Oxygen-stabilized cations

Mass Spectrometry Ethers like alcohols lose an alkyl radical from their molecular ion to give an oxygen stabilized cation Thus m/z 73 and m/z 87 are both more abun dant than the molecular ion m the mass spectrum of sec butyl ethyl ether... 

There is another oxygen stabilized cation of m/z 87 capable of... 

There is another oxygen-stabilized cation of mIz 87 capable of being formed by fragmentation of the molecular ion in the mass spectrum of sec-butyl ethyl ether. Suggest a reasonable structure for this ion. 

The second example depicted in Scheme 3.64 is the trioxotriphenylamine cation-radical. Kuratsu et al. (2005) compared structures of the cation-radical and its neutral counterpart. The neutral compound has a shallow bowl structure, whereas the cation-radical has a planar structure. In the latter, spin delocalization embraces a whole molecular contour, involving the oxygen atoms. This contribntes to the cation-radical stability. (The solid species is easily formed after oxidation of the neutral parent compound with tris(p-bromophenyl)aminiumyl hexafluorophosphate in methylene... 

The reaction of 1-isopropoxy-l-trimethylsiloxycyclopropane (1, R = i-Pr) with TiCl yields the titanium homoenolate of isopropyl propionate 2 (R = i-Pr) and Me3SiCl (Eq. see 9). That of 23, in contrast, yields a considerable fraction of the homoenolate of silyl propionate as well, Eq. (66) [19]. Apparently, the loss of an isopropyl group from an oxygen-stabilized cationic intermediate competes with that of the silyl group with bulky substituents. 

In a 2- (5-) substituted tetrahydrofuran loss of the 2-substituent is nearly always important because it leaves behind an oxygen-stabilized cation. Scheme 14 displays the fissions that allow a ready identification of the position of the tetrahydrofuran residue in a long chain but not the stereochemistry, the cis and trans isomers having nearly identical spectra (72MI31000). The series designated in that scheme was produced by the oxygenation of lipids... 

The molecular ion from sec-butyl ethyl ether can also fragment by cleavage of a carbon-carbon bond in its ethyl group to give an oxygen-stabilized cation of m/z 87. 

In the Au(I) catalysis of electron-poor alkynes such as 4, the catalytically active species is likely to be a cationic ligand-stabilized gold(I) Jt-complex, as in previously reported additions of oxygen nucleophiles to alkynes [5], Gold catalysts are very soft and thus carbophilic rather than oxophilic. On the basis of this assumption a plausible mechanism can be formulated as shown in Scheme 6. The cationic or strongly polarized neutral Au(I)-catalyst coordinates to the alkyne, and nucleophilic attack of the electron-rich arene from the opposite side leads to the formation of a vinyl-gold intermediate 7 which is stereospecifically protonated with final formation of the Z-olefm 8 [2, 4]. Regioselectivity is dominated by elec-... 

Another intermediate for which Die Is-Alder trapping provided convincing evidence is the oxy-allyl cation. This compound can be made from a,oc -dibromoketones on treatment with zinc metal. The first step is the formation of a zinc enolate (compare the Reformatsky reaction), which can be drawn in terms of the attack of zinc on oxygen or bromine. Now the other bromine can leave as an anion. It could not do so before because it was next to an electron-withdrawing carbonyl group. Now it is next to an electron-rich enolate so the cation is stabilized by conjugation. 

Problem 16.46 shows the mechanism of the addition of HBr to 1-phenylpropene and shows how the aromatic ring stabilizes the carbocation intermediate. For the methoxyl-substituted styrene, an additional resonance form can be drawn in which the cation is stabilized by the electron-donating resonance effect of the oxygen atom. For the nitro-substituted styrene, the cation is destabilized by the electron-withdrawing effect of the nitro... 

Another useful variation is the Pictet-Spengler isoquinoline synthesis, also known as the Pictet-Spengler reaction. The reactive intermediate is an iminium ion 49 rather than an oxygen-stabilized cation, but attack at the electrophilic carbon of the C=N unit (see 16-31) leads to an isoquinoline derivative. When a p-aryla-mine reacts with an aldehyde, the product is an iminium salt, which cyclizes with an aromatic ring to complete the reaction and generate a tetrahydroisoquinoline." ° A variety of aldehydes can be used, and substitution on the aromatic ring leads to many derivatives. When the reaction is done in the presence of a chiral thiourea catalyst, good enantioselectivity was observed." ... 

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