1.3- and 1,4-Pentadiene, relative

Preparative Method by sUylation of 3-hydroxy-1,4-pentadiene with chlorotriethylsilane. Other relatives of the title compound are avaUahle hy sUylation with chlorotrimethyl-silane or t-hutyldimethylchlorosUane 3-TrimethylsUyloxy-1,4-pentadiene is usuaUy prepared and used in situ. Storage of 3-trimethylsUyl-l,4-pentadiene at —30 °C is recommended. The dianion of the parent 3-hydroxy-1,4-pentadiene can also he prepared. ... 

Pentadiene and allylbenzene are about 10 times as reactive as 1-butene. The effect on reactivity of an alkyl group, relative to a methyl group at the same position, is nearly independent of whether the alkyl group is bonded to the 2, 3, or 4 position of the 1-alkene chain. The effects of unbranched alkyl substituents or reactivity correlate well with Taft s polar substituent constants , but isopropyl and tertiary butyl groups have much larger deactivating effects... 

The addition of the hydrogen halides to dienes can result in either 1,2 or 1,4 addition. The extra stability of the allyl cation which can be formed by proton transfer to a diene makes the ion pair mechanism relatively favorable. 1,3-Pentadiene, for example, gives a mixture of products favoring the 1,2 addition product by a ratio of from 1.5 1 to 3.4 1, depending on the temperature and solvent ... 

Recently, a more detailed study of this system has been carried out, together with the behavior of 3,4-pentadien-l-yl iodide, 157 (149), a system which has previously been shown to solvolyze through ion 156 (87,91). The products of reaction of 154-1 and 157 with AgOAc in acetic acid at 25° C are given in Table VIII. The similarity of the products and their relative amounts... 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

For application to thin film transistors (TFTs), which can operate electronic papers, high carrier mobility exceeding 0.01 cm2 V-1 s-1 is necessary. For this purpose, nematic semiconductors with low molecular order, resulting in relatively low carrier mobility, are not suitable. For application of thin films of semiconductors to TFTs, stabilization of the highly ordered smectic phases, which exhibit high carrier mobility, by photopolymerization may be desirable. Kreouzis et al. studied the carrier transport properties of photopolymer-izable phenylnaphthalene, diphenylbithiophene, and quaterthiophene derivatives having an oxetane moiety or l,4-pentadien-3-yloxy in their alkyl side chain (Fig. 21) [107,108],... 

Fig. 2.2 [64] shows chromatograms of commercial-grade 1,3-pentadiene, obtained on two columns connected in series, the first column containing a saturated solution of silver nitrate in ethylene glycol and the second containing chloromaleic anhydride reacting with a trans isomer. As a result of the reaction the relative content of the irons isomer decreases. The pseudo-first-order rate constant varied from 2.0-10 to 1.4 10 sec (the half-time of transformation changed from 30 to 100 sec) [64]. 

However, when the reaction of 1,3-pentadiene + DCl is carried out under kinetic control, essentially the same relative amounts of 1,2- and 1,4-addition products are obtained as are obtained from the kinetically controlled reaction of 1,3-butadiene + HBr. The transition states for formation of the 1,2- and 1,4-addition products from 1,3-pentadiene should both have the same stability because both resemble a contributing resonance stmcture in which the positive charge is on a secondary allylic carbon. Why, then, is the... 

Section IV,A,2,a Pentachloro-2H-pyrrole (21) behaves as a diene via the rearranged 3f/-pyrrole form 133 in its reaction with trans-1,3-pentadiene, giving a product 132 having R = trans-l-propenyl. However, the mode of cycloaddition is reversed with cyclopentadiene, the 2i/-pyrrole (21) reacting directly as a dienophile at the 3-4 C=C bond. With 1,3-cyclohexadiene, both modes of cycloaddition occur simultaneously. Steric factors rather than relative HOMO and LUMO energies are thought to determine the reaction course.147... 

Draw energy diagrams analogous to Figure 1 for simple addition of H" " to 1,3-pentadiene and 1,4-pentadiene so as to give the most stable carbonium ions possible. It turns out that the 1,3-isomer is both the more reactive and the more energetically stable diene. Explain how this information can be used to deduce the relative stabilities of the carbonium ions formed from these dienes. 

Record the absorption spectrum between 400 and 200 nm of (a) 1-octene, (b) 1,3-butadiene, and (c) a nonconjugated diolefin (e.g., 1,4-pentadiene). What is the effect of a conjugated system on the absorption spectrum What does the spectmm tell you about the relative energy of the molecular orbitals in each compound ... 

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