Allene derivatives hydrogenation

The protected propargylamines 224 (R = H, CH2C02Me or CH2CH2C02Me) react with aqueous formaldehyde under copper bromide catalysis to yield the allene derivatives 225. Deprotection with ethereal hydrogen chloride affords the free amines218. 

Elimination of two molecules of hydrogen fluoride affords B, an allene derivative, methyl 2-methoxycarbonyl-5,5,5-trifluoro-4-trifluoromethyl-penta-2,3-dienoate 84. ... 

The presence of a center of prochirality is not an obligatory condition for a molecule to be prochiral. Indeed, other elements of prochirality exist, namely axes and planes of prochirality. Thus, the prochiral allene derivative LX displays two geminal hydrogen atoms which are enantiotopic. 

As shown below, allene derivatives have been adopted as starting materials in the synthesis of furans. Palladium(II)-catalyzed cyclization of a-allenic ketones afforded 2,3-disubstituted furans in good yields <07EJO2844>. The substituent at the a-position of a carbonyl group influences the reaction pathway. Cyclization products were provided when the substituent was a methyl group or a phenyl group, while dimerization products were produced when the substituent was a hydrogen atom. 

Sol 2. (c) The reactant undergoes intramolecular ene reaction under thermal conditions. In this reaction, allyl hydrogen is transferred to alkyne system instead of the transfer of propargylic hydrogen to alkene part, which may give allene derivative. 

The vapour-phase infrared spectrum and structure of fulvenallene have been determined and hydrogen-bonding between phenol and acetylene or allene derivatives has bron studied using the hydroxyl stretching band. The basicity of acetylenes is greater than that of allenes and olefins which are of the same order of magnitude. ... 

Finally, allene derivatives were also convenient unsaturated substrates allowing carbon-carbon bond formation from benzylic alcohol via hydrogen transfer processes. With these substrates, the best catalytic systems were based on RuHCl (CO)(PPh3)3 in the presence of an equimolar amount of phosphine ligand such as bis(diisopropylphosphino)ferrocene (dippf) [63], bis(dicyclohexyphosphino) ferrocene or PCyPh2 [64], Some examples of selective formation of homoallylic alcohols using this reaction are reported in Scheme 59. 

We can narrow the difference from 10 kJmol-1 even further once it is remembered that in the comparison of meso-bisallene, 27, and (Z, Z)-diene, 29, there are two extra alkylallene and alkylolefin interactions for which a stabilization of ca 3 kJ mol-1 for the latter was already suggested. Admittedly, comparison with the corresponding 1,5-cyclooctadiyne suggests strain-derived anomalies. From the enthalpy of hydrogenation, and thus derived enthalpy of formation, of this diyne from W. R. Roth, H. Hopf and C. Horn, Chem. Ber., 127, 1781 (1994), we find 1/2S (bis-allene, bis-acetylene) equals ca — 80 kJ mol-1. We deduce that the discrepancy of this last 5 quantity from the others is due to strain in the cyclic diyne. 

The second large class of allenes with carbonyl groups as substituents is derivatives of carboxylic acids. Overall, the same principles as with the aldehydes and ketones apply, but not as many examples are known, which might originate in part from the lower acidity of the a-hydrogen atoms of the carbonyl group compared with aldehydes and ketones. 

The asymmetric synthesis of allenes via enantioselective hydrogenation of ketones with ruthenium(II) catalyst was reported by Malacria and co-workers (Scheme 4.11) [15, 16]. The ketone 46 was hydrogenated in the presence of iPrOH, KOH and 5 mol% of a chiral ruthenium catalyst, prepared from [(p-cymene) RuC12]2 and (S,S)-TsDPEN (2 equiv./Ru), to afford 47 in 75% yield with 95% ee. The alcohol 47 was converted into the corresponding chiral allene 48 (>95% ee) by the reaction of the corresponding mesylate with MeCu(CN)MgBr. A phosphine oxide derivative of the allenediyne 48 was proved to be a substrate for a cobalt-mediated [2 + 2+ 2] cycloaddition. 

Like the nitrone-olefin [3 + 2]-cycloaddition, the nitrone-allene [3 + 21-cycloaddition also takes place regioselectively to furnish methylene-substituted isoxazoli-dine derivatives. The substituents of the 3- and 4-positions of the cycloadducts 72a and 72b are disposed cis in contrast, the reaction of 70 with allenyl sulfone 71c gives rise to the trans-cycloadduct 72c exclusively (Table 12.4). On treatment with base or heating, methyleneisoxazolidines 72 readily rearrange to isoxazo-lines via a 1,3-hydrogen shift. 

Allenes with terminal double bonds are selectively reduced in the terminal position, whereas internal allenes afford a mixture of the corresponding olefins120. Some hydrogenations resulted in the cis alkene derivative (equation 46)121. 

Evidence derived from a study of the stereochemistry of hydrogenation of 1,2-cyclononadiene and 1,2-cyclodecadiene led Moore (108) to conclude that allyl complexes like those postulated above must be intermediates. He noted that, of the four ways in which either allene could be adsorbed on a surface, two, a and b, would yield via cis addition of hydrogen the cis-cycloalkene and two, c and d, the tram isomer. Examination of... 

It was reported that Pd(0)-catalyzed coupling reactions of allenic alcohols, amines and acids with hypervalent iodonium salts afforded cyclized heterocyclic tetrahydrofurans, tetrahydropyrans, pyrrolidines, piperidines, or lactones under mild conditions <99SL324>. Intramolecular 1,5-hydrogen atom transfer radical cyclization reaction of pyrrolidine derivatives was examined. Reaction of 3,4-dialiyloxy-JV-(0-bromobenzyl)pyrtolidine gave hexahydro-... 

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