Functionalized phosphonium salts

A variation of the Madelung cyclization involves installing a functional group at the o-methyl group which can facilitate cyclization. For example, a triphenylphosphonio substituent converts the reaction into an intramolecular Wittig condensation. The required phosphonium salts can be prepared by starting with o-nitrobenzyl chloride or bromide[9]. The method has been applied to preparation of 2-alkyl and 2-arylindoles as well as to several 2-alkenylindoles. Tabic 3.2 provides examples. 

The addition of P—H bonds across a carbonyl function leads to the formation of a-hydroxy-substituted phosphines. The reaction is acid-cataly2ed and appears to be quite general with complete reaction of each P—H bond if linear aUphatic aldehydes are used. Steric considerations may limit the product to primary or secondary phosphines. In the case of formaldehyde, the quaternary phosphonium salt [124-64-1] is obtained. 

A number of keto-carotenoids have been prepared from the bis-phosphonium salt (130) and substituted Cjs-aldehydes, the keto-functions... 

A simple cyclization was carried out to test the ability of chloro azidoalkene cyclization to generate quinolizidines. Reduction of 6-valerolactone to the corresponding lactol was followed by a Wittig reaction onto the carbonyl function employing an appropriate phosphonium salt, and the intermediate primary alcohol thus formed afforded the chloro... 

Heterocycles.—The phosphonium salt (59) is an effective three-carbon synthon, as demonstrated by its reaction with enolates of /9-keto-esters (Scheme 20) to give cyclopentenyl sulphides via an intramolecular Wittig reaction.63 Ylides are also intermediates in the synthesis of dihydrofurans (60) from the cyclopropylphos-phonium salt (61) and sodium carboxylates (Scheme 21).64 Cumulated ylides are very useful for the synthesis of heterocyclic compounds, e.g. (62), from molecules which contain both an acidic Y—H bond and a carbonyl or nitroso-function, as shown in Scheme 22.65... 

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... 

In a study which is relevant to the mechanism of hydrolysis of phosphonium salts, Glaser and Streitwieser297 studied the ions H4PO- and H3PFO- and their derivatives with Li +, NH4 and HF at the 6-31G level augmented by diffuse functions. They found that the structures of the anions are those of a hydride or fluoride ion solvated by or complexed with phosphine oxide, rather than phosphoranes297. A very important point is that earlier studies with diffuse functions yielded the pentacoordinated phosphoranes which they judged297 to be computational artifacts of the small basis set. 

The alkylation of tertiary phosphines is, in general, compatible with elaborate structures bearing various functions or chiralities, as illustrated by the preparation of a phosphonium salt (18), intermediate in the synthesis of pseudomonic acid248 (reaction 17). For the preparation of dialkylphosphonium salts, diphenylphosphine can be directly alkylated, but it is more advantageous to use triphenylphosphine as the starting material,... 

Two standard routes to y-functional phosphonium salts with electron-withdrawing groups consist in using the corresponding Michael alkenes to achieve either halide substitution in the -position to the functional group, in order to obtain y-functional vinylphosphonium salts, or Michael addition in the presence of acid to produce the y-functional alkylphosphonium salt (reaction 85). 

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