Phosphines quaternization

On treatment with aqueous or alcoholic alkali hydroxide or with tertiary amines the quaternary (hydroxymethyl)phosphonium salts are cleaved with loss of one equivalent of formaldehyde and formation of the tertiary phosphine 74-77 79,80 if the phosphonium salt contains more than one hydroxymethyl group, the amount of base used must not exceed one equivalent the fission occurs more easily the less basic is the resulting phosphine.75 As the tertiary (hydroxymethyl)phosphine quaternizes again with alkyl halides, any desired tertiary phosphine can be built up by successive quaternizations and basic fissions.75,77,79... 

The alkylation process possesses the advantages that (a) a wide range of cheap haloalkanes are available, and (b) the substitution reactions generally occur smoothly at reasonable temperatures. Furthermore, the halide salts formed can easily be converted into salts with other anions. Although this section will concentrate on the reactions between simple haloalkanes and the amine, more complex side chains may be added, as discussed later in this chapter. The quaternization of amines and phosphines with haloalkanes has been loiown for many years, but the development of ionic liquids has resulted in several recent developments in the experimental techniques used for the reaction. In general, the reaction may be carried out with chloroalkanes, bromoalkanes, and iodoalkanes, with the reaction conditions required becoming steadily more gentle in the order Cl Br I, as expected for nucleophilic substitution reactions. Fluoride salts cannot be formed in this manner. 

In principle, the quaternization reactions are extremely simple the amine (or phosphine) is mixed with the desired haloalkane, and the mixture is then stirred and heated. The following section refers to the quaternization of l-alkylimidazoles, as these are the most common starting materials. The general techniques are similar, however, for other amines such as pyridine [9], isoquinoline [10], 1,8-diazabi-cyclo[5,4,0]-7-undecene [11], 1-methylpyrrolidine [12], and trialkylamines [13], as... 

Quaternization of the phosphine (71) with methyl iodide gave the phosphonium salt in quantitative yield. 

HN=CR 2, the presence of triethylamine. When at least one of the groups is alkoxy, these phosphines may be converted into monophos-phazenes by a quaternization and subsequent pyrolysis ... 

Preparation.- Tetra-isopropylphosphonium salts are difficult to prepare by direct quaternization of tri-isopropylphosphine. However, a route to these compounds is provided by quaternization of the phosphine with iodoethane, followed by generation of the... 

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). 

Quaternary phosphonium iodides are also good choices for the iodide salt catalyst component because they are highly active and, in some cases, soluble in the reaction and recovery processes. The simple quaternization of tri(n-alkyl)phosphines or triarylphosphines with n-alkyl iodides produces a wide variety of low cost phosphonium iodide salts ... 

Bearing in mind that chemical shifts are usually not dependent on the solvent or concentration, but may be partly dependent on the anion37, quaternization of alkyl-phosphines causes an upfield shift (1-7 ppm) for the near carbons of the alkyl chain, the effect becoming only slightly detectable on C(y) carbons37,94. Where triarylphosphines are quaternized, the chemical shift of the phenyl ipso carbon Cf moves upfield whereas that of the Cpara carbon moves downfield, as expected from the polarization of the 71-electron density in the phenyl ring36,37. For the aromatic carbon atoms, the increments... 

Wittig reactions can also be performed with support-bound phosphorus ylides. Polystyrene-bound alkylphosphonium salts have been prepared from the corresponding alkyl mesylates or halides and trialkyl- or triarylphosphines (Figure 5.8 [60,80]). Because polystyrene is a hydrophobic support, salt formation does not proceed smoothly and quaternization of phosphines generally requires forcing conditions. The... 

The formation of charged molecules within hydrophobic supports does not proceed as smoothly as in polar solvents. For instance, high reaction temperatures are required to quaternize polystyrene-bound phosphines [335,336], and the N-benzylation of pyri-dines with Merrifield resin also proceeds sluggishly [37]. Quaternization of tertiary... 

As expected, carbon-13 shifts of phosphines (Table 4.49) are enhanced by increasing alkylation (a effect) and increasing number of jS alkyl carbons ([1 effect). In contrast to amines, however, quaternization of phosphines causes shieldings in a and fi posititions (Table 4.49). A particularly large a shielding is induced by quaternization of triphenylphosphine. 

