1.5- Hexadien-3-ones

We now turn to the isomeric hexadienes, of which three species qualify for consideration the 1,5- and the (Z)- and ( )-l,4- compounds, species 13, 14 and 15, respectively. If interaction between the two double bonds in 1,4-pentadiene is so small, we expect this as well for the 1,5-hexadiene. One test of this is to consider reaction 7 by analogy to reaction 6. 

Butadiene could also be trimerized to give cyclododecatriene. The trimer is again used by Hulls to manufacture nylon 12 and Vestamid . The codimerization of butadiene and ethylene is used by DuPont to manufacture 1,4-hexadiene, one of the monomers of EPDM (ethylene, propylene, diene, monomers) rubber. The role of the diene monomer in EPDM rubber is to provide with two double bonds of different reactivities. The more reactive, terminal double bond takes part in the polymerization with ethylene and propylene. The less reactive internal one is used later on for cross-linking. These important catalytic reactions are shown in Fig. 7.6. 

Another hydride transfer has been observed when dioxolane 25 was reacted with 14c to give the dioxolenium salt 26. A methoxyl-anion, however, is transferred when 14c is reacted with 1 -methoxy-1 -methyldioxolane (27) to give 57 % of 1 -methyldioxo-lenium-tetrafluoroborate 29 and 4-methoxy-2,4,6-tris-(4 -methoxyphenyl)cyclo-2,5-hexadiene-one-1 (28)205 ... 

Consider the following two isomers of 2,4-hexadiene. One isomer reacts rapidly as a diene in a Diels-Alder reaction, and the other does not. Identify which isomer is more reactive, and explain your choice. 

The codimerization of butadiene and ethylene is used to manufacture 1,4-hexadiene, one of the monomers of ethylene, propylene, diene (EPDM) rubber (see Section 6.7). As mentioned earlier, the role of the diene monomer in EPDM rubber is to provide two double bonds of... 

Keto esters are obtained by the carbonylation of alkadienes via insertion of the aikene into an acylpalladium intermediate. The five-membered ring keto ester 22 is formed from l,5-hexadiene[24]. Carbonylation of 1,5-COD in alcohols affords the mono- and diesters 23 and 24[25], On the other hand, bicy-clo[3.3.1]-2-nonen-9-one (25) is formed in 40% yield in THF[26], 1,5-Diphenyl-3-oxopentane (26) and 1,5-diphenylpent-l-en-3-one (27) are obtained by the carbonylation of styrene. A cationic Pd-diphosphine complex is used as the catalyst[27]. 

One of the butadiene dimeri2ation products, COD, is commercially manufactured and used as an intermediate in a process called FEAST to produce linear a,C0-dienes (153). COD or cyclooctene [931-87-3], obtained from partial hydrogenation, is metathesi2ed with ethylene to produce 1,5-hexadiene [592-42-7] or 1,9-decadiene [1647-16-1], respectively. Many variations to make other diolefins have been demonstrated. Huls AG also metathesi2ed cyclooctene with itself to produce an elastomer useful in mbber blending (154). The cycHc cis,trans,trans-tn.en.e described above can be hydrogenated and oxidi2ed to manufacture dodecanedioic acid [693-23-2]. The product was used in the past for the production of the specialty nylon-6,12, Qiana (155,156). 

The [3,3] sigmatropic reaction pattern is quite general for other systems that incorporate one or more heteroatoms in place of carbon in the 1,5-hexadiene unit. The... 

The vinylogous 3,5-hexadien-2-one (16) adds in a 1,4 cycloaddition with zl -dehydroquinolizidine (17) to form compound 18 (26). A similar 1,4-cycloaddition reaction takes place between pyrylium salts and the pyrrolidine or morpholine enamines of cycloalkanones (26a). 

Perkin pointed out that open chain compounds, which are analogous in structure to a terpene, show a certain similarity in behaviour thus the addition of an ethyl group to 2-methyl 1 5-hexadiene by converting it into 2-methyl 3-ethyl 1 5-hexadiene changes the unpleasant acrid odour into a pleasant one reminding of lemon and peppermint. 

A one-pot procedure [9] based on the cycloaddition of 4-aryl-2-silyloxybuta-dienes 7 and bisdiene 8 with alkynes, followed by oxidative aromatization of the cycloadducts, opened a route to polycyclic phenols without isolating the cyclo-hexadiene derivative intermediates (Scheme 2.5). 

One of these 1,3-diene complexes, a 1,3-hexadiene derivative, was also obtained in a less straightforward manner by thermally induced loss of "CgHig" from a bis(fj-hexyl) precursor (Scheme 95). ... 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

It is quite possible that more highly dehydrogented products (e.g. hexadiene or hexatriene) may also be involved in the reaction sequence. However, none of these species was observed in the GLC. This is not surprising since both these species are highly reactive and may not have accumulated to any measurable extent. One could have used labeled diolefins or triolefins in mixture with n-hexane to test this possibility. Although this experiment was not attempted, we would speculate that most of the radioactivity would have been quickly incorporated into the benzene with a small amount perhaps flowing temporarily upstream into the olefins and the paraffin. 

