Synthesis And Reactivity Of Metal Bis(alkoxide) Complexes In Nitrene Coupling And Polymerization Of Polar Monomers
Open Access Dissertation
Date of Award
The nitrene homocoupling to produce azoarenes was done using two different iron (II) alkoxide complexes, Fe(OCtBu2(3,5-Ph2C6H3)2(THF)2 and Fe[OO]Ph(THF)2. Due to the steric bulkiness of HOCtBu2(3,5-Ph2C6H3 ligand, this complex exhibited selectivity for the bis(alkoxide) ligation; no tris(alkoxide) complexes were observed. As a result, both bulky and non-bulky aryl nitrenes are coupled with Fe(OCtBu2(3,5-Ph2Ph)2(THF)2, albeit the coupling of the less bulky substrates requires higher temperatures and longer reaction times. Stoichiometric reactions of Fe(OCtBu2(3,5-Ph2C6H3)2(THF)2 with non-bulky aryl azides led to the observation of the iron(III) tetrazene radical anion complexes, that can produce azoarene products after heating. Tetrazene complexes likely serve as a “masked form” of the reactive nitrene complex based on these observations and the QM/MM modeling of the reaction mechanism. These calculations suggest that the tetrazene complex is more stable than nitrene. Fe[OO]Ph(THF)2 is selective for coupling aryl nitrenes lacking ortho substituents; no reactivity with ortho-substituted (i.e. mesityl) azide took place. The difference in the reactivity is hypothesized to be due to the sterically congested active site of Fe[OO]Ph, which interferes with the reactivity of putative ‘‘Fe[OO]Ph(=NMes)’’ species. Furthermore, heterocoupling reactivity of different nitrenes was investigeted to produce asymmetric azoarenes, using iron(II) alkoxide pre-catalysts. We have explored the heterocoupling reactivity of Fe(OCtBu2(3,5-Ph2C6H3))2(THF)2 (which previously exhibited wide substrate scope) by itself, as well as the combination of Fe(OCtBu2(3,5-Ph2C6H3))2(THF)2 and Fe[OO]Ph(THF)2 catalysts. Fe(OCtBu2(3,5-Ph2C6H3))2(THF)2 has demonstrated efficient heterocoupling reactivity for a combination of mono-ortho-substituted aryl azides with di-ortho substituted aryl azides. In contrast, any combination involving less bulky meta/para substituted aryl azide did not lead to the efficient production of the heterocoupled product in a good yield due to stable tetrazene complexes with para/meta substituted azides. Mixed catalyst reactivity of Fe(OCtBu2(3,5-Ph2C6H3))2(THF)2 and Fe[OO]Ph(THF)2 was not successful again likely due to the stable tetrazene formation of Fe(OCtBu2(3,5-Ph2C6H3))2(ArNNNNAr). A variety of new asymmetric azoarenes were isolated and their cis-trans isomerism was investigated. All azoarenes were shown to demonstrate the presence of both isomers in solution at room temperature, with the trans isomer being the predominant one. Thermal conditions lead to the full conversion of the mixture to the trans isomer only in all cases, while the irradiation of the mixture with the UV light leads to the predominant formation of the cis isomer. In a second major part of my dissertation, I have reported synthesis, ROP, and ROCOP with a new well-defined mononuclear magnesium complex Mg(OCtBu2Ph)2(THF)2. The complex led to active albeit not well controlled ROP of lactide precursor. Utilization of coordinating solvent (THF) or benzyl alcohol as a co-catalyst leads to better control of polymerization. In contrast, well-behaved ROCOP was obtained with a variety of different monomers. While the use of PPNCl as a nucleophilic initiator leads to an efficient copolymerization of cyclohexene oxide (CHO) with phtalic anhydride (PA) or succinic anhydride (SA), the structure of the resulting copolymers was found to be only moderately alternating, demonstrating small amount of ether linkages. In contrast, the use of BnOH as an initiator forms perfectly alternating copolymer of PA with CHO. More challenging biorenewable monomer limonene oxide (LO) was also co-polymerized with PA. The combination of PA with both CHO and LO leads to the formation of terpolymer. Finally, the combination of two biorenewable precursors, LO and dihydrocumarin, formed a fully biorenewable novel copolymer. No stereoselectivity was observed in all the above reactions, likely due to the achiral nature of the catalyst. Furthermore, several chiral ligands and a chiral magnesium complex were synthesized to investigate stereoselective polymerization of polar monomers for polyester synthesis. Bulky HOCAdtBuPh ligand led to the formation of diasteriomerically pure magnesium complex (Mg(OCAdtBuPh)2(THF)2) which was isolated as homochiral diastereomer of Mg(OCRAdtBuPh)2(THF)2 and Mg(OCSAdtBuPh)2(THF)2 enantiomers. (Mg(OCAdtBuPh)2(THF)2) was a very fast catalyst for polymerization of cyclic esters but it was not able to produce tactic polymers, likely due to the loss of the chiral alkoxides. (Mg(OCAdtBuPh)2(THF)2) was able to show very high reactivity for lactide, ε-caprolactone also showed reactivity for less reactive substrates including ω-pentadecalactone (PDL), and ω-hexadecenlactone (HDL). HOCAdMePh was unable to make diasteriomerically pure Mg complex due to drastic decrese of bulkyness of the ligand.
Wannipurage, Duleeka Chamini, "Synthesis And Reactivity Of Metal Bis(alkoxide) Complexes In Nitrene Coupling And Polymerization Of Polar Monomers" (2021). Wayne State University Dissertations. 3584.