Allyl cation 1,3-dimethyl

Further variations of the general scenario described in Scheme 4 consist in trapping adduct radical 48 before oxidation occurs7. This can be achieved if intramolecular radical additions are possible, as is the case in radical 62. Oxidation of 62 to the corresponding allyl cation is slower than 6-ew-cyclization to the chlorobenzene ring to form radical 63, which subsequently is oxidized to yield tetrahydronaphthalene 64 as the main product (equation 27). This sequence does not work well for other dienes such as 2,3-dimethyl-1,3-butadiene, for which oxidation of the intermediate allyl radical is too rapid to allow radical cyclization onto the aromatic TT-system. 

Dimethyl-l-phenylpropenylidene (15) was generated from the tosylhy-drazone sodium salt 11 as well as from 3,3-dimethyl-5-phenylpyrazole (12), by way of the diazo compound 14.17,18 The reaction of 15 with methanol gave a mixture of the isomeric ethers 18 and 19, pointing to intervention of the allylic cation 16 (Scheme 7). In order to assess the regioselectivity of 16, the solvolysis of the 4-nitrobenzoate in methanol was also studied. Although 19 prevailed in each case, the 19 18 ratio obtained from 11 (1.5) and from 12 (1.7) was inferior to that obtained from 13 (5.1). 

Solvolysis of cyclopropyl derivatives leads directly to the allyl cation the ring opening is disrotatory as predicted. The most direct demonstration is the transformation of the 2,3-dimethyl-1-chlorocyclopropanes at — 100°C in strong acid... 

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

Diprotonated 2,4-pentanediol 10 loses water and rearranges to form 1,3-dimethyl allyl cation [Eq. (4.6)]. Diprotonated 2,5-hexanediol 11, above — 30°C, rearranges to a mixture of protonated cis- and tra .v-2,5-dimethyltetrahydrofurans [Eq. (4.7)]. This would seem to indicate that there is a significant amount of the monoprotonated form present or that the carbocation formed can easily lose a proton before ring formation occurs. 

In this case, the ethanoic acid is a solvent with a high dielectric constant, but it is also a weak nucleophile, and so it provides ideal conditions for a long lived carbonium ion intermediate. This is then statistically attacked at either carbon atom at the ends of the delocalised 1,3-dimethyl allyl cation. As a result this gives rise to the 1 1 ratio of substituted products, i.e. equal amounts of the products that result from the SN1 and SN1 reactions. 

This reaction does not occur (up to 150°C) when the gew-dimethyl groups stabilizing the allylic cation are missing. ... 

In sharp contrast to the coupling of the l,2-bis(diisopropylamino)-3-lithiocyclopropenylium ion 10 with the chlorocyclopropenylium ion containing rather bulky alkylamino substituents, the reaction with the 3-chloro-l,2-bis(dimethylamino)cyclopropenyliuin ion 18 took a completely different course i.c. a nucleophilic attack at the amino-substituted carbon of the chlorocyclopropenylium ring, a ring cleavage, then a second nucleophilic attack at the other amino-substituted carbon, followed by protonation. The whole reaction afforded the 1,3-bis-[2,3-bis(diisopropylamino)cyclopropenyliumyl]-l,3-bis(dimethylamino)propenylium trication 20 142 Trication 20 was also prepared by the reaction of two equivalents of l,2-bis(diiso-propylamino)-3-lithiocyclopropenylium perchlorate (10) with l,3-dichloro-l,3-bis(dimethyl-amino)allyl cation 19. 

Hoffmaim, H.M.R., and Henning, R., Synthesis of 2-norzizaene and 9,10-dehydro-2-norzizaene (7,7-dimethyl-6-methylidenetricyclo[6.2.1.0 -5]undec-9-ene) via intramolecular allyl cation induced cycloaddition, Helv. Chim. Acta, 66, 828, 1983. 

Chemistry On the basis of the discussion by Matsumura and Ghlasuddin (6), several bridged bicyclic compounds were synthesized (Table III) Cycloaddition of allyl cations to isopropylidenecyclo-pentadiene yielded three epimers of 2,4-dimethyl-8-isopropylidenebicyclo[3.2.1]oct-6-en-3-one, i.e., the equatorial is-2,4-dimethyl [1], axial cis-2,4-dimethy1 [8], and trans-2,4-dimethyl [12] analogs, and two epimers of the 2,4-dibromo analog [7 and 11] Several epoxy and hydroxy analogs were prepared by epoxidation of double bonds and reduction of carbonyl groups, respectively ... 

Model calculations of the dimethyl vinyl cation system at the MP2/6-31G level show that the hydrogen-bridged cation is more stable than the secondary cation and that the barrier for the degenerate hydride transfer is very low (< 1 kcalmol Fig. 11). The isomeric allylic cation, which could be generated by a 1,2-hydride shift across the single bond of the secondary vinyl cation, is much more stable than the vinyl cation, but the barrier for this 1,2-hydride shift is very high since allylic conjugation cannot contribute to the stabilization of the transition state for the hydride shift. 

In contrast to the above additions A-allyl- and substituted A-allyl-amides, -urethanes, -ureas and -thioureas undergo intramolecular cyclization only in 6(3-96% sulfuric acid to give the corresponding oxazolinium and thiazolinium salts. Treatment of these cations with base yields 2-oxazolines and 2-thiazolines in moderate to good yields. The reaction is illustrated by the conversion of A-2-phenylallylacetamide (342) into 2,5-dimethyl-5-phenyl-2-oxazoline (343) in 70% yield 70JOC3768) (see also Chapter 4.19). 

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

Allylic CH bonds Aliphatic alkenes frequently undergo allylic substitution by oxidation of the double bond to a radical cation that undergoes deprotonation at the allylic position and subsequent oxidation of the resulting allyl radical to a cation, which finally combines with the nucleophiles from the electrolyte [21, 22]. The selectivity is mostly low. Regioselec-tive allylic substitution or dehydrogenation is, however, found in some cases with activated alkenes, for example, -ionone that reacts to (1) (Fig. 5) as a major product [23], menthone enolacetate that yields 90% (2) [24], and 3,7-dimethyl-6-octen-l-ol... 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

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