Cyclic cationic intermediate

The sulfenylation, on the other hand, has a moderate negative p value. No cation is formed that is delocalized round the ring, but electrons flow out of the ring and we suspect some loss of conjugation. All this fits well with the formation of a three-membered ring intermediate. From experiments like this we learn that PhSCl is much more likely than bromine to react stereo specifically with alkenes through cyclic cation intermediates. 

Acetyl nitrate (or nitryl acetate, Ac0NO2), prepared from nitric acid and acetic anhydride, reacts with simple acyclic and cyclic alkenes to give complex mixtures of nitro acetates, nitro nitrates and nitroalkenes, which are often difficult to separate12 103, The reaction with unsubstituted cycloalkenes was recently reexamined and a complex mixture of products, including allylic and homoallylic nitroalkenes, 1,2-, 1,3- and 1,4-nitro acetates, and 1,4-nitro nitrates, was identified104. These experimental observations are best accommodated by the proposal, supported by theoretical calculations, that the initial reaction of acetyl nitrate with alkene substrates is a [2 + 2] addition of the nitryl cation to the C-C double bond to form a cyclic cationic intermediate. 

Corey and coworkers suggested that cyclic cationic intermediates are involved in the formation of 3-methoxynorticyclene (68) from either endo- or exo-5-norbornene-2-carboxylic acid (67), and for the formation of gxo-2-methoxynorbornane (70) as the only volatile product from the electrolysis in MeOH (at Pt anode) of either exo- or endo-norbornane-2-carboxylic acid (69). These results are consistent with the suggested formation of the same bridged ions in the solvolysis reaction . ... 

The first step of the Provost reaction is the reaction of the alkene with iodine to form the cyclic iodonium ion. Next, the iodonium ion is stereospecifically opened by the silver carboxylate to form the corresponding frans-1,2-iodo carboxylate. The iodine is displaced intramolecularly by the carbonyl group of the carboxylate (anchimeric assistance) to form a cyclic cationic intermediate. In the absence of water, this cation is opened with the inversion of configuration by the second equivalent of silver carboxylate to afford the frans-1,2-dicarboxylate. However, in the presence of water Woodward-Brutcher modification) the common intermediate is converted to a c/s-orthocarboxylate which is hydrolyzed to the corresponding c/s-1,2-diol. 

Various heteroatoms have been used to facilitate carbenium ion formation at less substituted centres, including halogens,9"" nitrogen,12 selenium,13 and sulfur.14 These reactions generally proceed via activation of the alkene, through formation of a cyclic cationic intermediate 25.15... 

Methyl 2,3-dideoxy-2,3-epithio-a-D-allopyranoside (60) gave trideuteriomethyl 2,3-dideoxy-2,3-epithio-P-D-allofuranoside on treatment with a cation-exchange resin in [ H4]methanol. The pyranoside -> furanoside isomerization, caused by the inductive effect of the S-substituent at C-2, was explained by a mechanism involving the cyclic cationic intermediates shown in Scheme 11. The effect of the S-... 

Sulfur and selenium compounds in which the sulfur and selenium atoms are bound to more electronegative elements can react with alkenes to give addition products. The mechanism is similar to that in halogenation with a cyclic cationic intermediate being involved. In many synthetic applications the sulfur and selenium... 

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

Solvolyses of these cyclic vinyl triflates at 100 in 50% aqueous ethanol, buffered with triethylamine, lead exclusively to the corresponding cyclo-alkanones. Treatment of 176 with buffered CH3COOD gave a mixture of cyclohexanone (85%) and 1-cyclohexenyl acetate (15%). Mass spectral analysis of this cyclohexanone product showed that the amount of deuterium incorporation was identical to that amount observed when cyclohexanone was treated with CH3COOD under the same conditions. This result rules out an addition-elimination mechanism, at least in the case of 174, and since concerted elimination is highly unlikely in small ring systems, it suggests a unimolecular ionization and formation of a vinyl cation intermediate in the solvolysis of cyclic triflates (170). The observed solvent m values, 174 m =. 64 175 m =. 66 and 16 m =. 16, are in accord with a unimolecular solvolysis. 

The subjects of this section are two reactions that do not actually involve carbo-cation intermediates. They do, however, result in carbon to carbon rearrangements that are structurally similar to the pinacol rearrangement. In both reactions cyclic intermediates are formed, at least under some circumstances. In the Favorskii rearrangement, an a-halo ketone rearranges to a carboxylic acid or ester. In the Ramberg-Backlund reaction, an a-halo sulfone gives an alkene. 

