Electronic preference

The UHE wave function can also apply to singlet molecules. Usually, the results are the same as for the faster RHEmethod. That is, electrons prefer to pair, with an alpha electron sharing a molecular space orbital with a beta electron. Use the UHE method for singlet states only to avoid potential energy discontinuities when a covalent bond is broken and electrons can unpair (see Bond Breaking on page 46). 

Phosphinidenes have either a singlet ground state with two lone pairs and an empty p-orbital on the phosphorus atom or a triplet ground state in which the phosphorus has instead one lone pair and two singly occupied p-orbitals. Not surprisingly, the electronic preference, i.e., the singlet-triplet energy gap (AEst) and thus the stabUity and reactivity of a phosphinidene, is determined by its substituent. 

In the X-ray structures of both 3 and 4, the tetrahedral distortion is greater than that found in model A. Since there is an electronic preference for a square planar coordination, the pendant phenyl substituents in 3 and 4 likely result in further steric crowding and therefore in more distorted structures compared to model A. In the QM/MM model B an optimized 0 angle of 30° is found, close to the 34° angle of the X-ray structure. Since the phenyl and trimethyl phenyl groups are accounted for on a steric basis only in model B, the result supports the notion that the severely distorted coordination of the Pd center in 3 is due to a steric effect. 

The deviation from the square planar geometry at the Pd center in complexes 3 can be attributed wholly to steric factors. Calculations have shown that there is a moderate electronic preference favouring the planar coordination amounting to approximately 4.5 kcal/mol. However, steric interactions between the bulky trichlorosilyl ligands with both phenyl substituents of the phosphine and pyrazole ring result in a distortion away from the ideal square planar geometry. We further have found that the framework of the specific chelate backbone positions the pyrazole ring such that these interactions are enhanced. 

In the case of cyclooctatetraene, an electron prefers the isotopicaUy heavier material. At 173 K, the equilibrium constant for -F C Dg = CgHg + CgDg was found to be 1.16 (Stevenson 2007). However, when this anion-radical reacts with cyclooctatetraene dianion (not with the anion-radical), the transferred electrons prefer the isotopicaUy lighter material (Stevenson et al. 1990,1992) as follows CgHg -F = CgHg -F CgDg. The semiempirical quantum chemical consideration led to the... 

An interesting but still unexplained case refers to nitrobenzene. The reversible electron exchange between nitrobenzeneand sodium salt of the nitrobenzene- N anion-radical is characterized by the usual constant of 0.40. Stevenson et al. (1987b) used NH3(liq) as a solvent for these measurements at -75°C. Under the same conditions, they obtained the equilibrium constant of 2.1( ) for the electron exchange between nitrobenzene- N and the potassium salt of nitrobenzene- " N anion-radical. Perhaps, the difference between ion radii of sodium and potassium cations is crucial for the stability of the corresponding ion pair with nitrobenzene anion-radical. Such diversity can be pivotal when the electron prefers the heavy or light nitrobenzene. 

We propose that there is in fact a substantial electronic preference, not reflected in the Mechanics calculations, for the ester carbonyl and the C=Rh bond to be syn at the point of commitment to cychzation. This preference is strong enough to overcome the calculated steric preference (3.37 kcal moh ) for the anti transition state. The competition then, is between the syn transition state leading to (R,R)-29, and the syn transition state leading to (S,S)-29. The relative energies of these two transition states differ by slightly less than 1 kcal moh, so we predict, and observe, low diastereoselectivity. 

The stereoselectivity of monosubstituted dipolarophiles has also been studied with cyclic nitronates (Table 2.30) (84). In most cases, an exo selectivity was observed. The ratio between the endo and exo adducts can be correlated to the size of the substituents on the dipolarophile. Because of the endo preference observed with acrolein, it is believed that there is a slight electronic preference for the endo orientation in the transition state, in the absence of steric hindrance. In line with these results is the observation that, for 49, maleic anhydride reacts with complete exo selectivity, in contrast the cycloaddition with 47 (69). 

When electrons go into orbitals that have the same energy (degenerate orbitals), the state that has the highest number of equal spin quantum numbers will have the lowest energy. Thus electrons prefer to occupy different orbitals, if possible, since in many cases it is energetically favorable to avoid spin pairing. 

In the case of cyclo-octatetraene, an electron prefers the isotopically heavier material in the following equilibrium ... 

However, when this anion radical reacts with cyclo-octatetraene dianion (not with the anion radical), the transferred electrons prefer the isotopically lighter material (G. R. Stevenson, Peters et al. 1990, 1992) ... 

In contrast to the (E)-isomer, (Z)-alkenyl(phenyl)-A3-iodane 41 is labile and decomposes with a half-life time of 20 min to terminal alkynes in chloroform solution at room temperature [64]. Stereo electronically preferable reductive anti / -elimination accounts for this facile decomposition. In fact, the kinetic results for E2-type dehydrohalogenation of vinyl halides show that the relative rates of elimination decrease in the order anti /3->syn / - a-elimination [65]. Similar anti -elimination of vinyl-A3-iodane was proposed in the oxidation of methoxyallene with (diacetoxyiodo)benzene 4 to 3-acetoxy-3-methoxypropyne [66]. 

Notice that the three key reactions work brilliantly in this synthesis the hydrogenation of 113 is totally stereoselective and very high yielding while the two electrophilic substitutions on the pyrrole are perfectly regioselective acylation of 108 controlled by steric hindrance and alkylation of 112 controlled by electronic preference and because it is intramolecular. 

In the explanation favored today, the reason for this stereoelectronic effect is as follows The electronically preferred direction of attack of a hydride donor on the 0=0 double bond of cyclohexanone is the direction in which two of the C—H bonds at the neighboring a positions are exactly opposite the trajectory of the approaching nucleophile. Only the axial C—H bonds in the a positions can be in such an antiperiplanar position while the equatorial C—H bonds cannot. Moreover, these axial C—H bonds are antiperiplanar with regard to the trajectory of the H nucleophile only if the nucleophile attacks via a transition state B, that is, axially (what was to be shown). The antiperiplanarity of the two axial C—H bonds in the a positions is reminiscent of the antiperiplanarity of the electron-withdrawing group in the a position relative to the nucleophile in the Felkin-Anh transition state (formula C in Fig. 8.8 cf. Fig. 8.11, middle row). 

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