@article {44143, title = { Self-assembly of P-chiral supramolecular phosphines on rhodium and direct evidence for Rh-catalyst-substrate interactions}, journal = {Dalton Transactions}, volume = {46}, year = {2017}, month = {OCT}, pages = {13966-13973}, type = {Article}, abstract = {Supramolecular phosphine-derived catalysts are known to provide high enantioselectivity in asymmetric transformations such as hydrogenation, but direct evidence unravelling the role of secondary interactions is largely missing. As a representative case study, the role of hydrogen bonding in asymmetric hydrogenation catalysed by p-chiral supramolecular phosphines is investigated. To establish the nature of hydrogen bonding in the self-assembled Rh-complex, NMR experiments were performed at different concentrations and temperatures. It was found that with increasing concentration of 1-(3-(phenyl(o-tolyl)phosphanyl) phenyl) urea ligand (L1), the NH and NH2 peaks shift downfield. This indicated the presence of intermolecular hydrogen bonding in L1. This observation was further supported by variable temperature NMR experiments wherein, with decreasing temperature, the NH and NH2 resonances of L1 shifted downfield. The downfield shift once again suggests the existence of intermolecular hydrogen bonding in L1. In contrast, the chemical shift of NH and NH2 signals did not significantly change with increasing concentration of the self-assembled Rh-complex (C1). This observation suggested the existence of intramolecular hydrogen bonding in the self-assembled complex. The concentration experiment was further corroborated by variable temperature NMR experiments. No change in the chemical shift of NH2 resonance could be detected with decreasing temperature, which corroborates the existence of intramolecular hydrogen bonding in C1. In a stoichiometric experiment, C1 was treated with hydrogenation substrate N-acetyldehydrophenylalanine (S2) and the proton NMR was recorded. The NH2 protons of the selfassembled Rh-complex were found to shift downfield, as compared to untreated parent C1. These observations indicated that there is a hydrogen bonding interaction between the Rh-complex and the substrate. To further attest this hypothesis, NH and NH2 groups were exchanged with ND and ND2 groups, respectively, and a self-assembled Rh-complex was prepared using the deuterated supramolecular phosphine ligand L1.D. When the deuterated Rh-complex (C1.D) was treated with substrate S2, the ND and ND2 resonances were found to shift downfield. Thus, the labelling experiment further authenticated the existence of catalyst-substrate interactions. The presence of this catalyst-substrate interaction could be one of the parameters that leads to high enantiomeric excess in the asymmetric hydrogenation reaction of S2.}, doi = {10.1039/c7dt02923c}, author = {Koshti, Vijay S. and Sen, Anirban and Shinde, Dinesh and Chikkali, Samir H.} } @article {46995, title = {Highly enantioselective synthesis of sitagliptin}, journal = {Asian Journal of Organic Chemistry}, volume = {9}, year = {2020}, month = {FEB}, pages = {189-191}, type = {Article}, abstract = {

A highly enantioselective synthesis of sitagliptin, a potent DPP-4 inhibitor, is reported. Explicitly identified chiral FerroLANE ligands in the presence of rhodium catalyze the asymmetric hydrogenation of an enamine to yield sitagliptin with excellent enantioselectivity (98\% ee). The process was scaled up to 5 g and the final product was isolated as a phosphate salt with \>99\% ee.

