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Scheme 32

An interesting variant of Hofmann elimination on solid support was reported which involves two resins [66]. Besides the solid support for synthesis, an ion-exchange resin (Am-berlite) was applied to promote the Hofmann elimination in the presence of a catalytic amount of triethylamine.

3.1.3.2 Traceless Linkers Based on Silylfunctionalization

Silicon-based linkers can be applied to the immobilization of aromatic and heteroaromatic entities onto a solid support. Protodesilylation procedures used to cleave off the combinatorial compounds leave no functional groups on the final products [67-69]. One limitation of this can be the relatively complicated and multistep synthesis of the actual linker unit. Usually, the starting material must be bound to the linker unit first, after which the resulting construct is attached to the support material. In addition, the conditions for the protodesilylation depend on the substitution on the arene ring.

Newly developed silicon linkers such as 67 (Scheme 33) were synthesized using fewer steps as compared with the earlier reported traceless linkers based on silylfunctionalization. They allow for an easy binding to the support before performing the combinatorial synthesis [70, 71].

The linker was used in combination with alkylation, acylation, and Mitsunobu reaction according to Scheme 33, and the Si-phenyl bond was cleaved with either TFA vapor, neat TFA [70] or TFA/DCM (1:1) [71].

R'xj

Scheme 33

HOBt

iXOH

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