Neopentyl system

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. 

Branching at the a and (3 Carbons. For the Sn2 mechanism, branching at either the a or the (3 carbon decreases the rate. Tertiary systems seldom react by the Sn2 mechanism and neopentyl systems react so slowly as to make... 

The shift of the hydrogen in a neopentyl system (i.e., the hydrogen attached to carbon atom (4) in XXX) in preference to that of the methyl groups of the neopentyl system (i.e., one of the two methyl groups attached to carbon atom (6)) would probably not have been predicted. Such a migration of a methyl group would yield 2,3,5,5,6-pentamethyl-... 

The fact that about five times as much of 2,3,4,4-tetramethyl-l-pentene (XXXVIII) was obtained as its 2-isomer (XXXIX) indicates that the loss of a proton from either of the two methyl groups takes place about five times as easily as do the loss of the proton on the tertiary carbon atom that is part of the neopentyl system. Similarly, the relative amounts of 3,5,5-trimethyl-2-hexene and its 3-isomer (XL and XLI) indicates that the loss of a proton from the ethyl group occurs about five times as readily as from the neopentyl group no loss of a proton from the methyl group appears to have occurred. By analogy with the formation of the two isomeric diisobutylenes from the ear-bonium ion VI it would be expected that the carbonium ion XLII which leads to the formation of 2,4,4-trimethyl-2-hexene (XLIII) would yield the 1-isomer in about four to five times the amount of the 2-isomer. The failure to find any of the 1-isomer was little less than startling (Whitmore and Mixon, 47). 

The SNl-type process occurs mostly when B is a tertiary atom or has one aryl group and at least one other alkyl or aryl group. In other cases, the SN2-type process is more likely. Inversion of configuration (indicating an SN2-type process) has been shown for a neopentyl substrate by the use of the chiral neopentyl-l-d alcohol.18 On the other hand, there is other evidence that neopentyl systems undergo rearrangement by a carbocation (SNl-type) mechanism.19... 

Hydroxy-4,4-dimethyl steroids and triterpenes16 such as lanosterol possess a-substituted neopentyl systems which rearrange easily. The reagent of choice for this rearrangement on a preparative basis is phosphorus pentachloride. Mechanistic studies of the ring contraction have been carried out on C-3 acetates, mesylates and tosylates under solvolysis conditions.1... 

Figure 4.4 Transition state for SN2 substitution in the neopentyl system.

A final example of the utility of adamantyl substrates as model systems in mechanistic investigations is provided by 1-adamantylcarbinyl derivatives. Detailed product and rate constant determinations of 1-adamantylcarbinyl solvolyses have been interpreted relative to the mechanism of the solvolysis reactions of neopentyl systems in general. 

Neopentyl halides are among the most unreactive substrates in polar nucleophilic substitution reactions118. Due to the fact that the halogens are on a primary carbon atom, neopentyl halides seldom react by the S l mechanism. Beacuse of the steric hindrance by the t-butyl group to backside attack, the S 2 reaction in the neopentyl system is notoriously slow. However, with nucleophiles that are good electron donors, the ET reaction competes with the polar mechanism. 

In a study of the reduction of primary neopentyl and cycloalkyl mesylates and tosylates, two groups showed that LiBEtaH in THF is superior to LAH and L-selectride in both rate and selectivity for reduction versus elimination. While cyclohexyl tosylate still gave 20% alkene, elimination was not seen for the cyclo-pentyl, -heptyl and -octyl esters. The lower rate for neopentyl systems was used to advantage in the selective reduction of (5) to (6) at 25 °C. In a more complex system, LiBEtaH reduction of (7 =... 

Nucleophilic rearrangements formally occur in three steps. These steps may be illustrated by the rearrangement of a carbonium ion. A typical example is provided by the neopentyl system. It will be recalled that the SN2 mechanism at such a centre is slow, because of the steric hindrance caused by the /-butyl group in the (5-position. Furthermore, normal elimination is impossible, because there is no (5-hydrogen. 

Examination of models, however, reveals that in neopentyl chloride the fcr -butyl group protects the methylene carbon atom from a backside attack by the approaching reagent. It is presumably this fact that is responsible for the abnormally low reactivity of a neopentyl system to Sn2 reactions. 

Answer The neopentyl system would migrate a methyl to give a tertiary carbocation. Isobutyl systems migrate an H to give the fert-butyl carbocation. The strained ring compound produces the 1-methylcyclopentyl carbocation. (Relief of severe... 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleophilic substitution reactions. They are primary and do not form carbocation intermediates moreover the r-butyl substituent hinders back-side displacement. The rate of reaction of neopentyl bromide with iodide ion is 470 times less than that of n-butyl bromide. Under solvolysis conditions the neopentyl system usually reacts with rearrangement to the... 

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