C Newman projections

FIGURE 11-1 Structure of acetylcholine. (A) The three torsion angles, ly T2 and T3. (B) Newman projection of the gauche conformation. (C) Newman projection of the trans conformation. The molecule is viewed in the plane of the paper from the left side, and the bond angles around t2 are compared. 

FIGURE 10. (a) Z-Form and (b) -form of At-chloro-At-methoxyformamide computed at B3LYP/ 6-31G level (c) Newman projection along the O—N bond in (a)... 

Figure 1 (a) Octant projection of twisted cyclopentanone with a-hydrogen atoms, (b) Octant projection without a-hydrogen atoms, (c) Newman projection of 0=C—C —H to show torsion angle ai... 

Fig.C. (a) a proton (b) orbital diagram illustrating orbital interactions (c) Newman projection.

Figure 16. Chair conformations of frans-(a-axial)-methyl-4-rm-butylcyclohexanone (a) and (a-axial)-methylbicyclo[3.2.1]octanone (c). Newman projections showing relative orientations of methyl and C=0 groups (b) and (d), with torsion angles indicated.

Sight down the C 2—C 3 bond and draw Newman projection formulas for the... 

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 ... 

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... 

The stereoselectivity of elimination of 5-bromononane on treatment with potassium ethoxide was described in Section 5.14. Draw Newman projections or make molecular models of 5-bromononane showing the conformations that lead to c -4-nonene and tra i-4-nonene, respectively. Identify the proton that is lost in each case, and suggest a mechanistic explanation for the observed stereoselectivity. 

The dihedrals for the remaining two hydrogens are best visualized with a Newman projection. They are located above and below the plane of the C-C-C bond. H5 is the hydrogen below the plane, and its dihedral is 60°. The dihedral for H6 could be expressed as either 300° or -60° we ll use the latter to express the symmetry of the molecule. Here are the Z-matrix lines for these atoms ... 

Newman projections are helpful in determining the proper dihedral angles for the fluorines and hydrogens. Here are diagrams for the RR isomer, looking down the C-C bond in both directions ... 

A syn-selective asymmetiic nih o-aldol reaction has been reported for structurally simple aldehydes using a new catalyst generated from 6,6-bis[(tiiethylsilyl)ethynyl]BINOL (g in Scheme 3.18). The syn selectivity in the nitro-aldol reaction can be explained by steric hindrance in the bicyclic transition state as can be seen in Newman projection. In the favored h ansition state, the catalyst acts as a Lewis acid and as a Lewis base at different sites. In conbast, the nonchelation-controlled transition state affords anti product with lower ee. This stereoselective nitro-aldol reaction has been applied to simple synthesis of t/ireo-dihydrosphingosine by the reduction of the nitro-aldol product with H2 and Pd-C (Eq. 3.79). 

Conformational isomers are represented in two ways, as shown in Figure 3.6. A sawhorse representation views the carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all C-Tl bonds. A Newman projection views the carbon-carbon bond directly end-on and represents the two carbon atoms by a circle. Bonds attached to the front carbon are represented by lines to the center of the circle, and bonds attached to the rear carbon are represented by lines to the edge of the circle. 

Despite what we ve just said, we actually don t observe perfectly free rotation in ethane. Experiments show that there is a small (12 kj/mol 2.9 kcal/mol) barrier to rotation and that some conformers are more stable than others. The lowest-energy, most stable conformer is the one in which all six C-H bonds are as far away from one another as possible—staggered when viewed end-on in a Newman projection. The highest-energy, least stable conformer is the one in which the six C-H bonds are as close as possible—eclipsed in a Newman projection. At any given instant, about 99% of ethane molecules have an approximately staggered conformation... 

Figure 4.4 The structure of cyclopropane, showing the eclipsing of neighboring C-H bonds that gives rise to torsional strain. Part (b) is a Newman projection along a C-C bond.

Figure 4.5 The conformation of cyclobutane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C—H bonds are not quite eclipsed.

In Newman projection formulas (the two figures on the right) the observer looks at the C C bond head on. The three lines emanating from the center of the circle represent the bonds coming from the front carbon, with respect to the observer. 

Figure 4 Sketch of two possible stereochemical arrangements for a chiral monomer. P represents the polymer chain, R represents a vinyl substitutent on a carbon, H represents hydrogen, (a) Linear sketch showing one conformation and two configurations (bracketed and unbracketed). The apex of bonds is a tetrahedrally bonded carbon atom (solid and dashed circles), (b) Newman projection of the same monomer showing the free rotation about the C-C bond.

FIGURE 22. Top Labels of the four localized basis w-orbitals of [4.4]spirononatetraene 247 and Newman projection defining their relative phases. Middle Newman projections of the four linear combinations ld2(tr), b <7T) and 1 c(jt). defined in equation 59. Bottom Correlation diagram showing the splitting due to spiroconjugation between the butadiene moieties in 247... 

Fig. 11.— Newman Projection-Formulas, Viewed Along the C-4-C-5 Bond in a, Compound 86, and b, Compound 87 (from Ref. 100, with permission).

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