Resonance-stabilized carbon nucleophiles

The Mannich reaction is the prototype of carbon-carbon bond forming reactions that involve the addition of resonance-stabilized carbon nucleophiles to iminium salts and imines. In its original and most widely recognized form, the Mannich reaction consists of three components (i) ammonia, a primary amine, or a secondary amine (ii) a nonenolizable aldehyde, usually formaldehyde and (iii) an active... 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

Protonation of the enolate ion is chiefly at the oxygen, which is more negative than the carbon, but this produces the enol, which tautomerizes. So, although the net result of the reaction is addition to a carbon-carbon double bond, the mechanism is 1,4 nucleophilic addition to the C=C—C=0 (or similar) system and is thus very similar to the mechanism of addition to carbon-oxygen double and similar bonds (see Chapter 16). When Z is CN or a C=0 group, it is also possible for Y to attack at this carbon, and this reaction sometimes competes. When it happens, it is called 1,2 addition. 1,4 Addition to these substrates is also known as conjugate addition. The Y ion almost never attacks at the 3 position, since the resulting carbanion would have no resonance stabilization " ... 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

The results of ab initio calculations provide evidence that Me2NC(S)-[14+] is stabilized by resonance electron donation from the a-thioamide group (A, Scheme 12) and by covalent bridging of sulfur to the benzylic carbon (B, Scheme 12).96 Direct resonance stabilization of the carbocation will increase the barrier to the nucleophile addition reaction, because of the requirement for the relatively large fractional loss of the stabilizing resonance interaction (A, Scheme 12) at the transition state for nucleophile addition to a-substituted benzyl carbocations.8,13,28 91-93 If the solvent adds exclusively to an open carbocation that is the minor species in a mixture of open and closed ions, then... 

Besides the allylation reactions, imines can also undergo enol silyl ether addition as with carbonyl compounds. Carbon-carbon bond formation involving the addition of resonance-stabilized nucleophiles such as enols and enolates or enol ethers to iminium salt or imine can be referred to as a Mannich reaction, and this is one of the most important classes of reactions in organic synthesis.104... 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

If the potential leaving group is attached to unsaturated carbon, as in vinyl chloride or phenyl chloride, attack by nucleophiles is also extremely difficult, and these compounds are very unreactive in Sn2 reactions compared with simple alkyl halides. In these cases, the reason is not so much steric but electrostatic, in that the nucleophile is repelled by the electrons of the unsaturated system. In addition, since the halide is attached to carbon through an 5p -hybridized bond, the electrons in the bond are considerably closer to carbon than in an 5/ -hybridized bond of an alkyl halide (see Section 2.6.2). Lastly, resonance stabilization in the halide gives some double bond character to the C-Hal bond. This effectively strengthens the bond and makes it harder to break. This lack of reactivity is also tme for SnI reactions (see Section 6.2). 

Although both Sn2 and SnI mechanisms might be formnlated for such reactions, all the available evidence favours an Sn 1 process. This is rationalized in terms of formation of a favourable resonance-stabilized allylic cation by loss of the leaving gronp. In the majority of natnral prodnct structures, the nucleophile has attacked the allylic system on the same carbon that loses the diphosphate, bnt there are certainly examples of nncleophilic attack on the alternative tertiary carbon. 

The possibility of nucleophilic attack on different carbons in the resonance-stabilized carbocation facilitates another modification exploited by nature during terpenoid metabolism. This is a change in double-bond stereochemistry in the allylic system. The interconversions of geranyl diphosphate, linalyl diphosphate, and neryl diphosphate provide neat but satisfying examples of the chemistry of simple allylic carbocations. 

It system, thus creating resonance stabilization in the neutral amide. This effect also diminishes the reactivity of the carbonyl towards nucleophilic attack, since the resonance contribution actually means less carbonyl character and more carbon-nitrogen double bond character. 

Mechanism. Removal of an a-hydrogen from the acetaldehyde by NaOH produces a resonance-stabilized enolate anion. Nucleophilic addition of the enolate to the carbonyl carbon of another acetaldehyde gives an alkoxide tetrahedral intermediate. The resulting alkoxide is protonated by the solvent, water, to give 3-hydroxybutanal and regenerate the hydroxide ion. 

In the first step of the conversion catalyzed by pyruvate decarboxylase, a carbon atom from thiamine pyrophosphate adds to the carbonyl carbon of pyruvate. Decarboxylation produces the key reactive intermediate, hydroxyethyl thiamine pyrophosphate (HETPP). As shown in figure 13.5, the ionized ylid form of HETPP is resonance-stabilized by the existence of a form without charge separation. The next enzyme, dihydrolipoyltransacetylase, catalyzes the transfer of the two-carbon moiety to lipoic acid. A nucleophilic attack by HETPP on the sulfur atom attached to carbon 8 of oxidized lipoic acid displaces the electrons of the disulfide bond to the sulfur atom attached to carbon 6. The sulfur then picks up a proton from the environment as shown in figure 13.5. This simple displacement reaction is also an oxidation-reduction reaction, in which the attacking carbon atom is oxidized from the aldehyde level in HETPP to the carboxyl level in the lipoic acid derivative. The oxidized (disulfide) form of lipoic acid is converted to the reduced (mer-capto) form. The fact that the two-carbon moiety has become an acyl group is shown more clearly after dissocia-... 

When stabilized (and consequently less reactive) anions are employed as the nucleophile, more reactive electrophiles are needed for successful carbon-carbon bond formation. Nitronate anions, which are highly resonance stabilized, fail to react widi simple alkyl hahde electrophiles. On the other hand, /3-dicarbonyl compounds react effectively with primary and some secondary alkyl bromides and iodides to give monoalkylated products. 

Finally, Table 8.1 lists three primary alkyl chlorides—allyl chloride, benzyl chloride, and chloroacetone—that react considerably faster than other primary alkyl chlorides. This increase in reaction rate is due to resonance stabilization of the transition state. Each of these compounds has a pi bond adjacent to the reactive site and forms a transition state that is conjugated. The p orbital that develops on the electrophilic carbon in the transition state overlaps with the p orbital of the adjacent pi bond. The stabilization due to the conjugated transition state results in a significantly faster reaction. The transition state for the reaction of allyl chloride with a nucleophile is shown as follows ... 

Because of its resonance stabilization, the secondary benzylic carbocation is more stable than the secondary carbocation, so the product has the bromine (nucleophile) bonded to the carbon attached to the phenyl group. 

Rather than add as a nucleophile to the positive carbon, a base removes a proton from the arenium ion to regenerate the aromatic ring with its large resonance stabilization. 

---