Alkylation carbocations

These reactions provide examples of intermolecular carbocation alkylations. Despite the feasibility of this type of reaction, the requirements for good yields are stringent and the number of its synthetic applications is limited. 

In understanding these reactions, it is helpful to view the metal-alkene tt complex as an incipient carbocation (just as tt complexes of halogens are incipient carbocations). Alkyl and hydride shifts then bear analogy to carbocation rearrangements. This may be an oversimplification but it makes the chemistry easier to follow. 

A second terr-butyl carbocation alkylation forms BHT. 

Even with reactions of a non-radical active centre, the generated polymer is not always inert. Carbanions react with —C=N and —COOR sub-stitutents, carboxonium ions produce less acid centres by reaction with an ether-type chain (see Chap. 4, Sect. 2.3), carbocations alkylate aromatic groups, etc. All these reactions affect propagation. Sometimes the physical effect of the generated insoluble polymer is combined with its ability to react chemically in a certain way. 

The addition of H2 to a reaction is called hydrogenation. The heat of hydrogenation is the heat released in a hydrogenation reaction. The greater the stability of a compound, the lower is its energy and the smaller is its heat of hydrogenation. The more alkyl substituents bonded to the sp carbons of an alkene, the greater is its stability. Hence, carbocations, alkyl radicals, and alkenes are all stabilized by alkyl substituents. Trans alkenes are more stable than cis alkenes because of steric strain. 

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

Finding snch acids (called snperacids ) turned out to be the key to obtaining stable, long-lived alkyl cations and, in general, carbocations. If any deprotonation were still to take place, the formed alkyl cation (a strong Lewis acid) would immediately react with the formed olefin (a good TT-base), leading to the mentioned complex reactions. 

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

Numerous other studies have shown that alkyl groups directly attached to the pos itively charged carbon stabilize a carbocation Figure 4 13 illustrates this generalization for CH3+ CH3CH2 " (CH3)2CH+ and (CH3)3C" Among this group CH3+ is the least stable and (CH3)3C the most stable... 

FIGURE 4 13 The order of carbocation stability is methyl < primary < second ary < tertiary Alkyl groups that are directly attached to the positively charged car bon stabilize carbocations... 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Free radicals like carbocations have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that release electrons Consequently the order of free radical stability parallels that of carbocations... 

Carbocations are stabilized by alkyl substituents attached directly to the positively charged carbon Alkyl groups are electron releasing sub stituents Stability increases in the order... 

The rate at which alcohols are converted to alkyl halides depends on the rate of carbocation formation tertiary alcohols are most reactive primary alcohols and methanol are least reactive... 

In Chapter 4 you learned that carbocations could be captured by halide anions to give alkyl halides In the present chapter a second type of carbocation reaction has been introduced—a carbocation can lose a proton to form an alkene In the next section a third aspect of carbocation behavior will be described the rearrangement of one carbo cation to another... 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

There is a strong similarity between the mechanism shown m Eigure 5 12 and the one shown for alcohol dehydration m Eigure 5 6 The mam difference between the dehy dration of 2 methyl 2 butanol and the dehydrohalogenation of 2 bromo 2 methylbutane IS the source of the carbocation When the alcohol is the substrate it is the correspond mg alkyloxonmm ion that dissociates to form the carbocation The alkyl halide ionizes directly to the carbocation... 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

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