Carbocations alkene

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

The increased reactivity of the silyl- and stannyl-substituted alkenes enhances the synthetic utility of carbocation-alkene reactions. 

The acylation of alkenes with trifluoroacetic anhydride having only weak elec-trophilicity was reported to give a,p-unsaturated ketones in 19-49% yields.117 118 The reaction requires the use of the Me2S—BF3 complex as catalyst and takes place only with alkenes that readily form stable carbocations. Alkenes were also electrolyzed in CH2C12 solution of Et4NCl as the electrolyte at —5 to — 10°C in the... 

The first demonstration of a carbocation-alkene cyclization route to the lanesterol series has been described by Corey et al. <94TL9149> (Equation (7)). The failure of the analogue of (1) lacking the 7a-silyl substituent to cyclize underscores the crucial role of silyl assistance in alkene-oxirane cyclizations. 

Cyclopropanoxyl radicals have also been generated by treatment of cyclopropanols 38 with manganese(III) pyridine-2-carboxylate [Mn(pic)j]. The 3-oxopropyl radicals, formed by rapid -scission, added to a variety of alkenes before a second oxidation transformed them to the corresponding adduct carbocations. Alkenes containing trialkylsilyl ether groups were particularly useful because loss of the trialkylsilyl cation led to efficient formation of 1,5-diones... 

The enhanced reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions. Silyl enol ethers also show enhanced reactivity. Electrophilic attack is followed by desilylation to give an a-substituted carbonyl compound. The carbocations can be generated from tertiary chlorides and a Lewis acid, such as TiCU. This reaction provides a method for introducing tertiary alkyl groups alpha to a carbonyl. This transformation cannot be achieved by base-catalyzed alkylation because of the strong tendency for tertiary halides to undergo elimination. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Step 3 IS new to us It is an acid-base reachon m which the carbocation acts as a Br0n sted acid transferrmg a proton to a Brpnsted base (water) This is the property of carbo cations that is of the most significance to elimination reactions Carbocations are strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation... 

Write a structural formula for the carbocation intermediate formed in the dehydration of each of the alcohols in Problem 5 14 (Section 5 10) Using curved arrows show how each carbocation is deprotonated by water to give a mixture of alkenes... 

Water may remove a proton from either Cl or C 3 of this carbocation Loss of a proton from C 1 yields the minor product 2 3 dimethyl 1 butene (This alkene has a disubstituted double bond )... 

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... 

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

A mechanism for the formation of these three alkenes is shown m Figure 5 9 Dissociation of the primary alkyloxonmm ion is accompanied by a shift of hydride from C 2 to C 1 This avoids the formation of a primary carbocation leading instead to a sec ondary carbocation m which the positive charge is at C 2 Deprotonation of this carbo cation yields the observed products (Some 1 butene may also arise directly from the pri mary alkyloxonium ion)... 

Step (2) Ethanol acts as a base to remove a proton from the carbocation to give the alkene products (Deprotonation step)... 

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... 

We can gam a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier In Section 5 12 we pointed out that carbocations are the conjugate acids of alkenes Therefore strong acids such as HCI HBr and HI can protonate the double bond of an alkene to form a carbocation... 

Figure 6 4 shows the complementary nature of the electrostatic potentials of an alkene and a hydrogen halide We also know (from Section 4 8) that carbocations when gen erated m the presence of halide anions react with them to form alkyl halides... 

Let s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCH=CH2 (a) according to Markovnikov s rule and (b) opposite to Markovnikov s rule (a) Addition according to Markovnikov s rule... 

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes... 

CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES... 

Our belief that carbocations are intermediates m the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements sometimes occur For example the reaction of hydrogen chloride with 3 methyl 1 butene is expected to produce 2 chloro 3 methylbutane Instead a mixture of 2 chloro 3 methylbutane and 2 chloro 2 methylbutane results... 

The notion that carbocation formation is rate determining follows from our previous experience and by observing how the reaction rate is affected by the shucture of the aUcene Table 6 2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations Protonation of ethylene the least reactive aUcene m the table yields a primary carbocation protonation of 2 methylpropene the most reactive m the table yields a tertiary carbocation As we have seen on other occa sions the more stable the carbocation the faster is its rate of formation... 

Carbocation intermediates are not involved m hydroboration-oxidation Hydration of double bonds takes place without rearrangement even m alkenes as highly branched as the following... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

Step 2 The carbocation acts as an electrophile toward the alkene A carbon-carbon bond is formed resulting in a new carbocation—one that has eight carbons... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

Addition of water to the double bond of an alkene takes place in aqueous acid Addition occurs according to Mar kovnikov s rule A carbocation is an in termediate and is captured by a mole cule of water acting as a nucleophile... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

Not all the properties of alkenes are revealed by focusing exclusively on the func tional group behavior of the double bond A double bond can affect the proper ties of a second functional unit to which it is directly attached It can be a sub stituent for example on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene... 

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