Oxocarbenium cations

With respect to the initiation of cationic chain polymerizations, the reaction of chlorine-terminated azo compounds with various silver salts has been thoroughly studied. ACPC, a compound often used in condensation type reactions discussed previously, was reacted with Ag X , X, being BF4 [10,61] or SbFa [11,62]. This reaction resulted in two oxocarbenium cations, being very suitable initiating sites for cationic polymerization. Thus, poly(tetrahydrofuran) with Mn between 3 x 10 and 4 x lO containing exactly one central azo group per molecule was synthesized [62a]. Furthermore, N-... 

The Jencks clock, as well as pulse radiolysis experiments, indicate that oxocarbenium cations often can be quite stabilized, with a half-life on the order of milliseconds. For example, at room temperature, the structures shown here (left to right) have respective lifetimes of... 

C-Glycosylation Oxocarbenium cation Anomeric radical Anomeric anion Stereoelectronic effect... 

C-Glycosylation reactions of glycosyl-halide, -ester and -ether with carbon nucleophiles are divided into two types the Sn2 type of substitution with carbon nucleophiles such as Grig-nard reagents, and addition of carbon nucleophiles to oxocarbenium cations generated from... 

The stereochemical outcome is determined by the nucleophilic attack of a carbon nucleophile to an oxocarbenium cation in the case of hexopyranose as tri-O-acetyl-D-glucal, axial attack of the nucleophile takes place at the anomeric position due to stereoelectronic effects, giving a-C-glycoside as the major product. 

In contrast to the reaction of 1,2-anhydrosugar, C-glycosylation of 1,6-anhydropyranose requires a strong Lewis acid in order to open the anhydro bridge, generating the corresponding oxocarbenium cation (O Eq. 6). Addition of nucleophiles to the cation could be controlled by the stereoelectronic effect, chelation between the nucleophile and the Lewis acid coordinated to the hydroxyl group, or intramolecular delivery of nucleophiles as shown below. 

Cyclic oxocarbenium cations approximate the a- and jS-chelation models for allylations of substituted aldehydes. In this analogy, the nature and the degree of substitution of the carbon bound to the oxonium site replaces the role of coordinated bidentate Lewis acids. Aside from the obvious concerns about the relative reactivity of 126 (Scheme 5.2.27), as compared with metal cation activation of precomplexed aldehyde 127, the C—O bond of 126 is much shorter (1.514 A) than distances associated with the... 

Reactions of aiienyistannanes with oxocarbenium cations are also facile processes. Williams, and subsequently Nelson,have demonstrated the use of glycal acetate 313 in the course of the total synthesis of laulimalide, to generate a reactive oxocarbenium species in the presence of Lewis acids. Axial addition of aiienyistannanes 314 and 316 resulted in tranx-2,6-disubstituted dihydropyrans 315 and 317, respectively (Scheme 5.2.68). Similarly, the propargylation of lactol 318 is affected with allenylstannane 319 in the presence of BFjoOEta (Eq. 24)... 

Calculated energies of stabilized cations corresponding to the anchimeric assistance from the positions 3, 4, and 6 were compared with the energies of two possible conformations of the nonstabiUzed oxocarbenium cation (Scheme 5.21). Two most stable conformations of the nonstabiUzed cation, H4 and undergo a nucleophilic attack preferentially from P- and a-sides, respectively, giving products with the opposite stereochemistry [49-55]. Calculations showed that a-selective conformation of the oxocarbenium ion is by almost 3kcalmol more stable than P-selective one, that is, the a-selectivity is intrinsic to this... 

For instance, in polymerization of cyclic acetals the oxo-nium or oxocarbenium cation acting as the active center of propagation can react with the acetal groups located inside the polymer chain (backbiting) or with the end group, the nature of which depends on the used initiator. If... 

It is worthy of note that the stereoselectivities of the above glycosylations were demonstrated to be independent on the a/p ratio of the starting substrate, confirming the formation of the oxocarbenium cation intermediate and therefore the SnI type mechanism. 

The Nicholas reaction [103] has been successfully exploited for a novel glyco-sylation reaction based on the internal delivery concept (Scheme 31a). The suitably O-protected glycop3rranoside derivative 111 bearing the cobalt-complexed 4-alkoxy-6-phenyl-5-hexynoate residue at the anomeric position affords the corresponding propynyl cation 112 under Lewis acid catalysis. The intermediate 112 collapses spontaneously to form the oxoniiun ion delivering the cobalt-complexed y-lactone 113. The oxocarbenium cation is then intramolecularly captured by its counter alkoxy anion leading to glycoside 114. 

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