Elimination from cyclohexanes, effects

In cyclic systems, the usual simple requirements of Saytzev or Hofmann rules may be overridden by other special requirements of the system, e.g. the preference for elimination from the truns-diaxial conformation in cyclohexane derivatives (cf. p. 255). Another such limitation is that it is not normally possible to effect an elimination so as to introduce a double bond on a bridgehead carbon atom in a fused ring system (Bredt s rule), e.g. (47) (48) ... 

Elimination of hydrogen fluoride from cyclohexanes 34A and 34B illustrates the balance between electronic and conformational effects. Anti elimination is possible from cyclohexane 34A to give 1//-nonafluorocyclohexene (35), involving removal of fluorine from a fluoromethyl group rather than from a difluoromethyl group. Here electronic and conformational effects are in accord. 

The experimental data reported in the Table for gas phase have been extracted from measurements in dioxane solution by applying the Onsager reaction field model to eliminate the solvent effect [37], By contrast, the cyclohexane experimental dipole moments have been obtained from those reported in Ref. [37] re-including the proper reaction field factors. Once recalled these facts, we note that the observed solvent-induced changes on both ground and excited state dipole moments are quantitatively reproduced by the calculations. 

Tatlow and his co-workers conducted an extremely comprehensive programme of syntheses and structure derivations of a series of fluorinated cycloalkanes [24], and concluded that the reactivity of the system, as well as the orientation of the cycloalkene produced, are similarly influenced by electronic factors which have been outlined in the preceding sections of this chapter. Anti elimination is generally the more favourable process but conformational effects may make the synjanti rates nearly comparable. Elimination from the cyclohexanes 6.16A and 6.16B illustrates the balance between electronic and conformational effects [25]. Anti elimination is possible from 6.16A, involving removal of fluoride from >CHF rather than >Cp2 since in this case electronic (the carbon-fluorine bond in CFH is weaker than in CF2) and conformational effects (H and F are anf -periplanar) are in concert (Figure 6.16). In contrast, anti elimination from 6.16B can only occur with elimination of fluoride from the more stable >CF2 position and therefore anti and syn eliminations occur together. 

Bubble columns in series have been used to establish the same effective mix of plug-flow and back-mixing behavior required for Hquid-phase oxidation of cyclohexane, as obtained with staged reactors in series. WeU-mixed behavior has been established with both Hquid and air recycle. The choice of one bubble column reactor was motivated by the need to minimize sticky by-products that accumulated on the walls (93). Here, high air rate also increased conversion by eliminating reaction water from the reactor, thus illustrating that the choice of a reactor system need not always be based on compromise, and solutions to production and maintenance problems are complementary. Unlike the Hquid in most bubble columns, Hquid in this reactor was intentionally weU mixed. 

Diels-AUer reactions. This diene can serve as a precursor to the highly oxygenated cyclohexane derivative shikimic acid (3), as shown in Scheme 1. Oxidative desilylation of the Diels-Alder adduct 2 could not be effected with peracids, but was effected by cis-dihydroxylation (Upjohn procedure, 7, 256-257) followed by p-elimination of (CH3)3SiOH with TsOH. Introduction of the 4a,5P diol system was effected indirectly from the 4a,5a-epoxide in several steps, since direct hydrolysis of the epoxide resulted in a mixture of three triols.1... 

The acetal RCH(OMe)2 can have a total of nine conformers, 30a-30i. We may ignore the broken red bonds, which are included to allow a quick conformational match with that of the cyclohexane chair and, thus, ascertain the geometrical relationships rather easily. The conformers 30a and 30e have two methyl groups within van der Waals distance and, hence, their contributions to the overall conformational equilibrium will be small, if not zero. We can therefore eliminate these conformers from further discussion. The conformers 30b and 30d, 30c and 30 g, and 30f and 30 h are mirror images and, thus, we need to consider only one conformer of each pair. Thus, we are left with four distinct conformers, namely 30b, 30c, 30f, and 30i, to consider for acid hydrolysis. The relative contributions of these conformers could be estimated from the understanding that they are laced with two, one, one and zero stereoelectronic effects, respectively. The conformers 30b and 30i are, respectively, the most contributing and the least contributing. The conformers 30c and 30f contribute at the medium level. 

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