Allene-Olefin Cycloadditions

Kiefer and Okamura 73> demonstrated stereoselectivity with respect to the olefinic component in an allene-olefin cycloaddition. 1,1-Dimethyl-allene with dimethyl fumarate at 160—180 °C gave two cycloadducts, both retaining the trans disposition of ester functions with better than 99% stereoselectivity dimethyl maleate gave the isomeric cis adducts with... 

With the hypothesis that allene-olefin cycloadditions and methylenecyclobutane rearrangements have a common intermediate comes the prediction that common stereochemical modes for the various rotations involved will prevail. The cycloaddition model outlined above involves disrotatory ring closure with the same sense of rotation about C(4)C(5) and C(5)C(2). For the methylenecyclobutane rearrangement, the same stereochemistry is anticipated for the reverse reaction. 

The approximations inherent in the EH method make the small energy preference calculated for 4 (0, 90, 90) over 4 (0, 90, 0) seem inadequate grounds for firm conclusions. We simply note that the preference for 4 (0, 90, 90) as a stereochemical model for the intermediate in allene-olefin cycloadditions deduced from experimental data is consonant with the EH result. The very small dependence in energy upon the angle y for 4 (0, 90, y) may be cause for suspecting that at least in some substituted cases, rotation through y = 180 ° in the intermediate 4 (0, 90, y) may be kinetically competitive with ring closure. 

These new developments place allene-olefin and allene-allene thermal cycloadditions in a new context framed by the questions Are there reactive intermediates separating reactants and products in the cycloadditions, or in the degenerate rearrangements Might there be a common reactive intermediate for the allene-olefin addition and the methyl-enecyclobutane rearrangement, and another for the allene-allene addition and the 1,2-dimethylenecyclobutane rearrangement How may the experimentally observed stereochemical features of these four processes be reconciled with orbital symmetry theory ... 

Kinetic isotope effects indicate that both allene-olefin and allene-allene cydoadditions occur in at least two steps, and stereochemical results suggest they may be highly stereoselective. These facts and analysis of the categories included under the rubric of concertedness leads to rationalization of these (2 + 2) cycloadditions as orbitally concerted but energetically nonconcerted, or equivalently, as concerted two-step processes. 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

The ketone (265) was prepared via photo[2 + 2]cycloaddition between the corresponding olefin and allene 91). 

As exemplified in Eq. 8.38, thermal [2 + 2] cycloadditions of 4-vinylidene-2-oxazoli-dinone 287 and alkynes such as phenylacetylene result in the formation of 3-phenyl-substituted methylenecyclobutene 288 [149]. The authors confirmed by NMR analysis that only the Z-configuration isomer was formed. It is worth noting that the [2 + 2] cycloaddition of allenes 287 is not restricted to alkynes even olefins such as acrylic esters or silyl enol ethers furnish the corresponding methylenecyclobutanes... 

Electron-deficient olefins such as acrylonitrile can participate in the cross [2 + 21-cycloaddition with allenes. 3-Methylenecydobutanecarbonitrile (17) was obtained in 60% yield by the reaction of allene with a large excess of acrylonitrile under autogenous pressure at 200 °C [16]. Initial bond formation takes place between the central carbon of allene and the terminal carbon of acrylonitrile to give a diradical species, which cydizes to form the cydoadduct [17]. 

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. 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

With this revision in our original plans, both alkenes and allenes were found to undergo efficient cycloadditions to produce cyclooctenone products in a new [6+2] cycloaddition process. This novel cycloaddition has been shown to proceed efficiently with alkenes tethered with sulfonamide, ether, or geminal diester Hnkers (Tab. 13.15, see page 294). Isomerization of the olefin, a potential competing reaction in this process, is not observed. Methyl substitution of either alkene in the substrate is well tolerated, resulting in the facile construction of quaternary centers. Of mechanistic importance, in some cases cycloheptene byproducts were isolated from [6+2] cycloaddition reactions in addition to the expected cyclooctenone products (that is, entries 3 and 4). 

