Allylic carbenium ions

This reaction type has been intensely studied °. The application of highly polar solvents, catalysis with tertiary amines" or with acids mesomeric stabilization of intermediate carbenium ions " (allylic and benzylic systems propargylic systems" ) as well as derivatives of sulfinic acids with increasing acidity - usually indicate an ionic pathway (intra- and/or inter-molecular) ... 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

The initial step is the protonation of the aldehyde—e.g. formaldehyde—at the carbonyl oxygen. The hydroxycarbenium ion 6 is thus formed as reactive species, which reacts as electrophile with the carbon-carbon double bond of the olefinic substrate by formation of a carbenium ion species 7. A subsequent loss of a proton from 7 leads to formation of an allylic alcohol 4, while reaction with water, followed by loss of a proton, leads to formation of a 1,3-diol 3 " ... 

Some cyclopropylcarboxylic acids, namely 14 [125] and 15 [46] could be coupled to bicyclopropyl compounds, others led to allylic compounds via ring opening of an intermediate carbenium ion (see chap. 7). Tertiary alkanoates yield predominantly non-Kolbe products (see chap. 8). 

The various carbenium ions /erf-alkyl, bridgehead-, norbornyl-, allyl-, benzyl- or cyclopropylcarbinyl-cations, which are assumed to be intermediates in these decarboxylations are compiled in ref. [293]. 

There is good reason to believe that the potential of NMR studies of carbenium ions on solid metal halitks exceeds that of corresponding studies in superacid solutions. Of course the advantages of working in solids include Ae possibility of very low temperatures and the mass transport restrictions of frozen media. Thus, Mehre and Yannoni were able to characterize the sec-butyl cation in frozen SbF5 by NMR [20] and Schleyer and coworkers have obtained infrared evidence of the allyl cation in the same medium [21]. So far, we have been successful in every case in which we have tried to duplicate known solution carbenium ion chemistry on... 

The second is that the initiation consists of the addition to the monomer of some or all of the cationic species thus formed to give the growing carbenium ion (I) (reaction (8)). Even if the first part is proved, the second part must be tested separately, for it is possible, though at present it seems unlikely, that the cations in the initiator solution could generate cations from the monomer by reactions other than the addition reaction (8). For instance, they could generate an allylic cation (II) by abstracting H" (reaction (12)), or they could form a radical-cation (III) by abstracting an electron (reaction (13)), from the monomer ... 

According to this mechanism, the first formed ion pair is 19a. Owing to dispersal of charge in the allylic system, the bond between halogen and C(2) is weakened so that an open carbenium ion (19c) readily forms, allowing for the possibility of front-side attack by the anion with the resulting formation of syn 1,2-adducts. This intermediate explains the formation of the cis-],2-adducts by chlorine addition to cyclic systems. However, syn 1,2-dichlorides can also result from linear dienes by rotation around the C(l)—C(2) bond in 19c to produce 19d, followed by back-side attack by the anion with respect to its position in 19d. Syn 1,4-adducts should instead arise by attack of the anion on C(4) in either 19a, 19c or 19d. Formation of anti dichlorides (1,2- or 1,4-) can only occur when there is appreciable translocation in the ion pair 19a to give 19b. Attack by the anion at C(2) in 19b yields anti 1,2-dichloride and attack at C(4) yields anti 1,4-dichloride. 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

In the oxirane ring opening reaction (Eq. (2)), it is possible to prepare the ketone 5, the acetonide 10, and the allylic alcohol 11 selectively by controlling the reactivity of the intermediate carbenium ion 12 under some specified reaction conditions. In order to prepare 5 preferentially, a high concentration of perchlorate ion must be employed in CH2CI2, ClCHjCH Cl, or THF. In this system, an incipient carbenium ion 12 would be associated with perchlorate ion and stabilized so that the deprotona-... 

The allyl cation (9) is the simplest member of the class of resonance-stabilized cations that includes the alkyl-substituted cyclopentenyl cations. But one could also say that the carbenium ion (CH3) is the simplest member of a class of cations that includes the trityl cation. In each case, 10 or so orders of magnitude of acidity separate the primitive member from its more elaborate derivatives. 

The allyl cation has never been characterized as a persistent species in solution. If prepared, it would be the smallest carbenium ion universally accepted to have been formed in condensed media (this title has for many years been held by the isopropyl cation). No allyl cation derivatives, e.g., 10, were observed in a 1990 report of a CAVERN study of butadiene on HY and HZSM-5 (104). The same year, Hutchings reported a flow reactor study of allyl alcohol on HZSM-5 (105) and Gorte (106) reported a TPD study of allyl alcohol on the same zeolite. Hutchings and co-workers found that allyl alcohol had two reaction paths, one in which propanal was formed and another that formed hydrocarbons. The latter route was proposed to proceed through an allyl cation intermediate, but no claim for its persistence or spectroscopic observation was made or implied by Hutchings. Gorte... 

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

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