Iron complexes allyl dimerization

Cyclohexene oxidation in the presence of the molybdenum complex, [C5Hr)Mo(CO)3]2, gave two major products at low conversion VI and VII nearly 1 1 mole ratio, Table V. The ketone, VIII, was formed in very low yield in contrast to oxidations using the iron complex. This reaction is far more selective than the oxidation of cyclohexene in the presence of Mo02(acac)2 reported by Gould and Rado (24). When a cyclohexene solution of V was exposed to [CsHsMk COJs] at 70°C, VI and VII were formed in approximately equimolar amounts (Table VI). These data show that the molybdenum complex efficiently catalyzes the epoxidation of cyclohexene by V before the allylic hydroperoxide decomposes substantially. Reaction 16 represents the predominant course of cyclohexene oxidation in the presence of cyclopentadienyltricarbonyl molybdenum dimer. 

This product contains two molecules of butadiene and two molecules of carbon dioxide, and it is assumed that it is formed by intermole-cular dimerization of two allyl carboxylato groups. However, till now the synthesis of the C-ig-dicarboxylic acid occurs only stoichio-m trically. The synthesis requires an overall reaction time of 4 days and about 5 g of the starting iron complex are necessary to form one gram of the acid. 

Treating pentacarbonyliron with dicyclopentadiene affords the dicarbonyl (ri -cyclopentadienyl)iron dimer [Cp(CO)2pe]2. Reduction of the latter with sodium amalgam provides the nucleophilic dicarbonyl(r -cyclopentadienyl)iron anion [Cp(CO)2Fe]. Allylic halides or tosylates can be reacted with this Fe(0) complex to afford ri -allyl-Fp-iron complexes (Fp = Fe(CO)2Cp, Scheme... 

Reactions of anionic metal complexes with halide-bridged dimers can also be used to form a bond between two different metal atoms. Reaetion of (/j -C7H7)Fe(CO)3 with Mn2(CO)gBr2 gives ()j -C7H7)MnFe(CO)6 in which the cycloheptatrienyl ligand coordinates as a diolefin to manganese and as an allyl toward iron . 

The catalytic dimerization at 100 °C of butadiene (and isoprene) with dicarbonyldinitrosyliron and dicarbonylnitrosyl( r-allyl)iron [1—2 wt. %] to 4-vinylcyclohex-l-ene (CHs-substituted vinylcyclohexenes) can be carried out photochemically at —10 °C to 25 °C. It has been suggested, that intermediates formed by UV decomposition of the complexes may be the active catalysts, and that these intermediates could be the same as those produced thermally. u-Allyl iron tricarbonyl iodide was found to be ineffective as a thermal dimerization catalyst 90>. However, u-methallyl iron tricarbonyl bromide has been shown to be effective in the trimerization of isobutylene 284> [see section G2a],... 

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]... 

X-ray studies have revealed a long iron-iron bond (3.14 A) in [ Fe(CO)a-(i -allyl) 2], in agreement with the ability of the dimer to dissociate into Fe(CX))3-(allyl) radicals. The complex slowly decomposes in solution to give ferracyclo-pentadiene complexes such as (25) via hydrogen abstraction and carbon-carbon coupling reactions. "... 

---