Ruthenium carbene first-generation

For the last 2 decades ruthenium carbene complexes (Grubbs catalyst first generation 109 or second generation 110, Fig. 5.1) have been largely employed and studied in metathesis type reactions (see Chapter 3) [31]. However, in recent years, the benefits of NHC-Ru complexes as catalysts (or pre-catalysts) have expanded to the area of non-metathetical transformations such as cycloisomerisation. 

A concise total synthesis of dehydrohomoancepsenolide is achieved in an optically active form. The key steps are alkene metathesis and alkyne metathesis. A three-component coupling reaction affords dienyne 137, which undergoes ring-closing alkene metathesis in the presence of the first-generation ruthenium carbene complex to give 138,... 

The First and Second Generation of Grubbs Ruthenium Carbenes... 

Scheme 8.21 Preparation of the carbene ruthenium(II) complex 81, the first representative of the first generation Grubbs catalysts", from 79 and 3,3-diphenylcyclopropene via the related ruthenium carbene 80 as the intermediate...

The enyne metathesis generates the first ring and an intermediate ruthenium carbene species, which undergoes a second RCM process to produce the fused [n.m.0]bicyclic product (Scheme 2). 

Norbornene derivatives bearing two alkynes undergo cascade enyne metathesis reactions when treated with a first generation ruthenium carbene and ethylene, giving heterocyclic dienes [28]. The ROM of the norbornene moiety initiates the cascade enyne RCM reactions (Scheme 14). When ethylene is replaced by a monosubstituted alkene, a single enyne RCM takes place, after the initial ROM of norbornene. 

The synthesis of the new hybrid systems is realized with the help of a new generation of high-performance, reasonably stable, and, most importantly, exceedingly tolerant catalysts or catalyst precursors such as the ruthenium first generation carbene complex Cl2(PCy3)2Ru=CHPh introduced by Grubbs and co-workers [2, 7]. 

The short total synthesis of (+)-differolide based on a tandem enyne metathesis / [4+2] cycloaddition was accomplished by T.R. Hoye et al." The enyne metathesis was carried out on ally propynoate using Grubbs s first-generation metathesis catalyst. The catalyst was added to the substrate slowly to maintain high substrate and low ruthenium carbene concentrations. The initially formed 2-vinylbutenolide readily dimerized via a Diels-Alder cycloaddition in which the vinyl group participated as the dienophile to afford the natural product. 

When conducting the ROMP of norbornene or cyclooctadiene in miniemulsions [82], two approaches were followed (i) addition of a catalyst solution to a miniemulsion of the monomer and (ii) addition of the monomer to a miniemulsion of Grubbs catalyst in water. With the first approach it was possible to synthesize stable latexes with a high conversion, whereas for the second approach particles of >400 nm were created, without coagulum, but with 100% conversion. Subsequently, a water-soluble ruthenium carbene complex [poly(ethylene oxide)-based catalyst] was prepared and used in the direct miniemulsion ROMP of norbornene [83], whereby particles of 200-250 nm were produced. The catalytic polymerization of norbornene in direct miniemulsion was also carried out in the presence of an oil-soluble catalyst generated in situ, or with a water-soluble catalyst [84] the reaction was faster when using the oil-soluble catalyst. Helical-substituted polyacetylene could be efficiently polymerized in direct miniemulsion to yield a latex with particles that ranged between 60 and 400 nm in size, and which displayed an intense circular dichroism [85] that increased as the particle size decreased. The films were prepared from dried miniemulsion latexes that had been mixed with poly(vinyl alcohol) (PVA) in order to conserve the optical activity. 

---