Main Group Metal Complexes as Efficient Catalysts for Functionalization of Small Molecules

Green chemistry, also called sustainable chemistry, is an important area of chemistry as well as chemical engineering that focused on the designing of catalyst or the processes that minimize the use and generation of hazardous substances. To maintain the sustainable production of chemicals in any modern industrial processes it requires high efficiency in any catalytic reactions, by minimizing by-products and wastes also at the same time the metal which is used as a catalyst should be cheaper, earth-abundant, environmental friendly and non-toxic in nature. Thus introducing this earth-abundant metals as a novel catalyst, allows us to readily access a host of applications with lower environmental impact. Chemistry of main group metals are less developed and need to explore more. Due to earth abundant in nature of the alkaline-earth metals and aluminum metal, make it inexpensive, easily accessible along with its low toxicity and biocompatibility make it attractive for various type of application in chemical synthesis. Additionally, it can able to show as a versatile catalyst for a broad spectrum of reactions including polymerization reactions of cyclic esters, styrene and dienes, epoxidation of alkenes, hydrofunctionalization reactions and also atom economical reaction, cross-dehydrocoupling of silanes and amines, hydroelementation (such as hydroamination, hydrophosphination, hydrosilylation and hydroboration) reactions of unsaturtarated moieties such as carbonyl compounds, alkenes and alkynes etc. In my dissertation work, I have primarily focused on the syntheses and structural characterization of well-defined homo- and heteroleptic main group metal complexes supported by functionalized phosphinamino chalcogenide ligands in their coordination sphere as multi-dentate chelating ligands. All newly prepared homoleptic and heteroleptic metal complexes were utilized as active pre-catalysts in atom economical reactions such as ROP of cyclic ester Cross-dehydrocoupling reactions and hydroelementation (hydroboration) reactions

Alkali Metal Catalysed Double Hydrophosphorylation of Nitriles and Alkynes

Catalytic C–P and N–P bond formation via double hydrophosphorylation of nitriles with diphenylphosphane oxide using alkali metal precatalyst [MN(SiMe 3 ) 2 ] (M = Li, Na, K) is reported. The potassium congener was observed to be the most efficient catalyst for converting aryl nitriles to the corresponding N-((diphenyl-phosphoryl)(aryl)methyl)-P,P-diphenylphosphinic amide [ArCHP(O)-Ph 2 NHP(O)Ph 2 ] at mild temperature (60 °C) and with neat reactants. Double hydrophosphorylation of alkynes with HP(O)Ph 2 is also very effective when [KN(SiMe 3 ) 2 ] is used as a precatalyst at room temperature and gives the corresponding 1,2-diylbis(diphenyl-phosphane oxide [RCHP(O)Ph 2 CH 2 P(O)Ph 2 ] as the product. A wider substrate scope of both reactions is explored

Catalytic Hydroboration of Organic Nitriles Promoted by Aluminum Complex

We demonstrate an efficient protocol for the chemoselective hydroboration of organic nitriles with pinacolborane (HBpin) and catecholborane (HBcat) using aluminum alkyl complex [κ2‐{2‐F‐C6H4NP(Se)Ph2}2Al‐(Me)] as a pre‐catalyst to afford diboryl amines under solvent‐free and mild conditions (60°C) in high yield. The aluminum complex was prepared by the reaction of [2‐F‐C6H4NHP(Se)Ph2] and trimethylaluminum in toluene. The solid‐state structure of Al complex is established. Nitriles with a wide array of electron‐withdrawing and electron‐donating functional groups were easily converted to the desired products through the formation of aluminum hydride as an active species. A kinetic study of the catalytic reaction is also reported

Alkaline Earth Metal-Mediated Highly Iso-selective ROP of rac-Lactide

Alkaline earth (Ae) metal complexes of the amidophosphine borane ligand are highly active and iso-selective catalysts for the ring opening polymerization (ROP) of rac -lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac -LA catalyzed by all Ae metal complexes had first-order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex-mediated ROP of rac -LA is discussed, based on controlled kinetic experiments and molecular chain mobility

