Electrophilic Ring Bond Activation of 2H-Azaphosphirene Complexes
Electrophilic Ring Bond Activation of 2H-Azaphosphirene Complexes
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DissertationDate of Exam
22.04.2010Date of Publication
20.05.2010Advisor
Streubel, RainerCo-Referee
Niecke, EdgarMetadata
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Abstract
In combined experimental and computational studies the scope of different strategies for P,N bond activation and ring expansion of 2H-azaphosphirene complexes was investigated.
First, a protocol using nitriles and catalytic amounts of SET oxidants such as [FeCp2] [PF6] was explored, which leads to 2H-1,4,2-diazaphosphole complexes. Studies on the reaction course revealed evidence for a radical cation chain reaction mechanism. For reactions with heterocyclic carbonitriles it was demonstrated that the amount of [FeCp2][PF6] can be reduced to 0.05-0.025 equivalents. In the absence of nitriles symmetrically 3,5-disubstituted 2H-1,4,2-diazaphosphole complexes were formed, one of which could be structurally confirmed by single-crystal X-ray diffraction. Here, the nitrile fragment that is required for the formation of these products stems from the 2H-azaphosphirene complex. DFT calculations on the reaction mechanism revealed that the 2H-azaphosphirene complex is oxidized in the first step and the resulting cationic species is a metal-centered radical that shows ligand-centered reactivity, as the nucleophilic attack of a nitrile causes ring opening followed by facile cyclization of the acyclic intermediate.
The synthesis of various 2H-1,4,2-diazaphosphole complex derivatives with different P, C3, and C5 substituents as well as different metal fragments (Cr(CO)5, Mo(CO)5, W(CO)5) from 2H-azaphosphirene complexes and nitriles was achieved by consecutive reaction with the strong acid TfOH and a base (triethylamine or pyridine); all products were isolated and unambiguously identified. This protocol offers new synthetic perspectives as even nitriles with high steric demand could successfully be employed, and it enabled access to the first C-SiMe3 ring-functionalized 2H-1,4,2-diazaphosphole complex. By 31P NMR spectroscopy evidence was obtained for the formation of 2H-1,4,2-diazaphospholium complexes. Two derivatives thereof were isolated, and one was structurally confirmed by a single-crystal X-ray diffraction study. Reactions of 2H-azaphosphirene complexes with HCN afforded mixtures of κP- and κN-bonded 2H-1,4,2-diazaphosphole complexes. A chemical equilibrium between the two haptomeric complexes was evidenced by 31P{1H} NMR spectroscopic measurements at varying temperatures. In ring expansion reactions of molybdenum and chromium complexes with dimethyl cyanamide partial decomplexation was observed, but this could completely be prevented by adding the base at low temperature. On the other hand, after prolonged reaction times the N1-protonated liberated 2H-1,4,2-diazaphosphole ligand was quantitatively formed and could be characterized by NMR spectroscopy. Subsequent deprotonation afforded the neutral heterocycle. Although it decomposed during column chromatography, all relevant NMR spectroscopic information was obtained from the raw product. The reaction of a 2H-azaphosphirene complex with cyclohexyl isocyanide under the same conditions yielded a novel 2,3-dihydro-1,3-azaphosphete complex; its molecular structure was confirmed by single-crystal X-ray diffractometry. Investigations on the applicability of various Brønsted and Lewis acids to induce the ring expansion with dimethyl cyanamide showed that good results can also be obtained with CF3CO2H, B(C6F5)3, and Li[B(C6F5)sub>4].
The reaction with TfOH in the absence of trapping reagents resulted in partial desilylation of the P-substituent combined with ring opening, thus leading to coordination-isomeric N-protonated 1-aza-3-phosphabutadiene complexes. Their characterization was achieved by multinuclear NMR spectroscopy at low temperature. Subsequent reaction with a nitrile and deprotonation yielded a 2H-1,4,2-diazaphosphole complex with R(P) = CH2SiMe3, which was isolated and structurally confirmed. The first 2H-azaphosphirenium complex was observed in the reaction of a P-Cp* substituted 2H-azaphosphirene complex with TfOH. It was characterized by multinuclear one- and two-dimensional NMR experiments. Upon addition of a nitrile ring expansion occurred, and it was demonstrated that the protonation can be reversed through the addition of NEt3. On the basis of DFT calculations a mechanism for the acid-induced ring expansion of 2H-azaphosphirene complexes with nitriles and isonitriles is proposed. Upon N-protonation the resulting 2H-azaphosphirenium complex is prone to undergo spontaneous ring opening with formation of a phosphenium complex. Following nucleophilic attack of a nitrile or an isonitrile and subsequent cyclization and deprotonation yields the final products.
In their UV/Vis spectra 2H-1,4,2-diazaphosphole complexes show absorption bands at very long wavelengths. This was interpreted on the basis of TD-DFT calclulations, which revealed that the longest-wavelength absorption is assigned to a metal-ligand charge transfer (MLCT) process; a second band was assigned to a π-π* transition. Furthermore, the origin of the intense colors of 2H-1,4,2-diazaphospholium complexes as well as the photophysical properties of κ N-coordinated 2H-1,4,2-diazaphosphole complexes was elucidated by TD-DFT calclulations.
In the reaction of a mixture of κP- and κN-bound 2H-1,4,2-diazaphosphole complexes with MeOTf and TfOH methylation of the N4-center occurred in combination with partial desilylation at the exocyclic P-substituent. Both SiMe3 groups of such complexes could be removed via reaction with [nBu4N]F in the presence of [Et3NH][OTf], which afforded a P-methyl substituted 2H-1,4,2-diazaphosphole complex.
