Re/si system

Although in this volume only the Re/Si system is applied, transformation from one system into the other is frequently necessary. Fortunately, for the most common case of heterotopic hydrogen atoms (see Table 3). always last in ligand rank, the equivalence pro-R or HR = 1IR( holds. This is also true when the heterotopic ligands possess the highest rank. However, for heterotopic groups of intermediate rank, as is typical for the methyls of isopropyl groups, e.g.. in isopropyl alcohol and valine, there is no equivalence. 

In retrospect, it was a consistent and logical step for Seebach and Prelog to propose, in 1982 2 a reactant-based descriptor system (SP system) corresponding to the Izumi -Tai classification. The SP system makes use of the familiar Re Si system already discussed (see Section 1.1.2.2.). Again, relative configurational relationships must be incorporated, something which the SP system does by straightforward combination of descriptors into Ikjul pairs (relative topicities) (see Section 1.1.2.2. and 1.1.4.). 

FIGURE 14.11. The re/si system. A planar arrangement of groups around an hybridized carbon atom. Priorities are assigned to groups as in the R/S system. The re face of the plane through the carbon atom and its three substituents is the one in which the priorities follow a clockwise sense. This sequence is counterclockwise for the si face. 

No phase diagram has been established for the Dy-Re-Si system however, the existence of two ternary compounds has been reported. 

Only two ternary compounds have been characterized in the Gd-Re-Si system. According to X-ray powder data by Bodak et al. (1978) Gd2Rc3Si5 crystallizes with the Sc2Fc3 Si 5-type of structure [P4/mmc, a = 10.91(1), c = 5.571(5)]. Samples were arc melted however, no detailed conditions for heat treatment were given. Segre (1981) confirmed the structure type, but reported different lattice parameters a = 10.93(1), c = 5.550(8). Samples were arc melted = 1.2 K. 

The phase equilibria in the Si,Re-rich part of the Nd-Re-Si system have been investigated by Pecharskij (1979) by means of X-ray and metallographic analysis of 42 ternary alloys, which were arc melted and subsequently annealed in evacuated silica tubes for 400 -I- 600 h at 800 ° C and finally quenched in water. Starting materials were Nd 99.07%, Re 99.99% and Si 99.99%. 

The Re/Si system can also be used to specify enantiotopic groups or atoms. In this case the group from which the other three appear clockwise in order of decreasing priority is described as Re while the other, from which they appear anticlockwise is Si. The two examples (13) and (14) illustrate the application of this rule. 

Figure 6.62. Silicon nanowire growth from a gold nanocluster catalyst. Shown is (a) the phase diagram for the Au/Si system, showing the eutectic temperature/composition (b) SEM image and (c) high-resolution TEM image of the nanowires grown at a temperature of 450° C. The dark tip of the nanowire is from the gold nanocluster. Reproduced with permission from Hu, J. Odom, T. W. Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. Copyright 1999 American Chemical Society.

Gamer and co-workers have once again illustrated the power of tether-mediated synthesis. In their example, the intermolecular reaction gave the undesired exo cycloadducts with no diastereofacial selectivity. Connecting the two reacting systems provided access to endo addition products. With further refinement of the tether length, the re si diastereofacial selectivity was successfully addressed. 

The enolate alkylation process with simple aldehydes and ketones does not generally lend itself to enan-tioselective control, due to the planar nature of the enolate Jt system.206 inspection of 331 shows that the si-re face (face a) has no more steric hindrance than the re-si face (face b). When this enolate reacts with iodo-methane, therefore, no facial selectivity is anticipated and the product will be racemic. In general, enolate alkylation reactions produce chiral, racemic products. The reaction can be diastereoselective, however, when substituents attached to the molecule provide facial bias. In general, enolate alkylation proceeds by approach... 

The possible origin of the low stereoselectivity for the chain-end-controlled catalytic systems based on metallocenes including two cyclopentadienyl rings has been discussed by Corradini, Guerra, and co-workers.2 The two possible diastereomeric preinsertion intermediates for si and re coordinations of the monomer for the case of a si chain—that is, a growing chain in which the last monomeric unit has been obtained by addition of a si-coordinated propene—are shown in Figure 24, parts A and B, respectively. These models would lead to a si—si and re—si propene enchainments, that is, to an isotactic and syndiotactic diad, respectively. 

D,D% F, F faces - as in discussions of intermolecular selectivity (Figure D.4). The literature a/p system is inapplicable to these acyclic systems. Furthermore, the literature Re/Si (relsi), B/N (b/n) systems do not differentiate between enantiotopic faces, on the one hand, and diastereotopic faces, on the other. Thus, whether it is the enantiotopic faces of iii, or the diastereotopic faces of iv and v, the faces are described by the same Re/Si and B/N descriptors. In contrast, in the novel HED system, iii has enantiotopic faces E / 3, iv possesses diastereotopic faces D/F, and v incorporates chirodiastereotopic faces D /F. Thus, the HED system (a) identifies the relative stereotopicity of the paired faces, including homotopic ones, (b) differentiates between enantiotopic and diastereotopic faces, and (c) reveals the chirality of the molecular field - e.g. homotopic vs. chirohomotopic (i vs. ii), and, diastereotopic vs. chirodiastereotopic (iv vs. v). 

A comparison of different facial stereodescriptors for bicentric cases is in order (Figure D.6). The Si/Re (or Re/Si) descriptor may indiscriminately refer to homotopic (vii), or diastereotopic (xiii) faces. Similarly, Re-Re (or Si-Si) applies equally well to homotopic (ix), enantiotopic (x, xi), and diastereotopic (xiv) faces. Furthermore, the Re/Si nomenclature falls short for xii with diastereotopic faces, and is inapplicable to homotopic faces (vi,vii,viii). In contrast, as seen in the examples of Figure D.6, in the novel HED system presented here, every face gets a descriptor. In this manner, one is able to (a) specify the stereotopicity of the faces,... 

The binary systems Ce-Si and Re-Si have been discussed in context with the ternary systems Ce-Ni-Si and Y Re-Si, respectively. The mutual solid solubilities of Ce and Re silicides at 800 °C were found to be small and within 1-2 a/o of the third constituent. 

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