C-H bond activation is considered a major challenge in modern synthetic organic chemistry, and oxidation reactions the most straightforward way to achieve this purpose. In this context, the reaction of hydrocarbons with oxygen has always played a prominent role as it represents the cheapest way to access complex molecules starting from natural materials. To date, several methodologies have been developed for this purpose. Most of them rely on the use of transition metal complexes due to their high efficiency. However, today’s ecological standards impose the development of greener approaches. In this context, N-oxyl radicals are emerging as a valid alternative and, among them, an outstanding role is played by N-hydroxyphthalimide (NHPI). This is a cheap and commercially available organocatalyst capable of promoting, through the formation of the corresponding phthalimido-N-oxyl (PINO) radical, the free-radical functionalization of a wide variety of substrates. Despite the great interest earned and its high catalytic efficiency, three main limitations related to its solubility, stability, and recovery, prevent the final launch of NHPI as catalyst for industrial processes. In the present thesis, different possible ways to overcome these limitations have been investigated, through the development of: Homogeneous systems (Chapter 3): to improve its solubility in apolar aromatic hydrocarbons, a lipophilic NHPI derivative has been synthesized and applied to the solvent-free aerobic hydroperoxidation of ethylbenzene. The catalyst showed improved efficiency by affording conversion and selectivity values comparable to those of the industrial process but under much milder conditions. The addition of catalytic amounts of ionic liquids (ILs) has been used as alternative strategy to solubilize commercially available NHPI. The NHPI/IL catalytic system has been then successfully applied to both the oxidation of ethylbenzene to acetophenone and the hydroperoxidation of cumene. Heterogeneous systems (Chapter 4): the use of metal-organic frameworks (MOFs) as innovative non-inert materials to support the NHPI unit has been investigated. Twelve different MOFs have been synthesized and used as catalysts in the aerobic epoxidation of linear olefins as well as in the hydroperoxidation of cumene. Finally, a novel NHPI-appendant terephthalic acid ligand has been designed and employed for the synthesis of the first reported example of an NHPI-decorated UiO-66 MOF. The thus obtained material showed improved efficiency in catalysis the aerobic oxidation of tetralin to tetralone with respect to the unmodified UiO-66 MOF. “Multidesign” systems (Chapter 5): a novel imidazole-decorated NHPI derivative has been designed and synthesized. The molecule has been used as ligand for the synthesis of cobalt- and zinc-containing zeolitic imidazolate frameworks with potential applications as heterogeneous catalysts. Moreover, its derivatization through mono- or bis-alkylation have led to the synthesis of a small library of NHPI derivative to be used as lipophilic or ionic liquid catalysts in homogeneous oxidation reactions.
L'attivazione del legame C-H è considerata una sfida importante nella moderna chimica organica sintetica e le reazioni di ossidazione sono il modo più semplice per raggiungere questo scopo. In questo contesto, la reazione degli idrocarburi con l'ossigeno ha sempre giocato un ruolo di primo piano in quanto rappresenta il modo più economico per accedere a molecole complesse partendo da materiali naturali. Ad oggi, sono state sviluppate diverse metodologie per questo scopo. La maggior parte di loro si basa sull'uso di complessi di metalli di transizione a causa della loro alta efficienza. Tuttavia, gli standard ecologici odierni impongono lo sviluppo di approcci più verdi. In questo contesto, i radicali N-ossile stanno emergendo come valida alternativa e, tra questi, un ruolo di primo piano è svolto dall'N-idrossoftalimmide (NHPI). Si tratta di un organocatalizzatore economico e disponibile in commercio in grado di favorire, attraverso la formazione del corrispondente radicale ftalimmido-N-ossile (PINO), la funzionalizzazione radicalica di un'ampia varietà di substrati. Nonostante il grande interesse guadagnato e la sua elevata efficienza catalitica, tre principali limitazioni legate alla sua solubilità, stabilità e recupero, impediscono il lancio finale di NHPI come catalizzatore per processi industriali. Nella presente tesi sono stati indagati diversi modi possibili per superare questi limiti, attraverso lo sviluppo di: Sistemi omogenei (Capitolo 3): per migliorare la sua solubilità negli idrocarburi aromatici apolari, è stato sintetizzato un derivato NHPI lipofilo e applicato all'idroperossidazione aerobica senza solventi dell'etilbenzene. Il catalizzatore ha mostrato una migliore efficienza fornendo valori di conversione e selettività paragonabili a