The connection of radical and pericyclic transformations in one and the same reaction sequence seems to be on the fringe within the field of domino processes. Here, we describe two examples, both of which are highly interesting from a mechanistic viewpoint. The first example addresses the synthesis of dihydroindene 3-326 by Parsons and coworkers, starting from the furan 3-321 (Scheme 3.79) [128]. Reaction of 3-321 with tributyltin hydride and AIBN in refluxing toluene led to the 1,3,5-hexatriene 3-324 via the radicals 3-322 and 3-323. 3-324 then underwent an elec-trocyclization to yield the hexadiene 3-325 which, under the reaction conditions, aromatized to afford 3-326 in 51 % yield. 

In contrast to the spectrum of isotactic trans-l,4-hexadiene polymer (Figure 5), the 300 MHz -H-NMR spectra of the 5-methylhexadiene polymer in both CCI4 and CgDg solutions exhibit only one peak for its backbone methylene protons. As in the case of cis-l,4-hexadiene polymer (14), the backbone methylene protons were not resolvable. The absence of a doublet for the methylene protons in these polymers does not necessarily preclude the possibility that they are isotactic. 

Three EPDM samples were chosen, two containing 5-ethylidene-2-norbornene as the diene monomer, viz. samples D and K with 1.0 and 2.5 mol % respectively, and one, viz. sample N, comprising 1.0 mol % 1,4-hexadiene. Sample D contained 58 mol % ethylene, K and N both 65, and their Mjj was 65 kg/mol. Measurements with a Perkin Elmer DSC-2 confirmed that the K and N samples were amorphous at T > 283 K, whereas sample D was amorphous at T ) 240 K. 

The second termination reaction is alkyl chain end transfer from the active species to aluminium [155]. This termination becomes major one at lower temperatures in the catalyst systems activated by MAO. XH and 13CNMR analysis of the polymer obtained by the cyclopolymerization of 1,5-hexadiene, catalyzed by Cp ZrCl2/MAO, afforded signals due to methylenecyclopentane, cyclopentane, and methylcyclopentane end groups upon acidic hydrolysis, indicating that chain transfer occurs both by /Miydrogen elimination and chain transfer to aluminium in the ratio of 2 8, and the latter process is predominant when the polymerization is carried out at — 25°C [156]. The values of rate constants for Cp2ZrCl2/MAO at 70°C are reported to be kp = 168-1670 (Ms) 1, kfr = 0.021 - 0.81 s 1, and kfr = 0.28 s-1 [155]. 

Recently, a metallocene/MAO system has been used for the polymerization of non-conjugated dienes [204, 205]. The cyclopolymerization of 1,5-hexadiene has been catalyzed by Zieger-Natta catalyst systems, but with low activity and incomplete cyclization in the formation 5-membered rings [206]. The cyclopolymerization of 1,5-hexadiene in the presence of ZrMe2Cp2/MAO afforded a polymer (Mw = 2.7 x 107, Mw/Mn = 2.2) whose NMR indicated that almost complete cyclization had taken place. One of the olefin units of 1,5-hexadiene is initially inserted into an M-C bond and then cyclization proceeds by further... 

Catalysts for this codimerization reaction can be derived from prac-tially all the Group VIII transition metal compounds. Their catalytic properties, such as rate, efficiency, yield, selectivity, and stereoselectivity, vary from poor to amazingly good. Some better-known catalyst systems and their product distributions are listed in Table I. As one can see, the major codimerization product under the given condition is the linear 1 1 addition product, 1,4-hexadiene. The formation of this diene and its related C6 products will become the center of our discussions. The catalyst systems that have been investigated rather extensively are derived from Rh, Ni, Co, and Fe. We shall cover these systems in some detail. A lesser-known catalyst system based on Pd will also be briefly discussed. 

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. 

In the polymerization of butadiene, Teyssie (52-54) has shown that certain electron donors, such as alcohols or phosphines, can convert tt-allylnickel chloride from a catalyst which forms c/j-polybutadiene to one which produces frans-polybutadiene. These ligands presumably block a site on the nickel atom, forcing the butadiene to coordinate by only one double bond. While alcohols cannot be added directly to the hexadiene catalyst (as they deactivate the alkylaluminum cocatalysts), incorporation of the oxygen atom on the cocatalyst places it in an ideal position to coordinate with the nickel. The observed rate reduction (52) when the cri-polybutadiene catalyst is converted into a fra/w-polybutadiene catalyst is also consistent with the observed results in the 1,4-hexadiene synthesis. 

In the literature there are many reports of the formation of active catalyst for the 1 1 codimerization or synthesis of 1,4-hexadiene employing a large variety of Co or Fe salts, in conjunction with different kinds of ligands and organometallic cocatalysts. There must have been many structures, all of which are active for the codimerization reaction to one degree or another. The scope of the catalyst compositions claimed to be active as the codimerization catalysts is shown in Table XV (69-82). As with the nickel catalyst system discussed earlier, the preferred Co or Fe catalyst system requires the presence of phosphine ligands and an alkylaluminum cocatalyst. The catalytic property can be optimized by structural control of these two components. 

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