Allenyl cations cycloadditions

Similar to the cycloaddition of allyl cations30, allenyl cations have been found to undergo cycloadditions with alkenes to afford bicyclic compounds31. The allenyl cations were generated from propargyl chlorides by treatment with Lewis acids. This reaction sequence proceeds via the cyclization 34 -------> 3532, in spite of the fact that... 

The cycloaddition of allenyl cations with monoolefins lead to [2 + 2]- or [3 + 2]-cycloadducts based on the substituents in the allenyl cations as exemplified in equation 1332. 

The cycloaddition of allenyl cations with 1,3-dienes results in a number of intermediate cations from which different products result. The allenyl cations 38 are generated first by the reaction of propargyl chlorides with zinc chloride and are then allowed to react with cyclopentadiene or other 1,3-dienes. The products of cycloaddition depend on the substituents on the allenyl cations32,35. The products formed with cyclopentadiene are given in equation 14. 

In general, allenyl cations 38 attack at the sp2-carbon atom of 1,3-dienes and form vinyl cations 39 and 40 (R = H, alkyl) or (R = aryl). Although a concerted cycloaddition mechanism is possible, a stepwise mechanism is preferred34. If a nucleophilic attack at the sp-carbon atom of the allenyl cation takes place, then cation 41 and the resulting cations 42 and 43 are formed. Some examples of bicyclic products obtained from cyclic 1,3-dienes and propargyl chlorides are given in equation 1534. 

Allenyl cations generated from silver dehalogenations6 or acid-catalyzed dehydration of prop-2-yn-l-ols7 also undergo cycloaddition to alkenes under mild conditions. 

Allenyl cations. In the presence of a zinc chloride-ether complex allenyl cations (a) can be generated from a propargyl halide such as (1). The cation can undergo either [3 + 2]- or [2 + 2]cycloaddition with an alkene. When the R group is aryl, [2 + 2]cycloaddition is the major pathway (2) when R is CHj or C,H3, [3 + 2]-cycloaddition becomes the major reaction (3) (equation 1).1... 

A range of thiazoles with various combinations of Ri, R2, and R3 substituents was synthesized using this method. The silver-catalyzed cycloaddition of propargylic alcohol with thioamide was proposed to proceed through the intermediacy of a propargylic cation or the corresponding allenyl cation and its subsequent reaction with the nucleophilic sulfiir of thioamide, followed by a 5-exo-dig attack by nitrogen. 

In the presence of catalytic quantities of zinc halides, propargyl halides are converted into allenyl cations these undergo stepwise [2 + 2] or [3 + 2] cycloaddition to olefins to furnish moderate yields of a-halobenzylidenecyclobutanes or 1-halocyclopentenes depending on the substitution pattern of the propargyl halide (Scheme 29). ... 

Allenyl ions 372 undergo [4+2] cycloaddition reactions. For example, from allenyl cations, formed in the reaction of propargyl halides with zinc chloride at —30 to —50 °C and cyclopentadiene the [4+2] cycloadducts 373 (R = alkyl) or the [4+2] cycloadducts... 

The first report of allenyl cations undergoing [4 + 3] cycloaddition with a diene has appeared.Thus, treatment of a solution of propargyl halide and cyclo-pentadiene in pentane with silver trifluoroacetate provides, after hydrolysis, a... 

The reactions with tertiary amines or phosphines that have no active hydrogen atoms result in platinacyclobutene cations, a rare species for late transition metal (Scheme 39). Substituted carbanions are added to the jj -allenyl/propargyl platinum complex to yield the neutral substituted-jj -TMM derivatives that undergo further [3 + 2] cycloaddition with good TT-acids as TCNE or maleic anhydride to produce highly substituted cyclopentanoids (Schemes 40, 41). 

Protonation of the allenyl complex (16) with HPFg at — 20°C gives the cationic acetylene complex (17), and this observation, together with product stereochemistry, strongly suggests that the [2 + 3] cycloaddition reactions of (16) with tetracyanoethylene and toluene-p-sulphonyl isocyanate, which generate (18) and (19), respectively, proceed via an acetylene dipolar ion (20) rather than an allene (21). However, protonation of the related butynyl complex (22) generates stereospecifically a cationic allene complex (23), which on... 

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