Allene-acrylonitrile cycloaddition

The intramolecular and intermolecular deuterium isotope effects in the cycloaddition of acrylonitrile to allene (equation 98) have been studied by Dolbier and Dai231,232. The intramolecular KIEs in the allene-acrylonitrile system were found to be 1.21 0.02 at 206°C and 1.14 0.02 at 225°C. A negligible intermolecular SKIE was found in the reaction of the mixture of tetradeuteriated and undeuteriated allene using a limited amount of acrylonitrile (ku/ku) = 1.04 0.05 at 190-210 °C for D0/D4 allene. An equilibrium deuterium IE of 0.92 0.01 was found at 280-287 5°C (15-45 h reaction time). 

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]. 

Pasto and colleagues studied the stereochemical features of the [2 + 2] cycloadditions of chiral allenes. The formation of a diradical intermediate in the cycloadditions of enan-tiomerically enriched 1,3-dimethylallene (10) with acrylonitrile (11a) and methyl acrylate (lib) (equation 4) was shown to be irreversible. 1,3-Dimethylallene recovered from the reaction mixture was shown to have the same ee as the starting material. Interestingly,... 

Other unsaturated ketones to which allene 2 undergoes cycloaddition include cyclopent-2-enone, l-phenyl-but-2-enone, l,4-diphenyIbut-2-enone, l-phenyloct-2-enone and acrylonitrile in yields ranging from 60 to 92%. The use of a chiral titanium(IV) catalyst gives chiral methylenecyclobutanes 5 with optical yields of at least 94%.2... 

The stereoselectivity of the cycloaddition with respect to the allene component was learned through study of the reaction of -(-)-1,3-dimethyl-allene 69,133,136) and acrylonitrile 8>. The reaction gave four cycloadducts in approximately equal proportions, all having a predominance of the R configuration at C(2) of the ethylidenecyclobutanes. 

Introduction of an alkylthio group on the allene system increased the reactivity of the allene moiety in [2 + 2] cycloaddition reactions. It proved possible to conduct reactions of this allene at much lower temperatures. By adding Lewis acids, the reaction temperature could be decreased even more, as was illustrated by the Lewis acid catalyzed [2-1-2] cycioadditions of l-trimethylsilyl-l-methylthio-l,2-propadiene with a variety of electron-poor alkenes, including cyclic and non-cyclic enones, acrylates, methyl fumarate and acrylonitrile. When a chiral diol 21 based titanium catalyst was employed, the [2-1-2] cycloaddition reactions of /-acryloyl-l,3-oxazolidin-2-ones 17a and 17b with allenyl sulfides 18 yielded methylenecyclobutanes 19 and 20 with high optical purities (equation The highest yields were obtained with electron-poor allenophile 17b. 

Furan also undergoes cycloadditions with allenes," " with benzyne" and with simpler dienophiles, like acrylonitrile and acrylate various Lewis acidic catalysts can assist" in some cases, zinc iodide is one such, hafnium tetrachloride another, and improved endo exo ratios are obtained in an ionic liquid as reaction solvent. Maleate and fumarate esters react if the addition is conducted under high pressure. This device can also be used to increase markedly the reactivity of 2-methoxyfuran and 2-acetoxyfuran towards dienophiles. At higher reaction temperatures alkynes and even electron-rich alkenes will add to furan. 3- or 5-Halo-furans react faster in these cycloadditions. ... 

The regiochemistry of the cycloadditions of allenes is not easily explained by these frontier orbitals. Penta-2,3-diene and acrylonitrile give the adducts Z- and -6.404 in which the central carbon of the allene, with the smaller HOMO coefficient, has bonded to the / carbon of the Z-substituted alkene.894 Equally unexplained is the regiochemistry of the reaction between allene itself and diazomethane, which gives more of the adduct 6.405 than of its regioisomer 6.406.893... 

Furan also undergoes cycloadditions with allenes and even with simpler dienophiles, like acrylonitrile and acrylate (specifically enhanced by the presence of... 

An intramolecular secondary isotope effect has been measured by analysing the product of 1,2-cycloaddition of l,l-c(2-allene to acrylonitrile in benzene at 210-225°C. The result is (Ah/Ad)2s°c 1-18 per D atom it has the opposite direction of that found for a Diels-Alder reaction of M-r/j-allene, and is consistent with a two-step mechanism (see Section 4.1.5). A diradical intermediate seems probable. A similar result was obtained from the study of isotope effects for the dimerisation of allene. Since the reaction of / (-)-... 

The authors found just such an effect in the presumably concerted H- 2]-cycloaddition of allene to hexachlorocyclopentadiene, but not in its [ 2 H- 2]-cycloaddition to acrylonitrile. There, athough no isotope effect was observed on the rate of reaction, there was a substantial direct isotope effect ( h > d) on product formation protium was preferentially incorporated in the ring. The authors therefore concluded that the reaction takes place in two steps. 

Cycloadditions of Substituted Allenes with Acrylonitrile -With isochloroprene, the 3-methylene-2-chloromethylcyclobutanecar-bonitrile and l-chloro-l-cyclohexene-4-carbonitrile mixture was obtaineS in 21% yield, and 3-chloropropionitrile in 15% yield. 

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