Cumulated double-bond systems

These molecules also have large substituents, and it might seem surprising that the radialene rings avoid puckered conformations in these species. The nonbonded repulsions are, however, reduced in these molecules because of external ring closures (13) or because two of the exocylic CC double bonds involve cumulated double bond systems (14 and... 

These considerations led us to the conclusion that suitable triple-bond systems A=B (or compounds containing cumulative double-bond systems A=B=C) might be found that add pyranoses and furanoses under base... 

A dissociative elimination-addition pathway has also been proposed to account for the kinetics of alkaline hydrolysis of 2,4-dinitrophenyl 4 -hydroxyphenylpropionitrile in 40% (v/v) dioxane-water, although participation of the associative Bac2 mechanism cannot be ruled out since it may be facilitated by the electronic effect of the triple bond. Formation of intermediate (15), having a conjugated and cumulated double-bond system, should favour the ElcB mechanism and thereby account for the contrasting entropies of activation found for hydrolysis of (14) and the corresponding 4 -methoxyphenylpropionate. 

The number of the sp carbon atoms interposed between the two terminal sp2 carbon atoms conveniently classifies the cumulated double bond systems into two categories 1. even-numbered cumulenes of D2h symmetry (e.g. ethylene), and 2. odd-numbered cumulenes of D2d symmetry (e.g. allene). 

Cumulated Alkenes A cumulated double-bond system, as occurs in the allenes C=C=CH2j, absorbs near 2000-1900 cm-1. The absorption results from asymmetric C=C=C stretching. The absorption may be considered an extreme case of exocyclic C=C absorption. 

This reaction takes place because diimides, —N=C=N—, have reactive cumulated double-bond systems like those of ketenes, C=C=0 isocyanates, —N=C=0 and isothiocyanates, —N=C=S and are susceptible to nucleophilic attack at the central carbon. In the first step of the diimide-coupling reaction, the carboxyl function adds to the imide to give an acyl intermediate, 9. This intermediate is an activated carboxyl derivative RCO—X and is much more reactive toward an amino function than is the parent acid. The second step therefore is the aminolysis of 9 to give the coupled product and yV,N -dicyclohexylurea ... 

Since RN=S=NR and RN=S=0 are cumulated double bond systems they may be considered as heteroallene compounds. In the case of metal-allene compounds the allene may rotate about the metal- 2-allene bond and in addition the metal may jump intramolecularly from one n-C=C bond to the other and vice versa.1... 

These considerations suggested that pyranoses and furanoses should undergo base-catalyzed addition directly and in a stereocontrolled manner to suitable triple bond systems AsB (or compounds containing cumulative double bond systems A—B=C, Scheme 1, path c). Only instability of aldehydic intermediates in basic media and insufficient or undifferentiated reactivities of the a- and p-alkoxides seemed to lower the expectations for a stereocontrolled anomeric O-activation. 

Stannenes, doubly-bonded tin-carbon compounds, are viewed as bridge compounds to elucidate the similarities and differences between olefins (>C=C<) and distannenes (>Sn=Sn<). The bonding situations of stannenes are considered to be different from those of distannenes. In this section, the previously reported stannenes are classified into three types (i) typical examples, bearing three-coordinate tin and carbon atoms (except class 3), (ii) compounds bearing an Sn—C bond in a cumulative double-bond system, and (iii) compounds bearing a Sn—C bond in an aromatic ring. 

Additions of diazoalkanes to the cumulated double-bond systems of ketenes and allenes yield cyclopropanones (Houben-Weyl, Vol. 4/3, pp 65 and 66) and alkylidenecyclopropanes respectively. For example, methylenecyclopropane (53) was made by the addition of diazomethane to allene. The intermediate 4-methylene-4,5-dihydro-3//-pyrazole is exeeptionally sensitive to tautomerism and poor yields were obtained under all but strictly neutral conditions. 

Cumulated Double Bond Systems as Ligands. [. Dialkylsulfurdiimine Compounds of Platinum, J. Kuyper and K. Vrieze, J. Organometal. Chem., 74, 289(1974). 

If an a-halogen is available hydrogen chloride can be eliminated, thereby generating a cumulative double bond system or a 1,3-dipole. 

Thus, the carbodiimides CXIII, having a phosphorus atom attached to the cumulative double bond system, were obtained for the first time ( ). 

Cycloisomerization of Aiienenes and Alleneynes. Allenes are a special class of compounds that possess cumulated double-bond systems. For a long period of time, the development of the chemistry of allenes was impaired under the false notion that such cumulated double-bond systems are highly unstable (90-92). Unlike the chemistry of alkene and alkyne that have been comprehensively developed, allenes only started to emerge in recent decades as versatile precursors in organic synthesis. The Brummond Laboratory was the first to illustrate that cross-conjugated trienes 83 could be prepared by Rh-catalyzed allenic Alder-ene reaction of allenyne 82 (Scheme 42) (93). 

Allenes like other cumulative double bond systems do not show a normal double bond absorption. They have instead, a high frequency antisymmetric absorption which is usually of medium intensity in the infra-red, and a symmetric band at lower frequency which is usually very weak or forbidden in the infra-red and is best seen in the Raman spectra. The antisymmetric band occurs at 1980—1945 cm" in mono-substituted allenes, and at 1930—1955 cm" when di-substituted so that there is no terminal =CH2. Asymmetric di-substitution of the latter type gives this band at 1930— 1915 cm". The symmetric mode appears strongly in the Raman spectra near 1080 cm". ... 

In ketenimines, the cumulative double bond system absorbs in the infrared region at 2000-2050 cm . In cycloaddition reactions, ketenimines are less reactive than ketenes, and A-arylketenimines react faster than A-alkylketenimines. These reactions most likely proceed via a linear ionic intermediate, which can cyclize to give four-membered ring or six-membered ring cycloadducts, or react with a second mole of ketenimine to give a [2-I-2-I-2] cycloadduct. 

Sterically hindered 1-phosphaallenes, such as RP=C=CPh2 (R = 2,4,6-tris-t-butyl-phenyl) are stable at room temperature The resonance structure 1 indicates the polarity of the cumulative double-bond systems in 1-phosphaallenes. 

The lower band (the in-phase stretch) for all cumulated double-bond systems is near 1000 200 cm . It is not useful because it occurs in the fingerprint region and is often weak in the IR. 

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