Control of the Stereochemistry of Kinetic Protonation: Intramolecular Proton Delivery(1)(,)(2)

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Domino Aza-Annulations of Enynyl-/(Alkynyl)aryl-acetonitriles to Access Nitrogen-Enriched Heterocycles

Unprecedented domino aza-annulations of (E)-2-en-4-ynyl-acetonitriles (generated from the Morita-Baylis-Hillman acetates of propiolaldehydes for the first time) with sodium azide under metal- and oxidant-free conditions for the assembly of triazolo-pyridines are accomplished. The developed strategy offers broad substrate scope, extending to (2-alkynyl)aryl and indolyl-acetonitriles to provide the corresponding triazolo-fused isoquinolines and β-carbolines, respectively, in good yields. Additionally, the synthetic utility of the products is demonstrated via denitrogenative coupling of fused triazoles with different nucleophiles.

Asymmetric Radical Process for General Synthesis of Chiral Heteroaryl Cyclopropanes

A highly efficient catalytic method has been developed for asymmetric radical cyclopropanation of alkenes with in situ-generated α-heteroaryldiazomethanes via Co(II)-based metalloradical catalysis (MRC). Through fine-tuning the cavity-like environments of newly-synthesized D2-symmetric chiral amidoporphyrins as the supporting ligand, the optimized Co(II)-based metalloradical system is broadly applicable to α-pyridyl and other α-heteroaryldiazomethanes for asymmetric cyclopropanation of wide-ranging alkenes, including several types of challenging substrates. This new catalytic methodology provides a general access to valuable chiral heteroaryl cyclopropanes in high yields with excellent both diastereoselectivities and enantioselectivities. Combined computational and experimental studies further support the underlying stepwise radical mechanism of the Co(II)-based olefin cyclopropanation involving α- and γ-metalloalkyl radicals as the key intermediates.

Denitrogenative Transformations of Pyridotriazoles and Related Compounds: Synthesis of N-Containing Heterocyclic Compounds and Beyond

The high demand for new and efficient routes toward synthesis of nitrogen-containing heterocyclic scaffolds has inspired organic chemists to discover several methodologies over recent years. This Perspective highlights one standout approach, which involves the use of pyridotriazoles and related compounds in denitrogenative transformations. Readily available pyridotriazoles undergo ring-chain isomerization to produce uniquely reactive α-diazoimines. Such reactivity, enabled by metal catalysts, additives, or visible-light irradiation, can be applied in transannulation, insertion, cyclopropanation, and many other transformations.

Assembly of Thiosubstituted Benzoxazoles via Copper-Catalyzed Coupling of Thiols with 5-Iodotriazoles Serving as Diazo Surrogates

An efficient cascade approach to thiosubstituted benzoxazoles has been developed. The transformation starts with in situ generation of a diazo compound via annulation-triggered electrocyclic opening of the 1,2,3-triazole ring. The subsequent Cu-catalyzed trapping of diazo intermediates by various thiols affords the desired heterocycles in generally good yields of up to 91%. The protocol features very good functional group tolerance and is applicable to substrates with different electronic properties.

Light-induced metal-free transformations of unactivated pyridotriazoles

A highly efficient and practical method for incorporation of the arylmethylpyridyl moiety into diverse molecules has been developed. This method features the transition metal-free light-induced room temperature transformation of pyridotriazoles into pyridyl carbenes, which are capable of smooth arylation, X-H insertion, and cyclopropanation reactions. The synthetic usefulness of the developed method was illustrated in a facile synthesis of biologically active molecules.

Catalyst-free oxidative N–N coupling for the synthesis of 1,2,3-triazole compounds with tBuONO

A catalyst-free oxidative N–N coupling with tBuONO has been developed for the synthesis of 1,2,3-triazole compounds.

Diastereomeric Reissert Compounds of Isoquinoline and 6,7-Dimethoxy-3,4-dihydroisoquinoline in Stereoselective Synthesis

Chiral acid chlorides were reacted with isoquinoline and 6,7-dimethoxy-3,4-dihydroisoquinoline to form diastereomeric Reissert compounds 8-11 and 18-21, respectively. The best diastereoselectivity (80:20) was achieved in formation of the 9-phenylmenthyl derivative 20. The diastereomers of 2-l-menthoxycarbonyl-1,2-dihydroisoquinaldonitriles (S)-8/(R)-8), formed in equal amounts, were inseparable. However, the individual diastereomers of 2-cholesteryloxycarbonyl-1,2-dihydroisoquinaldonitriles ((R)-11 and (S)-11) and the 2-l-menthoxycarbonyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinaldonitriles ((S)-19/(R)-19)) were each readily purified. (S)-8/(R)-8 (1:1) via the corresponding anions (NaH, -40 degrees C, DMF) with pivaldehyde yielded in 82:18 predominance the S-diastereomer of 1-isoquinolyl tert-butyl carbinyl l-menthyl carbonate ((S)-12), which was obtained in pure form by a single recrystallization; hydrolysis produced 99% pure S-(-)-1-isoquinolyl tert-butyl carbinol [(S)-16]. Reactions of the anions of diastereomeric Reissert compounds, either as mixtures or pure single species, with aromatic aldehydes and alkyl halides proceeded with at best modest selectivity (diastereomeric ratios up to 66:34 and 72:28, respectively). Therefore, it is concluded that the Reissert anions are either planar or rapidly inverting tetrahedral structures.

