Phosphonium special

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

The reaction of a diphosphine with a dihalo compound, resulting in the formation of a di- or tetra-phosphonium salt2c, is a special case because if fits well with a cyclization on phosphorus, but also at the same time with a cyclization between two chains already linked to the second phosphorus (reaction 102). Another kind of ring closure on the phosphorus results from the biphilic character of halophosphines toward dienic392 or acetylenic28,393 systems (reaction 105). 

Tetraalkylammonium tosylates [74] and trifluoromethanesulfonates [72] are also excellent electrolytes. Although tetraalkylammonium ions are favored as the cations for supporting electrolytes because of their wide potential range, other cations are sometimes used for special applications—for example, methyltri-phenyl phosphonium, whose tosylate is freely soluble in methylene chloride, and other fairly nonpolar solvents [74] or metal ions (lithium salts tend to have the best solubility in organic solvents) where undesirable reactions of the tetraalkylammonium ion might occur [13,75]. The properties of many electrolytes suitable for nonaqueous use have been surveyed [76]. 

The epoxy-phosphonium salt adduct would then be vulnerable to attack by another epoxy molecule resulting in the formation of an oxonium ion (Eq. (26)) or it reacts with the anhydride to form a special monoester with hydroxy groups (Eq. (27)). 

Hydroboration.1 The usual hydroboration reagents, BH3THF and BH3-S(CH3)2, are sensitive to oxygen and moisture and require special handling. 1 he complexes of BH3 and phosphorus compounds are generally stable, but much less reactive. The complex of BH3 and triphenylphosphine, m.p. 189°, can be used for hydroboration if activated by addition of methyl iodide (to form a phosphonium iodide) or sulfur (to form a triphenylphosphine sulfoxide). The complex of borane and triphenyl phosphite does not require activation and hydroborates alkenes in a reasonable time in refluxing DME or THF. Trialkyl phosphite complexes are not useful. 

With bulky substituents attached to the phosphorus, none of these special measures are necessary because complex formation is precluded through steric effects (74). Therefore ethylidene triethylphosphorane may be obtained in high yield without complications from the tetraethyl-phosphonium salt and an alkyllithium reagent (74) ... 

Other most successful durable treatment is based on tetrakis (hydroxymethyl) phosphonium derivatives. Very well-known brand marketed as Proban CC (Rhodia, previously Albright Wilson) involves padding of tetrakis (hydroxymethyl) phosphonium chloride (THPC) urea solution onto the cotton fabric, curing with ammonia in a specially designed reactor to generate a highly cross-linked three-dimensional polymer network. The fabric is then treated with hydrogen peroxide, which converts P3+ to the P5+ state. The reactions are shown in Scheme 24.2. Other similar commercial product is Thor s Aflammit P. In literature many combinations of tetrakis (hydroxymethyl) phosphonium derivatives with other salts have been reported,50 but the most successful so far has been the THPC-urea-NH3 system discussed earlier. 

Triphenylarsonium ethylide. trans-Epoxides can be prepared stereoselec-tively via arsonium ylides under special conditions (10,445). A simpler method involves transylidation of a phosphonium ylide to an arsonium ylide (equation I).2... 

Tertiary phosphines, in the absence of special effects 2 ), have relatively high barriers 8) ca. 30-35 kcal/mol) to pyramidal inversion, and may therefore be prepared in otically stable form. Methods for synthesis of optically active phosphines include cathodic reduction or base-catalyzed hydrolysis 3° 31) of optically active phosphonium salts, reduction of optically active phosphine oxides with silane hydrides 32), and kinetic 3 0 or direct 33) resolution. The ready availability of optically pure phosphine oxides of known absolute configuration by the Grignard method (see Sect. 2.1) led to a study 3 ) of a convenient, general, and stereospecific method for their reduction, thus providing a combined methodology for preparation of phosphines of known chirality and of high enantiomeric purity. 

As illustrated in Eq. (10), a chiral phosphonium ion can undergo attack by a nucleophile at any one of four different faces or six different edges, thus placing the entering ligand in the a and e positions, respectively. In the general case, when all five ligands are different, and in the absence of special constraints (see Sect. 3.2) 20 isomeric phosphoranes, which are interconnected by 30 pseudorotation steps, are thus produced from both enantiomers of the phosphonium ion. Because of the possibility for reaction via this complex intermediate manifold, interpretation of the stereochemical consequences of... 

The compound is called diphenyliodonium hydroxide—a name which indicates its relation to ammonium, sulphonium, and phosphonium compounds. Diphenyliodonium hydroxide, like other strong bases, forms well characterized salts with acids, which resemble in solubility the analogous salts of thallium. The compound is of special interest as in it iodine, which under other circumstances is an acid-forming element, plays the part of a base-forming element. 

A number of methods used for preparing phosphonium compounds has already been indicated on p. 7. Some tetra-alkylphosphonium hydroxides may be produced by heating white phosphorus and the corresponding alcohol above 250° C. for a long period. The compound PgHgg reacts with alkyl iodides to form quaternary compounds, and the latter also occur when trialkylphosphines react with alkyl halides. There are also special methods of preparation applying only to individual derivatives. The iodides are converted into the hydroxides by treatment with moist silver oxide, and bromides, cyanides, carbonates, acetates, oxalates and sulphates are similarly obtained when the appropriate silver salt is used. Such salts also result when the hydroxides are treated with the corresponding acids. 

A significant increase in the volume of published work in the area of phosphine and related chemistry has been noted. Worthy of special mention are the first example of chiral resolution of a secondary phosphine which is chiral at phosphorus and, for followers of chemical fashion, inevitably the first phosphonium derivative of Buckminsterfullerene (C ) has been described. 

Synthesis of the carotenoid backbone by Wittig condensation between C(ll)/C(12), or C(H )/C(12 ) has been used most extensively. Apart from special cases, which will be discussed later, generally a C(1 l)-phosphonium salt is linked to a C( 12)-aldehyde (synthesis strategy 2). 

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