Benzyl spacers

The following subsection considers PBs featuring 2-benzyl spacers. Both conceivable arrangements (the phosphine or the borane moiety in ben-zylic position) have been demonstrated experimentally. 

More flexible systems are obtained with benzyl spacers due to free rotation around the CH2-aryl bond. The formation of intramolecular P-B interactions is also more favorable since it leads to unstrained five-membered ring structures. Accordingly, monomeric closed structures were authenticated spectroscopically for compounds 26a,44 26d, and 26e46 (Figure 15). 

The series of MPB, DPB, and TPB 27-29 further substantiates the propensity of the benzyl spacer to favor the closed form (Figure 16). 45 The spectroscopic 31P and 11B NMR data are diagnostic of P-B interactions, while X-ray diffraction studies performed on 27 and 28 revealed short P-B distances (2.11-2.13 A) and significantly pyramidalized environments around boron (XB, = 343-347°). In addition, low-temperature 31P NMR experiments indicated rather fast exchange of the two phosphorus atoms for the DPB 28 (activation barrier <10 kcal/mol), but in contrast to that observed for the orflto-phenylene-bridged system 8a,24 no indication of a tricoordinate boron center associated with the monomeric open structure was apparent in the 11B NMR spectrum. 

Fig. 20.2 Grafting methods used to generate site-isolated amines with the circle representing either the trityl or benzyl spacer group...

Hicks JC, Jones CW (2006) Controlling the density of amine sites on silica surfaces using benzyl spacers. Langmuir 22 2676... 

Kbnig prepared guanidinium-crown ether receptors (24) for the recognition of zwitterionic substrates. They investigated the effects of the spacer between the guanidinium and the crown ether units on the association of the guest. The 1,4-disubstituted benzyl spacer was optimal for binding peptides that contain carboxylates. 

Miller and co-workers have recently prepared nanometer-scaled molecular dumbbells based on poly(benzyl ether) dendrons (G1-G4) and oligoimide spacers [64], Their synthetic approach involved the coupling of amine-terminated oligoimides to dendrons with a carboxylic acid focal point. The resulting hybrid materials were found to be quite soluble thus allowing their analysis by cyclic voltammetry in DMF. Consistent with Roncali s observation, the kinetics of reduction of the oligoimide core was not found to be limited by the presence of the dendritic wedges. 

Spacer chain catalysts 3, 4, and 19 have been investigated under carefully controlled conditions in which mass transfer is unimportant (Table 5)80). Activity increased as chain length increased. Fig. 7 shows that catalysts 3 and 4 were more active with 17-19% RS than with 7-9% RS for cyanide reaction with 1-bromooctane (Eq. (3)) but not for the slower cyanide reaction with 1-chlorooctane (Eq. (1)). The unusual behavior in the 1-bromooctane reactions must have been due to intraparticle diffusional effects, not to intrinsic reactivity effects. The aliphatic spacer chains made the catalyst more lipophilic, and caused ion transport to become a limiting factor in the case of the 7-9 % RS catalysts. At > 30 % RS organic reactant transport was a rate limiting factor in the 1-bromooctane reations80), In contrast, the rate constants for the 1 -chlorooctane reactions were so small that they were likely limited only by intrinsic reactivity. (The rate constants were even smaller than those for the analogous reactions of 1-bromooctane and of benzyl chloride catalyzed by polystyrene-bound benzyl-... 

Alkylation of 2-naphthoxide ion (Eq. (6)) occurs mainly on carbon in aqueous solvents and mainly on oxygen in aprotic solvents. The product distribution is often used as a probe of the solvent environment in heterogeneous reactions. Brown and Jenkins 54) found that 40-100 % RS spacer chain catalysts 15 and 16 gave up to 98 % O-benzylation of 2-naphthoxide ion with benzyl bromide. The shorter spacer chain catalyst 16 gave 85% O-alkylation, and a conventional benzyltrimethylammonium ion resin 2 gave about 70 % O-alkylation. Because of low activity, product distribution data were obtained with varied amounts of catalyst and were extrapolated to equimolar amounts of catalyst and substrate to obtain the catalyzed O/C product ratios. Interpretation of the data also was complicated by independent evidence that catalysts 15 adsorbed 2-naphthoxide ion, in addition to that bound by ion exchange54). Essentially the same results were obtained with catalysts 24 which lack the ester link in the spacer chain 106). 

The first attempts to use macroporous polystyrene supports for onium ion catalysts were reported in the early papers of Brown and Jenkins 54) and of Regen 89). The lightly cross-linked macroporous Rohm and Haas XE-305, 46 % RS as spacer chain catalyst 15, gave O- and C-alkyl products from 2-naphthoxide ion and benzyl bromide... 

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