@article {42739, title = {Mechanistic insights into pincer-ligated palladium-catalyzed arylation of azoles with aryl Iodides: evidence of a Pd-II-Pd-IV-Pd-II pathway}, journal = {Organometallics}, volume = {35}, year = {2016}, month = {MAR}, pages = {875{\textendash}886}, abstract = {

Pincer-based (R2POCNR'2)PdCl complexes along with CuI cocatalyst catalyze the arylation of azoles with aryl iodides to give the 2-arylated azole products. Herein, we report an extensive mechanistic investigation for the direct arylation of azoles involving a well-defined and highly efficient (iPr2POCNEt2)PdCl (2a) catalyst, which emphasizes a rare PdII{\textendash}PdIV{\textendash}PdII redox catalytic pathway. Kinetic studies and deuterium labeling experiments indicate that the C{\textendash}H bond cleavage on azoles occurs via two distinct routes in a reversible manner. Controlled reactivity of the catalyst 2a underlines the iodo derivative (iPr2POCNEt2)PdI (3a) to be the resting state of the catalyst. The intermediate species (iPr2POCNEt2)Pd-benzothiazolyl (4a) has been isolated and structurally characterized. A determination of reaction rates of compound 4a with electronically different aryl iodides has revealed the kinetic significance of the oxidative addition of the C(sp2){\textendash}X electrophile, aryl iodide, to complex 4a. Furthermore, the reactivity behavior of 4a suggests that the arylation of benzothiazole proceeds via an oxidative addition/reductive elimination pathway involving a (iPr2POCNEt2)PdIV(benzothiazolyl)(Ar)I species, which is strongly supported by DFT calculations.

}, doi = {10.1021/acs.organomet.6b00003}, author = {Khake, Shrikant M. and Jagtap, Rahul A. and Dangat, Yuvraj B. and Gonnade, Rajesh G. and Vanka, Kumar and Punji, Benudhar} } @article {42738, title = {Unified strategy for nickel-catalyzed C-2 Alkylation of indoles through chelation assistance}, journal = {ACS Catalysis}, volume = {6}, year = {2016}, month = {JUL}, pages = {5666{\textendash}5672}, abstract = {

A nickel-catalyzed direct C-2 alkylation of indoles through monodentate-chelation assistance has been described. This reaction proceeds via an unusual strategy by the use of a well-designed and defined (quinolinyl)amido{\textendash}nickel catalyst, [{κN,κN,κN-Et2NCH2C(O)(μ-N)C9H6N}Ni(OAc)], providing a solution to the limitations associated with bidentate-chelate auxiliaries. The method allows coupling of indoles with various unactivated primary and secondary alkyl halides with ample substrate scope. This uniquely strategized alkylation proceeded through crucial C{\textendash}H activation and via an alkyl radical intermediate. The reaction by this approach represents a rare example of Ni-catalyzed monodentate-chelate-assisted C{\textendash}H functionalization.

}, doi = {10.1021/acscatal.6b02003}, author = {Soni, Vineeta and Jagtap, Rahul A. and Gonnade, Rajesh G. and Punji, Benudhar} } @article {43568, title = {Expeditious and solvent-free nickel-catalyzed C-H arylation of arenes and indoles}, journal = {ChemSusChem}, volume = {10}, year = {2017}, month = {APR}, pages = {2242-2248}, type = {Article}, abstract = {An efficient solvent-free nickel-catalyzed method for C-H bond arylation of arenes and indoles has been developed, which proceeds expeditiously through chelation assistance. The reaction is highly selective for mono-arylation and tolerates sensitive and structurally diverse functionalities, such as halides, ethers, amines, indole, pyrrole and carbazole. This reaction represents the first example of a nickel-catalyzed C-H arylation by monochelate assistance and symbolizes a rare precedent in solvent-free C-H arylation. Mechanistic investigations by various controlled reactions, kinetic studies, and deuterium labeling experiments suggest that the arylation follows a single electron transfer (SET) pathway involving the turnover-limiting C-H nickelation process.}, doi = {10.1002/cssc.201700321}, author = {Jagtap, Rahul A. and Soni, Vineeta and Punji, Benudhar} } @article {46415, title = {C-H functionalization of indoles by 3d transition-metal catalysis}, journal = {Asian Journal of Organic Chemistry}, year = {2019}, month = {DEC}, type = {Review; Early Access}, abstract = {

