Development of homogeneous palladium catalytic systems for selected transformations of terminal acetylenes

Virant, Miha (author), Košmrlj, Janez (mentor)

This thesis is aimed to describe several aspects and stages in the development of palladium catalytic systems through several case studies connected to functionalization of terminal acetylenes. Initially, the development of new ligand precursors is addressed through the synthesis of novel N-heterocyclic carbene (NHC) precursors, namely 1,3,4-triaryl-1H-1,2,3-triazolium salts. They were prepared by selective arylation of click triazoles with diaryliodonium salts in the presence of copper salts. The neat reaction protocol, which was also developed in a ‘one-pot’ fashion, allowed for the preparation of unsymmetrically substituted triazolium salts among which the examples containing pyridine substituent were particularly interesting. They were employed in the preparation of PdII, RuII and IrIII complexes. Additionally, Ru-NHC and Ir-NHC thus formed showed increased catalytic performance in transfer hydrogenation of benzophenone as compared to previously reported complexes with alkyl substituted Py-tzNHC ligands. Mechanism of copper-free Sonogashira reaction, an important cross-coupling process employing terminal acetylenes, was elucidated through a thorough experimental-computational study. It was demonstrated thorough an extensive 1D and 2D NMR study of reaction mixtures combined with reaction kinetics as well as density functional theory (DFT) calculations that the reaction operates through an interplay of two Pd catalytic cycles connected in the transmetallation step. The new mechanistic understanding of the catalytic process, that was previously overlooked, should inspire the development of new and improved catalytic systems for the preparation of internal acetylenes. The synthesis of highly substituted 1,3-enynes was investigated employing aryl halides and acetylenes. During the study, silyl-substituted geminal dimers of acetylenes were successfully coupled with aryl halides under the conditions for Heck coupling for the preparation of 1,2,4-trisubstituted analogues of 1,3-enynes. Finally, the DFT mechanistic study of an effective hydroamination reaction of acetylenes catalysed by Pd-iPEPPSI (internal pyridine-enhanced precatalyst preparation, stabilization, and initiation) catalyst offered insight in to the active role of pyridine substituent in pyridine substituted triazoliumligands (Py-tzNHCs). Noteworthy, pyridine substituent can act as an internal base and enable proton shuttling in this new type of catalysis.

• Language: English
• Keywords: palladium, triazolium salts, carbenes, catalysis, reaction mechanisms
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2019
• PID: 20.500.12556/RUL-113031
• COBISS.SI-ID: 303177472
• Publication date in RUL: 29.11.2019

Secondary language

• Language: Slovenian
• Title: Razvoj homogenih paladijevih katalitskih sistemov za izbrane pretvorbe terminalnih acetilenov

Abstract

Doktorsko delo obsega proučevanje in razvoj homogenih paladijevih katalitskih sistemov. Obravnava različne vidike v razvoju katalitskih procesov skozi obravnavo reakcij za funkcionalizacijo terminalnih acetilenov. Na začetku smo v delu razvili metodo za pripravo nove generacije prekurzorjev N-heterocikličnih karbenov (NHC), natančneje 1,3,4-triaril-1H-1,2,3-triazolijevih soli. Ariliranje klik triazolov z diariljodonijevimi solmi v prisotnosti bakrovega katalizatorja vodi do selektivnega nastanka N-3 ariliranih produktov z dobrimi izkoristki. Metoda omogoča pripravo nesimetrično substituiranih triazolijevih soli, med katerimi so posebej zanimivi derivati funkcionalizirani s piridinskim substituentom. Te smo uporabili v postopkih sinteze PdII, RuII in IrIII kompleksov. Medtem ko nam paladijevih kompleksov ni uspelo pripravitiv čisti obliki zaradi nastanka kompleksnih zmesi produktov, smo novi generaciji Ru-NHC in Ir-NHC kompleksov izolirali z dobrimi izkoristki. Katalitsko aktivnost kompleksov z novo serijo ligandov smo testirali na primeru katalitskega hidrogeniranja s prenosom vodika. Primerjave rezultatov testiranja s predhodnimi podatki iz študij z alkil-substituiranimi solmi so pokazale, da so kompleksi s triariliranimi triazolijevimi ligandi, ki vsebujejo piridinski substituent, precej bolj stabilni, kar vodi do boljših izkoristkov pretvorb po daljših reakcijskih časih. Sonogashirova reakcija je pomembna s paladijem katalizirana reakcija spajanja za funkcionalizacijo terminalnih acetilenov. Mehanizem njene različice brez uporabe bakrovih kokatalizatorjev je kljub intenzivnim študijam ostal neraziskan. Z novim vpogledom v reakcijski mehanizem, ki ga je omogočil tip katalizatorjev na osnovi iPEPPSI ligandov (interna s piridinom olajšana priprava, stabilizacija in iniciacija predkatalizatorja), smo uspeli to uganko razrešiti. Kombinirana eksperimentalno-teoretična študija je s prepletom NMR analiz, kinetičnih raziskav in kvantnomehanskih DFT izračunov pokazala, da reakcija poteka preko dveh paladijevih katalitskih ciklov, ki ju povezuje stopnja transmetalacije. Nov vpogled v reakcijski mehanizem omogoča razvoj novih katalitskih sistemov za pripravo internih acetilenov. V delu je opisana tudi študija tvorbe substituiranih 1,3-eninov. V njej je predstavljena s paladijem katalizirana metoda za pripravo tovrstnih eninov s sililnimi substituenti. Gre za Heckovo reakcijo med geminalnimi dimeri acetilenov in aril halogenidi. Na koncu je v DFT mehanistični študiji reakcije hidroaminiranja organskih acetilenov katalizirane s Pd-iPEPPSI kompleksom predstavljen nov tip katalize. Študija je pokazala, da piridin poleg klasične stabilizacijske vloge deluje tudi kot aktiven ligand, ki v vlogi interne baze omogoči intramolekularni prenos protonov.

• Keywords: paladij, triazolijeve soli, karbeni, kataliza, reakcijski mehanizem