17.5 ALLYLIC AND BENZYLIC OXIDATION

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1 17.5 ALLYLI AND BENZYLI XIDATIN 803 Nuc d d Nuc d overlap of 2p orbitals X d no p-orbital overlap X d (a) (b) Figure 17.2 Transition states for N 2 reactions at (a) an allylic carbon and (b) a nonallylic carbon. Nuc and X are the nucleophile and leaving group, respectively.the allylic substitution is faster because the transition state is stabilized by overlap (blue lines) of the 2p orbital at the site of substitution with the adjacent p bond. PBLEM 17.9 Explain how and why the product(s) would differ in the following reactions of trans-2- buten-1-ol. (1) eaction with concentrated aqueous Br (2) onversion into the tosylate, then reaction with NaBr in acetone 17.5 ALLYLI AND BENZYLI XIDATIN A. xidation of Allylic and Benzylic Alcohols Allylic and benzylic alcohols are oxidized selectively by a suspension of activated manganese(iv) dioxide, 2. Primary allylic alcohols are oxidized to aldehydes and secondary allylic alcohols are oxidized to ketones. 3 (4-methoxyphenyl)methanol (p-methoxybenzyl alcohol) () 2 (insoluble) 2 l 2 4-methoxybenzaldehyde (81 yield) (17.30) l (insoluble) () 2 (17.31) (E)-2-penten-1-ol (E)-2-pentenal (83 yield)

2 804 APTE 17 ALLYLI AND BENZYLI EATIVITY Activated 2 is obtained by the oxidation reduction reaction of potassium permanganate, K 4, with an 2 salt such as 4 under either alkaline or acidic conditions followed by thorough drying K K 2 4 manganese dioxide (17.32) Allylic and benzylic oxidation of alcohols takes place on the surface of the 2, which is insoluble in the solvents used for the reaction. Water competes with alcohol for the sites on the 2 and thus must be removed by drying to produce an active oxidant. The selectivity of 2 oxidation for allylic and benzylic alcohols is illustrated by the following example. the benzylic alcohol is selectively oxidized acetone (17.33) 3,4-dimethoxyphenyl-1,3-propanediol 3,4-dimethoxyphenyl-3-hydroxy-1-propanone (94 yield) (This example illustrates the selectivity for benzylic alcohols; a corresponding selectivity is observed for allylic alcohols.) The reaction is selective because allylic and benzylic alcohols react much more rapidly than ordinary alcohols. An understanding of this selectivity comes from the mechanism. In the first step of the mechanism, the group of the alcohol rapidly adds to 2 to give an ester (ec. 10.3). 2 + (in solution) 2 (on the 2 surface) (17.34a) (The solid-state structures of the -containing species are simplified in these equations.) In the next step, which is rate-limiting, (IV) accepts an electron to become (III), and, at the same time, a hydrogen atom is transferred from the allylic or benzylic carbon to an oxygen of the oxidant. The product has an unpaired electron on the allylic or benzylic carbon and is therefore a resonance-stabilized radical. (IV) (III).. an allylic or benzylic radical is resonance-stabilized (17.34b)

3 17.5 ALLYLI AND BENZYLI XIDATIN 805 The stability of the radical intermediate, by ammond s postulate (ec. 4.8), increases the rate of this step. The allylic/benzylic selectivity occurs because the analogous radical intermediate in the oxidation of an alcohol that is not allylic or benzylic is less stable and is formed more slowly. In the rapid final step, (III) is reduced to the more stable (II), and a strong A double bond is formed to give the aldehyde product, which is washed away from the oxidant surface by the solvent. (III) (II).. + (dissolves in the solvent) (17.34c) PBLEM Give the structure of the product expected when each of the following alcohols is subjected to 2 oxidation. (a) (b) 2 2 N 3 (c) (d) In each case, give the structure of a starting material that would give the product shown by 2 oxidation. (a) (b) (c) In each case, tell whether oxidation with pyridinium chlorochromate (P) and oxidation with 2 would give the same product, different products, or no reaction. Explain. (a) (b) (c) (d) B. Benzylic xidation of Alkylbenzenes Treatment of alkylbenzene derivatives with strong oxidizing agents under vigorous conditions converts the alkyl side chain into a carboxylic acid group. xidants commonly used for this purpose are r(vi) derivatives, such as Na 2 r 2 7 (sodium dichromate) or r 3 ; the (VII)

