A One-Pot Reaction of α-Imino Rhodium Carbenoids and Halohydrins: Access to 2,6-Substituted Dihydro-2H-1,4-oxazines

Dirhodium-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles to 2,3-Dehydropiperazines

The dirhodium(II)-catalyzed synthesis of a range of C2-substituted 2,3-dehydropiperazines using 1-mesyl-1,2,3-triazoles and β-haloalkylcarbamates is reported. The reaction is proposed to proceed through an α-imino rhodium carbene 1,3-insertion into N-H followed by a base-mediated cyclization. C-Substituted dehydropiperazines can also be conducted directly from terminal alkynes in a three-step, one-pot operation, forming the triazole in situ. This methodology has also been expanded to afford several 2,5-disubstituted 2,3-dehydropiperazines as well as a larger 4,5,6,7-tetrahydro-1H-1,4-diazepine derivative.

Synthesis of Oxazolidines and Dihydroxazines via Cyclization of α-Aminated Ketones

A new approach to oxazolidines and dihydroxazines was developed by regioselective cyclization of α-aminated ketones under transition metal-free conditions. Oxazolidine derivatives were generated in the presence of chloro benziodoxole and TFA, while dihydroxazines were formed without a hypervalent iodine reagent. The reaction was performed under room temperature and gave the products in good to excellent yields.

Ring-Opening Cyclization (ROC) of Aziridines with Propargyl Alcohols: Synthesis of 3,4-Dihydro-2H-1,4-oxazines

Activated aziridines react with propargyl alcohols in the presence of Zn(OTf)₂ as the Lewis acid catalyst following an S_N2-type ring-opening mechanism to furnish the corresponding amino ether derivatives. Those amino ethers further undergo intramolecular hydroamination via 6-exo-dig cyclization in the presence of Zn(OTf)₂ as the catalyst and tetrabutylammonium triflate salt as an additive under one-pot two-step reaction conditions. However, for nonracemic examples, ring-opening and cyclization steps were conducted under two-pot conditions. The reaction works well without any additional solvents. The final 3,4-dihydro-2H-1,4-oxazine products were obtained with 13 to 84% yield and 78 to 98% enantiomeric excess (for nonracemic examples).

Rhodium(II)-Catalyzed Highly Selective 1,3-Insertion Reactions Using N-Sulfonyl-1,2,3-Triazoles with Heteroaryl Ethers or Heteroaryl Alcohols

The transformation of N-sulfonyl-1,2,3-triazoles via insertion/rearrangement is achieved using 2-hydroxybenzimidazole or 2-alkoxybenzothiazole over 3 mol % Rh₂(Oct)₄ for the synthesis of α-((1H-benzo[d]imidazol-2-yl)amino) ketones or (Z)-2-alkoxy-2-phenylethenamines. This regio- and stereoselective reaction proceeds under mild conditions, is tolerant of functional groups, and has a broad substrate scope. Notably, while the general rhodium-catalyzed reaction involves sigmatropic rearrangement from an allyl vinyl ether, the present synthetic methodology prevents rearrangement owing to the benzimidazole group, allowing access to (Z)-olefins.

Annulation-Triggered Denitrogenative Transformations of 2-(5-Iodo-1,2,3-triazolyl)benzoic Acids

The ability of [1,2,3]triazolobenzoxazinones to act as a source of "hidden" diazo group was discovered. These diazo precursors can be easily prepared by the intramolecular cyclization of 2-(5-iodo-1,2,3-triazolyl)benzoic acids. The Cu-catalyzed capture of the hidden diazo group allows for further functionalization through the denitrogenative pathway. The transformations proceed via the formation of either diazoimine or diazoamide intermediates. Novel routes to various anthranilamides as well as thiolated benzoxazinones were developed using the one-pot cyclization/diazo capture procedure.