Over the past decade, the use of 3d transition metal for the regioselective C-H bond functionalization of indoles has significantly increased. Particularly, advances in manganese, iron, cobalt, nickel and copper catalysis have demonstrated the selective C(2)-H and C(3)-H arylation, alkenylation, alkynylation and alkylation to a greater extent. Similarly, the C-O and C-N bond-forming reactions are manifested via direct C-H bond activation by these earth-abundant metals. The emergence of 3d metals in selective functionalization of the biologically relevant indoles and related heteroarenes would make this protocol more attractive for practical applications. Herein, we provide a brief overview of 3d transition metal-catalyzed (mostly Mn, Fe, Co, Ni and Cu) C-H functionalization of indoles and related heteroarenes.

}, keywords = {3d transition metal, C-H functionalization, Indole, indoline}, issn = {2193-5807}, doi = {10.1002/ajoc.201900554}, author = {Jagtap, Rahul A. and Punji, Benudhar} } @article {45528, title = {Nickel-catalyzed straightforward and regioselective C-H alkenylation of indoles with alkenyl bromides: scope and mechanistic aspect}, journal = {ACS Catalysis}, volume = {9}, year = {2019}, month = {JAN}, pages = {431-441}, type = {Article}, abstract = {

Nickel-catalyzed regioselective C-H bond alkenylation of indoles and related heteroarenes with alkenyl bromides is accomplished under relatively mild conditions. This method allows the straightforward synthesis of C-2 alkenylated indoles employing an air-stable and well-defined nickel catalyst, (bpy)NiBr2, providing a solution to the limitations associated with hydroindolation and oxidative alkenylation. The reaction conceded the coupling of indole derivatives with various alkenyl bromides, such as aromatic and heteroaromatics, alpha- and beta-substituted as well as exo- and endo-cyclic alkenyl compounds. An extensive mechanistic investigation, including controlled study, reactivity experiments, kinetics and labeling studies, and EPR and XPS analyses, highlights that the alkenylation proceeds through a single-electron transfer process comprising an odd-electron oxidative addition of alkenyl bromide. Furthermore, the alkenylation operates via a probable Ni(I)/Ni(III) pathway involving the rate-limiting C-H nickelation of indole.

}, keywords = {alkenylation, C-H activation, indoles, Mechanism, Nickel, single-electron transfer}, issn = {2155-5435}, doi = {10.1021/acscatal.8b04267}, author = {Jagtap, Rahul A. and Vinod, C. P. and Punji, Benudhar} } @article {46079, title = {Scope and mechanistic aspect of nickel-catalyzed alkenylation of benzothiazoles and related azoles with styryl bromides}, journal = {Organometallics}, volume = {38}, year = {2019}, month = {JUN }, pages = {2422-2430}, type = {Article}, abstract = {

Alkenylation of benzothiazoles and related azoles with alkenyl bromides is achieved employing the well-defined and air-stable nickel complexes, (bpy)NiBr2 and [Ni(bpy)(3)] [NiBr4], as catalysts. Numerous electronically distinct alkenyl bromides efficiently coupled with substituted benzothiazoles, oxazoles, and benzimidazoles under the catalytic conditions to afford 2-alkenylated azoles. An extensive mechanistic study of the alkenylation of benzothiazole using (bpy)NiBr2 highlights a single-electron transfer process for the reaction involving the two-step one-electron oxidative addition of alkenyl bromide. The substrate benzothiazole plays a significant and diverse role in the reaction, and C-H bond cleavage is reversible in nature. Detailed kinetic analysis and control reactivity studies are indicative of a Ni(I)/Ni(III) pathway for the alkenylation comprising the rate-influencing reductive elimination step.

