Phosphine-Catalyzed Asymmetric Synthesis of α-Quaternary Amine via Umpolung γ-Addition of Ketimines to Allenoates

The Catalytic Asymmetric Allylic Alkylation of Acyclic Enolates for the Construction of Quaternary and Tetrasubstituted Stereogenic Centres

To facilitate the discovery and development of new pharmaceuticals, the demand for novel stereofunctionalised building blocks has never been greater. Whilst molecules bearing quaternary and tetrasubstituted stereogenic centres are ideally suited to explore untapped areas of chemical space, the asymmetric construction ofsterically congested carbon centres remains a longstanding challenge in organic synthesis. The enantioselective assembly of acyclic stereogenic centres is even more demanding due to the need to restrict a much wider range of geometries and conformations of the intermediates involved. In this context, the catalytic asymmetric allylicalkylation (AAA) of acyclic prochiral nucleophiles, namely enolates, has become an indispensable tool to access a range of linearα-quaternary andα-tetrasubstituted carbonyl compounds. However, unlike the AAA of cyclic enolates with a fixed enolate geometry, to achieve high levels of stereocontrol in the AAA of acyclic enolates, the stereoselectivity of enolisation must be considered. The aim of this review is to offer acomprehensivediscussion of catalytic AAA reactions of acyclic prochiral enolates and their analogues to generate congested quaternary and tetrasubstituted chiral centres using metal, non-metal and dual catalysis, with particular focus given to the control of enolate geometry and its impact on the stereochemical outcome of the reaction.

Copper-Catalyzed Direct Asymmetric Vinylogous Mannich Reaction between β,γ-Alkynyl-α-ketimino Esters and β,γ-Unsaturated N-Acylpyrazoles

We report a Cu(I)-Ph-BPE-catalyzed asymmetric vinylogous Mannich reaction of β,γ-alkynyl-α-ketimino esters with β,γ-unsaturated N-acylpyrazoles. In this process, the Cu(I)-Ph-BPE catalyst activates the β,γ-alkynyl-α-ketimino ester through N,O-coordination, enabling the subsequent nucleophilic addition of a dienolate generated from the β,γ-unsaturated N-acylpyrazole via α-position deprotonation with a catalytic amount of tertiary amine. The reactions gave useful products with very high enantioselectivities. A broad range of substrates with various substituents are tolerated in this reaction. The versatility of this method was demonstrated by a gram-scale reaction, and subsequent elaboration of the Mannich adducts was also provided.

Cu catalyzed [4 + 2] cycloaddition for the synthesis of highly substituted 3-fluoropyridines

A copper catalyzed annulation-aromatization of benzyl trifluoromethyl ketimines with 3-acryloyloxazolidin-2-ones for the synthesis of 3-fluoropyridines through double C-F bond cleavages has been developed. In this approach, the annulation occurred between the in situ formed dienes from trifluoromethyl ketimines via the first C-F bond cleavage and 3-acryloyloxazolidin-2-ones. Then the aromatization afforded 3-fluoropyridines in moderate yields through the second C-F bond cleavage. The 3-fluoropyridine products could be further hydrolyzed to multi-substituted 3-pyridinecarboxylic acids.

Practical asymmetric amine nucleophilic approach for the modular construction of protected α-quaternary amino acids

We report the first amine nucleophilic approach for the modular construction of enantioenriched protected α-quaternary amino acids. The key to success is the use of an alcohol solvent, which makes a rationally designed COOMe-bonded Cu-allenylidene electrophilic intermediate stable enough to couple with amine nucleophiles before its decomposition. The reaction features wide functional group tolerance with high enantioselectivity, typically >90% ee, and is amenable to the modification of commercially available bioactive molecules. The resultant protected α-amino acids could be readily converted into a number of precious enantioenriched amines featuring α-hindered tertiary carbon centers, which are otherwise synthetically quite challenging, including those of α-amino aldehyde, peptides or α-vinyl amino ester with >92% ee in excellent yields. This protocol could be utilized for the synthesis of the protected bioactive α-ethylnorvaline in 3 steps, a significant advancement in comparison to an 11-step sequence reported previously.

