Reaction with onium salts

Reaction between alkoxides or arox-ides and alkyl halides (Williamson) 0-14 Reaction between alkoxides or arox-ides and inorganic esters 0-15 Alkylation of alcohols or phenols with diazo compounds 0-16 Dehydration of alcohols 0-17 Transetherification 0-19 Alkylation of alcohols with onium salts... 

PTC with onium salts (TBAB or TBPB) in a two-phase system (HzO/NaOH/CIfcCh) afforded bis(aryloxynitrophenyl) sulfones (47) in good yields at room temperature, from the reaction of the coire-sponding chlorides with several phenols.159... 

It can be concluded that cationic photopolymerization of glycidyl ethers with onium salt initiators should be considered as a dual-step process 1) liberation of the active initiator species by irradiation and 2) heat treatment to complete the polymerization reaction... 

Oxetane undergoes ring-opening polymerization under the action of MAD in conjunction with onium salts, including quaternary ammonium and phosphonium halides, giving a narrow MWD polyether (Scheme 6.173) [221]. Use of MesAl in place of MAD resulted in no polymerization. The aluminum ate complex seemed to be an initiator, which underwent a trigger reaction involving halide transfer to the aluminum-oxetane complex. 

Electron-rich polyaromatic compounds such as anthracene, pyrene, and pery-lene [107] are suitable as photosensitizers as they give redox reactions with DPI salts through exciplex to finally yield the initiating species for photoinduced cationic polymerizations. Scheme 11.28 demonstrates the mechanism of a polymerization followed via exciplex formation through the excited sensitizer with the ground-state onium salt. 

Non-crosslinked polystyrene with salen ligand Non-crosslinked polystyrene with onium salt residues Non-crosslinked polystyrene copolymerised with a chiral phosphine ligand 14,765 0.5-3.8" 0.7-2.3 CH2Cl2,THF, EtOAc, DMF DMF,DMSO,DMA, toluene, anisole, MeOH,MeCN, diglyme Asymmetric epoxidation Regioselective addition reaction of phenyl glycidyl ether with S-phenyl thioacetate Pt-catalysed asymmetric hydroformylation of olefins Precipitation (methanol) (r) Precipitation (diethyl ether) (r) [68] [142] [143]... 

It was considered previously that the most effective phase transfer catalysts are quaternary ammonimn bases. However, preliminary experiments with crown-ethers had already shown that these compounds are more powerful phase transfer catalysts than quaternary ammonium bases and are more selective [106, 177]. This is explained by differences in the mechanism of catalytic action. The mechanisms of reaction acceleration in two-phase systems with crown-ethers are as yet little studied, but simple examination of salt extraction with crown-ethers shows that the salt in the aqueous phase G>oth anion and cation) passes into the organic layer, whereas only anions paired with the onium cation pass from the aqueous into the organic phase during extraction with onium salts. This considerable difference in the mechanism of action of the two groups of ion-carrying catalysts is the basis for the prospective use of crown-ethers and their analogs instead of quaternary ammonium bases in many fields. 

Mukaiyama has reviewed the various synthetic methods based on reaction of onium salts of aza-arenes [e.g. (50), (51)] with hydroxy-groups which results in an activation of the oxygen function, permitting nucleophilic attack at the adjacent carbon atom several such methods have been featured in these Reports (1,166, 174 2, 122, 128, 134 3, 153). 

SEE 01] Selvaraju C., Sivakumar A., Rammamurthy P., Excited state reactions of acridinedione dyes with onium salts mechanistic details , Journal of Photochemistry and Photobiology A Chemistry, vol. 138, no. 3, pp. 213-226, 2001. 

Selvaraju, C., Sivakumar, A., and Ramamurthy, R, Excited state reactions of acridinedione dynes with onium salts mechanistic details, /. Photochem. Photobiol, A Chem., 138, 213, 2001. 

However, the vast majority of research has been devoted to synthesis involving electrophilic substitution on the aromatic ring of hydroquinone. Hence, phenylhydroquinone can be obtained by the reaction of phenyl dia onium salts (18) with hydroquinone (82). 

The only example of this ring system to the best of our knowledge was formed by reaction of Mc3SiCH2SCH2Cl with quinoline to give the onium salt 693 which upon treatment with CsF in MeCN at room temperature afforded the thiazoloquinoline 694 (87JOC4423) (Scheme 118). 

A multiphase system consisting of a hydrocarbon solvent, a strong alkaline solution, and a quaternary onium salt, in the presence of a Pd/C catalyst with hydrogen that was bubbled at atmospheric pressure through the organic phase, allows the rapid displacement of chlorine from polyhalogenated benzenes. The onium salt, insoluble in both phases, is localized in the interfaces, coats the Pd/C catalyst, and constitutes the phase in which the reaction takes... 

Thiols like pyridine-2-thiol yield (carbene)gold thiolates, and onium salts give cationic (carbene)gold ylide complexes, isolated and characterized as the perchlorates.170 The reaction of dithiocatechol with (cyclohexylisocya-nide)gold chloride affords a carbene complex [(CyNC)AuC(NHCy)2]+Cl-, which co-crystallizes with a neutral tetranuclear complex [(CyNCAu)2Au2(S2C6H4)2].224... 

The chemical properties of trichalcogen dications are, in many ways, analogous to the properties of dichalcogen dications, but are considerably less studied. Hydrolysis of chalcogenurane dications occurs preferentially at the onium chalcogen atom. Similar to the usual dichalcogen dications, trichalcogen dications also display oxidative properties. For example, reduction of selenu-rane dication 135 (X = Y = Se) is observed in reactions with Sm(II) salts, triphenylphosphine or thiophenol (Scheme 54).135... 

Here A, lA, and3A represent anthracene in the ground state, the first excited singlet state and first excited triplet state, respectively. In addition, I represents the onium salt initiation, while Rs and Rt correspond to the reactive centers formed by reaction of the onium salt with the excited singlet and triplet state anthracene, respectively. 

These two phase reactions summarized in Scheme 1 are strongly promoted by catalytic amounts of the quaternary onium salts which will transfer between aqueous and organic phases, as shown in Scheme 2 for the substitution of halides with cyanides. The effect was termed by Starks... 

In an early example, Mukaiyama and coworkers used hetaryl onium salts for nucleoside synthesis. The active hetaryl onium salt is generated in situ from the reaction of 2-chloro-3-ethylbenzoxazolium tetrafluoroborate 77 and the glycosyl acceptor. With benzimidazole as glycosyl acceptor, the resulting 2-(l-benzimida-zoyl)benzoxazolium tetrafluoroborate 78 was obtained. The reaction between the hetaryl onium salt 78 and hemiacetal donor 1 occurs at 60 °C to activate the hemiacetal and thereby reveal the glycosyl acceptor. This procedure led to the formation of nucleoside 80 with exclusive 1,2-trans selectivity [139]. The nucleoside 81 was similarly prepared. Alternatively, 2-fluoro-l-methylpyridinium tosylate 79 directly... 

In an extension beyond hetaryl onium salt promoted hemiacetal activation, Ishido and coworkers have reported the carbodiimide activation of hemiacetals [141]. In the method (Scheme 3.13), the hemiacetal donor 1 is treated with a carbodiimide electrophile 83 and copper(I) chloride to provide glycosyl isourea intermediate 85. Highly susceptible to hydrolysis, the isourea 85 was not isolated but could be detected by 13C NMR and IR spectroscopy [142,143], Accordingly, the reaction between intermediate 85 and the glycosyl acceptor (NuH) provides glycoside product 3, along with urea by-product 84. 

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