Stabilization vinyl cations

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

Stabilized vinyl cations also can be formed by allylic double-bond participation in vinyl halide solvolysis. Grob and co-workers (151) have investigated the solvolysis of a number of substituted 2-bromo-l,3-butadienes, 165. Bromodienes 165 solvolyzed via first-order rates in 80% aqueous ethanol... 

A jS-silyl stabilized vinyl cation, the l-bis(trimethylsilyl)methyl-2-bis(trimethylsilyl) ethenyl cation (32) was investigated by dynamic 13C NMR spectroscopy (Fig. 6).55... 

Conjugated ketones and esters react with allenylsilanes to yield acylcyclopentenes (Eq. 9.60) [63]. These products are formed by initial 1,4-addition to the conjugated double bond to afford a silyl-stabilized vinyl cation intermediate. 1,2-Silyl migration gives rise to a second silyl-stabilized vinyl cation which cyclizes to the acyl cyclopen-tene (Scheme 9.14). 

Electrophilic addition of HC1 to triple bonds can apparently also go by bi-or termolecular mechanisms. Thus in acetic acid 3-hexyne (14) gives predominantly anti addition through an Ad3 pathway, but 1-phenylpropyne (15), which can form the resonance-stabilized vinyl cation (16), gives predominantly syn addition through an ion pair Ad 2 mechanism.27... 

Cationic vinylidene complexes can be considered to be metal-stabilized vinyl cations in purely organic chemistry, vinyl cations can be obtained by dissociation of the super leaving group, TfO-, from the esters obtained from enolate anions and Tf20. 

They are both reactions of vinyl cations, either as such 2.170, or highly stabilized vinyl cations in the case of ketene 2.167, where the carbonyl group is a stabilized carbocation. 

In suitable vinylic halides, nucleophilic photosubstitution occurs intramolecular- 295-297,324,330-332 por jnstance irradiation of 113 in methanol only yields benzofuran 114, and no vinyl ether product or 1,2-aryl rearrangement products are formed (equation 84a)296. Apparently, the reaction with an internal nucleophile is faster than the 1,2-aryl shift and also faster than the reaction with an external nucleophile. With the less stabilized vinyl cations derived from the a-methyl- and a-H-analogues of 113-OMe, 1,2-aryl shifts do... 

Homopropargylic alcohols are readily available substrates that can be used for the synthesis of 7-lactones. Cul-catalyzed selenation with PhSeBr at the alkyne terminus affords alkynyl aryl selenides. These react with an excess of /i-toluenesulfonic acid monohydrate, in dichloromethane at 60°C, to form a selenium-stabilized vinyl cation intermediate. The cation is then intramolecularly trapped by the tethered hydroxyl group to afford a cyclic selenoketene acetal, which readily adds a molecule of water to give the 7-lactone products (Scheme 55) <2006SL587>. 

With further increasing leaving ability of Y, the reaction becomes dissociative and becomes a vinylic SnI (SnVI) reaction involving a vinyl cation as intermediate. SnV 1 reactions have been extensively studied, both with substrates giving stabilized vinyl cations and/or with substrates with a good nucleofuge such as triflate (trifiuoromethanesulfonate, TfO ) and are the subjects of several reviews." Their stereochemical consequences are discussed in Section 2. 

Thus far, we have discussed typical vinylic Sn2 and SnI reactions. The former involve reactions of good nucleophiles with simple 1-alkenyliodonium salts (S Ycr) and unactivated j8-halo derivatives (SnVtt), while the latter reactions occur under poor nucleophilic conditions (solvolysis) if the substrates can give stabilized vinylic cations of secondary or bridged structure. In this section, reactions which have characteristics of both categories are presented. 

The stereochemistry of addition is usualy anti for alkyl-substituted alkynes, whereas the addition to aryl-substituted compounds is not stereospecific. This suggests a termolecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation intermediate in the aryl case. Aryl-substituted alkynes can be shifted... 

Although details are obscure, it is presumed that weakening of the carbon-bromine bond (n n absorption) precedes heterolysis to a stabilized vinyl cation and bromide anion (Br ). The cation then reacts with ethanol (CH3CH2OH) and a... 

Stabilized vinyl cation 393 is at about 5 and 4 ppm lower field compared with the /J-jr-resonance-stabilized phenylallenyl cation 399, or the sp -hybridized carbocation 400, respectively. 

Trifluoromethyl-substituted alkynes were found to give good yields of the hydroary-lation products in superacid (Scheme 21). ° i For example, the propyne derivative (91) gives a nearly quantitative yield of the addition product (92) by reaction in triflic acid. Protonation initially leads to the resonance-stabilized vinyl cation (93), which reacts with benzene. The diarylalkyl cation (94) is generated by a second protonation - its presence confirmed through direct observation by NMR - and this yields the olefin (92) upon workup of the reaction. 

Three-Carbon Synthon for [3 + 2] Annulations. Danheiser and co-workers have exploited allenylsilanes as the three-carbon components in a [3 + 2] annulation strategy for the synthesis of a variety of five-membered carbocycles and heterocycles. The pathway by which a typical annulation proceeds is shown in eq 6. Reaction of the 2-carbon component (the allenophile ) at C-3 of the allenylsilane is followed by rapid rearrangement of the silicon-stabilized vinyl cation. Ring closure then affords the five-membered product. 

An interesting and novel preparation of 5,5-disubstituted cyclopent-2-enones involves the addition of acetylenes to 3Y-unsaturated acid chlorides (35). The reaction is thought to involve cyclization of the silicon-stabilized vinyl cation (36)... 

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