Differential Transition State approach

Differential transition state/product stabilization approach... 

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

When the carbonyl groups are present, the transition state for syn attack is sta-bihzed by interactions between the in-phase combination of the NN lone pairs and the antisymmetric n orbital of the CO-X-CO bridge (100). Although the secondary effect (SOI) operates only during syn approach and contributes added stabilization to this transition state, the primary orbital interaction (see 103) between the HOMO of the cyclohexadiene moiety of 100 and the n orbital of the dienophile (NN, Fig. 16) is differentiated with respect to the direction of attack, i.e., syn or anti, of triazolinedione (NN, Fig. 16). 

Preliminary approximate calculations of the transition state in C2H6 + IIO reaction, also executed by the Partial Reservation of Double-Centered Differential Overlap (PRDDO) method [29, 30] indicate the interaction between masked ethane conformation (with a calculated internal rotation barrier equal 1.3kJ/mol) and the HO radical approaching it in the C—C plane. The distance between C—C and O—O bond sites was taken as the reaction coordinate. It is found that planar structure (II) corresponds to the transition state in the C2H6 + H02 reaction. 

The stereoselectivity of the allylic hydroperoxidation also depends on se eral factors. With chiral allylic alcohols or allylic amines 200, a hydrogen is developed between ]02 and the vicinal hydroxyl or amino group. The fac differentiation results from an approach of 102 in the transition state that mi mizes 1,3-allylic strain. 201 and 202 can be obtained with a diastereoisome excess higher than 90% in CCI4. As previously indicated for the formation dioxetanes and 1,4-endoperoxides, the selectivity decreases considerably in presence of hydroxylic solvents [123]. When hydrogen bonding is no more pos ble in 203, the stereofacial differentiation is steered by steric and electronic rep sion effects at the level of the possible diastereoisomeric transition states 204 is formed selectively (Scheme 54). 

A number of studies have compared the use of the multiple regression technique using semiempirical parameters such as tt and o-, and parameters calculated for the particular molecules from molecular orbital theory. Hermann, Culp, McMahon, and Marsh (23) studied the relationship between the maximum velocity of acetophenone substrates for a rabbit kidney reductase. These workers were interested in the reaction mechanism, and two types of quantum chemical calculations were made (1) extended Huckel treatment, and (2) complete neglect of differential overlap (CNDO/2). Hydride interaction energy and approaching transition-state energies were calculated from the CNDO/2 treatment. All these parameters plus ir and a values were then subjected to regression analysis. The best results are presented in Table II. 

Hey-Hawkins reported that the asymmetric Diels-Alder reactions of 2H-phospholes with the dienophile (5/ )-(l-menthyloxy)-(5//)-furanone allowed the generation of multiple stereogenic centers in 1-phosphanorbomadienes 256. The cycloaddition products were converted to their air-stable sulfur derivatives, which were isolated and the endo- and exo-isomers were separated by column chromatography (Scheme 81). In this case, the principle of face differentiation for a P=C bond is a synthetic tool for highly selective and efficient synthesis of P-chiral phosphanes from readily available starting materials. The observed selectivity was explained by the transition states of the two main isomers exo-A and endo-B (Scheme 82). The two molecules approach mainly in an endo fashion, which in consequence leads to the main diastereomer. Because of the favorable secondary orbital interactions between the C=0 functionality and the diene system, the endo product is kinetically favored, as is known for such systems with normal electron demand [170]. 

The enantioselectivity of [Cu(S,S)-tBu-box](OTf)2 (13b)Claisen rearrangement is explained as follows (Fig. 2.4). The alkoxycarbonyl and ether oxygens coordinate in a bidentate fashion to the Cu (box) complexes. The square planer geometry around the copper(II) cation has been proposed and a chair-hke transition-state model is suggested. The aUyUc ether moiety should approach the vinyl ether moiety from the opposite direction of the t-Bu substituents on the box-hgand. The Cu"(box) catalyst differentiates between two enantiomeric chair-hke transition state by selective coordination of enantiotopic lone pairs on oxygen to form (S,S,pro-S)-14a. 

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