Finally, the synthesis of the first Bis(2H-azaphosphirene complexes) is presented. Reaction of a 1,1'-ferrocenediyl-bis(aminocarbene complex) with a chloro(methylene)phosphane in the presence of NEt3 yielded first a Mono(2H-azaphosphirene complex) with an unreacted aminocarbene complex group at the other Cp-ring. The reaction of the 1,1'-ferrocenediyl-bis(aminocarbene complex) with one equivalent of ClP=C(SiMe3)2 and NEt3 in a dilute CH2Cl2 solution yielded a novel diaminophosphane-bridged [5]ferrocenophane bis(carbene complex), having two metal carbene centers that electronically communicate with the iron center, which was isolated and structurally confirmed. Subsequent reaction with ClP=C(SiMe3)2 and NEt3 afforded a mixture of the desired bis(2H-azaphosphirene complexes) and a 2,3-dihydro-1,2,3-azadiphosphete complex. The structure of the latter was determined by a by single-crystal X-ray diffraction study. It could completely be separated, and a purified mixture of the diastereomeric target complexes was obtained. Elektrophile Ringbindungs-Aktivierung von 2H-Azaphosphirenkomplexen
In dieser Arbeit wurde die Anwendungsbreite verschiedener Strategien zur P,N Bindungsaktivierung und Ringerweiterung von 2H-Azaphosphirenkomplexen mittels experimenteller und theoretischer Methoden untersucht.
Zuerst wurde ein Verfahren untersucht, bei dem Nitrile und katalytische Mengen an SET Oxidanzien eingesetzt werden und das zu 2H-1,4,2-Diazaphospholkomplexen führt. Studien zum Reaktionsverlauf lieferten Hinweise auf einen radikal-kationischen Kettenreaktionsmechanismus. Für Reaktionen mit heterocyclischen Carbonitrilen konnte gezeigt werden, dass die Menge an eingesetztem [FeCp2][PF6] auf 0.05-0.025 Äquivalente reduziert werden kann. In Abwesenheit von Nitrilen wurden symmetrisch 3,5-disubstituierte 2H-1,4,2-Diazaphospholkomplexe gebildet, von denen einer mittels Einkristall-Röntgendiffraktometrie strukturell charakterisiert werden konnte. In diesen Fällen stammt das Nitrilfragment, das zur Bildung der Produkte benötigt wird, aus dem 2H-Azaphosphirenkomplex. DFT-Berechnungen zum Reaktionsmechanismus ergaben, dass der 2H-Azaphosphirenkomplex im ersten Schritt oxidiert wird. Die resultierende kationische Spezies stellt ein Metall-zentriertes Radikal dar, das Ligand-zentrierte Reaktivität aufweist, da der nucleophile Angriff eines Nitrils die Ringöffnung des Liganden verursacht. Anschließend erfolgt die Cyclisierung des gebildeten acyclischen Intermediats.
Die Synthese zahlreicher 2H-1,4,2-Diazaphospholkomplex-Derivate mit verschiedenen P-, C3- und C5-Substituenten sowie verschiedenen Metallfragmenten (Cr(CO)5, Mo(CO)5, W(CO)5) gelang ausgehend von 2H-Azaphosphirenkomplexen und Nitrilen durch konsekutive Umsetzung mit der starken Säure TfOH und einer Base (Triethylamin oder Pyridin). Alle Produkte wurden isoliert und eindeutig identifiziert. Diese Methode bietet neue synthetische Perspektiven, da sogar Nitrile mit großem sterischen Anspruch erfolgreich eingesetzt werden konnten. Außerdem eröffnete sie Zugang zum ersten C-SiMe3 Ring-funktionalisierten 2H-1,4,2-Diazaphospholkomplex. Durch 31P-NMR Spectroskopie wurden Hinweise auf 2H-1,4,2-Diazaphospholiumkomplexe erhalten. Zwei Derivate wurden isoliert und eines wurde strukturell charakterisiert. Reaktionen von 2H-Azaphosphirenkomplexen mit HCN ergaben Mischungen aus κP- und κN-gebundenen 2H-1,4,2-Diazaphospholkomplexen. Durch 31P{1H} NMR spektroskopische Messungen bei verschiedenen Temperaturen konnte ein chemisches Gleichgewicht zwischen den beiden haptomeren Komplexen nachgewiesen werden. In Ringerweiterungsreaktionen von Molybdän- und Chromkomplexen mit Dimethylcyanamid wurde partielle Dekomplexierung beobachtet, was allerdings durch Zugabe der Base bei tiefer Temperatur unterdrückt werden konnte. Andererseits wurde nach längeren Reaktionszeiten quantitativ der N1-protonierte freie 2H-1,4,2-Diazaphospholligand gebildet, der mittels NMR Spektroskopie charakterisiert wurde. Anschließende Deprotonierung ergab den neutralen Heterocyclus, der sich jedoch während der Säulenchromatographie zersetzte. Allerdings konnten aus dem Rohprodukt alle relevanten NMR-spektroskopischen Informationen erhalten werden. Die Umsetzung eines 2H-Azaphosphirenkomplexes mit Cyclohexylisonitril ergab unter denselben Bedingungen einen neuartigen 2,3-Dihydro-1,3-azaphosphetkomplex, dessen Molekülstruktur mittels Einkristall-Röntgendiffraktometrie aufgeklärt wurde. Untersuchungen zur Einsetzbarkeit verschiedener Brønsted- und Lewis-Säuren zur Induktion der Ringerweiterung ergab, dass gute Resultate auch mit CF3CO2H, B(C6F5)3 und Li[B(C6F5)4] erzielt werden können.
