Carbonium ion analogs

Although it does not concern us here, it should be mentioned that organoboron and organoaluminum compounds exhibit anionoid (Grignard) and free radical reactivity as well as their behavior as carbonium ion analogs. 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

Suitably constructed bridged carbenes exhibit a tendency for structural rearrange ment and undergo the so-called foiled methylene reaction 141 142) with formation of bicyclo[3.3.0]octane derivatives. The behavior of P6143) and 97144) exemplifies such conversions. Carbonium ion analogies to such reactions are known. ... 

The direct dehydration of 3 j5-hydroxy-4,4-dimethyl steroids and triterpenoids (19), with contraction of ring A, is a well-known characteristic of this system [84]. It, occurs readily with phosphorus pentachloride and must proceed by initial ionisation of a chlorophosphate ester, with rearrangement (to give 20) of the resulting carbonium ion, analogous to the... 

This method is suitable only for the preparation of 4-substituted and/or 3,4-disubstituted derivatives, the substituents being only alkyl, aryl or heteroaryl groups. The presence of electron-withdrawing groups in the unsaturated side chain prevents the cyclization step. This is understandable if the influence of such groups on the stability of the intermediate carbonium ion is considered. Of more limited application is the analogous cyclization of diazotized o-aminophenylpropiolic acids, the reaction being referred to as the Richter synthesis (Scheme 70). A related synthesis (also referred to as the Neber-Bossel synthesis)... 

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

In analogy with carbonium ions, one would expect the linear form with sp hybridization, 172a, to be more stable. 

The vinyl cation analog of an allylic carbonium ion is an allenyl cation 242, where the empty p orbital on the unsaturated carbon overlaps with the perpendicular n bond of the allenyl system. Allenyl cation 242 is of course a resonance form of the well known alkynylcarbonium ion,... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

Oxygen and nitrogen electron-deficient intermediates will be discussed as analogs of carbonium ions in Chapter VIII. 

Not only water and alcohols, but also other oxygen compounds, are able to react covalently with acylium ions. In the case of hydroxy compounds the product is stabilized by loss of the proton from the hydroxyl group, but certain ethers give an analogous reaction in which the product is stabilized by loss of a carbonium ion.288 Using acetyl chloride with silver perchlorate in nitromethane as the source of acetyl... 

The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297... 

The possible nitrogen analogs of carbonium ions are the cation LIII and its conjugate base LIV. 

In this respect such reactions are analogous to the S 1 or limiting reactions of compounds producing carbonium ions, although the intermediate is a solvated carbanion rather than a solvated carbonium ion. In the base-catalyzed halogenation of ketones, for example, the rate is independent of the halogen concentration and is the same for the reaction with bromine as for the reaction with chlorine.384... 

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