The electrophilic bromonium ion adds to the diene at the position which yields the most stable cationic intermediate and the stereochemical relation of the Br and the MeO group in the product is always trans when the diene system is cyclic. The fact that 1,2-addition takes place selectively but 1,4-addition does not occur is explained by the formation of the bridged bromonium ion as the intermediate. 

Table 3.20 The effect of nature of base on direction of deprotonation of cationic intermediates derived from six-membered cyclic nitronates...

Detailed configurational and conformational analyses of the starting six-membered cyclic nitronates (356) of cationic intermediates (357), and (decoupling products (357 + Nu), taking into account slow nitrogen inversion (Ijv) in nitroso acetals (see Section 3.4.3.4.4), made it possible to propose a mechanistic model based on the stereochemical outcome of the C,C-coupling process (Chart 3.23). 

Cationic intermediates A can react with active nucleophiles Y- to give coupling products (399) in high yields, with the trans configuration of the nucleophile and the substituent R predominating. (As in the case of the [3+ 2]-cycloaddition reactions with six-membered cyclic nitronates (162), this stereoselectivity can be attributed to the favorable approach of the nucleophile to the plane of the cation which is distal with respect to the C-6 atom. It should be noted that this... 

The formation of five-membered cyclic nitronates (404) is explained in terms of ring-chain tautomerism of cationic intermediates A (A=A ). The presence of the alkoxy substituent (R4) at the C-6 atom could stabilize the open form (cation A7), which finally leads to the formation of functionalized five-membered cyclic nitronates (404) probably with the participation of water. 

The probable pathway giving rise to silylated cyclic nitrile (589), which is the most unusual reaction product, is shown on the left of Scheme 3.282. Apparently, this compound is generated through the cationic intermediate A. It undergoes cyclization at the terminal electron-rich C,C-double bond to form silylated oxime (587), which is transformed into nitrile (588). After silylation of the latter, nitrile (589) can be isolated. Desilylation of (589) according to standard procedures affords nitrile (588). 

The authors proposed two mechanisms. The first proceeds through a cationic intermediate and was ruled out after failed efforts to trap the cationic intermediate. The favored mechanism proceeds by allylic G-H activation, forming a 7r-allyl complex, which undergoes insertion and reductive elimination to give the cyclic product (Scheme 26). 

Hydride transfer from alkenes was also proposed to occur during sulfuric acid-catalyzed alkylation modified with anthracene (77). Then the butene loses a hydride and forms a cyclic carbocation intermediate, yielding—on reaction with isobutene—trimethylpentyl cations. This conclusion was drawn from the observation of a sharp decrease in 2,2,3-TMP selectivity upon addition of anthracene to the acid. 

The reaction with silyl enol ethers 3f and 3g gave only the [3 + 2] cycloadducts in comparison with effective formation of acyclic adduct 15 in the reaction with ketene silyl acetals 3a and 3e at lower reaction temperature. This can be explained by the reactivity of cationic intermediates 19 the intermediates from 3f and 3g are more reactive owing to lower stabilization by the oxy group than those from 3a and 3e, and react with the internal allene more efficiently to give the cycloadduct(s). Cyclic product 17a could be obtained at higher temperature via the reaction of 3a (entry 2). 

With -substituted- ,/i-unsaturated trimethylsilyl ketones, six-membered /i-(tri-mcthylsilyl)- ,/)-unsaturated cyclic enones 68 were formed, probably owing to the stabilization of the R1 group for the six-membered cationic intermediates 67 [35]. 

A type I one-electron photo-oxidation of methionine-methionine-containing peptides by triplet carboxybenzophenone in air-saturated aqueous solution has been reported the S+ radical cation that is formed then reacts with the other Met-S to form an S-S three-electron complex which reacts with superoxide radical anion before hydrolysis to Met(=0)-Met(=0) bis-sulfoxide. Alternatively, cyclization of the A-terminal NH2 on to the S can occur to give a three-electron S-N complex which can react with superoxide radical anion to give a cyclic sulfonium intermediate. 

Bromination of tetrafluorobenzobarrelene with Br2 has been reported to produce stereoisomers of the annulated tricyclic dibromide. However, when pyridine, 15-crown-5, or Me3S were present, the /ranv-dibromide was obtained as the main product. MNDO calculations suggested that different cationic intermediates are responsible for the two pathways cyclic bromonium ions lead to the fonner, whereas open cations give rise to the latter product.26... 

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