Diastereoselective Spirocyclization of Cyclic N-Sulfonyl Ketimines with Nitroalkenes via Iridium-Catalyzed Redox-Neutral Cascade Reaction

One-Step Syntheses of 3,4-Disubstituted Isochroman-1-ones by the Annulation of Benzoic Acids with Nitroalkenes

The Rh(III)-catalyzed annulation of benzoic acids with nitroalkenes was disclosed to afford a wide range of 3,4-disubstituted isochroman-1-ones with excellent regioselectivity and high catalytic efficiency. Both aromatic and aliphatic nitroalkenes participated in this cyclization reaction successfully. The synthetic value of 3,4-disubstituted isochroman-1-ones was proven by a series of derivatizations. Furthermore, a reliable mechanism is outlined on the basis of experimental investigations and related precedents.

[3+2] Cascade Ring-Closing/Ring-Opening Strategy: Access to N-Indene-Tethered Amino Alcohols

An unprecedented atom-economic redox neutral regioselective Rh(III)-catalyzed cascade [3+2] annulation of 2-aryl oxazoline with α,β-unsaturated nitro olefins has been accomplished, furnishing a novel set of nitro-functionalized indene-tethered amino alcohols through a synergistic ring-closing/ring-opening strategy via the formation of two new C-C bonds and the regioselective cleavage of the C2-O bond of oxazoline under silver free mild reaction conditions with a broad substrate scope.

Enantioselective Synthesis of 1-Aminoindanes via [3 + 2] Annulation of Aldimines with Alkenes by Scandium-Catalyzed C-H Activation

Multisubstituted chiral 1-aminoindanes are important components in many pharmaceuticals and bioactive molecules. Therefore, the development of efficient and selective methods for the synthesis of chiral 1-aminoindanes is of great interest and importance. In principle, the asymmetric [3 + 2] annulation of aldimines with alkenes through C-H activation is the most atom-efficient and straightforward route for the construction of chiral 1-aminoindanes, but such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. Herein, we report for the first time the enantioselective [3 + 2] annulation of a wide range of aromatic aldimines and alkenes via ortho-C(sp2)-H activation by chiral half-sandwich scandium catalysts, which provides a straightforward route for the synthesis of multisubstituted chiral 1-aminoindanes. This protocol features 100% atom-efficiency, broad functional group compatibility, and high regio-, diastereo-, and enantioselectivity (up to >19:1 dr and 99:1 er). Remarkably, by fine-tuning the sterics of the chiral ligand around the catalyst metal center, the diastereodivergent asymmetric [3 + 2] annulation of aldimines and styrenes has been achieved with a high level of diastereo- and enantioselectivity, offering an efficient method for the synthesis of both the trans and cis diastereomers of a novel class of chiral 1-aminoindane derivatives containing two contiguous stereocenters from the same set of starting materials. Moreover, the asymmetric [3 + 2] annulation of aldimines with aliphatic α-olefins, norbornene, and 1,3-dienes has also been achieved.

Synthesis of indene-fused spiro-dibenz(ox)azepines via Rh(III)-catalyzed cascade regioselective C-H activation/annulation

We report an unprecedented atom-economic one-pot Cp*Rh(III)-catalyzed regioselective [3+2]-spiroannulation reaction between dibenz(ox)azepines and ynones, allowing the synthesis of biologically relevant novel spirocyclic dibenz(ox)azepines under operationally simple and mild reaction conditions. The reaction proceeds without any silver additive or external oxidant implementing a redox-neutral pathway. A broad substrate scope with diverse functional group tolerance permitted the regioselective synthesis of a wide spectrum of indene-containing spirocyclic dibenz(ox)azepines in good to excellent yields. Also, we showcased detailed mechanistic studies to justify the formation of spirocycles. In addition, the synthetic utility of this process was also demonstrated by the modular synthesis of various steroid conjugates.

Synthesis of 2-(2-Nitroalkyl)indoles by Rhodium(III)-Catalyzed C-H Alkylation

The Rh(III)-catalyzed addition of the indole C2-H bond to nitroalkenes under an ambient atmosphere is disclosed, providing direct access to a wide range of 2-(2-nitroalkyl)indoles (33 examples) with excellent chemo- and regioselectivity. In addition, pyrrole derivatives also successfully participated in this Friedel-Crafts alkylation reaction. Representative nitroalkane products could be converted into structurally diverse and valuable indole derivatives. Furthermore, a series of control experiments were conducted, and a plausible mechanism was proposed.

