1:00 pm – 2:00 pm
Lecture Hall 55.22 (next to Café Urknall)
We are very happy to welcome within the CRC 1333 Colloquium Series:
– 2005 Nobel Laureate in Chemistry –
F G Keyes Professor of Chemistry, emeritus at MIT, USA
George K. Helmkamp Founder’s Chair of Chemistry, University of California, Riverside, USA
Topic: “Metallacyclopentane Ring-Contraction Yields Olefin Metathesis Catalysts”
Prof. Richard R. Schrock is Professor emeritus at MIT and currently holds a small working group at the University of California, Riverside.
His interests include the fundamental organometallic chemistry of high oxidation state molybdenum and tungsten alkylidene and alkylidyne complexes and catalytic reactions and mechanisms of reactions involving alkylidene complexes, especially olefin metathesis reactions. He is perhaps best known for his discovery of “high oxidation state carbene” (alkylidene complexes) by alpha hydrogen abstraction in high oxidation state metal alkyl complexes. In the last several years he has applied alkylidene chemistry toward the controlled polymerization of cyclic olefins via ring-opening-metathesis polymerization (ROMP) and to organic chemistry in collaboration with Amir H. Hoveyda.
The CRC cordially invites all who are interested to the lecture.
For almost sixty years an unanswered question in organometallic chemistry and catalysis has been how molybdenum and tungsten olefin metathesis catalysts are formed from olefins. We have now shown that five-coordinate tungstacyclopentane complexes will form from ethylene and “contract” to yield W(VI) methylene complexes. The simplest example is formation of a tungstacyclobutane complex, W(NAr)(OSiPh3)2(CH2CH2CH2) (Ar = 2,6-i-Pr2C6H3), from a tungstacyclopentane complex, W(NAr)(OSiPh3)2(C4H8), in the presence of ethylene. The intermediate is an a methyl tungstacyclobutane complex, W(NAr)(OSiPh3)2(MeC3H5), from which propylene is lost and W(NAr)(OSiPh3)2(CH2) is formed. Ring-contraction takes place in complexes bound to silica, either thermally, or through activation by room or LED (450 nm) light, for a variety of imido complexes (but not all), and for oxo complexes. Disubstituted tungstacyclopentane rings formed from propylene also ring-contract to form methylene complexes. Finally, ring-contraction has been shown to be a means of forming propylene directly from ethylene (E -> P) at 85°C without forming butenes. The metallacyclopentane complex that ring-contracts is proposed to be an unobservable trigonal-bipyramidal complex formed in an observable Berry-type five-coordinate rearrangement process.