Carbocations, from 1,2-diols

Like the pinacol rearrangement in the preceding problem, this one also begins with the formation of the more stable of two possible carbocations from a vicinal diol. A 99% yield of a single ketone was isolated. What is this ketone ... 

The pinacol rearrangement is frequently observed when geminal diols react with acid. The stmcture of the products from unsymmetrical diols can be predicted on the basis of ease of carbocation formation. For example, l,l-diphenyl-2-metltyl-l,2-propanediol rearranges to... 

Pinacol rearrangement is a dehydration of a 1,2-diol to form a ketone. 2,3-drmethyl-2,3-butanediol has the common name pinacol (a symmetrical diol). When it is treated with strong acid, e.g. H2SO4, it gives 3,3-dimethyl-2-butanone (methyl r-butyl ketone), also commonly known as pinacolone. The product results from the loss of water and molecular rearrangement. In the rearrangement of pinacol equivalent carbocations are formed no matter which hydroxyl group is protonated and leaves. 

Mechanism. The protonation of OH, followed hy the loss of H2O from the protonated diol, yields a tertiary carbocation, which rearranges with a 1,... 

Simple mono- and disubstituted alkenes react to yield 1,3-diols, when the Prins reaction is carried out at elevated temperature. Diols originate from the attack of water on carbocation 18, or through the acidolysis of dioxanes under the reaction conditions. When the reaction is conducted in acetic acid, monoacetates are formed by acetate attack on 18. Dienes resulting from the dehydration of intermediate diols are the products of the transformation of more substituted alkenes. Monoacetates and diols may react further to yield 1,3-diol diacetates. When the Prins reaction... 

The acid-catalysed hydrolysis of 5-methoxyacenaphthylene 1,2-oxide was shown to proceed via the carbocation (76).149 This cation reacted with water to give an approximately 3 2 ratio of cis.trans diols, despite the trans-diol being substantially more stable. Hence, transition state effects, and not thermodynamic stability, determine the products. A computational investigation examined carbocations derived from oxidized benzo aJanthraccncs.150 Examples of cations considered are (77), derived by epoxi-dation and ring opening, and (78), derived by methyl oxidation and subsequent C—O bond heterolysis. 

Complex rearrangements involving both the steroid backbone and the cholestane side chain were observed when either the Westphalen diacetate (252) or 5a-cholestane-2a,5-diol (253) was treated with HF at —60 °C. The 25-fluoro-product (254) was isolated in low yield from the 2,5-diol, and an analogous product resulted from the Westphalen diacetate. Inversion of the configuration at C-20 to give the unnatural (20S)-isomer is thought to involve stereospecific hydride transfer from C-25 to a C-20 carbocation subsequent nucleophilic attack at C-25 gives the 25-fluoro-compound. A hydride transfer from C-20 to C-13 is postulated to account for... 

The mechanism involves the loss of water molecule from a protonated diol, followed by 1,2-nucleophilic shift of a group. Since the diol is symmetrical, protonation and loss of water take place with equal probability at either hydroxyl group. The resulting 3°-carbocation is relatively stable but 1,2-methyl shift generates an even more stable carbocation in which the charge is delocalized by heteroatom resonance (Scheme 2.11). 

Since the formation of cyclic pyruvate acetals from alkyl pyruvates and diols under classical conditions (i.e. catalysis by acids) is expected to be unfavoured due to the necessity of the intermediate formation of a destabilized carbocation, early attempts to prepare 1-carboxyethylidene sugars used indirect procedures. Later, it was shown that under carefully controlled conditions the direct acetalation of a sugar diol with methyl pyruvate can also be used for preparative purposes. 

Solid acids also catalyze the reaction of epoxides with nucleophiles. Nafion-H promotes the hydrolysis of epoxides giving good yields of the diols at 20°C. 2 Nafion-H catalyzed reactions run in methanol give the methoxy alcohol with the methoxy group on the more substituted carbon atoms as expected from a reaction that proceeds by way of nucleophilic attack on the more stable carbocation (Eqn. 22.3 5). 2 Oxiranes are also cleaved by nucleophiles in the presence of... 

The evidence for carbocation intermediates in the pinacol rearrangement is compelling for some substrates and conditions. Bunton" found that pinacol itself that was recovered from O-enriched aqueous sulfuric acid had incorporated an appreciable amount of solvent oxygen. Under these conditions the reaction goes to completion, i.e. the incorporation cannot be due to reversible formation of diol from pina-... 

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