Cope rearrangements cations

Cartion-cartxin bond formation via tandem Mannich condensation, cationic aza-Cope rearrangement of homoaHylarrunes, leading to pyrolidmes... 

With certain substituents, such as methoxy150 or (substituted) phenyl53 functions, in the allylie position the reaction outcome completely changes, giving rise to predominant or exclusive formation of five-membered ring products via a preceding 2-aza-Cope rearrangement of the initially formed A -acyliminium ion. These substituents clearly stabilize the intermediary carbo-cation 3. 

An important improvement in the oxy-Cope reaction was made when it was found that the reaction is strongly catalyzed by base.212 When the C(3) hydroxy group is converted to its alkoxide, the reaction is accelerated by a factor of 1010-1017. These base-catalyzed reactions are called anionic oxy-Cope rearrangements, and their rates depend on the degree of cation coordination at the oxy anion. The reactivity trend is K+ > Na+ > Li+. Catalytic amounts of tetra-rc-butylammonium salts lead to accelerated rates in some cases. This presumably results from the dissociation of less reactive ion pair species promoted by the tetra-rc-butylammonium ion.213... 

There are few examples of other allowed sigmatropic shifts involving six ten electron transition states, but these are not common reactions. A [3, 4] shift is observed in competition with a [1, 2] shift in cations derived from cyclohexane diols. This is a cationic equivalent of the Cope rearrangement,... 

Barbaralene [85] undergoes a rapid Cope rearrangement with a doublewell potential. The radical cation was studied using CIDNP by Roth (1987) after one-electron oxidation of [85] by y or X-irradiation. On the time-scale of the CIDNP experiment ( 10 8s), a single-minimum potential energy surface was found, i.e. bishomoaromatic structure [156] was suggested. 

The mechanism involves conversion of V-homoallylamine 116a to imine 117a via a Lewis acid-catalyzed cationic aza-Cope rearrangement (equation 61). Various Lewis acids were tested with yields ranging from 40-99% with high diastereoselectivities. 

Apparently, the aromatization of the heterocyclic cation serves as a driving force of the Cope rearrangement in the transformation of the 3-formyl-4-allyl-4//-pyrane (481) into poly-substituted pyrylium salt 483 which presumably proceeds via 482 (equation 183)242. 

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... 

However, a better known version of the 2-aza-Cope rearrangement is that carried out by using 2-aza-l,5-hexadienes 619 (equation 269) and particularly their iminium ion counterparts, usually N-acyliminium cations 620 (equation 270)365,366 (for reviews, see also Reference 367). Aza-Cope rearrangement of the norbomene ester 621 leads to tetrahydropyridine ester 622 when allowed to stand in solution at room temperature for... 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

Radiolysis of the diacetylene hexa-l,5-diyne (50) generates the hexa-1,2,4,5-tetraene radical cation (51 +) via a Cope rearrangement in the Freon matrix. ... 

In an elegant approach to gelsemine [238] based on a similar strategy the cationic aza-Cope rearrangement was superseded by a simple Mannich reaction because the twistane skeleton is more highly strained. However, the desired transformation is achievable using the anionic version. 

Scheme 7.2 General scheme for the 3-aza-Cope rearrangement. The enammonium cation undergoes a [3,3] sigmatropic rearrangement to form an iminium cation that can be hydrolyzed in waterto give the associated aldehyde and dimethyl ammonium.

The fluorescence of DCA is also quenched efficiently by 2,5-diphenyl-l,5-hexadiene with a nearly diffusion-limited rate constant in MeCN (1.1 x 10lodm3 mol-1 s ), since the photoinduced electron transfer from the diene ( ° = 1.70 V vs. SCE) to DCA (E ed = 1.91 V vs. SCE) [170] is exergonic [184], The photoinduced electron transfer induces Cope rearrangement of the diene via the cyclohexane-1,4-radical cation intermediate. In... 

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