}, keywords = {asymmetric hydrogenation, catalysis, FerroLANE ligands, Rhodium, sitagliptin}, issn = {2193-5807}, doi = {10.1002/ajoc.201900709, Early Access Date = JAN 2020}, author = {Khopade, V. Kishor and Sen, Anirban and Birajdar, Rajkumar S. and Paulbudhe, Uday P. and Kavale, Dattatry S. and Shinde, Prashant S. and Mhaske, Santosh B. and Chikkali, Samir H.} } @article {47643, title = {C (1)-Symmetric diphosphorus ligands in metal-catalyzed asymmetric hydrogenation to prepare chiral compounds}, journal = {Organic \& Biomolecular Chemistry}, volume = {19}, year = {2021}, month = {NOV }, pages = {9095-9137}, type = {Review}, abstract = {Asymmetric hydrogenation has remained an important and challenging research area in industry as well as academia due to its high atom economy and ability to induce chirality. Among several types of ligands, chiral bidentate phosphine ligands have played a pivotal role in developing asymmetric hydrogenation. Although C-2-symmetric chiral bidentate phosphine ligands have dominated the field, it has been found that several C-1-symmetric ligands are equally effective and, in many cases, have outperformed their C-2-symmetric counterparts. This review evaluates the possibility of the use of C-1-symmetric diphosphorus ligands in asymmetric hydrogenation to produce chiral compounds. The recent strategies and advances in the application of C-1-symmetric diphosphorus ligands in the metal-catalyzed asymmetric hydrogenation of a variety of C=C bonds have been summarized. The potential of diphosphorus ligands in asymmetric hydrogenation to produce pharmaceutical intermediates, bioactive molecules, drug molecules, agrochemicals, and fragrances is discussed. Although asymmetric hydrogenation appears to be a problem that has been resolved, a deep dive into the recent literature reveals that there are several challenges that are yet to be addressed. The current asymmetric hydrogenation methods mostly employ precious metals, which are depleting at a fast pace. Therefore, scientific interventions to perform asymmetric hydrogenation using base metals or earth-abundant metals that can compete with established precious metals hold significant potential.}, issn = {1477-0520}, doi = {10.1039/d1ob01207j}, author = {Sen, Anirban and Chikkali, Samir H.} } @article {47899, title = {Mechanistically guided one pot synthesis of phosphine-phosphite and its implication in asymmetric hydrogenation}, journal = {European Journal of Organic Chemistry}, volume = {2022}, year = {2022}, month = {JAN}, pages = {e202101447}, type = {Article}, abstract = {

Although hybrid bidentate ligands are known to yield highly enantioselective products in asymmetric hydrogenation (AH), synthesis of these ligands is an arduous process. Herein, a one pot, atom-economic synthesis of a hybrid phosphine-phosphite (L1) is reported. After understanding the reactivity difference between an 0-nucleophile versus C-nucleophile, one pot synthesis of Senphos (L1) was achieved (72\%). When L1 was treated with [Rh], P-31 NMR revealed bidentate coordination to Rh. Senphos, in the presence of rhodium, catalyzes the AH of Methyl-2-acetamido-3-phenylacrylate and discloses an unprecedented turn over frequency of 2289, along with excellent enantio-selectivity (92\%). The generality is demonstrated by hydrogenating an array of alkenes. The AH operates under mild conditions of 1-2 bar H-2 pressure, at room temperature. The practical relevance of Ll is demonstrated by scaling-up the reaction to 1 g and by synthesizing DOPA, a drug widely employed for the treatment of Parkinson{\textquoteright}s disease. Computational insights indicate that the R isomer is preferred by 3.8 kcal/mol over the S isomer.

}, keywords = {asymmetric hydrogenation, catalysis, DOPA synthesis, One pot synthesis, Phosphine-phosphite ligand}, issn = {1434-193X}, doi = {10.1002/ejoc.202101447}, author = {Sen, Anirban and Kumar, Rohit and Pandey, Swechchha and Raj, K. Vipin and Kumar, Pawan and Vanka, Kumar and Chikkali, Samir H.} } @article {48924, title = {Iron-catalyzed alkoxylation, dehydrogenative-polymerization and tandem hydrosilylative-alkoxylation}, journal = {Chemistry- a european journal }, volume = {29}, year = {2023}, month = {AUG }, type = {Article}, abstract = {

Alkoxylation, hydrosilylative-alkoxylation, and dehydrogenative-polymerization are some of the most widely used transformations in synthetic chemistry. However, these transformations are traditionally catalyzed by precious, and rare late-transition metals. Presented here is a molecularly defined iron complex that catalyzes alkoxylation, tandem hydrosilylative-alkoxylation, and dehydrogenative polymerization of silanes under mild conditions. The iron complex [Fe(CO)(4)(H)(SiPh3)] 1 catalyzes a direct Si-O coupling reaction between an array of silanes and alcohols to produce desired alkoxysilanes in excellent yield, with H-2 as the only byproduct. The iron catalyst tolerates various functional groups and provides access to 20 alkoxysilanes, including essential molecules such as \& beta;-citronellol and cholesterol. Further, complex 1 catalyzes the polymerization of renewable diol and silane monomer to produce a renewable and degradable poly(isosorbide-silyl ether). Remarkably, complex 1 catalyzes a tandem hydrosilylative-alkoxylation of alkynes under mild conditions to yield unsaturated silyl ethers. The synthetic utility has been demonstrated by gram-scale alkoxylation and hydrosilylative-alkoxylation reactions.

