Tertiary position

Although the nitro group plays a crucial role in most of these SrnI reactions, reactions of this type have synthetic application beyond the area of nitro compounds. The nitromethyl groups can be converted to other functional groups, including aldehydes and carboxylic acids.Nitro groups at tertiary positions can be reductively removed by reaction with the methanethiol anion.This reaction also appears to be of the electron-transfer type, with the methanethiolate anion acting as the electron donor ... 

Free radicals may also react with a hydrocarbon molecule from the feed by abstracting a hydrogen atom. In this case the attacking radical is terminated, and a new free radical is formed. Abstraction of a hydrogen atom can occur at any position along the chain. However, the rate of hydrogen abstraction is faster from a tertiary position than from a secondary, which is faster than from a primary position. 

Alkanes, arylalkanes, and cycloalkanes can be aminated, at tertiary positions only,... 

TABLE 14.1 Relative Susceptibility to Attack by Cl- of Primary, Secondary, and Tertiary Positions at 100 and 600° C in the Gas Phase... 

Alkyl Side Chains of Aromatic Rings. The preferential position of attack on a side chain is usually the one a to the ring. Both for active radicals such as chlorine and phenyl and for more selective ones such as bromine such attack is faster than that at a primary carbon, but for the active radicals benzylic attack is slower than for tertiary positions, while for the selective ones it is faster. Two or three aryl groups on a carbon activate its hydrogens even more, as would be expected from the resonance involved. These statements can be illustrated by the following abstraction ratios ... 

N2, and bromine trifluoride at 25-35°C " are also highly regioselective for tertiary positions. These reactions probably have electrophilic, not free-radical mechanisms. In fact, the success of the F2 reactions depends on the suppression of free-radical pathways, by dilution with an inert gas, by working at low temperatures, and/or by the use of radical scavengers. 

The mechanism involves a simple 1,2 shift. The ion (52, where all four R groups are Me) has been trapped by the addition of tetrahydrothiophene. It may seem odd that a migration takes place when the positive charge is already at a tertiary position, but carbocations stabilized by an oxygen atom are even more stable than tertiary alkyl cations (p. 323). There is also the driving force supplied by the fact that the new carbocation can immediately stabilize itself by losing a proton. 

This dry ozonation procedure is a general method for hydrox-ylation of tertiary carbon atoms in saturated compounds (Table 1). The substitution reaction occurs with predominant retention of configuration. Thus cis-decalin gives the cis-l-decalol, whereas cis- and frans-l,4-dimethylcyclohexane afford cis- and trans-1,4-dimethylcyclohexanol, respectively. The amount of epimeric alcohol formed in these ozonation reactions is usually less than 1%. The tertiary alcohols may be further oxidized to diols by repeating the ozonation however, the yields in these reactions are poorer. For instance, 1-adamantanol is oxidized to 1,3-adamantane-diol in 43% yield. Secondary alcohols are converted to the corresponding ketone. This method has been employed for the hydroxylation of tertiary positions in saturated acetates and bromides. 

Answer In this problem, we are starting with an alkane. There are no leaving groups, so we cannot do a substitution or an elimination reaction. There are also no double bonds, so we cannot do an addition. It seems that we are stuck, with nothing to do. Clearly, our only way out of this situation is to introduce a functional group into the compound, via radical bromination. Radical bromination will place a Br at the most substituted position (the tertiary position), and then we can eliminate ... 

There are a few things to keep in mind when using this technique. First of all, radical bromination will selectively place a Br on the most substituted position. Therefore, you should always look for the tertiary position to see where the Br will go. Then, when doing the elimination, make sure to choose the base carefully, in order to achieve the desired regiochemistry. Let s get some practice with this. 

The latter method is improved by use of the 2,2-dimethyl derivative.273 The rearrangement is faster and the stability of the orthoester to hydrolysis is better. Isotopic labeling showed that the rearrangement occurs by ionization at the tertiary position. 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

When the reactant provides more than one kind of hydrogen for insertion, the catalyst can influence selectivity. For example, Rh2(acam)4 gives exclusively insertion at a tertiary position, whereas Rh2(02CC4F9)4 leads to nearly a statistical mixture.217aThe attenuated reactivity of the amidate catalyst enhances selectivity. 

Carboalkoxynitrenes are somewhat more selective than the corresponding carbenes, showing selectivities of roughly 1 10 40 for the primary, secondary, and tertiary positions in 2-methylbutane in insertion reactions. 

The tertiary position of the isobutylsilane was only about 70% deuter-ated, and deuterium was on every carbon atom in the system, so the i-butyltrichlorosilane contained an average of 2.5 D s and 6.5 H s per mole. This gives a D/H ratio in the i-butyl group of 0.39. If D s and H s... 

Reductive y-lactone ring opening, with concomitant desilylation at the tertiary position by LiAlH4, gave triol 17 in 80% yield. Finally, acetonide formation followed by oxidation with tetra-n-propylammonium perruthenate/A-methylmorpholine / /-oxide oxidation, led to the target aldehyde 19 in 80% overall yield. 

Nucleophilic displacement using [ F] fluoride works well in aUphatic systems where reactive haUdes or sulfonates esters can undergo substitution at unhindered sites. In order to introduce a F fluorine atom in a secondary or tertiary position, a two steps strategy was developed. It involves a F-bromofluorination of alkenes, followed by reductive debromination (n-BujSnH, AIBN). [ F]BrF is usually generated in situ from [ F]potassium fluoride and l,3-dibromo-5,5-dimethylhydantoin (DBH) in sulfuric acid. This methodology was successfully applied to label steroids at the 11 and 6a positions [245] (Scheme 60) and to prepare [ F]fluorocyclohexanes [246]. 

The mechanism most consistent with all the data is an ionic acid opening of the epoxide —apparently where the hydrocarbonyl is used as an acid to attack the epoxide— which is more sensitive to steric effects than to electronic factors. This conclusion may at first appear to be inconsistent with our previous finding that isobutylene reacted with cobalt hydrocarbonyl to give exclusively addition of the cobalt to the tertiary position. The inhibitory effect of carbon monoxide on that reaction, however, indicated that it was probably cobalt hydrotricarbonyl that was actually adding to the olefin and steric effects would be expected to be much less important with the tricarbonyl than with the tetracarbonyl (7) Apparently he feels now that the former reactions really involve the tricarbonyl, loss of CO being important to get the reaction running whereas epoxide attack perhaps involves a tetracarbonyl, steric factors are more important here. 

TABLE 14.1 Relative susceptibility to attack by Cl of primary, secondary, and tertiary positions at 100 and 600°C in the gas phase ... 

When a double bond has two different allylic positions, e.g., CHiCH=CHCHiCH , a secondary position is substituted more readily than a primary. The relative reactivity of tertiary hydrogen is not clear, though many substitutions at allylic tertiary positions have been performed.11B It is possible to brominate both sides of the double bond.119 Because of the electron-withdrawing nature of bromine, the second bromine substitutes on the other side of the double bond rather than a to the first bromine. 

This electrophilic substitution is unique among chemical transformations, in the sense that a reaction takes place on a completely deactivated site difficult to access by other reactions. With fluorine as a reagent, decalins, bicyclohexyls, alkylcycloalkane derivatives 6-8 and many more can be fluorinated at their tertiary position and subsequently dehydrofluorinated.37... 

Amination of hydrocarbons in allylic, benzylic, or tertiary position can be mediated... 

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