Allene Cycloadditions

AUene Cycloadditions. —The consensus of opinion is that allene dimerization to 1,2-dimethylenecyclobutanes is a two-step process which proceeds via an intermediate bisallyl biradical (315). Preference for formation of those stereoisomers having larger groups (R) on the inside positions of the double bonds of the product (316) can be explained on this model by assuming that they occupy the less hindered inward positions in (315). Dimerization of 1,3-diadamantylallene was studied since models suggested that a product such as (316 R = adamantyl) would be impossible to form. In fact dimerization 

Battioni, L. Vo-Quang, and Y. Vo-Quang, Tetrahedron Letters, 1972, 4803. J. E. Baldwin and R. H. Fleming, Portschr. Chem. Forsch., 1907, 15, 28. 

A study by Dolbier and Dai of secondary deuterium isotope effects in reactions of deuteriated allenes with various olefins and dienes led to the conclusion that all [2 + 2] four-electron thermal cycloadditions, including dimerization, are multistep processes, whereas [2 -I- 3] and [2 + 4] six-electron thermal cycloadditions are concerted. Thus allenes are demoted to the rank of relatively reactive alkenes.  

When cyclohexa-1,2-diene (318) is generated from dibromonorcarane it rapidly dimerizes to an intermediate diallylene (319) which cyclizes to diene (320) or dimerizes to two C24H32 stereoisomers. Hence, the formation of (320) is known to be a two-step process from (318). Using labelled dibromide (321), 

The structures of all seven dimers possible from the thermal dimerization of methylallene have been assigned.  

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Kanematsu K. Molecular Design and Syntheses of Biologically Active Compounds Via Intramolecular Allene Cycloaddition Reaction Strategy Rev. Heteroat. Chem. 1993 9 231-259... 

The study of the intramolecular nitrile oxide—allene cycloaddition shows, in particular, that dehydration of nitroallene 339 by PhNCO, generates a nitrile oxide in situ, which gives isoxazoline 340 (Scheme 1.36). Thus, the reaction of the more remote double bond with the formation of six-membered ring prevails (405). 

Angularly substituted bicyclo[4.3.0] systems can be synthesized efficiently through a rhodium-catalyzed intramolecular [4 + 2] diene-allene cycloaddition protocol (Scheme 16.74) [80]. 

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

A [2 + 2] cycloadduct 29 is also obtained with ethoxyallene (28) and 2-methylacrylaldehyde reflecting the opposite regiochemistry for allene cycloaddition.22... 

Allenic selenoketenes 129, generated by [3,3] sigmatropic rearrangements, react with primary amines to afford 2-imino-277-5,6-dihydroselenines 130 (Scheme 12) <2001JOC4099>. This reaction appears to be the first example of a selenoketene-allene cycloaddition. Uncyclized allenic selenoamides 131 are by-products in the reactions and are the only products when the reaction is carried out with secondary amines. 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

The complete omission of specific material on allene cycloadditions in the Woodward and Hoffmann review article 142> emphasizes the ticklish theoretical situation there may be no violations to the principle of maximum bonding 142>, but sure applications of the theory in its present early stage of development are more easily made for some types of reaction than for others. 

This TiCU-promoted allene cycloaddition has also been extended to the synthesis of five-membered ring heterocycles. In this case, increasing the steric shielding about the silicon atom seems to improve the cyclization process by suppressing the unproductive desilylative alkylation. For example, 1,3-di-methyl(r-butyldimethylsilyl)allene (39) reacts smoothly with cyclohexanecarbaldehyde to give in high yield the dihydtofuran predominantly as one stereoisomer (equation 31). Similar reaction with an N-acyl-iminium ion precursor (40) produces the pyrrolizidine system as a mixture of bicyclic isomers (equation... 

The picture of allene cycloadditions as [n2s+n2s+n2s reactions is by no means proved but it does provide an explanation for most of the puzzles in these remarkable reactions. 

Intramolecular enone-allene cycloadditions obey the rule of five and always take place across the 2,3-double bond of the allene (see Table 5, entry 12128). 

The IMDA-Grob fragmentation strategy that starts with an allene cycloaddition reaction is illustrated in Scheme 43. The allene-furan cycloadduct was formed together with the other exoadduct which had the opposite configuration at the i-propyl group. Partial isomerization of the other product into the thermodynamic product shown and conversion into the 10-membered ketone constituted a formal synthesis of periplanone-B <88TL650l> since that ketone had been used in an... 

Transition metal catalyzed intramolecular [4+2] diene-allene cycloadditions provide 6,6- and 6,5-fused ring systems in good yields under mild conditions. For example, from the tethered allene 404 in the presence of Ni(COD)2 as the catalyst a 97 % yield of the cycloadduct 405 is obtained resulting from addition to the terminal allene double bond. From the same substrate, using [Rh(COD)Cl]2 as the catalyst, the hydrindane derivative 406 is obtained in 90 % yield. 

Intramolecular nitrone-allene cycloaddition has been used to prepare the bicyclic isoxazolidine (195). ... 

Intramolecular photochemical [2+2] ene-allene cycloaddition of 10 or homo-logue 11 generates the tricyclic methylene cyclobutanes 12 and 13 in 60-99%... 

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