-Cope rearrangement. Various alkyl substituents on 2-alkylpyridines were tolerated in the reaction to give the allylation products in 26–91% yields. The developed method provides a straightforward and operational simple strategy for the allylic functionalization of 2-alkypyridine derivatives.