Phosphonium linkers

Hughes exploited a polymer-bound phosphonium-salt as a traceless linker for the synthesis of alkyl, alkenyl and heteroaryl products [263]. The linker system 383 was prepared from commercially available resin-bound triph-enylphosphine and nitrobenzyl bromide with following reduction of the nitro group and acylation. 

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Scheme 54 Multifunctional cleavage from the phosphonium linker hy Hughes...

Some 5-methyl-l,2-disubstituted benzimidazoles derivatives have been synthesized in an efficient and rapid solid-phase method with help of the phospho-nium linker. The phosphonium linker was prepared by reaction between polymer-supported triphenylphosphine and 4-fluoro-3-nitrobenzyl iodide, which underwent aromatic substitution with primary amines, followed by one pot reaction with aldehydes in the presence of SnCl2 2H20 under microwave irradiation. The products were isolated from resin using NaOH to give high purity and good overall yield (Rios et al., 2013). 

Two different synthetic strategies were proposed to obtain a variable linker between the phosphoester and the cationic entity. The first was based on the treatment of a fatty dialkylchlorophosphate (obtained from POCl3 and two equivalents of fatty alcohol) with phosphorous and arsenic ylides, which resulted, after acidification, in the phosphonium and arsonium methylenephosphonates. Tri-methylsilyl stabilized ylides... 

Figure 15.23 Synthesis of phosphonium and arsonium based cationic lipids linker with single carbon.

The nucleophilic curing system is most common and is used in about 80% of all applications. It is based on the cross-linker (bisphenol AF) and accelerator (phase transfer catalyst, such as phosphonium or amino-phosphonium salt). Both diaminic and bisphenol type cure systems are permitted by U.S. Food and Drug Administration (FDA) regulations governing rubber articles in contact with food. The diaminic curing system is also used in some coating and extrusion applications [42]. 

Abbreviations used Dde, N-[l-(4,4-dimethyl-2,6-dioxocyclohexadiene)]-ethyl DIEA, diisopropylethylamine DMF, N,N-dimethylformamide EDC, l-Ethyl-3-(3 -dimethylaminopropyl)carbodiimide hydrochloride HOBt, 1-hydroxybenzotriazole NHS, N-hydroxysuccinimide NMP, l-methyl-2-pyrroIidinone PAL, Peptide Amide Linker [5-(4-(9-fluorenylmethyloxycarbonyl) aminomethyl-3,5-dimethoxyphenoxy) valeric acid] PyBOP, Benzotriazole-l-yl-oxy-trispyrrolidino-phosphonium hexafluorophosphate TFA, trifluoroacetic acid. 

The phosphorous-carbon bond in phosphonium salts is readily cleavable by the aid of a base in the absence of aldehydes. Hence, the polymer-bound phosphonium salt 128 offers a direct access to methylarenes 129 (Scheme 16.31). An interesting feature of this linker is the fact that carbonyl compounds can be olefi-nated and this leads to a cleavage-olefmation linker system [126, 127]. 

The transformations and cleavage reactions of polymer-supported phosphonium salts of type 6 3 can be considered as nice examples of traceless linker strategies. 

A review article by Blumel describes classical and modern solid-state NMR methods that allow to gain insight into catalyst systems where one or two metal complexes are bound to oxide supports via bifunctional phosphine linkers, such as (EtO)3Si(CH2)3PPh2. It has been shown that many aspects of the immobilized molecular catalysts can be elucidated with the corresponding NMR technique. For example, the bulk of the support can be studied, as well as the interface of the support with the ethoxysilane. In addition, electrostatic bonding to the support via phosphonium groups can be proven by solid-state NMR. For the immobilized catalysts, leaching, and even horizontal translational mobility effects, as probed by HR MAS NMR under realistic conditions in the presence of solvents, are described. 

Elimination of the sulfone linker has also been used in the formation of aryl sulfonamides, dehydroalanine, and heterocyclic 2-substituted-4-piperidones." Pendant phosphonium sulfone intermediate 169 was subjected to Wittig reagent to form sulfone-bound vinyl ketones 170 (Scheme 12.35). Benzylamine as nucleophile and base was then... 

Wang and co-workers synthesised a phosphine-borane Lewis pair connected by a U-shaped linker (Fig. 2). The photo-physical properties of this compound were examined and compared with the corresponding phosphonium salt, obtained by methylation with Mel, and the Au(i) and Pt(ii) complexes where the phosphine-borane Lewis pair was used as a ligand. 

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