Bei der Reaktion mit TfOH in Abwesenheit von Abfangreagenzien erfolgte partielle Desilylierung des P-Substituenten in Kombination mit Ringöffnung unter Bildung koordinationsisomerer, N-protonierter 1-Aza-3-phosphabutadienkomplexe, die mittels Multikern-NMR Spektroskopie bei tiefer Temperatur charakterisiert werden konnten. Nach anschließender Reaktion mit einem Nitril und Deprotonierung bildete sich ein 2H-1,4,2-Diazaphospholkomplex mit R(P) = CH2SiMe3, der isoliert und strukturell abgesichert wurde. Der erste 2H-Azaphosphireniumkomplex wurde bei der Reaktion eines P-Cp*-substituierten 2H-Azaphosphirenkomplexes mit TfOH beobachtet und mittels ein- und zweidimensionaler Multikern-NMR Experimente charakterisiert. Nach Zugabe eines Nitrils erfolte Ringerweiterung und es konnte gezeigt werden, dass die Protonierung durch Zugabe von NEt3 Rückgängig gemacht werden konnte. Ein plausibler Mechanismus für die Säure-induzierte Ringerweiterung wurde auf der Basis von DFT Berechnungen vorgeschlagen. Nach N-Protonierung tendiert der gebildete 2H-Azaphosphireniumkomplex zur spontanen Ringöffnung unter Bildung eines Phospheniumkomplexes. Der darauf folgende nucleophile Angriff eines Nitrils oder Isonitrils und die anschließende Cyclisierung und Deprotonierung führt schließlich zu den Endprodukten.
Die UV/Vis-Spektren von 2H-1,4,2-Diazaphospholkomplexen weisen Absorptionsbanden bei sehr langen Wellenlängen auf. Diese konnten anhand von TD-DFT Berechnungen interpretiert werden. Dabei konnte die längst-wellige Absorption einem Metal-Ligand Charge-Transfer-Prozess (MLCT) zugewiesen werden. Eine weitere Bande wurde einem π-π* Übergang zugeordnet. Des weiteren wurden die intensive Farbigkeit der 2H-1,4,2-Diazaphospholiumkomplexe sowie die photophysikalischen Eigenschaften der κN-koordinierten 2H-1,4,2-Diazaphospholkomplexe mittels TD-DFT untersucht.
Bei der Reaktion einer Mischung von κP- und κN-gebundenen 2H-1,4,2-Diazaphospholkomplexen mit MeOTf and TfOH erfolgte Methylierung des N4-Zentrums in Kombination mit partieller Desilylierung am exocyclischen P-Substituenten. Beide SiMe3-Gruppen derartiger Komplexe konnten durch Reaktion mit [nBu4N]F in Gegenwart von [Et3NH][OTf] vollständig entfernt werden, wodurch ein P-Methyl-substituierter 2H-1,4,2-Diazaphospholkomplex erhalten wurde.
Schließlich wird die Synthese des ersten Bis(2H-azaphosphirenkomplexes) vorgestellt. Die Reaktion eines 1,1'-Ferrocendiyl-bis(aminocarbenkomplexes) mit einem Chlor(methylen)phosphan in Anwesenheit von NEt3 ergab zunächst einen Mono(2H-azaphosphirenkomplex) mit einer verbleibenden Aminocarbenkomplex-Gruppe am anderen Cp-Ring. Bei der Reaktion des 1,1'-Ferrocendiyl-bis(aminocarbenkomplexes) mit einem Äquivalent an ClP=C(SiMe3)2 und NEt3 wurde ein neuartiger Diaminophosphan-überbrückter [5]Ferrocenophan-bis(carbenkomplex) mit zwei Metall-Carbenzentren, die elektronisch mit dem Eisenzentrum kommunizieren, erhalten. Dieser wurde isoliert und strukturell abgesichert. Die anschließende Reaktion mit ClP=C(SiMe3)2 und NEt3 lieferte eine Mischung des gewünschten Bis(2H-azaphosphirenkomplexes) und einem 2,3-Dihydro-1,2,3-azadiphosphetkomplex. Seine Struktur wurde mittels Einkristall-Röntgendiffraktometrie aufgeklärt. Er konnte vollständig abgetrennt werden, wodurch eine gereinigte Mischung der diastereomeren Zielverbindungen erhalten wurde.
First, a protocol using nitriles and catalytic amounts of SET oxidants such as [FeCp2] [PF6] was explored, which leads to 2H-1,4,2-diazaphosphole complexes. Studies on the reaction course revealed evidence for a radical cation chain reaction mechanism. For reactions with heterocyclic carbonitriles it was demonstrated that the amount of [FeCp2][PF6] can be reduced to 0.05-0.025 equivalents. In the absence of nitriles symmetrically 3,5-disubstituted 2H-1,4,2-diazaphosphole complexes were formed, one of which could be structurally confirmed by single-crystal X-ray diffraction. Here, the nitrile fragment that is required for the formation of these products stems from the 2H-azaphosphirene complex. DFT calculations on the reaction mechanism revealed that the 2H-azaphosphirene complex is oxidized in the first step and the resulting cationic species is a metal-centered radical that shows ligand-centered reactivity, as the nucleophilic attack of a nitrile causes ring opening followed by facile cyclization of the acyclic intermediate.
