Catalysts Grubbs

Fig. 3.28 ROMP initiators, first, second and third generation Grubbs catalysts (complexes 71, 72, and 73, respectively) 74a-c 3-phenyl-indenylidene replaces the benzylidene carbene...

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. 

Olefin metathesis is one of the most important reaction in organic synthesis [44], Complexes of Ru are extremely useful for this transformation, especially so-called Grubbs catalysts. The introduction of NHCs in Ru metathesis catalysts a decade ago ( second generation Grubbs catalysts) resulted in enhanced activity and lifetime, hence overall improved catalytic performance [45, 46]. However, compared to the archetypal phosphine-based Ru metathesis catalyst 24 (Fig. 13.3), Ru-NHC complexes such as 25 display specific reactivity patterns and as a consequence, are prone to additional decomposition pathways as well as non NHC-specific pathways [47]. 

Scheme 4.14 Schreiber diversity-oriented synthesis plan (2000). (a) MeOH/THF, heat (57%) (b) 2KN(SiMe3)2, 2 CH2=CH2 Br (89%) (c) Grubbs catalyst (59%) (d) HF, pyridine (95%).

The most commonly used catalyst is the benzylidene complex of RuC12[P(c — C6Hn)3]2, F, which is called the Grubbs catalyst, but several other catalysts are also reactive. Catalyst H, which is known as the second-generation Grubbs catalyst, is used extensively. 

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

Other monomeric precursors similar to 6-hexynyl-decaborane such as 6-norbornenyl-decaborane (129) and 6-cyclooctenyl-decaborane (131) (Fig. 75) underwent ROMP in the presence of either first- or second-generation Grubbs catalysts to produce the corresponding poly(norbornenyl-decaborane) (130) (Fig. 75) and poly(cyclooctenyl-decaborane) (132) (Fig. 75) with Mn > 30 kDa and polydis-persities between 1.1 and 1.8.152 Electrostatic spinning and pyrolysis of poly (norbomenyl-decaborane) was discovered to produce nanoscale, free-standing porous boron-carbide/carbon, ceramic fiber matrices.153... 

Figure 5.2 The use of hollow PDMS thimbles to achieve site separation ofGrubbs catalyst and an osmium dihydroxylation catalyst [34], The solution of the Grubbs catalyst was placed on the interior of the PDMS thimble in which a metathesis reaction was then performed. After...

Bowden and co-workers demonstrated the utility of such a system in multiple cascade reactions, including the use of an otherwise incompatible combination of a Grubbs catalyst and an osmium dihydroxylation catalyst (Figure 5.2) [34],... 

Moreover, in this example, the solvent systems used are also incompatible. The Grubbs catalyst is used in a relatively dry, nonpolar solvent to dissolve the substrates, whereas the AD-mix is placed in various alcohol-water mixtures. 

Hoveyda and coworkers [227] used a domino process to give chromanes 6/3-8 by treatment of 6/3-7 in the presence of ethylene. One of the first-generation Grubbs catalyst 6/3-9 and one of Blechert s [228] early examples allowed the synthesis of bicyclic compounds of different sizes, depending on the length of the tether thus, the reaction of 6/3-10 led to 6/3-11 using 30 mol% of the Schrock Mo complex 6/3-12. 

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. 

Polyether-type structures such as 6/3-38 are frequently found in bioactive compounds (e. g., maitansine). Nicolaou and coworkers [239] have developed a new, efficient approach to these compounds, which is based on a domino ROM/RCM using the second-generation Grubbs catalyst 6/3-15. Thus, the cyclobutene derivative 6/3-37 could be transformed into 6/3-38 in 80% yield (Scheme 6/3.10). 

Thus, by using a mixture of the Grubbs catalyst 6/3-13 and Pd(OAc)2/PPh3, 6/3-84a was transformed into 6/3-85a in 65% yield. With the polystyrene-bound palladium catalyst 71 % yield was obtained in contrast, the use of a biphasic system... 

Although the metathesis of ene-ynes is a valuable method for the preparation of 1,3-butadienes, and may be used for Diels-Alder reactions, a problem arises from the need to employ either a high temperature or a Lewis add to accelerate the cycloaddition, which is usually not feasible with the Grubbs catalyst Therefore, the combination of metathesis and cycloaddition is usually performed in sequential fashion (as just shown, and highlighted earlier) [264]. However, Laschat and coworkers [265] have shown the Lewis acid BC13 to be compatible with the Grubbs I catalyst (6/3-13). Reaction of 6/3-92 and ethyl acrylate using a mixture of 2.5 equiv. of the Lewis acid and 10 mol% of 6/3-13 led to 6/3-93 in 60% yield (Scheme 6/3.27). 

In one example of application of the RCM strategy, summarized in Scheme 107, the key intermediate 454 was obtained from 451 by sequential Wittig methylenation to 452, reductive iV-deprotection to 453, and introduction of an alkenyl chain onto the secondary amine. The RCM reaction of 454 to 455 proceeded in quantitative yield in the presence of the second-generation Grubbs catalyst <20040L965>. 

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