Ruthenium first-generation

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 metathesis of ene-ynamides has been investigated by Mori et al. and Hsung et al. [80]. Second-generation ruthenium catalysts and elevated temperatures were required to obtain preparatively useful yields. Witulski et al. published a highly regioselective cyclotrimerization of 1,6-diynes such as 98 and terminal alkynes using the first-generation ruthenium metathesis catalyst 9... 

The cross metathesis of vinylsilanes is catalyzed by the first-generation ruthenium catalyst 9. This transformation has been extensively investigated from both preparative and mechanistic points of view by Marciniec et al. [86]. Interestingly, the same vinylsilanes obtained from cross metathesis may also result from a ruthenium-hydride-catalyzed silylative coupling and there might be some interference of metathesis and nonmetathesis mechanisms [87]. 

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

Scheme 14. Convergent synthesis of first-generation ruthenium-acetylide dendrimer 49...

Scheme 15. Rapid convergent approach of the synthesis of first-generation ruthenium-acety-...

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. 

The main reason for the rapid development of metathesis reactions on a laboratory scale (the reaction itself had been known for quite a long time) has been the development of active and robust second-generation ruthenium catalysts (6/3-14 to 6/3-16), which usually provide better yields than the first-generation Grubbs catalysts (6/3-9 or 6/3-13) (Scheme 6/3.2). This also reflects the huge number of domino processes based on ruthenium-catalyzed metathesis, which is usually followed by a second or even a third metathesis reaction. However, examples also exist where, after a metathesis, a second transition metal-catalyzed transformation or a pericyclic reaction takes place. 

BINAP was introduced by Noyori [18], It has been particularly explored for reduction with ruthenium catalysts. While the first generation rhodium catalysts exhibited excellent performance with dehydroamino acids (or esters), the second generation of hydrogenation catalysts, those based on ruthenium /BINAP complexes, are also highly enantioselective for other prochiral alkenes. An impressive list of rather complex organic molecules has been hydrogenated with high e.e. s. 

The allenylidene-ruthenium(arene) catalyst precursors I have been used for the synthesis of macrocycles by the RCM reaction and were revealed as active as the first generation Grubbs catalyst RuCl2(=CHPh)(PCy3)2 [35], depending on the nature of the diene functional groups and macrocyde size [32] (Scheme 8.3).These macrocyde syntheses show that the allenylidene mthenium catalysts I offer functional group tolerance. 

In the early syntheses of alkenyl alkylidene-mthenium catalysts, the first generation of Grubbs catalyst, it was observed that propargyl chloride could be a convenient source of the vinylcarbene initiator [53] with respect to the previous one arising from activation of cyclopropene [4] (Equation 8.3). In this synthesis the alkylidene hydrogen atom arises from the ruthenium hydride. 

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,... 

In order to investigate this point more fully, the rates of reaction of the two complexes with ethyl vinyl ether (EVE) were studied. This alkene was chosen as it is rather reactive towards ruthenium alkylidene complexes and forms an inert alkoxyalkylidene product in an essentially irreversible manner. This alkene, therefore, should rapidly capture any nascent complex from which a Cy3P ligand has dissociated (27 and 30 in Scheme 12.21). The two complexes displayed very different kinetics. The rate of reaction of the first generation pro-catalyst complex 24c with EVE was found to be dependent on EVE concentration (over a range of 30-120 equivalents of EVE) and did not reach pseudo-first-order conditions... 

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