Rh-Catalyzed [3+2] Annulation of Cyclic Ketimines and Alkynyl Chloride: A Strategy for Accessing Unsymmetrically Substituted and Highly Functionalizable Indenes

Alkynyl chlorides were found to be extraordinarily novel electrophiles, which could afford a single regioisomer of the [3+2] annulation adducts with cyclic ketimines by rhodium catalysis. The alkenyl chloride moiety in the products provided a valuable functional handle for further diverse transformations. Therefore, this research provided not only a synthetic protocol for accessing unsymmetrically substituted indenyl amines but also a highly divergent solution for decorating the substituting group by postmanipulation of the chloride.

Regio- and Diastereoselective [3 + 2]-Spiroannulation of Benzoxazines with Chalcones: A Rh(III)-Catalyzed Redox-Neutral Approach to α-Aroyl Spiro-Indanamines

We report an atom-economic Rh(III)-catalyzed [3 + 2]-spiroannulation reaction between cyclic ketimines and α,β-unsaturated carbonyl compounds, allowing the synthesis of novel spirocycles with concomitant generation of three stereogenic centers in one pot. The reaction does not require any silver additives or external oxidants and is believed to proceed in a redox-neutral manner. A broad substrate scope with good functional group tolerance permitted the synthesis of a vast spectrum of spirocyclic 1,4-benzoxazine derivatives containing polysubstituted α-aroyl-indanamines in good to excellent yields with high diastereoselectivity.

Construction of C2-Quaternary-indol-3-ones via RhIII-Catalyzed [3+2] Spirocyclization from Indole Ketones and Nitroolefins

Novel complex C2-quaternary-indol-3-one units bearing versatile nitro groups have been successfully developed from pseudo-indolones and α,β-unsaturated nitroolefins through rhodium-catalyzed C-H activation/[3 + 2] spirocyclization. Notably, four diastereomers could be selectively obtained in the reaction by condition control.

Rh(III)-Catalyzed spiroannulation of ketimines with cyclopropenones via sequential C-H/C-C bond activation

An unprecedented Rh(III)-catalyzed [3+3]-spiroannulation of ketimines with cyclopropenones to access spiro[4,5]dienones has been developed. Sequential C-H/C-C bond activation and subsequent nucleophilic addition are disclosed in this process. This procedure represents the first example of the construction of spirolactams utilising cyclopropenones as 3C synthons. The remarkable advantages of this protocol are excellent chemo- and regio-selectivity, wide functional group tolerance, high reaction yields, and tolerance towards H₂O.

A rearrangement of saccharin-derived cyclic ketimines with 3-chlorooxindoles leading to spiro-1,3-benzothiazine oxindoles

An unusual rearrangement of saccharin-derived cyclic ketimines (SDCIs) and 3-chlorooxindoles has been developed to provide a series of spiro-1,3-benzothiazine oxindoles. The reaction features simple manipulations, short reaction times, mild reaction conditions and inexpensive reagents. It is the first example where SDCIs serve as a ring-opening reagent in organic synthesis.

Unmasking the reverse reactivity of cyclic N-sulfonyl ketimines: multifaceted applications in organic synthesis

The chemistry related to the exploration of cyclic N-sulfonyl ketimines and their derivatives has attracted significant attention in the last few decades because of their intriguing structures and properties. They serve broadly as reactive synthons in various reactions to create a diverse set of synthetically and biologically attractive molecules. Furthermore, these moieties, which possess multiple heteroatoms (N, O and S), display or can enhance many biological activities. In the case of synthetic reactions, chemists mainly focus on the chemical manipulation of the highly reactive prochiral CN bond of N-sulfonyl ketimines. Besides their traditional role as electrophiles, N-sulfonyl ketimines possess α-Csp³-H protons, and thus behave as potential carbonucleophiles, where they can undergo several C-X (X = C, N and O) bond-forming reactions with different types of electrophiles under various conditions to form a wide range of fascinating asymmetric and non-asymmetric versions of fused heterocycles, carbocycles, spiro-fused skeletons, pyridines, pyrroles, etc. Herein, we highlight the recent examples from our research work and others covering the scope of cyclic N-sulfonyl ketimines as useful carbonucleophiles. In addition, the detailed mechanistic studies of the above-mentioned reactions are also presented.

Straightforward Access to Anthrone Functionalized Benzylic Amines via Organocatalytic 1,2-Addition of Anthrones to Imines at Ambient Temperature

Activation of anthrone via benzylic deprotonation in the presence of triethylamine paves the way for the 1,2-addition reaction with imines to provide the desired functionalized anthrones in good to excellent yields under mild and operationally simple reaction conditions with a broad range of substrate scopes without using any external additives or toxic stoichiometric reagents.