}, issn = {0276-7333}, doi = {10.1021/acs.organomet.9b00060}, author = {Patel, Ulhas N. and Jagtap, Rahul A. and Punji, Benudhar} } @article {46673, title = {Expeditious and solvent-free nickel-catalyzed c-h arylation of arenes and indoles (vol 10, pg 2242, 2017)}, journal = {ChemSusChem}, volume = {13}, year = {2020}, month = {NOV }, pages = {5771}, type = {Correction}, keywords = {Arylation, C-H activation, heterocycles, Homogeneous catalysis, Nickel}, issn = {1864-5631}, doi = {10.1002/cssc.202002313, Early Access Date = OCT 2020}, author = {Jagtap, Rahul A. and Soni, Vineeta and Punji, Benudhar} } @article {46798, title = {Iron-catalyzed C(sp(2))-H alkylation of indolines and benzo[h]quinoline with unactivated alkyl chlorides through chelation assistance}, journal = {ACS Catalysis}, volume = {10}, year = {2020}, month = {JUL }, pages = {7312-7321}, type = {Article}, abstract = {

Regioselective C-H bond alkylation of indolines and benzo[h]quinoline with a wide range of unactivated and highly demanded primary and secondary alkyl chlorides is accomplished using a low-cost iron catalyst. This reaction tolerates diverse functionalities, such as C(sp(2))-Cl, fluoro, alkenyl, silyl, ether, thioether, pyrrolyl, and carbazolyl groups including cyclic and acyclic alkyls as well as alkyl-bearing fatty-alcohol and polycyclic-steroid moieties. The demonstrated iron-catalyzed protocol proceeded via either a five-membered or a six-membered metallacycle. Intriguingly, the C-7-alkylated indolines can be readily functionalized into free-NH indolines/indoles and tryptamine derivatives. A detailed mechanistic investigation highlights the participation of an active Fe(I) catalyst and the involvement of a halogen-atom transfer process via a single-electron-based mechanism. Deuterium labeling and kinetics analysis indicate that the C-H metalation of indoline is the probable turnover-limiting step. Overall, the experimental and theoretical studies supported an Fe(I)/Fe(III) pathway for the alkylation reaction comprising the two-step, one-electron oxidative addition of alkyl chloride.

}, keywords = {alkyl chlorides, Alkylation, C-H activation, chelation assistance, indolines, iron}, issn = {2155-5435}, doi = {10.1021/acscatal.0c02030}, author = {Jagtap, Rahul A. and Samal, Pragnya Paramita and Vinod, C. P. and Krishnamurty, Sailaja and Punji, Benudhar} } @article {46818, title = {MnBr2-catalyzed direct and site-selective alkylation of indoles and benzo[h]quinoline}, journal = {Organic Letters}, volume = {22}, year = {2020}, month = {JUN}, pages = {4643-4647}, type = {Article}, abstract = {

Manganese-catalyzed regioselective C-H alkylation of indoles and benzo[h]quinoline with a variety of unactivated alkyl iodides is reported. Unlike other Mn-catalyzed C-H functionalization, this protocol does not require a Grignard reagent base and employs a simple and inexpensive MnBr2 as a catalyst. This method tolerates diverse functionalities, including fluoro, chloro, bromo, iodo, alkenyl, alkynyl, pyrrolyl, and carbazolyl groups. The alkylation proceeds through a single-electron transfer pathway comprising reversible C-H manganesation and involving an alkyl radical intermediate.