We report the first amine nucleophilic approach for the modular construction of enantioenriched protected α-quaternary amino acids.

The Regiocontrollable Enantioselective Synthesis of Chiral Trifluoromethyl-Containing Spiro-Pyrrolidine-Pyrazolone Compounds via Amino-Regulated 1,3-Proton Migration Reaction

An amino-controlled regiodivergent asymmetric synthesis of CF₃-containing spiro-pyrrolidine-pyrazolone compounds is described. With alkaloid-derived squaramide as catalyst, the 1,3-dipolar cycloaddition of α,β-unsaturated pyrazolone with diethyl 2-((2,2,2-trifluoroethyl)imino) malonate offered adducts in excellent yields, dr, and ee. While the cyclohexanediamine-derived squaramide was employed, the reaction afforded a series of structure isomers through a switched umpolung reaction.

Asymmetric Catalytic Ketimine Mannich Reactions and Related Transformations

The catalytic enantioselective ketimine Mannich and its related reactions provide direct access to chiral building blocks bearing an α-tertiary amine stereogenic center, a ubiquitous structural motif in nature. Although ketimines are often viewed as challenging electrophiles, various approaches/strategies to circumvent or overcome the adverse properties of ketimines have been developed for these transformations. This review showcases the selected examples that highlight the benefits and utilities of various ketimines and remaining challenges associated with them in the context of Mannich, allylation, and aza-Morita-Baylis-Hillman reactions as well as their variants.

Cobalt-Catalyzed Umpolung Alkylation of Imines To Generate α-Branched Aliphatic Amines

Here we report a general and mild approach to prepare α-branched aliphatic amines from imines. This method capitalizes on a cobalt-catalyzed umpolung alkylation of imines, employs easily available reaction partners, and demonstrates a broad substrate scope. Mechanistic studies suggest this transformation occurs by a radical pathway.

Catalytic asymmetric umpolung reactions of imines via 2-azaallyl anion intermediates

The imine umpolung is a relatively new and interesting strategy, especially in catalytic asymmetric synthesis. A significant development in organo- and transition metal-catalyzed umpolung of imines took place only in the recently concluded decade. A majority of the reports on the asymmetric umpolung of imines involve the initial generation of 2-azaallyl anion intermediates with the chiral catalysts, which serve as a significant driving force for the umpolung addition/substitution reactions. A variety of organocatalysts such as bifunctional cinchona alkaloids including squaramides and thioureas, chiral BINOL derived phosphoric acids, phase transfer catalysts (PTCs), phosphines, and transition metal-complexes of iridium, copper and palladium have been employed to achieve the excellent level of asymmetric induction in such types of umpolung reactions. The asymmetric imine umpolung strategy has been applied successfully to synthesize synthetic amino-acid derivatives and other useful chiral amines, including drugs and potentially bioactive molecules. This review summarizes all the significant recent development in catalytic umpolung reactions of imines involving a 2-azaallyl anion intermediate.

Phosphine-catalyzed γ-addition of nitroacetates to allenoates for enantioselective creation of α,α-disubstituted α-amino acid precursors

Enantioselective γ-addition of readily available α-substituted nitroacetates to allenoates has been achieved.

