Butatrienyl cation

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. 

Butatrienyl cations 61 were also implied in the solvolysis of the w-propynyl vinyl triflate 6243. The vinyl triflate solvolyzes 35-70 times faster than the corresponding simple vinyl analogues and gives products resulting mainly from the a-alkynyl vinyl cation 63. A small amount of the allenic ketone 64 derived from the butatrienyl cation is also detected (equation 22). 

Higher homologues than the butatrienyl cations are not known to the best of my knowledge. The summary given here provides strong evidence for the existence of the cumulated vinyl cations, i.e. allenyl and butatrienyl cations. A leap into the preparatory domain, harvesting the potentials of these cations, should definitely be a fruitful venture. 

The interaction of (PPh3)4Pt with alkylnylvinyl triflates results in formation of their reactions were also reported145,146. Vinyl triflates reacted with PPh3 in the presence of catalytic amounts of Pd(0), with formation in good yields of vinyl phosphonium salts with high stereospecificity as shown in equation 132. 

Both the butatrienyl halides 54a and 54b gave the alkyne (56) as the sole product in 97-100% yield41. The kinetics described above fit the mechanistic sequence shown in equation 20 for the formation of the product 56. The mesomeric butatrienyl vinyl cation 55... 

If one compares the solvolyses of 2-bromo-l,l-diphenyl-4-(p-methoxyphenyl)-but-l-en-3-yne (57) and 4.4-diphenyl-1 -bromo-1 -(/ -mcthoxyphcny l)-buta-1,2,3-tricncs (58, X = Br) in aqueous ethanol (equation 21), the destabilization of the intermediate cation 59 by the large inductive effect of the triple bond as compared to its conjugative effect is evident42. Only in the case of 58 could the substitution product butatrienyl enol ether 60 be isolated in 40% yield, while it was only detected by UV and IR spectroscopy in the solvolysis product of 57. The faster observed reaction rate of 58 as compared to 57 was ascribed to a difference in their ground-state energies42. 

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