Catalysts silyl cation based

Finally, a few examples of the Morita-Baylis-Hilhnan reaction are provided, where a silyl species functions as a Lewis acid co-catalyst. These examples could have been presented in the previous section about silyl cation-based catalysts. Since the enantiomeric induction originated in the present examples from a Lewis base, we have listed these examples in this section. 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

Asymmetric Diels-Alder reaction of acryloyl oxazolidinone and 1,3-cyclohexa-diene using the chiral silyl cationic catalyst (5)-29 (Sch. 24, TPFPB = tetrakis(penta-fluorophenyl)borate) was recently reported by Jprgensen and Helmchen [41], This work was based on two concepts ... 

The vapor phase synthesis of methacrylic acid from propionic acid and formaldehyde was studied [42]. In particular, the choice of alkali metal cation and loading were evaluated for their effect on the activity and selectivity of silica supported catalysts. Experiments were carried out in 0.5 in. (o.d.) quartz reactors equipped with 0.125 in. thermowells. Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid. Silica surfaces exchanged with alkali metal cations are capable of chemisorbing propionic acid yielding surface-bound silyl propionate esters and metal propionate salts. The alkali metal cation influences the temperature at which desorption of the ester occurs (Cs < Na < Li < support). For silica catalysts of equimolar cation loading, activity and selectivity to methacrylic acid show the opposite trend, Cs > K. > Na > Li. Methacrylic acid selectivity reaches a maximum at intermediate cation loadings where interaction of adjacent silyl esters is minimized [42]. 

In the catalytic cycle, a simplified version of which is shown in Scheme 5.72 for the acetate aldol addition of 246, the highly electrophilic silyl cation 251 plays a key role, as assumed by the authors. It forms from the reaction of tetrachlorosilane with the corresponding phosphoramide ((Me2N)3PO symbolizing the catalyst 235). When loaded with benzaldehyde, silicon enlarges its coordination sphere and adopts an octahedral geometry in 252. After the carbon-carbon bond has been established, cation 253 forms. It then decomposes to liberate phosphoramide 235, chlorotrialkylsilane, and the aldolate 254. By NMR studies, it was shown that the intermediate of this procedure is the tric/i/orosilyl-protected aldolate 254. This makes a substantial mechanistic difference to conventional Lewis acid-catalyzed Mukaiyama aldol protocols that deliver tri /Ay/silyl-protected aldolates. In accordance with the catalytic cycle shown in Scheme 5.72, tetrachlorosilane is consumed and therefore required to be used in stoichiometric amounts. Thus, the reaction is catalyzed by phosphoramides and mediated by tetrachlorosilane or, more generally, by Lewis base-activated Lewis acids [126]. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

ILs can be immobilized on a functionalized support which contains one component of the IL or a precursor to such a component. The IL may be immobilized via the anion by treating a support with an anion source, e.g., an inorganic halide, before the IL is applied or formed. Alternatively, the IL may be immobilized by having the cation covalently bound to the support, e.g., through silyl groups, or incorporated in the support by synthesizing the support in the presence of a suitable base. The immobilized ILs are of use as catalysts, for example for the Friedel-Crafts reaction. 

The activated a-trimethylsilyl group on the pyrimidine moiety reacts with the triflate ion 26 to regenerate the triflate catalyst. Under reversible and thus thermodynamically controlled conditions, the nucleophilic silylated base 23 attacks the carbohydrate cation 25 only from the top (/(-side) to afford exclusively the /(-nucleoside. 

MacMillan and co workers have significantly expanded the scope of this enamine-mediated procedure by the addition of stoichiometric amounts of oxidant that leads to the in situ formation of a radical cation (12.57). This intermediate then undergoes enantioselective radical-based addition with a range of unsaturated substrates (12.58). For example, a-allylation with allylsilanes such as (12.60) can be effected with high ee using CAN as oxidant in the presence of imidazohdinone (12.61) as catalyst, while an a-heteroarylation occurs using N-Boc pyrrole. Furthermore, an asymmetric a-enolation of a range of aldehydes can be achieved by addition of silyl enol ethers such as (12.64). 

Mechanistic studies have been carried out for neutral and cationic Cu systems [12,13b]. The proposed mechanism for [Cu(Cl)(NHC)j complexes involves the formation of [Cu(0 Bu)(NHC)] by reaction of the chloride complex with the base (Scheme 8.3). [Cu(H)(NHC)j would be formed in situ by o-bond metathesis between the terf-butoxide copper complex and the hydrosilane. The hydride copper complex is highly unstable (observable by NMR) however, it is the active species. Hence, by addition of the hydride species to the carbonyl, a second o-bond metathesis with the silane affords the expected silyl ether and regenerates the active catalyst. In the case of cationic derivatives, dissociation of one NHC occurs as the first step, which is displaced by the fert-butoxide moiety, and is the direct precursor of the active species. The hydrosilane is activated by the nucleophilic NHC, leading to the formation of the silyl ether. The activation of the silane appears to be the decisive step for this transformation. 

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