Spacer chain catalysts

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... 

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... 

The DMAP analogues bound to cross-linked PS are active in non-polar solvents for esterification of sensitive tertiary alcohols as in equations (21) and (22), dimerization of phenyl isocyanate as in equation (23), nucleophilic acyl rearrangements as in equation (24), and synthesis of dipalmitoylphosphatidylcholine from palmitic anhydride as in equation (25). The polymeric catalysts are slightly less active than DMAP, but they have been recovered and recycled three times with no loss of activity. " The spacer chain catalyst (58 n = 3, DF = 0.16-0.48, 2% DVB) was more active than catalyst (58 n= ) for acetylation of 1-methylcyclohexanol. Spacer chains (n = 4,7) and DF 0.15-0.20 gave highest activity for acyl rearrangements. A mixture of catalyst (58) and cross-linked poly(V,V-diethylaminomethylstyrene), as a proton acceptor, was more active for acetylation of linalool (equation 21), than catalyst (58) alone. 

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.)...

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. 

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. 

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)... 

A related catalyst type where the metal centre is supported by a longer spacer chain has been reported (Figure 4.3).64,65 This material was prepared from aminopropyl-silica by reaction with terephthalaldehyde to form the mono-imine (attachment of the amine to the surface precludes reaction at both ends of... 

The reactivity of a liquid-solid-liquid triphase reaction (i.e., polymer-supported catalytic reaction) is influenced by the structure of the active sites, particle size, degree of cross-linkage, degree of ring substitution, swollen volume, and spacer chain of a catalyst pellet. In the past, the characteristics of a triphase reaction, subjected to the mass transfer limitation of the reactants and ion-exchange rate in the aqueous phase, have been discussed [146,158,162,178,179]. The ion-exchange rate in the aqueous phase affects the reactivity of the triphase reaction. 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

Quaternary ammonium and phosphonium salts bonded both directly and through spacer chains to silicic matrices operate as PTC catalysts in analogy to quaternary salts bonded to polymers. Many types of silica have been used jjj... 

In a Darzens condensation the optical yield increased up to 23 % in the presence of chiral catalysts bonded to a polymer. This yield is much higher than that obtained with analogous soluble catalysts In other cases, the progressive increase of the spacer chain length resulted in no optical yield... 

The most commonly studied resin support is polystyrene crosslinked with divinylbenzene (DVB) (1-2% crosslinking) in its microporous form, though it has also been used in its macroporous and popcorn forms. The PT catalyst can be physically adsorbed or chemically bound on the support with or without a spacer chain between the support and the PT catalyst. These supported catalysts are active in a variety of reactions but find limited commercial applications due to lower reactivity than the soluble analogs, which is mainly due to diffusional limitations in the... 

Full details are now available of a study of polystyrene-supported quaternary ammonium and phosphonium catalysts (80) for triphase catalysis (TC) in a liquid-solid-liquid mode. Long spacer chains between the quaternary centre and the polymer backbone were found to be unnecessary for TC activity, and phosphonium catalysts were more effective than ammonium thus (80 n = 3, A = P) compared favourably with soluble octadecyltributylphosphonium bromide as a PT catalyst for reactions such as halide to halide interconversions. A test reaction, that of potassium acetate with 1-bromo-octane, has been used to examine the TC activity of silica in the solid-solid-liquid mode. Silica impregnated with a cationic surfactant appears to be as effective as alumina (4,163), whereas silica bearing covalently bound quaternary ammonium groups exhibits increased effectiveness. 

Brown and Jenkins found that 98% ring substituted catalyst gave 98% 0-benzylation of 2-naphthoxide ion, and that catalyst (2% cross-linked, >90% substituted) gave about 65% 0-benzyl product (eq 5). The long aliphatic spacer chain creates a more hydrophobic... 

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