The synthesis of various 2H-1,4,2-diazaphosphole complex derivatives with different P, C3, and C5 substituents as well as different metal fragments (Cr(CO)5, Mo(CO)5, W(CO)5) from 2H-azaphosphirene complexes and nitriles was achieved by consecutive reaction with the strong acid TfOH and a base (triethylamine or pyridine); all products were isolated and unambiguously identified. This protocol offers new synthetic perspectives as even nitriles with high steric demand could successfully be employed, and it enabled access to the first C-SiMe3 ring-functionalized 2H-1,4,2-diazaphosphole complex. By 31P NMR spectroscopy evidence was obtained for the formation of 2H-1,4,2-diazaphospholium complexes. Two derivatives thereof were isolated, and one was structurally confirmed by a single-crystal X-ray diffraction study. Reactions of 2H-azaphosphirene complexes with HCN afforded mixtures of κP- and κN-bonded 2H-1,4,2-diazaphosphole complexes. A chemical equilibrium between the two haptomeric complexes was evidenced by 31P{1H} NMR spectroscopic measurements at varying temperatures. In ring expansion reactions of molybdenum and chromium complexes with dimethyl cyanamide partial decomplexation was observed, but this could completely be prevented by adding the base at low temperature. On the other hand, after prolonged reaction times the N1-protonated liberated 2H-1,4,2-diazaphosphole ligand was quantitatively formed and could be characterized by NMR spectroscopy. Subsequent deprotonation afforded the neutral heterocycle. Although it decomposed during column chromatography, all relevant NMR spectroscopic information was obtained from the raw product. The reaction of a 2H-azaphosphirene complex with cyclohexyl isocyanide under the same conditions yielded a novel 2,3-dihydro-1,3-azaphosphete complex; its molecular structure was confirmed by single-crystal X-ray diffractometry. Investigations on the applicability of various Brønsted and Lewis acids to induce the ring expansion with dimethyl cyanamide showed that good results can also be obtained with CF3CO2H, B(C6F5)3, and Li[B(C6F5)sub>4].
The reaction with TfOH in the absence of trapping reagents resulted in partial desilylation of the P-substituent combined with ring opening, thus leading to coordination-isomeric N-protonated 1-aza-3-phosphabutadiene complexes. Their characterization was achieved by multinuclear NMR spectroscopy at low temperature. Subsequent reaction with a nitrile and deprotonation yielded a 2H-1,4,2-diazaphosphole complex with R(P) = CH2SiMe3, which was isolated and structurally confirmed. The first 2H-azaphosphirenium complex was observed in the reaction of a P-Cp* substituted 2H-azaphosphirene complex with TfOH. It was characterized by multinuclear one- and two-dimensional NMR experiments. Upon addition of a nitrile ring expansion occurred, and it was demonstrated that the protonation can be reversed through the addition of NEt3. On the basis of DFT calculations a mechanism for the acid-induced ring expansion of 2H-azaphosphirene complexes with nitriles and isonitriles is proposed. Upon N-protonation the resulting 2H-azaphosphirenium complex is prone to undergo spontaneous ring opening with formation of a phosphenium complex. Following nucleophilic attack of a nitrile or an isonitrile and subsequent cyclization and deprotonation yields the final products.
In their UV/Vis spectra 2H-1,4,2-diazaphosphole complexes show absorption bands at very long wavelengths. This was interpreted on the basis of TD-DFT calclulations, which revealed that the longest-wavelength absorption is assigned to a metal-ligand charge transfer (MLCT) process; a second band was assigned to a π-π* transition. Furthermore, the origin of the intense colors of 2H-1,4,2-diazaphospholium complexes as well as the photophysical properties of κ N-coordinated 2H-1,4,2-diazaphosphole complexes was elucidated by TD-DFT calclulations.
In the reaction of a mixture of κP- and κN-bound 2H-1,4,2-diazaphosphole complexes with MeOTf and TfOH methylation of the N4-center occurred in combination with partial desilylation at the exocyclic P-substituent. Both SiMe3 groups of such complexes could be removed via reaction with [nBu4N]F in the presence of [Et3NH][OTf], which afforded a P-methyl substituted 2H-1,4,2-diazaphosphole complex.
Finally, the synthesis of the first Bis(2H-azaphosphirene complexes) is presented. Reaction of a 1,1'-ferrocenediyl-bis(aminocarbene complex) with a chloro(methylene)phosphane in the presence of NEt3 yielded first a Mono(2H-azaphosphirene complex) with an unreacted aminocarbene complex group at the other Cp-ring. The reaction of the 1,1'-ferrocenediyl-bis(aminocarbene complex) with one equivalent of ClP=C(SiMe3)2 and NEt3 in a dilute CH2Cl2 solution yielded a novel diaminophosphane-bridged [5]ferrocenophane bis(carbene complex), having two metal carbene centers that electronically communicate with the iron center, which was isolated and structurally confirmed. Subsequent reaction with ClP=C(SiMe3)2 and NEt3 afforded a mixture of the desired bis(2H-azaphosphirene complexes) and a 2,3-dihydro-1,2,3-azadiphosphete complex. The structure of the latter was determined by a by single-crystal X-ray diffraction study. It could completely be separated, and a purified mixture of the diastereomeric target complexes was obtained.
In dieser Arbeit wurde die Anwendungsbreite verschiedener Strategien zur P,N Bindungsaktivierung und Ringerweiterung von 2H-Azaphosphirenkomplexen mittels experimenteller und theoretischer Methoden untersucht.
