Grubbs’s second-generation catalyst

Grubbs II Grubb s second generation catalyst (ruthenium[l,3-bis(2,4,6- trimethylphenyl)-2-imidazolidinylidene]dichloro(phenylmethylene) (tricyclohexylphosphane)... 

Scheme 26.12 shows examples of the synthesis of pyrroles. Dieltiens et al. obtained pyrroles 43 by an RCM/oxidation sequence of diallyamines 42 in which Grubbs s second-generation catalyst 2 and chloranil (44) were used simultaneously [19]. At first, Dieltiens et al. used an RuCls catalyst system for the oxidation [19b]. However, there was a modification to use chloranil (44) because of the slow reaction rate of the RuCb catalyst system. 

Two tandem alkene metathesis-oxidation procedures using Grubb s second-generation ruthenium catalyst resulted in unique functional group transformations. Use of sodium periodate and cerium(III) chloride, in acetonitrile-water, furnished cis-diols. Oxidation with Oxone, in the presence of sodium hydrogencarbonate, yielded a-hydroxy ketones.296 Secondary alcohols are oxidized to ketones by a hydrogen... 

The same catalyst system works well in hetero-allylic asymmetric alkylations (h-AAA Scheme 1-16). Substrates such as enol esters 163 provide entry to nonracemic esters of allylic alcohols. Remarkably, competing 1,2-addition and/or acyl transfer were not issues yields are good (80-99%). In these cases, catalyst loading can go as low as 0.8%, and ee s are mostly >95%. Additional chemoselectivity has been noted in the case of cinnamyl ester 163, where the desired Sn2 AAA takes place without competing Cu-catalyzed 1,4-addition to the enoate. This sets the stage for a subsequent metathesis (GH-2 = Grubbs-Hoveyda second-generation catalyst) en route to butenolide 164. 

Arisawa et al. reported the synthesis of quinolines 41 from dienes 39 (Scheme 26.11) [18]. RCM of dienes 39 protected by Bn, Ac, or Boc on nitrogen with Grubbs s first-generation catalyst 1 or second-generation catalyst 2 gave dihydroquinoUnes 40... 

The sulfone and sulfoxide alkenes 134 underwent the RCM reaction in the presence of Grubbs second-generation catalyst 77 (see Scheme 11, Section 14.06.3.4.1) to form the corresponding 1,4-thiazocines 135 and 136 (Scheme 15) <2004JOC2750>. Sulfanyl derivatives 134 (X = S) were inactive in this transformation and did not produce the... 

In 2002, Kiddle et al. showed that Ru catalysed olefin metathesis in [bmim][BF4] or dichloromethane could be accelerated upon microwave heating [33]. For instance, the RCM of diethyl diallylmalonate under microwave was complete in 15 s in [bmim][BF4] when using the Grubbs second-generation catalyst 2 (Scheme 2). 

Figure 4 Synthesis of permanently bonded polymeric Grubbs second-generation catalyst In a fused-sllica capillary, (a) 3-Bromo-prop-1-ene, NaH, tetrahydrofuran (THF), 64°C, 24h. (b) HMPS, Karstedt s catalyst,...

The reaction temperature was set to 50 C, and helium was used as the inert carrier gas (inlet pressure 50 kPa). From the obtained chromatographic data, the reaction rate constants were directly accessible by assuming a pseudo-first-order reaction law for the ring-closing metathesis (RCM). The contact time Af of the reactant on the catalytically active column was determined to be only 1.75 s, which corresponds to a reaction rate constant of 0.54 s and an activation barrier AG (323 k) of 81 kj mol . The high activity of the permanently bonded polymeric Grubbs second-generation catalyst is corroborated by the activation barrier determined for the formation of Af-trifluoroacetamide-3-pyrroline. 

Figure 27 Grubb s first and second generation catalysts.

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