quelli del processo industriale ma in condizioni molto più miti. L'aggiunta di quantità catalitiche di liquidi ionici (IL) è stata utilizzata come strategia alternativa per solubilizzare NHPI disponibile in commercio. Il sistema catalitico NHPI / IL è stato quindi applicato con successo sia all'ossidazione dell'etilbenzene ad acetofenone che all'idroperossidazione del cumene. Sistemi eterogenei (Capitolo 4): è stato studiato l'uso di strutture metallo-organiche (MOF) come materiali innovativi non inerti a supporto dell'unità NHPI. Dodici MOF differenti sono stati sintetizzati e utilizzati come catalizzatori nell'epossidazione aerobica delle olefine lineari e nell'idroperossidazione del cumene. Infine, un nuovo ligando acido tereftalico appendente NHPI è stato progettato e impiegato per la sintesi del primo esempio riportato di un MOF UiO-66 decorato con NHPI. Il materiale così ottenuto ha mostrato una migliore efficienza nella catalisi dell'ossidazione aerobica della tetralina in tetralone rispetto all'UiO-66 MOF non modificato. Sistemi “Multidesign” (Capitolo 5): è stato progettato e sintetizzato un nuovo derivato NHPI decorato con imidazolo. La molecola è stata utilizzata come ligando per la sintesi di strutture imidazolate zeolitiche contenenti cobalto e zinco con potenziali applicazioni come catalizzatori eterogenei. Inoltre, la sua derivatizzazione mediante mono- o bis-alchilazione ha portato alla sintesi di una piccola libreria di derivati NHPI da utilizzare come catalizzatori lipofilici o liquidi ionici in reazioni di ossidazione omogenea.
N-oxyl radicals as catalysts in aerobic oxidations : the route from homogenenous to effective heterogenenous conditions
Caruso, Manfredi
2020/2021
Abstract
C-H bond activation is considered a major challenge in modern synthetic organic chemistry, and oxidation reactions the most straightforward way to achieve this purpose. In this context, the reaction of hydrocarbons with oxygen has always played a prominent role as it represents the cheapest way to access complex molecules starting from natural materials. To date, several methodologies have been developed for this purpose. Most of them rely on the use of transition metal complexes due to their high efficiency. However, today’s ecological standards impose the development of greener approaches. In this context, N-oxyl radicals are emerging as a valid alternative and, among them, an outstanding role is played by N-hydroxyphthalimide (NHPI). This is a cheap and commercially available organocatalyst capable of promoting, through the formation of the corresponding phthalimido-N-oxyl (PINO) radical, the free-radical functionalization of a wide variety of substrates. Despite the great interest earned and its high catalytic efficiency, three main limitations related to its solubility, stability, and recovery, prevent the final launch of NHPI as catalyst for industrial processes. In the present thesis, different possible ways to overcome these limitations have been investigated, through the development of: Homogeneous systems (Chapter 3): to improve its solubility in apolar aromatic hydrocarbons, a lipophilic NHPI derivative has been synthesized and applied to the solvent-free aerobic hydroperoxidation of ethylbenzene. The catalyst showed improved efficiency by affording conversion and selectivity values comparable to those of the industrial process but under much milder conditions. The addition of catalytic amounts of ionic liquids (ILs) has been used as alternative strategy to solubilize commercially available NHPI. The NHPI/IL catalytic system has been then successfully applied to both the oxidation of ethylbenzene to acetophenone and the hydroperoxidation of cumene. Heterogeneous systems (Chapter 4): the use of metal-organic frameworks (MOFs) as innovative non-inert materials to support the NHPI unit has been investigated. Twelve different MOFs have been synthesized and used as catalysts in the aerobic epoxidation of linear olefins as well as in the hydroperoxidation of cumene. Finally, a novel NHPI-appendant terephthalic acid ligand has been designed and employed for the synthesis of the first reported example of an NHPI-decorated UiO-66 MOF. The thus obtained material showed improved efficiency in catalysis the aerobic oxidation of tetralin to tetralone with respect to the unmodified UiO-66 MOF. “Multidesign” systems (Chapter 5): a novel imidazole-decorated NHPI derivative has been designed and synthesized. The molecule has been used as ligand for the synthesis of cobalt- and zinc-containing zeolitic imidazolate frameworks with potential applications as heterogeneous catalysts. Moreover, its derivatization through mono- or bis-alkylation have led to the synthesis of a small library of NHPI derivative to be used as lipophilic or ionic liquid catalysts in homogeneous oxidation reactions.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/171276