Supported Organocatalysts in Hydrosilylation of Imines

The formamide type catalysts 22-35, developed by Malkov and Kocovsky 

The reaction was carried out at 0.1 mmol scale with 2.0 equiv of CbSiH atO C for 24 h in toluene with 10mol% catalyst loading unless stated otherwise see Ref. [Ilf]. 

To address these problems, a fluorous tag was appended to the catalyst (48) [12c], which reduced the separation to an ordinary filtration through a pad of fiuorous silica gel that retained the catalyst, whereas the product was eluted. Subsequent change of the solvent resulted in elution of the catalyst that could be reused. The classical chromatography of the crude mixture after the workup was thus avoided. The presence of the fiuorous tag had practically no effect on the catalytic activity (Table 4.11 compare entries 1 with 3 5) [12c]. However, the polyfiuorinated starting material required for the synthesis of 48 and the fiuorous silica gel are rather expensive, which may become prohibitive for large scale applications therefore, other options were also explored. 

Further simplification was attained by immobilization of the catalyst on a solid polymer (49), so that its separation from the product was reduced to mere mechanical filtration the resin bound catalyst could then be used for another run. Ofthe number of polymeric supports investigated, Merrifield (49) and Wang resins were identified as most suitable, exhibiting the same behavior and efficiency [18]. However, owing to the heterogeneous nature of the system, the enantioselectivity of the reaction decreased by 10 15% ee (Table 4.11, entries 6 12). In view ofthe swelling properties, toluene was found to be less suitable than chloroform (compare entries 6 and 7), which makes the method less environment friendly. Furthermore, a conditioning effect was observed for these systems the second run was always found to be more enantioselective than the first one by 10% ee and this level was maintained in the subsequent runs (compare entries 7 with 8 12). The latter effect stems from the 

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