1.4- Pentadiene hydrogenation

Procedures are avail le for preparing isotactic-cis-, isotactic trans- and syndiotactic-cis- varieties of poly-l,3-pentadiene. Hydrogenation of these polymers yields polymers that may be considered to be stereoregular alternating copolymers of ethylene and propylene. The cmr spectra of such copolymers have been investigated [14,15,18]. Differences observed in the methyl and methylene carbon resonances were observed for the isotactic and syndiotactic polymers. The parent polymers may be considered to be alternating stereoregular copolymers of acetylene with propylene. Their spectra also revealed stereochemical differences. 

Let us finally consider two Z-matrices for optimization to transition structures, the Diels-Alder reaction of butadiene and ethylene, and the [l,5]-hydrogen shift in Z-1,3-pentadiene. To enforce the symmetries of the TSs (Cj in both cases) it is again advantageous to use dummy atoms. 

The [l,5]-hydrogen shift in Z-l,3-pentadiene is an example of a narcissistic reaction, with the reactant and product being identical. The TS is therefore located exactly at half-way, and has a symmetry different from either the reactant or product. By suitable constraints on the geometry the TS may therefore be located by a minimization within a symmetry consti ained geometry. 

Figure E.6 Atom numbering for the transition structure for the [l,5]-hydrogen shift in 1,3-pentadiene...

Interestingly, benzonitrile oxide does not react with thiirene dioxide 19b even in boiling benzene, whereas the electron-rich diene l-piperidino-2-methyl-l, 3-pentadiene (177) does react under the same reaction conditions to give the expected six-membered [4 + 2] cycloadduct 178, accompanied by sulfur dioxide extrusion and 1,3-hydrogen shift to form the conjugated system 179175 (equation 70). 

Energy level diagram for the hydrogenation of 1,3- and 1,4-pentadiene. The difference in these energies, 28 ... 

Does the delocalized tt system of 1,3-pen tadiene stabilize the molecule relative to 1,4-pentadiene Each molecule reacts with two molecules of H2 to give a common product, pentane. Thus, a comparison of their energies of hydrogenation allows us to determine the relative stabilities of two delocalized tt bonds compared with two isolated tt bonds. The hydrogenation of 1,4-pentadiene releases 252 kJ/mol, but the hydrogenation of... 

Another type of steric effect results from interactions between diene substituents. Adoption of the s-cis conformation of the diene in the TS brings the d.v-oricnlcd 1- and 4-substituents on a diene close together. /(-1,3-Pcnladicnc is 103 times more reactive than 4-methyl-l,3-pentadiene toward the very reactive dienophile tetracyanoethylene. This is because the unfavorable interaction between the additional methyl substituent and the C(l) hydrogen in the s-cis conformation raises the energy of the TS.20... 

Isomerization versus Reduction Reaction Pathways in the Hydrogenation of 3-Buten-2-ol and l,4-Pentadien-3-ol on Pd-Black... 

The general experimental procedure employed in the study here has been described previously (7), thus only a brief overview is presented here. For all experiments, 45 mL deionized water and catalyst (50 mg Pd-black for 3-buten-2-ol and 25 mg for l,4-pentadien-3-ol) were added to the reaction cell. For ultrasound-assisted, as well as stirred (blank) experiments, the catalyst was reduced with hydrogen (6.8 atm) in water for 5 minutes at an average power of 360 W (electrical 90% amplitude). The reagents (320 mg 3-buten-2-ol or 360 mg l,4-pentadien-3-ol) were added to the reduced catalyst solution to achieve... 

Figure 2 Time-dependent composition data is shown for the hydrogenation of aqueous l,4-pentadien-3-ol for both (a) ultrasound irradiated and (b) magnetically stirred systems. The symbols correspond to experimental measurements (l,4-pentadien-3-ol 14PD30L-solid circles l-penten-3-ol lP30L-open hourglass 3-pentanone 3PONE-open triangles 3-pentanol 3POL-crossed squares). The lines in the ultrasound experiment simply connect the data points, whereas for the stirred experiment the lines correspond to a modeled fit (see text).

Something that will become useful are the hydrogenation reaction schemes for 3-buten-2-ol and l,4-pentadien-3-ol are illustrated below. 

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