Zuerst wurde ein Verfahren untersucht, bei dem Nitrile und katalytische Mengen an SET Oxidanzien eingesetzt werden und das zu 2H-1,4,2-Diazaphospholkomplexen führt. Studien zum Reaktionsverlauf lieferten Hinweise auf einen radikal-kationischen Kettenreaktionsmechanismus. Für Reaktionen mit heterocyclischen Carbonitrilen konnte gezeigt werden, dass die Menge an eingesetztem [FeCp2][PF6] auf 0.05-0.025 Äquivalente reduziert werden kann. In Abwesenheit von Nitrilen wurden symmetrisch 3,5-disubstituierte 2H-1,4,2-Diazaphospholkomplexe gebildet, von denen einer mittels Einkristall-Röntgendiffraktometrie strukturell charakterisiert werden konnte. In diesen Fällen stammt das Nitrilfragment, das zur Bildung der Produkte benötigt wird, aus dem 2H-Azaphosphirenkomplex. DFT-Berechnungen zum Reaktionsmechanismus ergaben, dass der 2H-Azaphosphirenkomplex im ersten Schritt oxidiert wird. Die resultierende kationische Spezies stellt ein Metall-zentriertes Radikal dar, das Ligand-zentrierte Reaktivität aufweist, da der nucleophile Angriff eines Nitrils die Ringöffnung des Liganden verursacht. Anschließend erfolgt die Cyclisierung des gebildeten acyclischen Intermediats.
Die Synthese zahlreicher 2H-1,4,2-Diazaphospholkomplex-Derivate mit verschiedenen P-, C3- und C5-Substituenten sowie verschiedenen Metallfragmenten (Cr(CO)5, Mo(CO)5, W(CO)5) gelang ausgehend von 2H-Azaphosphirenkomplexen und Nitrilen durch konsekutive Umsetzung mit der starken Säure TfOH und einer Base (Triethylamin oder Pyridin). Alle Produkte wurden isoliert und eindeutig identifiziert. Diese Methode bietet neue synthetische Perspektiven, da sogar Nitrile mit großem sterischen Anspruch erfolgreich eingesetzt werden konnten. Außerdem eröffnete sie Zugang zum ersten C-SiMe3 Ring-funktionalisierten 2H-1,4,2-Diazaphospholkomplex. Durch 31P-NMR Spectroskopie wurden Hinweise auf 2H-1,4,2-Diazaphospholiumkomplexe erhalten. Zwei Derivate wurden isoliert und eines wurde strukturell charakterisiert. Reaktionen von 2H-Azaphosphirenkomplexen mit HCN ergaben Mischungen aus κP- und κN-gebundenen 2H-1,4,2-Diazaphospholkomplexen. Durch 31P{1H} NMR spektroskopische Messungen bei verschiedenen Temperaturen konnte ein chemisches Gleichgewicht zwischen den beiden haptomeren Komplexen nachgewiesen werden. In Ringerweiterungsreaktionen von Molybdän- und Chromkomplexen mit Dimethylcyanamid wurde partielle Dekomplexierung beobachtet, was allerdings durch Zugabe der Base bei tiefer Temperatur unterdrückt werden konnte. Andererseits wurde nach längeren Reaktionszeiten quantitativ der N1-protonierte freie 2H-1,4,2-Diazaphospholligand gebildet, der mittels NMR Spektroskopie charakterisiert wurde. Anschließende Deprotonierung ergab den neutralen Heterocyclus, der sich jedoch während der Säulenchromatographie zersetzte. Allerdings konnten aus dem Rohprodukt alle relevanten NMR-spektroskopischen Informationen erhalten werden. Die Umsetzung eines 2H-Azaphosphirenkomplexes mit Cyclohexylisonitril ergab unter denselben Bedingungen einen neuartigen 2,3-Dihydro-1,3-azaphosphetkomplex, dessen Molekülstruktur mittels Einkristall-Röntgendiffraktometrie aufgeklärt wurde. Untersuchungen zur Einsetzbarkeit verschiedener Brønsted- und Lewis-Säuren zur Induktion der Ringerweiterung ergab, dass gute Resultate auch mit CF3CO2H, B(C6F5)3 und Li[B(C6F5)4] erzielt werden können.
Bei der Reaktion mit TfOH in Abwesenheit von Abfangreagenzien erfolgte partielle Desilylierung des P-Substituenten in Kombination mit Ringöffnung unter Bildung koordinationsisomerer, N-protonierter 1-Aza-3-phosphabutadienkomplexe, die mittels Multikern-NMR Spektroskopie bei tiefer Temperatur charakterisiert werden konnten. Nach anschließender Reaktion mit einem Nitril und Deprotonierung bildete sich ein 2H-1,4,2-Diazaphospholkomplex mit R(P) = CH2SiMe3, der isoliert und strukturell abgesichert wurde. Der erste 2H-Azaphosphireniumkomplex wurde bei der Reaktion eines P-Cp*-substituierten 2H-Azaphosphirenkomplexes mit TfOH beobachtet und mittels ein- und zweidimensionaler Multikern-NMR Experimente charakterisiert. Nach Zugabe eines Nitrils erfolte Ringerweiterung und es konnte gezeigt werden, dass die Protonierung durch Zugabe von NEt3 Rückgängig gemacht werden konnte. Ein plausibler Mechanismus für die Säure-induzierte Ringerweiterung wurde auf der Basis von DFT Berechnungen vorgeschlagen. Nach N-Protonierung tendiert der gebildete 2H-Azaphosphireniumkomplex zur spontanen Ringöffnung unter Bildung eines Phospheniumkomplexes. Der darauf folgende nucleophile Angriff eines Nitrils oder Isonitrils und die anschließende Cyclisierung und Deprotonierung führt schließlich zu den Endprodukten.
