Polyenes cumulated—

Using standard references and protocol, we find the three reactions are respectively endothermic by ca 2, 8 and 6 kJmol-1, or ca 2, 4 and 3 kJmol-1 once one remembers to divide by 2 the last two numbers because the allene is dialkylated. So doing, from equations 10 and 11 we find an average ca 3 kJmol-1 (per alkyl group) lessened stability for alkylated allenes than the correspondingly alkylated alkenes. This is a small difference that fits most naturally in the study of substituted cumulenes such as ketenes and ketenimines, i.e. not in this chapter. But it is also a guideline for the understanding of polyenes with more cumulated double bonds. 

In view of the above we have arranged the material according to the structural features by using a subdivision into cumulated, conjugated and non-conjugated dienes and polyenes. 

In a cumulated triene, or any cumulated polyene with an odd number of double bonds, the atoms connected to the terminal carbons lie in the same plane, just as they do in an ordinary alkene. Van t Hoff pointed out that suitably substituted cumulated polyenes of this type should then have cis and trans forms ... 

The construction of models emphasises all these stereoisomeric features also note that in a homologous sequence of cumulative polyenes, the presence of a chiral axis, or of geometric isomerism, alternates [i.e. diene (R/S), triene (E/Z), tetraene (R/S), etc.]... 

Classify the following dienes and polyenes as isolated, conjugated, cumulated, or some combination of these classifications. 

The title reactions are discussed in a series of reviews . However, the most complete and detailed description of rearrangements involving allenes was presented in Huntsman s comprehensive survey, wherein the cumulated systems were considered as either the starting materials, the intermediates or the reaction products. More recently, very detailed reviews devoted to vinyl cations and containing numerous examples of rearrangements of cumulenes and other polyenes were published . Therefore, this section will cover only relatively recent publications. 

Monocyclic polyenes containing the maximum number of non-cumulative double bonds (for short called mancude systems) and having the general composition C H or C H + i n> 6) can also be named as [n]annulenes. 

Alkenes or polyenes with isolated or coupled double bonds that are devoid of chiral atoms are not optically active. Such activity occurs in cyclic alkenes where double bonds occur in the trans form, such as in trans-cyclooctene (Figure 2.18). Another group of alkenes that includes representatives having optical activity is that of cumulenes. The name refers to cumulation of double bonds in such molecules. The best-characterized group of these compounds is that of allenes in which two double bonds occur next to each other [54]. Compounds of this type have a so-called chirality axis determined by cumulated double bonds. Besides allenes, higher optically active cumulenes are also known. An example of optically active cyclic allene is provided by 1,2-cyclononadiene, which was synthesized in 1972 [55]. 

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