Allenes synthesis, mechanisms

For DFT and ab initio quantum mechanical computations of the cydopropylidene ringopening process, the decisive step of this allene synthesis, see H. F. Bettinger,... 

Warren and coworkers have reported an interesting synthesis of nonracemic allenes by reaction of vinylphosphine oxides with aldehydes in the presence of chiral lithium [(R)-l-phenylethyl](benzyl)amide to give hydroxyvinylphosphine oxides in 33-87% yields (0-51% ee) [38]. These products underwent a Horner-Wittig elimination reaction to produce nonracemic allenes. A mechanism similar to the Baylis-Hillman reaction was suggested. 

Braverman, S., Duar, Y. Thermal rearrangements of allenes. Synthesis and mechanisms of cycloaromatization of it and heteroatom bridged diallenes. J. Am. Chem. Soc. 1990,112, 5830-5837. 

This is, as the numerous references will show, probably the most important method for the synthesis of allenes by isomerizations (Reaction A in Scheme 1.1). Examples following a different mechanism but overall delivering analogous products will also be covered in this section. 

Abstract The basic principles of the oxidative carbonylation reaction together with its synthetic applications are reviewed. In the first section, an overview of oxidative carbonylation is presented, and the general mechanisms followed by different substrates (alkenes, dienes, allenes, alkynes, ketones, ketenes, aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, phenols, amines) leading to a variety of carbonyl compounds are discussed. The second section is focused on processes catalyzed by Pdl2-based systems, and on their ability to promote different kind of oxidative carbonylations under mild conditions to afford important carbonyl derivatives with high selectivity and efficiency. In particular, the recent developments towards the one-step synthesis of new heterocyclic derivatives are described. 

Heating of thietanes can often lead to a ring contraction. Thermal treatment (150°C, in vacuo) of the tosylhydrazide salt 213 gives the allene epi-sulfide 214, a unique synthesis for this molecule. The mechanism is suggested to proceed either via the intermediate bicyclobutane ylide 215 or the mesomeric structure 216. Dodson et al. have noticed the rearrangement... 

One of the most versatile methodologies of EPC synthesis of allenes is the chirality transfer reaction which involves highly stereoselective, mechanism-controlled, metal-mediated propyn-yl-allenyl transpositions. Of these, the organocopper(l)-mediated reactions are especially useful and provide a relatively straightforward route to a broad variety of allenes of high enantiomeric purity. 

In a recently reported synthesis of pyridines, lithiated methoxyallenes react with nitriles in the presence of trifluoroacetic acid (Scheme 107) <2004CEJ4283>. The mechanism is postulated to proceed via initial protonation followed by nucleophilic addition of the trifluoroacetate ion with subsequent intramolecular acyl transfer and aldol condensation to give the pyridine. An additional pyridine formation starting from azaenyne allenes forms a-5-didehydro-3-picoline diradicals, which can be trapped by 1,4-cyclohexadiene, chloroform, and methanol to produce various pyridines <20040L2059>. 

When pyrolyzed, p-hydroxy alkenes cleave to give alkenes and aldehydes or ketones." Alkenes produced this way are quite pure, since there are no side reactions. The mechanism has been shown to be pericyclic, primarily by observations that the kinetics are first order" and that, for ROD, the deuterium appeared in the allylic position of the new alkene." This mechanism is the reverse of that for the oxygen analog of the ene synthesis (16-54). p-Hydroxyacetylenes react similarly to give the corresponding allenes and carbonyl compounds." " The mechanism is the same despite the linear geometry of the triple bonds. 

As part of a synthesis of santalene sesquiterpenoids, Barrett and McKenna (1971) treated cyclopentadiene (PI5.1) with the allenic acid (P15.2). This gave a mixture of two isomeric acids, (PI 5.3) and (P15.4). One of these, (PI 5.3) rearranged to the lactone (PI 5.5) on treatment with formic acid. The other remained unchanged. What are the structures of (PI5.3) and (PI5.4) and what are the mechanisms of the reactions (Figure PI 5). 

Freedman and Huber showed that the cyclization of 253 with NaH/DMSO did not afford 3-methyl-l,4-benzoxazepin-5(4//)-one 255, as previously reported. Instead, they isolated 2-(2-hydroxyphenyl)-5-methyloxazole 256 and confirmed the structure by independent synthesis from 2-hydroxy-A -(2-propynyl)benzamide 257 (Scheme 1.69). The authors considered an intermediate allene but were not able to unequivocally establish the mechanism for conversion of 253 to 256. 

Scheme 3 Nenitzescu and Allen Mechanism for this Indole Synthesis...

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