Grubbs metathesis

Handbook of Metathesis, Grubbs, R. H., ed. John Wiley Sons, New York, 2003. 

Trimmer, M. S. Commercial Applications of Ruthenium Olefin Metathesis Catalysts in Polymer Synthesis. In Handbook of Metathesis-, Grubbs, R. H., Ed. Wiley-VCH Weinheim, 2003 Vol. 3, pp 407-H8. 

Olefin Metathesis (Grubbs Reaction) allows the exchange of substituents between different olefins - a transalkyiidenation. 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

The mechanism for olefin metathesis is complex, and involves metal-carbene intermediates— intermediates that contain a metal-carbon double bond. The mechanism is drawn for the reaction of a terminal alkene (RCH=CH2) with Grubbs catalyst, abbreviated as Ru=CHPh, to form RCH = CHR and CH2 = CH2. To begin metathesis, Grubbs catalyst reacts with the alkene substrate to form two new metal-carbenes A and B by a two-step process addition of Ru=CHPh to the alkene to yield two different metallocyclobutanes (Step [1]), followed by elimination to form A and B (Steps [2a] and [2b]). The alkene by-products formed in this process (RCH=CHPh and PhCH=CH2) are present in only a small amount since Grubbs reagent is used catalytically. 

General aspects and new metathesis catalysts. For alkene metathesis Grubbs I (1) and Grubbs II (2, 3) complexes, and the Grubbs-Hoveyda catalyst (4A) and Grela catalyst (4B) remain the workhorses. 

The direct ruthenium catalysed allylation with allylic alcohol derivatives of various aromatic compounds and heterocycles such as furans and thiophenes was performed by Nishibayashi with cationic thiolate-bridged diruthenium(III, II) catalysts. The reaction is consistent with an electrophilic aromatic substitution by the electrophilically activated allyl moiety [68]. Allylation also takes place with the alkene metathesis Grubbs catalyst [69]. More importantly using (phosphine-sulfonate)ruthenium(II) catalyst Bmneau et al. have recently shown that allyl alcohols are activated generating an allyl-ruthenium(IV) intermediate leading to C3-allylation of indole with high regioselectivity in favour of the branched allyl derivative [(Eq. 84)] [167]. 

In this chapter, we mainly discussed, selective industrial scale organic synthetic applications on C-C and C-0 bond forming ractions involving transition metal catalysis and asymmetric catalysis. It focused on recent advancements of catalytic reactions developed by Nobel laureates, viz., cross-coupling reactions (Heck, Suzuki and Negishi), asymmetric oxidation (Sharpless), metathesis (Grubbs ) and two other noteworthy reactions, Jacobsen and Shi ep-oxidations. 

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