17.3 REACTIONS INVOLVING ALLYLIC AND BENZYLIC ANIONS

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1 798 HAPTER 17 ALLYLI AND BENZYLI REATIVITY Because the unpaired electron is shared by two different carbons, this radical can react in the final propagation step to give two different products. Reaction of at the radical site shown in structure A gives product (1), and reaction at the radical site shown in structure B gives product (): H A H L H L H H H LH A H LH H (1) () Notice that product (1) is chiral, and product () can exist as both cis and trans stereoisomers. Hence, bromination of 1-hexene gives racemic (1) as well as cis- and trans-(), although the trans isomer should predominate because of its greater stability What product(s) are expected when each of the following compounds reacts with one equivalent of NBS in l 4 in the presence of light and peroxides? Explain your answers. (a) cyclohexene (b) 3,3-dimethylcyclohexene (c) trans--pentene (d) 4-tert-butyltoluene (e) 1-isopropyl-4-nitrobenzene 17.3 REATIONS INVOLVING ALLYLI AND BENZYLI ANIONS The prototype for allylic anions is the allyl anion, and the simplest benzylic anion is the benzyl anion. H A H LH H LH A H allyl anion (17.18) cl H na H 3 xa H /A H 1 benzyl anion (17.19) Further Exploration 17. Polar Effects of Double Bonds Allylic and benzylic anions are about 59 kj mol 1 (14 kcal mol 1 ) more stable than their nonallylic and nonbenzylic counterparts. There are two reasons for the stabilities of these anions. The first is resonance stabilization, as indicated by the preceding resonance structures. The second reason is the polar effect (Sec. 3.6) of the double bond (in the allyl anion) or the phenyl ring (in the benzyl anion). The polar effect of both groups stabilizes anions. (Opinions differ about the relative importance of resonance and polar effects.)

2 17.3 REATIONS INVOLVING ALLYLI AND BENZYLI ANIONS 799 The enhanced stability of allylic and benzylic anions is reflected in the pk a values of propene and toluene (B3 = a base): H + B 3 A H L H L H H A H L H + B L H propene pk a 43 (17.0) i H LH H + B3 i + B L H (17.1) toluene pk a 41 Although these compounds are very weak acids, their acidities are much greater than the acidities of alkanes that do not contain allylic or benzylic hydrogens. Recall from Sec. 14.7A that ordinary alkanes have pk a values in the range of Free benzylic or allylic carbanions are rarely involved as reactive intermediates. However, a number of reactions involve species that have carbanion character. Two of these are the reactions of Grignard and related organometallic reagents, and E eliminations. The following sections show how these reactions are affected when carbanion character occurs at benzylic or allylic positions. A. Allylic Grignard Reagents Recall that Grignard reagents have many of the properties expected of carbanions (Sec. 8.8B). Thus, allylic Grignard reagents resemble allylic carbanions. H A H L H L resembles H A H L H (17.) Allylic Grignard reagents undergo a rapid equilibration in which the L group moves back and forth between the two partially negative carbons at a rate of about 1000 times per second. H# H H H H H very fast ( d+ d+ (17.3) The transition state for this reaction can be envisioned as an ion pair consisting of an allylic carbanion and a cation. H H H allylic carbanion Because the allylic carbanion is resonance-stabilized, this transition state has relatively low energy, and consequently, the equilibration occurs rapidly.

3 800 HAPTER 17 ALLYLI AND BENZYLI REATIVITY The equilibration in Eq is an example of an allylic rearrangement. An allylic rearrangement involves the simultaneous movement of a group G and a double bond so that one allylic isomer is converted into another. ( G G ( G = any group (17.4) Notice that these two structures are not resonance structures; they are two distinct species in rapid equilibrium. The rapid allylic rearrangement of an unsymmetrical Grignard reagent, such as the one shown in Eq. 17.3, means that the reagent is actually a mixture of two different reagents. This has two consequences. First, the same mixture of reagents is obtained from either of two allylically related alkyl halides: A H LH L LH A H Mg Mg AH LH fast LH AH (17.5) Second, when the Grignard reagents undergo a subsequent reaction, a mixture of products is usually obtained, and the same mixture of products is obtained regardless of the alkyl halide used to form the Grignard reagent. For example, protonolysis of the mixture of equilibrating Grignard reagents in Eq gives the following result: fast A H LH LH A H H O AH L + LH AH + HO L H (17.6) mixture of products

