Decarboxylative Generation of 2-Azaallyl Anions: 2-Iminoalcohols via a Decarboxylative Erlenmeyer Reaction

Semistabilized Diazatrienyl Anions from Pyridine Imines and Acetylenes: An Access to (Z)-Stilbene/Imidazopyridine Ensembles, Benzyl Imidazopyridines, and Beyond

Semistabilized diazatrienyl anions are generated by the reaction of 2-pyridylarylimines with arylacetylenes in superbase systems MO^tBu (M = Li, Na, K)/DMSO at ambient temperature for 15 min. The initial intermediate N-centered propargyl-1,3-diaza-1,3,5-trienyl anions undergo intermolecular cyclization to benzyl imidazopyridine anions (formally [3 + 2] cycloaddition), further intercepting a second molecule of the starting pyridylimines or a proton of medium to afford (Z)-stilbene/imidazopyridine ensembles and benzyl imidazopyridines. The charge distribution in all intermediate anions and their synthetic evolution are consistent with quantum-chemical analysis (B2PLYPD/6-311+G**//B3LYP/6-31+G*).

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.

Formal α-Allylation of Primary Amines by a Dearomative, Palladium-Catalyzed Umpolung Allylation of N-(Aryloxy)imines

N-(Aryloxy)imines, readily accessible by condensation/tautomerization of (pseudo)benzylic primary amines and 2,6-di-tert-butyl-1,4-benzoquinone, undergo efficient allylation to afford a wide range of homoallylic primary amines following hydrolytic workup. Deprotonation of N-(aryloxy)imines generates a delocalized 2-azaallyl anion-type nucleophile that engages in dearomative C-C bond-forming reactions with allylpalladium(II) electrophiles generated from allylic tert-butyl carbonates. This reactivity umpolung enables the formal α-allylation of (pseudo)benzylic primary amines. Mechanistic studies reveal that the apparent regioselectivity of the desired bond-forming event is a convergent process that is initiated by unselective allylation of N-(aryloxy)imines to give several regioisomeric species, which subsequently rearrange via stepwise [1,3]- or concerted [3,3]-sigmatropic shifts, ultimately converging to provide the desired regioisomer of the amine products.

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.

Decarboxylative Umpolung Synthesis of Amines from Carbonyl Compounds

2-Azaallyl anions are valuable intermediates which have versatile applications in functionalization with various electrophiles. Decarboxylation of the imines formed from aromatic aldehydes and α,α-diphenylglycine provides an interesting and efficient way to generate delocalized 2-azaallyl anions, which display high reactivity toward different electrophiles with excellent regioselectivity at the diphenylketimino aryl carbon of the 2-azaallyl anions. The transformation produces various amines in good yields under very mild conditions. This Synpacts article highlights the recent advances on the decarboxylative umpolung synthesis of amines from carbonyl compounds.

1 Introduction

2 Decarboxylative Umpolung Reactions of Carbonyl Compounds with Different Electrophiles

2.1 Reaction with π-Allyl–Pd(II) Species

2.2 Reaction with Morita–Baylis–Hillman Adducts

2.3 Reaction with Imines

2.3.1 Intermolecular Reaction with N-Ts Imines

2.3.2 Intramolecular Reaction with Chiral N-tert-Butanesulfinyl Imines

2.4 Reaction with Aldehydes and Ketones

3 Decarboxylative Umpolung Reaction of α,β-Unsaturated Aldehydes with Aldehydes

4 Conclusion

Intramolecular Umpolung Synthesis of Exocyclic β-Amino Alcohols through Decarboxylative Amination

An intramolecular aminative Umpolung cyclization strategy has been developed by using α,α-diphenylglycine (2) as the amination and Umpolung reagent. Aldehydes (1) bearing an additional carbonyl group underwent condensation with α,α-diphenylglycine to form an imine, decarboxylation to generate a delocalized 2-azaallylanion, and subsequent intramolecular Umpolung cyclization to produce a variety of exocyclic β-amino alcohols (6) in 60-93% yields with up to >20:1 trans/cis selectivity under mild conditions.

Aminative Umpolung cyclization for synthesis of chiral exocyclic vicinal diamines

Chiral exocylic vicinal diamines are biologically and chemically important compounds, but they are not easy to make. In this paper, an interesting aminative Umpolung cyclization process has been developed. Aromatic aldehydes 6 bearing an electrophilic chiral sulfinimine group underwent imine formation with 2,2-diphenylglycine (2), decarboxylation, and subsequent Umpolung cyclization, producing various trans-diamines 10 in 84-96% yields with high trans/cis ratios under very mild conditions. This work not only provides an efficient, clean, and mild method for the synthesis of chiral exocyclic vicinal diamines in one step but also represents a new application of aminative Umpolung strategy on intramolecular reactions.