Die UV/Vis-Spektren von 2H-1,4,2-Diazaphospholkomplexen weisen Absorptionsbanden bei sehr langen Wellenlängen auf. Diese konnten anhand von TD-DFT Berechnungen interpretiert werden. Dabei konnte die längst-wellige Absorption einem Metal-Ligand Charge-Transfer-Prozess (MLCT) zugewiesen werden. Eine weitere Bande wurde einem π-π* Übergang zugeordnet. Des weiteren wurden die intensive Farbigkeit der 2H-1,4,2-Diazaphospholiumkomplexe sowie die photophysikalischen Eigenschaften der κN-koordinierten 2H-1,4,2-Diazaphospholkomplexe mittels TD-DFT untersucht.
Bei der Reaktion einer Mischung von κP- und κN-gebundenen 2H-1,4,2-Diazaphospholkomplexen mit MeOTf and TfOH erfolgte Methylierung des N4-Zentrums in Kombination mit partieller Desilylierung am exocyclischen P-Substituenten. Beide SiMe3-Gruppen derartiger Komplexe konnten durch Reaktion mit [nBu4N]F in Gegenwart von [Et3NH][OTf] vollständig entfernt werden, wodurch ein P-Methyl-substituierter 2H-1,4,2-Diazaphospholkomplex erhalten wurde.
Schließlich wird die Synthese des ersten Bis(2H-azaphosphirenkomplexes) vorgestellt. Die Reaktion eines 1,1'-Ferrocendiyl-bis(aminocarbenkomplexes) mit einem Chlor(methylen)phosphan in Anwesenheit von NEt3 ergab zunächst einen Mono(2H-azaphosphirenkomplex) mit einer verbleibenden Aminocarbenkomplex-Gruppe am anderen Cp-Ring. Bei der Reaktion des 1,1'-Ferrocendiyl-bis(aminocarbenkomplexes) mit einem Äquivalent an ClP=C(SiMe3)2 und NEt3 wurde ein neuartiger Diaminophosphan-überbrückter [5]Ferrocenophan-bis(carbenkomplex) mit zwei Metall-Carbenzentren, die elektronisch mit dem Eisenzentrum kommunizieren, erhalten. Dieser wurde isoliert und strukturell abgesichert. Die anschließende Reaktion mit ClP=C(SiMe3)2 und NEt3 lieferte eine Mischung des gewünschten Bis(2H-azaphosphirenkomplexes) und einem 2,3-Dihydro-1,2,3-azadiphosphetkomplex. Seine Struktur wurde mittels Einkristall-Röntgendiffraktometrie aufgeklärt. Er konnte vollständig abgetrennt werden, wodurch eine gereinigte Mischung der diastereomeren Zielverbindungen erhalten wurde.
Subjects
Phosphorheterocyclen, Ringerweiterung, DFT-Berechnungen, TD-DFT-Berechnungen, protonierte Heterocyclen, Elektronentransfer, Einelektronentransfer-Reaktionen, Ligand-zentrierte Reaktivität, Carbene, Phospheniumkomplexe, phosphorus heterocycles, ring expansion, DFT calculations, TD-DFT calculations, protonated heterocycles, electron transfer, single electron transfer reactions, ligand-centered reactivity, carbenes, phosphenium complexes
Classification (DDC)
540 ChemieHelten, Holger: Electrophilic Ring Bond Activation of 2H-Azaphosphirene Complexes. - Bonn, 2010. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5N-21464
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5N-21464
@phdthesis{handle:20.500.11811/4581,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5N-21464,
author = {{Holger Helten}},
title = {Electrophilic Ring Bond Activation of 2H-Azaphosphirene Complexes},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2010,
month = may,
note = {In combined experimental and computational studies the scope of different strategies for P,N bond activation and ring expansion of 2H-azaphosphirene complexes was investigated.
First, a protocol using nitriles and catalytic amounts of SET oxidants such as [FeCp2] [PF6] was explored, which leads to 2H-1,4,2-diazaphosphole complexes. Studies on the reaction course revealed evidence for a radical cation chain reaction mechanism. For reactions with heterocyclic carbonitriles it was demonstrated that the amount of [FeCp2][PF6] can be reduced to 0.05-0.025 equivalents. In the absence of nitriles symmetrically 3,5-disubstituted 2H-1,4,2-diazaphosphole complexes were formed, one of which could be structurally confirmed by single-crystal X-ray diffraction. Here, the nitrile fragment that is required for the formation of these products stems from the 2H-azaphosphirene complex. DFT calculations on the reaction mechanism revealed that the 2H-azaphosphirene complex is oxidized in the first step and the resulting cationic species is a metal-centered radical that shows ligand-centered reactivity, as the nucleophilic attack of a nitrile causes ring opening followed by facile cyclization of the acyclic intermediate.
