Decennary Update on Oxidative-Rearrangement Involving 1,2-Aryl C-C Migration Around Alkenes: Synthetic and Mechanistic Insights

In recent years, numerous methodologies on oxidative rearrangements of alkenes have been investigated, that produce multipurpose synthons and heterocyclic scaffolds of potential applications. The present review focused on recently established methodologies for oxidative transformation via 1,2-aryl migration in alkenes (2013-2023). Special emphasis has been placed on mechanistic pathways to understand the reactivity pattern of different substrates, challenges to enhance selectivity, the key role of different reagents, and effect of different substituents, and how they affect the rearrangement process. Moreover, synthetic limitations and future direction also have been discussed. We believe, this review offers new synthetic and mechanistic insight to develop elegant precursors and approaches to explore the utilization of alkene-based compounds for natural product synthesis and functional materials.

Metal-Free Syntheses of α-Ketothioamide and α-Ketoamide Derivatives from Sulfoxonium Ylides

An efficient base, additive and metal-free synthetic methods for α-ketothioamide and α-ketoamide derivatives from readily available sulfoxonium ylides have been described. Sulfoxonium ylides with primary or secondary amines afforded α-ketothioamides in the presence of elemental sulfur, whereas α-ketoamides were produced when I2 and TBHP were present. The reaction proceeded well at room temperature and generated the corresponding molecules in good to excellent yields. The reaction can be scaled-up and tolerated by a range of functional groups with simple operational procedures.

Iodine-TBHP mediated efficient synthesis of α-ketoamides from vinyl azides and amines under mild conditions

A convenient and practical synthetic approach for α-ketoamides has been developed under mild conditions. The facile synthesis of α-ketoamides has been accomplished using aryl vinyl azides and secondary amines at room temperature. The inexpensive and readily available iodine and TBHP easily promoted the oxidative amidation process to afford diketoamide derivatives in high yields. This method involves the synthesis of ketoamide compounds via sequential C-O and C-N bond formation. Moreover, metal-free and aerobic conditions and wide substrate scope are the notable advantages of this protocol.

Mg(OtBu)2-catalyzed C–H oxidation of α-azido arylethanones using TBHP as the oxidant and carbonyl oxygen source: facile access to primary α-ketoamides

A Mg(OtBu)2-catalyzed C(sp3)–H oxidation of α-azido arylethanones to straightforwardly produce primary α-ketoamides is presented.

Microwave-Enhanced Coupling of Carboxylic Acids with Liquid Ketones and Cyclic Ethers Using Tetrabutylammonium Iodide/t-Butyl Hydroperoxide

The oxidative coupling of carboxylic acids with liquid ketones and cyclic ethers has been accomplished in minutes using t-butyl hydroperoxide in the presence of tetrabutylammonium iodide under microwave irradiation in the absence of a solvent. In addition to drastically shortening the reaction times, the use of microwaves resulted, in general, in yields equal to or higher than those obtained by conventional heating.

Copper-Catalyzed Oxidative Synthesis of α-Ketoamides from Aryl Methyl Ketones and N-Bromobutanimide Using N,N-Dimethylformamide as Dimethylamine Source

A novel and practical Cu(OAc)2-catalyzed oxidative synthesis of α-ketoamides from aryl methyl ketones and N-bromobutanimide (NBS) using N,N-dimethylformamide (DMF) as dimethylamine (HNMe2) source and solvent has been developed under mild conditions. DMF was used as a HNMe2 source and can be easily converted into HNMe2 by acid hydrolysis. The mechanistic studies indicate that Cu(OAc)2 plays a dual role in providing both catalyst and oxidant.

Mechanistic Insights into La-Catalyzed Amidation of Aldehyde with Amine

A new mechanism of La-catalyzed amidation of N-methylbenzylamine with p-chlorobenzaldehyde has been computationally proposed, where L₂La[NR₁R₂] (I) rather than previously proposed L₂La[OCHRNR₁R₂] (II) is the catalytically active species. Interestingly, the side-product alcohol acting as a proton relay to reduce reaction energy barrier could participate in the regeneration of I. Besides, DFT calculations suggest that an addition of alcohol additive into the initial reaction system could accelerate the reaction, which has been further verified by experiments.

Direct intramolecular double cross-dehydrogentive-coupling (CDC) cyclization of N-(2-pyridyl)amidines under metal-free conditions

A facile transition-metal-free protocol to form 2-iminoimidazo[1, 2-a]-pyridines bearing a -CHBr₂ group and an aza-quaternary carbon center at the 3 position from N-(2-pyridyl)amidines substrates, in which the new heterocyclic skeletons constructed from amidines via radical reactions or nucleophilic substitution reactions are promoted only by CBr₄ under mild conditions, is demonstrated. The reactions were realized by intramolecular CDC reaction involving C-N and C-C bond formation via the sequential C(sp³)-H bifunctionalization mode on the same carbon atom under mild conditions. Moreover, this work also provides an excellent and representative example for CBr₄ as an efficient reagent to initiate radical reactions under initiator-free conditions or to give rise to nucleophilic substitution reactions only by base.

Direct synthesis of 2-oxo-acetamidines from methyl ketones, aromatic amines and DMF via copper-catalyzed C(sp3)-H amidination

A convenient method for the synthesis of 2-oxo-acetamidines from methyl ketones using aromatic amines and DMF as nitrogen sources is reported via copper-catalyzed C(sp3)-H amidination. Various methyl ketones react readily with aromatic amines and DMF, producing 2-oxo-acetamidines in yields of 47 to 92%. This protocol features the simultaneous formation of C-N and C[double bond, length as m-dash]N bonds using DMF and aromatic amines as two different nitrogen sources. It thus provides an efficient approach to construct acyclic amidines via three C(sp3)-H bond amidination. Based on the preliminary experiments, a plausible mechanism of this transformation is disclosed.

