Hydration of allenes

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

Hydration of allene in sulfuric acid yields acetone, presumably via a vinyl cation intermediate (55, 56). Addition of HCl and HBr to allene results in... 

The intermolecular hydration of allenes catalysed by [AuCl(lPr)]/AgOTf (1 1,5 mol%) in dioxane/waler at room temperature, has also been studied. In most cases, low to modest yields (25-65%) of fi-aUylic alcohols were obtained by selective addition of the water to the terminal C atom of the aUene group [89]. 

Hydration of allene and mono- and disubstituted allenes leads to ketones through the rearrangement of the intermediate enols.29 Further details of the mechanism are not known, but protonation of the terminal carbon in monosubstituted allenes is probable. Although the formation of isomeric ketones may be expected, only ketones possessing the keto function on the central carbon of the allene bond system were found to form.30 When alcohols add to allenes, enol ethers of the corresponding ketones are usually the products. They may further react to form acetals. Hg2+ salts may be used as catalysts.31... 

The addition of water to allenes has been reviewed recently.298-299 The product obtained from the acid-catalyzed hydration of allene, alkylallenes and 1,3-dialkylallenes is usually the ketone. The intermediate is a vinyl cation formed by protonation on the terminal carbon of the allene moiety (equation 198). 

Alcohols will add to allenes in the presence of trace amounts of acids to give vinyl ethers (or acetals) or allylic ethers.401 Analogous to the hydration of allenes, protonation occurs on the terminal carbon of the allenic functionality in 1,2-propadiene, 3-alky 1-1,2-propadienes and 1,3-dialkyl- 1,2-propadienes (equation 248).402 Addition of an alcohol to the resulting vinylic cation produces a vinylic ether, which may on further reaction form an acetal of the corresponding ketone. 

Catalytic conversions of allenes are sometimes considered models for catalytic reactions of alkenes, even though allene reactivity is more closely comparable to that of alkynes rather than alkenes. The catalytic hydration of allenes was achieved by means of a cationic gold(I) complex with a carbene steering ligand, (IPr)AuCl/ AgOTf (5 mol%), in dioxane (rt, 4—9 h) in fair yield [180]. Attack of water is selective for the terminal carbons, whereas regioselectivity in nonsymmetric substrates is controlled by steric, electronic, and solvation factors. 

Most of the examples of gold-catalysed hydration of allenes are performed at high temperatures. There has been an interest in reducing the temperature... 

Hydration of allenes catalyzed by gold has been much less studied. The interest of the addition of primary and secondary alcohols to allenes using gold complexes... 

The hydration of simple ketenes (RCH=C=0—> RCH2COOH) also shows relatively constant values of oh w which are quite low (100-1000) (Tidwell, 1990 Allen et al., 1992), implying p/fj = 11 to 12 for the transition state for water attack. Corresponding to this, the Leffler index and the /3nuc are both about 0.25. Whether these low values really indicate an early transition state or arise because water and hydroxide ion react quite differently is not yet clear. However, it appears possible that water attack proceeds through a cyclic mechanism involving two (or more) water molecules (Allen et al., 1992) whereas hydroxide ion probably attacks conventionally as a nucleophile (Tidwell, 1990). Of course, any mechanism for the water reaction which is superior to simple nucleophilic attack will elevate kw and necessarily lead to low kOH/kw ratios. 

Investigations of base-catalyzed isomerizations of allene derivatives have been recently continued. For instance, the rearrangement of allene ethers 53 under superbasic conditions (KOH-DMSO) is considered as one of the steps in hydration of acetylene derivatives (equation 17)31,32. 

However, the enzymology of alkene and alkyne hydration is not well known. Recently, Meckenstock et al. (1999) discovered that the enzyme responsible for anaerobic hydration of acetylene contains a tungsten atom and an [Fe-S] cluster. This may hint that the enzyme uses the tungsten as a Lewis acid to activate the double bond. Possibly, the [Fe-S] cluster then serves to deliver a hydroxide as known in many common metabolite hydrations (Flint and Allen, 1996). Having introduced an oxygen moiety in an initial hydration, anaerobic bacteria may now be able to continue the biodegradation of such compounds. 

However, the site of protonation changes to the central catbon when electron-donating substituents such as acetates, alkoxides, arenes and fluoride are attached to the allene moiety. Thus, the hydration of allenyl ethers provides unsaturated aldehydes showing deuterium incorporation at the central carbon (equation 199).300... 

As demonstrated below, a Lewis acid-mediated reaction was utilized in the synthesis of dihydro[b furan-based chromen-2-one derivatives from l-cyclopropyl-2-arylethanones and allenic esters <070L4017>. The TiCh-catalyzed anti-Markovnikov hydration of alkynes, followed by a copper-catalyzed O-arylation was applied to the synthesis of 2-substituted benzo[6]furan <07JOC6149>. In addition, benzo[6]furan-based heterocycles could be made from chloromethylcoumarins <07SL1951>, substituted cyclopropanes <07AGE1726>, as well as benzyne and styrene oxide <07SL1308>. On the other hand, DBU-mediated dehydroiodination of 2-iodomethyl-2,3-dihydrobenzo[6]furans was also useful in the synthesis of 2-methylbenzo[Z>]furans <07TL6628>. 

Ethynyl carbinols (propargylic alcohols) such as 134 (Scheme 2.58) represent another important group of oxidation level 3 compounds. Their preparation involves nucleophilic addition of acetylides to the carbonyl group, a reaction that is nearly universal in its scope. Elimination of water from 134 followed by hydration of the triple bond is used as a convenient protocol for the preparation of various conjugated enones 135. Easily prepared O-acylated derivatives are extremely useful electrophiles in reactions with organocuprates, which proceed with propargyl-allenyl rearrangements to furnish allene derivatives 136. 

Hydration of attenic nitriles. 4,4-Dialkyl-substitutcd allenic nitriles (1) are converted into allenic amides (2) in 60 70% yield by reaction with hydrogen peroxide in alcoholic sodium hydroxide. 

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