Anti geometry

Problem 29.12 ls the pro-R or pro-5 hydrogen removed from citrate during the dehydration in step 2 of the citric acid cycle Does the elimination reaction occur with syn or anti geometry ... 

Triphenylformazan can be isolated in two forms the red form is assigned to the tautomeric structure 178 or 179 and the yellow form is assigned to the trans-anti geometry 181.303,304 This assignment is supported by IR, NMR, X-ray diffraction as well as kinetic studies.245,305-315... 

For neutral nucleophiles the electrostatic interaction between N and X will stabilize the syn more than the anti geometry due to the proximity of the two oppositely charged groups while the opposite will obtain when N is charged. 

From the previous discussions, we expect the anti geometry to be stabilized due to the larger overlap integral between the C—C sigma bond and the empty sp3 orbital. 

Ab initio calculations on these conformers indicate that the anti geometry is 6.04 kcal/mol more stable than the syn291 Some of this energy difference is undoubtedly due to the eclipsing interaction of the hydrogens but this can not account for the total rotational barrier. Thus, the a—p+ interaction effect must be responsible for some major component of the rotational barrier. 

The model system FCH2—CH2F will illustrate our approach. Here the dominant interaction is aCH— °cf The geometry which maximizes this interaction is the gauche geometry. Note that the anti geometry maximizes the much more inferior °cF ffCF and °ch-0ch interactions. 

The mechanism for the oxidative photofragmentation of a,/ -amino alcohols is consistent with a preference for anti geometry in the cleavage step (Ci, X. Kellett, M. A. Whitten, D. G., J. Am. Chem. Soc., 1991, 113, 3893). Provide a rationalization based on the frontier orbitals of the system. 

The pro-R hydrogen is removed during dehydration, and the reaction occurs with anti geometry. 

Addition of H+ For carbon 3, the top face is Si, and H+ adds from the bottom, or Re, face to give an S configuration at carbon 3. The reaction occurs with anti geometry. 

In step 2 of flic citric acid cycle (Figure 29.12), c/.v-aconitate reacts with water to give (2jR,35 )-isocitrate. Does —OH add from the Re face of the double bond or from the Si face What about - H Does the addition of water occur with syn or anti geometry ... 

For purposes of comparison, the anti-anti and syn-syn geometries were also studied by the authors . The configurations pictured in Fig. 6.10b and 6.10c were optimized. The anti-anti geometry was found to be more stable than syn-syn by about 3 kcal/mol. At the SCF level, the anti-anti is 4 kcal/mol higher in energy than anti-syn geometry, but this difference is reduced to only 1 kcal/mol at MP2. 

In summary, there is little distinction between the syn and anti lone pairs of the car-boxylate oxygen atom with regard to forming H-bonds with proton donors. When a proton has approached closely enough to form a covalent bond, the syn position is favored, but only in the gas phase. The syn and anti conformers of the carboxylic acid are close in energy in aqueous solution. Even in the gas phase, the preference for the syn configuration of the isolated carboxylic acid can be eliminated when it forms a H-bond, due to more favorable electrostatic interactions between the partner and the anti geometry of the carboxyl. 

SnCU is effective in promoting the addition of nucleophiles to simple aldehydes. Among the most synthetically useful additions are allylstannane and -silane additions. The product distribution in the stannane reactions can be influenced by the order of addition, stoichiometry, and reaction temperature. The anti geometry of the tin-aldehyde complex is favored because of steric interactions. Furthermore, the six-coordinate 2 1 complex is most probably the reactive intermediate in these systems. The use of crotylstaimanes provides evidence for competing transmetalation pathways (Eq. 35) [60]. TiCU results in superior selectivity. 

Heteronuclear NOE experiments provided evidence for the anti geometry in solution. Enhancement at the aldehydic (but not the phenyl) proton absorption was observed upon irradiation of the fluorine atoms, see ref. 67. 

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