Orientation, cyclopentadiene complexes

An unusual Diels-Alder cycloaddition involving the Cp=Cy bond has been described. The reaction took place by treatment of the electron-deficient allenylidene moiety in complex [RuCp(=C=C=CPh2)(CO)(P -Pr3)][BF4] (46) with a 20-fold excess of isoprene at room temperature affording the cycloadduct 90 (Scheme 33) [287]. This Diels-Alder cycloaddition in which the allenylidene moiety acts as a dienophile was completely regioselective, only the Cp=Cy bond of the allenylidene skeleton being implicated. Furthermore, it was also regioselective with regard to the orientation of the diene with the exclusive attack of C(l) and C(4) carbons at the Cp and Cy positions, respectively. Allenylidene 46 also underwent Diels-Alder reactions with cyclopentadiene and cyclohexadiene to afford the... 

Metal complexes of bis(oxazoline) ligands are excellent catalysts for the enantioselective Diels-Alder reaction of cyclopentadiene and 3-acryloyl-l,3-oxa-zolidin-2-one. This reaction was most commonly utilized for initial investigation of the catalytic system. The selectivity in this reaction can be twofold. Approach of the dienophile (in this case, 3-acryloyl-l,3-oxazolidin-2-one) can be from the endo or exo face and the orientation of the oxazolidinone ring can lead to formation of either enantiomer R or S) on each face. The ideal catalyst would offer control over both of these factors leading to reaction at exclusively one face (endo or exo) and yielding exclusively one enantiomer. Corey and co-workers first experimented with the use of bis(oxazoline)-metal complexes as catalysts in the Diels-Alder reaction between cyclopentadiene 68 and 3-acryloyl-l,3-oxazolidin-2-one 69 the results are summarized in Table 9.7 (Fig. 9.20). For this reaction, 10 mol% of various iron(III)-phe-box 6 complexes were utilized at a reaction temperature of —50 °C for 2-15 h. The yields of cycloadducts were 85%. The best selectivities were observed when iron(III) chloride was used as the metal source and the reaction was stirred at —50 °C for 15 h. Under these conditions the facial selectivity was determined to be 99 1 (endo/exo) with an endo ee of 84%. 

Diels-Alder reaction of 2-bromoacrolein and cyclopentadiene using 10 mol% of titanium catalyst 74 gave the synthetically versatile (R)-bromoaldehyde adduct 75 in 94% yield, 67 1 exo. endo diastereoselectivity, and 93% ee. The absolute stereochemical outcome of the reaction is consistent with the proposed transition state assembly 76 in which the dienophile coordinates at the axial site of the metal, proximal to the indane moiety through Ji-attractive interactions. In this complex, the 7t-basic indole and the Ji-acidic dienophile can assume a parallel orientation facilitated by the octahedral geometry of the transition metal. The aldehyde would then react through a preferential s-cis conformation (Scheme 17.27).54... 

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

Consideration of the dipolarity of the two activated complexes can explain the observed trend. If the reactants are pictured as lying in roughly parallel planes, the dipole moments for the exo orientation are seen to be nearly opposite in direction, whereas for the endo orientation they are parallel. Therefore, the net dipole moment for the endo transition state is greater than that for the exo. Thus, the solvation of the endo activated complex will be more pronounced as the polarity of the solvent increases. This leads to a lowering of the activation enthalpy and preferential formation of the endo adduct. The logarithm of the endojexo product ratio in various solvents has been used to define an empirical solvent polarity scale [124] [cf. Section 7.3). Analogous solvent-dependent endolexo product ratios have been obtained in [4 -1- 2]cycloadditions of cyclopentadiene to other acrylic acid derivatives [560]. Theoretical calculations on exoj endo structures for activated complexes of [4 + 2]cycloadditions have shown that the observed endo preference in polar solvents is due to the influence of the medium, and that secondary orbital interactions are not involved [808]. The solvent has the decisive influence on the exo/endo selectivity. 

FIGURE 1. Possible orientations of phosphine derivatives of cyclopentadiene iron and nickel complexes. The Fe(II) and Ni(II) ions lie behind the circle of Newman s rendition of the complexes... 

Cyclopentadiene is obtained from the light oil from coal tar distillation but exists as the stable dimer, dicyclopentadiene, which is the Diels-Alder adduct from two molecules of the diene. Thus, generation of cyclopentadiene by pyrolysis of the dimer represents a reverse Diels-Alder reaction. See Figs. 1 and 2 for nmr and infrared spectra of dicyclopentadiene. In the Diels-Alder addition of cyclopentadiene and maleic anhydride the two molecules approach each other in the orientation shown in the drawing above, as this orientation provides maximal overlap of ir-bonds of the two reactants and favors formation of an initial ir-complex and then the final e do-product. Dicyclopentadiene also has the endo-configuration. 

Huisgen has also studied the effects of substitution in the keten in the reactions of a series of alkylphenyl ketens to ethyl cis- and trans-propenyl ethers. With the cis-enol ethers the thermodynamically less stable cyclobutanone is always produced. This is the same result as that found in the addition of ketens to cyclopentadiene and other cis-olefins, and the mechanistic implications are the same. With the trans-enol ether, the thermodynamically more stable product is formed, and this observation can be rationalized in terms of a [tc2 + k2 J cycloaddition if the preferred orientation complex has the substituent on the keten between the alkoxy-group and a hydrogen rather than between a methyl group and a hydrogen on the enol ether. In all the cases studied, the cis-enol ether reacted more rapidly than its trans-isomer. This cis trans reactivity ratio is not found in [2 + 2] additions proceeding via zwitterionic intermediates. For example, the rate ratio for the reaction of TCNE with cis- and trans-1-alkenyl ethers is very close to unity. 

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