Methyl, trans effect

After several years of effort it has been possible to reproduce Schmidbaur fs addition of methyliodide W The problem appears to be that the methyliodide product is extremely photosensitive in solution. It decomposes with the production of methyl radicals and the ultimate formation of a mixture of the Au(I), and Au(II) diiodide products. The X-ray crystal structure of the methyliodide product is presented in Figure 5. Two features are to be noted. Firstly, the metal-metal distance lengthens only a small amount, 0.04A, but the Au iodide distance lengthens by approximately 0.20. The structural trans effect caused by the methyl group is not attenuated by the Au-Au bond. 

In fact, a similar intramolecular cyclisation was studied by Reusch [11] and he found a remarkable methyl substituent effect on the aldol equilibrium. Starting from the cis-decalones 25 (easily prepared from the Wieland-Miescher ketone), in which the angular methyl group prevents isomerisation to the more stable trans-decalone, it was found that other methyl groups may exert profound but less... 

A report of House, Respess and Whitesides (47) showed that the reaction of lithium dimethyl cuprate with the unsaturated ketone 135 gave exclusively the ketone J36 having the methyl trans to the t-butyl group. In this case where the double-bond is exocyclic, stereoelectronic effects allow equal attack from either face. Thus, the exclusive formation of 136 must be due to steric reasons only, the equatorial approach being favored. 

X-ray diffraction studies performed on complex 6d (Figure 12.2) revealed a rearrangement of the phosphorus atom and the methyl group, as a consequence of the classical trans-effect occurring in the coordination sphere of square planar d8 metal ions [16]. 

Table 19 shows results from the a-acetoxylation of two 2-alkyl-indans under different conditions analogous homogeneous reactions are included for comparison. Table 19 demonstrates that, at most, a weak effect is noticeable in the case of 2-t-butylindan, in that anodic oxidation on platinum, a strongly adsorbing metal, gives a cis/trans ratio that is about 10 times that observed from a 2-t-butyl-1-indanyl cation generated by solvolysis. Ratios on the weaker adsorbing electrode materials, carbon and lead dioxide, fall in between. For a smaller substituent (R = methyl) no effect is observable. 

The directionality property of the parameters is most relevant to conformationally flexible molecules and to the description of differences in reactivity of optical isomers. The steric substituent constant reflects the weighted average of the steric effects in variously populated conformations that contribute to the observed property. For example, an indication of different steric effects in conformationally locked compounds can be obtained from the hydrolysis of methyl trans- and cis-4-t-butyl cyclohexane-carboxylates (100) ... 

Methyl anion (CHs"), see also Carbanions gas phase, 109 trans effect, 181... 

Acidic resins [1, 514, after citation of ref. 17]. The biogenetically patterned cyclization of polyenes by Lewis acid catalysts can also be effected by cation-exchange resins (Amberlite IR-20 and XE-100).17a Thus methyl trans.trans-iarntsate (1) is cyclized by XE-100 in acetic acid to (2), (3), and (4), separable by gas chroma-... 

The stereochemistry of the 1,3-dipolar cycloaddition of the heteroaromatic iV-imines has been investigated in some detail by using the reaction of phenanthridine N-benzoylimine with a series of activated olefins such as JV-methylmaleimide, maleic anhydride, diethyl maleate, methyl acrylate, methyl methacrylate, and methyl trans-crotonate (e.g., Eq. 30).202 The adducts from the former three have the all-cis stereochemistry. These results are rationalized in terms of secondary molecular orbital interactions. With acrylates such stereospecificity is lost, suggesting that this effect is of lesser importance in these cases (see Table II). 

Metal-nitrogen distances, as observed in a very large number of crystal structure determinations, are almost the same as those observed for the azole ligands (see Table 2) within experimental error. Comparing pyridine with substituted ligands, it is observed that substituents at position 2 (and 6) next to the donor atom have a dramatic effect upon the stoichiometry and properties of the compounds formed. It appears that a stoichiometry M(Rpy)4(anion)2 with a trans orientation of the anions is not possible, because the methyl group effectively blocks the axial coordination sites. This results in square-planar low-spin compounds for Ni and tetrahedral geometry for some other metals such as Co " and... 

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