Α-aminoalkyl Radicals Research Articles

On the complex •OH/•O−-induced free radical chemistry of arylalkylamines with special emphasis on the contribution of the alkylamine side chain

A full account of the •OH-induced free radical chemistry of an arylalkylamine is given taking all the possible reaction pathways quantitatively into consideration. Such knowledge is indispensable when the alkylamine side chain plays a crucial role in biological activity. The fundamental reactions are investigated on the model compound N-methyl-3-phenypropylamine (MPPA), and extended to its biologically active analog, to the antidepressant fluoxetine (FLX). Pulse radiolysis techniques were applied including redox titration and transient spectral analysis supplemented with DFT calculations. The contribution of the amine moiety to the free radical-induced oxidation mechanism appeared to be appreciable. •O− was used to observe hydrogen atom abstraction events at pH 14 giving rise to the strongly reducing α-aminoalkyl radicals (∼38% of the radical yield) and to benzyl (∼4%), β-aminoalkyl (∼24%), and aminyl radicals (∼31%) of MPPA. One-electron transfer was also observed yielding aminium radicals with low efficiency (∼3%). In the •OH-induced oxidation protonated α-aminoalkyl (∼49%), β-aminoalkyl (∼27%), benzyl radicals (∼4%), and aminium radicals (∼5%) are initially generated on the side chain of MPPA at pH 6, whereas hydroxycyclohexadienyl radicals (∼15%) were also produced. These initial events are followed by complex protonation–deprotonation reactions establishing acid–base equilibria; however, these processes are limited by the transient nature of the radicals and the kinetics of the ongoing reactions. The contribution of the radicals from the side chain alkylamine substituent of FLX totals up to ∼54% of the initially available oxidant yield.

Photoredox/Brønsted Acid Co-Catalysis Enabling Decarboxylative Coupling of Amino Acid and Peptide Redox-Active Esters with N-Heteroarenes

An iridium photoredox catalyst in combination with a phosphoric acid catalyzes the decarboxylative α-aminoalkylation of natural and unnatural α-amino acid-derived redox-active esters (N-hydroxyphthalimide esters) with a broad substrate scope of N-heteroarenes at room temperature under irradiation. Dipeptide- and tripeptide-derived redox-active esters are also amenable substrates to achieve decarboxylative insertion of a N-heterocycle at the C-terminal of peptides, yielding molecules that have potential medicinal applications. The key factors for the success of this reaction are the following: use of a photoredox catalyst of suitable redox potential to controllably generate α-aminoalkyl radicals, without overoxidation, and an acid cocatalyst to increase the electron deficiency of N-heteroarenes.

Polarity-Reversed Allylations of Aldehydes, Ketones, and Imines Enabled by Hantzsch Ester in Photoredox Catalysis.

The polarity reversal (umpolung) reaction is an invaluable tool for reversing the chemical reactivity of carbonyl and iminyl groups, which subsequent cross-coupling reactions to form C-C bonds offers a unique perspective in synthetic planning and implementation. Reported herein is the first visible-light-induced polarity-reversed allylation and intermolecular Michael addition reaction of aldehydes, ketones, and imines. This chemoselective reaction has broad substrate scope and the engagement of alkyl imines is reported for the first time. The mechanistic investigations indicate the formation of ketyl (or α-aminoalkyl) radicals from single-electron reduction, where the Hantzsch ester is crucial as the electron/proton donor and the activator.

ChemInform Abstract: Synthetic Utilization of α‐Aminoalkyl Radicals and Related Species in Visible Light Photoredox Catalysis

AbstractReview: 68 refs.

Synthetic Utilization of α-Aminoalkyl Radicals and Related Species in Visible Light Photoredox Catalysis.