2-Azaallyl Anions, 2-Azaallyl Cations, 2-Azaallyl Radicals, and Azomethine Ylides

This review covers the use of 2-azaallyl anions, 2-azaallyl cations, and 2-azaallyl radicals in organic synthesis up through June 2018. Particular attention is paid to both foundational studies and recent advances over the past decade involving semistabilized and nonstabilized 2-azaallyl anions as key intermediates in various carbon-carbon and carbon-heteroatom bond-forming processes. Both transition-metal-catalyzed and transition-metal-free transformations are covered. Azomethine ylides, which have received significant attention elsewhere, are discussed briefly with the primary focus on critical comparisons with 2-azaallyl anions in regard to generation and use.

Intramolecular Umpolung Allylation of Imines

An intramolecular umpolung allylation of imines is reported. This reaction occurs via the intermediacy of 2-azaallyl anions. It could proceed either under transition-metal-catalyzed conditions or under transition-metal-free conditions. Importantly, this approach afforded trans-3-vinyl-4-aminochromanes with high diastereoselectivity, while conventional, nonumpolung methods often display high cis-selectivity.

Exploring the Steric and Electronic Factors Governing the Regio- and Enantioselectivity of the Pd-Catalyzed Decarboxylative Generation and Allylation of 2-Azaallyl Anions

The impact of the steric and electronic factors in both the para-substituted benzaldimine and 2,2-diarylglycine components on the regioselectivity and enantioselectivity of the palladium-catalyzed decarboxylative allylation of allyl 2,2-diarylglycinate aryl imines was explored. These studies revealed that using 2,2-di(2-methoxyphenyl)glycine as the amino acid linchpin allowed for the exclusive synthesis of the desired homoallylic benzophenone imine regioisomers, independent of the nature of the imine moiety, in typically high yields. The resulting enantiomeric ratios, however, are slightly decreased in comparison to the transformations involving the corresponding allyl 2,2-diphenylglycinate imines, but this is more than balanced out by the increases in yield and regioselectivity. Overall, these studies suggest a general strategy for the highly regioselective functionalization of 2-azaallyl anions.

Umpolung Synthesis of 1,3-Amino Alcohols: Stereoselective Addition of 2-Azaallyl Anions to Epoxides

We report the direct preparation of 1,3-amino alcohols that contain up to three contiguous stereogenic centers by the umpolung coupling of imines and epoxides. Nucleophilic 2-azaallyl anions, generated from imines, are stereoselectively added to epoxides to furnish 1,3-amino alcohols after hydrolysis of the product imine. Transformations afford amino alcohols with >98% site selectivity with respect to both reaction partners and in up to >98% yield and >20:1 dr.

Transition-metal-free chemo- and regioselective vinylation of azaallyls

Direct C(sp³)-C(sp²) bond formation under transition-metal-free conditions offers an atom-economical, inexpensive and environmentally benign alternative to traditional transition-metal-catalysed cross-coupling reactions. A new chemo- and regioselective coupling protocol between 3-aryl-substituted-1,1-diphenyl-2-azaallyl derivatives and vinyl bromides has been developed. This is the first transition-metal-free cross-coupling of azaallyls with vinyl bromide electrophiles and delivers allylic amines in excellent yields (up to 99%). This relatively simple and mild protocol offers a direct and practical strategy for the synthesis of high-value allylic amine building blocks that does not require the use of transition metals, special initiators or photoredox catalysts. Radical clock experiments, electron paramagnetic resonance studies and density functional theory calculations point to an unprecedented substrate-dependent coupling mechanism. Furthermore, an electron paramagnetic resonance signal was observed when the N-benzyl benzophenone ketimine was subjected to silylamide base, supporting the formation of radical species upon deprotonation. The unique mechanisms outlined herein could pave the way for new approaches to transition-metal-free C-C bond formations.

Decarboxylative Umpolung of conjugated enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde

Decarboxylative Umpolung converts enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde group with excellent regio- and diastereoselectivities.

Catalytic Asymmetric Synthesis of Chiral γ-Amino Ketones via Umpolung Reactions of Imines

The first direct catalytic asymmetric synthesis of γ-amino ketones was realized by the development of a highly diastereoselective and enantioselective C-C bond-forming umpolung reaction of imines and enones under the catalysis of a new cinchona alkaloid-derived phase-transfer catalyst. In a loading ranging from 0.02 to 2.5 mol %, the catalyst activates a broad range of trifluoromethyl imines and aldimines as nucleophiles to engage in chemo-, regio-, diastereo- and enantioselective C-C bond-forming reactions with acyclic and cyclic enones, thereby converting these readily available prochiral starting materials into highly enantiomerically enriched chiral γ-amino ketones in synthetically useful yields. Enabled by this unprecedented umpolung reaction of imines, conceptually new and concise routes were developed for the asymmetric synthesis of nitrogen-heterocycles such as pyrrolidines and indolizidines.