Molecular Mechanics Predictions and Experimental Testing of Asymmetric Palladium-Catalyzed Allylation Reactions Using New Chiral Phenanthroline Ligands

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

A Quantum-Guided Molecular Mechanics Force Field for the Ferrocene Scaffold

Ferrocene derivatives have a wide range of applications, including as ligands in asymmetric catalysis, due to their chemical stability, rigid backbone, steric bulk, and ability to encode stereochemical information via planar chirality. Unfortunately, few of the available molecular mechanics force fields incorporate parameters for the accurate study of this important building block. Here, we present a MM3* force field for ferrocenyl ligands, which was generated using the quantum-guided molecular mechanics (Q2MM) method. Detailed validation by comparison to DFT calculations and crystal structures demonstrates the accuracy of the parameters and uncovers the physical origin of deviations through excess energy analysis. Combining the ferrocene force field with a force field for Pd-allyl complexes and comparing the crystal structures shows the compatibility with previously developed MM3* force fields. Finally, the ferrocene force field was combined with a previously published transition-state force field to predict the stereochemical outcomes of the aminations of Pd-allyl complexes with different amines and different chiral ferrocenyl ligands, with an R² of ∼0.91 over 10 examples.

Ln(iii) complexes with a chiral 1H-pyrazolo[3,4-b]pyridine derivative fused with a (−)-α-pinene moiety: synthesis, crystal structure, and photophysical studies in solution and in the solid state

Novel chiral luminescent Ln(iii) complexes with a pyrazolo[3,4-b]pyridine derivative containing a natural monoterpene (−)-α-pinene moiety are reported.

Application of Q2MM to predictions in stereoselective synthesis

Quantum-Guided Molecular Mechanics (Q2MM) can be used to derive transition state force fields (TSFFs) that allow the fast and accurate predictions of stereoselectivity for a wide range of catalytic enantioselective reactions. The basic ideas behind the derivation of TSFFs using Q2MM are discussed and the steps involved in obtaining a TSFF using the Q2MM code, publically available at github.com/q2mm, are shown. The applicability for a range of reactions, including several non-standard applications of Q2MM, is demonstrated. Future developments of the method are also discussed.

Design of bifunctional chiral phenanthroline ligand with Lewis basic site for palladium-catalyzed asymmetric allylic substitution

Conceptually new bifunctional chiral ligands with Lewis basic site were developed for Pd-catalyzed asymmetric allylic substitution.

Synthesis of highly rigid phosphine–oxazoline ligands for palladium-catalyzed asymmetric allylic alkylation

Rigid phosphine–oxazoline ligands with a spirocarbon stereogenic center were developed, which exhibited excellent catalytic performance for Pd-catalyzed allylic alkylation reactions.

Ln(iii) complexes (Ln = Eu, Gd, Tb, Dy) with a chiral ligand containing 1,10-phenanthroline and (–)-menthol fragments: synthesis, structure, magnetic properties and photoluminescence

Novel chiral luminescent Ln(iii) complexes are reported.

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.

Prediction of Stereochemistry using Q2MM

The standard method of screening ligands for selectivity in asymmetric, transition metal-catalyzed reactions requires experimental testing of hundreds of ligands from ligand libraries. This "trial and error" process is costly in terms of time as well as resources and, in general, is scientifically and intellectually unsatisfying as it reveals little about the underlying mechanism behind the selectivity. The accurate computational prediction of stereoselectivity in enantioselective catalysis requires adequate conformational sampling of the selectivity-determining transition state but has to be fast enough to compete with experimental screening techniques to be useful for the synthetic chemist. Although electronic structure calculations are accurate and general, they are too slow to allow for sampling or fast screening of ligand libraries. The combined requirements can be fulfilled by using appropriately fitted transition state force fields (TSFFs) that represent the transition state as a minimum and allow fast conformational sampling using Monte Carlo. Quantum-guided molecular mechanics (Q2MM) is an automated force field parametrization method that generates accurate, reaction-specific TSFFs by fitting the functional form of an arbitrary force field using only electronic structure calculations by minimization of an objective function. A key feature that distinguishes the Q2MM method from many other automated parametrization procedures is the use of the Hessian matrix in addition to geometric parameters and relative energies. This alleviates the known problems of overfitting of TSFFs. After validation of the TSFF by comparison to electronic structure results for a test set and available experimental data, the stereoselectivity of a reaction can be calculated by summation over the Boltzman-averaged relative energies of the conformations leading to the different stereoisomers. The Q2MM method has been applied successfully to perform virtual ligand screens on a range of transition metal-catalyzed reactions that are important from both an industrial and an academic perspective. In this Account, we provide an overview of the continued improvement of the prediction of stereochemistry using Q2MM-derived TSFFs using four examples from different stages of development: (i) Pd-catalyzed allylation, (ii) OsO4-catalyzed asymmetric dihydroxylation (AD) of alkenes, (iii) Rh-catalyzed hydrogenation of enamides, and (iv) Ru-catalyzed hydrogenation of ketones. In the current form, correlation coefficients of 0.8-0.9 between calculated and experimental ee values are typical for a wide range of substrate-ligand combinations, and suitable ligands can be predicted for a given substrate with ∼80% accuracy. Although the generation of a TSFF requires an initial effort and will therefore be most useful for widely used reactions that require frequent screening campaigns, the method allows for a rapid virtual screen of large ligand libraries to focus experimental efforts on the most promising substrate-ligand combinations.

