Phosphonium ion catalysts

Comparison of polystyrene-supported phosphonium ion catalysts 1 in the reaction of 1-bromooctane with iodide ion showed a 4.5 % CL catalyst to be only half as active as a 2% CL catalyst74 . Use of decane, toluene and o-dichlorobenzene as solvents gave rate constants that increased as the swelling ability of the solvent increased. Swelling ratios were measured at 90 °C, the reaction temperature. 

A slow non-competing liquid/liquid reaction with no catalyst present gave only 78 % O-alkylation. Thus the active site of the lipophilic phosphonium ion catalysts appears to be aprotic, just as in analogous phase transfer catalyzed alkylations with soluble quaternary ammonium salts 60), Regen 78) argued that the onium ion sites of both the 17% and the 52% RS tri-n-butylphosphonium ion catalysts 1 are hydrated, on the basis of measurements of water contents of the resins in chloride form. Mon-tanari has reported measurements that showed only 3.0-3.8 mols of water per chloride ion in similar 25 % RS catalysts 74). He argued that such small hydration levels do not constitute an aqueous environment for the displacement reactions. No measurements of the water content of catalysts containing phenoxide or 2-naphthoxide ions have been reported. 

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... 

No experiments with variation in particle size of the silica gel have been done to study intraparticle diffusion effects. In silica gel such diffusion would be only through the pores (analogous to the macropores of a polystyrene) since the active sites lie on the internal surface. The silica gel used by Tundo had a surface area of 500 m2/g and average pore diameter of 60 A.116). Phosphonium ion catalyst 28 gave rates of iodide displacements that decreased as the 1-bromoalkane chain length increased from C4 to Cg to C16, The selectivity of 28 was slightly less than that observed with soluble catalyst hexadecyltri-n-butylphosphonium bromide U8). Consequently the selectivity cannot be attributed to intraparticle diffusional limitations. 

When the reactions of alkyl bromides (n-Q-Cg) with phenoxide were carried out in the presence of cosolvent catalyst 51 (n = 1 or 2,17 % RS) under triphase conditions without stirring, rates increased with decreased chain length of the alkyl halide 82). The substrate selectivity between 1-bromobutane and 1-bromooctane approached 60-fold. Lesser selectivity was observed for polymer-supported HMPA analogue 44 (5-fold), whereas the selectivity was only 1,4-fold for polymer-supported phosphonium ion catalyst 1. This large substrate selectivity was suggested to arise from differences in the effective concentration of the substrates at the active sites. In practice, absorption data showed that polymer-supported polyethylene glycol) 51 and HMPA analogues 44 absorbed 1-bromobutane in preference to 1-bromooctane (6-7 % excess), while polymer-supported phosphonium ion catalyst 1 absorbed both bromides to nearly the same extent. 

The major disadvantages of the polymer-supported quaternary ammonium and phosphonium ion catalysts are 1) They have higher initial cost. Unless they can be used in a flow system or recovered from batch reactors and reused many times, they will be more expensive to use than soluble catalysts. 2) In most cases the insoluble catalysts are less active than their soluble analogues. Their lesser activity is an intrinsic property of heterogeneous catalysts. If activity is the sole criterion for choice of a catalyst, one should use a soluble catalyst. The reasons for use of supported catalysts are ease of separation and reuse. 

For reaction of n -decyl methanesulfonate with aqueous sodium chloride (eq 3), five percent ring substituted phosphonium ion catalysts have activities 0.6-0.7 times that of the highly active soluble benzyltributylphosphonium bromide.The activities decrease rapidly with increasing ring substitution to <0.3 times that of the soluble catalyst at 20% iring-substitution. 

The rate of reaction of 1-bromooctane with aqueous potassium iodide using phosphonium ion catalysts decreased with decreasing polarity of the organic solvent in the order j -dichlorobenzene > toluene > decane.Tomoi obtained similar results for the reaction of 1-bromooctane with aqueous sodium cyanide and catalyst 3a, shown in Figure 3. In both cases the reactivity decreased in the same order as the degree of swelling of the catalyst by the solvent. Swelling of the catalyst should promote reactant transport to the active sites. More polar solvents also should increase the intrinsic reactivity between nucleophile and 1-bromooctane. 

There is one report of triphase liquid/liquid/solid catalysis in a continuous flow reactor. Ragaini and Saed l passed a mixture of 1-bromooctane in o-dichlorobenzene and aqueous potassium iodide upward through a bed of polystyrene-bound phosphonium ion catalyst. 

Another substituted derivative of BINAP was used by Lemaire et al. [109]. The ammonium salt catalysts (7 and 8, Fig. 41.10) were prepared in situ from the bro-mohydrates and [Ru( /3-2-methylallyl)2(/72-COD)], and immobilized in several ionic liquids. By comparative studies of the hydrogenation of ethyl acetoacetate, the best results were obtained with imidazolium- and pyridinium-containing ionic liquids. No significant ee was observed with the phosphonium salt. This observation was attributed to problems of solubility and to the ability of complexation for the phosphonium ion. From the anionic side, use of the [BF4] anion appeared superior compared to [PF6] and [(CF3S02)2N]A... 

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 dependence of kobsd on stirring speed for Br-I exchange reactions with polymer-supported crown ethers 34 and 35 has been determined under the same conditions as with polymer-supported phosphonium salts 1 and 4149). Reaction conditions were 90 °C, 0.02 molar equiv of 100-200 mesh catalyst, 16-17% RS, 2% CL, 20 mmol of 1-bromooctane, 200 mmol of KI, 20 ml of toluene, and 30 ml of water. Reaction rates with 34 and 35 increased with increased stirring speed up to 400 rpm, and were constant above that value. This result resembles that with polymer-supported onium ion catalysts and indicates that mass transfer as a limiting factor can be removed in experiments carried out at stirring speeds of 500-600 rpm, whatever kind of polymer-supported phase transfer catalyst is used. 

Crown ether, cryptand, and poly(ethylene glycol) catalysts are more stable in base than the quaternary ammonium and phosphonium ions. Only the polyethylene glycols) are likely to meet industrial requirements for low cost, although a number of more efficient, lower cost crown ether syntheses have appeared recently, such as those of sila-crowns 64 bound to silica1B9). 

Polymer-supported multi-site phase-transfer catalysis seems to require the use of less material in order to provide activity comparable to others253 (Table 27). Quaternary phosphonium ions on polystyrene latices, the particles of which are two orders of magnitude smaller than usual, were shown to be capable of higher activity coagulation of the catalysts under reaction conditions was minimized by specific treatment904. The spacers may also contain ether linkages. 

Transition metal oxidants such as permanganate, ruthenium tetroxide and diromium(VI) oxide are convenient and efficient reagents for routine cleavage reactions. The use of phase transfer catalysts (quaternary ammonium and phosphonium ions, primarily) has made it possible to solubilize transition metal oxides such as permanganate and chromatt in nonaqueous solvents, and to therdry increase the scope of these reactions substantially. ... 

Within this mechanisms the phosphonium ions function as a methyl-group transfer agent and the critical step for the conversion of methanol - the splitting-off of water - is facilitated by taking place outside the direct catalyst cycle. 

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