Spacer intermediate size

The requirements for solid-phase synthesis are diverse. The support must be insoluble, in the form of beads of sufficient size to allow quick removal of solvent by filtration, and stable to agitation and inert to all the chemistry and solvents employed. For continuous-flow systems, the beads also must be noncompressible. Reactions with functional groups on beads imply reaction on the inside of the beads as well as on the surface. Thus, it is imperative that there be easy diffusion of reagents inside the swollen beads and that the reaction sites be accessible. Accessibility is facilitated by a polymer matrix that is not dense and not highly functionalized. A matrix of defined constitution allows for better control of the chemistry. Easier reaction is favored by a spacer that separates the matrix from the reaction sites. Coupling requires an environment of intermediate polarity such as that provided by dichloromethane or dimethylformamide benzene is unsuitable as solvent. 

An acid chloride functionalized liquid crystalline material was tried to bond to the silanol group in silica. The direct reaction does not seem to be successful also, a spacer with a substituted dichlorodimethyl silane is not successful. The reaction scheme is shown in Figure 16.8. However, a separation performance is observed rather by deposition than by bonding of the liquid crystals. The liquid crystalline materials used in this study was based on cholesteric moieties. It is suspected that the reaction failed because of the large size of the cholesteryl group. In another study, dimeth-ylchlorosilane was added to the allyl group of 4-methoxyphenyl-4-allyloxy benzoate. This intermediate could be bonded to silica. Several other routes to fix liquid crystalline moieties on silica have been reviewed. ... 

Figure 13 Calix[4]arene-derived carcerand used by Warmuth et al. to stabilize reactive intermediates, such as o-benzyne and a singlet carbene. Note the butylene spacers that account for the larger size of the inner void space, compared to Cram s original structure.

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