Hexadiene Synthesis

The synthesis of hexa-1,4-diene has been achieved by the nickel-catalyzed homogeneous addition of ethylene to butadiene. The nickel is introduced in the form of the complex Ni[P(OEt)3]4. The reaction is carried out in acid media, and the active catalyst is the cationic complex NiH[P(OEt)3]3 which is a 16-electron molecule. In Fig. 7 the sequence of reactions that leads to the catalytic formation of isomeric hexadienes is 

The equilibrium constant KCj using H2SO4 in methanol at 0° was determined spectrophotometrically as 33 3 M h This value of was obtained in a solution containing excess P(OEt)3. An excess of ligand was introduced since this caused suppression of the decomposition of the hydro complex, without in any way affecting the absorption spectrum used for the measurement of Ki- 

The kinetics of the protonation reaction were measured by stop-flow techniques. The reaction is endothermic with AH° = + 5 2 kcal and AS° = + 24 6 eu. The driving force was considered to come, not from the heat change, but from the increase in entropy associated with the freeing of solvent molecules bound to the proton. Addition of a large excess of P(OEt)3 had no effect on the rate of hydro complex formation. This protonation reaction of Ni[P(OEt)3]4 occurs much more rapidly than does the dissociation reaction of this compound (226), clearly indicating that hydro formation occurs before dissociation of the 

The equilibrium constant for the reaction shown in Eq. (82) has been obtained for a range of different phosphorus ligands (285). The value of Ki was found to increase as the ligand L became more basic (280) (Table I). The large error for Ni(diphos)2 (258) is due to the extremely low solubility of the complex in methanol. 

Formation Constants for NiL4 with HaSOi IN CH3OH at 0°C 

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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...

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. 

The du Pont 1,4-Hexadiene Synthesis. An important industrial process for the synthesis of 1,4-hexadiene, a component of ternary rubbers, illustrates a different type of mechanism, which is more closely related to the processes -discussed- in-the -previous Sections. The-synthesis involves-the -reaction- of- -ethylene and butadiene and may be carried out by using rhodium chloride in ethanolic hydrogen chloride solution or by nickel(O) phosphite complexes in acid solution.80... 

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... 

Relative Rate of Hexadiene Synthesis Using Various Aluminum Cocatalysts with NilCODt + Fh,P... 

The rhodium-catalyzed addition of ethylene to 1,3-butadiene to yield 1,4-hexadiene (5a, 151) proceeds via a similar mechanism (151) with the exception that, upon formation of the alkylrhodium(III) species, the hexadiene synthesis proceeds without further change in the oxidation state of the metal. In these reactions with butadiene the coordinated alkyl groups are either chelate or 7r-allyl structures which appear to stabilize Rh(III) (151). The addition of propylene to butadiene and isoprene to produce [Pg.297]

Fig. 24 Kharasch bis-addition of CCI4 to 1,5-hexadiene synthesis of 1,1,1,3,6,8,8,8-Octachlo-rooctane [22]...

Hexaazadecalin photoelectron spectra, 3, 543 2,4,12,20,22,30-Hexaaza-1,5,13,29-tetrathia[l,2-30]-[5.2.3.2]paracyclophane nomenclature, 1, 27 Hexabenzof 18]crown-6 complexes, 7, 742 c/s, trans-2,4-Hexadiene synthesis, 1, 431 trans, trarts-2,4-Hexadiene synthesis, 1, 431 Hexahelicene... 

Previous reports have based the synthesis on the reaction of hexamethylbicyclo[2.2.0] hexadiene with Rh(III)CI].3H20. It has been suggested that pentamethylcyclopentadiene, which was not then commercially available, was the actual reactant in the hexa-methylbicyclo[2.2.0]hexadiene synthesis see B. L. Booth, R. N. Haszeldine, and M. Hill, J. Chem. Soc. (A), 1969, 1299 and P. M. Maitlis, Chem. Soc. Rev., 10, 1 (1981). 

It is still unclear how the initiation step in alkene metathesis occurs and how the initial carbene forms. Commercial applications of metathesis include the triolefin process, in which propylene is converted to ethylene and butene, the neohexene process, in which the dimer of isobutylene, Me3CCH=CMe2, is metathesized with ethylene to give Me3CCH=CH2, an intermediate in the manufacture of synthetic musk, and a 1,5-hexadiene synthesis from 1,5-cy-clooctadiene and ethylene. Two other applications, SHOP and ROMP (Shell higher olefins process and ring-opening metathesis polymerization), are discussed in the next section. 

Ho, S. C. H. Wu, M. M. Xiong, Y. Novel cyclopolymerization polymers from nonconjugated dienes and 1-alkenes. PCT International Patent Application WO 95/06669 (Mobil Oil Corp.), March 9,1995. Hustad, P. D. Coates, G W. Insertion/isomerization polymerization of 1,5-hexadiene synthesis of functional propylene copolymers and block copolymers. J. Am. Chem. Soc. 2062,124, 11578-11579. Hustad, P. D. Tian, J. Coates, G. W. Mechanism of propylene insertion using bis(phenoxyimine)-based titanium catalysts an unusual secondary insertion of propylene in a group IV catalyst system. J. Am. Chem. Soc. 2002,124,3614-3621. 

Hustad, P.D. and Coates, G.W. (2002) Insertion/isomerization polymerization of 1,5-hexadiene Synthesis of functional propylene copolymers and block copolymers. Journal of the American Chemical Society, 124,11578-11579. 

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]. 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

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. 

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). 

For the synthesis of permethric acid esters 16 from l,l-dichloro-4-methyl-l,3-pentadiene and of chrysanthemic acid esters from 2,5-dimethyl-2,4-hexadienes, it seems that the yields are less sensitive to the choice of the catalyst 72 77). It is evident, however, that Rh2(OOCCF3)4 is again less efficient than other rhodium acetates. The influence of the alkyl group of the diazoacetate on the yields is only marginal for the chrysanthemic acid esters, but the yield of permethric acid esters 16 varies in a catalyst-dependent non-predictable way when methyl, ethyl, n-butyl or f-butyl diazoacetate are used77). 

A striking example for the preferred formation of the thermodynamically less stable cyclopropane is furnished by the homoallylie halides 37, which are cyclopro-panated with high c/s-selectivity in the presence of copper chelate 3891 The cyclopropane can easily be converted into cw-permethric acid. In contrast, the direct synthesis of permethric esters by cyclopropanation of l,l-dichloro-4-methyl-l,3-pentadiene using the same catalyst produces the frans-permethric ester (trans-39) preferentially in a similar fashion, mainly trans-chrysanthemic ester (trans-40) was obtained when starting with 2,5-dimethyl-2,4-hexadiene 92). 

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-... 

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. 

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. 

The diacetoxylation works well with a number of cyclic and acyclic conjugated dienes and has been applied to the synthesis of natural products33,34. For example, the meso diacetate from 2,4-hexadiene was used for the enantiodivergent synthesis of the carpenter bee pheromone343. 

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