Allenyl cations generation

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

However, until recently no data were available on the direct generation of allenyl cations, 242, via solvolysis of suitable precursor allenyl substrates. 

Allenyl cations have been generated by solvolysis of allenic derivatives, by photolysis of allenyl halides and by reaction of metal salts with allenyl and propargyl halides. This review will delineate these reactions. The related butatrienyl cations are not many and they will be only briefly described. 

Ferrocenyl-substituted allenyl cations 28 were generated when 1,3-diferrocenyl-substituted secondary and ferrocenyl-substituted tertiary alcohols 29 were treated with trifluoroacetic acid27. These were rapidly converted into trifluoroacetoxyallylic ions by solvent addition the ions gave ferrocenyl-substituted enones by reaction with water (equation 8). 

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 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. 

Substituted allenyl cations 47 have been generated from propargyl alcohols 48 under stable carbocation conditions (Sbf s/f SOsII in SO2CIF) (equation 17). On the basis of 13C-NMR chemical shifts, the positive charge has been found to be extensively delocalized with the mesomeric allenyl cations contributing highly to the total ion structure36,37. 

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

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... 

Other than the stabilized allenyl 242 and related 248 cations, there is also to date no report on the solvolytic generation of a primary 256 vinyl cation. Such species have, however, been implicated in the decomposition of some... 

The ratios of nucleophilic substitution versus [3,3]-sigmatropic rearrangement for the collapse of allenyl(aryl)iodine(III), generated from the reaction of aryliodanes with propargylsilanes in the presence of Bp3.0Et2 in alcohols, have been determined. The mechanism proposed by the authors ° involves the generation of propargyl cations from the allenyliodine (III) via a unimolecular pathway. 

Only a few cyclization processes of this type have been described up to now. Most of them result from inter- or intramolecular attacks of anionic nucleophiles containing at least two reactive heteroatoms. Thus, sodium dimethyldithiocarbamate reacts with the cationic allenylidene complex [RuTp(=C=C=CPh2)(PPh3)2][PF6] (32) to generate the allenyl-metallacycle 33 (Scheme 2.12), as the result of the nucleophilic addition of one of the sulfur atoms at the carbon and subsequent coordination of the second... 

A new type of triaryl phosphine-functionalized imidazolium salt containing cations such as (6) has been prepared. Palladium complexes of (6) generated in situ have been used successfully in Heck-type reactions of aryl halides with acrylates and of 4-bromotoluene with styrene derivatives.34 The first Heck-type reaction of aryl halides with allenes has been reported. 1,3-Double arylations were observed with 3-substituted-l,2-allenyl sulfones, while 1-monoarylation was favoured with 3,3-disubstituted-l,2-allenyl sulfones.35 It has been shown that the a-arylation of methane-sulfonamides (7) may be achieved using palladium catalysis reaction proceeds through the sulfonamide enolates.36 It is also reported that palladium cross-coupling of alkynes with /V - (3 - i odophe n y I an i I i ncs) may lead to the formation of substituted carbazoles.37... 

This transformation was further studied and the catalyst load could be decreased to 0.2 equiv.35 33 A mechanism was proposed through deuterium incorporation experiments, and the conclusion was that there was no 1,2 shift of the deuterium present in the starting material (A, Scheme 5.3) since exclusive formation of furans D deuterated on position 3 could be explained by the presence of an external source of deuterium (such as D20). Therefore, it is believed that after silver(I) coordination to the allenyl system (A, Scheme 5.3), the attack by the carbonyl oxygen may lead to an oxo cation intermediate B. Finally, proton lost would generate silver furan C that would lead to furan D after silver release (Scheme 5.3).39... 

Propargyl cations may also be generated by heterolytic fission of the C—X bond of allenyl derivatives (178). Early attempts to solvolyse chloride 178 (R = H R = Me) in alcoholic solution, even in the presence of silver nitrate, did not furnish reliable conclusions concerning the possibility of unimolecular solvolysis through the propargyl cation (Pudovik, 1951 Hennion and Maloney, 1951). The apparently slow... 

Acetylenic ketones are viable substrates as well for gold-catalyzed cyclizations to furans. Whereas alk-3-yn-l-ones readily cyclize to substituted furans in the presence of gold(I) or gold(III) chloride (possibly via isomerization to a-allenyl ketones see Section 5.2), the corresponding transformation of alk-4-yn-l-ones takes place in the presence of the cationic gold catalyst generated in situ from PhsPAuCl and AgOTf in toluene (Scheme 4-83). The cyclization is accelerated by... 

Planar chiral orf/to-substituted (arene)Cr(CO)3-stabilized a-propargylic cations (67) may be generated from (66) in 5n1 fashion mediated by Lewis acid. They are in principle ambident electrophiles, susceptible to nucleophilic attack at the propar-gyl or allenyl positions. However, they react with 5-, N-, and jr-nucleophiles to give regioselectively the propargyl derivatives (68) in good yields and excellent diastereoselectivities, overall a process of retention by double inversion. Without the orf/to-substituent nucleophilic attack at both possible positions occurs. 

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