Cis-1,3-Pentadiene

The extreme stereoselectivity toward the synthesis of cis-1,4-hexadiene is attributed to the fact that only cisoid-coordinated 1,3-diene can undergo the addition reaction (65, 66). 1,3-Dienes whose cisoid conformations are stoically unfavorable do not react with ethylene under the dimerization conditions. For example, Hata (65) was able to show that, using an Fe-based catalyst system, l-tra/is-3-pentadiene (40) and 2-methyl-1 -trans-3-pentadiene (41) reacted readily with ethylene to form the expected 1 1 addition products, while l-c/s-3-pentadiene (42) and 4-methyl- 1,3-penta-diene (43) failed to interact with ethylene. The explanation is that the cisoid conformations of 40 and 41 are stoically favorable while those for 42 and 43 are not. 

Dimethylfuran, in a sensitized reaction, gave 1,3-dimethylcyclopropene (the main product), isoprene, cis- and rran.s-l,3-pentadiene, 2-pentyne, and 1-methylcyclopropenyl methyl ketone (Scheme 7) (70JPC574) the ring contraction showed a high selectivity. 

The irradiation of 2,5-dimethylfuran in the presence of mercury vapor gave a complex mixture of products. Carbon monoxide and propene were removed as gaseous products. Then, cis- and rran.s-l,3-pentadiene, isoprene, 1,3-dimethylcyclopropene, 2-pentyne, 2-ethyl-5-methylfuran, hexa-3,4-dien-2-one, 1-methyl-3-acetylcyclopropene, and 4-methylcyclopent-2-enone were obtained (Scheme 8) (68JA2720 70JA1824). The most abundantproduct was the cyclopentenone 19, the second was the 1,3-pentadiene 12, while the third product was the cyclopropenyl derivative 18. 

French researchers [38c] have investigated the /zetero-Diels-Alder reaction of methylglyoxylate and glyoxal monoacetal with 2-methyl-1,3-pentadiene in a microwave oven under various reaction conditions (Table 4.9). The microwave (MW) irradiation does not affect the diastereoisomeric ratio of adducts trans/cis = 70 30) but dramatically reduces the reaction time. The glyoxal monoacetal, for instance, gives 82 % adducts after 5 minutes when submitted to irradiation with an incident power (IP) of 600 W in PhH and in the presence of ZnCL (Table 4.9, entry 1), while no reaction occurs if carried out for 4h at 140 °C in sole PhH. Similarly, methylgloxylate in water at 140 °C gives 82% adducts after 3h, whereas microwave irradiation reduces the reaction time to 8 minutes (Table 4.9, entry 5). 

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... 

On the other hand, l-ethoxy-l,3-pentadiene (44a) and l-(trimethylsilyloxy)-1,3-pentadiene (44b), as mixtures of four isomers, gave low yields of 45a and 45b [16], respectively, besides a complex mixture of products including cis and tram-46, head-to-head dimers of 4 [7a, 17] (see Sect. 4.1.1) (Scheme 9). 

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