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Introduction to Organometallics
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Samir H. Chikkali, Sandeep Netalkar
Apart from oxidative addition and reductive elimination, a very frequently encountered reaction in metal-catalyzed polymerization (especially Ziegler–Natta type polymerization) is insertion of monomer in a M-E (E = CR3, H, etc.) bond to create a long chain molecule called polymer. An insertion reaction in organometallic chemistry represents the interposition of a neutral unsaturated (π) 2e donor ligand L (such as ethylene) into the precoordinated metal E-type ligand bond, leading to the creation of new covalent σ bond between the two ligands (L and E) and between the inserting ligand and metal (M-L) to produce a species M-L-E. The inserted ligand L now functions as X-type ligand. If the inserting ligand is from the same complex then the insertion is referred as migratory insertion. The insertion reaction causes no change in the oxidation state of the metal as the neural unsaturated ligand L reinserts as X-type ligand in between the precoordinated X-type ligand (E) and metal M. However, the loss of L-type coordination causes a decrease in the valence electron count by two units and creation of a vacant site on the metal is enabled (Figure 1.24).
Understanding oxidative addition in organometallics: a closer look
Published in Journal of Coordination Chemistry, 2022
Nabakrushna Behera, Sipun Sethi
In 1986, the British Petroleum (BP) Chemicals took over the Monsanto process, and in 1996, it emerged with a new as well as more effective commercial synthetic process for acetic acid known today as Cativa process [5, 53]. Fundamentally, it utilizes an iridium complex, [IrI2(CO)2]− (49), as catalyst along with a metal-based iodide promoter, InI3 (Scheme 21). In contrast to the Rh-analogue, the in situ generated methyl iodide adds to 49 oxidatively almost 150 times faster. Subsequently, InI3 abstracts one I− ligand from 50 to facilitate solvent coordination, after which the process is completed by coordination of CO, migratory insertion to build an acetyl group, and following reductive elimination of acetyl iodide, which upon hydrolysis gives acetic acid with restoration of HI. Various steps involved in the catalytic cycle are represented in Scheme 21.
Rhodium catalysis in the synthesis of fused five-membered N-heterocycles
Published in Inorganic and Nano-Metal Chemistry, 2020
Navjeet Kaur, Neha Ahlawat, Yamini Verma, Pranshu Bhardwaj, Pooja Grewal, Nirmala Kumari Jangid
The tetrasubstituted stereocenters found in natural products, like FR901483 and cylindricines A, C-F, were generated upon substitution on the tethered alkene (186) as substituted olefins were utilized. The desired cycloadducts were formed in good selectivity and yield from a number of 1,1-disubstituted olefins (186)[153,154] (Scheme 47). Both aryl and alkyl alkynes (185) were tolerated in the reaction. Ligand provided the best enantio- and product selectivity for lactam with alkyl alkynes. Various alkyl olefin substitutions were tolerated; however, increased 2-pyridone (187 and 188) formation and lower yields were observed with sterically bulky substituents which disfavored migratory insertion of the alkene. The desired cycloadduct was formed in 19% yield with cyclohexyl substitution while the yield increased to 75% with butenyl- and methyl-substituted olefins. The size of substituent not diminished the enantioselectivities which remain greater than 87%.
Subtle variation of stereo-electronic effects in rhodium(I) carbonyl Schiff base complexes and their iodomethane oxidative addition kinetics
Published in Journal of Coordination Chemistry, 2020
Pennie P. Mokolokolo, Alice Brink, Andreas Roodt, Marietjie Schutte-Smith
The synthesis and characterization of a series of [Rh(N,O-ScBa)(CO)(PR3)] (PR3 = PPh3, PPh2Cy, PPhCy2 and PCy3) complexes (N,O-ScBaH = Schiff bases) including two crystal structure determinations (1a and 2a) were performed, followed by a detailed mechanistic investigation into the iodomethane oxidative addition thereto. The iodomethane oxidative addition, typically proceeding via an associatively activated mechanism (large negative ΔS≠ values) is accepted to be due to the electron-rich Rh(I) metal center acting as nucleophile. It is manifested in a relatively fast equilibrium for the oxidative addition/reductive elimination in the range of [Rh(5-Me-SalCyP)(CO)(PR3)] (R = phenyl or cyclohexyl) complexes (1a-d), as well as to [Rh(Sal-Cy)(CO)(PPh3)] (2a) and [Rh(5-Me-Sal-IProp)(CO)(PPh3)] (3a). The reaction was well-defined for all complexes, leading exclusively to formation of the octahedral RhIII-alkyl species, [Rh(N,O-ScBa)(CO)(Me)(I)(PR3)] as final products. The reason for this lies in the subtle steric effect exerted primarily by the N,O-ScBa ligand, whereupon the tertiary phosphine ligands adjust themselves to accommodate the alkyl formation, yet does not allow the further migratory insertion to produce the square pyramidal RhIII-acyl species [Rh(N,O-ScBa)(COMe)(I)(PR3)].