Titanium catalyzed one-pot multicomponent coupling reactions for direct access to substituted pyrimidines

Modeling Complex Ligands for High Oxidation State Catalysis: Titanium Hydroamination with Unsymmetrical Ligands

A method for modeling high oxidation state catalysts is used on precatalysts with unsymmetrical and symmetrical bidentate ligands to get a more detailed understanding of how changes to ancillary ligands affect the hydroamination of alkynes catalyzed by titanium. To model the electronic donor ability, the ligand donor parameter (LDP) was used, and to model the steric effects, percent buried volume (% Vbur) was employed. For the modeling study, 7 previously unpublished unsymmetrical Ti(XX')(NMe2)2 precatalysts were prepared, where XX' is a chelating ligand with pyrrolyl/indolyl linkages. The rates of these unsymmetrical and 10 previously reported symmetrical precatalysts were used with the model kobs = a + b(LDP)1 + c(LDP)2 + d(% Vbur)1 + e(% Vbur)2, where a-e were found through least-squares refinement. The model suggests that (1) the two attachment points of the bidentate ligand XX' are in different environments on the metal (e.g., axial and equatorial in a trigonal bipyramidal or square pyramidal structure), (2) the position of the unsymmetrical ligand on the metal is determined by the electronics of the ligand rather than the sterics, and (3) that one side of the chelating ligand's electronics strongly influences the rate, while the other side's sterics more strongly influences the rate. From these studies, we were able to generate catalysts fitting to this model with rate constants larger than the fastest symmetrical catalyst tested.

Multicomponent syntheses of 5- and 6-membered aromatic heterocycles using group 4-8 transition metal catalysts

In this Perspective, we discuss recent syntheses of 5- and 6-membered aromatic heterocycles via multicomponent reactions (MCRs) that are catalyzed by group 4-8 transition metals. These MCRs can be categorized based on the substrate components used to generate the cyclized product, as well as on common mechanistic features between the catalyst systems. These particular groupings are intended to highlight mechanistic and strategic similarities between otherwise disparate transition metals and to encourage future work exploring related systems with otherwise-overlooked elements. Importantly, in many cases these early- to mid-transition metal catalysts have been shown to be as effective for heterocycle syntheses as the later (and more commonly implemented) group 9-11 metals.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.

Catalyst design insights from modelling a titanium-catalyzed multicomponent reaction

High oxidation state transition metal catalysis touches our daily lives through bulk chemical production, e.g. olefin polymerization, and through specialty chemical reactions common in organic synthesis, e.g. the Sharpless asymmetric epoxidation and olefin dihydroxylation. Our group has been expanding the reaction chemistry of titanium(iv) to produce a host of nitrogen-based heterocycles via multicomponent coupling reactions. One such multicomponent coupling reaction discovered in our laboratory is iminoamination, involving an amine, an alkyne, and an isonitrile. However, the experimental modeling of high oxidation state reactions lags far behind that of low oxidation state systems, where a great deal is known about ligands, their donor properties and how their structures affect catalysis. As a result, we have developed an experimental method for determining the donor abilities of anionic ligands on high oxidation state systems, which is based on the chromium(vi) nitride system NCr(NiPr2)2X, where X = the ligand being interrogated. The parameters obtained are simply called ligand donor parameters (LDP). In this contribution, a detailed optimization of the Ti(NMe2)2(dpm)-catalyzed iminoamination reaction was carried out, where dpm = 5,5-dimethyldipyrrolylmethane. During the course of these studies, dimeric {Ti(μ-N-tolyl)(dpm)}2 was isolated, which is proposed as the resting state of the catalyst. To destabilize this resting state, a more electron-rich bis(aryloxide) catalyst system was investigated. The more electron-rich system is somewhat more active for iminoamination under some conditions; however, the catalyst is prone to disproportionation. A study of heteroleptic titanium complexes revealed that the disproportionation equilibrium constant can be effectively modeled as a function of the square of the difference in LDP between the ligands, (ΔLDP)2. Using this methodology, one can estimate the stability of titanium complexes toward disproportionation.

Synthesis, molecular docking studies, and biological evaluation of novel alkyl bis(4-amino-5-cyanopyrimidine) derivatives

A series of bis(4-amino-5-cyano-pyrimidines) was synthesized and evaluated as dual inhibitors of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). To further explore the multifunctional properties of the new derivatives, their antioxidant and antibacterial activities were also tested. The results showed that most of these compounds could effectively inhibit AChE and BChE. Particularly, compound 7c exhibited the best AChE inhibitory activity (IC₅₀  = 5.72 ± 1.53 μM), whereas compound 7h was identified as the most potent BChE inhibitor (IC₅₀  = 12.19 ± 0.57 μM). Molecular modeling study revealed that compounds 7c, 7f, and 7b showed a higher inhibitory activity than that of galantamine against both AChE and BChE. Anticholinesterase activity of compounds 7h, 7b, and 7c was significant in vitro and in silico for both enzymes, since these compounds have hydrophobic rings (Br-phenyl, dimethyl, and methoxyphenyl), which bind very well in both sites. In addition to cholinesterase inhibitory activities, these compounds showed different levels of antioxidant activities. Indeed, in the superoxide-dimethyl sulfoxide alkaline assay, compound 7j showed very high inhibition (IC₅₀  = 0.37 ± 0.28 μM). Also, compound 7l exhibited strong and good antibacterial activity against Staphylococcus epidermidis and Staphylococcus aureus, respectively. Taking into account the results of biological evaluation, further modifications will be designed to increase potency on different targets. In this study, the obtained results can be a new starting point for further development of multifunctional agents for the treatment of Alzheimer's disease.

