Vinyl cation, cycloadditions

The frontier orbital treatment for vinyl cation cycloadditions, such as those of ketenes, has some merits. It satisfyingly shows that the bond forming between C-l and C-l develops mainly from the interaction of the LUMO of the ketene (n of the C=0 group) and the HOMO of the alkene 6.178, and that the bond between C-2 and C-2 develops mainly from the interaction of the HOMO of the ketene (i/j2 of the 3-atom linear set of orbitals analogous to the allyl anion) and the LUMO of the alkene 6.179. 

In general, allenyl cations 38 attack at the sp2-carbon atom of 1,3-dienes and form vinyl cations 39 and 40 (R = H, alkyl) or (R = aryl). Although a concerted cycloaddition mechanism is possible, a stepwise mechanism is preferred34. If a nucleophilic attack at the sp-carbon atom of the allenyl cation takes place, then cation 41 and the resulting cations 42 and 43 are formed. Some examples of bicyclic products obtained from cyclic 1,3-dienes and propargyl chlorides are given in equation 1534. 

A regioselective [3 + 2]-cycloaddition approach to substituted 5-membered carbo-cycles was made available by the use of allenylsilanes [188]. The reaction involves regioselective attack of an unsaturated ketone by (trimethylsilyl)allene at the 3-position. The resulting vinyl cation undergoes a 1,2-silyl migration. The isomeric vinyl cation is intercepted intramolecularly by the titanium enolate to produce a highly substituted (trimethylsilyl)cyclopentene derivative. 

As in the case of catalyzed cycloadditions of alkenes, reactions of allenes catalyzed by Lewis acids or via vinyl cations proceed more efficiently than their thermal counterparts. Even nonactivated allenes can be induced to react with nonactivated alkenes to give good yields of cyclobutanes. Such reactions could not be carried out under noncatalyzed conditions.1... 

Cycloaddition. In the presence of either C2H5AICI21 or AICI3,2 esters of 2,3-butadienoic acid (1) undergo [2 + 2] cycloadditions at the 3,4-double bond with acyclic or cyclic alkenes to give cyclobutylideneacetic esters. The reaction is considered to involve the vinyl cation H2C— CH - C(OR)OAlCl,. A mixture of... 

Related to ketene cycloadditions are the group of cycloadditions with vinyl cation intermediates. The reaction between 2-butyne 6.120 and chlorine giving the dichlorocyclobutene 6.122 is the Smirnov-Zamkow reaction, and there is a similar reaction between allene 6.123 and hydrogen chloride giving the... 

Lewis acid catalyzed reactions of allenes with alkenes generally give cyclobutanes rather than ene adducts. AlCb catalyzed reactions of alkylallenes with alkenes give alkylidenecyclobutanes. Similarly, AICI3 catalyzed reactions of alkynes with alkenes give cyclobutenes. These reactions are believed to occur by stereospecific cycloaddition of the alkene with the vinyl cation formed by complexation of AICI3 to the allene or alkyne. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

The presence of the orthogonal vacant p-orbitals in the vinyl cation results in the formation of strong bonding interactions with the occupied rr-system of the simple olefinic reactant. Thus, the normal symmetry-allowed combination of a cation with an olefin in this particular situation sets the stage for the [tt s + TT a] cycloaddition reaction. However, these secondary interactions are absent from the [rr s + rr a] reaction path for simple olefins. 

These systems are known to behave as vinyl cations displaying, as a consequence, a potentially interesting chemistry towards a variety of olefmic substrates[2]. Provided that this similarity applies, it will confer a promising outlook to alkynylcarbene metal complexes in cycloaddition chemistry concerning versatility and, what is even of the utmost importance, a strict control of the stereochemistry of the products may be expected[3]. 

Lewis acids, such as EtAlCL, GaCb and AlBrs, catalyze the cycloaddition of allenes to alkenes to give methylenecyclobutanes 151. 1-Alkenes and sterically hindered olefins, such as 4,4-dimethyl-2-pentene, do not react, and from butadiene only polymeric products are obtained. Substituted olefins react more readily, i.e. 1,1,2,2-tetraalkylethylene > 1,1,2-trialkylethylene > 1,2-dialkylethylene. The acid catalyzed [2+2] cycloaddition reaction most likely proceeds via a vinyl cation. The slightly exothermic reaction proceeds at room temperature in chlorobenzene and EtAlCb was used as a catalyst in the examples listed in Table 6.7... 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Enol ether additives were used to probe the protonation of 3-cyclopen-tenylidene (127). Treatment of A-nitroso-A-(2-vinylcyclopropyl)urea (124) with sodium methoxide generates 2-vinylcyclopropylidene (126) by way of the labile diazo compound 125 (Scheme 25). For simplicity, products derived directly from 126 (allene, ether, cycloadduct) are not shown in Scheme 25. The Skat-tebpl rearrangement of 126 generates 127 whose protonation leads to the 3-cyclopentenyl cation (128). In the presence of methanol, cyclopentadiene (130) and 3-methoxycyclopentene (132) were obtained.53 With an equimolar mixture of methyl vinyl ether and methanol, cycloaddition of 127 (—> 131)... 

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