1,4-hexadiene, from olefin insertion

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-... 

Olefin insertion to 7c-allyl derivatives similarly may proceed through a (7-allyl intermediate. A good example is given by the formation of 1,4-hexadiene from ethylene and butadiene in the presence of rhodium chloride, equation (6-85). (See Zeigler-Natta catalyst in Chapter 7). 

The cyclopolymerisation of unsymmetrical a, co-diolefins such as 2-methyl-1,5-hexadiene in the presence of catalysts such as Cp2 ZrMc2 M(Me)0 x, [Cp2 , ZrMe]+ [B(C6F5)4] or [Cp2 ZrMe]4 [McB(C6 F5)2] yields highly regiore-gular cyclopolymers [501]. The perfectly head-to-tail linked monomeric units in the formed poly[methylene-l,3-(l-methylcyclopentane)] arises from the chemo-selective insertion of the less hindered terminus of 2-methyl-1,5-hexadiene into the active Mt—P bond, followed by cyclisation involving the insertion of the disubstituted olefinic bond (Figure 3.50) [497]. The insertion of the disubsti-tuted olefinic bond is made easier by its intramolecular nature. 

The polymerization tests with ethylene and 1-olefines as well as with dienes showed a good ability of the metallocene catalyst for copolymerization. Interesting results from practical and theoretical point of view could be gained in the copolymerization of ethylene and 1,5-hexadiene. During polymerization first a complexation of one of the double bonds of 1,5-hexadiene takes place at the vacant coordination side of the transition metal. After insertion into the polymer chain the complexation of the second double bond occurs followed by intramolecular cyclisation of the 5-membered ring. Analysis of the 13C-NMR spectra reveals an incorporation of 4.2 mole% 1,5-hexadiene and a predominance of trans rings caused by the diastereoselectivity of the cyclisation step. 

In allylic oxidation, an olefin (usually propylene) is activated by the abstraction of a hydrogen a to the double bond to produce an allylic intermediate in the rate-determining step (Scheme 1). This intermediate can be intercepted by catalyst lattice oxygen to form acrolein or acrylic acid, lattice oxygen in the presence of ammonia to form acrylonitrile, HX to form an allyl-substituted olefin, or it can dimerize to form 1,5-hexadiene. If an olefin containing a jS-hydrogen is used, loss of H from the allylic intermediate occurs faster than O insertion, to form a diene with the same number of carbons. For example, butadiene is fonned from butene. 

---