Polymer-supported borohydride

The N-substituted aminoacids required could be prepared by microwave-assisted reductive amination of aminoacid methyl esters with aldehydes, and although in the Westman report soluble NaBH(OAc)3 was used to perform this step, other reports have shown how this transformation can be performed in using polymer-supported borohydrides (such as polymer-supported cyanoborohydride) under microwave irradiation [90]. An additional point of diversity could be inserted by use of a palladium-catalyzed reaction if suitably substituted aldehydes had been used. Again, these transformations might eventually be accomplished using supported palladium catalysts under microwave irradiation, as reported by several groups [91-93]. 

Aryl azides are converted into the corresponding anilines by polymer-supported borohydride [33]. Simple aliphatic azides are not reduced under similar conditions and the reduction of benzyl azides is slow. 

Undecafluorocyclohexane (20) on dehydrofluorination using either potassium hydroxide, nickel at 840°C or polymer-supported borohydride (e.g., Amberlitc IRA 400) gives decafluorocyclo-hexene (21).18,72,73... 

In addition, supported reagents have been demonstrated to be effective under reaction conditions when either thermal or microwave heating - is employed. They have also been utilised in traditional batch synthesis, stop-flow methods and continuous flow processes. ° However, one caveat is that the immobilisation of reagents can change their reactivity. For example, polymer-supported borohydride selectively reduces a,P-unsaturated carbonyl compounds to the a,P-unsaturated alcohoF in contrast to the behaviour of the solution-phase counterpart, which additionally causes double bond reduction. 

One of the most widely used supported reagents for reduction is polymer-supported borohydride. An interesting example is the final step of the total synthesis of polysphorin 55, an anti-malarial natural product (Scheme 4.15) [62]. The stereoselective reduction of ketone 54 occurred with excellent diastereomeric excess when PS-BEMP was used, affording the 55 in 90% yield. 

The Ni Bt2(bpy) complex was found to be an efficient catalyst precursor, not only in the synthesis of ArSAr derivatives, but also in the catalytic formation of ArSeAr compounds (via the cross-coupling reaction of aryl selenols). Polymer-supported borohydride was used as a reducing agent and polymer-supported aryl iodide as a reagent. The reaction was carried out with high yield and resulted in high purity of the products leading to the construction of a library of supported species (Scheme 3.21) [48]. 

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