Allene-propyne rearrangement

A detailed, extensive review of cyclopropene has appeared. Cyclopropene is the last of the small strained ring hydrocarbons to have its thermal decomposition subjected to intensive investigation.158 Cyclopropenes can decompose by a variety of mechanisms involving diradicals, vinylcarbenes, and vinylidenes. Cyclopropene itself has been shown to be an intermediate in the allene-propyne rearrangement. 

In the second example, analysis of the neutral obtained via halide abstraction by the ionic intermediate allowed one to establish the structure (2-propenyl vs allyl) of the gaseous c3h5+ ionic species formed in the protonation of allene and propyne in radiolytic experiments7. Using 1,4-dibromobutane as the trapping agent it was demonstrated that 2-propenyl ions are formed almost exclusively and trapped before they can rearrange to the more stable allyl cation (Scheme 2)7 (See also Section II.B.l). 

Allene 46 is prepared from l-(phenylthio)-l-propyne 45 [37a] by deprotonation [37b] with LDA, followed by quenching with chlorotrimethylsilane (TMSCl). Hydroboration of 46 with 9-BBN leads to allylborane. The facile al-lylic rearrangement of allylic dialkylboranes allows the formation of more stable Z isomer 47. Condensation of 47 with aldehydes proceeds smoothly at room temperature, and the intermediate 48, on treatment with sulfuric acid gives the Z-diene 49 and with 4 N sodium hydroxide affords the -diene 50 (Table 24.18) [36]. 

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