Phosphine-Catalyzed (4+1) Annulation: Rearrangement of Allenylic Carbamates to 3-Pyrrolines through Phosphonium Diene Intermediates

We have developed a phosphine-catalyzed (4+1) annulative rearrangement for the preparation of 3-pyrrolines from allenylic carbamates via phosphonium diene intermediates. We employed this methodology to synthesize an array of 1,3-disubstituted- and 1,2,3-trisubstituted-3-pyrrolines, including the often difficult to prepare 2-alkyl variants. A mechanistic investigation employing allenylic acetates and mononucleophiles unexpectedly unveiled that a phosphine-catalyzed (4+1) reaction for the construction of cyclopentene products, previously reported by Tong, might not occur through a phosphonium diene, as had been proposed, but rather through multiple mechanisms working in concert. Consequently, our phosphine-catalyzed rearrangement is most likely the first transformation to involve the unequivocal formation of a phosphonium diene intermediate along the reaction pathway. To demonstrate the synthetic utility of this newly developed reaction, we have completed concise formal syntheses of the pyrrolizidine alkaloids (±)-trachelanthamidine and (±)-supinidine.

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.

Regio- and Enantioselective Palladium-Catalyzed Asymmetric Allylation of N-Fluorenyl Trifluoromethyl Imine

A palladium-catalyzed asymmetric allylation of N-fluorenyl trifluoromethyl imine with allylic acetates is disclosed. This method provides scalable and efficient access to polysubstituted chiral α-trifluoromethyl amines bearing two adjacent stereocenters and one allyl group in high yields with excellent regio-, diastereo-, and enantioselectivity. Importantly, this method also provides a powerful strategy for the synthesis of both regioisomeric products and the regioselectivity is controlled by the chiral catalysts and optically active substrates.

Catalytic Enantiodivergent Michael Addition by Subtle Adjustment of Achiral Amino Moiety of Dipeptide Phosphines

Over the past decades, asymmetric catalysis has been intensely investigated as a powerful tool for the preparation of numerous chiral biologically active compounds. However, developing general and practical strategies for preparation of both enantiomers of a chiral molecule via asymmetric catalysis is still a challenge, particularly when the two enantiomers of a chiral catalyst are not easily prepared from natural chiral sources. Inspired by the biologic system, we report herein an unprecedented catalytic enantiodivergent Michael addition of pyridazinones to enones by subtle adjustment of achiral amino moiety of dipeptide phosphine catalysts. These two dipeptide phosphine catalysts, P5 and P8, could deliver both enantiomers of a series of N2-alkylpyridazinones in good yields (up to 99%) with high enantioselectivities (up to 99% ee) via the catalyst-controlled enantiodivergent addition of pyridazinones to enones.

Regio- and Enantioselective Synthesis of Trifluoromethyl-Substituted Homoallylic α-Tertiary NH2 -Amines by Reactions Facilitated by a Threonine-Based Boron-Containing Catalyst

A method for catalytic regio- and enantioselective synthesis of trifluoromethyl-substituted and aryl-, heteroaryl-, alkenyl-, and alkynyl-substituted homoallylic α-tertiary NH₂ -amines is introduced. Easy-to-synthesize and robust N-silyl ketimines are converted to NH-ketimines in situ, which then react with a Z-allyl boronate. Transformations are promoted by a readily accessible l-threonine-derived aminophenol-based boryl catalyst, affording the desired products in up to 91 % yield, >98:2 α:γ selectivity, >98:2 Z:E selectivity, and >99:1 enantiomeric ratio. A commercially available aminophenol may be used, and allyl boronates, which may contain an alkyl-, a chloro-, or a bromo-substituted Z-alkene, can either be purchased or prepared by catalytic stereoretentive cross-metathesis. What is more, Z-trisubstituted allyl boronates may be used. Various chemo-, regio-, and diastereoselective transformations of the α-tertiary homoallylic NH₂ -amine products highlight the utility of the approach; this includes diastereo- and regioselective epoxide formation/trichloroacetic acid cleavage to generate differentiated diol derivatives.

Design, synthesis and application of a new type of bifunctional in highly enantioselective γ-addition reactions of N-centered nucleophiles to allenoates

A novel class of cyclic phosphine derived bifunctional catalysts (Le-Phos) is reported, which can be readily prepared from inexpensive and commercially available starting materials and exhibit good performances in enantioselective γ-addition reactions of N-centered nucleophiles and allenoates under mild conditions. The salient features of this reaction include high product yields, good enantioselectivity, mild reaction conditions, and broad substrate scope and gram-scale scalability.

