Acid catalyzed, addition acids, with ethers

Further improvements in alkylation can be achieved when an MTBE unit (acid-catalyzed addition of methanol to isobutylene to form tert-butyl methyl ether) is added before the alkylation unit.299 The MTBE unit removes the lowest-octane-producing isomer, isobutylene, from the C4 stream (and produces a very-high-octane number component, MTBE). The H2S04 alkylation unit then can be operated under better conditions to produce an alkylate with a somewhat increased octane number (0.75-1). Higher acid consumption, however, may be experienced as a result of methanol, MTBE and dimethyl ether impurities in the olefin feed. The combination of MTBE with HF alkylation offers no apparent advantages. 

Exercise 15-39 Write the mechanistic steps involved in the acid-catalyzed addition of alcohols to the cyclic ether 16. Why does cyclohexene react far less readily than 16 with alcohols under acidic conditions Write equations for the steps involved in the hydrolysis of 17 with aqueous acid. 

These reactions are carried out in the presence of acidic and basic catalysts. The acid-catalyzed addition of ethyl alcohol to acetylene or to a vinyl ether produces acetals (diethers of 1,1-dihydroxyethane). The acid-catalyzed reaction of ethyl alcohol with an aldehyde or ketone also gives acetals. 

The mechanism of the formation of the tetrahydropyranyl ether (see Figure 23.1) is an acid-catalyzed addition of the alcohol to the double bond of the dihydropyran and is quite similar to the acid-catalyzed hydration of an alkene described in Section 11.3. Dihydropyran is especially reactive toward such an addition because the oxygen helps stabilize the carbocation that is initially produced in the reaction. The tetrahydropyranyl ether is inert toward bases and nucleophiles and serves to protect the alcohol from reagents with these properties. Although normal ethers are difficult to cleave, a tetrahydropyranyl ether is actually an acetal, and as such, it is readily cleaved under acidic conditions. (The mechanism for this cleavage is the reverse of that for acetal formation, shown in Figure 18.5 on page 776.)... 

Stereoselective additions to chiral a- and -alkoxy aldehydes. Lewis-acid-catalyzed additions of enol silyl ethers to chiral ct-alkoxy or (3-alkoxy aldehydes can proceed with high 1,2- and 1,3-asymmetric induction. Moreover, the sense of induction can be controlled by the Lewis acid. Thus BF, which is nonchelating, can induce diastereo-... 

Ituramokeidar addition of t noketones to oiefims. Erman and Stone have reported the first example of an acid-catalyzed addition of a diazokeione to an olefln. The reaction has useful applications for the synthe.sis of bicyclic ketones, particularly in the sesquiterpene series. Thus treatment of the diazoketone (I) with boron trifluoride etherate for 3 hr. at 0-27° yields the ketones (2) and (3) in yields of about 30 and 3%,... 

A preparation of y-oxygenated allyhc stannanes in which the chirality resides in the alkoxy function has been described (Eq. 41) [61]. The starting alcohol is derived from tri-O-acetyl D-glucal. Acid-catalyzed addition of (Z)-l-methoxy-3-tributylstannyl-l-propene afforded the mixed acetal which was converted to the (Z) enol ether with TMSI and hexamethyldisilazane (HMDS). 

Acid-catalyzed addition of aliphatic, aromatic or heteroaromatic cyanohydrins to ethyl vinyl ether, n-butyl vinyl ether or dihydro-4//-pyran provides base stable, protected cyanohydrin derivatives. Phase transfer catalyzed alkylation of aliphatic cyanohydrins with allylic bromides gave a-substituted a-allyl-oxyacetonitrile. Carbonyl compounds react wiA cyanide under phase transfer catalysis to give cyanohydrin anions, which are trapped by an acyl chloride or ethyl chloroformate to give acyl- or alkoxycarbonyl-protected cyanohydrins respectively. The reduction of the carbonyl group of an acyl cyanide by NaBH4 under phase transfer conditions followed by esterification serves as an alternative route to aldehyde-derived cyanohydrin esters. ... 

The enol content of simple aldehydes and ketones is low under standard acid-catalyzed conditions. Silyl enol ethers, often available free of regioisomers, are an important source of enol equivalents for nucleophilic addition reactions. The reaction of silyl enol ethers with carbonyl compounds in the presence of BF3 Et20, SnCl4, TiCl4 or InCl3 proceeds through an open transition state instead of a closed transition state and leads, after hydrolytic workup, to aldol products. 

The second chapter, by David A. Oare and Clayton H. Heathcock, deals with the stereochemistry of uncatalyzed Michael reactions of enamines and of Lewis acid catalyzed reactions of enol ethers with a,/ -unsaturated carbonyl compounds. It is effectively a continuation of their definitive review of base-promoted Michael addition reaction stereochemistry that appeared in the preceding volume of the series. 

The acid-catalyzed addition of such olefins as isobutylene to the hydroxyl group at C-1 was also studied. After 24 hours in a bomb at 100°, the sirupy 1-0-isobutylene derivative was obtained in 58% yield. A similar product was formed during treatment of the acetal with ethane and nitrogen in a bomb at 150 to 160°. The resulting vinyl ether had m.p. 43-45°. 

