With phosphonium salts

A general review of synthesis and reactivity of silica-immobilized onium catalysts, which deals mainly with phosphonium salts, is available [42]. 

These derivatives were described as uronium salts by analogy with phosphonium salts, but X-ray analysis demonstrated that HBTU, HATU, and HAPyU crystahize as aminium salts (guanidinium A-oxides) rather than uronium salts.t NMR studies of HAPyU in solution show the same structure as that observed in the crystahine form.P This discovery changes the nomenclature of these compounds, but their acronyms are unchanged, and they are described in the literature as uronium or aminium salts without discrimination. HBTU should be reformulated as A-[(l//-benzotriazol-l-yl)(dimethylamino)methylene]-A-methyl-methanaminium hexafluorophosphate A-oxide, HATU as A-[(dimethylamino)(l//-l,2,3-triazolo[4,5-b]pyridin-l-yl)methylene]-A-methylmethanaminium hexafluorophosphate A-oxide, and HAPyU as 0-(7-azabenzotriazol-l-yl)-A,A,A, A -bis(tetramethylene)uronium hexafluorophosphate. 

The synthetic strategy leading to formation of the unsymmetrical rt-exTTF 805 involved two successive olefina-tion reactions of phosphonium salts 800 and 801 with glyoxal through two possible routes, A and B. In route A, the aldehyde 802 was further reacted with 801, while in route B the aldehyde 803 was reacted with phosphonium salt 800. Since yields in both approaches did not exceed 40%, the phosphonate 804 was reacted with the aldehyde 802 to give 805 in 84% yield (Scheme 121) <1995TL1645>. 

Condensation of the D-galactopyranosyl-derived phosphonium salt 60 and aldehyde 61 provides the unsaturated dimer 62 after desilylation and oxidation. Iteration of the sequence twice with phosphonium salt 60 leads to tetramer 63 with, however, low coupling efficiencies (36% for the trimer and 11% for the tetramer) mostly due to the base-induced a,/3-unsaturation of the aldehydic substrates. Protecting group removal and hydrogenation of the carbon-carbon double bonds furnish tetramer 64, a C-mimetic of the /3-(1 6) tetragalactoside similar to the one shown in O Scheme 3. 

Reaction of [CpFe(CO)2] with phosphonium salts (Ph3PCH2R) Br in the presence of excess... 

Carbonyl-stabilized phosphorus ylides are less nucleophilic and hence do not react with (phosphine)gold(I) halides, but their gold(I) complexes can be generated from precursors such as [Au(acac)(PPh3)] or [AuCl(SC4Hg)] by reaction with phosphonium salts and ylides, respectively, and again both mono- and bis(yhde) complexes have been obtained (equation 48 and Scheme 8)206 207. The ylide carbon atoms in these complexes are centres of chirality, but no stereospecificity was observed in the coordination process and racemic mixtures are formed throughout. 

T. Takata, K. Takuma, and T. Endo, Photoinitiated cationic polymerization of epoxide with phosphonium salts as novel photolatent initiators. Makromol. Chem. Rapid Commun. 1993, 14(3), 203-206. 

K. Takuma, T. Takata, and T. Endo, Latent cationic initiator Photoinitiated polymerization of epoxides and vinyl monomers with phosphonium salts. J. Photopolym. Sci. Technol. 1993, 6, 67-74. 

Activation of a-hydroxycarboxylic acids with TBTU leads to the formation of iV,iV-dimethylcarbamoyl derivatives [197]. The coupling of this class of carboxylic acids can be safely accomplished with phosphonium salts. 

The Wittig reaction of 91 with crocetindialdehyde (536) and lithium methoxide resulted in the Cso-aldehyde 92. Lycoxanthin (109) was obtained by a Wittig reaction of 92 with phosphonium salt 15 and BuLi to give the carotenoid ester 93 (which is not known to occur in Nature) followed by reduction with LiAlH4 to give 109 in an overall yield of 25 % referred to 92 (Scheme 26). 

In a more recent variant by Gelmi et al. [80], 5(4H)-oxazolones 143 and munchnones 144 were used as sources for 1,3-dipoles in thermal cycloaddition reactions with phosphonium salt 128. 3-Methylpyrroles and 3-pyrrole carboxylic acids were obtained likewise from substituted vinylphos-phonium salts. The cycloaddition reactions proceed with high regioselectivity (Scheme 30). 

Phase-transfer catalysts, such as the classic onium salts, crown ethers, and cryptands, have been immobilized on insoluble polymer matrices with various degrees of cross-linking. Their activity remains reasonably high if the catalytic centre is sufficiently far from the polymer backbone or if the resin is very porous. However, with phosphonium salts immobilized on silica gel die length of the hydrophobic chain between the active centre and the matrix and the solvent determine the adsorption capacity of the polar support, which then controls the rate of reaction. ... 

A variation of the Wittig reaction was described [14, 15] in which a-hydroxysulfonates (13) were used, instead of aldehydes, to react with phosphonium salts. Polymers with only traru-vinylene units were formed (Scheme 5). 

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