Enantioselective Recognition of Chiral Carboxylic Acids by a β-Amino Acid and 1,10-Phenanthroline Based Chiral Fluorescent Sensor

A novel chiral 1,10-phenanthroline-based fluorescent sensor was designed and synthesized from optical active β-amino acids. It used 1,10-phenanthroline moiety as a fluorescent signaling site and binding site, with optically active β-amino acids as a chiral barrier site. Notably, the optically active β-amino acids were obtained by a Lewis base catalyzed hydrosilylation of β-enamino esters according to our former work. The chiral sensor has been used to conduct the enantioselective recognition of chiral mono and dicarboxylic acids derivatives. Using this fluorescent sensor, a moderate “turn-off” fluorescence-diminishment response towards enantiomer of tartaric acids, and proline was observed. It found that l-enantiomers quench the chiral fluorescence sensor more efficiently than d-enantiomers due to the absolute configuration of the β-amino acid.

DNA Binding Test, X-Ray Crystal Structure, Spectral Studies, TG-DTA, and Electrochemistry of [CoX 2 (dmdphphen)] (Dmdphphen Is 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline, X = Cl, and NCS) Complexes

Two new neutral mixed-ligand cobalt(II) complexes, [CoCl₂(dmdphphen)] 1 and [Co(NCS)₂(dmdphphen)]  2, where dmdphphen is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, were synthesized and characterized by an elemental analysis, UV-Vis, IR, TG/DTA, cyclic voltammetry CV, and single X-ray diffraction. Complex 2 crystallized as monoclinic with a space group P2₁/c. Co(II) ions are located in a distorted tetrahedral environment. TG/DTA result shows that these complexes are very stable and decomposed through one-step reaction. The two complexes exhibit a quasireversible one-electron response at −550 and 580 mV versus Cp₂Fe/Cp₂Fe⁺, which has been assigned to Co(I)/Co(II) and Co(II)/Co(III) couples. Absorption spectral studies reveal that such complexes exhibit hypochromicity during their interaction with CT-DNA.

Breaking Conjugation: Unusual Regioselectivity with 2-Substituted Allylic Substrates in the Tsuji-Trost Reaction

η³-Allyl palladium complexes are key intermediates in Tsuji-Trost allylic substitution reactions. It is well known that (η³-1-aryl-3-alkyl substituted allyl)Pd intermediates result in nucleophilic attack at the alkyl substituted terminus. In contrast, the chemistry of (η³-1,2,3-trisubstituted allyl)Pd intermediates is relatively unexplored. Herein we probe the regioselectivity with 1,2,3-trisubstituted allylic substrates in Tsuji-Trost allylic substitution reactions. DFT investigation of cationic (η³-1-Ph-2-B(pin)-3-alkyl-allyl)Pd(PPh₃)₂ intermediates predict that nucleophilic attack should occur preferentially on anti-allyls rather than the syn-isomers to generate benzylic substitution products under Curtin-Hammett conditions. Experimentally, systematic studies with 1,2,3-trisubstituted allylic substrates revealed that a Linear Free Energy Relationship (LFER) is observed when Charton steric parameters of the C-2 substituents are plotted against the log of the ratio of regioisomers. Bulkier C-2 substituents in 1,2,3-trisubstituted η³-allyl palladium intermediates provide stronger preference for nucleophilic attack at anti-oriented benzylic termini. Additionally, the geometry of 1,4-elimination products supports the presence of anti-allyl palladium intermediates.

Design of novel chiral N,N,O-tridentate phenanthroline ligands and their application to enantioselective addition of organozinc reagents to aldehydes

The novel chiral N,N,O-tridentate ligands (BinThro) containing binaphthyl and phenanthroline units were developed. The enantioselective organozinc addition to aldehydes was performed in the presence of BinThro (S)-1 with up to 95% ee.

