Penta-1:3-diene, hydrogenation

Another example, in which the piperidine cycle is generated de novo, exploits a hetero Diels-Alder cycloaddition of 1 -/r-tolylsulfinyl-1,3-penta-diene 91 with benzylnitrosoformate, that generates an oxazine 92 with complete regioselectivity and 7i-facial diastereoselectivity.69 Osmilation of the double bond inserts stereoselectively two hydroxyl groups on the oxazine skeleton, protection and catalytic hydrogenation finally afforded the enantiomerically pure imino sugars 94 (Fig. 38). 

According to Midorikawa,310 treatment of 2-furylacrylic acid with 35% hydrogen peroxide in dilute hydrochloric acid yields 1,4-penta-diene-3-one-l,5-dicarboxylic acid in 50% yield. 

A. P. Mishchenko, V. M. Gryaznov, M. E. Sarylova, Effect of thermodifftision and chemical treatment of the surface of membrane catalysts made from palladium alloys with ruthenium and rhodium on penta-l,3-diene hydrogenation. Bull. Acad. Sci. USSR 1991, 40(6 (1)), 1154-1157. 

Figure D.6 Atom numbering for the transition structure for the [l,5]-hydrogen shift in (Z)T,3-penta diene...

Chirality about an axis is exemplified by allenes, such as the 2,3-penta-diene enantiomer 23. In 23 the methyl and hydrogen substituents on C2 lie in a plane (in the page) that is perpendicular to the plane containing the methyl and hydrogen substituents on C4. Here the axis of chirality is coincident with the C2—C3—C4 bond axis. The structure has a C2 symmetry element, so it is dissymmetric, not asymmetric. The C2 rotation axis is perpendicular to the axis of chirality, as illustrated in Figure 2.13. It must be emphasized that not all structures that are chiral about an axis have a C2 rotation axis. For example, the 2,3-hexadiene enantiomer 24 also has an axis of chirality coincident with the C2-C3-C4 bond axis, but it does not have a C2 rotation axis. The adamantane 25 and appropriately substituted spiro compounds, such as 26, are also chiral about an axis. ... 

Suprafacial (allowed) and antarafacial (forbidden) [1,5] hydrogen shifts in ground state 1,3-penta-diene-1,1-c/2-... 

The Diels-Alder addition of unstable thiadiazoline 498 to 1,4,5,5-tetrafluorocyclo-penta-l,3-diene provided the corresponding 4-thia-2,6-diazatricyclo[5.2.1.0 2,6 ]dec-8-ene-3,5-dione, which upon hydrogenation and following irradiation gave l,4,7,7-tetrafluoro-2,3-diazabicyclo[2.2.1]hepta-2-ene (see Scheme 85) <2006JFC(127)688>. 

A density functional theory computational approach has been used " to investigate the [1,5]-hydrogen shift in (z)-penta-l,3-diene. Ab initio calculations of the activation barriers to proton transfer in nitrogen derivatives have been computed and these values used to show that the proton transfer in pyrazole is formally a... 

Initial distributions of products observed in the hydrogenation of penta-1 3-diene over various alumina-supported metals [215]... 

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. 

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]. 

The same discrepancy is found in the abilities of metals to afford one of the other isomers in hydrogenation of cw-penta-1,3-diene. Half-reduction over Co leads to 90% yield of rrans-pent-2-ene, whereas over Cu 70% pent-l-ene is obtained even though the disubstituted double bond is more difficult to hy ogenate than the terminal one ... 

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