The formation of acetals, by the acid catalyzed addition of hydroxyl compounds to a,/8-ethylene ethers is a useful method of protecting the hydroxyl group in reactions effected in basic media.—E 3-Chloro-l-propanol and dihydropyran, with a few drops of coned. HC1, allowed to stand for 3 hrs. with occasional shaking —> 2-(y-chloropropoxy)-tetrahydropyran. Y 78%. (F. e. s. W. E. Parham, E. L. Anderson, Am. Soc. 70, 4187 (1948).)... 

The reactivity of carbon-carbon double bonds toward acid-catalyzed addition of water is greatly increased by ERG substituents. The reaction of vinyl ethers with water in acidic solution is an example that has been carefully studied. With these reactants, the initial addition products are unstable hemiacetals that decompose to a ketone and alcohol. Nevertheless, the protonation step is rate determining, and the kinetic results pertain to this step. The mechanistic features are similar to those for hydration of simple alkenes. Proton transfer is rate determining, as demonstrated by general acid catalysis and solvent isotope effect data. ... 

The corresponding reaction of but-3-yn-l-ols or pent-4-yn-l-ols with primary or secondary alcohols in the presence of catalytic amounts of Ph3PAuBF4 and p-TsOH afforded tetrahydrofuranyl ethers (Scheme 4-76). This tandem 5-endo-cycloisomerization/hydroalkoxylation proceeds via 2,3-dihydrofurans, which then undergo an intermolecular Bronsted acid-catalyzed addition of the external alcohol. The transformation is not restricted to internal alkynols but can be applied to terminal acetylenes as well. Application of the method to the s thesis of bicyclic heterocycles with a P-lactam structure was reported recently.Under the same conditions, epoxyalkynes undergo a sequence of epoxide opening, 6-exo-cycloisomerization, and nucleophilic addition to afford tetrahydropyranyl ethers. In a closely related transformation, cyclic acetals were obtained from alk-2-ynoates bearing a hydroxy group in 6- or 7-position by treatment with AuCU and MeOH. ... 

The commercial production of f-butyl methyl ether has become important in recent years. In 2002, worldwide consumption of MTBE was about 7 billion gallons. With an octane value of 110, it is used as an octane number enhancer in unleaded gasolines. It is prepared by the acid-catalyzed addition of methanol to 2-methyl-propene. The reaction is related to the hydration of alkenes (Sec. 3.7.b). The only difference is that an alcohol, methanol, is used as the nucleophile instead of water. 

The addition of an acid such as HX to an alkene leads to formation of an alkyl halide. If the acid catalyzed addition of water or an alcohol is examined, the product is an alcohol or an ether. Oxymercuration also leads to addition of water to an alkene. The functional group exchange can be generalized as shown, where X = Cl, Br, I, OH, OR. When an alkene reacts with a halogen, the product is a vicinal dihalide. The functional group exchange is shown where X = Cl, Br, I. When bromine in water or chlorine in water is reacted with an... 

As shown in Table 8.1, the preparation of some alcohols (with one or more hydroxyl [-OH] groups) has been provided earlier. Table 8.2 reminds the reader that (a) enols are in equilibrium with their corresponding carbonyl tautomers (Chapter 5) (b) acid-catalyzed addition of alcohols (R-OH R H) and thiols (R-SH R H) to, for example, alkenes can be used to produce ethers (R-O-R R, R H) and thioethers (R-S-R R, R H), respectively (Chapter 6) (c) ethers are readily formed by nucleophilic substitution of halogen (X = Cl, Br, I) by alkoxide (RO ) (Chapter 7) under the appropriate (SnI or Sn2) conditions and (d) substitution of a halogen on an aromatic ring (Ar-X, X = Cl, Br) can be effected by fusion with molten alkali (e.g., NaOH) (Chapter 7). 

Enol ethers with potential leaving groups at the allylic position (e. g. compounds of general structure 3) can be subject to acid-catalyzed additions or nucleophilic displacements, protonic acids commonly giving products (5) of the former reactions while Lewis acids favour the formation of unsaturated products (7,8)... 

In the first phase the vinyl ether 51 is prepared from the acetate 50 by methylene-transfer from the Tebbe-reagent. Next, the thioglycoside 51 and the aglycone 52 are connected by acid-catalyzed addition of the latter to the vinyl ether moiety to afford the acetal-tethered intermediate 53. In the final phase, NIS/TfOH-induced anomeric activation proceeds in concert with the attack of the tethered oxygen at the anomeric carbon to afford the P-linked disaccharide 54 (Scheme 16) [114]. 

The initial studies centered on the 1-norbomyl halides 8. On irradiation in CH3OH, they afford a mixture of the reduction product norbornane (9) and the ether 10, with the former predominating from bromide 8b and the latter from iodide 8a. The reduction product 9 arises via abstraction of a hydrogen atom by 1-norbornyl radical from the medium and ether 10 via nucleophilic trapping of the 1 -norbornyl cation. Irradiation of either hahde in CH3OD afforded ether 10 with no detectable incorporation of deuterium, indicating that it does not arise via acid-catalyzed addition of the alcohol to an initially formed unsaturated intermediate such as the bridgehead alkene 12 or the propeUane 13. 

---