Trans complexes, substitution

Bartoli recently discovered that by switching from azide to p-anisidine as nucleophile, the ARO of racemic trans- 3-substituted styrene oxides could be catalyzed by the (salen)Cr-Cl complex 2 with complete regioselectivity and moderate selectivity factors (Scheme 7.36) [14]. The ability to access anti-P-amino alcohols nicely complements the existing methods for the preparation of syn-aryl isoserines and related compounds [67] by asymmetric oxidation of trans-cinnamate derivatives [68]. 

In the case of the trans-complex, only the two chloride ions are substituted, the trans-effect of ammonia being too low to give substitution with the result that white needle crystals of trans-[Pt(NH3)2(tu)2]Cl2 are formed [73],... 

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

Complex formation between Ru and ligand (12) has been investigated [Ru(12)3] is easily prepared and can be oxidized to the corresponding Ru complex. Substitution by PPh3, py, or im (L) occurs very slowly, and in [Ru (12)3] , it occurs with concomitant reduction to yield trans- [Ru(12)2L2]-. ... 

Cyanide anation in DMSO has proved useful in the preparation of cyano complexes (Scheme 29). 25 Cyanide displaces coordinated DMSO cleanly but deprotonates rather than displaces coordinated water. The preparation of trans- and ds-[Cr(CN)2(NH3)4]C104 starts from [Cr(DMSO)2(NH3)4]3+ or [CrCl(DMSO)(NH3)4]2+. The insolubility of trans-[Cr(CN)2(NH3)4]C104 permitted its isolation from solution, but it was necessary to separate the cis isomer by ion exchange. Rapid trans to cis isomerization of bww-[Cr(DMSO)2(NH3)4]3+ occurs on the addition of CNT before anation produces the dicyano complexes stepwise, both apparently via c -[CrCN(DMSO)(NH3)4]2+, which in an uncommon rearrangement gives a mixture of trans- and cis-dicyano complexes. Substitution of DMSO by CN" in water is stereoretentive. 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

It must be noted that the approach of the monomer to the reactive metal-carbon bond should occur from the less hindered side. The greater stability of the trans complex should arise because the substituted carbon atom of the last unit is involved in the formation of the four-membered ring, so that reaction is hindered. The structure of copolymers of deuteropropylene from cis- and trans-(U,)-propylene proved that, even in syndiotactic polymerization, cis-addition to the double bond occurs, because with 1,2-disubstituted monomers, structures of ery-thro and threo di-syndiotactic polymers degenerate (see Fig. 4). 

Stang et al. [94JA4981, 9403776, 95JA1667, 95JA6273] described the self assembly of a series of unique cationic, tetranuclear, Pd(II)- and Pt(II)-based macrocyclic squares (e.g., 25), utilizing different bidentates as the sides of these structures. The similar self-assembly complexation of dipyridyl porphyrins by cis- and tran.v-substituted Pd(II) or Pt(II) ions possessing square planar coordination afforded multiporphyrin arrays with a square... 

If the intermediates (or transition states) are drawn correctly in Figure 23-1, substitution by dissociation should convert the trails complex shown into another trans complex, but substitution by direct displacement should convert it partially (for there are paths other than the one shown) to a cis complex. Unfortunately, however, there is still some question concerning the geometry of such transition states, and stereochemical results do not often lead to straightforward mechanistic conclusions. Mechanistic information concerning substitution reactions of octahedral complexes may be obtained, but its interpretation is, in general, less direct than we would like. 

Stereochemical Considerations. Much attention has been directed toward the stereochemistry of substitution reactions of alkyl complexes. Substitution of MeMn(CO)s leads to cw-MeC(0)Mn(C0)4L initially, although the cis complex isomerizes to the trans eventually ... 

The preparation of (/ra ,< -l,3-pentadieiie)Fe(CO).3 by reaction of 1,4-pentadiene with Fe3(CO),2 has been reviewed (47, reference 160). Although earlier attempts to prepare the cis analog by direct interaction of cw-l,3-pentadiene with Fe(CO)5 lead only to formation of the trans complex (188, 387), it has been successfully jwepared wdth Fe2(CO)B (363). Although, in general, the cis-substituted dienes undergo isomerization upon interaction with Fe(CO)5, cis,/mw -2,4-hexadiene has been observed to retain its configuration (188). There may be a relationship between the effect of steric hindrance on complex stability and the tendency for isomerization to occui (188). 

Cyclic amines react with oxirane to introduce 2-hydroxyethyl TV-substituents. The TV-hydroxyethyl-substituted cyclic amines form complexes with a wide range of cations, the formation constants being smaller than for the unsubstituted amine for the later (/-cations, larger for hard and large cations. Structures usually have some OH groups coordinated. Tetrakis(2-hydroxyethyl)-substituted cyclic tetraamines insert cations much more rapidly than the unsubstituted amines, and the complexes are also more rapidly demetallated in add. Studies with differing patterns of TV-hydroxyethyl substitution showed that at least two civ-hydroxyethyl groups were required for this rate enhancement, and no enhancement was observed for mono- or trans-bis-substituted tetraamines.122... 

Even back in 1912, Werner was keenly aware of the need to understand metal complex substitution. Stereochemical changes [e.g., cis-trans conversions, equation (1.2)] in cobalt(III) complexes were critical to the development of his coordination theory and he sought to understand how these occurred. 

Q6.2 Why is substitution in the trans complex much faster than in the cis complex ... 

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