Resin-bound phosphonium salt

Phosphonium salts can be dealkylated by treatment with alkoxides to yield alkanes. Although the hydrolytic cleavage of phosphonium salts in solution has been well investigated, the solid-phase variant of this reaction has not yet found broad application. One example, in which traceless linking was based on the alkoxide-induced dealkylation of a resin-bound phosphonium salt, is given in Table 3.48 (Entry 1). 

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. 

However, thermolysis of the phosphonium salts (X=+PPh3) leads directly to the indolic products without need of acid catalyst or PPh3, and thus may not proceed via a normal Wittig pathway. Alternatively, Hughes has effected a solid-phase version of this reaction employing a polymer-hound phosphonium salt and potassium tert-butoxide as base <96TL7595>. In this case, the phosphine oxide by-product remains bound to the polymer resin. 

Polystyrene-bound benzaldehydes can be smoothly olefinated with benzyl- or cin-namylphosphonium salts in DMF or THF using sodium methoxide as a base (Entry 1, Table 5.5 [64-67]). Alkylphosphonium salts, however, only react with resin-bound aldehydes upon deprotonation with stronger bases, such as butyllithium [30,68-70]. The more acidic acceptor-substituted phosphonium salts, on the other hand, even react with resin-bound aldehydes and ketones upon treatment with tertiary amines, DBU, sodium ethoxide, or lithium hydroxide [71-75], but stronger bases are also used occasionally [76]. 

Polymer-bound triphenylphenylphosphine [354] has been converted to a phosphonium salt 8i by reaction with 2-nitrobenzyl bromide. The nitro group was then reduced to an amine and acylated. The resulting product formed a resin-bound ylide in the presence of base, which underwent a Wittig reaction with aldehydes that released alkenes into solution with the phosphine oxide by-product remaining bound to the solid support [355]. 

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