11.5 PREPARATION AND OXIDATIVE CLEAVAGE OF GLYCOLS

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1 11.5 PEPAATIN AND XIDATIVE EAVAGE F GY 50 These reactions also provide methods for the formation of carbon carbon bonds. eactions that form carbon carbon bonds are especially important in organic chemistry because they can be used to lengthen carbon chains. We ll explore this point further in ec PBEM (a From what Grignard reagent can -methyl-1-pentanol be prepared by reaction with ethylene oxide, then aqueous acid? (b From what epoxide and what higher-order cuprate reagent can -ethyl--heptanol be prepared? (c Give the structure of another epoxide and another higher-order cuprate that could be used to prepare the alcohol in Eq omplete the following reactions by giving the structures of the alcohol products. In part (b, show the stereochemistry of the product as well. (a Mg bromocyclopentane ether (b 2 i un ether PEPAATIN AND XIDATIVE EAVAGE F GY Glycols are compounds that contain hydroxy groups on adjacent carbon atoms. general structure of a glycol ( = alkyl, aryl, or Example: 2 1,2-propanediol (propylene glycol Although glycols are alcohols, some glycol chemistry is quite different from the chemistry of alcohols. ome of this unique chemistry is the subject of this section. A. Preparation of Glycols You have already learned that some glycols can be prepared by the acid-catalyzed reaction of water with epoxides (Eq This is one of two important methods for the preparation of glycols. The other important method for the preparation of glycols is the oxidation of alkenes with s 4. 2 Na (or other reducing agent A 2 s 4 2 a glycol (90 95 yield reduced forms of s (11.46

2 504 APTE 11 TE EMITY F ETE, EPXIDE, GY, AND UFIDE The osmium in s 4 is in a 8 oxidation state. Metals in high oxidation states (such as Mn(VII and r(vi, as you ve learned are oxidizing agents because they attract electrons. This electron-attracting ability of s(viii results in a concerted (that is, one-step cycloaddition reaction between s 4 and an alkene to give an intermediate called an osmate ester: Further Exploration 11.1 Mechanism of s 4 Addition s(viii s 2 A 2 s accepts electrons s an osmate ester s(vi (11.47a (The osmate ester is another example of an organic ester derivative of an inorganic acid; ec The curved-arrow notation shows that in this reaction osmium accepts an electron pair. As a result, its oxidation state is decreased to 6. A glycol is formed when the cyclic osmate ester is treated with water. Two water molecules, acting as nucleophiles, displace the glycol oxygens from the osmium. A mild reducing agent such as sodium bisulfite, Na, is often added to convert the osmium-containing byproducts into reduced forms of osmium that are easy to remove by filtration. (The Na is converted into sodium sulfate, Na 2 4. s nucleophilic substitution at s by s 2 Na reduced forms of osmium (11.47b Two practical drawbacks to the use of the s 4 oxidation are that osmium and its compounds are very toxic, and they are quite expensive. owever, the reaction of s 4 with alkenes is so useful that chemists have devised ways for it to be used with very small amounts of s 4. This is done by including in the reaction mixture an oxidant that recycles the s(vi by-product back into s 4. Among the common oxidants used for this purpose are amine oxides, which are compounds of the form N. Two amine oxides used commonly are the following: ( N trimethylamine-n-oxide (TMA N N-methylmorpholine-N-oxide (NMM 1 1 In other words, once a small amount of s 4 is used up, the s(vi by-product is oxidized within the reaction mixture by the amine oxide to re-form s 4. Thus, a catalytic amount of s 4 can be used and the amine oxide acts as the ultimate oxidant. 2 A ( N 2,-dimethyl-2-butene (0.025 mole TMA (0.04 mole s 4 (10 4 mole water/tert-butyl alcohol pyridine ( N 2,-dimethyl-2,-butanediol (85 yield (11.48

3 11.5 PEPAATIN AND XIDATIVE EAVAGE F GY 505 The s 4 oxidation is particularly useful because of its stereochemistry. The formation of glycols from alkenes is a stereospecific syn-addition. 2 y 0 N s 4 (0. mole acetone/water 0 N (11.49 cyclohexene NMM cis-1,2-cyclohexanediol (89 yield The mechanism of this reaction provides a simple explanation for the syn stereochemistry. The five-membered osmate ester ring is easily formed when two oxygens of s 4 are added to the same face of the double bond by a concerted mechanism. This process closely resembles the concerted cycloaddition mechanism of ozonolysis. (ee Eq. 5.4, p ydrolysis of the osmate ester gives the glycol. s A syn-addition s 2 2 an osmate ester a 1,2-glycol (11.50 n the other hand, an anti-addition by a concerted mechanism would be very difficult, if not impossible: the two reacting oxygens of s 4 cannot simultaneously reach opposite faces of the p bond. The hydrolysis of epoxides and the s 4 oxidation are complementary reactions because they provide glycols of different stereochemistry. This point is explored in tudy Problem tudy Problem 11.4 utline preparations of cis-1,2-cyclohexanediol and ( -trans-1,2-cyclohexanediol from cyclohexene. olution As Eq shows, the direct oxidation of cyclohexene by s 4 yields cis-1,2- cyclohexanediol by a syn-addition. In contrast, conversion of cyclohexene into the epoxide with a peroxycarboxylic acid (see Problem 11.11a, followed by acid-catalyzed hydrolysis (Eq. 11.6, gives the trans-diol. Epoxide hydrolysis gives the trans-diol because it occurs with inversion of configuration. y (for example, mpba, 2 (occurs with inversion (11.51 cyclohexene cyclohexene oxide ( -trans-1,2-cyclohexanediol (emember the following convention: Although we draw a single enantiomer of the product for convenience, it should be understood to be the racemate; ec. 7.8A.

