Allenes hydration

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]. 

One of the most dramatic developments in the chemistry of N2 during the past 30 years was the discovery by A. D. Allen and C. V. Senoff in 1965 that dinitrogen complexes such as [Ru(NH3)5(N2)1 could readily be prepared from aqueous RUCI3 using hydrazine hydrate in aqueous solution. Since that time virtually all transition metals have been found to give dinitrogen complexes and several hundred such compounds are now characterized.Three general preparative methods are available ... 

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... 

Even substituted allenes, RCH=C=CH2, are protonated at the 1 position, giving methyl ketones by hydration (60-63) and mostly 2-halo-2-alkenes, RCH=CXCH3, by the addition of hydrogen halides (62, 64, 65). Jacobs and Johnson (65), in a careful study, have shown that addition of HCl to... 

Allen JW, Collins BW, Evansky PA. 1994. Spermatid micronucleus analyses of trichloroethylene and chloral hydrate effects in mice. Mutat Res 323 81-88. 

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]. 

Debierne (1914) was the first to suggest a radical reaction theory for water radiolysis (H and OH). In various forms, the idea has been regenerated by Risse (1929), Weiss (1944), Burton (1947, 1950), Allen (1948), and others. Platzman (1953), however, criticized the radical model on theoretical grounds and proposed the formation of the hydrated electron. Stein (1952a, b) meanwhile had suggested that both electrons and H atoms may coexist in radiolyzed water and proposed a model in which the electron digs its own hole. Later, Weiss (1953, 1960) also favored electron hydration with ideas similar to those of Stein and Platzman. In some respects, the theoretical basis of these ideas is attributable to the polaron (Landau, 1933 Platzman and... 

On the experimental side, evidence was accumulating that there is more than one kind of reducing species, based on the anomalies of rate constant ratios and yields of products (Hayon and Weiss, 1958 Baxendale and Hughes, 1958 Barr and Allen, 1959). The second reducing species, because of its uncertain nature, was sometimes denoted by H. The definite chemical identification of H with the hydrated electron was made by Czapski and Schwarz (1962) in an experiment concerning the kinetic salt effect on reaction rates. They considered four... 

Discovery of the hydrated electron and pulse-radiolytic measurement of specific rates (giving generally different values for different reactions) necessitated consideration of multiradical diffusion models, for which the pioneering efforts were made by Kuppermann (1967) and by Schwarz (1969). In Kuppermann s model, there are seven reactive species. The four primary radicals are eh, H, H30+, and OH. Two secondary species, OH- and H202, are products of primary reactions while these themselves undergo various secondary reactions. The seventh species, the O atom was included for material balance as suggested by Allen (1964). However, since its initial yield is taken to be only 4% of the ionization yield, its involvement is not evident in the calculation. 

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. 

FIG. 3. Geometry of hydrated molecules cylinders associate to a lamellar liquid crystal, cones to a hexagonal and an inverse hexagonal. Adapted from The Physical Chemistry ofMembranes (Silver, B., ed.), Allen Unwin, Inc. Solomon Press, Winchester, MA, 1985. 

P. Walden34 has studied soln. of iodine in acetaldehyde, hydrazine hydrate, and acetonitrile J. H. Mathews, soln. of iodine in pyridine, ethyl, allyl, and plienyl isothiocyanic esters, and phenyl isocyanate while H. A. Allen has studied soln. of bromine and iodine in various oils. 

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. 

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). 

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... 

Allenes undergo mercury-catalyzed hydration to give a variety of products depending upon the substituents and the substitution pattern (equations 222 and 223).344-347... 

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. 

Of more than 130 compounds that are known to form clathrate hydrates with water molecules, the majority form either si, sll, or sH, with exceptions such as (1) bromine (Allen and Jeffrey, 1963 Dyadin et al., 1991), (2) dimethyl ether (Gough et al., 1974, 1975 Udachin et al., 2001a), (3) ethanol (Brownstein et al., 1967 Calvert and Srivastava, 1967), and (4) very high pressure hydrate phases (Dyadin et al., 1997 Loveday et al., 2001b, 2003b Kursonov et al., 2004). Detailed emphasis is given to si, sll, and sH hydrates since these are by far the most common natural gas hydrate structures. 

The reaction of 2-aminopyridine hydrochlorides and allenic nitriles either neat at 90°C for 20 hours or in boiling methylene chloride for 15 hours afforded unstable 2-imino-2//-pyrido[l,2-a]pyrimidine hydrochlorides 71, which could be isolated as hydrates in about 50% yield [88JCS(P1)975]. If the reactions were carried out in boiling 95% ethanol for 72 hours, the initially formed 2-imino-2//-pyrido[ 1,2-a Ipyrimidines 71 were hydrolyzed and 2-pyridyl ketones 73 could be isolated. Under the latter conditions 3-hydroxy-2-aminopyridine gave 2-iminopyridopyrimidines 74, which were stabilized by the formation of zwitterionic structures (Scheme 5). [Earlier the ring-opened products 73 (R = H, 3-NH2) were described as a 4-imino-4/7-pyrido[ 1,2-a]pyrimidine derivative 72 (81TL4127)]. 

The hydrated electron reacts with many compounds which are capable of releasing an anion by dissociative electron capture [e.g., reaction (8)], and, among others, it was this property which allowed the differentiation between eaq and H" [reactions (9) and (10)] (Armstrong et al. 1958 Hayon and Allen 1961 Jortner and Rabani 1962). 

This sequence is particularly well characterized for fluoride complexes of high-spin cations of the first-series transition elements (Allen and Warren, 1971). Moreover, between successive transition metal series, values of Ac increase by about thirty to fifty per cent. For example, in hydrated cations of the first and second transition series, Ac for [CftHjO) 3 and [Mo(H20)6]3+ are 17,400 cm-1 and 26,110 cm-1, respectively. 

Simple and complex hydrazides, with the general formula RCONHNHCOR, were readily dehydrogenated by DIB to the corresponding azo compounds, some of which were used as dienophiles for in situ Diels-Alder reactions [65,66]. Hydrazine itself in the form of its hydrate was similarly converted into diimide, NH — NH, which served for some in situ hydrogenations [67], Further reactivity accompanied by solvent participation was observed in some cyclic derivatives of hydrazine, i.e. pyrazolones. At low temperature these underwent ffagmentive loss of dinitrogen to give either methyl alkynoates or allenic esters [68] ... 

A key configuration in these water clusters, in ice and in clathrate hydrates is the pentamer (Fig. 4) in which one water molecule at the centre of a tetrahedron is hydrogen-bonded to four other water molecules at the vertices. Here two hydrogen bonds are formed by H-transfer and two by electron transfer . The calculated average bond energy is similar to that for the dimer (Kollmann and Allen, 1970 Hoyland and Kier, 1969). More sophisticated calculations on... 

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