Protonated diamine complexes

Figure 16. Proton magnetic resonance spectra of substituted cobalt(III)-bis(ethylene-diamine) complexes. (100-mc TMS external reference extreme right. The spectra are referred to the 400 cps TMS side band or the shifts in ppm are given.)...

The second and final example of a computational study of a reaction mechanism that will be considered here is drawn from work carried out by the author s group and serves to illustrate some of the points discussed in the previous section. The reaction in question is the catalytic hydrogenation of ketones by ruthenium(bisphosphine) (diamine) complexes. This reaction was developed by the group of Professor Ryoji Noyori20 and was also studied by the group of Professor Robert Morris. The initial computational work discussed here was a collaboration with Professor Morris. It was motivated by the desire to test the feasibility of a proposed mechanism, involving a key ruthenium dihydride complex, that would transfer a hydride (from Ru) and a proton (from N) in a concerted step to the ketone (Figure 10.9). 

Figure 1 The hydride-metal-nitrogen-proton motif in the catalyst attacking a ketone in a bifunctional manner (a) and the source of enantioselectivity in the ADH of ketones catalyzed by trans-RuHJ(R)-binap) (diamine) complexes (b) ...

Clearly the ammonia-formaldehyde reaction competes with the process but, using an excess of these reagents, the condensation can be made almost quantitative with respect to the tris(ethylene-diamine) complex. The specificity is decided by the chirality of the parent tris(ethylenediamine) complex since this decides the orientation of the gem-diamine and subsequent additions of the amino group to the adjacent imine. Unless the gem-diamine is oriented in the apical position, condensation to give the cap is prohibited. The A or A configuration of the ethylenediamine chelates then decides the orientation of the secondary proton if the amino methylene moiety is required to be apical A(S) or A(R). 

More recently, a two-handed Ceo derivative - carrying two protonated diamine side chains - was shown to bind through electrostatic interaction to duplex DNA. ° It also condenses and allows the complexed DNA to be delivered into, and transiently expressed in, the target cell. It is an interesting fact that, despite the intrinsic photoactivity of Ceo, no differences were noted when the transfection assays were carried out under ambient light or black light. 

The first structurally characterized example of a platinum(II) derivative containing a saturated tetraamine macrocycle, 6,13-dimethyl-l,4,8,ll-tetraazacyclotetradecane-6,13-diamine has been reported (80).251 The species crystallizes as the colorless tetra-cationic complex from dilute HC104 solution by slow evaporation, where the two pendant primary amines are protonated. Other macrocyclic tetraamine complexes including [Pt([14]aneN4)]Cl2 have also been described.252... 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

The approach to polyketide synthesis described in Scheme 5.2 requires the relatively nontrivial synthesis of acid-sensitive enol acetals 1. An alternative can be envisioned wherein hemiacetals derived from homoallylic alcohols and aldehydes undergo dia-stereoselective oxymercuration. Transmetallation to rhodium could then intercept the hydroformylation pathway and lead to formylation to produce aldehydes 2. This proposal has been reduced to practice as shown in Scheme 5.6. For example, Yb(OTf)3-cata-lyzed oxymercuration of the illustrated homoallyhc alcohol provided organomercurial 14 [6]. Rhodium(l)-catalyzed hydroformylation of 14 proved successful, giving aldehyde 15, but was highly dependent on the use of exactly 0.5 equiv of DABCO as an additive [7]. Several other amines and diamines were examined with variation of the stoichiometry and none proved nearly as effective in promoting the reaction. This remarkable effect has been ascribed to the facilitation of transmetallation by formation of a 2 1 R-HgCl DABCO complex and the unique properties of DABCO when both amines are complexed/protonated. 

In our NMR studies 143,147,148,322-324) of amine and other adducts of Ni[R-dtp]2 complexes neat amines were employed in order to eliminate variations in extent of association (H-bonding) of the amines, to permit observation of NH proton shifts, and to maximize the concentration of the preferred adduct. The use of high concentration of primary amines in solutions with Ni[R-dtp]2 complexes can lead to products other than those expected, e.g., with aliphatic diamines, the R-dtp anion salts of f/zs(diamine)nickel(ll) chelates are obtained ). Furlani and co-workers ) have shown that Ni-(ethyl-dtp)2 reacts with n-butyl amine to yield complexes containing the NiS2N4 chromophore, presumably with monodentate ethyl-dtp. In all work with adducts it is necessary to assure that the complexes, adduct molecules and solvent systems are anhydrous. A number of authors 132,284,295,329) shown that Ni[ R-dtp ]2 complexes decompose when in contact with water. 

A, A, A",/V-Tetramethylnaphthalene-l,8-diamine ( Proton Sponge ) Hydrogen Fluoride Complex (2) 3H... 

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