Synthesis of Pd complexes directly linked to the light-absorbing [(bpy)(3)Ru](2+) unit and their photochemical reactions toward styrenes

A series of bipyridyl-Pd and Rh complexes containing a [(bpy)(3)Ru](2+) or [(bpy)(2)Ru(phen)](2+) (bpy = 2,2'-bipyridyl, phen = 1,10-phenanthroline) moiety as visible-light absorbing unit was synthesized. The complexes were synthesized via a Suzuki-Miyaura coupling reaction between the Ru complexes having a 4-bromo-2,2'-bipyridyl ligand and a 2,2'-bipyridyl-4-boronic acid and a subsequent reaction with various mononuclear Pd and Rh precursors. There was a noticeable structural difference between the QP (2,2':4',4'':2'',2'''-quaterpyridyne) and PB (5-(2,2'-bipyridyl)-yl-1,10-phenanthroline) complexes, which involved the dihedral angles within the bridging ligand; the PB complexes possessed large dihedral angles but the QP complexes showed small values. This structural difference clearly indicated a strong pi-conjugation through the QP ligand. The electrochemical and photophysical properties of the QP and PB complexes were compared with the parent mononuclear Ru complexes, such as [(bpy)(3)Ru](2+), [(bpy)(2)Ru(phen)](2+), and [(bpy)(2)Ru(bpm)](2+) (bpm = 2,2'-bipyrimidine). The QP and PB complexes showed a (3)MLCT life time that was similar to [(bpy)(3)Ru](2+) and [(bpy)(2)Ru(phen)](2+), which was about 10 times longer (ca. 1 mus) than the corresponding bpm complexes. Reactivity studies with Pd complexes toward styrene dimerizations were examined. The reaction proceeded under visible-light irradiated conditions and the reactivity of the QP complexes was much higher than the corresponding PB complexes. Substantial acceleration of the reaction was observed with the introduction of an Me substituent on the bipyridyl ligand.

Studies toward the Total Synthesis of Dumsin. 2. A Second Generation Approach Resulting in Enantioselective Construction of a Functionalized ABC Subunit of the Tetranortriterpenoid Insect Antifeedant

A synthesis of the ABC framework of dumsin is described. The optically active intermediate 9b, which is expeditiously assembled from 5-oxobornyl pivalate by the sequential implementation of an oxy-Cope rearrangement and an intramolecular ene reaction, proved to be suitably functionalized for ultimate conversion to 5. The synthesis plan relies on two approaches to this targeted intermediate. In the first, the exocyclic double bond introduced during EtAlCl2-promoted closure of aldehyde 10b is cleaved to leave a carbonyl group that is amenable to hydride reduction and elimination of water. Cleavage of the resulting double bond with ruthenium tetroxide provided the seco ketoacid. The same advanced intermediate was obtained by initially positioning the double bond internal to the five-membered ring in advance of transient ring expansion via diketone formation and intramolecular aldolization. Both of these approaches bypass the complications arising from the substantial steric congestion prevailing in these structural networks. The task of covalently positioning an oxygen atom adjacent to the gem-dimethyl-substituted carbon in 5 was properly realized by oxidative decarboxylation. The stereochemical assignments to many of the intermediates were confirmed by an X-ray crystallographic analysis of 43.

Deconvoluting the Memory Effect in Pd-Catalyzed Allylic Alkylation: Effect of Leaving Group and Added Chloride

An analysis of product distributions in the Tsuji-Trost reaction indicates that several instances of reported "memory effects" can be attributed to slow interconversion of the initially formed syn- and anti-[Pd(eta3-allyl)] complexes. Addition of chloride triggers a true memory effect, in which the allylic terminus originally bearing the leaving group has a higher reactivity. The latter effect, termed regioretention, can be rationalized by ionization from a palladium complex bearing a chloride ion, forming an unsymmetrically substituted [Pd(eta3-allyl)] complex. DFT calculations verify that the position trans to the phosphine ligand is more reactive both in the initial ionization and in the subsequent nucleophilic attack.

Synthesis and application in asymmetric catalysis of camphor-based pyridine ligands

This tutorial review deals with the synthesis and application in asymmetric catalysis of camphor-based pyridine ligands. These ligands can be roughly divided into two groups: those in which the camphor is annulated in the 2,3-positions to the beta-face of the pyridine ring and those in which the pyridine is contained as a pendant on the C2 or C3 of the camphor framework. Camphor-based pyridine ligands can also contain other donor centers located on the pyridine ring or camphor skeleton. Some of these ligands have provided interesting enantioselectivities in several asymmetric reactions, such as S(N)2' reactions, allylic oxidations, carbonyl additions with organozinc reagents and hydrogenations. This review contains a lot of chemistry on ligand synthesis and readers will find it of value and also perhaps an inspiration for the development of more active and improved versions.

Synthesis of Readily Cleavable Immobilized 1,10-Phenanthroline Resins

1,10-Phenanthroline is derivatized and ultimately immobilized on two different polystyrene/divinylbenzene solid supports using convenient methodology. All syntheses are amenable to semiautomatic processing and are scalable for high-throughput screening. A domino copper-catalyzed coupling-cyclization reaction is used to illustrate applicability in catalytic studies.