Single electron oxidation of amines provides an efficient way to access synthetically useful α-aminoalkyl radicals as reactive intermediates. After the single electron oxidation of amines, fragmentation of the resulting radical cations proceeds to give the α-aminoalkyl radicals along with generation of a proton. In the synthetic utilization of the α-aminoalkyl radicals, precise control of single electron transfer is essential, because further oxidation of the α-aminoalkyl radicals occurs more easily than the starting amines and the α-aminoalkyl radicals are converted into the corresponding iminium ions. As a result, photoinduced single electron transfer is quite attractive in the synthetic utilization of the α-aminoalkyl radicals. Recently, visible light-photoredox catalysis using transition metal-polypyridyl complexes and other dyes as catalysts has attracted considerable attention, where useful molecular transformations can be achieved through the single electron transfer process between the excited catalysts and substrates. In this context, MacMillan et al. ( Science 2011, 334 , 1114 , DOI: 10.1126/science.1213920 ) reported an aromatic substitution reaction of cyanoarenes with amines, where α-aminoalkyl radicals work as key reactive intermediates. Pandey and Reiser et al. ( Org. Lett. 2012 , 14 , 672 , DOI: 10.1021/ol202857t ) and our group ( Nishibayashi et al. J. Am. Chem. Soc. 2012 , 134 , 3338 , DOI: 10.1021/ja211770y ) independently reported reactions of amines with α,β-unsaturated carbonyl compounds, where addition of α-aminoalkyl radicals to alkenes is a key step. After these earliest examples, nowadays, a variety of transformations using the α-aminoalkyl radicals as reactive intermediates have been reported by many groups. The α-aminoalkyl radicals are usually produced from amines by single electron oxidation and the subsequent deprotonation of the C-H bond adjacent to the nitrogen atom. In addition, the α-aminoalkyl radicals are also produced from α-silylamines and α-amino acids in high efficiency through desilylation or decarboxylation after the single electron oxidation. The generated α-aminoalkyl radicals are utilized in a variety of reaction systems. In fact, reactions based on the addition of α-aminoalkyl radicals to alkenes and other unsaturated bonds have been extensively studied. Aromatic and other types of substitution reactions have also been investigated. Some of these transformations are achieved by combination of photoredox catalysts and other catalysts such as Brønsted and Lewis acids, organocatalysts, and transition metal catalysts. It is also noteworthy that the enantioselective reactions have been accomplished by combination of photoredox catalysts and chiral catalysts. The strategy for the generation of α-aminoalkyl radicals can be applied to utilize other types of alkyl radicals. In the generation of α-aminoalkyl radicals, the bond dissociation of the radical cations occurs at the α-position of amines. In relation to this process, synthetic utilization of other types of alkyl radicals generated by the bond dissociation of the radical cations at a remote position has been also investigated. These alkyl radicals have been applied to molecular transformations in a manner similar to the α-aminoalkyl radicals. Recently, organic synthesis using the α-aminoalkyl radicals and related alkyl radicals has been studied extensively. In this Account, we describe recent advances in photoredox-catalyzed synthetic utilization of these alkyl radicals.

Monofluoroalkenylation of Dimethylamino Compounds through Radical–Radical Cross‐Coupling

An unprecedented and challenging radical-radical cross-coupling of α-aminoalkyl radicals with monofluoroalkenyl radicals derived from gem-difluoroalkenes was achieved. This first example of tandem C(sp(3) )-H and C(sp(2) )-F bond functionalization through visible-light photoredox catalysis offers a facile and flexible access to privileged tetrasubstituted monofluoroalkenes under very mild reaction conditions. The striking features of this redox-neutral method in terms of scope, functional-group tolerance, and regioselectivity are illustrated by the late-stage fluoroalkenylation of complex molecular architectures such as bioactive (+)-diltiazem, rosiglitazone, dihydroartemisinin, oleanic acid, and androsterone derivatives, which represent important new α-amino C-H monofluoroalkenylations.

Catching α-aminoalkyl radicals: cyclization between tertiary alkylanilines and alkenes

A practical synthesis of polycyclic heterocycles by radical annulation of tertiary arylamine derivatives is described. Radical annulation occurs via formation of α-aminoalkyl radicals. The reaction is initiated by tetrabutylammonium iodide using tert-butyl hydroperoxide as oxidant. The developed method allows functionalization of diverse tertiary alkylanilines with different alkenes.

