1,3-Cycloaddition of allene

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... 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

Intramolecular [2 + 2]-cycloadditions of allenes and enones occur with fair to good site- and regioselectivities. Thus, cyclohexenone 40 bearing an allenic side-chain at the C3 position was irradiated and a single product 41 was isolated in 95% yield [45],... 

The most recent review54 about [2 + 2] cycloadditions of allenes covers the literature up to 1992. This section deals with recent results combined with some representative results from the past decade. 

An ab initio study of the 2 + 2-cycloadditions of allene to isocyanic acid and ketene to vinylimine found the reactions to be concerted and mostly asynchronous.28,29 The diastereoselective 2 + 2-cycloaddition of dichloroketene with a chiral enol ether (26) produced the cyclobutanone (27), which leads to a key intermediate (28) in (g) the total synthesis of the natural alkaloid (-)-Swainsonine (29) (Scheme 8).30 The... 

Cycloaddition of allenes to alkenes. This cycloaddition to give methylenecyclobutanes (3) can be realized in the presence of certain Lewis acids as catalysts (listed in approximate order of decreasing reactivity) C2H5AICI2, GaCls, AlBrj, AICI3, FeClj. 

An important reaction that is on the spiropentane/methylenecyclobutane energy surface is the 2 + 2 cycloaddition of allene and appropriately substituted ethylenes to give methylenecyclobutane. 

Snider and coworkers125 have reported the Lewis acid catalyzed [2 + 2]cycloaddition of (phenylsulfonyl)allene 112. The reaction with methylenecyclohexane in dichloro-methane gives a 25% yield of an 8 1 mixture of 210 and 211 (equation 132). An addition reaction of l-(p-tolylsulfonyl)ethylene with enamines gives aminocyanobutanes via the zwitterionic intermediate (212) as shown in equation 133126. 

Less frequently applied are [3 + 2] and [2 + 2] cycloadditions of oxygen-substituted allenes [102-104], Battioni et al. described only a limited number of [3 + 2] cycloadditions of phenyloxy- and methoxyallene with diphenyldiazomethane (157) and the nitrile imine derived from diphenylhydrazonoyl chloride (159) (Scheme 8.40) [102], Both 1,3-dipoles exclusively attack the terminal C=C bond, furnishing cycloadducts 158 and 160. Padwa et al. reported [3 + 2] cycloadditions of methoxyallene 145 with two nitrones which afforded isoxazolidines in low yield [103]. 

Good regioselectivities were observed in the [2 + 2]-cycloaddition of heteroatom-substituted allenes [58],... 

Scheme 16.75 Rh-catalyzed intramolecular [5 + 2]-cycloadditions of allenes and vinylcyclopropanes.

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,... 

In addition to the concerted [2 + 2] cycloadditions of cyclic allenes reported by Elliot and colleagues, Kimura and coworkers22 reported [2 + 2] cycloadditions of several 4-ethenylidene-l,3-oxazolidin-2-ones 35 with alkenes and alkynes (equation 11). The... 

A review of the mechanism of thermal 2 + 2-cycloadditions of activated alkenes to allenes and ketenes has been published. " Stereoselective intramolecular 2 + 2-cycloadditions of alkene-keteniminium salts (19) derived from L-glutamic salts yield... 

Alcaide and coworkers have reported the thermolysis of (3-1 actam-tethered enallenyl alcohols to give tricyclic ring structures (Fig. 12) via a formal [2+2] cycloaddition of the alkene with the distal bond of the allene [273]. 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

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