Gauche open conformation

Figure 6.6 Tentative model for the transition state of the catalytic conjugate addition of a carbon nucleophile to a nitroalkene with the cupreine derivative in a gauche open conformation [34],...

A cross-linked polymer has a density of 0.94 g cm" at 25°C and a molecular weight between crosslinks of 28,000. The conformation of one bond in the middle of the molecule changes from trans to gauche, and the molecule opens up by 120°. In w-butane, the trans to gauche transformation requires about 3.3 kJ mol". Estimate a value for AH of stretching based on this model, and use the law of cosines to estimate the magnitude of the opening up that results. 

The gauche-conformation is the most stable for all betaines of the first group and open forms of compounds from the third group. Table VIII contains the most important geometric parameters of these betaines. 

However, there are many cases where an open-chain radical gives stereoselectivity that is less obvious. The radical 7.103 can be expected to attack anti to the donor substituent, but the major product 7.104 in the capture of a bromine atom implies a conformation which is no longer obviously the preferred one, since the two alkyl substituents are gauche rather than anti.1035 This is another inside methyl effect (see pp. 233 and 242) allowing the bromide to attack close to the small substituent, the hydrogen atom. 

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