Drugs with susceptible sites for free radical induced oxidative transformations: the case of a penicillin

Penicillins, as bactericidal antibiotics, have been widely used to treat infections for several decades. Their structure contains both aromatic and thioether moieties susceptible to free radical oxidation. The •OH induced oxidation mechanism of amoxicillin was investigated by pulse radiolysis techniques and by final product analysis performed after steady-state γ-irradiation. The predominant sites of the •OH attack are suggested to be the thioether group, initially yielding an •OH adduct to the sulfur, and the aromatic ring. This adduct to the sulfur converts to sulfur radical cation, which has three competitive reaction paths: (1) by deprotonation at the adjacent carbon α-(alkylthio)alkyl radicals form, which undergo disproportionation leading presumably to sulfoxide as main product; (2) via the pseudo-Kolbe mechanism it may transform to α-aminoalkyl radicals; (3) the radical cation can be stabilized through intramolecular S.˙.O bond formation. The reaction mechanism suggests the presence of a short-living and a stabilized (via hydrogen bonding) long-living •OH adduct to the sulfur. The three-electron bonded dimers of amoxicillin were not formed owing to steric hindrance. Thiyl radicals were also present in equilibrium with α-aminoalkyl radicals. In the presence of dissolved oxygen, aromatic ring hydroxylation occurred along with complex reactions resulting in e.g. oxidation of the methyl groups. The formation of the sulfoxide is especially effective in the presence of dissolved oxygen, under anaerobic condition, however, it is also generated owing to H2O2 and α-(alkylthio)alkyl radicals. The thioether moiety appears to be more sensitive to oxidation compared to the aromatic ring in case of amoxicillin.

A Highly Efficient Gold-Catalyzed Photoredox α-C(sp(3))-H Alkynylation of Tertiary Aliphatic Amines with Sunlight.

A new α-C(sp(3))H alkynylation of unactivated tertiary aliphatic amines with 1-iodoalkynes as radical alkynylating reagents in the presence of [Au2(μ-dppm)2](2+) in sunlight provides propargylic amines. Based on mechanistic studies, a C-C coupling of an α-aminoalkyl radical and an alkynyl radical is proposed for the C(sp(3))-C(sp) bond formation. The mild, convenient, efficient, and highly selective C(sp(3))-H alkynylation reaction shows excellent regioselectivity and good functional-group compatibility. A scale-up to gram quantities is possible with sunlight used as a clean and sustainable energy source.

Visible light photoredox catalysis: regioselective radical addition of aminoalkyl radicals to 2,3-allenoates

Visible-light-mediated regioselective addition of α-aminoalkyl radicals to 2,3-allenoates catalyzed by a photoredox catalyst was developed. The scope and generality of the substrates were broad.

Inside Cover: Interception of Cobalt-Based Carbene Radicals with α-Aminoalkyl Radicals: A Tandem Reaction for the Construction of β-Ester-γ-amino Ketones (Angew. Chem. Int. Ed. 4/2015)

Inside Cover: Interception of Cobalt-Based Carbene Radicals with α-Aminoalkyl Radicals: A Tandem Reaction for the Construction of β-Ester-γ-amino Ketones (Angew. Chem. Int. Ed. 4/2015)

A new synthesis of indoles via intramolecular cyclization of ο-alkynyl N,N-dialkylanilines promoted by KOt-Bu/DMSO

2-Aryl indoles could be prepared in excellent yields via the intramolecular cyclization of ο-alkynyl N,N-dialkylanilines. The reaction is efficiently promoted by the catalytic amount of KOt-Bu in DMSO at room temperature. A reaction mechanism involving α-aminoalkyl radical intermediates is suggested.

Interception of cobalt-based carbene radicals with α-aminoalkyl radicals: a tandem reaction for the construction of β-ester-γ-amino ketones.