Cu-Catalyzed carbamoylation versus amination of quinoline N-oxide with formamides

Carbamoylation and amination of quinoline with formamides was developed via copper-catalyzed C–C and C–N bond formations.

Selective synthesis of aryl thioamides and aryl-α-ketoamides from α-oxocarboxylic acids and tetraalkylthiuram disulfides: an unexpected chemoselectivity from aryl sulfonyl chlorides

Tosyl chloride-controlled generation of thioamides and α-ketoamides via base-promoted decarboxylative reaction of α-oxocarboxylic acids with tetraalkylthiuram disulfides has been established.

TBHP-mediated oxidative synthesis of substituted pyrimido[4,5-d]pyrimidines fromN-uracil amidines and methylarenes under metal free conditions

An efficient and operationally simple protocol has been demonstrated for the synthesis of 1,3,5,7-tetrasubstituted pyrimido[4,5-d]pyrimidines via TBHP-mediated direct oxidative coupling of N-uracil amidines and methylarenes under metal-free conditions. Due to the inherent stability of methylarenes compared to aldehydes, the presented synthetic protocol is adaptable to a broad substrate scope, is operationally simple, has no need for stringent protection in the whole preparation process, and has the potential to prepare valuable products that are currently inaccessible or challenging to prepare using conventional methods. It is a significantly important complement to the conventional synthetic methods. The reaction possesses an efficient tandem oxidation–imination–cyclization process.

An efficient and operationally simple protocol has been described for the synthesis of 1,3,5,7-tetrasubstituted pyrimido[4,5-d]pyrimidines via TBHP-mediated direct oxidative coupling of N-uracil amidines and methylarenes under metal-free conditions.

Copper-catalyzed coupling of anthranils and α-keto acids: direct synthesis of α-ketoamides

Copper-catalyzed coupling of α-keto acids with anthranils is reported for the synthesis of α-ketoamides bearing an aldehyde group via N–O/C–O bond cleavages and C–N bond formation.

Metal-Free Oxidative Thioesterification of Methyl Ketones with Thiols/Disulfides for the Synthesis of α-Ketothioesters

A direct C_sp3-H bond oxidative thioesterification of methyl ketones with aromatic thiols/disulfides promoted by TBAI/K₂S₂O₈ has been developed. The reaction provides successfully a simple and efficient method for the synthesis of functionalized α-ketothioesters of aromatic thiols. This practical methodology exhibits readily available starting materials, large-scale applicability, synthetic application, and broad functional group tolerance. A possible mechanism for the transformation is proposed.

Direct defluorinative amidation-hydrolysis reaction of gem-difluoroalkenes with N,N-dimethylformamide, and primary and secondary amines

A novel and efficient method for the synthesis of arylacetamides by the reactions of gem-difluoroalkenes with N,N-dialkylformamides, and primary and secondary amines with the assistance of KOtBu and water was developed.

A metal-free synthesis of 2-aminobenzothiazoles through aminyl radical addition to aryl isothiocyanates

A convenient synthesis of 2-aminobenzothiazoles, starting from aryl isothiocyanates and formamides under metal-free conditions, is described. Various secondary and tertiary amine- and even α-amino acid-derived formamides can be used as amino sources in this process. Mechanistic studies suggest that the reaction is initiated by decarbonylative aminyl radical formation in the presence of n-Bu4NI and TBHP, followed by aminyl radical addition to isothiocyanates and cyclization via sulfur centred radical intermediates.

Solvent-promoted catalyst-free N-formylation of amines using carbon dioxide under ambient conditions

An unprecedented catalyst-free formylation of amines using CO2 and hydrosilanes was developed. The solvent plays a vital role in promoting the interaction of amines with hydrosilanes and subsequent CO2 insertion, thus facilitating the simultaneous activation of N-H and Si-H bonds. Based on relevant mechanistic studies, a plausible mechanism involving a silyl carbamate intermediate is proposed.

nBu4NI-catalyzed oxidative cross-coupling of carbon dioxide, amines, and aryl ketones: access to O-β-oxoalkyl carbamates

The first nBu₄NI-catalyzed oxidative cross-coupling reaction of carbon dioxide, amines and arylketones leading to O-β-oxoalkyl carbamates is reported.

nBu4NI-Mediated oxidation of methyl ketones to α-ketoamides: using ammonium, primary and secondary amine-salt as an amine moiety

Presented is the first example of synthesizing an array of primary-, secondary-, and tertiary-α-ketoamides in moderate to excellent yields with a catalyst nBu₄NI from methyl ketones and amine/ammonium salts under mild conditions using oxidant TBHP.

Copper(ii)-mediated formation of oxazole-4-carbonitrile from acetophenone and coordinated cyanide anion via a radical coupling

A protocol for the direct synthesis of 4-aryloxazole-5-carbonitrile from acetophenone was first described.

Synthesis of benzyl esters from the commercially available alcohols catalyzed by TBAI via C(sp3)–H bond functionalization

The direct esterification of alcohol catalyzed by TBAI via C(sp³)–H bond functionalization was achieved under mild and clean conditions with good to excellent yields.