}, issn = {1523-7060}, doi = {10.1021/acs.orglett.0c01398}, author = {Jagtap, Rahul A. and Verma, Suryadev K. and Punji, Benudhar} } @article {47635, title = {Achiral and chiral NNN-pincer nickel complexes with oxazolinyl backbones: application in transfer hydrogenation of ketones}, journal = {New Journal of Chemistry}, volume = {45}, year = {2021}, month = {JUL}, pages = {11927-11936}, type = {Article}, abstract = {We describe the synthesis of new NNN-oxazolinyl-pincer nickel complexes and their application in the transfer hydrogenation of ketones. Achiral NNN-ligands, R {\textquoteleft}(2)-oxazolinyl-2-C6H4-NH-C(O)CH2NEt2 [((R {\textquoteleft} 2-OxNNNEt2))-H; R{\textquoteright} = H (3a), R {\textquoteleft} = Me (3b)], and chiral ligands, (R)-R {\textquoteleft}-oxazolinyl-2-C6H4-NH-C(O)CH2NEt2 [(R)-((R {\textquoteleft}-OxNNNEt2))-H; R {\textquoteleft} = Ph (3c), R {\textquoteleft} = CH2Ph (3d), R {\textquoteleft} = Pr-i (3e), R {\textquoteleft} = (CH2Pr)-Pr-i (3f)], were efficiently synthesized. Treatment of these ligands with (DME)NiCl2 afforded the desired amido-pincer nickel complexes, ((R {\textquoteleft} 2-OxNNNEt2))NiCl [R {\textquoteleft} = H (4a), R {\textquoteleft} = Me (4b)] and ((R {\textquoteleft}-OxNNNEt2))NiCl [R {\textquoteleft} = Ph (4c), R {\textquoteleft} = CH2Ph (4d), R {\textquoteleft} = Pr-i (4e), R {\textquoteleft} = (CH2Pr)-Pr-i (4f)], in good yields. All the ligand precursors and nickel complexes were thoroughly characterized by various analytical techniques. The molecular structures of 4a, 4d and 4f were established by X-ray crystallography. The developed nickel complexes were found to be efficient catalysts for the transfer hydrogenation of ketones using (PrOH)-Pr-i as a viable hydrogen source. Enantioselectivity in hydrogenation was not observed with the developed chiral catalysts.}, issn = {1144-0546}, doi = {10.1039/d1nj01698a}, author = {Jagtap, Rahul A. and Ankade, Shidheshwar B. and Gonnade, Rajesh G. and Punji, Benudhar} } @article {47412, title = {Nickel-catalyzed C-H bond functionalization of azoles and indoles}, journal = {Chemical Record}, volume = {21}, year = {2021}, month = {DEC}, pages = {3573-3588}, type = {Article}, abstract = {

Direct C-H functionalization of privileged and biologically relevant azoles and indoles represents an important chemical transformation in molecular science. Despite significant progress in the palladium-catalyzed regioselective C-H functionalization of azoles and indoles, the use of abundant and less expensive nickel catalyst is underdeveloped. In the recent past, the nickel-catalyzed regioselective C-H alkylation, arylation, alkenylation and alkynylation of azoles and indoles have been substantially explored, which can be applied to the complex organic molecule synthesis. In this Account, we summarize the developments in nickel-catalyzed regioselective functionalization of azoles and indoles with a considerable focus on the reaction mechanism.

}, keywords = {azoles, C-C Bond formation, C-H functionalization, indoles, Nickel}, issn = {1527-8999}, doi = {10.1002/tcr.202100113, Early Access Date = JUN 2021}, author = {Jagtap, Rahul A. and Punji, Benudhar} } @article {48083, title = {Efficient route to 3,3 {\textquoteleft}-biindolinylidene-diones by iron-catalyzed dimerization of isatins}, journal = {Chemistry-an Asian Journal}, volume = {17}, year = {2022}, month = {AUG}, pages = {e202200414}, type = {Article}, abstract = {

Iron-catalyzed dimerization of various isatin derivatives is described for the efficient synthesis of 3,3 {\textquoteleft}-biindolinylidene-diones (isoindigos). The reaction provides easy access to self-coupled and cross-coupled 3,3 {\textquoteleft}-indolinylidene-diones that have high relevance to biology and materials. This Fe(0)- or Fe(II)-catalyzed dimerization reaction tolerates a wide range of functionalities, such as fluoro, chloro, bromo, alkenyl, nitrile, ether, ester, pyrrolyl, indolyl and carbazolyl groups, including cyclic and acyclic alkyls as well as an alkyl-bearing fatty-alcohol moiety. Especially, the coupling between two distinct isatins provided excellent selectivity for the cross-dimerization with trace of self-couplings. The single-crystal X-ray diffraction study established the molecular structure of eight dimerized products. A preliminary mechanistic study of the Fe-catalyzed dimerization supported the radical pathway for the reaction.

}, keywords = {Cross-coupling, dimerization, iron, Isatin, Isoindigo, Mechanism}, issn = {1861-4728}, doi = {10.1002/asia.202200414}, author = {Jagtap, Rahul A. and Pradhan, Chandini and Gonnade, Rajesh G. and Punji, Benudhar} }