Steric test

The basis of ALADDIN is the Daylight Chemical Information Systems software, particularly GENIE, a substructure specification language. When GENIE finds a query substructure in an input SMILES structure, it can return to the user those atoms in the structure that correspond to those hit. Since in a MENTHOR database the coordinates of the atoms are stored in the order in which they occur in the SMILES for that molecule, the coordinates of the atoms of interest are thereby identified. Thus, our geometric objects are established from this set of atoms, and geometric tests are performed on them. Steric tests are performed on molecules that meet the geometric criteria. 

The roles of phosphines are not clearly understood and are unpredictable. Therefore, in surveying optimum conditions of catalytic reactions, it is advisable to test the activity of all these important types of phosphines and phosphites. which have different steric effects and electron-donating properties. 

Pure, low temperature organic Hquid viscosities can be estimated by a group contribution method (7) and a method combining aspects of group contribution and coimectivity indexes theories (222). Caution is recommended in the use of these methods because the calculated absolute errors are as high as 100% for individual species in a 150-compound, 10-family test set (223). A new method based on a second-order fit of Benson-type groups with numerous steric correctors is suggested as an alternative. Lower errors are claimed for the same test set. 

Aimulene offers a particularly significant test of the Hiickel rule. The internal cavity in [18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most MO calculations find the delocalized structure to be more stable than the polyene. ... 

The lack of steric hindrance is also shown by the kinetic data for p-xylene, mesitylene, and durene, the observed reactivities being close to those calculated by the additivity principle. The additivity principle has also been tested for the last seven compounds in Table 177, and for the first five of these it holds very well. If one assumes a value for/3Me0 of ca. 4.0 and takes the average of the values listed in the table for the methyl substituent partial rate factors, then the observed calculated reactivity ratios are 1.6, 0.85, 0.75, 1.4 and 1.0. For the last two compounds in the table the ratios are 5.3 and 4.1, the reason for this being unknown. 

Given the above possible reaction mechanism, it is then intriguing to speculate that another approach to the same stereoselective reduction of a vinyl sulphone could be achieved by the use of a suitably sterically hindered organosilane, as outlined in equation (64). Such a reaction would provide an interesting test for the stereoelectronics of a conjugate addition reaction by a second-row heteroatom to a vinyl sulphone. 

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. 

In order for an experimental test of the kinetic behaviour to be as informative as possible, the system investigated should fulfil various specific requirements. From the experimental point of view, the reaction should cause a minimum of change in the reaction medium and be without side-reactions as far as possible, in order for accurate and well-defined rate measurements to be feasible. For the same reason an accOTate physical method which can be applied without distiubing the reacting system is to be preferred. From the theoretical point of view, it is desirable that the steric effects play as important a role in the reaction as possible, because only then is a sizeable effect to be expected. Finally, a transition state of well-known conformation is a necessary prerequisite for the quantitative application of the theory. 

As already indicated above, kinetic studies of steric isotope effects afford a possibility of testing our present ideas about the transition state. Bartell s perturbation treatment (Section II, A) involves a timeaveraging of the repulsion energy with respect to the ordinary vibrational motion within the transition state. The passage through the... 

Solvolytic experiments specifically designed to test Bartell s theory were carried out by Karabatsos et al. (1967), who were primarily interested in an assessment of the relative contributions of hyperconjugation and non-bonded interactions to secondary kinetic isotope effects. Model calculations of the (steric) isotope effect in the reaction 2- 3 were performed, as well as that in the solvolyses of acetyl chloride... 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

PMc3 to give the complex salts 21 as shown in Scheme 12. Compounds of type 20 and 21 have successfully been tested for their catalytic efficiency. However, it was observed that fBuaPNSiMea failed to show similar adduct formation and this failure was attributed to steric crowding. 

The intermediates 74 and 76 can now lose OR to give the acid (not shown in the equations given), or they can lose OH to regenerate the carboxylic ester. If 74 goes back to ester, the ester will still be labeled, but if 76 reverts to ester, the 0 will be lost. A test of the two possible mechanisms is to stop the reaction before completion and to analyze the recovered ester for 0. This is just what was done by Bender, who found that in alkaline hydrolysis of methyl, ethyl, and isopropyl benzoates, the esters had lost 0. A similar experiment carried out for acid-Catalyzed hydrolysis of ethyl benzoate showed that here too the ester lost However, alkaline hydrolysis of substimted benzyl benzoates showed no loss. This result does not necessarily mean that no tetrahedral intermediate is involved in this case. If 74 and 76 do not revert to ester, but go entirely to acid, no loss will be found even with a tetrahedral intermediate. In the case of benzyl benzoates this may very well be happening, because formation of the acid relieves steric strain. Another possibility is that 74 loses OR before it can become protonated to 75. Even the experiments that do show loss do not prove the existence of the tetrahedral intermediate, since it is possible that is lost by some independent process not leading to ester hydrolysis. To deal with this possibility. Bender and Heck measured the rate of loss in the hydrolysis of ethyl trifluorothioloacetate- 0 ... 

HOMO, destabilizing the transition state. The compound 3-formylcyclobutene provided a test. Steric factors would cause the CHO (an electron-withdrawing group) to rotate outward electronic effects would cause it to rotate inward. The experiment showed inward rotation. 

The data to be examined are correlated with eq. (24). Successful correlation with eq. (24) is a necessary but not sufficient condition for the existence of case (a). Strong evidence for case (a) is provided by a confidence level of Ij greater than or equal to 90.0. The confidence level of p is obtained by means of a student t test of. If is not significant, then this fact implies either the existence of cases (b), (c), or (d), or the use of an incorrect steric parameter. The data are now correlated with eq. (2). If the correlations with... 

One phase of these studies involved steric assistance to ionization in highly branched tertiary halides and related derivatives. This concept was tested and fully supported by a number of studies. 

Davies [30] studied the PyBOx-induced conformational effects by testing several ligands sterically hindered on the oxazoUne moieties (Scheme 11, structures 18 and 19). However, these new ligands gave poorer results in terms of yields and enantioselectivities than ligand 16 for the Ru-catalyzed cyclopropanation reaction, indicating unfavorable steric interactions between styrene and the carbene complex. 

Salvadori et al. [62] tested the same strategy but derived the bis(oxazohne) ligands in such a way that they minimized the steric hindrance at the bridging methylene carbon (structure 53 in Scheme 25). The polymer was used affording enantiomeric excesses superior to 90% and was reused at least five times with almost no loss in enantioselectivity or activity. 

Numerous articles have been devoted to the synthesis of structurally modified bis(oxazoline) ligands and to their ability to promote enantios-elective Diels-Alder transformations. For example, Davies et al. [74] synthesized and tested several Evans-type auxiliaries, i.e., bis(oxazolines) or pyridine-bis(oxazolines), bearing various sterically-hindering substituents. The best results were obtained according to the conditions presented in Scheme 26, and afforded the endo diastereomer with 95% ee by using ligand 58 (Scheme 28). 

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