The synthesis of various 2H-1,4,2-diazaphosphole complex derivatives with different P, C3, and C5 substituents as well as different metal fragments (Cr(CO)5, Mo(CO)5, W(CO)5) from 2H-azaphosphirene complexes and nitriles was achieved by consecutive reaction with the strong acid TfOH and a base (triethylamine or pyridine); all products were isolated and unambiguously identified. This protocol offers new synthetic perspectives as even nitriles with high steric demand could successfully be employed, and it enabled access to the first C-SiMe3 ring-functionalized 2H-1,4,2-diazaphosphole complex. By 31P NMR spectroscopy evidence was obtained for the formation of 2H-1,4,2-diazaphospholium complexes. Two derivatives thereof were isolated, and one was structurally confirmed by a single-crystal X-ray diffraction study. Reactions of 2H-azaphosphirene complexes with HCN afforded mixtures of κP- and κN-bonded 2H-1,4,2-diazaphosphole complexes. A chemical equilibrium between the two haptomeric complexes was evidenced by 31P{1H} NMR spectroscopic measurements at varying temperatures. In ring expansion reactions of molybdenum and chromium complexes with dimethyl cyanamide partial decomplexation was observed, but this could completely be prevented by adding the base at low temperature. On the other hand, after prolonged reaction times the N1-protonated liberated 2H-1,4,2-diazaphosphole ligand was quantitatively formed and could be characterized by NMR spectroscopy. Subsequent deprotonation afforded the neutral heterocycle. Although it decomposed during column chromatography, all relevant NMR spectroscopic information was obtained from the raw product. The reaction of a 2H-azaphosphirene complex with cyclohexyl isocyanide under the same conditions yielded a novel 2,3-dihydro-1,3-azaphosphete complex; its molecular structure was confirmed by single-crystal X-ray diffractometry. Investigations on the applicability of various Brønsted and Lewis acids to induce the ring expansion with dimethyl cyanamide showed that good results can also be obtained with CF3CO2H, B(C6F5)3, and Li[B(C6F5)sub>4].
The reaction with TfOH in the absence of trapping reagents resulted in partial desilylation of the P-substituent combined with ring opening, thus leading to coordination-isomeric N-protonated 1-aza-3-phosphabutadiene complexes. Their characterization was achieved by multinuclear NMR spectroscopy at low temperature. Subsequent reaction with a nitrile and deprotonation yielded a 2H-1,4,2-diazaphosphole complex with R(P) = CH2SiMe3, which was isolated and structurally confirmed. The first 2H-azaphosphirenium complex was observed in the reaction of a P-Cp* substituted 2H-azaphosphirene complex with TfOH. It was characterized by multinuclear one- and two-dimensional NMR experiments. Upon addition of a nitrile ring expansion occurred, and it was demonstrated that the protonation can be reversed through the addition of NEt3. On the basis of DFT calculations a mechanism for the acid-induced ring expansion of 2H-azaphosphirene complexes with nitriles and isonitriles is proposed. Upon N-protonation the resulting 2H-azaphosphirenium complex is prone to undergo spontaneous ring opening with formation of a phosphenium complex. Following nucleophilic attack of a nitrile or an isonitrile and subsequent cyclization and deprotonation yields the final products.
In their UV/Vis spectra 2H-1,4,2-diazaphosphole complexes show absorption bands at very long wavelengths. This was interpreted on the basis of TD-DFT calclulations, which revealed that the longest-wavelength absorption is assigned to a metal-ligand charge transfer (MLCT) process; a second band was assigned to a π-π* transition. Furthermore, the origin of the intense colors of 2H-1,4,2-diazaphospholium complexes as well as the photophysical properties of κ N-coordinated 2H-1,4,2-diazaphosphole complexes was elucidated by TD-DFT calclulations.
In the reaction of a mixture of κP- and κN-bound 2H-1,4,2-diazaphosphole complexes with MeOTf and TfOH methylation of the N4-center occurred in combination with partial desilylation at the exocyclic P-substituent. Both SiMe3 groups of such complexes could be removed via reaction with [nBu4N]F in the presence of [Et3NH][OTf], which afforded a P-methyl substituted 2H-1,4,2-diazaphosphole complex.
Finally, the synthesis of the first Bis(2H-azaphosphirene complexes) is presented. Reaction of a 1,1'-ferrocenediyl-bis(aminocarbene complex) with a chloro(methylene)phosphane in the presence of NEt3 yielded first a Mono(2H-azaphosphirene complex) with an unreacted aminocarbene complex group at the other Cp-ring. The reaction of the 1,1'-ferrocenediyl-bis(aminocarbene complex) with one equivalent of ClP=C(SiMe3)2 and NEt3 in a dilute CH2Cl2 solution yielded a novel diaminophosphane-bridged [5]ferrocenophane bis(carbene complex), having two metal carbene centers that electronically communicate with the iron center, which was isolated and structurally confirmed. Subsequent reaction with ClP=C(SiMe3)2 and NEt3 afforded a mixture of the desired bis(2H-azaphosphirene complexes) and a 2,3-dihydro-1,2,3-azadiphosphete complex. The structure of the latter was determined by a by single-crystal X-ray diffraction study. It could completely be separated, and a purified mixture of the diastereomeric target complexes was obtained.},
url = {https://hdl.handle.net/20.500.11811/4581}
}
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5N-21464,
author = {{Holger Helten}},
title = {Electrophilic Ring Bond Activation of 2H-Azaphosphirene Complexes},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2010,
month = may,
note = {In combined experimental and computational studies the scope of different strategies for P,N bond activation and ring expansion of 2H-azaphosphirene complexes was investigated.
First, a protocol using nitriles and catalytic amounts of SET oxidants such as [FeCp2] [PF6] was explored, which leads to 2H-1,4,2-diazaphosphole complexes. Studies on the reaction course revealed evidence for a radical cation chain reaction mechanism. For reactions with heterocyclic carbonitriles it was demonstrated that the amount of [FeCp2][PF6] can be reduced to 0.05-0.025 equivalents. In the absence of nitriles symmetrically 3,5-disubstituted 2H-1,4,2-diazaphosphole complexes were formed, one of which could be structurally confirmed by single-crystal X-ray diffraction. Here, the nitrile fragment that is required for the formation of these products stems from the 2H-azaphosphirene complex. DFT calculations on the reaction mechanism revealed that the 2H-azaphosphirene complex is oxidized in the first step and the resulting cationic species is a metal-centered radical that shows ligand-centered reactivity, as the nucleophilic attack of a nitrile causes ring opening followed by facile cyclization of the acyclic intermediate.
