Conformational diagrams

Fig. 13. Conformation diagrams for three models of p 4GT considered. (O) Positions of hydrogen atoms (a) Hall-Pass model 6 (b) Hall-Pass model 7 (c) Yokouchi model. Reproduced from Polymer by permission of the publishers Butterworth Co (Publishers) Ltd. (C)...

Problem 4.42 How would the energy-conformation diagrams of ethane and propane differ ... 

If you follow all the guidelines above, you will soon be drawing good conformational diagrams. However, a few... 

Six-membered rings with more than one sp2 C atom do lose their chair conformation—they become flattened to some degree when there are one or more double bonds included in the ring. Cyclohexene, with just one double bond in the ring, has a half-chair conformation similar to that of its related epoxide, cyclohexene oxide. The usual conformational diagram of cyclohexene is shown below. The barrier for ring inversion of cyclohexene is around 22 kj mol-1 (about half that for cyclohexane ). 

Draw conformational diagrams for these compounds. State in each case why the substituents have the positions you state. To what extent could you confirm your predictions experimentally ... 

This part of the discussion Is a revision of the material in Chapter 18, When dealing with six-membered rings, you should always aim to draw their conformation, though in this base you can explain the result adequately without conformational diagrams. 

One proton at the ring junctions appears clearly in the NMR spectrum as it is next to two oxygen atoms (shown in black on the conformational diagrams alongside). In one compound it is a doublet, = 7.1 Hz, and in the other a doublet / = 1.3 Hz. Which is which ... 

This result is more impressive because the large electrophile ends up on the same side of the ring as the t-butyl group, so the stereoselectivity cannot be based on any simple idea of reaction on the less hindered side of the ring. It is genuine axial attack, as the conformational diagram of the product confirms. 

We can use the same conformational diagram to show how the orbitals overlap as the new bond is formed. 

Fig. 35. Schematic conformation diagram (lna/lnN vs InP/InN) for N-mer brush exposed to P-mer chains, structurally identical but with a different isotopic status (%-dependent details of the diagram calculated for the data from [245], describe well the situation also for other experimental reports). The brush height L is described as the power law Lp=oq,with the exponent q characterizing different regimes q=0 (I and II), 1/3 (III wet), 1/2 (III t), 1 (III dry). The cross over between different regions occurs for coverage values o N-1 (I/II), o2=PN 3/2 /(l-2Px) (II/IH wet),o3=(l-2Px)2P 1/2 (HI wet/III t),a4sP 1/2 (III t/III dry),o5=lsr1/2 (II/III dry)...

Practically all observations [241-243, 245,249, 261, 262,264] of the entropy driven brush stretching (see Eq. 59) have been performed for polystyrene brushes immersed in an identical, except for the isotopic status, PS matrix. The change in the conformational properties of the brush has been observed as a function of surface coverage o [241-243,245,249,261,262,264] and/or host matrix degree of polymerization P [241,243]. These studies were initiated in 1992 by us [241] and two other groups [249,262]. Here we resume the results of these reports focusing on the verification of the schematic conformation diagram (InP/ln N vs lno/ln N) predicted by theory [232, 239, 267] and presented in Fig. 35. 

Fig. 38. The comparison of the predicted by theory conformation diagram (see Fig. 35) with the experimental observations of brush stretching made for equi-P situation ((A, ) for [241], (V, ) for [249], (O, ) for [245], and (O, ) for [264]) and for a=const ((A, ) for [241] and ( , ) for [243]). Dashed lines terminated by symbols mark the o or P ranges. The progress of a brush swelling goes from open to filled symbols. Numbers are the observed exponents q of the L°=aq variation...

A palladium-catalysed reaction of an allylic acetate (chapter 19) was used for the first C-C bond formation and the planned Michael addition for the second though the intermediate 79 was not isolated. The conformational diagram of 80 should help you to see why that diastereoisomer was favoured. 

This simple Robinson annelation (chapter 5) produces a cis cyclohexene 33 because that alone is possible. The step in which the double bond appears is the ElcB elimination 32 at the end. The enolate will always have one lobe of the p-orbital shown in the conformational diagram correctly aligned to expel the hydroxide leaving group and form the cis alkene. 

We need to consider how the conformation of the half-chair changes as it reacts. Enolate 91 is drawn as a half-chair with the tertiary butyl group at the back of the molecule in an equatorial position. If the electrophile attacks from the top face then, as a bond forms to the electrophile and the sp2 hybridisation changes to sp3, a chair 92 starts to take shape (for clarity, we ve omitted the carbonyl double bond in the conformational diagram). We see that the electrophile occupies an axial position. However, if the electrophile attacks from below, then a twist boat 93 starts to develop instead. If this boat were flipped into a chair, the new electrophile would be in the equatorial position.22... 

We now draw the conformational diagrams 99a and 100a of the intermediates and consider the reverse reaction that would lead back to the same epoxide 95. In the reverse reaction, the alkoxide would have to come in behind the R group and displace it. 

Figure 7 The nine possible conformational diagrams for a six-membered ring in the SCRIPT program. The torsional angles of the ring bonds are only defined by their sign ( + /—) or zero (0) for a planar bond.

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