Phosphine-catalyzed (3+2)/(2+3) sequential annulation involving a triple nucleophilic addition reaction of γ-vinyl allenoates

A phosphine-catalyzed (3+2)/(2+3) sequential annulation involving a triple nucleophilic addition reaction of γ-vinyl allenoates was successfully developed. The reaction provided efficient and more practical access to functionalized hydropyrroloimidazolones with good to excellent yields under mild reaction conditions. Notably, γ-vinyl allenoate served as a triple-electrophilic intermediate in this protocol.

2-Azadienes as Enamine Umpolung Synthons for the Preparation of Chiral Amines

The development of new strategies for the preparation of chiral amines is an important objective in organic synthesis. In this Synpacts, we summarize our approach for catalytically accessing nucleophilic aminoalkyl metal species from 2-azadienes, and its application in generating a number of important but elusive chiral amine scaffolds. Reductive couplings with ketones and imines afford 1,2-amino tertiary alcohols and 1,2-diamines, respectively, whereas fluoroarylations of gem-difluoro-2-azadienes deliver α-trifluoromethylated benzylic amines.

NH2OH-HCl-Mediated Umpolung α-Methylsulfonylation of α-Sulfonyl Ketones with Methylsulfoxides: Synthesis of α,β-Bis-sulfonyl Arylketones

In this paper, a novel and efficient route for the synthesis of α,β-bis-sulfonyl arylketones via an NH₂OH-HCl-mediated intermolecular umpolung α-methylsulfonylation of α-sulfonyl ketones with methylsulfoxides is described. A plausible mechanism is proposed and discussed. Various reaction conditions for this efficient, one-pot, environmentally friendly transformation were investigated.

Direct Catalytic Asymmetric Synthesis of Trifluoromethylated γ-Amino Esters/Lactones via Umpolung Strategy

Enabled by the discovery of new cinchonium salts and coadditives, a direct and efficient asymmetric access to trifluoromethylated γ-amino esters/lactones has been realized through the enantioselective and diastereoselective umpolung reaction of trifluoromethyl imines with acrylates or α,β-unsaturated lactones as carbon electrophiles. At 0.5-5.0 mol % catalyst loadings, the newly developed catalytic system activates a variety of imine substrates as unconventional nucleophiles to mediate highly chemo-, regio-, diastereo-, and enantioselective C-C bond forming reactions. The developed synthetic protocol represents an excellent strategy to target a series of versatile and enantiomerically enriched γ-amino esters/lactones in good to excellent yields from the readily available starting materials. Additionally, we found that the epi-vinyl catalysts based on cinchonidine and quinine promote a similarly high enantioselective reaction generating the opposite configuration of chiral products in a highly efficient manner, which allows convenient access to either the R- or S-enantiomer of the chiral amine products in high yields and excellent enantioselectivities.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Phosphine-catalyzed [4 + 2] annulation of γ-benzyl allenoates: facile synthesis of benzothieno[3,2-b]pyran derivatives

An efficient tris(4-methoxyphenyl)phosphane catalyzed domino reaction between γ-benzyl allenoate and ethyl (Z)-2-(3-oxobenzo[b]thiophen-2(3H)-ylidene)acetate has been developed, which produces a series of 2H-benzo[4,5]thieno[3,2-b]pyran derivatives in high yields.

Phosphine-catalysed α-umpolung addition of nucleophiles to δ-acetoxy allenoates: stereoselective synthesis of 2,4-dienoates

The first example of phosphine-catalyzed α-umpolung addition of nucleophiles to allenoates is described, which features the use of δ-acetoxy allenoate to generate a 3-phosphonium-2,4-dienoate intermediate, thus facilitating the α-umpolung addition of nucleophiles.