Heteroatom-metal interaction

Highly enantioselective hydrogenation of functionalized ketones has been achieved with chiral phosphine-Rh(I) and -Ru(II) complexes [1,162], The presence of a functional group close to the carbonyl moiety efficiently accelerates the reaction and also controls the stereochemical outcome. The heteroatom-metal interaction is supposed to effectively stabilize one of the diastereomeric-transition states and/or key intermediates in the hydrogenation. 

I.4.2.6. 1-Deuterio Aldehydes Asymmetric hydrogenation of 1-deuterio benzaldehydes with Ru(OCOCH3)2[(5)-binap] in the presence of 5 equiv. of HC1 gives the corresponding chiral 1-deuterio alcohols with up to 89% ee (Scheme 1.75) [280]. The heteroatom substitution at the C2 position in the benzene ring tends to increase the enantioselectivity due to the heteroa-tom/metal interaction. This catalyst is less reactive in deuteration of benzaldehyde. 

The above mechanism is novel in that it does not require the interaction of a carbonyl moiety with the metal center. Neither a ketone/Ru complex nor a Ru alkoxide is involved in the mechanism, and the alcohol forms directly from the ketone. This non-classical mechanism also explains the high functional selectivity for the C=0 group. When the chiral molecular surface of the Ru hydride recognizes the difference of ketone enantiofaces, asymmetric hydrogenation is achieved. This is different from the earlier BINAP Ru chemistry where the enantio-face differentiation is made within the chiral metal template with the assistance of heteroatom/metal coordination. Similar heterolyses of H2 ligands have been shown by Morris and others (92) to be the critical step in the mechanism of reaction processes related to the Noyori systems. 

The surface chemistry of carbon is rather complex. At a single adsorption site several chemically inequivalent types of heteroatom bonds may form. Strong interactions between surface functional groups further complicate the list of surface chemical structures as derived for the most relevant carbon-oxygen system An additional dimension of complexity is presented by the large variety of substrate structures of carbon which arise from anisotropic covalent bonding rather than by a isotropic metallic interaction. 

Metal-metal interactions can be consolidated by additional steric interactions to ensure the linearity of infinite metal-metal chains. For example, Pt-Pt scaffolding with sufficiently bulky ligands interact to occupy the space around the polymetallic core. Additional weak interactions can be suspected when heteroatoms are in close contact with C-H groups, as is the case with the cyano /erfto-butyl ligand in Figure 13. However, the effects of steric bulk seem predominant. The Pt-Pt interactions in the wires obtained by this approach are responsible for the luminescent character of the wires and the emission can be modulated by exposme to acetonitrile that introduces small distortions along the Pt metallic chain. 

The metal-vapor technique was applied to cobalt atoms and r-BuC = P (01JOM(635)212). The mixture of products that resulted includes the mixed-ligand sandwiches 170 and 171. Further interaction of complex 170 with [W(C0)5(THF)] leads to the coordination of the W(CO)5-group via the phosphorus heteroatom of the four-membered ring to yield 172. 

Ionophores constitute a large collection of structurally diverse substances that share the ability to complex cations and to assist in the translocation of cations through a lipophilic interface.1 Using numerous Lewis-basic heteroatoms, an ionophore organizes itself around a cationic species such as an inorganic metal ion. This arrangement maximizes favorable ion-dipole interactions, while simultaneously exposing a relatively hydrophobic (lipophilic) exterior. 

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

The availability of different metal ion binding sites in 9-substituted purine and pyrimidine nucleobases and their model compounds has been recently reviewed by Lippert [7]. The distribution of metal ions between various donor atoms depends on the basicity of the donor atom, steric factors, interligand interactions, and on the nature of the metal. Under appropriate reaction conditions most of the heteroatoms in purine and pyrimidine moieties are capable of binding Pt(II) or Pt(IV) [7]. In addition, platinum binding also to the carbon atoms (e.g. to C5 in 1,3-dimethyluracil) has been established [22]. However, the strong preference of platinum coordination to the N7 and N1 sites in purine bases and to the N3 site in pyrimidine bases cannot completely be explained by the negative molecular electrostatic potential associated with these sites [23], Other factors, such as kinetics of various binding modes and steric factors, appear to play an important role in the complexation reactions of platinum compounds. 

It is assumed that most of the electron spin density resides on the metal, but that a certain small part of it, given by the quantity p , is delocalized to the ligand heteroatom L. The first term is the point-dipole interaction term, the second corresponds to the dipolar interaction between the nuclear spin under consideration and the spin-density on the atom L and the last term describes the cross-correlation of the two dipolar interactions (we discuss the issue of cross-correlation phenomena in more general terms in Section II. D and III.B). The quantity is the effective distance from the nuclear spin... 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

The HO-energy of a ir-system can be changed, e.g. by occupied orbitals of hetero-atoms which are relative donors or acceptors for the HO s. This can also be true of interactions of the LUMO s of the tr-system with vacant AO s of heteroatoms. The consequences e.g. for the preference of certain conformers relative to the type and position of perturbations in tricarbonyl-chromium benzene complexes (organo-metallic example) are described in Figure 2 of Scheme 2.1-4 together with the consequences for the reactivities in benzene derivatives (example of organic chemistry) due to rektive donor- or acceptor-perturbations (see also Scheme 2.1-2 Fig. 2). 

The oxygen as heteroatom in ethers or carbonyl compounds is weak to moderate Lewis base. Nevertheless, a highly reactive metal carbene complex can interact with the oxygen to generate oxygen ylide. The interaction between ether and metal carbene functional groups is believed to be rather weak as demonstrated by the facts that other metal carbene reactions, such as G-H insertion and cyclopropanation, can proceed in ethereal solvents." These experiments demonstrate that the formation of the metal ylide is much less favored in the equilibrium shown in Equation (1). ... 

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