Cyclooctadiene complexes with iron

In an ethanol solution of RhCl3, cis,irons-1,5-cyclodecadiene is converted to its cts,cis-l,6-isomer with subsequent formation of the dimeric rhodium complex [(l,6-CioHie)RhCl]2 which can also be prepared by direct interaction of the 1,6-olefin with RhCl3 in ethanol (579, 582). Spectral evidence suggests a configuration (192) much like that of the 1,5-cyclooctadiene complex with the 1,6-CioHie rings in a boat conformation. 

The best known example is the cyclization of butadiene and acetylene 121 14°). Butadiene forms cyclooctadiene and cyclododecatriene by the catalytic action of nickel, iron, and other metal complexes. By an experiment using an iron complex with deuterated butadiene, it was proved that no hydrogen shift takes place in the cyclization reaction 70>. 

Similar to the iron chemistry (compare Chapter 2.3), also nickel complexes allow the reaction of one molecule of butadiene with two molecules of CO2 yielding a,u-dicarboxylic acids [48]. In the reaction of butadiene and CO2 in the presence of nickelbis(cyclooctadiene) and tetramethylethylenediamine first a nickelamonocarboxylate is formed (Figure 19). By further treatment with carbon dioxide and by addition of pyridine a nickeladicarboxylate complex is obtained in yields up to 72 %. Decomposition of the complex with methanol/hydrochloric acid gives cis-dimethyl-3-hexenedioate. 

Ironcarbonyl compounds are liable to form 7r-complexes with unsaturated compounds such as monoenes, butadiene, cyclobutadienes, and cyclopentadiene. In the case of diene, the two double bonds of 1,5-cyclooctadiene are able to coordinate to the metal, without a sterical strain as shown in Table 15.2. The two double bonds of 1,4-cyclohexadiene are difficult to bond to iron without strain, so 1,4-cyclohex-adiene isomerized to 1,3-cyclohexadiene. The iron atoms have such a high reactivity with diene compounds that the isomerization of the diene occurs [13,19,20,25]. 

Dihydromesitylene likewise gives a 1,3 complex (34) and 1,4-cyclohexadiene gives 1,3-cyclohexadieneiron tricarbonyl. 1,5-Cyclo-octadiene on treatment with catalytic quantities of Fe(CO)g gives 1,3-cyclooctadiene (35), as the iron tricarbonyl complex is probably not very stable and is continuously displaced by fresh 1,5-diene until isomerization is complete. 

Tris (triphenylphosphine) nickel, tris (tri-p-tolylphosphine) nickel, and bis (1,3-diphenylphosphinepropane) nickel proved to be good catalysts, the first being slightly more effective. The tricyclohexylphosphine complex was a very poor catalyst, and bis (cyclooctadiene) nickel did not catalyze cyanation. Cyanation of several substituted aromatic halides in the presence of Ni[P(C6H5)3]3 prepared by reducing dichlorobis (triphenylphosphine) nickel (II) 2 with a powdered manganese iron (80 20) alloy (Reaction 3) is reported in Table II. 

Scheme 4.17 Hydrogenation with low-valent iron complexes (cod = 1,5-cyclooctadiene coe = cyclooctene).

Careful application of the three simple postulates listed above can yield insight into the mechanism and stereochemistry of biradical reactions as complex as the thermal dimerization of cis, irons-1,5-cyclooctadiene [26] or the isomerization of allyl-substituted cyclopropanes via internal [2 + 2]-cycloaddition [27]. An attempt to do so here would take us too far afield, in view of the ease with which biradical intermediates interconvert. Instead let us move on to the considerably more stereoselective cycloaddition of reactant pairs with complementary polarity, that proceeds stepwise along a zwitterionic pathway. [23]... 

Cyclooctadiene, when heated with Fe(CO)s, does not produce an iron carbonyl complex but becomes quantitatively isomerized to 1,3-cyclo-octadiene 43). A compound of formula CgHi2 Fe(CO)3 has been obtained from 1,5-cyclooctadiene and either Fe3(CO)i2 in benzene (34) or with Fe(CO)s in the presence of sunlight 44) this material is thought to be a l,5-diene-Fe(CO)3 complex though, in view of the above rearrangements, it may well prove to be l,3-cyclooctadiene-Fe(CO)3. The complex is found... 

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