Takemoto and coworkers extended their palladium-catalyzed asymmetric allylic alkylation strategy using allyl acetate and chiral phase-transfer catalyst to the quaternization of 13 [23b]. A correct choice of the achiral palladium ligand, (PhO P, was again crucial to achieve high enantioselectivity and hence, without chiral phosphine ligand on palladium, the desired allylation product 15 was obtained with 83% ee after hydrolysis of the imine moiety with aqueous citric acid and subsequent benzoylation (Scheme 2.12). 

Monocyclic Phosphoranide Anion. The intramolecular oxidative addition of hydroxyalkyl phosphites, which gives P-H phosphoranes, is well known (10). Some P-H phosphoranes are so stable that the open-chain P(III) tautomers cannot be detected spectroscopically or even by attempted H2O2 oxidation (8). Thus, it is surprising to find no evidence for an equilibrium between phosphine alcohol 1 and its closed-ring tautomer phosphorane 2. Phosphine 1 is quaternized by alkyl halides giving phosphonium halides such as 3. These in turn are converted to alkoxyphosphor-anes, such as 4 by NaH (Scheme I). 

Tricoordinated phosphorus compounds react with a wide range of substrates and a great deal of mechanistic information is now available within this area.J-.3 The classical work involved the quaternization of phosphines with alkyl halidesll and the famous Arbusov reaction. . which prompted intensive studies in the field of organophosphorus chemistry. 

All quaternization reactions were performed under the same conditions, e.g., in ca. 0.08 M solution in acetone, at ambient temperature, for 20 h, and a pressure of 10 kbar was maintained during the reaction course. Without exception colourless crystalline solids precipitate in the reaction cell, and after a simple work-up (washing with hexane and drying in vaccuo) the isolated bisquaternary salts were analytically pure. Demethylation was carried out by heating the respective salt with triphenyl-phosphine in boiling DMF, followed by column chromatography on alumina. Yields obtained for the quaternization and demethylation reactions are listed in Table 2. 

A suspension of nutshell particles in DMF was quatemized with tributyl-phosphine. This converted the encapsulated poly(VBC) into poly[(vinylben-zyl)-tributylphosphonium chloride]. The EDS experiments indicated a low surface concentration of phosphorus in the quaternized samples. Since the elemental analysis indicated a high content of phosphorus in all the samples, one can infer that the quaternary onium cations are hidden inside the polystyrene cages . 

The more conventional ionic liquids are generally prepared in a two-step procedure from the corresponding amine or phosphine (Figure 6.2). Alkylation leads to quaternization of the heteroatom and then anion metathesis can be performed if desired. The most effective way to perform the quaternization is in solvent free conditions under microwave irradiation. As they need to be prepared, RTILs are less green than many other alternative solvents in terms of life cycle... 

The ratio of the two Zr(IV) products that one observes depends on the nature of RX primary alkyl halides highly favor the formation of the Zr alkyl complex, tertiary alkyl and acyl halides almost exclusively form the Zr dihalide complex, while mixtures of the two organometallic complexes are observed for secondary halides. The coordinated phosphine ligands invariably are quaternized to phosphonium salts, but this conversion is slower than the rates of oxidation of Zr. 

The incorporation of an alkylammonium group can readily be achieved, as for (128), by reduction of the protected tertiary phosphine (as its oxide) with HSiCl3.251 The analogous phosphonium salt (129) is also known.2 2 The chiral quaternized phosphine (130) has been documented,253 whilst protonation of the rhodium(I) complex containing (131) with aqueous HBF4 gave a water-soluble complex.254 Addition of a weak base (NEt3) cleanly reversed this reaction. 

Another way to use tertiary phosphines as precursors for phosphine oxides is to quaternize them with alkyl halides and then decompose the quaternary ion by alkaline hydrolysis (usually... 

Reactions.—Nucleophilic Attack at Carbon. A number of studies of the kinetics of quaternization of phosphines have been reported, all of which lend support to an earlier suggestion that the transition state for such reactions is reactant-like. From the rates of quaternization of a series of heteroaryldiphenylphosphines (54) with a-bromoacetophenone, it was concluded that the r-excessive heterocyclic substituents are not significantly involved in pj,-d conjugative stabilization of the developing phosphonium centre in the transition state of the reaction. Similarly, there is little evidence of conjugative effects in the transition state for quat nization of... 

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