An exceptionally interesting example of an intramolecular [3 + 2] cycloaddition, in which the diazo dipole and the olefinic C=C bond are separated by only one carbon atom, is outlined in Scheme 8.65. The thermal decomposition of the allenic tosylhydrazone sodium salt 267 produced 1,4-dihydropyridazine 269 (57). It is assumed that diazabicyclohexene 268 is a short lived reaction intermediate. This suggestion is supported by the observation that the generation of the diazocumulene l-diazo-2-methyl-l-propene in the presence of 3,3-dimethylcyclopropene also leads to 269. 

The thermal reaction between two molecules of olefin to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the olefins are the same or different, but the reaction is not a general one for olefins.921 Dimerization of like olefins occurs with the following compounds F2C=CX2 (X = F or Cl) and certain other fluorinated alkenes (though not F2C=CH2), allenes (to give derivatives of 97),922 benzynes (to give biphenylene deriv-... 

The formation of four-membered rings through 2 + 2 cycloaddition is a well-established reaction and the most generally effective synthetic approach to cyclobutanes. Most olefins cannot be induced to undergo this reaction thermally, a finding that is readily rationalized by the forbidden nature of the 2s + 2s addition and the steric difficulties associated with the allowed 2s + 2a pathway. There are nevertheless exceptions. Olefins substituted by two or more fluorine atoms undergo thermal 2 + 2 additions under relatively mild conditions,16 as do ketenes and allenes. 

The cycloaddition reactions of captodative olefins all are considered to proceed through the intermediacy of a 1,4-diradical, due to the captodative stabilization of the terminal radicals. In cross-cycloadditions captodative olefins easily give cyclobutanes when heated with fluoroolefins [141]. They also react with allenes to give methylenecyclobutanes [142], and with methylenecyclopropane to give spiro[2.3]hexanes [143]. 

Allene and ketene, though isoelectronic, have quite different photoelectron spectra and electronic structures 4> that they undergo (2 + 2) cycloadditions with olefins by quite distinct mechanisms 6-8> is not altogether surprising. 

Recently, we reported an entirely new and efficient formal [3 + 2] cycloaddition based on the hydrocarbonation reaction of allenes. The palladium-catalyzed reaction of the activated olefins 4 with allenes 26, bearing an acti-... 

Cycloaddition Reactions Across C—C Multiple Bonds. Cycloadducts derived from carbodiimides and olefins or allenes are not known. However, the [2+2] cycloaddition of ketenes, R2C=C=0, to carbodiimides affords 4-imino-2-azetidinones (/3-lactames) 239 in high yield. Aliphatic carbodiimides show higher reactivity in comparison to aromatic carbodiimides, and the reaction proceeds across the aliphatic C=N bond in N-alkyl-N -arylcarbodiimides. The cycloadducts obtained in this reaction are listed in Table 2.3. 

The methods for generating acyl ketenes (Scheme 7-V) and their subsequent in situ participation in [4 + 2] cycloadditions with a wide range of hetero- or olefinic and acetylenic dienophiles (Scheme 7-VI), including acyl ketenes,185 186,197 carbonyl compounds, 86-188 nitriles,1874,189,191 isocyanates and isothiocyanates,1864,190,191 ketenes,191 imines,1864,1874,191,192 carbo-diimides,l87c 190,191,193 ynamines,194 ketene acetals,1864,195 enol ethers,1864,191,196 and V-sulfinylamines197 have been extensively reviewed.5,9,12 Two reports have detailed the 4-n- participation of allenic ketones in [4 + 2] cycloaddition reactions [Eq. (51)].198,199... 

Aromatic and aliphatic acyl isocyanates participate in a similar range of [4 + 2] cycloadditions although [2 + 2] and simple addition reactions often are observed. The acyl isocyanate substituents may determine or alter the observed course of the reaction, and the substituent effects have been detailed in extensive reviews.7,71 Observed [4 + 2] cycloadditions of acyl isocyanates with selected olefins, p-quinones, allenes, the carbon-carbon double bond of ketenes, electron-rich acetylenes, imines, dianils, ethy-lenediimines, enamines, enol ethers, ketene acetals, carbodiimides, azirines, and vinyl sulfides have been described.7 0 The reaction of aromatic acyl isocyanates with carbodiimides is not a simple, direct [4 + 2] cycloaddition but proceeds by a kinetic [2 + 2] cycloaddition followed by a subsequent rearrangement to provide the observed [4 + 2] cycloadduct [Eq. (40)].97... 

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