The interception of cobalt-based carbene radicals with α-aminoalkyl radicals was combined with the Kornblum-DeLaMare reaction and provides β-ester-γ-amino ketones, which are otherwise difficult to obtain in high chemoselectivity. Mechanistically, this transformation is an interplay of cobalt-based carbene radicals, organoradicals, and ionic intermediates and involves the construction of two C-C bonds and one C=O bond in a one-pot process. The reaction also features a wide substrate scope and is highly efficient and insensitive to moisture and air.

ChemInform Abstract: Visible‐Light‐Mediated Addition of α‐Aminoalkyl Radicals to [60]Fullerene by Using Photoredox Catalysts.

The functionalization of fullerene has been extensively studied and various fullerene derivatives have been synthesized. We have succeeded in the functionalization of [60]fullerene by using α-aminoalkyl radicals generated by visible-light-mediated single-electron oxidation of α-silylamines as synthetic intermediates. In these reactions, the introduction of diarylamino groups, which are useful electron donors, has been easily achieved.

Synthesis of 3-acylindoles by visible-light induced intramolecular oxidative cyclization of o-alkynylated N,N-dialkylamines.

A visible-light photoredox synthesis of 3-acylindoles through intramolecular oxidative cyclization of o-alkynylated N,N-dialkylamines is developed. The reaction proceeds effectively under mild reaction conditions using air as the oxidant, and only water is generated as a side product. A plausible mechanism involving the addition of α-amino alkyl radicals to alkynes, followed by C-O bond formation, is proposed.

ChemInform Abstract: Intramolecular Dehydrative Coupling of Tertiary Amines and Ketones Promoted by KO‐t‐Bu/DMF: A New Synthesis of Indole Derivatives.

A new synthesis of indole derivatives has been achieved through intramolecular dehydrative coupling of tertiary amines and ketones promoted by KO-t-Bu/DMF. The reaction probably proceeds via an α-amino alkyl radical pathway.

Visible‐Light‐Mediated Addition of α‐Aminoalkyl Radicals to [60]Fullerene by Using Photoredox Catalysts

The functionalization of fullerene has been extensively studied and various fullerene derivatives have been synthesized. We have succeeded in the functionalization of [60]fullerene by using α-aminoalkyl radicals generated by visible-light-mediated single-electron oxidation of α-silylamines as synthetic intermediates. In these reactions, the introduction of diarylamino groups, which are useful electron donors, has been easily achieved.

Synthesis of nitrogen heterocycles via α-aminoalkyl radicals generated from α-silyl secondary amines under visible light irradiation.

We have succeeded in the visible light-mediated synthetic use of α-aminoalkyl radicals derived from α-silyl secondary amines toward addition to α,β-unsaturated carbonyl compounds. The resulting γ-aminocarbonyl compounds are converted into γ-lactams and pyrroles in a one-pot process.

Nucleophilic Attack of α‐Aminoalkyl Radicals on CarbonNitrogen Triple Bonds to Construct α‐Amino Nitriles: An Experimental and Computational Study

A new reactivity pattern of α-aminoalkyl radicals, involving nucleophilic attack on C≡N triple bonds under thermal conditions, has been developed to construct α-amino nitriles. In contrast to previous C-H functionalization of tertiary amines involving α-aminoalkyl radicals, this methodology does not require the use of photocatalytic conditions or a transition-metal catalyst. Inexpensive and nontoxic phenylacetonitrile was chosen as cyano source for this α-aminonitrile forming reaction. A plausible mechanism is proposed based upon experimental and computational results. An α-aminoalkyl radical intermediate and benzoyl cyanide have been shown to be key intermediates in this green and mild radical process. Nucleophilic attack of the α-aminoalkyl radical on the C≡N bond of PhCOCN followed by an elimination step forms the desired α-aminonitrile and an acyl radical.

Intramolecular Dehydrative Coupling of Tertiary Amines and Ketones Promoted by KO-t-Bu/DMF: A New Synthesis of Indole Derivatives

A new synthesis of indole derivatives has been achieved through intramolecular dehydrative coupling of tertiary amines and ketones promoted by KO-t-Bu/DMF. The reaction probably proceeds via an α-amino alkyl radical pathway.