}, keywords = {alkoxylation, dehydrogenative polymerization, earth abundant catalysts, hydrosilylative-alkoxylation, Iron Catalyst}, issn = {0947-6539}, doi = {10.1002/chem.202301375}, author = {Sen, Anirban and Kumar, Rohit and Tewari, Tanuja and Gonnade, Rajesh G. and Chikkali, Samir H.} } @article {48925, title = {Rh-catalyzed asymmetric hydroformylation of olefins using phosphorus ligands}, journal = {Journal of chemical sciences }, volume = {135}, year = {2023}, month = {NOV }, pages = {108}, type = {Article}, abstract = {

Easily accessible BINOL-based monodentate phosphite ligand L1 and hybrid ligand L2 have been synthesized in good yield by following a one-pot, two-step protocol. A single 31P resonance at 146.6 ppm confirmed the formation of L1. Subsequent 1-2D NMR and mass spectrometric analysis authenticated the existence of L1. These ligands afforded excellent activity and regio-selectivity in the Rh-catalyzed AHF of styrenic substrates. L1 showed excellent regioselectivity but did not discriminate between the two enantiomers, while L2 displayed an enantiomeric excess (ee) of up to 20\%. In our attempts to understand the reasons for low ee, the coordination behavior of L2 was investigated. The coordination study revealed that L2 coordinates with the Rh as a monodentate ligand, although there are two P-sites. It was found that only the phosphine arm was coordinated to the Rh and the phosphite arm stayed away from the Rh core at the ambient temperature, leading to moderate ee.Graphical abstractOne-step synthesis of a phosphite (L1) and a phosphine-phosphite (L2) ligand and their implication in the asymmetric hydroformylation of olefins with excellent regioselectivity (\>95\%) and moderate enantioselectivity (up to 20\%) is reported.

}, keywords = {catalysis, coordination, Hydroformylation, Phosphine-phosphite ligand, Phosphite ligand}, issn = {0974-3626}, doi = {10.1007/s12039-023-02227-5}, author = {Kumar, Rohit and Sen, Anirban and Chikkali, Samir H.} } @article {49017, title = {Iron-catalyzed (E)-selective hydrosilylation of alkynes: scope and mechanistic insights}, journal = {Catalysis Science and Technology}, volume = {14}, year = {2024}, month = {MAY}, type = {Article}, abstract = {

Iron-catalyzed hydrosilylation of internal alkynes has been rarely reported. Even in these rare cases, additives have been used for the success of the reaction, which often creates a problem for the functional group tolerance of the reaction. Herein, we report an additive-free iron-catalyzed (E)-selective hydrosilylation of internal alkynes in the presence of a phosphine ligand. A low-valent Fe(0) complex [Fe(CO)(3)(BDA)] {[Fe-1]} catalyzed the hydrosilylation of alkynes at 60 to 120 degrees C, exhibited a broad substrate (24 substrates) scope and tolerated different functional groups. The synthetic utility of the reaction was demonstrated by a gram scale experiment, preparing alkenes, and by chemo-selective hydrosilylation. The modus operandi of the reaction has been investigated by i) homogeneity test, ii) radical trapping experiments, iii) X-ray photoelectron spectroscopy, and iv) by preparing a Fe(II) complex as catalyst control. These mechanistic investigations revealed a two-electron pathway for the hydrosilylation of alkynes. In addition, kinetic investigations were undertaken to shed light on the rates of the reaction. Kinetic studies suggest the absence of an induction period, and the reaction is first order with respect to the concentration of iron catalyst [Fe-1] and zeroth order with respect to the substrate (alkyne). The Hammett plot suggests that strongly electron-withdrawing groups on the alkyne favour the hydrosilylation reaction. Meanwhile Eyring analysis suggests that the rate-determining step likely involves an associative pathway. Based on the findings of the mechanistic and kinetic investigation, a plausible Chalk-Harrod-type mechanism is likely to be operative. The proposed mechanism is substantiated by computational investigations, which suggested that the Chalk-Harrod mechanism is kinetically more favored by 15.8 kcal mol(-1) over the modified Chalk-Harrod mechanism.

}, issn = {2044-4753}, doi = {10.1039/d3cy01775c}, author = {Sen, Anirban and Tewari, Tanuja and Kumar, Rohit and Vinod, C. P. and Sharma, Himanshu and Vanka, Kumar and Chikkali, Samir H.} }