Control of Stereoselective Protonation of Enols1,2

A decalyl framework with a siloxy enolic moiety and proximate proton transferring groups was synthesized. On enolate generation with fluoride two competitive reaction modes were possible: (a) intermolecular protonation, and (b) intramolecular proton transfer by the proximate group. Control of the protonation stereochemistry proved possible by varying the proximate group and by changing the acidity of the medium. With the groups -CH2OH, -CH=O, and -CH2OCH2OCH3 as the proximate groups, only intermolecular proton transfer was observed with no dependence on acidity. In contrast, with -COO- and COOH, only intramolecular protonation resulted but again with no dependence on acidity of the medium. In contrast, with -CH2NH2 as the proximate group, intramolecular proton transfer predominated with a dependence on the effective pH of the medium. A kinetic analysis provided a linear-log relationship of the ratio of the two stereoisomers with the medium acidity. The analysis revealed that two acetic acid molecules are involved in providing the proton to the enolate moiety. A theoretical analysis was developed paralleling the experimental results. In the ketonization transition state, the hybridization was shown to be close to sp2 hybridized at the alpha-enolate carbon.

Control of Proton Transfer: Intramolecular vs Intermolecular1,2

[reaction: see text] Proton transfer in ketonization of enolates is a critical step in a myriad of organic reactions. Its stereochemistry has been the object of our studies since we reported kinetic protonation from the less hindered face of the molecule under kinetic control some decades ago. Very recently, we have succeeded in reversing the stereochemistry using 2-pyridyl groups to deliver the proton. We now report intramolecular delivery by other moieties and control of intramolecular versus intermolecular proton delivery.

The α-Effect in the Stereochemistry of Kinetic Ketonization of Enols1,2

Kinetic control of the stereoselectivity of protonation of enolates and other strongly delocalized anionic species is involved in a large number of organic reactions. Protonation occurring from the less hindered side of the, e.g., enolic system affords the less stable of two diastereomers. However, one apparent discrepancy has been in the synthesis of prostaglandins. The present research deals with the source of this behavior. A curious effect of the substituent at the enolic alpha carbon was uncovered. In certain instances an alpha substituent is forced to twist into a conformation blocking the proton donor from its side, thus reversing the stereochemistry of protonation. In the course of this research, a number of five-ring enols of varying structure were investigated. Finally, the ketonization reaction course has been studied theoretically.

Inter- and intramolecular stereoselective protonation of enols(1,2)

Kinetic control of the stereoselectivity of protonation of enolates and other delocalized species commonly affords the less stable diastereomer as a consequence of the considerable exothermicity and resulting sp(2) transition-state hybridization of the alpha-carbon. Protonation then is from the less hindered face of the enolate. The present study is aimed at reversing this phenomenon by intramolecular delivery of the proton. The approach employed required the synthesis of two enolate precursors, one with a 2-pyridyl group strategically close to the alpha-carbon and the other with a phenyl group in the same location. The synthesis required 15 steps and involved new methodology. Intramolecular proton transfer, reversing the usual stereoselectivity, was successful. The selectivity proved to depend on several factors including the exo versus endo configuration of the diastereomer reacting, the proton donor employed, and the concentration of the proton donor. A kinetic analysis permitted the determination of the relative reaction orders of the protonation on the two faces of the enolate.

Intramolecular proton transfer

[structure: see text] Reversal of the normal kinetic protonation stereochemistry results as a consequence of intramolecular delivery.

Nitrile anions: solvent-dependent cyclizations

Extensive cyclizations in hydrocarbon and polar solvents demonstrate a profound solvent sensitivity for intramolecular nitrile anion alkylations. S(N)i cyclizations enforce very precise steric constraints in the transition state, allowing correlation of the cyclization stereochemistry with the orbital orientation of the nitrile anion. Collectively the cyclizations suggest a continuum of nitrile anion transition states, varying from planar to fully pyramidal, that selectively cyclize to cis- and trans-decalins, respectively.