Esters cyclohexyl, conformation

In 2-trimethylsilylethyl 4-phenylbenzoate 44, the C—Si and C—O ester bonds are in an antiperiplanar conformation, and the C—O ester bond is significantly longer than expected for an alkyl ester bond at a primary centre. This is consistent with a oq si T c o interaction in the ground state. In /raw.v-2-(di methyl phenyl si ly I )cyclohexyl-3,5-dinitrobcnzoate 45, the dihedral angle between the C—Si and C—O ester bonds is found to be just over 60°, and there appears to be little lengthening of the C—O bond. 

TABLE 9. Ratio of diequatorial (46a) and diaxial (46b) conformers of 2-trimethysilyl cyclohexyl esters... 

Analysis of the crystal structure of the p-nitrobenzoate and 2,4-dinitrobenzoate derivatives of ewrfo-3-(trimethylsilyl)-ewrfo-2-norborneol 47, in which the Si—C and C—O bonds are in the synperiplanar geometry, showed that there was no significant lengthening of the C—O bond in these cases64. By comparison, the cyclohexyl ester 48, in which the Si—C and C—O bonds are in the antiperiplanar conformation, did show a significant lengthening of the C—O bond compared to model compounds64. 

With a more powerful donor than an H—C bond, cyclohexyl esters carrying a [3 silyl group demonstrate the preference for the donor and the acceptor bonds to be anti. The equilibrium proportion of the alcohol 2.89 (R = H) is in favour of the diequatorial isomer, but with esters (R = acyl) the equilibrium shifts to favour the diaxial conformation 2.90. Furthermore, the equilibrium constant correlates... 

The influence of 7C-stacking on the conformation of substrates and transition states has been a matter of debate [44, 46-49] (see above). Nonetheless, the efficiency of 8-phenylsubstituted menthyl or /ra s-2-phenylsubstituted cyclohexyl esters of a,p-unsaturated acids as substrates in asymmetric induction has been ascribed to such effects [66], Both NMR spectroscopy and calculations [67, 68] have shown that 7c-stacking can stabilize the s-cis-syn conformation of crotonates 10, especially when Ar = 2-Np or 4-PhOCgH4. An X-ray determination shows that 10 (Ar = 2-Np) adopts the s-cis conformation in the solid state [67], Interactions with remote substituents of the substrate can cause a preferred reactive conformation to intervene. For example, the reduction of y-ketoboronates by borane is highly stereoselective and probably takes place via an internal complexed intermediate 11, even though tins could not be characterized in the ground state [69, 70]. 

Dienol esters of chiral acids have been proposed as chiral dienes. Trost and coworkers [73, 170] have shown that cycloaddition of quinone 9.41 (X - OH) with l-(0-methyImandeloxy)butadiene 1.113 (R = H) is highly stereoselective when catalyzed by B(OAc)3 (Figure 9.47). The cycloaddition of the same diene with acrolein is also selective [387] (Figure 9.47). The replacement of the phenyl substituent of 1.113 by a cyclohexyl group does not affect the selectivity, so x-stacldng does not intervene in transition state [387]. Instead, the diene is probably in a nonfolded conformation (Figure 9.47). 

In 1,2-iraws-disubstituted compounds Reeves and Stromme (1961a) have shown from low-temperature intensity measurements of the separated adjacent proton resonances that conformational preferences ee or aa are a function of the polarity of the solvent. Deuteriation of both sets of adjacent CH2 protons at the 2- and 6-positions simplifies the appearance of the —CHX proton in cyclohexyl compounds. First-order deuterium couplings contribute to a considerable apparent line width of 3-6 c.p.s. (Allan et al., 1963 Premuzic and Reeves, 1962). At low temperature the position of the adjacent proton resonance is a reliable measurement and it has been shown, using the time-weighted average method, that the acetate, formate, trifluoroacetate and nitrate esters have the following equatorial preferences 76%, 61%, 76% and 73% with probable errors of + 1 %. 

For four- or five-membered cyclic transition states, pyrolysis only leads to beta-elimination if the C -H and C -X bonds can eclipse so as to ensure the necessary proximity for reaction. The syn nature of the amine oxide decomposition has been demonstrated by elimination from the diastereoisomeric pairs of the 1,2-diphenylpropyl system. In the cyclohexyl series, an eclipsed transition state is only achieved if elimination proceeds through the boat conformer and consequently the pyrolysis of 1-methylcyclohexylamine oxide gives an almost quantitative yield of the less stable exo-olefin. This orientation contrasts markedly with that of the six-membered cyclic transition states of the corresponding esters and xanthates which yield predominantly the endo-olefin, syn-clinal stereochemistry being preferred. For all the other ring... 

Figure 6.84 Hyperconjugative interactions responsible for stabilization of axial conformers in cyclohexyl ethers and esters.

A non-chair conformation of the cyclohexyl ring in cis-2,trans-3-6ibromo-cis-4-t-butylcyclohexyl p-nitrobenzoate (3) has been found in an X-ray structural study of the molecule. In the solid a twist conformation with a staggered t-butyl group and ester moiety in pseudo-equatorial positions exists the bromine atoms are located in... 

Substituent polarity effects are apparent in the conformational equilibria of cyclohexyl acetate and monochloro- dichloro-, and trichloro-acetates as evidenced by their H n.m.r. spectra. The inversion barriers decrease, and the proportions of axial ester conformer increase, with successive chlorine substitution of the acetate. The conformations of various trans-1,2-disubstituted cyclohexanes have been examined by n.m.r. and a discussion of the observed inversion barriers in terms of steric and dipole-dipole interactions has been given. A new description of these conformational effects is proposed. 

The photochemical a-cleavage of bicyclo[3,2,l]octan-6-ones (402) has been investigated in order to determine the conformational behaviour of the biradicals so formed, and the influence of the conformational equilibration on the distribution of the products (Scheme 2). ° In cases where the substituents in the cyclohexane ring are expected to stabilize the conformation (403), the major products (methyl esters because of the presence of added methanol) are derived from the ketene intermediate formed by cyclohexyl radical hydrogen abstraction from the side-chain. 

Side group rotations become important in polymers with ester side chains, of which there are very many. Of technological significance is the low temperature transition in poly(methyl methacrylate). Another interesting conformation change is the boat-chair switch in cyclohexyl groups (Figure 5.5). 

---