Hexadienes isomerization

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 reaction of S2CI2 with 1,5-hexadiene gives cycHc sulfide containing chlorines 1 and 11 which at 20°C give upon isomerization the stable mixture of 35% (I) and 65% (II) (143). 

Azabicyclo[2.2.0]hexa-2,5-diene, pentakis-(pentafluoroethyl)-synthesis, 2, 304 2-Azabicyclop.2.0]hexadiene reactivity, 7, 360 thermal isomerization, 7, 360 2-Azabicyclo[2.2.0]hexa-2,5-diene synthesis, 2, 304 1 -Azabicyclo[3.2.0]hexadiene synthesis, 7, 361 1 - Azabicyclo[2.2.0]hexane reactions, 7, 344 ring strain... 

Diazabieyelo[2.2.0]hexa-2,5-diene, perfluoro-IR speetra, 7, 360 Diazabieyelo[2.2.0]hexadienes thermal isomerization, 7, 360 Diazabieyelo[3.1.0]hexane, 7, 56... 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

Das 4,5,6,6-Tetrachlor-l,2-dimethyl-spiro[2.3]hexadien-(l,4) liefert nurein Isomeres ... 

In 15-58, we used the principle of conservation of orbital symmetry to explain why certain reactions take place readily and others do not. The orbital symmetry principle can also explain why certain molecules are stable though highly strained. For example, quadricyclane and hexamethylprismane are thermodynamically much less stable (because much more strained) than their corresponding isomeric dienes, norbomadiene and hexamethylbicyclo[2.2.0]hexadiene (108). Yet the... 

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

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-... 

Table 9.3. Quantum Yields of Sensitized Isomerization of 2,4-Hexadienes

The photostationary state composition for the benzophenone-sensitized isomerization of 2,4-hexadienes is given in Table 9.2. Table 9.3 gives the measured quantum yields for benzophenone-sensitized isomerization of 2,4-hexadienes along with the calculated quantum yields based on Eqs. (9.47)-(9.49) and the pss values given in Table 9.2. 

Alkenes with two reactive carbon-carbon double bonds per molecule like 1,5-hexadiene or diallyl ether are used in the synthesis of silicone compounds which can be later crosslinked by hydrosilylation. A sufficiently high excess of double bonds helps to prevent the dienes from taking part in silane addition across both olefmic ends, but trouble comes from double bond isomerization (Eq. 2). 

Polymerization/lsomerization. The polymerization of 5-methyl-1,4-hexadiene (>99% pure) was carried out in n-pentane with a (5-TiCl3/Et2AlCl catalyst at 0°C according to the procedure described previously (14). To assess monomer disappearance and identify isomerization products, samples were withdrawn at specified intervals from the reaction mixture for GLC analysis (14). The final polymer conversion was determined by precipitation in excess methanol. 

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...

The thermolysis of ladderanes has been studied in detail (Scheme 1). On heating, bicyclo[2.2.0]hexane and its derivatives exhibit skeletal inversion and cleavage to 1,5-hexadiene derivatives.26 The thermolysis of anti- and yyft-tricyclo[4.2.0.02,5]octanes and their derivatives gives cis,cis- and cis, trans-1,5-cyclooctadienes, cis- and trans-1,2-divinylcy clobutanes, and 4-vinylcyclohexene as ring-opening products.27-29 Furthermore, syn-tricyclo-[4.2.0.02,5]octane isomerizes to aw//-tricyclo[4.2.0.02,5]octane.29c,d The thermodynamic parameters and the reaction mechanisms for these thermal reactions have been discussed. 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

Hexadiene is the immediate product found in the codimerization reaction described above in a mixture of ethylene and butadiene. However, the reaction will not stop at this stage unless there is an overwhelming excess of butadiene and an adequate amount of ethylene present. As the conversion of butadiene increases, some catalyst begins to isomerize... 

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. 

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. 

In the model study by Tolman discussed earlier, the half-life of syn-to-anti isomerization measured by H NMR was found to be 0.36 hours at 30°C. This rate of isomerization is far too slow to affect the stereoselectivity of the hexadiene formed with the catalyst considered here. With the bimetallic catalyst, reaction rates frequently approach 4000 molecules of hexadiene/Ni atom/hour at 25°C (or ca. 1 hexadiene/Ni/second). The rate of insertion reaction d must be at least as fast as this, and the isomerization reaction would have to be even faster to affect the trans/ cis ratio of the product. 

It is entirely possible that isomerization may proceed much faster with this catalyst than with the model system considered by Tolman. To test this possibility, reactions were run at reduced ethylene concentrations. This should slow down the insertion reaction (d ) relative to the isomerization reaction (c). No effect on the trans/cis ratio of the product was observed, while the rate of hexadiene formation was reduced over 200-fold (39). So, unlike the Rh systems, the syn-to-anti isomerization appeared too slow to be a controlling factor for the stereoselectivity. 

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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