Titanium-catalyzed multicomponent couplings: efficient one-pot syntheses of nitrogen heterocycles

Nitrogen-based heterocycles are important frameworks for pharmaceuticals, natural products, organic dyes for solar cells, and many other applications. Catalysis for the formation of heterocyclic scaffolds, like many C-C and C-N bond-forming reactions, has focused on the use of rare, late transition metals like palladium and gold. Our group is interested in the use of Earth-abundant catalysts based on titanium to generate heterocycles using multicomponent coupling strategies, often in one-pot reactions. To be of maximal utility, the catalysts need to be easily prepared from inexpensive reagents, and that has been one guiding principle in the research. For this purpose, a series of easily prepared pyrrole-based ligands has been developed. Titanium imido complexes are known to catalyze the hydroamination of alkynes, and this reaction has been used to advantage in the production of α,β-unsaturated imines from 1,3-enynes and pyrroles from 1,4-diynes. Likewise, catalyst design can be used to find complexes applicable to hydrohydrazination, coupling of a hydrazine and alkyne, which is a method for the production of hydrazones. Many of the hydrazones synthesized are converted to indoles through Fischer cyclization by addition of a Lewis acid. However, more complex products are available in a single catalytic cycle through coupling of isonitriles, primary amines, and alkynes to give tautomers of 1,3-diimines, iminoamination (IA). The products of IA are useful intermediates for the one-pot synthesis of pyrazoles, pyrimidines, isoxazoles, quinolines, and 2-amino-3-cyanopyridines. The regioselectivity of the reactions is elucidated in some detail for some of these heterocycles. The 2-amino-3-cyanopyridines are synthesized through isolable intermediates, 1,2-dihydro-2-iminopyridines, which undergo Dimroth rearrangement driven by aromatization of the pyridine ring; the proposed mechanism of the reaction is discussed. The IA-based heterocyclic syntheses can be accomplished start to finish (catalyst generation to heterocyclic synthesis) in a single vessel. The catalyst can be formed in situ from commercially available Ti(NMe2)4 and the protonated form of the ligand. Then, the primary amine, alkyne, and isonitrile are added to the flask, and the IA product is synthesized. The volatiles are removed (if necessary), and the next reagent is added. A brief video showing the process for the simple heterocycle 4-phenylpyrazole from phenylacetylene, cyclohexylamine, tert-butylisonitrile, and hydrazine hydrate is included. Further development in this field will unlock new, efficient reactions for the production of carbon-carbon and carbon-nitrogen bonds. As an example of such a process recently discovered, a catalyst for the regioselective production of pyrazoles in a single step from terminal alkynes, hydrazines, and cyclohexylisonitrile is discussed. Using titanium catalysis, many heterocyclic cores can be accessed easily and efficiently. Further, the early metal chemistry described is often orthogonal to late metal-based reactions, which use substrates like aryl halides, silyl groups, boryl groups, and so forth. As a result, earth-abundant and nontoxic titanium can fulfill important roles in the synthesis of useful classes of compounds like heterocycles.

Ultrasound-promoted one-pot five-components synthesis of biologically active novel bis((6-alkyl or phenyl-2-phenylpyrimidine-4-yl) oxy) alkane or methyl benzene derivatives

We have described a synthesis of novel five-component reaction (5-CR) of bis((6-alkyl or phenyl-2-phenylpyrimidine-4-yl) oxy) alkane or methyl benzene derivatives under ultrasound irradiation. A useful ultrasound effect was observed and title products were obtained with high yields after 25-40 min sonication. Structural confirmation and characterization of the products based on the analytical, chemical, and spectral analysis. The results show obviously, that to reach a high efficiency of ultrasonic systems in synthetic chemical processes. Our procedure compared to the conventional heating method has the benefit of higher reaction yields and shorter reaction times. The in vitro antibacterial tests show that some of the target compounds have promising activities. Structure-activity relationships (SAR) were established for the substitutions on the pyrimidine rings and related length of linear chain linker.

The Flögel-three-component reaction with dicarboxylic acids - an approach to bis(β-alkoxy-β-ketoenamides) for the synthesis of complex pyridine and pyrimidine derivatives

An extension of the substrate scope of the Flögel-three-component reaction of lithiated alkoxyallenes, nitriles and carboxylic acids is presented. The use of dicarboxylic acids allowed the preparation of symmetrical bis(β-ketoenamides) from simple starting materials in moderate yields. Cyclocondensations of these enamides to 4-hydroxypyridine derivatives or to functionalized pyrimidines efficiently provided symmetrically and unsymmetrically substituted fairly complex (hetero)aromatic compounds containing up to six conjugated aryl and hetaryl groups. In addition, subsequent functionalizations of the obtained heterocycles by palladium-catalyzed couplings or by oxidations are reported. We also describe the simple synthesis of a structurally interesting macrocyclic bispyrimidine derivative incorporating a 17-membered ring, whose configuration was elucidated by DFT calculations and by subsequent reactions.

Titanium-mediated cross-coupling reactions of 1,3-butadiynes with α-iminonitriles to 3-aminopyrroles: observation of an imino aza-Nazarov cyclization

Ti(OⁱPr)₄/2 ⁿBuLi-mediated highly efficient cross-coupling reactions of 1,3-butadiynes with α-iminonitriles are described, which provide convenient access to functionalized 3-aminopyrroles.