Cis-1,4 hexadiene

We have reported earlier (14) that during the polymerization of trans-l,4-hexadiene with a Et3Al/6-TiCl3 catalyst (Al/Ti atomic ratio = 2) at 25°C, a major portion of the consumed monomer was converted to isomerized products, thereby accounting for the relatively low conversion to isotactic 1,2-polymer (Figure 1). The relative amounts of the hexadiene isomerization products were in the following order cis-2-trans-4-hexadiene> trans-2-trans-4-hexadiene> 1,3-hexadiene > 1,5-hexadiene >cis-2-cis-4-hexadiene. 

The diacetoxylation of E,E)- and ( ,Z)-2.4-hexadiene (351 and 353) is stereospecific, and 2,5-dimethylfurans (352 and 354) of different stereochemistry have been prepared from the isomers. Two different carboxylates are introduced with high cis selectivity by the reaction of 1,3-cyclohexadiene and... 

In MeOH, l,4-dimethoxy-2-cyclohexene (379) is obtainejl from 1,3-cydo-hexadiene[315]. Acetoxylation and the intramolecular alkoxylation took place in the synthesis of the naturally occurring tetrahydrofuran derivative 380 and is another example of the selective introduction of different nucleo-philes[316]. In intramolecular 1,4-oxyacetoxylation to form the fused tetrahy-drofurans and tetrahydropyrans 381, cis addition takes place in the presence of a catalytic amount of LiCI, whereas the trans product is obtained in its absence[317]. The stereocontrolled oxaspirocyclization proceeds to afford the Irons product 382 in the presence of Li2C03 and the cis product in the presence of LiCl[ 318,319]. 

The explanations for the relative rates of reaction have been based on three factors (1) The rate of reaction increases as the electron density in the diene system increases thus isoprene reacts faster than butadiene and a complex electron-rich 2-silylmethylbutadiene reacts even faster. (2) The rate of reaction increases as the steric hindrance due to the diene substituents decreases thus frans-piperylene reacts more slowly than dimethylbu-tadiene or isoprene. (3) A decrease in the equilibrium concentration of the cisoid conformer results in a slower reaction rate thus cw-piperylene or cis/trans-2,4-hexadiene react more slowly than /rans-piperylene or transltrans-2,4-hexadiene, respectively.175177... 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

We showed (14) that formation of the isomerization products is kinetically controlled and that it depends on the catalyst system employed, the principal conjugated diene isomer being either the trans-2-trans-4-hexadiene, cis-2-trans-4-hexadiene, or 1,3-hexadiene. 

Figure 1. GLC data for the polymerization and isomerization of trans-1,4-hexa-diene at 25°C with a Et3Al/S-TiCls catalyst (Al/Ti atomic ratio is 2). n-Hexane was used as the internal standard. Key O, trans-7,4-hexadiene , polymer A, cis-2-tzans-4-hexadiene tTans-2-trans-4-hexadiene A, 1,3-hexadiene. Reproduced, with permission, from Ref. 14. Copyright 1980, John Wiley Sons, Incorporated...

Cyclopentadiene (6) reacted with 105 at 0°C to give 108 (entry 1). At 100°C, butadiene (12) afforded 109 (entry 2). No [2 + 2] cycloadduct was formed in either reaction. Perfluoromethylenecyclopropane (105) failed to react with cis,cis- or cis,trans-2,4-hexadiene at 100 °C, although 110 was readily formed from trans,frans-2,4-hexadiene (106) under these conditions [29] (entry 3). Anthracene (107) added to 105 at 100 °C. The dienophilicity of 105 is exceptional when compared with the reactivity of simple fluoroolefins, such as perfluoro-isobutylene, which require 150 and 200 °C to undergo cycloaddition to cyclopentadiene [30] and anthracene, respectively. 

Catalyst trans-1,4- Hexadiene cis-1,4-Hexadiene 3-Methyl 1,4-pentadiene 2,4- Hexadiene... 

The simplest diene that satisfies this requirement is 1,4-hexadiene, and indeed it has been adopted as the cure site monomer in commercial ethylene-propylene-diene rubber. Because 1,4-hexadiene exists in both trans and cis configurations, significant amounts of work have been devoted to find ways to control the selectivity of the catalysts for one of the isomers over the other. 

The conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

Fig. 5. Effect of donor concentration on rate and trans/cis ratio of 1,4-hexadiene synthesis. Graph a ethanol 1 mmole Rh 2 mmoles HC1 reaction time, 15 minutes. Graph O...

Su and Collette found that oxygen donors with p Ka values between -2 and 0 are capable of increasing the trans/cis ratio of the 1,4-hexadiene formed. The effect on the trans/cis ratio is again proportional to donor strength and donor concentration. 

The key intermediates in the formation of both cis- and trans-1,4-hexadienes are the anti-7r-crotyl rhodium 17a and the syn isomer, 17b. The ratio of trans/cis products is determined by (a) the ratio of 17b to... 

BD = butadiene 1,4-HD = 1,4-hexadiene T/C = ratio of trans-1,4- to cis-1,4-hexa-diene 3-MeP = 3-methyl-1,4-pentadiene 2,4-HD = 2,4-hexadiene. r The yield of 2,4-hexadiene increases with increasing conversion. 

The isomer distributions were measured at <25% butadiene conversion. T 1C = trans/ cis ratio of 1,4-hexadiene 3-MeP=3-methyl-l,4-pentadiene. 

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. 

The trans/cis ratio of the product must, therefore, be determined at an earlier reaction stage and most probably by the ratio of species 27a and 27b. Steric or electronic factors affecting this ratio will influence the trans/cis ratio of the resulting 1,4-hexadiene. The phosphine and the cocatalyst effect on the stereoselectivity can thus be interpreted in terms of their influence on the mode of butadiene coordination. Some earlier work on the stereospecific synthesis of polybutadiene by Ni catalyst can be adopted to explain the effect observed here, because the intermediates that control the stereospecificity of the polymerization should be essen-... 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

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