Carbocation equivalents

Inter- and intramolecular reactions between a propargyiic carbocation equivalent stabilized by Co2(CO)6-coordination and enol derivatives also provide a good method for the carbon-carbon bond formation at the propargyiic carbon of propargyiic alcohols and their derivatives. Many diastereoselective and enantioselective propargyiic alkylation reactions at the propargyiic position take place between chiral propargyiic cation equivalents and enol derivatives. 

The /rans-3 -benzylth io-3-ch Ioro- ( -1 actams 192, the appropriate (3-lactam car-bocation equivalents, were prepared by stereospecific chlorination of their corresponding frans-3-benzylthio-(3-lactams 191 using IV-chlorosuccinimide and catalytical amount of AIBN [125]. These (3-lactam carbocation equivalents 192 on treatment with propargyl alcohol or allyl alcohol in the presence of ZnCVSiCL were further transformed to suitable substrates, such as, m-3-benzylthio-3-(prop-2-ynyloxy/enyloxy)-(3-lactams 193 and 195 respectively [126]. 

The seleno-(3-lactam 207, required for this study, was prepared from 2-benzyl-selenoethanoic acid [130] 205 and appropriate imine 206 in the presence of phosphorus oxychloride as the condensing reagent and triethylamine as the base. This was further transformed to appropriate (3-lactam carbocation equivalent,... 

In 2000, a route to novel C-3 substituted 2-azetidinones (I, Fig. 3) has been reported involving a reaction of a (3-lactam carbocation equivalent with active aromatic nucleophiles in the presence of a Lewis acid [245]. 

Earlier we mentioned the Wurtz reaction as being one of the simplest approaches to the formation of C-C bonds. In this reaction, the alkyl halide serves as the electrophile (carbocation equivalent) and the organometallic derivative plays the role of the nucleophile (carbanion equivalent). We have also seen that this old reaction has recently become a feasible route for the creation of C-C bonds due... 

Besides being an enolate anion equivalent, the lithium dimethylhydrazone can also serve as an acyl carbocation equivalent (equation 24). In this way, the a-substituted... 

D.iii. Trimethylenemethane Equivalents. Palladium catalysts can be used to convert trimethylsilyl acetate 390 to a trimethylene methane (TMM, 391) equivalent. Reaction with alkenes via [3+2]-cycloaddition (sec. 11.11) generates cyclopentanes (this process constitutes a quinane annulation reaction).229 in this reaction, the trimethylsilyl unit is a carbanion equivalent and acetate is a carbocation equivalent. In one example, Trost reacted 390 and 392 with palladium acetate and triisopropyl phosphite [P(Oi-Pr)3] to generate 393 in... 

Pandey, G. and Sekhar, B.B.V.S., Photoinduced electron transfer initiated activation of organose-lenium substrates as carbocation equivalents sequential one-pot selenylation and deselenylation reaction, /. Org. Chem., 59, 7367, 1994. 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

The major influence of the methyl group is electronic The most important factor IS relative carbocation stability To a small extent the methyl group sterically hinders the ortho positions making attack slightly more likely at the para carbon than at a single ortho carbon However para substitution is at a statistical disadvantage because there are two equivalent ortho positions but only one para position... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

In the present instance, protonation of the C1-C2 double bond gives a carbo-cation that can react further to give the 1,2 adduct 3-chloro-3-methylcyclohexene and the 1,4 adduct 3-chloro-L-methylcyclohexene. Protonation of the C3-C4 double bond gives a symmetrical carbocation, whose two resonance forms are equivalent. Thus, the 1,2 adduct and the 1,4 adduct have the same structure 6-chloro-l-methyl-cyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a tertiary allylic cation rather than a secondary allylic cation. 

Although five equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion should be aromatic. The four-77-electron cyciopentadienyl carbocation and the five-7r-electron cyciopentadienyl radical are predicted to be unstable and antiaromatic. 

