Alkylation, enolate ions carbocations

The mechanism of these reactions is usually Sn2 with inversion taking place at a chiral RX, though there is strong evidence that an SET mechanism is involved in certain cases, ° especially where the nucleophile is an a-nitro carbanion and/or the substrate contains a nitro or cyano group. Tertiary alkyl groups can be introduced by an SnI mechanism if the ZCH2Z compound (not the enolate ion) is treated with a tertiary carbocation generated in situ from an alcohol or alkyl halide and BF3 or AlCla, or with a tertiary alkyl perchlorate. ... 

A rare species of salts consisting of a heteroatom-stabilized carbocation and a heteroatom-stabibzed carbanion has been formed by deprotonating methyl (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoate with tetrakis(dimethylamino)methane the resonance stabilization of the cation [(CH3)2N]3C+ and enolate anion, which is of E-configuration exclusively, since the guanadinium ion is incapable of forming a chelate, prevents a spontaneous O- or C-alkylation.12... 

You may protest that these last two species are not carfoo-cations at all but rather sulfonium ions, and you would be right. However, they can be used in place of carbocations as they are electrophilic at carbon so it is useful to think of them as modified carbocations as well as sulfonium ions. Sulfur-stabilized a-cations are easily made from a-chlorosulfides and are useful in alkylation of silyl enol ethers. 

SOLUTION TO 15a The reaction takes place by an 8 2 pathway because neither alkyl group will form a relatively stable carbocation. Iodide ion attacks the carbon of the ethyl group because the other possibility is to attack an sp hybridized carbon, and sp carbons are generally not attacked by nucleophiles (Section 10.8). Thus, the products are ethyl iodide and an enol that immediately rearranges to an aldehyde (Section 6.6). 

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