4 806 APTE 17 ALLYLI AND BENZYLI EATIVITY reagent K 4 (potassium permanganate); or 2 and special catalysts, a procedure that is used industrially (Eq , p. 778). l v $ L 3 K 4 100, 3 4 h 2 l v $ L 2 (17.35) o-chlorotoluene 2-chlorobenzoic acid (77 yield) cl r 3, h, 100 cl 2 (17.36) propylbenzene benzoic acid (55 yield) TUDY GUIDE LINK 17.1 ynthetic Equivalence Notice that the benzene ring is left intact, and notice from Eq that the alkyl side chain, regardless of length, is converted into a carboxylic acid group. This reaction is useful for the preparation of some carboxylic acids from alkylbenzenes. xidation of alkyl side chains requires the presence of a benzylic hydrogen. onsequently, tert-butylbenzene, which has no benzylic hydrogen, is resistant to benzylic oxidation. Although we won t consider the mechanisms of these benzylic oxidations, they occur in many cases because resonance-stabilized benzylic intermediates such as benzylic radicals are involved. The conditions for this side-chain oxidation are generally vigorous: heat, high concentrations of oxidant, and/or long reaction times. It is also possible to effect less extensive oxidations of side-chain groups. Thus 1-phenylethanol is readily oxidized to acetophenone under milder conditions the normal oxidation of secondary alcohols to ketones (ec. 10.6A) but it is converted into benzoic acid under more vigorous conditions. r(vi) vigorous cl L cl L 3 benzoic acid r(vi) vigorous (17.37) 1-phenylethanol r(vi) mild cl L 3 acetophenone You do not need to be concerned with learning the exact conditions for these reactions; rather, it is important simply to be aware that it is usually possible to find appropriate conditions for each type of oxidation. (ee tudy Guide Link 16.3.) PBLEM Give the products of vigorous K 4 oxidation of each of the following compounds. (a) p-nitrobenzyl alcohol (b) 1-butyl-4-tert-butylbenzene

5 17.6 TEPENE (a) A compound A has the formula After vigorous oxidation, it yields phthalic acid. What is the structure of A? 2 i 2 phthalic acid (b) A compound B has the formula After vigorous oxidation, it yields benzoic acid (structure in Eq ). What is the structure of B? 17.6 TEPENE Further Exploration 17.3 Essential ils A. The Isoprene ule People have long been fascinated with the pleasant-smelling substances found in plants for example, the perfume of a rose and have been curious to learn more about these materials, which have come to be called essential oils. An essential oil is a substance that possesses a key characteristic, such as an odor or flavor, of the natural material from which it comes. (ee Further Exploration 17.3.) Essential oils, particularly oil of turpentine, were known to the ancient Egyptians. owever, not until early in the nineteenth century was an effort made to determine the chemical constitution of the essential oils. In 1818, it was found that the : ratio in oil of turpentine was 5:8. This same ratio was subsequently found for a wide variety of natural products. These related natural products became known collectively as terpenes, a name coined by August Kekulé (p. 47). The similarity in the atomic compositions of the many terpenes led to the idea that they might possess some unifying structural element. In 1887, the German chemist tto Wallach ( ), who received the 1910 Nobel Prize in hemistry, pointed out the common structural feature of the terpenes: they all consist of repeating units that have the same carbon skeleton as the five-carbon diene. This generalization subsequently became known as the rule. 2 # # 2 3 carbon-4 the carbon skeleton of carbon-1 (17.38) For example, citronellol (from oil of roses and other sources) incorporates two units: citronellol carbon skeleton carbon skeleton