Catalytic Insertion Reactions of α-Imino Carbenoids

Over the past decade, α-imino carbenoids generated via transition metal (such as rhodium, nickel, copper, palladium, silver) catalyzed denitrogenative ring-opening of N-sulfonyl-1,2,3-triazoles have found an extensive account of applications in synthetic organic chemistry. Particularly, they have been widely utilized as a donor/acceptor carbene complex in a range of transformations leading to diverse nitrogen containing compounds and heterocycles. Along the same direction, 3-diazoindolin-2-imines were successfully applied as an alternative source of α-imino carbenoid precursors for the development of a number of methodologies to access diverse indole derivatives. This review summarizes the insertion reactions of α-imino metal carbenes derived from N-sulfonyl-1,2,3-triazoles and 3-diazoindolin-2-imines.

Doyle-Kirmse reaction using 3,3-difluoroallyl sulfide and N-sulfonyl-1,2,3-triazole: an efficient access to gem-difluoroallylated multifunctional quaternary carbon

A Doyle-Kirmse reaction of N-sulfonyl-1,2,3-triazole with 3,3-difluoroallyl sulfide through a Rh(ii)-catalyzed [2,3]-sigmatropic rearrangement has been developed, which provides an efficient access to multifunctional quaternary centers containing aryl, imino, thio, and brominated gem-difluoroallyl groups. The reaction features broad substrate scope with moderate to excellent yields. The applicability of the method is confirmed by gram-scale synthesis and further transformations.

Synthesis of Difluorinated Halohydrins by the Chemoselective Addition of Difluoroenolates to α-Haloketones

α-Haloketones are valuable intermediates in the synthesis of pharmaceuticals and natural products because they display two electrophiles. Although chemoselective additions to each of these functional groups are known, the use of fluorinated nucleophiles has not been characterized, except for the dimerization of fluorohalomethyl ketones. Our studies demonstrate the use of difluoroenolates to create difluorinated bromohydrins and chlorohydrins from α-haloketones without any cyclization or rearrangement due to the mild conditions.

Rhodium-Catalyzed Denitrogenative Diazole-Triazole Coupling toward Aza-Bridged Structures and Imidazole-Based Chelating Ligands

1,4,8-Triazaocta-1,3,5,7-tetraenes, generated in situ by Rh₂(Piv)₄-catalyzed denitrogenative coupling of pyrazoles with 1-sulfonyl-1,2,3-triazoles, smoothly form 2,6,8-triazabicyclo[3.2.1]octa-3,6-dienes via intramolecular aza-Diels-Alder cycloaddition. This domino reaction, combined with the subsequent thermal or acid-catalyzed rearrangement of the cycloadducts, provides direct and flexible access to N-sulfonylated (Z)-2-(2-aminovinyl)imidazoles.

Rh(II)-catalyzed Denitrogenative Cascade of 1,2,3-Triazolyl Propiolates and Indoles: Access to Butenolide Tethered Homotryptamines

An efficient Rh(II)-catalyzed denitrogenative reaction of indoles with 1,2,3-triazolyl propiolates has been developed. This methodology provides facile access to butenolide tethered homotryptamines in good to excellent yields under operationally simple conditions and features a broad substrate scope. Overall, the reaction sequence involves the formation of three new bonds (two C-C and one C-O) in a nucleophilic cascade manner. Additionally, an intramolecular rearrangement of these derivatives to thermodynamically more stable butenolides is also demonstrated.

Rh(ii)-Catalyzed denitrogenative 1-sulfonyl-1,2,3-triazole-1-alkyl-1,2,3-triazole cross-coupling as a route to 3-sulfonamido-1H-pyrroles and 1,2,3-triazol-3-ium ylides

The reaction of 1-alkyl-1H-1,2,3-triazoles with rhodium(ii) azavinyl carbenes, generated from 1-sulfonyl-1H-1,2,3-triazoles, was utilized to prepare 3-sulfonamido-1H-pyrroles and 1,2,3-triazol-3-ium ylides in good yields.