2-methylallyl cation

Electron impact ionization of both disubstituted cyclopropanes 145 and 146 lead to the elimination of the Br radical. From the CA spectra of the resulting [M—Br]+ ions and those of C4H+ ions generated from other suitable precursors it must be concluded that both cyclopropane derivatives give only the 1-methylallyl cation 147. There is no experimental evidence for the generation of the isomeric 2-methylallyl cation 148. Moreover, it was shown that 147 and 148 do not interconvert under the experimental conditions used (36). 

The MINDO/3 calculations [215] on (C4H9)+ considered the 2-butyl cation decomposing to the 1-methylallyl cation and hydrogen. The transition state contained a pentacoordinated carbon atom and the loss of H2 from the transition state formally resembled a 1,1 elimination. The large proportion of the reverse critical energy observed to be released as translation was attributed to repulsive forces and tightness of the transition state. 

Ar aryl, Fc) have been discussed. Treatment of the 1,3-di-ferrocenyl-1-methylallyl cation with PhNMe2 or py causes cyclo-... 

In alkyl cations, hyperconjugation (e.g., from methyl substituents) increases the electron density in the central p orbital. In contrast, a methyl group attached to an allyl or other n delocalized carbocation results in a decreased r-density of the attached carbon. This is due to the polarization of the electron density in the r-system away from the alkyl substituent. The 1-methylallyl cation demonstrates this effect well (MMP2 Cl 0.367, C2 1.113, C3 0,521 MP2(full)/6-31G Cl 0.438, C2 1.118, C3 0,518),... 

The most stable conformer of the 1-methylallyl cation, with an eclipsed conformation of the methyl group with the planar allyl system, is a consequence of a a-a hyperconjugative interaction of the C-H bond parallel to the allyl system with an allyl C-C [Pg.203]

The Cs 1-methyl-l,3-pentadien-5-yl cation has one C-H bond of the methyl group eclipsed with the r-system, analogously to the 1-methylallyl cation. Rearrangements of this cation to cyclopenten-3-yl derivatives have long been known experimentally, The 1-methyl-l,3-pentadien-5-yl cation can... 

This scheme shows the reactions that can occur in the apparent proton-transfer equilibrium reaction between 2-butyne (PAapp = 778 kJ mol" ) and acetonitrile (PA = 787 kJ mol ). The scheme shows an equilibrium with 1,3-butadiene (PA = 787 kJ mol" ) typical of normal thermoneutral proton-transfer reactions in the gas phase with formation of complexes, 7 and 8, between acetonitrilium ion and 1,3-butadiene, and 1-methylallyl cation and acetonitrile, respectively. Formation of the ion molecule complexes 7 and 8 would be expected to be exothermic by about 60 kJ mol" as a result of charge-induced dipole and charge-dipole interactions. From the... 

The overall effect of this scheme would then result in a rapid forward proton transfer to 2-butyne to form 2 and an approximately equal reverse rate to give butadiene, rather than 2-butyne, for a pseudoequilibrium constant close to unity, as observed. The fact that the proposed reaction scheme mimics equilibijum behavior so closely for both 2-butyne and 1,2-butadi e is a result of a completely fortuitous coincidence of the proton affinity of 1,3-butadiene and the apparent proton affinity limit for protonation of 2-butyne and 1,2-butadiene with rearrangement to the allylic cation. Remarkably, 1-butyne, 1,1-dimethylallene, 3-methyl-1-butyne, and 2-pentyne all show a very similar pattern of behavior in proton-transfer experiments, with apparent heats of formation 25-50 kJ mol" too low for the vinyl cation products shown in Table 2 and 59-71 kJ mol" too high for the corresponding allylic ion products. Thus, the PA of 1-butyne is near to that of butadiene and the ultimate reaction product is apparently 1-methylallyl cation 2, as with 2-butyne and by the same sort of reversible proton exchanges within the ion-neutral reaction complexes. 

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