Cumulene chirality

In the D2h cumulenes, chirality is generated only by trans double bridging. The doubly bridged ethylene 43 ( bis(fn is-polymethylene)ethylene ) cited first in Cahn, Ingold, Prelog s paper (7) is the simplest example of a trans doubly bridged D2h cumulene (42) where p = 0 and m = n = 6. 

Adjacent polyenes are hydrocarbons with double bonds between each pair of atoms in the polyene system they are called cumulenes2. Two types of cumulenes exist those with an even number of adjacent double bonds and those with an odd number. The former can exhibit chirality, the latter geometric isomerism. Allenes (propadienes) are the simplest members of the even-numbered type of cumulenes. 

While the even-numbered cumulene gives (Z)-111 of C2y symmetry and ( )-112 of C2h symmetry, the odd-numbered cumulene gives two enantiomers 113 and 114 both of C2 symmetry. Bridging these two substituents A and A in 113 or 114 by a chain gives rise to a single-bridged allene of C2 symmetry whose chirality can be specified following the axial chirality rule 11 ... 

The enantioselective intramolecular formal 2+4-cycloaddition of acrylates and a, -unsaturated imines (99) catalysed by chiral phosphines (100), derived from amino acids, produced A-heterocycles (101) (Scheme 31). Chiral dirhodium(II) carboxamidates (102) catalysed the hetero-Diels-Alder reactions between 2-aza-3-silyloxy-l,3-butadienes and aldehydes to yield all cw-substituted l,3-oxazinan-4-ones in high yields and high enantioselectivity (98% ee)P The nickel-catalysed 4 + 2-cycloaddition of a, -unsaturated oximes with alkynes yielded 2,3,4,6-tetrasubstituted pyridine derivatives. The reaction of isoquinoline, an activated alkyne, and 4-oxo-4//-l-benzopyran-3-carboxaldehyde (103), in ionic solvents, produced 9a//,15//-benzo[a][l]benzopyrano[2,3-/t]quinolizine derivatives (105) via the zwitterion (104) selectively and in good yields (Scheme 32).The Diels-Alder cycloaddition of ethyl 3-(tetrazol-5-yl)-l,2-diaza-l,3-butadiene-l-carboxylates with -rich heterocycles, nucleophilic olefins, and cumulenes formed 3-tetrazolyl-l,4,5,6-tetrahydropyridazines regioselectively. The silver-catalysed formal inverse-electron-demand Diels-Alder... 

Ketiminium salts have been shown to participate readily in [2-r 2]-cyclo-additions with olefins [35]. Ghosez reported a classic in this area, involving the use of prolinol-derived ketiminium species (Scheme 18.28) [122]. Treatment of a-chloroenamine 172 with ZnCl2 facilitated chloride abstraction under mild conditions to yield an intermediate ketiminium salt 173. The reactive cumulene participated in a diastereoselective cycloaddition with cyclo-pentene to afford the chiral ketone 174 (> 97 % ee), after hydrolytic removal of the auxiliary. These diastereoselective [2 2]-cycloadditions of Ghosez provide a rare example of a chiral auxiliaiy-based approach to chiral cyclobutanones. 

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]. 

Axial chirality the atoms are placed in two perpendicular planes that are nonsymmetrical and cannot rotate freely (allenes, cumulenes, spiranes, and substituted ortho-biphenyls). 

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