Allenes diene conjugation

To illustrate the importance of vicinal connectivity of conjugating units, we consider two dienes in non-vicinal relationships 1,5-hexadiene, 9, and allene, 10. As shown in Table 3.19, the direct diene conjugations are negligible in both species, on account of spatial separation in 9 and symmetry-imposed orthogonality of the two pi planes in 10. Consistently with the essential absence of conjugation, the unsaturated C—C bonds of 9 and 10 have calculated bond orders characteristic of ethylene or other unconjugated systems and the ficc NLMOs have essentially localized character ... 

Table 8 Allenes and conjugated dienes reacted with SO3...

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. 

The reactive compound chlorosulfonyl isocyanate796 CIS02NC0 forms 3-lactams even with unactivated alkenes,797 as well as with allenes,798 conjugated dienes,799 and cyclopropenes.8 l, OS V, 673 65, 135, 140. 

The chemistry of unsaturated organoboranes often differs markedly from that of their saturated analogues. Both vinylic and allylic boranes react readily with many substrates toward which trialkylboranes are inert90 . Allylic boranes can be synthesized selectively via direct hydroboration of an appropriate allene or conjugated diene and are of immense synthetic importance 91 93). Mikhailov in his book and review 7,94) has documented the synthetic applicability of allylic boranes with caution of high thermal reactivity with respect to allylic rearrangement. For example, (l-methyl-2-propenyl)dialkylboranes rearrange spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 40). 

Silver(I), Cu(II) and Hg(II) have been employed in the dimerization of 1 and discussed elsewhere as has the role of the first of these in the addition of alkenes, alkynes, allenes and conjugated dienes to cyclopropabenzene. It is more than probable that product formation with Ag(I) is dictated by electrophilic addition of the metal and subsequent interaction of the organosilver benzylic cation with the hydrocarbon reagent to give ring-opened or ring-expanded products. 

Diazopropane is a potential source of ge w-dimethyl groups. It undergoes 1,3-dipolar addition to acetylenes and allenes/ and the adducts can be photolyzed to give cyclopropenes or methylenecyclopropanes/ respectively. In certain cases the adducts from a-substituted acetylenes give good yields of allenes and conjugated dienes on photolysis. ... 

The additives of water can serve as mechanistic probes and aid in obtaining true mechanistic understanding in some organocatalytic reactions [123]. The water is nucleophile in palladium-catalyzed oxidative carbo-hydroxylation of allene-substituted conjugated dienes [124]. This is an example of Pd- catalyzed oxidation leading to C -C bond formation in water with subsequent water attack on a (rr-allyl) palladium intermediate. The different effect of the water concentration on the intra- and extra-diol oxygenations of 3,5-di-tert-butylcatechol with O2, catalyzed by FeC in tetrahydrofuran-water indicates that the intermediates for two reactions are different (model for Catechol-2,3-dioxygenases) [125]. 

As discussed in Sect. V.3.5, halide ions can also add to the palladium 7r-complexes, and the process may be termed halopalladation. Addition of Pd and a halogen, such as Cl and Br, to olefins, acetylenes, allenes, and conjugated dienes has been reported. However, most of the currently useful reactions initiated by halopalladation involve the use of alkynes as the substrates. Halopalladation of aUcynes gives vinylpalladium intermediates, which can undergo intramolecular carbopalladation to form cyclized products. Pd-catalyzed cyclization of ally lie aUcynoates can produce 7r-alkylidene-y-butyrolactones, which represent a basic structural unit in a wide variety of biologically active natural products. 

A series of arylynamides with alkyloxy groups at the ortho position of the aryl group has been prepared by an alkylation/cross-coupling/amidation sequence and a subsequent gold-catalysed reaction provides benzofurans with amine functionalities at the 2-position and alkyl groups at the 3-position. The mechanism of gold(III)-catalysed isomerization of alkyl-substituted allenes to conjugated dienes in the presence of a... 

Hate 3. All glassware used for the work-up and distillation must be rinsed with a dilute solution of triethylamine in diethyl ether or acetone in order to be sure that traces of acids on the glass walls have been neutralized. Allenic sulfides with the structure C=C=C(SR)-CH- isomerize under the influence of acids to give conjugated dienes, C=C-C(SR)=C. 

Tertiary acetylenic halides give unsatisfactory results owing to further isomerization of the allenic halide into a conjugated diene system under the influence of the copper salt. Bromo- and iodoallenes with the structures R R2C=C=CH-X can also be synthesized by an aqueous procedure, consisting of reaction between... 

Isomerization of alkynes to allenes and then to conjugated dienes is catalyzed by Pd(0) species such as Pd(OAc)2 3.nd PhjP or Pdi(dba)3. The conjugated ketoalkyne 286 was converted into the conjugated dienone 287[141,I42]. 

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