4 506 APTE 11 TE EMITY F ETE, EPXIDE, GY, AND UFIDE Glycol formation from alkenes can also be carried out with potassium permanganate (KMn 4, usually under aqueous alkaline conditions. This reaction is also a stereospecific syn-addition, and its mechanism is probably similar to that of s 4 addition. 2, KMn 4 Mn acetone 2 (purple (brown ppt solution (45 yield (11.52 Although the use of KMn 4 avoids the expense of s 4, a problem with the use of KMn 4 is that yields are low in many cases because over-oxidation occurs; that is, the glycol product is oxidized further. onditions have to be carefully worked out in each case to avoid this side reaction. The manganese in Mn 4 is in the 7 oxidation state. It is converted into Mn(IV as a result of the reaction. Visually, when oxidation occurs, the brilliant purple color of the permanganate ion is replaced by a murky brown precipitate of manganese dioxide (Mn 2. This color change can be used as a test for functional groups that can be oxidized by KMn 4. PBEM 11.2 What organic product is formed (including its stereochemistry when each of the following alkenes is treated with NMM in the presence of 2 and a catalytic amount of s 4? (a 1-methylcyclopentene (b trans-2-butene From what alkene could each of the following glycols be prepared by the s 4 or KMn 4 method? (a (b (c meso-4,5-octanediol V how a curved-arrow mechanism for the first step, and the structure of the cyclic intermediate formed, when an alkene is treated with KMn 4. A ewis structure for the permanganate ion is as follows: 1 1 Mn permanganate ion 1 1 B. xidative leavage of Glycols The carbon carbon bond between the groups of a glycol can be cleaved with periodic acid to give two carbonyl compounds: 5 I 6 dilute Ac 2 2 I 8 2 an aldehyde a ketone periodic (77 8 yield acid a glycol (11.5 Periodic acid (pronounced PU-eye--dik is the iodine analog of perchloric acid. l 4 perchloric acid I 4 periodic acid

5 11.5 PEPAATIN AND XIDATIVE EAVAGE F GY 507 Periodic acid is commercially available as the dihydrate, I , often abbreviated, as in Eq. 11.5, as 5 I 6 (sometimes called para-periodic acid. Its sodium salt, NaI 4 (sodium metaperiodate, is sometimes also used. Periodic acid is a fairly strong acid (pk a = The periodate cleavage reaction has been used as a test for glycols as well as for synthesis. The formulas I 4 or 5 I 6 are used interchangeably for periodic acid. The cleavage of glycols with periodic acid takes place through a cyclic periodate ester intermediate (ec. 10. that forms when the glycol displaces two groups from 5 I 6. a glycol I # I # 5 I 6 contains iodine (VII a cyclic periodate ester; contains iodine (VII 2 2 (11.54a The cyclic ester spontaneously breaks down by a cyclic flow of electrons in which the iodine accepts an electron pair. (The direction of electron flow is arbitrary. I # iodine accepts an electron pair aldehydes and/or ketones I # I 4 (or I 8 2 contains iodine (V (11.54b A glycol that cannot form a cyclic ester intermediate is not cleaved by periodic acid. For example, the following compound is not cleaved because it is impossible for both oxygens to be part of the same cyclic periodate ester. (If you can t see why, build a model and try connecting the two oxygens with one other atom. Do not confuse osmium tetroxide, permanganate, and periodate oxidations, all of which occur through cyclic ester intermediates (ec. 11.4A. Periodate oxidizes glycols, but the other two reagents oxidize alkenes to give glycols. In all of these reactions, oxidation occurs because an atom in a highly positive oxidation state can accept an additional pair of electrons. In the periodate oxidation, the reduction of the iodine occurs during the breakdown of a cyclic ester; in the permanganate and osmium tetroxide oxidations, the metals are reduced during the formation of a cyclic ester.

6 508 APTE 11 TE EMITY F ETE, EPXIDE, GY, AND UFIDE PBEM Give the product(s expected when each of the following compounds is treated with periodic acid. (a (b (c 2 2 V What glycol undergoes oxidation to give each of the following sets of products? (a (b A A 11.6 XNIUM AND UFNIUM AT A. eactions of xonium and ulfonium alts If the acidic hydrogen of a protonated ether is replaced with an alkyl group, the resulting compound is called an oxonium salt. The sulfur analog of an oxonium salt is a sulfonium salt: a protonated ether a protonated sulfide a trialkyloxonium ion xonium and sulfonium salts react with nucleophiles in N 2 reactions: a trialkylsulfonium ion BF 4 trimethyloxonium tetrafluoroborate (an oxonium salt N trimethylsulfonium nitrate (a sulfonium salt BF ( 2 BF 4 4 (89 yield 2 Br 2 Br heat 1 ( N ( 2 N ( 4 N ( N (11.55 (11.56 (11.57 xonium salts are among the most reactive alkylating agents known, and they react very rapidly with most nucleophiles. Because of their reactivity, oxonium salts must be stored in the absence of moisture. For the same reason, these salts are stable only when they contain