Neopentyl cation rearrangement

Formation of the bicyclo[3.2.0]heptane system was visualized as a neopentyl-type rearrangement with the oxoalkyl group acting as the migrating group. However, neopentyl rearrangements occur in acid media via cations. 

Clearly, both spectra are of the tertiary 2-methylbutyl cation and the neopentyl cation never saw the light of day. The reaction is the same rearrangement that you saw in the substitution reaction of neopentyl iodide, but here the rate of rearrangement can be measured and it is extremely fast. Ncopentyl tosylate reacts to form a cation under these conditions about 104 times as fast as ethyl tosylate, even though both tosylates are primary. This massive rate difference shows that if migration of an alkyl group can allow rearrangement to a more stable carbocation, it will happen, and happen rapidly. 

Rearrangement of the neopentyl cation labeled with deuterium in the 1 position (11) gave only tert-pentyl products with the label in the 3 position (derived from 13), though if 12 were an intermediate, the cyclopropane ring could just as well cleave the other way to give lert-pentyl derivatives labeled in the 4 position (derived from 14). ° Another experiment that led to the same conclusion was the generation, in several ways, of Me3C CH2. In this case, the only tert-pentyl products isolated were labeled in C-3, that... 

Problem 16.2 Because of the great tendency of the neopentyl cation to rearrange, neopentyl chloride cannot be prepared from the alcohol. How might neopentyl chloride be prepared ... 

Clearly, both spectra are of the tertiary 2-methylbutyl cation and the neopentyl cation never saw the light of day. The reaction is the same rearrangement that you saw in the substitution reaction of neopentyl iodide, but here the rate of rearrangement can be measured and it is extremely fast. 

The effect of substituents on the equilibrium between a carbonium ion and an isomeric n complex can easily be predicted. In the carbonium ion, one atom carries a full unit of formal charge while the others are neutral. In the n complex, the charge is shared between the apical carbon atom in the acceptor and the two basal carbon atoms in the olefin moiety. The apical carbon atom and one basal carbon atom are therefore more positive in the 71 complex than in the carbonium ion, while the other basal atom is much more positive in the carbonium ion. If then we replace one of both hydrogen atoms at a basal position in (49) by — / groups, e.g., alkyl, we will stabilize one of the classical ions very strongly relative to the n complex. There is little doubt that in the rearrangement of neopentyl cation [equation (5.195)], the intermediate n complex immediately rearranges to the more stable r-amyl... 

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

ControUed-potential oxidations of a number of primary, secondary, and tertiary alkyl bromides at platinum electrodes in acetonitrile have been investigated [10]. For compounds such as 2-bromopropane, 2-bromobutane, tert-butyl bromide, and neopentyl bromide, a single Ai-alkylacetamide is produced. On the other hand, for 1-bromobutane, 1-bromopentane, 1-bromohexane, 1-bromo-3-methylbutane, and 3-bromohexane, a mixture of amides arises. It was proposed that one electron is removed from each molecule of starting material and that the resulting cation radical (RBr+ ) decomposes to yield a carbocation (R" "). Once formed, the carbocation can react (either directly or after rearrangement) with acetonitrile eventually to form an Al-alkylacetamide, as described above for alkyl iodides. In later work, Becker [11] studied the oxidation of 1-bromoalkanes ranging from methyl to heptyl bromide. He observed that, as the carbon-chain length is increased, the coulombic yield of amides decreases as the number of different amides increases. 

Dauben and coworkers observed unusual rate accelerations and ring-expansion rearrangements in the solvolysis of bicyclo[2.2.0]hexane-l-methyl / -nitrobenzoate, in agreement with Winstein s proposed nonclassical cyclobutylmethyl cations (Figure 3). ° Thus, the rate of the solvolysis of bicyclo[2.2.0]hexane-l-methyl p-nitrobenzoate is 7 x 10 times faster than that of the corresponding extrapolated rate for neopentyl derivative. The lack of scrambling of the label in the solvolysis of the 0-labeled... 

The following example illustrates this point. We shall see (Sec. 16.5) that the neopcntyl cation is particularly prone to rearrange to the more stable ten-pentyl cation. Neopentyl bromide reacts (slowly) with ethoxide ion by an 8 2 mechanism to yield neopentyl ethyl ether it reacts (slowly) with ethyl alcohol by an SnI reaction to yield almost entirely rearranged products. 

Cann, P.E, Howells, D., and Warren. S.. Rearrangements of a cation of the neopentyl-type containing a diphenylphosphinyl substituent, J. Chem. Soc.. Rerkin Trans. 2, 304, 1972. 

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