Bisphosphonium salts

Only one example of the potentially diatropic [19]annulenones is known. Wittig reaction of 5-(/3-formylvinyl)-2-furaldehyde 135 with the bisphosphonium salt 120... 

Oxidative coupling55 of the acetylenic alcohol 146 under Eglinton conditions followed by acidic treatment of the product gave the aldehydes 147 (37%) and 148 (8%). Wittig reaction of aldehyde 147 and the bisphosphonium salt 120 with 1,5-diazabicyclo[4.3.0]non-5-ene as base gave the [21]annulenone 149. This on reduction with lithium aluminium hydride-aluminium chloride gave the homoannulene... 

Convenient routes to several new sterically crowded chelating diphosphines have been described.48-50 Thus, e.g., m-xylylene dibromide, on treatment with di-t-butylphosphine, affords a bisphosphonium salt, which on treatment with a weak base affords the diphosphine (58).48... 

The interesting hydrocarbon (104), having both aromatic and anti-aromatic character, could be isolated in low yield by treatment of a bis-ylide with freshly distilled glyoxal.96 Precursors for the preparation of helicenes, e.g. (105), have been synthesized in one-step reactions. The yield of the double Wittig reaction is better using a di-aldehyde rather than a bisphosphonium salt.97 The [12]-annulene (106) is obtained from the reaction of the bis-ylide derived from (107) and a di-aldehyde.98 The condensation of (107) with two moles of 3-methylpent-2-en-4-yn-l-al (108) was also successful.99... 

By 1989 Mukaiyama had already explored the behaviour of phosphonium salts as Lewis acid catalysts. It was possible to show that the aldol-type reaction of aldehydes or acetals with several nucleophiles and the Michael reaction of a,j3-unsatu-rated ketones or acetals with silyl nucleophiles gave the products in good yields with a phosphonium salt catalyst [116]. In addition, the same group applied bisphosphonium salts as shown in Scheme 45 in the synthesis of ]3-aminoesters [117]. High yields up to 98% were obtained in the reaction of A-benzylideneaniline and the ketene silyl acetal of methyl isobutyrate. Various analogues of the reaction parteers gave similar results. The bisphosphonium salt was found to be superior to Lewis acids like TiCl and SnCl, which are deactivated by the resulting amines. 

For a-halo ketones, bisphosphonium salt formation can also take place by vinylation or arylation of the phosphine (reaction 90). Bisphosphonium salts can also be obtained, in moderate yield, by the introduction of two phosphines on the same aromatic or heterocyclic ring, at high temperature358 (reaction 91). For pyridine, activation by triflic anhydride allows successive additions of two phosphonio groups in the 2,4-positions247 (reaction 92). 

Unsaturated bisphosphonium salts have been also studied182,763,764. On mercury cathodes, in unbuffered or acidic aqueous-organic media, the electrochemical reduction was shown to be controlled by the ylide character of the first-formed radicals, which then exclusively induced a heterolytic P—C bond breaking on the unsaturated bridge182 (Scheme 10). 

SCHEME 10. Reduction mechanism of 1,2-vinylene and buta-l,4-dienylene bisphosphonium salts (counterion omitted)182. 

The one-electron electrochemical reduction of 1,2-vinylene and buta-l,4-dienylene bisphosphonium salts at a mercury cathode produces an ylide character by the reaction pathway depicted in reactions 11—13. The mechanism is altered when OH is generated in the unbuffered aqueous-organic medium this reaction is depicted as reaction 14. The electrochemical reduction of phosphonium salt in the presence of tri-p-anisylphosphine produces a mixture of the saturated or semi-saturated bisphosphonium salts through either reaction scheme 15 or alternatively 16. 

And also, conjugated polymers can be prepared through Heck s coupling between dihalide monomer and divinylbenzene monomer. This coupling reaction is very useful when the synthesis of dialdehyde monomer and bissulfonium or bisphosphonium salt monomer is difficult or impossible. But Heck s coupling has a problem in that very often the preparation of divinylbenzene monomers is... 

The treatment of [bis(phenyliodonium)]ethyne ditriflate with triphenylphosphine has also been investigated40,41. The products depend on the stoichiometric ratios of the reactants. Thus, with one equivalent of triphenylphosphine, a monoiodonium-monophos-phonium derivative of acetylene is obtained, but with two equivalents of the phosphine, [bis(triphenylphosphonium)]ethyne ditriflate is produced (equation 72). When three equivalents of triphenylphosphine are employed in acetonitrile spiked with H20 or D20, a fram -alkenediyl bisphosphonium salt is generated (equation 73)41. The reduction of the triple bond and formation of triphenylphosphine oxide in this reaction is thought to proceed at the bisphosphonium alkyne stage41. 

The di-trans isomer 206 was isolated from the products of the reaction of 190 with the bisphosphonium salt 205 in the presence of lithium ethoxide. The reaction of 207 (obtained by the oxidative coupling of 206, with dimethyl sulphate followed by reduction with sodium hydrosulphite) yielded the cyclic compound, containing the dihydropyridine nucleus, 208a. Lithium aluminium hydride reduction of 207 yielded... 

An alternative route to these same two isomeric hexathial36]hexaphyrin- 1.22.2.2.2) macrocycles (7.82a and 7.82b) has also been reported. Using Wittig-type chemistry, 3,4-diethyl-2,5-diformylthiophene 7.81 was reacted with the diethyl-substituted bisphosphonium salt 7.83 to afford macrocycles 7.82a and 7.82b in 0.4% and 0.1% yield, respectively (Scheme 7.7.3). It is of interest to note that in this reaction the formation of cyclotetramers was not observed. This is in direct contrast to the analogous reaction using 3,4-unsubstituted thiophene starting materials wherein only cyclic tetramers were found to form (see Chapter 4). This discrepancy is most likely directly attributable to the steric effects caused by the presence of the thiophene P-ethyl substituents. 

Scheme 1 The Formation of the Bisphosphonium Salt 1 and Its Use as a Coupling Reagent - ...

In the base hydrolysis of the unsymmetrical fluoromethylene bisphosphonium salt (35) the regioselectivity of C—bond cleavage can be accounted for by the susceptibility of the phosphonium center to nucleophilic attack by hydroxide ion and not by the relative stability of the newly formed fluoromethylene ylide (Scheme 40). ... 

Wittig reaction.1 Aldehydes when refluxed with the bisphosphonium salt (1) in a benzene-water solvent containing sodium hydroxide are converted into a,j3-unsaturated... 

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