Electrophiles 1,2-alkyl shifts

In the CH3CH=CH2- -NO+ complex, the nitrosyl cation retains the characteristic canted geometry indicative of strong 7tcc-7txo interaction (Fig. 5.46(c)). However, the electrophilic attack shifts toward the terminal C of the pi bond, away from the methyl substituent. Such anti-Markovnikov complexation is, of course, to be expected from the relative polarization of the propylene pi bond toward the terminal C (so that the 7tCc antibond is polarized toward the alkyl pi-donor). 

Ethers can react with electrophilic carbene complexes to yield oxonium ylides, which usually undergo either elimination reactions or 1,2-alkyl shifts to yield products of a formal carbene C-O bond insertion (Figure 4.11) [1020,1255-1259]. 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

The resonance stabilization of an allylic carbocation is estimated to be 15 kcal/mol. The loss of this resonance stabilization means that the vinyl cation is of comparable stability. When the R group is hydrogen or alkyl, the electrophile adds to the end as shown below. If the R group on the allene is capable of additional resonance stabilization of the carbocation, then electrophile attack shifts to the middle carbon. 

Substituted aromatics are essential chemical feedstocks. Among the xylenes, for example, p-xylene is in great demand as a precursor to terephthalic acid, a polyester building block. The pura-isomer is therefore more valuable than the o- and m-xylenes, so there is a powerful incentive for conversion of o- and m-xylene to p-xylene. Isomerisation over solid acids occurs readily as a result of alkyl shift reactions of the carbenium-ion-like transition state. The initial protonation occurs by interaction of the Bronsted acid site with the aromatic 71 system, by an electrophilic addition. Over non-microporous solid acids, at high conversion, xylenes are produced at their thermodynamically determined ratios, which favour the meta rather than the ortho or para isomers. In addition, unwanted transalkylation reactions occur, giving rise, for example, to toluene and trimethylbenzenes. Zeolite catalysts can be much more selective. 

Protonated cyclopropane has been reported in the gas phase" "" to be ca 8 kcal mol" in energy above the isopropyl cation. The bent bonds of the cyclopropane ring are susceptible to electrophilic attack leading to the expectation that cyclopropane will be more basic than saturated alkanes and that protonation will occur on the C—C bond, i.e. the edge-protonated isomer will have the lowest energy. There is, however, considerable evidence from solution chemistry that corner-protonated cyclopropanes exist as intermediates in 1,2-alkyl shifts in carbocations. There have been several reviews of protonated cyclopropanes " and, in the current work, only the very recent theoretical work will be reviewed. 

Lewis acid complexes of p-substituted a,p-unsaturated ketones and aldehydes are unreactive toward alkenes. Crotonaldehyde and 3-penten-2-one can not be induced to undergo ene reactions as acrolein and MVK do. 34 The presence of a substituent on the p-carbon stabilizes the enal- or enone-Lewis acid complex and sterically retards the approach of an alkene to the p-carbon. However, we have found that a complex of these ketones and aldehydes with 2 equivalents of EtAlQ2 reacts reversibly with alkenes to give a zwitterion. 34 This zwitterion, which is formed in the absence of a nucleophile, reacts reversibly to give a cyclobutane or undergoes two 1,2-hydride or alkyl shifts to irreversibly generate a p,p-disubstituted-o,p-unsaturated carbonyl compound (see Figure 19). The intermolecular addition of an enone, as an electrophile, to an alkene has been observed only rarely. The specific termination of the reaction by a series of alkyl and hydride shifts is also very unusual. 35 The absence of polymer is remarkable. 

As described in Section 11.5.12 with reference to SnI reactions, a hydride or alkyl shift in a carbenium ion is a common rearrangement. It entails the movement of a hydrogen, alkyl, or vinyl / aryl group from a carbon adjacent to a carbenium ion to the electrophilic center in order to create a more stable carbenium ion. Many rearrangements have a similar migration as a key step, with the additional feature of a heteroatom on the p-carbon that stabilizes the newly formed electron deficient center (Eq. 11.46). Two prototype examples are the pinacol rearrangement and the benzilic acid rearrangement. 

Gagosz et al. reported Au(l) catalyst 147-catalyzed alkylation of alkynyl ethers which produced cyclohexane 146 as major product (Scheme 54) [127]. Theoretically, the electrophilic activation of the alkyne 145 by Au(l) initiates a [1,5]-hydride shift to furnish oxocarbenium ion 1, interaction of which with the pendant nucleophilic vinyl-gold moiety affords cyclopropenium intermediate II. Carboca-tion IV, which would finally collapse into cyclohexene 146 after elimination of the gold(I) catalyst might be generated via a [1,2]-alkyl shift on Au-carbene intermediate III. 

The carbocation rearrangements that accompany Friedel-Crafts reactions are like those that accompany electrophilic additions to alkenes (Section 7.10) and occur either hy hydride shift or alkyl shift. For example, the relatively unstable primary hutyl carhocation produced hy reaction of 1-chlorohutane with AICI3 rearranges to the more stable secondary butyl carbocation by shift of a hydrogen atom and its electron pair (a hydride ion, H ") from C2 to Cl. Similarly, alkylation of benzene with l-chloro-2,2-dimethylpropane yields (l,l-dimethylpropyl)benzene. The initially formed primary carbocation rearranges to a tertiary carbocation by shift of a methyl group and its electron pair from C2 to Cl. 

Generation of Electrophilic Cations. 1 2 Aldehyde- or ketone-MeAlCl2 complexes add intramolecularly to alkenes to give zwitterions that undergo 1,2-hydride and alkyl shifts to regenerate a ketone (eqs 3 and 4). Et AICI2 can transfer a hydride to the zwitterion to give the reduced product (eq 3). ... 

Similar to the alkylation and the chlorination of benzene, the nitration reaction is an electrophilic substitution of a benzene hydrogen (a proton) with a nitronium ion (NO ). The liquid-phase reaction occurs in presence of both concentrated nitric and sulfuric acids at approximately 50°C. Concentrated sulfuric acid has two functions it reacts with nitric acid to form the nitronium ion, and it absorbs the water formed during the reaction, which shifts the equilibrium to the formation of nitrobenzene ... 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

Figure 7-9 illustrates a typical Friedel-Crcifts alkylation. Once formed, the carbocation is a very strong electrophile. A complication that may occur is the rearrangement of the carbocation to a more stable carbocation, as seen in Sfjl mechanisms of alkyl halides. These rearrangements may involve a hydride or other shift. 

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