Reactions cyanosilylation

Figure 9.45 MOF catalysed cyanosilylation reaction - when Ar is phenyl or napthyl the product can pass freely through the pores giving yields of over 90 per cent. When Ar = 4-phenoxyphenyl or biphenyl the yields are below 20 per cent because the molecules do not fit readily within the cavities.

The extension of this route to the case of ( )-camptothecin [61] was followed by a series of improvements [62,63], where the key intermediate 21 was obtained via the Sharpless dihydroxylation previously proposed by Fang [64] or via an asymmetric cyanosilylation reaction [65] (Scheme 16.16). 

In the addition reaction of cyanotrimethylsilane [147] to aliphatic aldehydes, another synthetic application of a BINOL-Ti catalyst was reported by Reetz [88]. In this instance, however, BINOL-TiCh was prepared by treatment of the lithium salt of BINOL with TiCU in ether (vide supra). The BINOL-TiCh thus obtained was used as a catalyst for the cyanosilylation reaction to give the cyanohydrins in up to 82 % ee (Sch. 62). 

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. 

A further example of a cooperative catalytic system is a bifunctionalized mesoporous silica nanosphere material (MSN) containing the ureidopropyl group (UDP) and 3-[2-(2-aminoethylamino)ethylamino]propyl group (AEP). All catalysts were synthesized by a sol-gel procedure similar to the previously mentioned one by using different AEP/UDP molar ratios. The MSN-AEP/ UDP catalyst was employed for the aldol, Henry and cyanosilylation reactions (Scheme 3.8 R = NO2, R" = H, Schemes 3.35 and 3.36, respectively) and TON values were compared with those observed by using MSN-AEP and MSN-UDP catalysts (Table 3.10). 

Table 3.10 TONs for the MSN-catalysed aldol, Henry and cyanosilylation reactions (see Schemes 3.8, 3.35 and 3.36, respectively). ...

The secondary amine of the AEP group is responsible for the supported enamine formation with acetone (aldol reaction), the deprotonation of nitromethane (Henry reaction) and the generation of a potential nucleophile from trimethylsilyl cyanide through hypervalent silicate formation (cyanosilylation reaction). Therefore, the presence of both AEP and UDP groups in close proximity can cooperatively activate the electrophile (through hydrogen bond) and the nucleophile by enamine formation, thus enhancing the reaction rate. 

As already mentioned, the addition of TMS-CN (255) across the carbonyl group is a very reasonable reaction (see C, 3). It is not a silylation in the usual sense of the term but its versatility and its properties as a protection group gives evidence of many parallels which only silylation normally possesses. Early attempts to synthesize 255 were undertaken by means of a KCN-18-crown-6-complex Recently, two very simple preparations of 255 have been reported, simplifying the access to 255 and furthermore a one-pot-synthesis for the cyanosilylation reaction as a whole Aldehydes ketones (incl. some o ,/J-unsaturated ones) react with 255 (in presence of catalytic amounts of Znl2) So do... 

Kaskel and coworkers carried out similar cyanosilylation reactions with coor-dinatively unsaturated metal coimecting points in 3D MOFs as heterogeneous catalysts [20]. The 3D framework [Cu3(BTC)2(H20)3] (2) (BTC benzene-1,3,5-tricarboxylate) used in this study was first reported by Williams et al. [21]. The open framework of 2 is built from dimeric cupric tetra-carboxylate units (paddle-wheels) with aqua molecules coordinating to the axial positions and BTC bridging... 

As-synthesized 2 was first activated by heating at 373 K under vacuum to remove the H2O molecules on the axial positions of the copper paddle wheels. The resulting coordinatively unsaturated Lewis acidic Cu(II) centers can coordinate to the aldehyde substrates to promote the cyanosilylation reaction (Fig. 5c). Activated 2 catalyzes the trimethylcyanosilylation of benzaldehydes with a very low conversion (<5% in 24 h) at 293 K. As the reaction temperature was raised to 313 K, a good conversion of 57% with a selectivity of 89% was obtained after 72 h. In comparison, less than 10% conversion was observed for the background reaction (without 2) under the same conditions. The heterogeneous nature of this catalyst system was demonstrated by removing the solid from the reaction mixture after 8 h of initial reaction (8% conversion at this point). The conversion of the colorless filtrate did not increase any further after the removal of the solid catalyst. 

Cr3F(H20)20(bdc)3] (MIL-101 bdc = 1,4-benzenedicarboxylate), each of which catalyse cyanosilylation reactions, with the latter acting as a stronger Lewis acid than the former due to the greater relative acidity of Cr overCu ... 

Following the same basic principle of exploiting the coordinate bond between Cd(ii) and bpy in a coordination complex targeted at cyanosilylation reaction of imines Natarajan et al. explored the catalytic performances of... 

Figure 17.4 Representation of the crystal structures accompanying the formation of desolvated 20 (catalyst for Knovenagel condensation) from 20 (catalyst for cyanosilylation reaction).

Scheme 17.7 (a) Cyanosilylation reaction of aromatic aldehydes to cyanohydrins (b) Knovenagel condensation of aromatic aldehydes to... 

Kaskel et al. [145] also studied the catalytic action of MIL-101 in the cyanosilylation reaction. The coordinated water molecules are easily detached, thereby a coordination unsaturation at the Cr(III) sites is created. Owing to the fact that the Lewis acidity of the Cr(III) sites is much higher than that of Cu(II), MIL-101 turned out to be much more active than HKUST-1 in this reaction. In addition, the catalytic sites of MIL-101 are resistant to the side reaction of benzaldehyde reduction, unlike HKUST-1 sites. 

Recently, hydrocyanation and cyanosilylation reactions with other type of chiral aluminum complexes were reported. In 1999, Shibasaki and Kanai reported enantioselective cyanosilylation of aldehydes catalyzed by Lewis acid-Lewis base bifunctional catalyst (64a) [56, 57]. In this catalyst, aluminum center works as a Lewis acid to activate the carbonyl group, and the oxygen atom of the phosphine oxide works as a Lewis base to activate TMSCN. Asymmetric induction was explained by the proposed transition state model having the external phosphine oxide coordination to aluminum center, thus giving rise to pentavalent aluminum... 

Scheme 12.4 Proposed mechanism of the cyanosilylation reaction of ketones [137],...

Air-stable and easy-to-prepare imidazolium-2-carboxylates (122) and (benz)imidaz-olium hydrogen carbonates (123) have been evaluated for their potential as organic precatalysts for diverse NHC-mediated reactions.Both (122) and (123) proved efficient as NHC sources and have been shown to catalyse benzoin condensation as well as transesterification and cyanosilylation reactions. Interestingly, the catalytic activity of (122) has been found to be higher than for (123). 

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