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Cyanosilylation, ketone

Zuend, S. J. Jacobsen, E. N. Cooperative Catalysis by Tertiary Amino-Thioureas Mechanism and Basis for Enantioselectivity of Ketone Cyanosilylation. J. Am. Chem. Soc. 2007,129,15872-15883. [Pg.222]

The in situ cyanosilylation of p-an1saldehyde is only one example of the reaction which can be applied to aldehydes and ketones in general. - The simplicity of this one-pot procedure coupled with the use of inexpensive reagents are important advantages over previous methods. The silylated cyanohydrins shown in the Table were prepared under conditions similar to those described here. Enolizable ketones and aldehydes have a tendency to produce silyl enol ethers as by-products in addition to the desired cyanohydrins. The... [Pg.199]

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. [Pg.81]

Corey reported a catalytic enantioselective cyanosilylation of methyl ketones using combination of a chiral oxazaborolidinium and an achiral phosphine oxide, [Eq. (13.23)]. An intermolecular dual activation of a substrate by boron and TMSCN by the achiral phosphine oxide (MePh2PO) is proposed as a transition-state model (54). The same catalyst was also used for cyanosilylation of aldehydes ... [Pg.400]

Song et al. extended this methodology to include cyanosilylation of aldehydes and ketones (Eq. 32) [160], They propose that NHC 276 interacts with TMSCN to form complex LXXVIII followed by cyano group transfer to the aldehyde (Scheme 48). The carbene is then regenerated and the desired product is obtained when LXXIX fragments. Concurrently, Kondo, Aoyama and co-workers describe similar reaction conditions for the synthesis of cyanohydrins in high yields [161, 162], while Suzuki and co-workers reported a cyanosilylation of aromatic and aliphatic aldehydes in good yields [163]. [Pg.135]

Scheme 48 Proposed mechanism for cyanosilylation of aldehydes and ketones... Scheme 48 Proposed mechanism for cyanosilylation of aldehydes and ketones...
Scheme 6.84 Typical silylated cyanohydrins prepared from various ketones under asymmetric 72-catalysis (cyanosilylation). Scheme 6.84 Typical silylated cyanohydrins prepared from various ketones under asymmetric 72-catalysis (cyanosilylation).
Figure 6.23 Proposed transition states of the asymmetric 72-catalyzed cyanosilylation of ketones describe two alternative mechanistic pathways for cooperative catalysis Addition via thiourea-bound ketone (TS 1, preferred) and addition via thiourea-bound cyanide (TS 2). Figure 6.23 Proposed transition states of the asymmetric 72-catalyzed cyanosilylation of ketones describe two alternative mechanistic pathways for cooperative catalysis Addition via thiourea-bound ketone (TS 1, preferred) and addition via thiourea-bound cyanide (TS 2).
Scheme 6.85 Product range of the 73- and 74-catalyzed asymmetric cyanosilylation of ketones. Scheme 6.85 Product range of the 73- and 74-catalyzed asymmetric cyanosilylation of ketones.
The computations suggested that the enantioselectivity of the cyanosilylation arose from direct interactions between the ketone substrate and the amino-acid derived unit of the catalyst type represented by thiourea 72. On the basis of this insight, the Jacobsen group designed thiourea catalysts 73 and dipepetide thiourea catalyst 74 [67]. These optimized catalysts gave access to a broader spectrum of silylated cyanohydrins (e.g., 1-6) and proved to be more active (88-97% yield) and more enantioselective (98-98% ee) than 72 (Scheme 6.85) [242]. [Pg.229]

Systematic investigations of the catalyst structure-enantioselectivity profile in the Mannich reaction [72] led to significantly simplified thiourea catalyst 76 lacking both the Schiff base unit and the chiral diaminocyclohexane backbone (figure 6.14 Scheme 6.88). Yet, catalyst 76 displayed comparable catalytic activity (99% conv.) and enantioselectivity (94% ee) to the Schiff base catalyst 48 in the asymmetric Mannich reaction of N-Boc-protected aldimines (Schemes 6.49 and 6.88) [245]. This confirmed the enantioinductive function of the amino acid-thiourea side chain unit, which also appeared responsible for high enantioselectivities obtained with catalysts 72, 73, and 74, respectively, in the cyanosilylation of ketones (Schemes 6.84 and 6.85) [240, 242]. [Pg.231]

