Direkt zu

 

Photoinduced Ring Opening Metathesis of Cyclic Olefines

Ring opening metathesis is a catalyzed metathesis reaction of cyclic olefines and used in most cases for chain growth polymerization. Commonly used catalysts are Grubbs and Shrock type metal complexes. A commonly used monomer is norbonene, which is shown in a simplified catalytic cycle in Figure 1. The metal catalyst is opening and joining cyclic olefines. Until now, a wide variety of catalysts and monomers is known, opening the possibility to produce tailor made polymers.[1]

promp1Figure 1: Simplified ring opening metathesis polymerization cycle with norbonene.

With the use of “switchable” catalyst for ring opening metathesis polymerization (ROMP), the polymerization can be triggered exactly at the desired moment. This is desirable for many technical applications. A common technique is to prepare a monomer-precatalyst mixture, that is storable over a period even at elevated temperatures (usually < 45C). Most used precatalysts can be activated thermally. However, there are also UV-triggerable precatalysts.[2;3] These systems are of interest for example for photolithography in microfabrication.[4] A UV-triggerable ruthenium sandwich catalyst system was presented by KARLEN ET AL. in 1995.[5] The Model for the formation of the active species is shown in Figure 2. The precatalyst is a ruthenium sandwich complex, that decomposes into a complex with solvent molecules as ligands when irradiated with UV/VIS radiation (330 nm to 500 nm, Hg-lamp). The polymerization rate is determined by the degree of decomposition of the precatalyst. Hence, the overall reaction rate can be controlled by the light intensity or the period of irradiation.

Karlen,trigger

Figure 2: Model for the formation of the active ROMP catalyst species from a sandwich precatalyst.[5]

A more advanced system was developed by LEMCOFF et al. (see Figure 4a). The sulfur chelated ruthenium precatalysts can be isomerized by UV-light with a wavelength of 350 nm from a cis-dichloro to a catalytic active trans-dichloro isomer. As a reason for the isomerization, the modification of the sulfur chelatation is proposed. This type of catalyst can be used for different metathesis reactions such as cross, ring-closing and ring-opening metathesis.[6;7]

 

Hoveyda

Figure 3: General reaction schemes of cross metathesis, ring-closing metathesis and ring opening metathesis.[6]

These types of metathesis are shown in Figure 3. By functionalizing the N-heterocyclic carbene (NHC) with supersilyl endgroups, not only the activation, but also the decomposition of the catalyst and thus the end of the reaction can be triggered. The catalyst shown in Figure 4b can be transferred to the catalytic active trans state with UV-light with a wavelength of 350 nm. In addition, the catalyst deactivates when irradiated with UV-light with a wavelength of 254 nm.[8] A reactivation of the catalyst by UV-light was not yet reported, however, the development of systems that can be triggered with different wavelengths for different reactions is desirable.[9]

 

Sutar,ROMP Sutar,ROMP2

(a)                                                                            (b)

Figure 4: (a) Proposed photoisomerization of a sulfur chelated ruthenium complex.[7] (b) Activation of a sulfur-chelated ruthenium catalyst with supersilyl NHC endgroups with 350 nm light.[8]

  1. GRUBBS, R. H.: Handbook of Metathesis, 2003, Wiley-VCH Verlag GmbH, ISBN 9783527619481, DOI: 1002/9783527619481.
  2. WANG, D.; UNOLD, J.; BUBRIN, M.; FREY, W.; KAIM, W.; BUCHMEISER, M. R.: Ruthenium(IV)-Bis(methallyl) Complexes as UV-Latent Initiators for Ring-Opening Metathesis Polymerization, 2012, 4, 1808–1812, DOI: 1002/cctc.201200183.
  3. NAUMANN, S.; BUCHMEISER, M. R.: Latent and delayed action polymerization systems, 2014, 35, 682–701, DOI: 1002/marc.201300898.
  4. WEITEKAMP, R. A.; ATWATER, H. A.; GRUBBS, R. H.: Photolithographic olefin metathesis polymerization, 2013, 135, 16817–16820, DOI: 1021/ja4093083.
  5. KARLEN, T.; LUDI, A.; MÜHLEBACH, A.; BERNHARD, P.; PHARISA, C.: Photoinduced ring opening metathesis polymerization (PROMP) of strained bicyclic olefins with ruthenium complexes, 1995, 33, 1665–1674, DOI: 1002/pola.1995.080331013.
  6. HOVEYDA, A. H.; ZHUGRALIN, A. R.: The remarkable metal-catalysed olefin metathesis reaction, 2007, 450, 243–251, DOI: 1038/nature06351.
  7. BEN-ASULY, A.; AHARONI, A.; DIESENDRUCK, C. E.; VIDAVSKY, Y.; GOLDBERG, I.; STRAUB, B. F.; LEMCOFF, N. G.: Photoactivation of Ruthenium Olefin Metathesis Initiators, 2009, 28, 4652–4655, DOI: 1021/om9004302.
  8. SUTAR, R. L.; LEVIN, E.; BUTILKOV, D.; GOLDBERG, I.; REANY, O.; LEMCOFF, N. G.: A Light-Activated Olefin Metathesis Catalyst Equipped with a Chromatic Orthogonal SelfDestruct Function, 2016, 55, 764–767, DOI: 1002/anie.201508966.
  9. LEVIN, E.; MAVILA, S.; EIVGI, O.; TZUR, E.; LEMCOFF, N. G.: Regioselective chromatic orthogonality with light-activated metathesis catalysts, 2015, 54, 12384–12388, DOI: 1002/anie.201500740.

 


Found an error or want to submit a new article? Please send your correction or article in by mail in word or pdf to ac113924@itc.uni-stuttgart.de.