Spacer chain

Yonemura, H., Noda, M., Hayashi, K, Tokudome, H., Moribe, S. andYamada, S. (2002) Photoinduced intramolecular electron transfer reactions in fiillerene-phenothiazine linked compounds effects of magnetic field and spacer chain length. Mol. Phys., 100, 1395-1403. 

Example 1 Attachment of an enzyme to a quartz surface is usually effected via a spacer chain.1119]... 

Quartz surfaces 16-Carbon spacer chain, urease [119]... 

The kinetics of iron(III) dissociation from a series of dihydrox-amate siderophores and siderophore mimics, including rhodo-torulic acid (3) and alcalagin, have been investigated (52,127,128, 177,178). ESI-MS studies show that these systems form multiple species as a function of pH and siderophore/iron ratio (128). The lability of these systems and the resultant multiple species leads to several parallel paths to iron(III) dissociation (177). Both the distribution of structures and kinetics of dissociation were shown to be dependent on the length of the spacer chain between the dihydroxamate donor groups (52,127). 

If the rays possess ideal flexibility to allow application of Gaussian statistics, the resultant structure will resemble a soft sphere. This was the reason why the present author introduced the soft sphere model [38]. This model reduces to dendrimers in the narrow sense when no spacer chains between the branching units are present. 

The direct quaternization of chloromethylated polystyrenes by tertiary amines or phosphines represents the easiest way to obtain polymer-supported quaternary onium salt (12,13). A lipophilic character of quaternary cation and a topology allowing sufficient cation-anion separation also play an important role (35,36). A linear spacer chain (of about 10 carbon atoms) between the catalytic site and the polymer backbone substantially increases the reaction rates. The loading of quaternary onium groups also affects catalytic efficiency, the influence being different for directly bonded and spaced groups, e.g. 10 and 11, respectively (37). 

In consideration of the above points, we have previously reported (18) the syntheses of two insoluble, polymer-supported, multi-site PTCs (II and III, Scheme 1) and a limited study of their effectiveness in two simple Sn2 reactions. The impetus for this investigation was provided in part by a report by Reeves (13), who in contrast to reports (19) at that time by other workers, had demonstrated that it was not necessary to separate the PTC site of a polymer-supported PTC from the polymer backbone by long (l e., 8-39 atoms) spacer chains in order to achieve activity. For example, Reeves reported (13) that the polystyrene-backbone material I... 

Figure 5.5 Architecture of a layered SAM consisting of a rigid biphenyl unit (BP) on top of an aliphatic spacer chain of m methylene units. The surface-terminating tail group can be either chemically inert (e.g., X = CH3, H) or active (e.g., X = CN).

Number indicates length of alkane spacer chain. 

Polystyrene-supported quaternary phosphonium ions with spacer chains between the active site and the aromatic ring (5, 4, 17-19% RS)... 

Table 4. Effect of Spacer Chains on Activity for Reaction of 1-Bromooctane in Toluene with Iodide Ion at 90 °C 931...

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-... 

Fig. 8. Effects of % RS and spacer-chain length on the amount of solvents imbibed into 100/200 mesh 2% CL catalysts at 25 °C on the basis of g solvent/g dry catalyst. Left, catalyst 1 center, catalyst 3 right, catalyst 4. (Reprinted with permission from Ref.80>. Copyright 1982 John Wiley and Sons. Inc.)...

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... 

Quaternary onium ions bound to silica gel were reported as phase transfer catalysts initially by Tundo 1I4) and by Rolla and co-workers115 . The short spacer chain catalyst 27 (1.0 mmol/g) was more active than the longer spacer chain phosphonium ion catalyst 28... 

Activities of tri-n-butylammonium and tri-n-butylphosphonium ions with two different spacer chain lengths are compared in Table 8 1I8). The greater activity of the phosphonium ions is opposite to what has been reported for analogous soluble phase transfer catalysts119). Activities of the catalysts bound to silica gel were as high as activities of soluble catalysts adsorbed to silica gel118). Without some independent determination of the role of intraparticle diffusion it is not possible to determine whether the reduced activity of the adsorbed catalysts is due to lower intrinsic activity at the silica gel surface or to diffusional limitations. The size selectivity for alkyl bromides suggests that intraparticle diffusion was not a problem. 

The introduction of a spacer chain between the polymer backbone and the active site is expected to facilitate intraparticle diffusion. Increased swelling power of sol-... 

Spacer chains affect intrinsic reactivity as well as intraparticle diffusion. Rates for Br-I exchange reactions with spacer-modified catalyst 41 were larger than those with catalyst 35 containing no spacer (Fig. 11). An aliphatic spacer makes the catalyst more lipophilic and the intrinsic reactivity of the active site larger, though the intraparticle diffusity of an inorganic reagent is reduced. It is not known at this time how intrinsic reactivity contributes to the rate increase. 

Fig. 67. Schematic representation of three types of anionic porphyrins in a cast multibilayer film of 34. The overall bilayer organization is assumed to be the same as that of Fig. 64. For clarity, counterions are not shown and the bilayer units are separated from each other. The spacer portion is also not shown in Stereogram . Type I porphyrins (Fig. 66) are inserted into the bilayer along the molecular axis of the spacer chain. Type II porphyrins are randomly placed on the bilayer surface. Type III porphyrins lie flat on the bilayer [445]...

FIGURE 13. Effect of the spacer-chain length on obsd f°r the reaction of C8H17Br with NaCN in the presence of catalysts la-c with 14-17% ring substitution (% RS). (A) Catalyst la with 16% RS (O) catalyst lb with 14% RS ( ) catalyst lc with 15% Rs. r-Mean particle size (radius)... 

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