Theoretical Studies of Rare-Earth-Catalyzed [3 + 2] Annulation of Aromatic Aldimine with Styrene: Mechanism and Origin of Diastereoselectivity

The synthesis of multisubstituted 1-aminoindanes through catalyst-controlled diastereodivergent [3 + 2] annulation of aromatic imines with alkenes is of great interest and importance. An understanding of the exact reaction mechanism, especially for the origin of diastereoselectivity, is an essential aspect for further development of such reactions. In this study, density functional theory calculations have been carried out on the rare-earth-catalyzed diastereodivergent [3 + 2] annulation of benzaldimine with styrene. The results show that the reaction mainly involves generation of active species, olefin insertion, cyclization, and protonation steps. The noncovalent interactions, such as C-H···π and metal···π interactions, play an important role in stabilizing the key transition state or intermediate. Both steric and electronic factors account for the diastereoselectivity. The preferred cis-diastereoselectivity could be ascribed to more efficient orbital interaction, while the crowded space will induce the formation of a C-H···π interaction between the N^tBu group and benzene ring in a trans-diastereoselectivity manner, thus stabilizing the trans-selective transition state. Therefore, the stereospecific product could be obtained by fine-tuning the ligand/metal combination of the catalysts.

Reducing-Agent-Free Convergent Synthesis of Hydroxyimino-Decorated Tetracyclic Fused Cinnolines via RhIII-Catalyzed Annulation Using Nitroolefins

A mild Rh-catalyzed method was developed for the synthesis of hydroxyimino functionalized indazolo[1,2-a]cinnolines and phthalazino[2,3-a]cinnolines by reductive [4 + 2] annulation between 1-arylindazolones and 2-aryl-2,3-dihydrophthalazine-1,4-diones with varied nitroolefins. The targeted oxime decorated tetracyclic fused cinnolines were synthesized via sequential C-H activation/olefin insertion/reduction under reducing-agent-free conditions.

Iridium-catalyzed direct C-H arylation of cyclic N-sulfonyl ketimines with arylsiloxanes at ambient temperature

An iridium-catalyzed ortho-selective C-H arylation of cyclic N-sulfonyl ketimines has been achieved with environmentally benign aryl siloxanes. The reaction is highly efficient and proceeds at ambient temperature which is the key feature of the methodology considering the weak coordination nature of the substrate as well as the sluggish reactivity of siloxanes. A wide array of pharmaceutically relevant novel biaryls has been synthesized under operationally simple conditions.

Ruthenium(II)-Catalyzed Regioselective [3 + 2] Spiroannulation of 2H-Imidazoles with 2-Alkynoates

The C═N double bond of 2H-imidazole has been employed as a C-electrophile for the ruthenium(II)-catalyzed [3 + 2] spiroannulation reaction of 4-phenyl-2H-imidazoles and 2-alkynoates to synthesize spiroimidazole-4,1'-indenes. This strategy features high regioselectivity, broad functional group tolerance, and use of ruthenium as a catalyst, providing a new method to synthesize spirocycles with potential applications in pharmaceuticals.

The regioselective annulation of alkylidenecyclopropanes by Rh(iii)-catalyzed C–H/C–C activation to access spirocyclic benzosultams

The synthesis of spirocyclic benzosultams fromN-sulfonyl ketimine and alkylidenecyclopropanes under Rh(iii) catalysis has been developed. This transformation enables the formation of two C–C bonds and a double bond with highE-selectivity.

The dual alkylation of the C(sp3)–H bond of cyclic α-methyl-N-sulfonyl imines via the sequential condensation/hydride transfer/cyclization process

The dual alkylation of the C(sp³)–H bond of the cyclic α-methyl-N-sulfonyl imine has been achieved through the piperidine-promoted cascade condensation/[1,5]-hydride transfer/cyclization from cyclic α-methyl-N-sulfonyl imine and o-aminobenzaldehyde in trifluoroethanol.

Spiro[indene-1,4'-oxa-zolidinones] Synthesis via Rh(III)-Catalyzed Coupling of 4-Phenyl-1,3-oxazol-2(3H)-ones with Alkynes: A Redox-Neutral Approach

Transition-metal-catalyzed C-H activation synthesis of heterocyclic spiro[4,4]nonanes has persistently witnessed the use of additional stoichiometric transition-metal oxidant when employing C═C bond as the spiro ring closure site. Herein, we have addressed the issue by reporting a redox-neutral strategy for spiro[indene-1,4'-oxa-zolidinones] synthesis via Rh(III)-catalyzed coupling of 4-phenyl-1,3-oxazol-2(3H)-ones with alkynes. The synthesis features a broad substrate scope and high regiospecificity.