Allenic intermediates

It has been used to establish the basicity scale of nuclei by means of the deviations calculated from This basicity has in fact a concrete physical sense, corresponding to the pK. of the heterocycloammonium (25. 55) regarding Mills views about the role of an active allenic intermediate (56). such compound could never be isolated or even identified (49). 

Triple bonds can also migrate in the presence of bases, but through the allene intermediate ... 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

A sequential 1,2-dechlorination/S - S bond fission of the 3,4-disubstituted 2-butenoate 49 is performed by cat. PbBr2/Al in DMF, giving the 2-exo-methylenepenam 51 through the allene intermediate 50 (Scheme 30) [70]. 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

This system was described in one report and has been synthesized by a copper-assisted cycloisomerization of alkynyl imines. The authors proposed the following mechanism at first, 372 could undergo a base-induced propargyl-allenyl isomerization to form 373 next, coordination of copper to the terminal double bond of the allene (intermediate 374) would make it subjected to intramolecular nucleophilic attack to produce a zwitterion 375. The latter would isomerize into the more stable zwitterionic intermediate 376, which would be transformed to the thiazole 377 (Scheme 55) <2001JA2074>. 

Another interesting allenic intermediate is generated when di(9-anthryl)diazo-methane (256) is deazotized by irradiation in benzene (Scheme 5.39). The carbene produced, 257, belongs to the long-sought persistent triplet carbenes, although its... 

The various products obtained from acetylenic diols 54 in the presence of acids suggest the formation and interconversion of acetylene-allene-diene cationoid intermediates (equation 18)33. The allene intermediates can be sometimes isolated and they were reported as participants in the acid-catalyzed reactions of alkynylpyrylium salts 55, a driving force of which is an aromatization of the pyrane ring (equations 19-21)34,35. 

The rearrangement of arylethynyl carbinols 58 that occurs via allene intermediates 59 and 60 in the presence of a polymeric silylvanadate catalyst37 (equation 23) is noteworthy. 

A new benzannulation methodology was developed in order to overcome the limitations of electrocyclic ring closure of divinylindoles. The cyclization is achieved via an allene-mediated electrocyclic reaction of 2,3-difunctionalized indoles. This method is more efficient for the synthesis of highly substituted 2-methyl carbazole alkaloids (559). The 3-alkenyl-2-propargylindole 557, a precursor for the allene intermediate, was prepared from 2-formylindole over several steps using simple functional group transformations (536,537) (Scheme 5.20). 

Again there is a number of interesting syntheses of di- and tetrahydrofuian derivatives. Due to space limitations only a few examples can be mentioned. The transformation of enantio-metically enriched butyn-l-ol monoesters into 2,2,5-ttisubstituted 3-acyloxy-2,5-dihydroftirans with complete stereospecificity was achieved by a Ag(I)-mediated rearrangement via allenic intermediates. The starting materials were prepared by an enantioselective reduction of the corresponding ketone, followed by acylation <99BCJ279>. 

The cyclization of enynes containing propargylic carbonate fragment involves allene intermediate, which takes part in intramolecular acylpalladation by endo-frig-modc to give a six-membered ring (Scheme 23). " ... 

A mechanism via an allene intermediate 135 was considered (equation 62), whereby elimination and re-addition of TMSZnCl could account for the rearrangement. Alternatively, a cyclopropylzinc intermediate 136 was proposed which, depending on the electrophile, could undergo different reactions (equation 63). This could account for the observed dependence of the product distribution on the electrophile (see entries 2 and 5 in Table 11). 

According to the authors an allene intermediate is not involved in this complex transformation [85]. If true, allenes are also unlikely to be produced in the dehalogenation reactions of some related dihalocyclopropanes [86],... 

The process may be reversed by treating 2-alkynes with sodium to get 1-alkynes after acidification of the product. The presence of an alkyl substituent at C(3) of a 1-alkyne prevents its isomerization to the corresponding 2-alkyne. Instead, an allene is formed [Eq. (4.27)]. This observation led to the suggestion of the involvement of allene intermediates to interpret the shift of the triple bond in the interconversion of acetylenes [Eq. (4.28)] ... 

It should be noted that there is a kinetic isotope effect on the normal reaction (9.11) when the a-deuterated compound is used as the substrate. A similar effect is found when the deuterated suicide inhibitor is used. Thus, both reactions involve a proton transfer in the rate-determining step of the reaction. It has also been shown that a sample of the allenic intermediate that is prepared chemically does in fact irreversibly inhibit the enzyme.18... 

Perfluoropenta-1,4-diene (10) isomerizes in the presence of antimony(V) fluoride to an E jZ mixture of 1,3-diene 11.41 Heating 11 at 90 C with antimony(V) fluoride gives cyclodimer 12, presumably via allene intermediate 13. 

The alternative /3,y-elim [nation-addition route, forming the cyclo-allenic intermediate (161, n = 2, 3) could not be dismissed, since the equilibrium between a cycloallene and the corresponding cycloalkyne... 

Irradiation of the trisilane system in the presence of l,4-bis(trimethylsilanyl)buta-l,3-diyne affords the as- and // ///Av-isomcnc l,3-dimethylene-2,4-disilacyclobutane derivatives, presumably via a 1-sila-allene intermediate (Equation 32) < 1999EJI2301 >. 

The diyne 66 undergoes an intramolecular dehydro Diels-Alder reaction in toluene under reflux to afford naphtho[2,3-c]chromene derivatives in reasonable yield (Scheme 20). The reaction is presumed to proceed via a [4+2]-cycloaddition of the alkyne to the aryl alkyne group to form the cyclic allene intermediate 67, which then isomerizes to the aromatic product <2003SL1524>. 

Note that alkynyl derivative 9 (Scheme 5.7) could not be isolated since during its synthesis, it directly formed allenyl substrate 12. Also, allene intermediate 11 could be isolated (in 56% yield) when the reaction was conducted at room temperature.42... 

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