Penta-2,4-diene

When the cyclopropane homolog 24, with the same 1,1,2,2-tetrabromide array, was treated with zinc, the analogous fragmentation reaction occurred, leading in this case to an 84% yield of 2,4-dimethyl-penta-1,4-diene, 25 4g). This is in contradistinction to the reaction of the corresponding tetramesylate 16 with sodio-diethyl malonate 4f>, (vide supra). 

In addition they noted the formation of some penta-1,4-diene. This appears to be a primary product formed by a parallel isomerization of the bicyclopentane. The Arrhenius equation for this reaction path is... 

It appears likely that the peak reported by the earlier workers as having the same retention time as methylenecyclobutane, was in fact penta-1,4-diene. Some support for this comes from the observation that highly energetic bicyclopentane (produced by the addition of methylene to cyclobutene) isomerizes to cyclopentene and to penta-1,4-diene at almost equal rates, but no methylenecyclobutane is formed (Elliott and Frey, 1965b). 

Synonyms Allyl ethene Allyl ethylene Divinyl methane EINECS 209-736-9 NSC 73902 Penta-1,4-diene UN 1993. 

For our unknown piperidine the other product is a penta-1,4-diene, the constitution of which can now identified by ozonolysis (in the presence of a reducing agent such as Zn dust, or triphenylphosphine, to prevent the oxidation of the ozonolysis fragments). Here ozonolysis will yield formaldehyde (methanal) (an indication that C-6 is unsubstituted) plus another aldehyde, XCHO (showing that C-2 bears the group X). The other product is a propanedial in which the groups Y and Z must occupy the central carbon. Clearly then, C-4 is substituted by both Y and Z. 

The above example was selected because the penta-1,4-diene cannot isomerize to a more conjugated isomer however, in many other examples this is not the case and, for example, a Hofmann exhaustive methylation reaction upon piperidine itself eventually leads to penta-2,4-diene (Scheme 2.33). 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

The Lewis acid-promoted tandem inter[4 + 2]/intra[3 + 2]-cycloaddition of the (fumaroyloxy)nitroalkene (124) with the chiral /i-silylvinyl ether (125) is the key step in the total synthesis of (+)-crotanecine (126), the necine base of a number of pyrrolizidine alkaloids (Scheme 46).237 The tandem inter[4 + 2]/intra[3 + 2]-cycload-ditions of nitroalkenes (127) with dipolarophiles attached to the /f-carbon of a vinyl ether (128) provides a method of asymmetric synthesis of highly functionalized aminocyclopentanes (129) (Scheme 47).238 trans-2-( 1 -Methyl-phenylethyl)cyclohex-anol has been developed as a new auxiliary in tandem 4 + 2/3 + 2-cycloadditions of nitroalkenes.239 The scope and limitations of the bridged mode tandem inter-[4 + 2]/intra[3 + 2]-cycloadditions involving simple penta-1,4-dienes are described in detail.240 A tandem intermolecular/intramolecular Diels-Alder cycloaddition was successfiilly used to synthesize a B/C cA-fused taxane nucleus (130) in 50% overall... 

Acetate pyrolysis is illustrated by the synthesis of penta-1,4-diene (Expt 5.14) from 1,5-diacetoxypentane. This is prepared from 5-chloropentyl acetate which is conveniently obtained from tetrahydropyran by treatment with acetyl chloride this ring-opening reaction offers interesting comparison with the methods for the preparation of a, cu-dihalides discussed in Section 5.5.1, p. 555. 

Morishige, K., Hattori, H. and Tanabe, K. Pronounced catalytic activity of magnesium oxide and calcium oxide in isomerization of penta-1,4-diene to penta-1,3-diene, J. Chem. Soc., Chem. Commun., 1975, 559-560. 

Double bonds can interact with each other if they are separated by just one single bond. Such interacting double bonds are said to be conjugated. Double bonds with two or more single bonds separating them have little interaction and are called isolated double bonds. For example, penta-1,3-diene has conjugated double bonds, while penta- 1,4-diene has isolated double bonds. 

When a molecule has two isolated double bonds, the heat of hydrogenation is close to the sum of the heats of hydrogenation for the individual double bonds. For example, the heat of hydrogenation of penta-1,4-diene is -252 kJ/mol (-60.2 kcal/mol), about twice that of pent-l-ene. 

Because penta-1,2-diene has a larger heat of hydrogenation than penta-1,4-diene, we conclude that the cumulated double bonds of allenes are less stable than isolated double bonds and much less stable than conjugated double bonds. Figure 15-1 summarizes the relative stability of isolated, conjugated, and cumulated dienes and compares them with alkynes. 

Because they have no interaction with each other, isolated double bonds do not contribute to shifting the UV absorption to longer wavelengths. Both their reactions and their UV absorptions are like those of simple alkenes. For example, penta-1,4-diene absorbs at 178 nm, a value that is typical of simple alkenes rather than conjugated dienes. 

Hydrolytically unstable polyethylene was prepared by Wilson [4] by copolymerizing ethylene with acid-labile crosslinkers 1 -allyloxy-penta-1,4-diene, (XI), tetraallyloxysilane, (XII), or 3,9-divinyI-2,4,8,I0-tetraoxaspiro [5,5] undecane, (XIII). 

If there is one or more double bond in the compound then their locations must be shown by the lowest numbered carbon atom to which the bonds are attached, e.g penta-1,4-diene. 

When applied to penta-1,4-diene, the Sharpless asymmetric dihydroxylation forms a 1 1 mixmre of (25,45)- and (25,4/ )-penta-l,2,4,5-tetrols 478 and 479, which can be converted to diepoxides 480 and 481, respectively [214] (Scheme 13.110). A stereo- and enantioselective synthesis of 480 is possible starting from l,5-dichloropenta-2,4-dione applying Noyori s asymmetric hydrogenation [215]. 

F )-BINAP (F )-(+)-2,2 -bis(diphenylphosphino)-1,1 -binaphthyl SCHEME 13.110 Conversion of penta-1,4-diene into pentitol derivatives. 

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