2.3- dimethyl-2-butene, hydration

Dimethyl-2-butanol is the major product in the acid-catalyzed hydration of 3,3 -dimethyl-1 -butene. 

Di-/i-chloro-bis(7)4-1,5-cyclooctadiene)dirhodium(l), [RhCl(l,5-C8Hi2)]2, has been prepared in 60% yield by reducing rhodium trichloride hydrate in the presence of excess olefin in aqueous ethanol.1 In the present preparation the yield has been greatly increased (to 94%). Two related complexes, [RhCl(l,5-C6Hio)]22 and [RhCl(C6H12)2]2, are similarly prepared in high yield from 1,5-hexadiene and 2,3-dimethyl-2-butene, respectively. 

Clinoptilolite Isomerization of n-butene, the dehydration of methanol to dimethyl ether, and the hydration of acetylene to acetaldehyde... 

It can be considered that these pillared products will be intercalated by accompanying with proton to produce a solid acid catalyst, because they exhibited acidity as shown in Table 1. To examine the acidic property of the catalysts dehydrations of methanol and 1-butanol were attempted by a flow reactor. The dehydration products of methanol were dimethyl ether and water, and those of 1-butanol were 1-, cis-2-, and trans-2-butenes and water. At relatively low temperature (250°C to 300°C) in hydration of 1-butanol a... 

Among the synthetically useful reactions of Hg(II) salts with organic compounds, the most familiar is a two-stage procedure for alkene hydration called oxymercuration-demercuration. Its application in the conversion of 3,3-dimethyl-l-butene to 3,3-dimethyl-2-butanol illustrates the procedure. 

Like other reactions that involve carbocation intermediates, hydration may take place with rearrangement. For example, when 3,3-dimethyl-l -butene undergoes acid-catalyzed hydration, the major product results from rearrangement of the carbocation intermediate. 

Similarly, oxymercuration-demercuration of 3,3-dimethyl-l-butene gives the Markovnikov product, 3,3-dimethyl-2-butanol, in excellent yield. Contrast this unre-arranged product with the rearranged product formed in the acid-catalyzed hydration of the same alkene in Section 8-4B. Oxymercuration-demercuration reliably adds water across the double bond of an alkene with Markovnikov orientation and without rearrangement. 

General acid catalysis was observed in the hydration of both trans-cyclooctene and 2,3-dimethyl-2-butene, which helped to establish the Ase2 mechanism for hydration of alkenes. Furthermore, 1,1-dicyclopropyl-ethene was foimd to react much faster than either cis- or trans-l,2-dicyclo-propylethene, which indicates that substituent location (and not just the total electron-donating ability of the substituents) affects the rate constant for hydration reactions. It appears that the approaching proton is undergoing bond formation to one of the olefinic carbon atoms in the transition structure but not to the other, so the regiochemistry of the hydration reaction is determined by the approach that leads to the development of the more stable carbocation. ... 

Steric effects may play a role in the regiochemistry of the reaction as well. While the oxymercuration-demercuration reaction is generally considered to give only Markovnikov hydration, as shown by predominant formation of 43, this regioselectivity is not absolute. For example, methoxymercuration of 3,3-dimethyl-l-butene produced 2% of 3,3-dimethylbutyl methyl ether (44, equation 9.42). ... 

Alkenes lacking the phenyl group are somewhat less convenient to study by kinetic methods, but such data as observation of general acid catalysis and solvent isotope effects are also consistent with rate-limiting protonation in simple alkenes such as 2-methylpropene and 2,3-dimethyl-2-butene. The observation of general acid catalysis rules out an alternative mechanism for alkene hydration, namely, water attack on an alkene-proton complex. The preequilibrium would be governed by the... 

Analysis of the mixture of products by gas chromatography failed to reveal the presence of any 2,3-dimethyl-2-butanol. The acid-catalyzed hydration of 3,3-dimethyl-1-butene, by contrast, gives 2,3-dimethyl-2-butanol as the major product. 

You might compare the product of oxymercuration-reduction of 3,3-methyl-l-butene with the product formed by acid-catalyzed hydration of the same alkene (Section 6.3C). In the former, no rearrangement occurs. In the latter, the major product is 2,3-dimethyl-2-butanol, a compound formed by rearrangement. The fact that no rearrangement occurs during oxymercuration-reduction indicates that at no time is a free carbocation intermediate formed. 

Acid-catalyzed hydration of 3,3-dimethyl-1-butene produces 2,3-dimethyl-2-butanol. Show a mechanism for this reaction. 

FIGURE 3.81 The acid-catalyzed addition of water to 2,3-dimethyl-2-butene —a hydration reaction. 

The oxymercuration-demercuration of 3,3-dimethyl-l-butene is a striking example illustrating this feature. It is in direct contrast to the hydration of 3,3-dimethyl-l-butene we studied previously (Section 8.5B). 

Hydration of 2,3-dimethyl-2-butene is a slower reaction than the hydration of 2,3-dimethyl-l-butene under the same reaction conditions. Suggest a possible explanation. 

Studies on the photochemistry of tetramethylcyclopropanone 17 have revealed efficient a-cleavage and decarbonylation reactions to give 2,3-dimethyl-2-butene 19 as the only product (Scheme 7). Interestingly, cyclopropanone 17 is prepared by an efficient decarbonylation of 2,2,4,4-tetramethylcyclobu-tanedione 16. Cyclopropanones, in general, are highly reactive compounds that can thermally equilibrate with oxyallyl, the open-shell 1,3-diyl 18. While cyclopropanones are highly susceptible to nucleophilic attack to form hydrates and hemiketals, oxyallyl was shown to react rapidly with oxygen and furan. 

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