Phosphonium salts exchange

Various new phosphonium salts were prepared in a one-pot reaction of carbon disulfide, phosphines, and 1 followed by an exchange reaction of the zirconated metal fragment with 2 equiv. of electrophiles (Scheme 8-23) [206],... 

Before examining the reaction of deactivated alkenes, the phosphonium salt synthesis was applied to 1,3-dienes.21 When ( )-6-phenyl-l,3-hexadiene was treated with equimolar amounts of PPh3 and CF3S03H in the presence of RhH(PPh3)4 (2.5 mol%) in THF at 0 °C for 3 h, ( )-(6-phenyl-3-hexenyl)triphenylphosphonium salt was obtained in 89% yield after anion exchange with LiPF6 and recrystallization (Scheme 16). The addition of phosphine and hydrogen occurred at the 1- and 2-carbon atoms of the 1,3-diene, respectively. The reaction of (7)-1,3-dienes was then performed for comparison. 

Reaction with a first aldehyde transforms 176 into the vinylphosphonium chloride 177, which for practical reasons is subjected to an anion-exchange process, leading to the phosphonium salt 178. From this, phenyllithium treatment liberates the allenic phosphorane 179, an intermediate that has previously been used to prepare allenes from aldehydes [69], in the present case providing the products 180. The same protocol has also been applied to o-alkynylbenzaldehydes to yield allenes of interest as model compounds for the study of Schmittel and Myers-type cyclization reactions [70]. 

More recently a variation of this mechanism was reported by Novak [37], The mechanism involves nucleophilic attack at co-ordinated phosphines and it explains the exchange of aryl groups at the phosphine centres with the intermediacy of metal aryl moieties. After the nucleophilic attack the phosphine may dissociate from the metal as a phosphonium salt. To obtain a catalytic cycle the phosphonium salt adds oxidatively to the zerovalent palladium complex (Figure 2.38). Note where the electrons go . 

The first catalysts utilized in phase transfer processes were quaternary onium salts. In particular, benzyltriethylammonium chloride was favored by Makosza (7 ) whereas Starks utilized the more thermally stable phosphonium salts (6,8). In either case, the catalytic process worked in the same way the ammonium or phosphonium cation exchanged for the cation associated with the nucleophilic reagent salt. The new reagent, Q+Nu , dissolved in the organic phase and effected substitution. 

Other Degraded Carotenoids. A new synthesis of the fungal sex hormone ( )-(7 , 9 )-trisporic acid B methyl ester (114) utilized as the key step a Michael-aldol sequence on the /3-keto-ester (115) to yield the highly functionalized cyclo-hexenone (116). The latter underwent Wittig reaction with the phosphonium salt (117) to give (114). After basic alumina-catalysed hydrogen exchange in tritiated... 

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. 

Complexation constants of crown ethers and cryptands for alkali metal salts depend on the cavity sizes of the macrocycles 152,153). ln phase transfer nucleophilic reactions catalyzed by polymer-supported crown ethers and cryptands, rates may vary with the alkali cation. When a catalyst 41 with an 18-membered ring was used for Br-I exchange reactions, rates decreased with a change in salt from KI to Nal, whereas catalyst 40 bearing a 15-membered ring gave the opposite effect (Table 10)l49). A similar rate difference was observed for cyanide displacement reactions with polymer-supported cryptands in which the size of the cavity was varied 141). Polymer-supported phosphonium salt 4, as expected, gave no cation dependence of rates (Table 10). 

In some cases, the anion exchange can also take place in solid-liquid biphasic conditions either for monomeric or dimeric phosphonium salts, in suspension or by using an anion-exchange resin, or for polymeric phosphonium salts, by washing with solutions of salt MX. Finally, it must be pointed out that the new anion Y can be generated in situ by a classical preparation of such anions (e.g. addition of cyanide anion to CS2 for NCCS2-527 or aromatic substitution with fluoride anion on substituted nitrobenzene derivatives for N02-528). Several recent examples of the various anions exchanges are shown in Table 10. 

The kinetic acidity of phosphonium salts was first studied by NMR measurements of the hydrogen-deuterium exchange rate for a series of phosphonium salts565 ... 

Adopting an alternative convergent strategy, the two key halves of (446) were readily assembled as shown. Catalytic reduction of (219) with phenylhydrazine produced an intermediate phenylhydrazone which smoothly exchanged with /i-nitrobenzaldehyde to give (444). Conversion of /i-toluoyl chloride to the phosphonium salt (445) was straightforward. Wittigcondensation of (445) with (444) followed by catalytic reduction and saponification afforded (446). 

Convenient alternatives to direct deprotonation of ethers are tin-lithium exchange [199, 258-261], halogen-magnesium exchange [262], or reductive cleavage of 0,Se-acetals [263, 264], Another synthetic equivalent of a-metalated ethers are (alkoxymethyl)phosphonium salts [265]. 

Anion exchangers, such as lipophilic quaternary ammonium salts (e.g., see Fig. 5.11) or phosphonium salts, have been employed for the preparation of... 

Kawashima et al. have devised a new method for the synthesis of monocyclic phosphoranes by the reaction of the thiophosphinate (33) with triethyloxonium tetrafluoroborate to form the phosphonium salt (34) which, on exchange of the CH2CI2 solvent for Et20, was converted quantitatively to (35) by fluoride abstraction from the counterion. 

Mechanistic Studies.- A number of substitution reactions of alcohols, phenols, or amines with 1,3,2-dioxaphospholans, e.g. (73), oxazaphospholans, and diazaphos-pholans have been followed by n.m.r. and shown to involve H-phosphoranes, e.g. (74). The reactions are run in toluene or without solvent, and without addition of an acidic catalyst in some systems, with 4-chlorophenol as the nucleophile, an equilibrium was established between an H-phosphorane (75) and a phosphonium salt (76), but the authors still favour (75) as the true intermediate. The equilibrium constants for a series of exchange reactions between (thio)phosphites (77) and (thio)phosphoro-dichloridites (78), and some analogous bromidites and a fluoridite, have been measured.The constants increase with increased electron donor ability of R. 

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