Conjugated dienes from allenes

The thiylation of conjugated dienes and allenes proceeds as expected. Thus, the acid-catalyzed reaction of thiols with 1,3-dienes affords the 1,4-addition product,552 whereas the nucleophilic addition to allene provides the product arising from sulfur attack at the central carbon (equations 303 and 304).553... 

The olefin synthesis has been extended to the preparation of conjugated dienes from the tosylhydrazoncs of /3-unsatnrated ketones.61 The reaction is particularly useful for preparation of 1,3-cyclohexadienes as shown by the preparation of (8). /3-lononc tosylhydrazone was converted into the triene (9). The method cannot be used to prepare allenes. Further experiments indicated that the eliminated a-hydro-gen must be part of a methylene or methyl group. Methyllithium was found to be superior to -butyllithium. 

The comparatively low yield of the initial dimerization of allene is also caused by further addition of 1 to 126 and other allene oligomers produced subsequently in the pyrolysis. A reinvestigation of the reaction has revealed that not only are new tetramers such as 287 and 288 formed in the reaction, but also numerous hexamers such as 289-292, the latter certainly not giving an indication that it originates from 1 [119]. Since some of these products still contain conjugated diene subunits - see, e.g., 291 - further growth appears likely tert-butylallene behaves similarly [120]. 

The reaction of a series of cumulative and conjugated dienes (cf. Table 8 and Scheme 48) with SO3 was studied over the temperature range from —60°C to ambient temperature using CH2CI2 as solvent and l.Sequiv of dioxane as reactivity moderator <1993RTC201>. In the case of the allenes 171, the / -sultones 172, that were first obtained... 

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). 

The addition of iodine azide to double bonds gives p-iodo azides. The reagent can be prepared in situ from KI—NaNa in the presence of Oxone -wet alumina. The addition is stereospecific and anti, suggesting that the mechanism involves a cyclic iodonium ion intermediate. The reaction has been performed on many double-bond compounds, including allenes and a,p-unsaturated ketones. Similar reactions can be performed with BrNa and CfNa. 1,4-Addition has been found with acyclic conjugated dienes. In the case of BrNa, both electrophilic and free-radical mechanisms are important, whUe with CIN3 the addi-... 

In this account, an overview of the methods employed for the synthesis of conjugated dienes and polyenes is presented. Dienes and polyenes with isolated double bonds are excluded, as they are accessed through methods usually employed for alkene synthesis. Oligomerizations and polymerization reactions leading to polyenes are also not covered. Synthesis of 1,2-dienes, i.e. allenes, is excluded from the purview as there is a volume in the present series devoted to this functional group. Synthesis of heterodienes, conjugated enol ethers, [n]-annulenes and related compounds are also not covered here. However, enynes, dienynes and enediynes syntheses have been included in a few cases in view of their emerging importance. 

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. ... 

Interesting variations of potential synthetic utility include the use of a-acetylenic epoxides and a-allenic alcohol derivatives. " The latter, which can be obtained from the former, have been converted to conjugated dienes for use in the Diels-Alder and other reactions (Scheme 33). 

The bonding in conjugated dienes is not so obviously exotic as it is in cumulated dienes, such as the allenes. Nevertheless, there are some subtle effects with far-reaching implications. The source of these effects can be seen from simple, schematic orbital drawings of 1,3-butadiene and 1,4-pentadiene. In a conjugated diene, the orbitals of the two double bonds overlap in an unconjugated diene, they do not (Fig. 12.14). 

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