Oxonium dimerization

In the crystals containing acetic add and water the solvent layers are included in between monolayers of the host, as illustrated in Fig. 8. Molecules of the host now interact simultaneously with two neighboring bands of the guest species. The solvent zone itself consists of cyclic dimers of the acetic acid surrounded by water and oxonium ions. 

The intramolecular addition of acylcarbene complexes to alkynes is a general method for the generation of electrophilic vinylcarbene complexes. These reactive intermediates can undergo inter- or intramolecular cyclopropanation reactions [1066 -1068], C-H bond insertions [1061,1068-1070], sulfonium and oxonium ylide formation [1071], carbonyl ylide formation [1067,1069,1071], carbene dimerization [1066], and other reactions characteristic of electrophilic carbene complexes. 

Though these alkaloids are not truly composed of two identical monomeric units, they are popularly named dimers or dimeric alkaloids. We prefer to avoid this incorrect nomenclature and would like to encourage the use of the more adequate binary terminology. In another consideration of nomenclature, we describe quaternary salts derived from an imine functionality as imonium salts, in accord with the descriptor for other onium salts (ammonium, oxonium, etc.), rather than by the frequently used iminium terminology. This nomenclature was suggested earlier (/). 

Most recently, we have investigated the use of iterative oxonium ylide [1,2]- or [2,31-shifts as a convenient approach to the polypyran domains often found in the marine polyether ladder toxins (Scheme 18.8) [21]. Initial studies indicated that [l,2]-shifts of O-benzyl oxonium ylides such as 19 a or 19 b were inefficient. Alternative metallocarbene processes including C-H insertion and dimerization were found to predominate in these cases, again suggesting that carbene-ylide equilibration may occur [21b]. On the rationale that concerted [2,3]-shifts of the corresponding O-allyl oxonium ylides might occur more readily, the allyl ethers 19 c, 19 d were then examined. These examples were much more effective, especially in conjunction with the optimized catalyst Cu(tfacac)2 [21a]. However, rhodium(II) triphenylacetate (Rh2(tpa)4) [22] was found to... 

Reaction of halides with oxide ion 0-68 Cleavage of oxonium salts 0-79 Reduction of acetals or ketals 0-92 Reaction between Grignard reagents and acetals or ketals dimerization of acetals... 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

The tris(triphenylphosphinegold) oxonium ion is a particularly significant oxo complex, prepared most conveniently by reaction (16)178,180 and which crystallizes in dimeric units.179... 

Although no mechanism is proposed for the dimerization of lactic acid in the FSOsH-SbFs solution, the process may very well involve the superelectrophile 147. Other oxonium-carbenium 1,3-dications have been suggested in superacid promoted pinacolone rearrangments of diprotonated aliphatic glycols and alkoxy alcohols (eqs 49-50).58... 

In the oligomerization of 47 e, the neighboring group participation of the equatorially located bromine atom is impossible, unless the oxonium ion is unimolecularly ring-opened to form an oxacarbenium ion. The electron withdrawing bromine atom disfavors such an unimoleeular ring-opening reaction. Therefore, the cyclic dimer cannot be formed by a mechanism similar to that described for the cyclodimerization of 47 a. 

The 18 —> 20-hemiacetal (160) gave novel dimers (163) on reaction with toluene-p-sulphonic acid in benzene, through attack of the vinyl ether (161) on the oxonium ion (162), followed by an intramolecular hydride transfer from C-16 to C-20.16°... 

The rearranged olefin 50 may be used in a variety of synthetic applications. For instance it can be converted into the novel di-bis-tetrahydrofuran-acetal 95 in a one-pot operation using trimethylsilyl iodide in dichloromethane at 22 °C (Scheme 16) (12). The mechanism involves the formation of an oxonium intermediate 96 which undergoes a Prins cyclization to form the cation 97. Subsequent pinacol rearrangement generates 98 which cyclizes to 99. This acetal dimerizes under elimination of trimethylsilylbenzyl ether and benzyliodide. The structure of 95 has been elucidated by X-ray analysis (Fig. 3) which nicely shows the C2-symmetry of the dimeric structure. 

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