Having control over an entity or even an entire process is arguably the ultimate demonstration of its understanding and it will enable its potential to be fully exploited. With this in mind, chemists have not only been creating and optimizing a myriad of different catalysts for most (relevant) chemical reactions over the past decades, but have recently started to implement controlling elements into the catalyst design. These incorporated gates operate upon exposure to suitable control stimuli, and light represents perhaps the scientifically and technologically most attractive stimulus. In principle, irradiation can thereby induce activity and selectivity in a given catalyst system with high spatial and temporal control, leading to an overall localization and amplification of an optical signal and translation into chemical action. While nature has developed and utilized this concept, in particular in the processes of vision and photomovement, such artificial photocontrolled catalyst systems offer unique opportunities and have high potential for future applications. In this Review, we outline the general concept of light-gated catalysis based on photocaged and also photoswitchable systems, and discuss relevant examples of the past and recent literature.

Artificial light-gated catalyst systems.

Metathesis chemistry and, in the context of polymer chemistry, ring-opening metathesis polymerization (ROMP) have gained a strong position in chemistry and materials science. ROMP is strongly associated with two classes of well-defined metal alkylidene based initiators, molybdenumbased Schrock and ruthenium-based Grubbs type initiators. Despite the tremendous achievements in catalyst development, both families of initiators are still experiencing ongoing, vivid development. Most Grubbs type initiators work at room temperature or require only gentle warming to work properly. More recently, an increasing number of reports on latent Ru-based initiators has appeared. Such precatalysts are of particular interest in technical applications of ROMP, since they allow for premixing, that is, the preformulation of a monomer/precatalyst mixture, its storage over a longer period of time even at elevated temperatures (usually less than 45 8C), and, most importantly, the shaping and profiling of such mixtures prior to polymerization (“curing”). Numerous latent Grubbs type initiators have been reported recently; however, all these precatalysts are triggered thermally. By contrast, surface modification and functionalization require UV-triggerable precatalysts. Few such systems have been reported to date. The synthesis of photoactive Schrock type tungsten-based compounds as well as ruthenium and osmium arene compounds of the general formula [Ru(p-cymene)Cl2(PR3)] and [Os(p-cymene)Cl2(PR3)] (R= cyclohexyl, etc.) were first reported by van der Schaaf et al. They also investigated the photoinduced polymerization of different functionalized norbornenes and 7-oxanorbornenes using various [Ru(solvent)n]X2 complexes, (X= tosylate, trifluoromethanesulfonate) as well as Ru half-sandwich and sandwich complexes. Noels and co-workers reported on the visiblelight-induced ROMP of cyclooctene using [RuCl2(IMes)(pcymene)] (IMes= 1,3-dimesitylimidazol-2-ylidene). Some of these systems were also used in ring-closing metathesis reactions. Most of the systems available to date, however, have significant disadvantages. They either show low activity, resulting in low polymer yields (less than 30%) in the photochemically triggered process, or the irradiation wavelength necessary to trigger ROMP is 360 nm or higher. In the latter case, the initiatorsA thermal stability is generally poor, thus discouraging their application in photoinduced ROMP. Thus, none of the systems reported to date was entirely thermally stable above or even at room temperature. Therefore, these systems do not fulfill the requirements of a truly latent photocatalyst. Herein, we report the development of the first thermally stable, truly UV-triggerable precatalysts for ROMP and their application in surface functionalization. We commenced our investigations with [Ru(IMesH2)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-1) and [Ru(IMes)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-2), which were prepared from [Ru(CF3CO2)2(L)(p-cymene)] [31,32] (L= IMes or IMesH2, 1,3-dimesityl-4,5-diyhdroimidazolin-2-ylidene) by reaction with excess tBuCN. Both compounds can be handled in air. H and C NMR spectroscopy data and elemental analysis reveal the presence of one N-heterocyclic carbene (NHC) ligand, two inequivalent trifluoroacetate groups, and four tBuCN ligands, suggesting cationic Ru complexes. The structures of PI-1 and PI-2 were confirmed by X-ray analysis; the structure of PI-1 is shown in Figure 1 (see also the Supporting Information). Upon mixing of either PI-1 or PI-2 with monomers 3–8 (Scheme 1), no reaction was observed at room temperature within 24 h. Even highly reactive (distilled) dicyclopentadiene (4) did not react with PI-1 or PI-2 at room or elevated temperature (RT<T< 45 8C) in the absence of light. Heating a mixture of 8 with PI-1 or PI-2 in 1,2-dichloroethane to 60 8C resulted in the formation of much less than 10% polymer within 24 h. However, exposing mixtures of either PI-1 or PI2 in chloroform with these monomers to 308-nm light at room temperature resulted in the formation of the corresponding polymers. Yields were between less than 5 and 99% (Table 1). Increasing the energy of the light by switching from 308 nm to a 254-nm Hg lamp gave raise to high, in most cases virtually quantitative, yields (Table 1). The molecular weights [*] Dr. D. Wang, Dr. W. Knolle, Dr. U. Decker, Dr. L. Prager, Dr. S. Naumov, Prof. Dr. M. R. Buchmeiser Leibniz-Institut f.r Oberfl/chenmodifizierung e.V. (IOM) Permoserstrasse 15, 04318 Leipzig (Germany) Fax: (+49)341-235-2584 E-mail: michael.buchmeiser@iom-leipzig.de Homepage: http://www.iom-leipzig.de/index_e.cfm