In fl-trimethylsilylcarboxylic acids the non-Kolbe electrolysis is favored as the carbocation is stabilized by the p-effect of the silyl group. Attack of methanol at the silyl group subsequently leads in a regioselective elimination to the double bond (Eq. 29) [307, 308]. This reaction has been used for the construction of 1,4-cyclohexa-dienes. At first Diels-Alder adducts are prepared from dienes and P-trimethylsilyl-acrylic acid as acetylene-equivalent, this is then followed by decarboxylation-desilyl-ation (Eq. 30) [308]. Some examples are summarized in Table 11, Nos. 12-13. 

The intermediacy of a carbocation or complex-equivalent is attractive, if one considers that the nucleophilic ambident cyanide ion may be accomodated on secondary or tertiary cationic sites. Where exceptions (e.g., 125,126,134-136 cf. Sect. 4.3) exist, the cationic intermediate resides on a primary allylic carbon. The following skeletal types are examples of some biogenetic schemes offered in conjunction with the structural determination of isocyanoterpenoids ... 

Normally, only a small stoichiometric excess (2-30 mol%) of silane is necessary to obtain good preparative yields of hydrocarbon products. However, because the capture of carbocation intermediates by silanes is a bimolecular occurrence, in cases where the intermediate may rearrange or undergo other unwanted side reactions such as cationic polymerization, it is sometimes necessary to use a large excess of silane in order to force the reduction to be competitive with alternative reaction pathways. An extreme case that illustrates this is the need for eight equivalents of triethylsilane in the reduction of benzyl alcohol to produce only a 40% yield of toluene the mass of the remainder of the starting alcohol is found to be consumed in the formation of oligomers by bimolecular Friedel-Crafts-type side reactions that compete with the capture of the carbocations by the silane.129... 

Aluminum chloride, used either as a stoichiometric reagent or as a catalyst with gaseous hydrogen chloride, may be used to promote silane reductions of secondary alkyl alcohols that otherwise resist reduction by the action of weaker acids.136 For example, cyclohexanol is not reduced by organosilicon hydrides in the presence of trifluoroacetic acid in dichloromethane, presumably because of the relative instability and difficult formation of the secondary cyclohexyl carbocation. By contrast, treatment of cyclohexanol with an excess of hydrogen chloride gas in the presence of a three-to-four-fold excess of triethylsilane and 1.5 equivalents of aluminum chloride in anhydrous dichloromethane produces 70% of cyclohexane and 7% of methylcyclopentane after a reaction time of 3.5 hours at... 

A mixture of exo- and endo-isomers of 5-methylbicylo[2.2.1]hept-2-ene is hydrogenated with the aid of five equivalents of triethylsilane and 13.1 equivalents of trifluoroacetic acid to produce a 45% yield of < <7o-2-methylbicylo[2.2.1] heptane (Eq. 71). The same product is formed in 37% yield after only five minutes. The remainder of the reaction products is a mixture of three isomeric secondary exo-methylbicylo[2.2.1]heptyl trifluoroacetates that remains inert to the reaction conditions. Use of triethylsilane-l-d gives the endo-2-methylbicylo-[2.2.1]heptane product with an exo-deuterium at the tertiary carbon position shared with the methyl group. This result reflects the nature of the internal carbocation rearrangements that precede capture by the silane.230... 

Alkenes with a 1,1-disubstitution pattern form tertiary carbocations upon treatment with a Brpnsted acid. Consequently, such compounds are often easily reduced (Eq. 72). An example of this is the formation of 2-methylpentane in 93% yield after only 5 minutes when a dichloromethane solution of 2-methyl-1-pentene and 1.4 equivalents of triethylsilane is treated with 1.4 equivalents of trifluoromethanesulfonic acid at —75°.216 Similar treatment of 2,3-dimethyl-l-butene gives a 96% yield of 2,3-dimethylbutane.216... 

Bicycloalkylidenes. a-Delocalization in bicyclic carbocations provides the opportunity to detect the protonation of bicycloalkylidenes. An obvious choice was the 2-norbomyl cation (66), for which symmetrical bridging (equivalence... 

In this chapter, die term carbene includes the species called carbenoid which is often considered as an a-metal-substituted carbocation or its equivalent. In some cases the term carbenoid also includes encumbered carbenes complexed with metal halides. 

The synthetic equivalents of the carbocations can be, of course a carbonyl group or a carbon with a leaving group -X, an epoxide or an activated double bound. 

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