Newman-like projection

Fig. 43. Newman-like projection of 74a, viewed along the N-Si-N axis. The A-methyl groups are labeled A, A, B, B. 73 Reproduced with permission from the American Chemical Society.

Scheme 17.2 Reaction mechanism of the r2.31-Wittig rearrangement on a Sp-Qx/Q/ propargyloxy-hexahydroindene ring system. The Newman-like projections in the dashed circles indicate transition state bond formation.

One conformation appears to be lowest in energy. This conformation is shown as molecular model 48D in the same Newman-type projection as 48C. This conformation is thought to look a little like an easy chair, and it is called a chair conformation. The chair shape can be seen in 48B or in molecrdar model 48E, which views the conformation from the side. Note that 48D is the molecular model of48B, and that 48D and 48E are identical, but simply viewed from a different perspective. 

Higher alkanes having unbranched carbon chains are, like butane, most stable in theh all-anti conformations. The energy difference between gauche and anti conformations is similar- to that of butane, and appreciable quantities of the gauche conformation are present in liquid alkanes at 25°C. In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end-on as in a Newman projection. Viewed from this perspective, the most stable conformations of pentane and hexane... 

Now we place the three groups that are on the back carbon atom into our drawing. There is one methyl group and two hydrogen atoms. If we put them into our drawing, then we get our Newman projection. It looks like this ... 

The many different conformers resulting from rotation around the carbon-carbon bonds in simple molecules like ethane and w-butane may be shown by Newman projections (Figure 2.7). The most stable is the anti or trans projection where the steric hindrance is minimized. There are a number of eclipsed and gauche arrangements of which only one of... 

Enolate species 6, derived from 1-oxopropyl complex 5, reacts similarly with monosubstituted epoxides. Under the influence of diethylaluminum chloride, only the diastereomers 7 and 8 were observed in the reaction mixture 7 was the major product. The use of boron trifluoride - diethyl ether complex instead of diethylaluminum chloride caused a complete loss of stereocontrol at C , producing a 50 50 mixture of diastereomers 7 and 8, but stereocontrol at C was retained as no other diastereomers were produced. The major diastereomer produced is consistent with the intermediacy of a transition state like that represented in Newman projection C which has the usual anti-E-snolate geometry and lacks the R methyl gauche interaction of structure D. 

Like the chair conformation, all of the C—C—C bond angles of the boat conformation are 109.5°, so it has no angle strain. However, it does have other types of strain. The two red hydrogens, called flagpole hydrogens, approach each other too closely and cause some sceric strain. In addition, the conformations about the green bonds are eclipsed. This can be seen more easily in the Newman projection down these bonds ... 

Let us make a model of the conformation of methylcyclohexane with axial methyl. If we hold it so that we can sight along the Ci—C2 bond, we see something like this, represented by a Newman projection ... 

If the two lone pair sp orbitals on oxygen are drawn in the customary skittle shapes, the major lobes in a Newman projection look like two rabbit s ears. The direct electrostatic effect of lone pair electrons was therefore dubbed the rabbits ears effect, but failed to find acceptance, as much from the overtones of Beatrix Potter as from the fact that the shape of electron density resembled not two, but one ear (Figure 2.11). 

Also likely is that this cation suffers intramolecular proton transfer to the oxygen since a bimolecular deprotonation would not be expected to result in significant selectivity between the diastcrcomeric products. The Newman projection and chair representations for the orientations that lead to the relative configuration are illustrated here. While either of these is applicable to analysis of a concerted reaction process, the former can also be used to represent the first step in a two-step process, proceeding through a cation, while the latter can be adapted to the analysis of the transition state for intramolecular proton transfer. 

Do (b) and (c) together You can t draw the diagram umil you know what all the conrormations look like It doesn t matter where you start (what you define as the 0° conformation). Here are lour Newman projections showing 180" rotation of C3 ... 

When we do conformational analysis, we will find that certain types of structural formulas are especially convenient to use. One of these types is called a Newman projection formula and another type is a sawhorse formula. Sawhorse formulas are much like dash-wedge three-dimensional formulas we have used so far. In conformational analyses, we will make... 

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