The Newman Projection

We can view this reaction as the replacement of one or the other of the two methylene protons at C 2 of butane These protons are prochiral atoms and as the red and blue protons m the Newman projection indicate occupy mirror image environments... 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

Substituent effects calculated for structure B lead to values which are not perfect but which agree more closely than for A with the measured C shifts of the benzene ring carbon atoms. The dia-stereotopism of the NC//2 protons in the H NMR spectrum also points to B as the Newman projection C along the C/fj-ammonium-N bond shows ... 

A similar process, using the Newman projection diagram on the right, which places C2 in front of Cj, and uses Clj as the third atom for the dihedral angles, results in the last two lines of the Z-matrix ... 

Figure 3.6 A sawhorse representation and a Newman projection of ethane. The sawhorse representation views the molecule from an oblique angle, while the Newman projection views the molecule end-on. Note that the molecular model of the Newman projection appears at first to have six atoms attached to a single carbon. Actually, the front carbon, with three attached green atoms, is directly in front of the rear carbon, with three attached red atoms.

PROBLEMS Draw Newman projections for each of the following compounds. In each case the skeleton of the Newman projection is drawn for you. You just need to fill in the six groups in their proper places. 

So, we expect this reaction to be stereospecific, rather than stereoselective. That is, we expect only one alkene, rather than a mixture of stereoisomeric alkenes. In order to determine which alkene is obtained, we begin by drawing the Newman projection ... 

Stable conformation for CHjSOH and CH3SOCH3 may be illustrated by the Newman projections of 32 and 33, while those of the carbanions are given by 34 and 35. This means that the a-sulfinyl carbanion is nonplanar and the Hj proton on the opposite side of the S—O bond is removed preferentially. The same calculation also indicated that conformer 32 is 16.5kcalmol and 17.2kcalmol" more stable than conformers 29 and 31, respectively . This new MO calculation thus contradicts the earlier suggestion that the most stable carbanionic conformer is 29 but emphasizes the importance of the contribution of 3d-orbitals for the stabilization of a-sulfinyl carbanions. 

Figure 4.5 a) The staggered conformation of ethane, b) The Newman projection formula for the staggered conformation. 

The two representations shown here are actually two different conformers of ethane there will be an infinite number of such conformers, depending upon the amount of rotation about the C-C bond. Although there is fairly free rotation about this bond, there does exist a small energy barrier to rotation of about 12kJmol due to repulsion of the electrons in the C-H bonds. By inspecting the Newman projections, it can be predicted that this repulsion will be a minimum when the C-H bonds are positioned as far away from each other... 

The steric effects become clear on inspecting the Newman projections of the transition states. The conformation 43 a leads to cyclopentane formation, while the conformation 43 b would give / -H elimination. As the rhodium carbenoid becomes larger, conformation 43 b is increasingly favored. Thus, as the steric bulk of the ligands on the Rh carbenoid increases on going from acetate (entry 1) to the TPA catalyst (entry 4), there is a significant increase in the proportion of /9-hydride elimination. 

The Newman projection along the central carbon-carbon bond is shown below. 

Other conformations encountered in the literature are categorised in terms of the Newman projection, as shown below. 

The stereochemical outcome of this sequence traces back to the Grignard addition step. The rotamer of the starting ketone (10-1), which involves the fewest nonbonding interactions, is that which opposes the two aromatic rings as shown in the Newman projection below. Isomer (10-2a) will be formed if it is assumed that the organo-metallic adds from that direction, which will lead to the fewest steric interactions in the product giving (10-2a). The rotamer (10-2b) from this product, which has the tmns hydroxyl-proton relation required for dehydration, leads to the observed product (9-3). 

Visualization of conformers There are four conventional methods for visualization of three-dimensional structures on paper. These are the ball and stick method, the sawhorse method, the wedge and broken line method and the Newman projection method. Using these methods, the staggered and eclipsed conformers of ethane can be drawn as follows. 

Figure 2.3 Fourier decomposition of the torsional energy for rotation about the C-O bond of fluoromethanol (bold black curve, energetics approximate). The Fourier sum (A) is composed of the onefold (o), twofold (o), and threefold ( ) periodic terms, respectively. In the Newman projection of the molecule, the oxygen atom lies behind the carbon atom al center...

In the transition state of the enantio-determining cyclometalation step, F - G, the Rh, nitrogen, C(l), and one of the hydrogens would be roughly coplanar, which requires either clockwise or counterclockwise rotation about the N—C(l) axis (see the Newman projection formula F in Scheme 16). The transition state models la and lb explain the prevailing clockwise rotation of the conformer Fa with respect to the coun-... 

Figure 2-2 Description of conformations about a single bond in the terminology of Klyne and Prelog10 11 using the Newman projection. Group A is on the front atom at the top the conformation is given for each possible position of group B on the other atom.

The stereochemical course of the Anil Synthesis is such that exclusive formation of trans products has been observed.11 This can be explained by consideration of the conformations of the intermediate addition product. The Newman projections (lOa-c) represent the three possible extreme conformations, and it can be seen that 10a, being the least sterically hindered, would be expected to be the most energetically... 

Norin (71) and Dauben and collaborators (72-74) have shown that the bond of the cyclopropane ring which is reductively cleaved corresponds to the bond which better overlaps with the n system of the adjacent carbonyl group. Thus, when the cyclopropyl ketone exists preferentially in the cisofd and transold conformations described by the Newman projections 238 and 239, the Cq — Cg bond is always cleaved in preference to the Cq— C3 bond. 

The Newman projections 304 and 305 illustrate the required transition state geometries for a concerted process leading to the trans and cis-bromo olefins 302 and 303 respectively. Models show that severe non-bonded steric interactions between the C- and C3 methylene groups and the R group strongly destabilize arrangement 305 relative to 304 so that the predominant formation of the trans olefin 302 (from 304) is readily explained. 

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