We began our synthesis by finding the optimum reaction conditions for the catalytic asymmetric cyanosilylation of ketone 28 (Table 1). Based on previous studies,30 the titanium complex of a D-glucose derived ligand (catalyst 32 or 33) generally gives (/ )-ketone cyanohydrins, which is required for a synthesis of natural fostriecin. [Pg.355]

Our proposed transition state model for this catalytic enantioselective cyanosilylation of ketone is shown as 35.30a The titanium acts as a Lewis acid to activate the substrate ketone, while the phosphine oxide acts as a Lewis base to activate TMSCN. The intramolecular transfer of the activated cyanide to the activated ketone should give the ( )-cyanohydrin in high selectivity. The successful results described above clearly demonstrate the practicality of our asymmetric catalyst for cyanosilylation of ketones. [Pg.356]

Deng et al. later found that dimeric cinchona alkaloids such as (DHQ AQN (8, Scheme 6.6) and (DHQD PHAL (9, Scheme 6.7) - both well known as ligands in the Sharpless asymmetric dihydroxylation and commercially available - also catalyze the highly enantioselective cyanosilylation of acetal ketones with TMSCN... [Pg.136]

Lithium chloride is a convenient catalyst for cyanosilylation of a range of ketones and aldehydes by trialkylsilyl cyanide, under solvent-free conditions the silylated cyanohydrin product can be directly distilled out.266 As little as a microequivalent of catalyst proved effective. Evidence for nucleophilic chloride, generating a pentavalent silicon (65) as reactive species, is presented. [Pg.29]

Strong base (66, R = Me, pAT, of conjugate acid = 32.9) catalyses the cyanosilylation of aldehydes and ketones, using TMSCN in THF at 0 °C.267 Even better results are obtained with isopropyl as R group, but the isobutyl case is a much poorer catalyst, indicating a very fine balance between basicity and steric bulk in the action of these catalysts. [Pg.29]

Cyanosilylation of methyl ketones has been carried out using diphenylmethylphos-phine oxide and trimethylsilyl cyanide, generating a phosphorus isonitrile-type species, Ph2MeP(OTMS)(N=C ), as the reactive intermediate.271 A chiral oxazaborolidinium ion catalyst renders the reaction enantioselective. [Pg.30]

Asymmetric cyanosilylation of ketones and aldehydes is important because the cyanohydrin product can be easily converted into optically active aminoalcohols by reduction. Moberg, Haswell and coworkers reported on a microflow version of the catalytic cyanosilylation of aldehydes using Pybox [5]/lanthanoid triflates as the catalyst for chiral induction. A T-shaped borosilicate microreactor with channel dimensions of 100 pm X 50 pm was used in this study [6]. Electroosmotic flow (EOF) was employed to pump an acetonitrile solution of phenyl-Pybox, LnCl3 and benzal-dehyde (reservoir A) and an acetonitrile solution of TMSCN (reservoir B). LuC13-catalyzed microflow reactions gave similar enantioselectivity to that observed in analogous batch reactions. However, lower enantioselectivity was observed for the YbCl3-catalyzed microflow reactions than that observed for the batch reaction (Scheme 4.5). It is possible that the oxophilic Yb binds to the silicon oxide surface of the channels. [Pg.61]

Table I illustrates the cyanosilylation of several representative ketones and aldehydes. Table I illustrates the cyanosilylation of several representative ketones and aldehydes.
The high sensitivity of lanthanide reagents to steric factors is also observed in the cyanosilylation reaction of ketones catalyzed by ytterbium cyanide, Yb(CN)3 (Eq. 7) [10], Other reactions, for example epoxide and the aziridine opening by tri-methylsilyl cyanide, TMSCN, are also efficiently catalyzed by Yb(CN)3 [11]. This Yb reagent is not regarded as a Lewis acid but as the active species in these reactions. [Pg.916]

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. [Pg.918]

Finally, carbohydrate ligands of enantioselective catalysts have been described for a limited number of reactions. Bis-phosphites of carbohydrates have been reported as ligands of efficient catalysts in enantioselective hydrogenations [182] and hydrocyanations [183], and a bifunctional dihydroglucal-based catalyst was recently found to effect asymmetric cyanosilylations of ketones [184]. Carbohydrate-derived titanocenes have been used in the enantioselective catalysis of reactions of diethyl zinc with carbonyl compounds [113]. Oxazolinones of amino sugars have been shown to be efficient catalysts in enantioselective palladium(0)-catalyzed allylation reactions of C-nucleophiles [185]. [Pg.494]


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See also in sourсe #XX -- [ Pg.136 ]

See also in sourсe #XX -- [ Pg.334 ]




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