Oxonium rearrangement

According to Ref. [88], the activation of methanol via a trimethyl oxonium rearrangement for first carbon/carbon bond formation is relevant only during an incubation period at low temperature. They suggested that the impurity-driven mechanism may gain more importance at higher temperatures [88]. 

Quaternization is difficult benzofuroxan is unaffected by triethyl-oxonium fluoroborate. With methyl trifluoromethanesulfonate, an interesting rearrangement occurs, and l-hydroxybenzimidazole-3-oxide (39, R = H) is formed, probably via the N-quaternized derivative (38). Compound 39 (R = Ceils) has been prepared similarly. 

Several ways to suppress the 2-oxonium-[3,3]-rearrangements might be envisioned. Apart from the introduction of a bulky substituent R at the aldehyde (Scheme 23) a similar steric repulsion between R and R might also be observed upon introduction of a bulky auxiliary at R. A proof-of-principle for this concept was observed upon by using of a trimethylsilyl group as substituent R in the alkyne moiety (Scheme 25, R = TMS). This improvement provided an efficient access to polysubstituted dihydropyrans via a silyl alkyne-Prins cyclization. Ab initio theoretical calculations support the proposed mechanism. Moreover, the use of enantiomerically enriched secondary homopropargylic alcohols yielded the corresponding oxa-cycles with similar enantiomeric purity [38]. 

In addition to cyclopropane 145 and the expected [2,3] rearrangement product 143 of an intermediary oxonium ylide, a formal [1,2] rearrangement product 144 and small amounts of ethyl alkoxyacetate 146 are obtained in certain cases. Comparable results were obtained when starting with dimethyl diazomalonate. Rh2(CF3COO)4 displayed an efficiency similar to Rh2(OAc)4, whereas reduced yields did not recommend the use of Rh6(CO)16 and several copper catalysts. Raising the reaction temperature had a deleterious effect on total product yield, as had... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Concerning the mechanism of O/H insertion, direct carbenoid insertion, oxonium ylide and proton transfer processes have been discussed 7). A recent contribution to this issue is furnished by the Cu(acac)2- or Rh2(OAc)4-catalyzed reaction of benz-hydryl 6-diazopenicillanate 237) with various alcohols, from which 6a-alkoxypenicil-lanates 339 and tetrahydro-l,4-thiazepines 340 resulted324. Formation of 340 is rationalized best by assuming an oxonium ylide intermediate 338 which then rearranges as shown in the formula scheme. Such an assumption is justified by the observation of thiazepine derivatives in reactions which involved deprotonation at C-6 of 6p-aminopenicillanates 325,326). It is possible that the oxonium ylide is the common intermediate for both 339 and 340. 

An additional example of an oxonium ion generated via the acid catalyzed rearrangement has been used to prepare a dihydropyran <06TL6149>. The oxonium ion 54 generated by the reaction of an epoxide with ZrCl4 can be trapped by a nucleophile such as butynol to prepare dihydropyran 55. A variety of mono- and disubstituted epoxides have been used in this reaction. 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

The reactivity of alkynylstannanes toward electrophiles is one element in the oxygen-to-carbon rearrangement of alkynylstannane derivatives of furanyl and pyranyl lactols (e.g., Equation (85)). The cleavage of the anomeric C-O bond is assisted by the Lewis acid to give an oxonium ion, which is trapped in situ by the nucleophilic stannylalkyne. The utility of this process has been demonstrated in the synthesis of the natural product muricatetrocin C, and the drug substance CMI-977.246... 

There are no reported studies of this rearrangement on the zeolite surface and we argued that it could give some clues to the alkyl-aluminumsilyl oxonium ion/carbocation equilibrium. In this work we show experimental and theoretical results on the rearrangement of the cyclopropylcarbinyl chloride over NaY zeolite, aiming at demonstrating the equilibrium between the carbocation and the alkyl-aluminumsilyl oxonium ion. 

Rearrangement of the cyclopropylcarbinyl chloride takes place over NaY zeolite, indicative of the formation of the bicyclobutonium cation. Theoretical calculations show that the bicyclobutonium is an intermediate on the zeolite surface and might be in equilibrium with the alkyl-aluminumsilyl oxonium ion. 

The results of cyclopropylcarbinyl chloride rearrangement over NaY impregnated with NaBr suggest that there is an equilibrium between the bicyclobutonium cation and the alkyl-aluminumsilyl oxonium ion, explaining the preferred formation of the allylcarbinyl bromide in the rearranged products. It also suggests that zeolites may act as solid solvents, providing unsymmetrical solvation for the ions inside the cavities. 

As shown in Scheme 3.88, an initially formed Rh-carbene species reacted with 169 to give either 172 or an oxonium ylide, 173. The latter underwent a [2,3]sigma-tropic rearrangement to form the allene 171 (Scheme 3.88). 

A related dienediol-phenol rearrangement which can occur by different pathways was reported as a new method for synthesis of the oxepine system180. Protonation of the starting diol 344 produces a cation 345 which can follow normal dienone-phenol rearrangement (path a) when the substituents R2 = Me, Ph and R1 = t-Bu are eliminated in the step 346 — 347. However, when R1 = t-Bu and R2 is a substituted phenyl which decreases the nucleophility, the cationoid intermediate 345 cyclizes to the oxonium ion 348 (path b) which then undergoes deprotonation to give the oxepine 349 (equation 124)180. 

A disputable problem of the cationic oxa-Cope rearrangement (equation 258) is whether open-chain oxonium ions are formed during transformations of 4-vmyl-l,3-dioxoianes 604 into acyltetrahydrofurans 605 (equation 259) as well as of methyl... 

The observation that essentially the same rate constants are measured in methane and propane at 40 and 100 °C demonstrates that the starting oxonium ion 35 is in thermal equilibrium with the bulk gas and that its unimolecular rearrangement depends exclusively on the reaction temperature. 

The limited extent of intramolecular rearrangements undergone by the chiral oxonium ions 35 and 36 at 720 torr and at 40 °C (Table 22) allows their use for probing the regio- and stereochemistry of the displacement reactions of Scheme 19. In this case, the allylic alcohol, precursor of the chiral oxonium ions 35 and 36, acts as the nucleophile NuH. The relevant results are condensed in Scheme 21. 

The fragment ions at m/z 149, [CgHsOs], and 167, [C8H704], are especially prominent in the El spectra of phthalates. The formation of the [CgHsOs]" ion has initially been attributed to a McLafferty rearrangement followed by loss of an alk-oxy radical and final stabilization to a cyclic oxonium ion. [104] However, it has been revealed that four other pathways in total lead to its formation excluding the above one. [105,106] The two most prominent fragmentation pathways are ... 

Experimental Procedure 4.2.6. Oxonium Ylide Formation and 2,3-Sigmatropic Rearrangement Ethyl 2,5-Dimethoxy-4-pentenoate [1264]... 

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

Rearrangement Processes of Oxonium and Ammonium Ylides Formed by Rhodium(ll)-Catalyzed Carbene Transfer... 

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