Protonated methylcyclopropane

Protonated methylcyclopropane [23] might be a comer-, edge-, or face-protonated compound, but, for our purpose, it is sufficient to neglect the distinction between the last two. On the basis of the exchange information, we can make a choice between the comer and edge species. 

If the intermediate were corner-protonated, however, and undergoing exchange with the acid, it would be most reasonable to expect all the protons at the comer to exchange with the acid before ring cleavage occurred. Thus, corner protonated methylcyclopropane would be expected to exchange three protons with the acid if it exchanged any. 

Methylcyclopropane 6 reacts selectively (> 98 %) with ZnCl2 at the less hindered site to give the homoenolate of 2-methylpropionate, whereas phenyl substituted analogue 7 is cleaved selectively (>98%) at the more substituted site [27], The latter homoenolate however suffers in situ protonation under the reaction conditions and could not be isolated. 

In the same medium methylcyclopropane is protonated to give the methylethylcarbonium ion... 

The processes are the monomolecular reaction through a protonated cyclopropane produced by the abstraction of H" over Lewis acid sites and the bimolecular mechanism where an olefin takes part in the reaction. The olefin is produced over Bronsted acid sites, in the case of butane in the monomolecular mechanism, isobutane is formed through protonated methylcyclopropane with an activation energy of 8.4kcalmoT followed by the formation of the primary isobutyl cation with high energy [134]. 

TABLE 5. Structures (bond lengths (A) from 6-3IG calculations) and energies of protonated methylcyclopropanes (kcal mol ) compared with 2-butyl cation... 

Protonated methylcyclopropanes are considered to be involved in the carbon scrambling of the 2-butyl cation which occurs (Scheme 18) with a measured activation energy of 7.5 kcalmol" a value in good agreement with the calculated energy difference... 

Fig. 4.11. Energy profile for the scrambling and rearrangement of C4H9 cation. (A) H-bridged (B) methyl-bridged (C) edge protonated methylcyclopropane (D) classical secondary (E) classical primary (F) tertiary. Adapted from J. Am. Chem. Soc., 112, 4064 (1990) J. Am. Chem. Soc., 115, 259 (1993) and J. Phys. Chem., 100, 633 (1996), by permission of the American Chemical Society.

Methylcyclopropane results from a 1,3- or y-elimination where the proton is removed from a carbon not adjacent to the positively charged carbon. 

Baldwin and Widdison prepared ethyl a-deuterodiazoacetate by reacting ethyl-diazoacetate with two different batches of sodium deuteroxide in deuterium oxide containing hexadecyltrimethylammonium bromide as a phase transfer catalyst. A proton NMR analysis showed the product was more than 98% deuterated at the a-carbon. The deuter-ated diazoacetate was then converted into two diastereomeric ethyl-(2-bromo-2-methylcyclopropane-1 -d) carboxylates by reaction with 2-bromopropene and a rhodium(II)... 

In postulating mechanisms for the carbon skeleton rearrangement reaction (31), one could consider the protonated methylcyclopropane... 

C-labeled t-butyl carbocation (CH3)3 C, obtained at 195 K in FS03H/SbF5/S02FCl from labeled t-BuCl, exhibits complete scrambling of the label after 20 h at 343 K. No deprotonation occurs, and the scrambling is thought to involve a delocalized protonated methylcyclopropane intermediate or transition state. "... 

Under mild conditions, the label scrambling was also observed in oct-l-ene on H-ZSM-5 [131] and in propane on H-ZSM-5 at523-573K [132]. Becausethe scrambling of the label between the 1- and 2-positions is impossible without the formation of protonated methylcyclopropane, which is in the equilibrium with the butyl cation [127] (Scheme 1), one can conclude that carbenium ions as transient intermediates are formed on solid acid catalysts. Recent theoretical estimates of the stability of some cations in zeolites proved this possibility [133-135]. 

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