Isolated dienes

Scheme 26 Synthesis of (3R,5 )-2,6-dimethyl-2,3-epoxy-octa-5,7-diene, isolated from male Amblypelta nitida [121]...

Nakamura, H., Kobayashi, J., and Ohizumi, Y., Novel bisabolene-type sesquiterpenoids with a conjugated diene isolated from the Okinawan sea sponge Theonella cf. swinhoei, Tetrahedron Lett., 25, 5401, 1984. 

Lutz and Chi-Kang Dien isolated three products from the oxidation of 2,5-diphenylfuran with 30% hydrogen peroxide in glacial acetic acid ... 

CH = CH — CH = CH — are said to have conjugated double bonds and react somewhat differently from the other diolefins. For instance, bromine or hydrogen is often added so that a product of the type -CHBr-CH=CH-CHBr- is formed. Also, these hydrocarbons participate in the Diels-Alder reaction see diene reactions). They show a tendency to form rubber-like polymers. Hydrocarbons not falling into these two classes are said to have isolated double... 

A hydrocarbon that contains two double bonds is called an alkadiene, and the rela tionship between the double bonds may be described as isolated conjugated or cumu lated Isolated diene units are those m which two carbon-carbon double bond units are separated from each other by one or more sp hybridized carbon atoms 1 4 Pentadiene and 1 5 cyclooctadiene have isolated double bonds... 

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... 

Laboratory syntheses of conjugated dienes can be achieved by elimination reactions of unsaturated alcohols and alkyl halides In the two examples that follow the conjugated diene is produced m high yield even though an isolated diene is also possible... 

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

What dienes containing isolated double bonds are capable of... 

Dienes with isolated double bonds can be formed when the structure of the alkyl halide doesn t permit the formation of a conjugated diene... 

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 5 Dienes are classified as having isolated, conjugated, or cumulated dou ble bonds... 

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

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

Isobutyl group (Section 2 13) The group (CH3)2CHCH2— Isoelectric point (Section 27 3) pH at which the concentration of the zwittenonic form of an amino acid is a maximum At a pH below the isoelectric point the dominant species is a cation At higher pH an anion predominates At the isoelec tnc point the ammo acid has no net charge Isolated diene (Section 10 5) Diene of the type... 

The addition proceeds in three discrete steps and the intermediates can be isolated. Simple alkenes are less reactive than alkynes and do not undergo the addition to aHylic boranes, but electron-rich alkyl vinyl ethers react at moderate temperatures to give 1,4-dienes or dienyl alcohols (440). 

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

The success of the cycloaddition reaction of maleic anhydride varies gready depending on which heterocyclic diene is used. The cycloaddition of maleic anhydride to furan [110-00-9] occurs ia a few seconds under ambient conditions (42,43). Although the endo adduct (14) is favored kiaeticaHy, the exo adduct (13) is isolated. 

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