Sterically-favored isomer

Addition of cyanide to androst-4-ene-3,17-dione gives the cyanohydrin 35-1 with a 17)8-nitrile (Scheme 5.35). The stereochemical result is a reflection of the reversible nature of the transformation in which the sterically favored isomer predominates. Reaction of the cyanohydrin with methanolic hydrogen chloride converts the nitrile to the hnino ether... 

Evidently, the structure of the transition state is such that cis as well as trans compounds can be formed with, more or less, a preference for the cis compounds. The reason for the latter may be that coordination of cis isomers is sterically favored. In agreement with this are the observations that cis compounds are more reactive than the corresponding trans compounds (49, 106). 

Estimated Electronic and Steric Contributions to the Thermodynamic Stabilities of the Favorable Isomers of 2a and 4a of the [Nih(octadienediyi, )LJ Complex3... 

The efficiency and limitations of 3-oxabicyclo[3.2.0]hept-6-ene-2,4-dione 386 (cyclobut-3-ene-l,2-dicarboxylic anhydride) as an acetylene equivalent in 1,3-dipolar cycloadditions has been reported. It reacted readily with a variety of reagents, including nitrile oxides. In all cases, the sterically favored anti-isomers... 

Both steric and electrostatic factors appear to favor the trans configuration, in which the bulkier and anionically charged F atoms are separated as far as possible. Nevertheless, 27a is actually found to be the energetically favored isomer by 0.80 kcal mol-1, a counterintuitive result that has been attributed to steric attraction. 83... 

In contrast, when l-bromo-2-butyne is employed in this sequence, allenylcarbi-nols are the major adducts (Table 9.63). In the former case, the allenyl antimony reagent is presumed to prevail whereas in the latter sequence, the terminal Me substituent causes the equilibrium to shift toward the sterically favored propargylic isomer. 

As shown in Scheme 2, ti-o-ji isomerization can result in syn-anti interconversion by rotation around the o-(C-C) bond in the r -intermediate syn and anti refer to the positions syn and anti to the substituent at C2). The syn position is sterically favored and, consequently, in 1 -monosubstituted allyl systems the syn-isomer is more stable than the anfi-isomer.In general, the thermodynamic equilibrium lies far on the side of the syn-isomer and only if a substituent is sufficiently small (e.g., R=Me) is the anfi-isomer present in notable amounts. However, certain ligands, that exert strong steric hindrance in the coordination plane toward the [Pg.792]

The peculiar temperature sendtivity of sulfonation respecting variation in-the ratio of isomers formed has been discussed in the light of the above reaction mechanism. It is known from experiment that low temperatures and/or short reaction times (i.e., nonequilibrivun conditions) favor formation of the sterically strained isomers. This has been noted, for example, with toluene (more ortho isomer at low temperature) and Avith naphthalene (more 1 iscsner at low temperature). In intermediate (A) above, the entering sulfonate group is out of the plane of the ring, while in the final product... 

The structure of [FeCo(CO)g] , with idealized Cg symmetry, is closely related to that of the lowest energy M2(CO)g isomer (which has Z)2d symmetry) but does not correspond to a minimum of the PES computed from steric forces alone (Lauher, to force the calculation toward the experimental stereogeometry, had to constrain the Fe-Co-C angle of the semibridging carbonyl group to its experimental value). As a matter of fact, the sterically favored D2d structure, when the metals contribute a different number of valence electrons, is highly polarized and distortion must occur to quench charge polarization partially (Fig. 2). 

The stereochemically most favorable structure for [Rh6(CO)i4L2] is shown in Eig. 5b and, when L = P(OPh)3, is the structure adopted in the solid state and in solution. There are, however, four other isomers which are theoretically possible and there is chromatographic and spectroscopic evidence for two of these isomers when L = P(OPh)3. P COSY measurements and simulations suggest that these are the next two least sterically hindered isomers. Both of these isomers, on heating, are transformed into the thermodynamically more stable isomer shown in Pig. 5b. When L = py or NCMe, separation into individual isomers was impossible because of their rapid interconversion. ... 

Berry pseudorotation. The stability of the abovementioned intermediates is additionally limited by steric factors and intramolecular overcrowding in trigonal bypiramidal structures. The more favorable isomers are A and A° due to the equatorial position of the methyl substituent of the phospholane ring. The presence of pentacoordinated intermediates was confirmed by P NMR. Nevertheless, the relative stability of isomeric isomers A -A"" is not a factor determining the stereochemical outcome of this reactions, since the substituent derived from mono-Grignard reagent is not yet present in these structures. 

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