Trans-l,3-pentadiene

Cyclobuten-Bildung findet auch bei zahlreichen substituierten 1,3-Buta-dienen statt (z.B. bei Isopren (92,277), 2,3-Dimethylbutadien-(l,3) (92, 277), trans-l,3-Pentadien (277), l-Cyclohexylbutadien-(l,3) (92), Sorbin-alkohol (92) und 2,3-Diphenyl-butadien (322)). Besonders erwahnt sei das 4-Methyl-l,3-pentadien (Formel 60), das bei der UV-Bestrahlung nicht 3,3-Dimethylcyclobuten-(l) (Formel 59), sondem 1,3-Dimethyl-cyclobuten-(l) (Formel 62) (92,126) liefert. 

A detailed simulation of the way in which the local interior environment of the channel of DCA induces direct asymmetric induction in the polymerization pathway of 2-methyl-trans-l,3-pentadiene was reported [17]. 

It is known that CoCl2/Al(C2H5)Cl can polymerize trans-l,3-pentadiene but not the cis isomer [263]. This suggests that a two-point coordination is required. Several reaction schemes that provide for an attack at either Ci or at C4 positions were proposed over the years [263]. One mechanism for polymerization of butadiene suggests that complexes of the catalysts in solvents of low dielectric constant will either act as ion pairs or as independent solvated entities. Also, the growing chain may be bound by either a 71 or a a linkage, and it is suspected that a crnitinuous u n isomerism is possible [264] ... 

FIGURE 14.2-6 Preparation of pure cis- and rrans-l. 3-pentadiene. (ti) Analysis of the feedstock. Column diameter = j in., 6 m long packed with 80-100 mesh Chromosorb P coated with 10% 2.2 oxydipropio-nitrile temperature 30°C. (1) isoprene (2) rrans-1,3-pentadiene (3) c/s-l,3-pentadiene (4) cyclopenta-diene. (b) Preparative chromatogram. Column diameter = 4 cm, 4 m long packed with 20% squalane on 60-80 mesh Chromosoib P temperature 35°C, helium flow-rate 3 L/min (5 cm/s). Injection 4 mL every 20 min. The isoprene of one injection is eluted at about the same time as the cyclopentadiene of the previous injection, (c) Analysis of the trans-l,3-pentadiene prepared (purity 99.8%, cyclopentadiene —1.5 ppm). Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, Inc. 

Pyrolysis of cis and trans-lj.-methyl-2-pentene does not yield isoprene. The i. -methyl-2-pentenyl radical decomposes primarily to cis- and trans-l,3-pentadiene ... 

Using data from Table 20.1, estimate the extra stability that results from the conjugation of double bonds in trans-l,3-pentadiene. 

Compare the sum of heats of hydrogenation of 1-pentene and trans-2-butene with the heat of hydrogenation of trans-l,3-pentadiene. Conjugation of double bonds in trans-l,3-pentadiene imparts an added stability of approximately 15 kJ (3.6 kcal)/mol. 

Polydienes such as 1,2-polybutadiene (3.125) 3,4-polyisoprene (3.126) 1,2-poly(trans-l,3-pentadiene) (3.127) and l,2-poly(l,4-hexadiene)s (3.128) under UV irradiation may undergo photocyclization according to the following mechanisms [795, 798, 799] ... 

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