Carbon nucleophiles, carbocation reactivity addition reactions

Nucleophilic substitutions of tertiary substrates follow more a complicated mechanism that consists of two steps. The first step is the dissociation of the bond between the carbon atom and the leaving group. The result of this dissociation is the formation of short-lived reactive intermediate called carbocation. In this book we have already discussed the carbocations in the section about addition reactions. We have mentioned that the most stable are the tertiary carbocations, in which the positive carbon is bound to three neighboring C-atoms. In substrates such as 1-bromopropane in the last example, only the C-Br bond can be cleaved to form a primary carbocation. Since such a primary carbocation is highly unstable, the reaction proceeds via the concerted Sn2 mechanism in which the primary carbocation does not appear. 

Scheme 2.2.9 demonstrates these different options for a Ce-carbocation that carries its positive charge at carbon number 3 (C3). While the reaction with the nucleophile HO leads to 3-hexanol, abstraction of a proton will produce 3-hexene. Addition of ethylene or any alkene would result in an addition reaction forming a new, very reactive carbocation. As a consequence, cationic polymerization would... 

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. 

In addition to steric effects, there are other important substituent effects that influence both the rate and mechanism of nucleophilic substitution reactions. As we discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in these systems than in alkyl systems. Direct displacement reactions also take place particularly rapidly in benzylic and allylic systems for example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone." These enhanced rates reflect stabilization of the Sjv2 TS through overlap of the /2-type orbital that develops at carbon." The tt systems of the allylic and benzylic groups provide extended conjugation. This conjugation can stabilize the TS, whether the substitution site has carbocation character and is electron poor or is electron rich as a result of a concerted Sjv2 mechanism. 

The carbocations you met in this chapter are reactive intermediates not only in S l substitutions but in other reactions too. One of the most convincing pieces of evidence for their formation is that they undergo reactions other than simple addition to nucleophiles. For example, the carbon skeleton of the cation may rearrange, as we will discuss in Chapter 36. 

Kinetic and product structure studies of the reaction of cyclopropanes and bicyclic compounds containing three-membered rings have shown that the protonation of the cyclopropane ring is followed by addition of the nucleophile at the most substituted carbon. The product composition is determined by the ability of the more highly substituted carbons to sustain more of the positive charge. The substitution at the incipient carbocation is the most important factor in determining the degree of reactivity. The relative rates of cyclopropane and its methyl, 1,1-... 

It is clear that atoms other than hydrogen can be electron deficient and function as electron pair acceptors. Can a carbon atom function as a Lewis acid The answer is yes, if the definition is modified somewhat. Various reactions generate carbocation intermediates (see 55 and 58) and a Lewis base can certainly donate electrons to that positive carbon. A species that donates electrons to carbon is called a nucleophile (see Section 6.7), so an electron donor that reacts with 55 or with 58 is a nucleophile. In addition to carbocations, which are charged species, the carbon atom in a polarized bond is electron deficient, and a nucleophile could donate electrons to the 6+ carbon. This is the basis of many organic reactions to be discussed, particularly in Chapter 11. The fundamental concept of a species donating electrons to a carbon is introduced in this section, with the goal of relating this chemical reactivity to the Lewis acid-Lewis base definitions used in previous sections. 

---