Cationic RuII Complexes with N‐Heterocyclic Carbene Ligands for UV‐Induced Ring‐Opening Metathesis Polymerization

Kontrolle uber einen Gegenstand oder einen gesamten Prozess auszuuben, ist zweifelsohne der ultimative Beweis fur dessen Verstandnis und ermoglicht erst die vollstandige Nutzung seines Potenzials. Vor diesem Hintergrund haben Chemiker in den vergangenen Jahrzehnten fur die meisten (relevanten) chemischen Reaktionen nicht nur eine Vielzahl von Katalysatoren entwickelt und optimiert, sondern vor kurzem auch damit begonnen, Steuerungselemente in das Katalysatordesign einzufuhren. Diese integrierten Kontrolleinheiten werden durch Einwirkung geeigneter Stimuli reguliert, wobei Licht aus wissenschaftlicher und technologischer Sicht vermutlich am attraktivsten ist. Bestrahlung kann hierbei prinzipiell sowohl Aktivitat als auch Selektivitat in einem gegebenen katalytischen System mit hoher raumlicher und zeitlicher Prazision steuern, die zu einer allgemeinen Lokalisierung und Verstarkung des optischen Signals und dessen Umwandlung in chemische Aktivitat fuhrt. Wahrend in der Natur dieses Konzept bereits erfolgreich Anwendung findet, im Speziellen beim Sehprozess und bei der Photobewegung, eroffnen derartige kunstliche lichtgesteuerte katalytische Systeme einzigartige Moglichkeiten und verfugen uber ein hohes Potenzial fur zukunftige Anwendungen. In diesem Aufsatz beschreiben wir die grundsatzliche Idee der lichtgesteuerten Katalyse basierend auf photoaktivierbaren und photoschaltbaren Systemen und diskutieren relevante Beispiele aus der alteren und neueren Literatur.

Künstliche lichtgesteuerte Katalysatorsysteme

Alkene metathesis: new developments in catalyst design and application

Light activation is a most desirable property for catalysis control. Among the many catalytic processes that may be activated by light, olefin metathesis stands out as both academically motivating and practically useful. Starting from early tungsten heterogeneous photoinitiated metathesis, up to modern ruthenium methods based on complex photoisomerisation or indirect photoactivation, this survey of the relevant literature summarises past and present developments in the use of light to expedite olefin ring-closing, ring-opening polymerisation and cross-metathesis reactions.

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-6-127.pdf

Light-induced olefin metathesis

Metathesis polymerisation can be effected with many catalysts under a variety of conditions. The system described here, viz., W(CO) 6 CBr 4 / hv , induces polymerisation by in situ generation of the active species and has not been reported in the literature so far. In this paper, optimisation studies of this system, yields obtained with various monomers and characterisation of the active species are discussed. Variation of different parameters such as the ratio of W(CO) 6 :CBr 4 , the time of photolysis and the effect of solvents as well as that of the metal carbonyls have been studied. In addition, copolymers of phenylacetylene and norbornene have been synthesised. We have also attempted to characterise the active species responsible for the polymerisation by reaction with acetophenone.

B Gita

Papers

W(CO)6/CBr4/hv — a new system for metathesis polymerisation