4 17.3 REATIONS INVOLVING ALLYLI AND BENZYLI ANIONS What product(s) are formed when a Grignard reagent prepared from each of the following alkyl halides is treated with D O? (a) H (b) y 1-(bromomethyl)cyclohexene q 6-bromo-1-methylcyclohexene B. E Eliminations Involving Allylic or Benzylic Hydrogens Recall that the S N (bimolecular substitution) and E (bimolecular elimination) reactions of alkyl halides are competing reactions, and that the structure of the alkyl halide is one of the major factors that determine which reaction is the dominant one (Sec. 9.5G). A structural effect in the alkyl halide that tends to promote a greater fraction of elimination is enhanced acidity of the b-hydrogens. It is found that a greater ratio of elimination to substitution is observed when the b-hydrogens of the alkyl halide have higher than normal acidity. Such a situation can occur when the b-hydrogens are allylic or benzylic. (Recall from the introduction to this section that allylic and benzylic hydrogens are more acidic than ordinary alkyl hydrogens.) For example, the E reaction of the alkyl bromide in Eq is more than 100 times faster than the E reaction of isopentyl bromide [( ) HH H ], a comparably branched alkyl halide. benzylic hydrogens Na H 5 O H 5 OH L H H L L H A H + L H H L O H 5 (17.7) (95 elimination) (5 substitution) (Elimination predominates because the E reaction is particularly fast; the S N component of the competition occurs at a normal rate.) Why should an acidic b-hydrogen increase the rate of an E reaction? In the transition state of the E reaction, the base is removing a b-proton, and the transition state of the reaction has carbanion character at the b-carbon atom. H 5 O double-bond character H cl H carbanion character This partially formed carbanion is stabilized in the same way that a fully formed carbanion is; a more stable transition state results in a faster reaction. Another reason that benzylic E reactions are faster is that the alkene double bond, which is partially formed in the transition state, is conjugated with the benzene ring; recall that conjugated double bonds are more stable than unconjugated double bonds (Sec. 15.1A). H transition state for E reaction

5 80 HAPTER 17 ALLYLI AND BENZYLI REATIVITY Let s summarize the structural characteristics of alkyl halides or sulfonate esters that favor E reactions over S N reactions. Elimination reactions are favored by: 1. branching at the a-carbon (Sec. 9.5G). branching at the b-carbon (Sec. 9.5G) 3. greater acidity of the b-hydrogens (this section) 17.8 Predict the major product that is obtained when each of the following alkyl halides is treated with potassium tert-butoxide. Explain your reasoning. (a) L (b) O L H L H I 17.4 ALLYLI AND BENZYLI S N REATIONS S N reactions of allylic and benzylic halides are relatively fast even though they do not involve reactive intermediates. The following data for allyl chloride are typical: relative rate H A H LH L l + I 50 H A H LH L I + l 73 (17.8a) 50 H H l I L L + H H I l L L + 1 (17.8b) An even greater acceleration is observed for benzylic halides. relative rate cl H L l + I 60 cl H L I + l 100,000 (17.9a) $ 60 $ H L H L l + I L H L H I + l 1 ) ) (17.9b) Allylic and benzylic S N reactions are accelerated because the energies of their transition states are reduced by p-orbital overlap, shown in Fig. 17., for an allylic S N reaction. In the transition state of the S N reaction, the carbon at which substitution occurs is sp -hybridized (Fig. 9., p. 390); the incoming nucleophile and the departing leaving group are partially bonded to a p orbital on this carbon. Overlap of this p orbital with the p orbitals of an adjacent double bond or phenyl ring provides additional bonding that lowers the energy of the transition state and accelerates the reaction.