Allenes vinyl cations

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Hydration of allene in sulfuric acid yields acetone, presumably via a vinyl cation intermediate (55, 56). Addition of HCl and HBr to allene results in... 

A summary of electrophilic additions to triple bonds and allenes involving a vinyl cation is given in Table IV. 

Summary of Electrophilic Additions to Acetylenes and Allenes Involving Vinyl Cations... 

Unlike triple-bond participation, no remote cumulative double-bond (allenic) participation in solvolysis has so far been observed or reported. A summary of triple-bond and allenic participation in solvolysis involving possible vinyl cations is given in Table VI. 

All of the above mentioned examples of vinyl cation intermediates have involved electrophilic additions to triple bonds or allenes or participation in solvolyses of such multiple bonds. In a sense, these reactions derive from analogies in normal... 

Finally photolysis of substituted pyrazolenines 140 in ether is reported (118) to yield allene 142 and diene 143 as products via vinyl cation 141. 

The first example of a carbon migrating across a double bond of a vinyl cation generated by solvolysis was reported by Hanack, Schleyer, Stang, and co-workers (177) with the vinyl substrate 230. Solvolysis of 230 in 80% aqueous ethanol gave, besides a small amount of allene, exclusively the rearranged ketone... 

Butatrienyl cations 61 were also implied in the solvolysis of the w-propynyl vinyl triflate 6243. The vinyl triflate solvolyzes 35-70 times faster than the corresponding simple vinyl analogues and gives products resulting mainly from the a-alkynyl vinyl cation 63. A small amount of the allenic ketone 64 derived from the butatrienyl cation is also detected (equation 22). 

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. 

Many valuable reviews of the chemistry of these species are given in the new book Dicoordinated Carbocations An introduction by Grob " is followed by reviews of various theoretical studies of vinyl cations, their gas-phase chemistry, their generation by nuclear decay, and their NMR spectroscopic characterization. Vinyl cation production by addition to acetylenes and allenes, by solvolysis, and photolytically are covered, together with the chemistry of the species generated in these various ways. The next chapter deals with the synthetic applications of vinyl cations,and alkynyl and aryl cations are covered in the last chapter. A review of the NMR spectroscopic and quantum-chemical investigation of vinyl cations in superacid media (also of dienyl and 1-cyclopropylvinyl cations) is published separately,as is a review of alkynylcar-... 

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... 

Carbocations with trivalent carbon may have carbon with coordination number 2. Acyl ions have already been mentioned the vinyl cations, or car-bynium ions (26), have been detected as intermediates in addition of electrophiles to acetylenes and allenes and in solvolysis reactions with the highly reactive trifluoromethanesulfonate (triflate) leaving group.84 Vinyl cations are expected... 

The addition of water to allenes has been reviewed recently.298-299 The product obtained from the acid-catalyzed hydration of allene, alkylallenes and 1,3-dialkylallenes is usually the ketone. The intermediate is a vinyl cation formed by protonation on the terminal carbon of the allene moiety (equation 198). 

Alcohols will add to allenes in the presence of trace amounts of acids to give vinyl ethers (or acetals) or allylic ethers.401 Analogous to the hydration of allenes, protonation occurs on the terminal carbon of the allenic functionality in 1,2-propadiene, 3-alky 1-1,2-propadienes and 1,3-dialkyl- 1,2-propadienes (equation 248).402 Addition of an alcohol to the resulting vinylic cation produces a vinylic ether, which may on further reaction form an acetal of the corresponding ketone. 

The formation of vinyl cations has also been observed [61]. The acyclic vinyl iodide (24) was irradiated at 254 nm in CH3OH. This afforded reduction, nucleophilic-trapping, and allene intermediate products (Scheme 18). In the presence of CH3OD the allylic ether had 72% incorporation of deuterium at the vinylic position. On irradiation in CH2C12 or pentane, reduction and 1,3-diene product were observed. 

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... 

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