1 - l-penten-4-yne

When this reaction is carried out under 1 atm of nitrogen or GO atmosphere, a cyclopentane 276 is formed selectively in a minute at 25 °G (Scheme 13, mode 2). Although the Pauson-Khand reaction of 1,6-enyne 273 (Scheme 13, mode 3) gives 21H, this transformation is completely suppressed under the conditions of mode 1. Even simple alkyne silylformylation product 277 is not detected at all. This contrasts sharply to the silylformylation of l-penten-4-yne 48 carried out under similar conditions (Equation (12)). These results can be explained by a pathway similar to the reaction of 1,6-diynes (i) stepwise insertion of the acetylenic and olefmic moieties into the Rh-Si bond in this order, and (ii) subsequent interaction of GO and Mc2PhSiH with the resultant intermediate to give 275. The... 

This is seen in its examples of 3-Penten-l-yne vs. l-Penten-4-yne, where the combined numbers 1,3 are selected as the priority numbering in the first of these names however, since the same numbering 1,4 would result in the second of these compounds, the "-ene" is given preference. Moreover, in writing the IUPAC canonical name, the suffix "-ene" always precedes the "-yne". 

Pentadiene (two double bonds) > l-penten-4-yne (one double bond) -... 

Selectivity between alkenes and alkynes is exemplified by the reaction of l-pentene-4-yne to give mainly 4,5-dibromo-l-pentyne (Figure 11.36). This selectivity derives from the poor stability of vinyl cations. 

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