Onium routes

Onium routes (method G) Onium compounds have been used as the aryl source for the preparation of triarylbismuthines. 

A suspension of p-toluenediazonium salt-bismuth chloride complex (32.5 g) in cold acetone (100 ml) was treated with copper powder (52.8 g). An evolution of gas took place in several minutes and the solution became dark brown in color. After 1.5 h, an equal volume of 25% aqueous ammonia was added and the mixture was diluted to 450 ml with water. After standing for 1 h, the solid precipitate was filtered, dried, and extracted with dry chloroform. The extract was evaporated and the solid residue was crystallized from methanol to give tris(4-methylphe-nyl)bismuthine (3.07 g), m.p. 116-117°C [39JA3586]. 

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A new route has been developed for the efficient formation of the variably substituted indolo[2,3-a]quinolizine ring system, starting from a properly substituted 2-piperidone (103, 104). For the preparation of octahydroindolo-quinolizine (1), the unsubstituted 2-piperidone 139 was treated with triethox-onium tetrafluoroborate. Then the corresponding lactim ether 140 was alkylated with 3-chloroacetylindole followed by a subsequent two-step reduction process and Bischler-Napieralski ring closure. Finally, reduction of the C=N bond afforded ( )-l (104). 

Cleavage reactions are best carried out in aqueous solution. In aprotic solvents, electrogenerated bases lead to the conversion of onium salts to the ylids which are not reducible [49]. The sequence of reactions shown in Scheme 5.2 shows that the bond cleavage process for phosphonium salts proceeds with retention of configuration around the phosphorus atom [50]. Retention of configuration at arsenic is also observed [51]. This electrochemical process is a route to asymmetric trisub-stituted phosphorus and arsenic centres. 

A number of general methods for the synthesis of meso-ionic 1,2,4-triazol-3-ones are available. Sodium ethoxide-catalyzed cyclization of 1-benzoyl-l,4-diphenylsemicarbazide (201, R = R = R = Ph, X = O) yielded anhydro-3-hydroxy-1,4,5-triphenyl-1,2,4-triazolium hydroxide (200, R = R = R = Ph). A general route to meso-ionic 1,2,4-triazol-3-ones (200) is exemplified by the formation of the 1,4,5-triphenyl derivative (200, R = R = R = Ph) from A-amino-MA -diphenylbenzamidine (202, R = R = R = Ph) and phosgene. In contrast with this ready meso-ionic compound formation, the corresponding reaction of the iV-methylbenzamidine (202, R = Me, R = R = Ph) did not yield the meso-ionic 1,2,4-triazol-3-one (200, R = Me, R = R = Ph). The product was in fact 3,4-diphenyl-2-methyl-l,2,4-triazol-5-onium chloride (203), which on heating gave 3,4-diphenyl-1,2,4-triazol-5-one (204, R = Ph). The formation of the A-methyl derivative (200, R = Me, R = R = Ph, yield 79%) by heating the 7V-thiobenzoyl semicarbazide (201, R = Me, R = R = Ph, X = S) with potassium carbonate in methyl cyanide has been reported. Another synthesis of A-methyl derivatives (200, R = Me) involves methylation of 3-methyl-4-phenyl-l,2,4-triazol-5-one (204,... 

Dialkylhalonium ions are reactive alkylating agents. The alkylation of Jt-donor (aromatic and olefinic) and w-donor bases with dialkylhalonium ions has been studied.353 Alkylation of aromatics with dialkylhalonium ions was found to be not significantly different from conventional Friedel-Crafts alkylations, showing particular similarities in the case of alkylation with alkyl iodides. Alkylation of w-donor bases with dialkylhalonium salts provides a simple synthetic route to a wide variety of onium ions. 

In addition to reaction with carbenium ions as in Eq. (20), nucleophiles may react directly with onium ions in an exchange reaction [Eq. (21)]. Although this exchange reaction is often faster than dissociation of onium ions [57], the latter route dominates with more stable secondary carbenium ions [67]. 

Neither the Viehe nor the Arens routes to the ethynyl ether are plausible for this system. The l-phenyl-l-methoxy-2-haloalkenes of equation (259), for example, may be recovered intact when treated in MeOH with 4m NaOMe at 155 °C . Though these conditions are presumably suitable for the generation of or C vinyl anions, no onium process (equation 253) seems to have occurred. Further, it seems improbable that phenylacetylide could be a precursor of the ethynylether (Arens mechanism), since this ion abstracts protons from protic solvents (/r lO M s at 25 °C in water) and halogen from hypohalite (OX") [k(Cl) = 2-3 x IQ- m" s" at 25 °C in water] . Thus, the possibility that there is an Arens ion-molecule intermediate, which can survive long enough in methanol to rearrange and form the alkynyl ether... 

All silica immobilized phase transfer catalysts previously reported involve two or more steps for the immobilization. Problems with preparations of this type include the difficulty in obtaining maximum functionality on the substrate and residual substrate bond intermediates which may interfere in final applications. The purpose of this work was to prepare well-characterized functionalized phase transfer catalysts that could be immobilized on siliceous substrates in a single step. As will be shown the preparation of functionalized onium catalysts proceeds readily. The route to facile immobilization of crown ether was not so direct. Avenues for high yield chemistry employing accessible or economic intermediates were not available. A new class of crown ethers which are readily functionalized during synthesis was developed. We have designated than "silacrowns". This report concentrates upon the properties and characterization of these new phase transfer catalysts. 

Coinitiators according to route A are electron-deficient materials. Representative examples are onium compounds and triazines [269, 563], The free energy of photoinduced electron transfer (AGei) between a photosensitizer and a coinitiator is described by Eq. (64), in which /i, /2 is the half-wave oxidation potential (route A e J2 is representative for the sensitizer route B Fox" stands for the coinitiator), is the half-wave reduction potential (route A red is representative for the coinitiator route B stands for the sensitizer), and oo is the... 

Photoinitiated cationic polymerization of benzoxazines by onium salts was investigated mechanistically. It was postulated that the first step involves the addition of photocemically generated proton (or carbocation) either to oxygen or to nitrogen atom. Then, the polymerization proceeds via two different routes leading to the formation of different structures (Scheme 11.34) [2,118]. 

It seems that the ester route may provide practical conditions also for the formation of random copolymers from monomers giving, in purely cationic polymerization, cations differing in their chemical nature (e.g. carbenium and onium). 

Scheme 10.13 Possible initiation routes in the cationic polymerization of monomer M. Initiating system onium salt/sensitizer.

Most investigations of polymer-supported onium ion phase transfer catalysts have used cross-linked polystyrenes. Not all of them have the same structure, even when they have the same formal degree of cross-linking with divinylbenzene. (The effect of percent cross-linking is considered in a later section). Two principal methods have been used to functionalize polystyrene for phase transfer catalysts, chloromethylation of pre-formed beads and copolymerization of chloromethylstyrene monomer with styrene and divinylbenzene. The chloromethylation route employs chloromethyl methyl ether (a cancer suspect agent), and a Lewis acid, usually stannic chloride.Substitution proceeds >90% para and is accompanied by some intrapolymer alkylation, which puts additional cross-links into the polymer... 

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