2-Picolylamino(diphenylphosphinoselenoic)amide supported zinc complexes: Efficient catalyst for insertion of N–H bond into carbodiimides, isocyanates, and isothiocyanate

We report here the hydroamination of heterocumulenes such as carbodiimides, isocyanates, and isothiocyanates by zinc complexes supported by the ligand 2-picolylamino-(diphenylphosphinoselenoic)amide [{(Ph2P-(˭Se)}2NCH2(C5H4N)] (1). A series of zinc complexes [κ2-{(Ph2P-(˭Se)}2NCH2(C5H4N)ZnX2] [(X˭Cl (2), Br (3a), I (4)] were prepared from ligand 1 and the corresponding zinc dihalide in a 1:1 molar ratio at 60°C in a chloroform solvent. The reaction of ligand 1 and ZnBr2 in methanol yielded another zinc complex [κ2-{(Ph2P-(˭Se)}2NCH2(C5H4N)ZnBr2(CH3OH)] (3b). The molecular structures of compounds 3a, 3b, and 4 were established through single-crystal X-ray diffraction analyses. The solid-state structures of all the complexes revealed a κ2- chelation through pyridine nitrogen and selenium atoms of ligand 1 to the zinc ion. Complex 2 proved to be a competent pre-catalyst for the addition of the amine N–H bond to carbodiimides, isocyanates, and isothiocyanates. The reaction scope was expanded to reactions of aliphatic and aromatic amines with phenylisocyanate and phenylisothiocyanate in toluene solvents, which proceeded rapidly at room temperature with 5 mol% catalyst loading to yield (up to 99%) the corresponding derivatives of urea and thio-urea. However, a temperature of 90°C was needed for the hydroamination of N,N′ dicyclohexylcarbodiimide. We also report the most plausible mechanism of the hydroamination reaction

Highly Chemoselective Hydroboration of Alkynes and Nitriles Catalyzed by Group 4 Metal Amidophosphine–Borane Complexes

We report a series of titanium and zirconium complexes supported by dianionic amidophosphine–borane ligands, synthesized by amine elimination and salt metathesis reactions. The TiIV complex [{Ph2P(BH3)N}2C6H4Ti(NMe2)2] (1) was obtained by the reaction between tetrakis-(dimethylamido)titanium(IV) and the protic aminophosphine–borane ligand [{Ph2P(BH3)NH}2C6H4] (LH2) at ambient temperature. Both the heteroleptic zirconium complexes—[η5-(C5H5)2Zr{Ph2P(BH3)N}2C6H4] (2) and [[{Ph2P(BH3)N}2C6H4]ZrCl2] (3)—and the homoleptic zirconium complex [[{Ph2P(BH3)N}2C6H4]2Zr] (4) were obtained in good yield by the salt metathesis reaction of either zirconocene dichloride [η5-(C5H5)2ZrCl2] or zirconium tetrachloride with the dilithium salt of the ligand [{Ph2P(BH3)NLi}2C6H4] (LLi2), which was prepared in situ. The molecular structures of the complexes 1, 2, and 4 in their solid states were confirmed by single-crystal X-ray diffraction analysis. Of these complexes, only titanium complex 1 acts as an effective catalyst for the facile hydroboration of terminal alkynes, yielding exclusive E-isomers. The hydroboration of organic nitriles yielded diborylamines with a broad substrate scope, including broad functional group compatibility. The mechanism of hydroboration occurs through the formation of titanium hydride as an active species

Homoleptic Zinc-Catalyzed Hydroboration of Aldehydes and Ketones in the Presence of HBpin

