Cumulated diene

Cumulated dienes are those m which one carbon atom is common to two carbon-carbon double bonds The simplest cumulated diene is 1 2 propadiene also called allene and compounds of this class are generally referred to as allenes... 

Thus the order of alkadiene stability decreases m the order conjugated diene (most stable) isolated diene cumulated diene (least stable) To understand this ranking we need to look at structure and bonding m alkadienes m more detail... 

Because of the linear geometry required of cumulated dienes cyclic allenes like cycloalkynes are strained unless the rings are fairly large 1 2 Cyclononadiene is the smallest cyclic allene that is sufficiently stable to be isolated and stored conveniently... 

We will not discuss the preparation of cumulated dienes They are prepared less readily than isolated or conjugated dienes and require special methods... 

Our discussion of chemical reactions of alkadienes will be limited to those of conju gated dienes The reactions of isolated dienes are essentially the same as those of individual alkenes The reactions of cumulated dienes are—like their preparation— so specialized that their treatment is better suited to an advanced course m organic chemistry... 

Section 10 6 Conjugated dienes are more stable than isolated dienes and cumulated dienes are the least stable of all... 

Propadiene (H2C=C=CH2) also called allene, is the simplest cumulated diene The two tt bonds m an allene share an sp hybridized carbon and are at right angles to each other Certain allenes such as 2 3 pentadiene (CH3CH=C=CHCH3) possess a chirality axis and are chiral... 

Elimination is typically regioselective and gives a conjugated diene rather than an isolated or cumulated diene system of double bonds... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Cumulated diene (Section 10.5) Diene of the type C=C=C, in which a single carbon atom participates in double bonds with two others. 

The thermochemistry of totally cumulated trienes, i.e. species with the C=C=C=C substructure, is very limited. Indeed, the sole examples we know are those reported by Roth, namely (Z)- and ( )-2,3,4-hexatrienes MeCH=C=C=CHMe, species 17 and 18. Their enthalpies of formation are identical to within experimental error, 265 kJ mol-1. This equality is altogether reasonable given the small Me—Me interaction across the 4-carbon, linear, cumulene chain in contradistinction to the 4.3 kJ mol-1 difference that is found for the isomeric (Z)-and (E)-2-butenes with their significantly smaller Me...Me distance. Are cumulated trienes unstable relative to cumulated dienes much as cumulated dienes are unstable relative to simple olefins Briefly regressing to cumulated dienes, this assertion is corroborated by the finding that species 3, i.e. 1,3-dimethylallene, has an enthalpy of decarbonization 18 of 144.5 kJmol-1 (reaction 12)... 

The cumulated Jt-system of allenes has been described as consisting of two comparatively unperturbed double bonds with regard to its reactivity towards nucleophiles or electrophiles [10]. Early reports on radical additions to 1,2-dienes, however, already pointed to peculiarities of the allene system concerning its reactivity towards intermediates with unpaired electrons [11-14], It was soon realized that no such correlation between polar and steric substituent effects existed, similar to what had been uncovered for the reaction of radicals with olefins, in order to predict selectivities in radical additions to cumulated dienes [4, 15],... 

Ruthenium complexes B also undergo fast reaction with terminal alkenes, but only slow or no reaction with internal alkenes. Sterically demanding olefins such as, e.g., 3,3-dimethyl-l-butene, or conjugated or cumulated dienes cannot be metathesized with complexes B. These catalysts generally have a higher tendency to form cyclic oligomers from dienes than do molybdenum-based catalysts. With enol ethers and enamines irreversible formation of catalytically inactive complexes occurs [582] (see Section 2.1.9). Isomerization of allyl ethers to enol ethers has been observed with complexes B [582]. 

The calculated AH, assuming no interaction between the double bonds, is 2(-126) = -252. The more negative the observed value of AH compared to -252, the less stable the diene the less negative the observed value, the more stable the diene. Conjugated dienes are most stable and cumulated dienes are least stable under the prof>er conditions allenes tend to rearrange to conjugated dienes. 

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