Gear effect

In subsequent reports [87], the principle of asymmetric glycolate alkylation/ RCM sequence was applied to the first total synthesis of isolaurallene (172), that contains a densely functionalized A5-oxonene core (Scheme 32). Anticipating that the gearing effect created by two synergistic gauche effects at C6-C7 and... 

It may be pertinent to mention here a recent criticism of the gear effect by methyl groups, since most of the known examples used in the study of the gear effect are in aromatic molecules. Based on dynamic NMR data on the internal rotation of the 9-methyl group in 42, which indicated a far lower barrier for 42a than for 42b, Oki... 

Then the gear effect Van der Waals radius of a tetrahedral MZ3 group is taken to be... 

In considering the importance of the gear effect it is convenient to examine the quantity Pgr defined by the expression... 

Pp represents the percent change in the magnitude of the minimum perpendicular Van der Waals radius of MZs due to the gear effect. Values of Pgr are given in Table 5. 

Fig. 9 A. Gear effect. Top view of conformation 1 (meshed) Fig. 9B. Gear effect. Top view of conformation 2 (nonmeshed)...

Fig. 10. Gear effect. Vertical cross section through an MZj group...

Many values of rv for groups have been obtained from kinetic data where the experimental errors is probably at last 5 % in log kr. It seems likely then, that the gear effect in the Me, CF3, CMe3, NH3, and NMe is negligible in most cases. The gear effect will clearly be much more important in MZ3 groups with M = Si, Ge, and As. 

Trimethoxybenzene-l,3,5-tricarbaldehyde (79) and its keto homologue (80) involve three symmetry-equivalent carbonyl centres, each in a 1,5-relationship to their neighbours.236 Two diastereoselective reactions have been performed (i) the trial can be trimethylated to give triol (81) with methyllithium in THF, and (ii) the triketone can be reduced (to the same product), in both cases with >95% de (anti, syn). Chelation and steric (gearing) effects about the crowded aromatic core are discussed to explain the observed diastereoselectivity. 

This has been explained on the basis of the greater stability of the rotamer (A) than that of the rotamer (B) due to the gear effect between the two isopropyls (or cyclopropyls) at geminal positions, which generates distinct energy minima with respect to rotation of the groups (Sch. 69). 

The interdependence of these conformational states and the possibility of transmission of conformational modification through a series of interdependent conformational states led to the definition of the gear-effect a conformational transmission, which is caused by interaction between polyhedral substituents and which depends on their polyhedral shapes. As a corollary of this definition, it was possible to outline the difference between the gear-effect and the classical buttressing effect (76JA2847). The two effects depend on different modes of minimization of the steric strain introduced by two... 

The borane complexes of acrylic acid and methyl acrylate were studied next (Table 5). The syn, s-trans conformers are again preferred. Notably, the steric arguments discussed previously can nicely rationalize these results. Single point calculations on a linearly restricted BH3 complex (a = 3 = 180°) revealed only a 0.1 kcal mol preference for the s-trans structure, thereby supporting the notion that, in contrast to the Li case, the steric gearing effect is dominant in determining the structure here. 

In agreement with ab initio predictions, Lewis acid complexation with a,p-unsaturated carbonyls seems to encourage adoption of the s-trans conformation. In the complexes of conjugated esters and ketones the gearing effect discussed previously may be responsible for the abundance of the (Z)-syn-s-trans conformation. 

Interactions of the carbonyl group with Lewis acids are colorful and varied, but by no means unpredictable. The structural information gathered in this chapter points, fairly consistently, to the same set of principles. Rules of complexation such as coordination syn to the small substituent, s-trans preference due to gearing effects and distortions towards the ir-cloud in response to steric strain are among these principles. Section 1.10.7 illustrated a few examples of how the predictive power of such structural rules have been applied to the design of novel and highly selective Lewis acidic catalysts. 

Iroff and Mislow (25) earned out empirical force-field calculations on this molecule and confirmed that the expected nonbonded interaction forces the molecule to undergo a coordinated twist motion ( gear effect ). Their calculations also indicated that the molecule exists, not only in this ground state conformation with T symmetry, but also in a higher energy conformer with achiral S4 symmetry. The energy gap, A T, between the T and S4 forms was found to... 

The methyl group plays a special role in the sense that it provides a reference point in many scales of effective size. Is there any evidence, then, that the perceived possibility of gearing has a noticeable conformational effect on the effective size of the methyl group In a critical examination of the existence of the proposed gearing of methyl groups with other groups in various molecular situations, Mislow and coworkers concluded that there is no experimental evidence for a dynamic gear effect (Hounshell el al., 1980). 

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