Here, we report the reaction between N-phenyl-o-phenylenediamine and pyrrole-2-carboxaldehyde to afford the N-phenyl-o-phenyl-enediiminopyrrole ligand {L-H2} in quantitative yield. A one-pot reaction between {L-H2} and diethylzinc (ZnEt2) in a 2:1 ratio afforded the homoleptic zinc metal complex [{L-H}2Zn] (1). The solid-state structures of ligand {L-H2} and zinc complex 1 were confirmed using X-ray crystallography. Further, complex 1 was used for chemoselective hydroboration of aldehydes and ketones in the presence of pinacolborane (HBpin) at ambient temperature to produce the corresponding boronate esters in high yield

Highly Active Dinuclear Titanium(IV) Complexes for the Catalytic Formation of a Carbon–Heteroatom Bond

A series of mononuclear titanium(IV) complexes with the general composition κ3-[R{NHPh2P(X)}2Ti(NMe2)2] [R = C6H4, X = Se (3b); R = trans-C6H10, X = S (4a), Se (4b)] and [{κ2-N(PPh2Se)2}2Ti(NMe2)2] (6b) and two dinuclear titanium(IV) complexes, [C6H4{(NPh2PS)(N)}Ti(NMe2)]2 (3c) and [{κ2-N(PPh2Se)}Ti(NMe2)2]2 (6c), are reported. Dinuclear titanium(IV) complex 6c acts as an efficient catalyst for the chemoselective addition of an E–H bond (E = N, O, S, P, C) to heterocumulenes under mild conditions. The catalytic addition of aliphatic and aromatic amines, alcohol, thiol, phosphine oxide, and acetylene to the carbodiimides afforded the corresponding hydroelemented products in high yield at mild conditions with a broader substrate scope. The catalytic efficiency of the dinuclear complex depends on the cooperative effect of the TiIV ions, the systematic variation of the intermetallic distance, and the ligand’s steric properties of the complex, which enhances the reaction rate. Most interestingly, this is the first example of catalytic insertion of various E–H bonds into the carbodiimides using a single-site catalyst because only the titanium-mediated insertion of E–H into a C═N unsaturated bond is reported to date. The amine and alcohol insertion reaction with the carbodiimides showed first-order kinetics with respect to the titanium(IV) catalyst as well as substrates. A most plausible mechanism for hydroelementation reaction is also proposed, based on the spectroscopic data of the controlled reaction, a time-course study, and the Hammett plot

Hydroboration, Cyanosilylation, and Sequential Cyanosilylation-Hydroboration of Carbonyl Compounds using Ti(IV) Amido Complex as an Efficient Catalyst

Catalytic hydroboration using pinacolborane (4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane) and cyanosilylation using trimethylsilyl cyanide of a wide range of aldehydes and ketones in the presence of titanium (IV) complexes [2‐{(C5H4N)CH2N‐P(=Se)Ph2}Ti(NMe2)3] (1) and [{2‐(C6H4F)NP(=Se)Ph2}Ti(NMe2)3] (2) are reported. The catalytic hydroboration and cyanosilylation processes are high‐yielding, general, and result in efficient conversion of carbonyl compounds to corresponding products under mild conditions. Titanium catalyst 1 exhibits high tolerance toward several functional groups. First‐order kinetics with respect to the catalyst and each of the substrates was observed for the titanium‐catalyzed hydroboration reaction of carbonyl compounds. First principles' calculations were performed to explore the catalytic hydroboration reaction mechanism. In addition, a combination of both reactions and sequential cyanosilylation‐hydroboration was reported. Ti(IV) complexes 1 and 2 were prepared by the aminolysis reaction of [Ti(NMe2)4] with protic ligands [(C5H4N)CH2NHP(=Se)Ph2] (L1H) and [(C6H4F)NHP(=Se)Ph2] (L2H) respectively. The third titanium complex [4‐{(Ph2P(=Se)NCH(C5H4N)CH(C5H4N)‐P(=Se)Ph2}Ti(NMe2)2] (3) was obtained by the reaction of [Ti(NMe2)4] and ligand L1H in a 1:2 molar ratio at room temperature. Molecular structures of complexes 1 and 3 in the solid state are reported