1,4-Hexadiene synthesis nickel

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... 

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. 

Before the modem era of organotransition metal reactivity, it was observed that nickel carbonyl reacted with 2-methallyl chloride in methanol to give methyl 3-methyl-3-butenoate and 2,5-dimethyl-1,5-hexadiene as a by-product. In THF at 25 °C, the diene was the exclusive product. This mild formation of a carbon carbon bond, and the interest in the synthesis of terpene-based natural products led to efforts to test the scope and limitations of the process. An obvious pathway involves stepwise oxidative addition of each aUyl unit followed by Reductive Elimination. As discussed below, the key intermediates (left vague in Scheme 48) are likely to involve Ni -Ni couples. 

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... 

A similar insertion of ethylene into a C—M bond of a w-butenyl complex seems to occur in the industrial synthesis of hexadienes from butadiene and ethylene, at least with rhodium and nickel compounds. These catalysts give tra f-l,4-hexadiene as the initial product 178). Cobalt and iron catalysts give the cis isomer 179,180), probably by a different mechanism. 

A diorganonickel complex generally undergoes reductive elimination more rapidly than the corresponding palladium complex. Nickel can mediate retro-allylation, which is involved in allylation of allylic carbonate with homoallylic alcohols for efficient synthesis of 1,5-hexadienes [25]. A combination of Ni(cod)2 (cod = 1,5-cyclooctadiene) and triethyl phosphite catalyzes the allylation reaction of Boc-protected cinnamyl alcohol (Boc = f-butoxycarbonyl) with homoallylic alcohols (Scheme 5.34). The reactions with alkyl-substituted homoallylic alcohols (R = alkyl) are not regiospecific but are sterically controlled. The highest linear... 

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