Oxonium species

Ether bond formation via reduction of the oxonium species with Et3SiH. 

Heteronuclear /x-oxo complexes have been prepared by oxo/halide exchange reactions, then the reaction of metal halide complexes with the gold oxonium species gives complexes of the Jy [Rh2(dien)2 0(AuPPh3)2 (BF4)2 (476) (dien = COD, NBD) or [Pt(COD)2 OAu(PR3) 2] 2725 J "... 

The activation of NPGs during a glycosylation reaction (Scheme 5.7a) depends on electrophilic addition to the olefin (—>111), followed by intramolecular displacement by the anomeric oxygen to form the oxonium species IV. Trapping with a glycosyl... 

The mechanistic rationale for the high stereoselectivity is provided by the intermediate production of the trans-iodonium species 279 and its collapse to the bridged oxonium species 280 prior to the introduction of the toxyloxy ligand. 

The particular ease of O-glycosidation (see step 1) cannot he overlooked as an important factor in the process, serving to trap the highly reactive oxonium species to form an O-glycoside. This is in contrast with the Friedel-Crafts approach A, in which the attack of the aromatic generally needs considerable energy, and the oxonium species often undergoes various side reactions. 

The Bronsted acid can then react with both the ECH promotor and the THF monomer to form the dialkyl oxonium ions shown. Either of these can react with THF to produce the propagating trialkyl oxonium species. But Saegusa et al., argue that the ECH species will undergo... 

Terpenoid alcohols, such as 25, are cyclized in superacids (FSO3H/SO2) under amixture of kinetic and thermodynamic control. Intermediate oxonium species were identified by 13C NMR59. 

Acetals and ketals 14 are the most often used precursors for the generation of oxonium species 16. Their main advantages lie in their resistance towards various basic reagents, their easy conversion into the desired oxonium intermediates in the presence of a suitable Lewis acid and in their ability to produce, by reaction with allylsilanes, enantiomerically pure homoallylic alcohols 26. For example, both enantiomers of 26 can be obtained starting from the enantiopure acetals 24 (Scheme 13.11). 

The cyclohexadiene complex 29 has been further elaborated to afford either the cydo-hexenone 34 or the cyclohexene 36 in moderate yields (Scheme 1) [21]. The addition of HOTf to 29 generates the oxonium species 33, which can be hydrolyzed and treated with cerium(IV) ammonium nitrate (CAN) to release the cyclohexanone 34 in 43 % yield from 29. Alternatively, hydride reduction of 33 followed by treatment with acid eliminates methanol to generate the r 3-allyl complex 35. This species can be trapped by the conjugate base of dimethyl malonate to afford a cyclohexene complex. Oxidative decomplexation of this species using silver trifluoromethanesulfonate liberates the cyclohexene 36 in 57 % overall yield (based on 29). 

The question as to whether the reactive intermediate is the phenol-metal/leaving group complex 21/22 or the free phenoxonium ion 17 has been studied in the particular case of hypervalent iodine. Pelter and co-workers presented permissive evidence in support of a mechanism involving the free oxonium species 17 (Scheme 7) Phl(OAc) is an extremely good nucleofuge, no transfer of chirality is observed when homochiral hypervalent iodine compounds are used, and calculations made on the cation species correctly predict the re-gioselectivity of the substitution reaction [32, 33]. 

The reaction takes place with cleavage of the anomeric C-0 bond by electrophilic addition to the olefin followed by intramolecular displacement by the ring oxygen and eventual expulsion of the pentenyl chain, in the form of a halomethyltetrahydrofuran, to form an oxonium species (O Scheme 74). Trapping with water then leads to the reducing sugar. This transformation has also been extended to the use of n-pentenoyl esters [406,407]. 

The pinacol rearrangement of sulfonate esters derived from a-hydroxy acetals proceeds by way of intermediate oxonium species, which upon hydrolysis are transformed to the corresponding esters. Sulfonate 24, prepared in optically pure form by classical resolution of the diastereomer-ic mixture obtained from reaction of (-)-camphorsulfonyl chloride with the racemic naph-thenyl alcohol, undergoes thermal [1,2] rearrangement to yield the corresponding ester29. 

Three mechanisms can be envisioned for oxidation of the dioxymethylene bridge to the iron-coordinated carbene. In one mechanism, elimination of a molecule of water after hydroxylation of the dioxymethylene bridge yields an acidic oxonium ion that upon deprotonation gives the carbene (Figure 7.12, path a). In a second mechanism, formation of the oxonium species could... 

The tendency of the AUPR3+ cations to form oxonium species in the reactions that would be expected to lead to the hydrate, [Au(OH2)(PR3)]+, hydroxide [Au(OH)(PR3)] or oxide [0(AuPR3)2] suggest the existence of structural factors enhancing the stability of the [0(AuPR3)3j+ ion compared to the above complexes, which are not known in the monomeric state. Such a factor could easily be the existence of inter- and intramolecular interactions of gold atoms. 

CROP is only reported for a limited number of cyclic ethers that exhibit enough ring strain to be readily opened. In addition, the rather similar nucleophilicity of the ether moieties in the monomers and the ring-opened polymers together with the reactive cationic oxonium species, often leads to the occurrence of transetherification reactions, complicating the development of living CROP methods. This section will focus on the living CROP of ethylene oxide, oxetane, and tetrahydrofuran. 

In contrast to these initial reports on the living CROP of tetrahydrofuran which were performed without additional solvents, Penczek and coworkers demonstrated that the solvent plays an important role in the cationic ROP of tetrahydrofuran since it controls the proximity and stability of the ion pair at the living chain end [100, 101]. The polymerization rate increases in more polar solvents because of stabilization of the ion pair, whereby it was demonstrated that the methyl-triflate-initiated CROP of tetrahydrofuran involves an equilibrium between the cationic propagating oxonium species and the covalent triflic acid adduct, which can be shifted by the solvent choice as depicted in Scheme 8.19. Nonetheless, as a result of the much higher reactivity of the cationic propagating species, the polymerization rate is almost exclusively determined by the concentration of oxonium ions. 

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