The synthesis of various 2H-1,4,2-diazaphosphole complex derivatives with different P, C3, and C5 substituents as well as different metal fragments (Cr(CO)5, Mo(CO)5, W(CO)5) from 2H-azaphosphirene complexes and nitriles was achieved by consecutive reaction with the strong acid TfOH and a base (triethylamine or pyridine); all products were isolated and unambiguously identified. This protocol offers new synthetic perspectives as even nitriles with high steric demand could successfully be employed, and it enabled access to the first C-SiMe3 ring-functionalized 2H-1,4,2-diazaphosphole complex. By 31P NMR spectroscopy evidence was obtained for the formation of 2H-1,4,2-diazaphospholium complexes. Two derivatives thereof were isolated, and one was structurally confirmed by a single-crystal X-ray diffraction study. Reactions of 2H-azaphosphirene complexes with HCN afforded mixtures of κP- and κN-bonded 2H-1,4,2-diazaphosphole complexes. A chemical equilibrium between the two haptomeric complexes was evidenced by 31P{1H} NMR spectroscopic measurements at varying temperatures. In ring expansion reactions of molybdenum and chromium complexes with dimethyl cyanamide partial decomplexation was observed, but this could completely be prevented by adding the base at low temperature. On the other hand, after prolonged reaction times the N1-protonated liberated 2H-1,4,2-diazaphosphole ligand was quantitatively formed and could be characterized by NMR spectroscopy. Subsequent deprotonation afforded the neutral heterocycle. Although it decomposed during column chromatography, all relevant NMR spectroscopic information was obtained from the raw product. The reaction of a 2H-azaphosphirene complex with cyclohexyl isocyanide under the same conditions yielded a novel 2,3-dihydro-1,3-azaphosphete complex; its molecular structure was confirmed by single-crystal X-ray diffractometry. Investigations on the applicability of various Brønsted and Lewis acids to induce the ring expansion with dimethyl cyanamide showed that good results can also be obtained with CF3CO2H, B(C6F5)3, and Li[B(C6F5)sub>4].
The reaction with TfOH in the absence of trapping reagents resulted in partial desilylation of the P-substituent combined with ring opening, thus leading to coordination-isomeric N-protonated 1-aza-3-phosphabutadiene complexes. Their characterization was achieved by multinuclear NMR spectroscopy at low temperature. Subsequent reaction with a nitrile and deprotonation yielded a 2H-1,4,2-diazaphosphole complex with R(P) = CH2SiMe3, which was isolated and structurally confirmed. The first 2H-azaphosphirenium complex was observed in the reaction of a P-Cp* substituted 2H-azaphosphirene complex with TfOH. It was characterized by multinuclear one- and two-dimensional NMR experiments. Upon addition of a nitrile ring expansion occurred, and it was demonstrated that the protonation can be reversed through the addition of NEt3. On the basis of DFT calculations a mechanism for the acid-induced ring expansion of 2H-azaphosphirene complexes with nitriles and isonitriles is proposed. Upon N-protonation the resulting 2H-azaphosphirenium complex is prone to undergo spontaneous ring opening with formation of a phosphenium complex. Following nucleophilic attack of a nitrile or an isonitrile and subsequent cyclization and deprotonation yields the final products.
In their UV/Vis spectra 2H-1,4,2-diazaphosphole complexes show absorption bands at very long wavelengths. This was interpreted on the basis of TD-DFT calclulations, which revealed that the longest-wavelength absorption is assigned to a metal-ligand charge transfer (MLCT) process; a second band was assigned to a π-π* transition. Furthermore, the origin of the intense colors of 2H-1,4,2-diazaphospholium complexes as well as the photophysical properties of κ N-coordinated 2H-1,4,2-diazaphosphole complexes was elucidated by TD-DFT calclulations.
In the reaction of a mixture of κP- and κN-bound 2H-1,4,2-diazaphosphole complexes with MeOTf and TfOH methylation of the N4-center occurred in combination with partial desilylation at the exocyclic P-substituent. Both SiMe3 groups of such complexes could be removed via reaction with [nBu4N]F in the presence of [Et3NH][OTf], which afforded a P-methyl substituted 2H-1,4,2-diazaphosphole complex.
Finally, the synthesis of the first Bis(2H-azaphosphirene complexes) is presented. Reaction of a 1,1'-ferrocenediyl-bis(aminocarbene complex) with a chloro(methylene)phosphane in the presence of NEt3 yielded first a Mono(2H-azaphosphirene complex) with an unreacted aminocarbene complex group at the other Cp-ring. The reaction of the 1,1'-ferrocenediyl-bis(aminocarbene complex) with one equivalent of ClP=C(SiMe3)2 and NEt3 in a dilute CH2Cl2 solution yielded a novel diaminophosphane-bridged [5]ferrocenophane bis(carbene complex), having two metal carbene centers that electronically communicate with the iron center, which was isolated and structurally confirmed. Subsequent reaction with ClP=C(SiMe3)2 and NEt3 afforded a mixture of the desired bis(2H-azaphosphirene complexes) and a 2,3-dihydro-1,2,3-azadiphosphete complex. The structure of the latter was determined by a by single-crystal X-ray diffraction study. It could completely be separated, and a purified mixture of the diastereomeric target complexes was obtained.},
url = {https://hdl.handle.net/20.500.11811/4581}
}
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