Carbonium ions allylic, structure

The retrosynthetic operations that we have addressed thus far have not resulted in significant structural simplification. After all, intermediate 6 still possesses a linear fusion of four rings and six contiguous asymmetric carbon atoms. But, nevertheless, intermediate 6 could potentially be derived in one step from intermediate 8, a polyunsaturated monocyclic compound containing only one stereogenic center. Under conditions that would be conducive to a heterolytic cleavage of the C-OH bond in 8, it is conceivable that the resultant tertiary allylic carbonium ion 7 would participate in a... 

An explanation not easily distinguishable from the one involving resonance with a carbonium ion structure in the transition state is that the reactive species is an ion pair in equilibrium with the covalent molecule. This is quite likely in a solvent insufficiently polar to cause dissociation of the ion pairs. Examples of second order nucleophilic displacements accelerated by the sort of structural change that would stabilize a carbonium ion are of fairly frequent occurrence. Allyl chloride reacts with potassium iodide in acetone at 50° seventy-nine times as fast as does -butyl chloride.209 Another example is the reaction of 3,4-epoxy-1 -butene with methoxide ion.210... 

The isomerization of olefins via er-alkyl species is a structure-requirement reaction, while the isomerization via alkyl cation is a structure-nonrequirement reaction. By the same reasoning, the isomerization via allyl carbanion or allyl carbonium ion is expected to be a structure-nonrequirement reaction. In fact, the formation of alkylallyl carbanions on basic surfaces seems to... 

The inherent plausibility of metastable bridged carbonium ions as intermediates is supported by two independent types of observation. One is the extensive rearrangements which can occur in allylic systems under conditions in which ionic displacement reactions are possible. A second is the existence of stable bridged compounds, including, in the case of boron compounds, pentavalent atoms. Thus diborane and substituted diboranes have stable bridged structures. 

Some new examples of cyclosteroid formation by homo-allylic participation have appeared recently. Hydrolysis of the A -19-mesylate (4) proceeded with z -electron participation to give the 5j(5,i9-cyclo-6 -ol (5) [6sa] or the related A -olefin (6) [64] according to the reaction conditions. Further studies on these compounds [63,65,66,6 ]] revealed other complicated transformations which must proceed through carbonium ion intermediates. Recent work by Tadanier [66,6]] indicates that two distinct non-classical carbonium ions are involved. Buffered solvolysis of the A -ig-mesylate (4) gives the ion represented by resonance between the canonical structures (A), or by the non-classical structure (B). Stereoelectronic factors of the type discussed for i-steroids ensure 6jS-attachment of a nucleophile in forming the 5jS, 19-cyclosteroid (5). This appears to be the initial product of a kinetically-controlled process, for a further rearrangement occurs in weakly acidic... 

The moiety that has the cationic centre in the carbonium ion adjacent to a carbon/carbon double bond is called an allylic cation. The diagram represents one of the canonical structures for this cation. 

Enzymes catalyze the formation of carbon-carbon bonds between allylic and homoallylic pyrophosphate species by mechanisms that are very different from those for carbonyl compounds. Here, carbonium ions, stabilized as ion pairs and generated from allylic pyrophosphates, are likely to be the intermediates that add to the TT-electron density of carbon-carbon double bonds to form new carbon-carbon single bonds. Reaction patterns are consistent with model systems and the mechanisms are based on analogies with the models, stereochemical information (which is subject to interpretation), and the structural requirements for inhibitors. Detailed kinetic studies, including isotope effects, which provide probes in the aldolase and Claisen enzymes discussed in Section II, have not yet been performed in these systems. The possibility for surprising discoveries remains and further work is needed to confirm the proposed mechanisms and to generalize them. 

All of these data may be reconciled with the formation of substituted allylic (alkenyl) carbonium ions. Structures of this type can be derived from any substituted olefin by removal of a hydride ion from a carbon atom adjacent to the double bond (a-hydrogen). Since an olefin may be derived by proton removal from an alkyl carbonium ion, a substituted alkenyl ion may be derived from any alkyl carbonium ion precursor containing at least four carbon atoms and hence the spectra of their solutions should be very similar. Sulfur dioxide evolution from sulfuric acid solutions of olefins would accompany the formation of the alkenyl... 

Any other structural effect which reduces the electron deficiency at a carbonium ion center will have the effect of stabilizing the carbocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. The 7r-electron... 

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