Compact transition

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

Historically, isotherms have been classified as favorable (concave downward) or unfavorable (concave upward). These terms refer to the spreading tendencies of transitions in fixed beds. A favorable isotherm gives a compact transition, whereas an unfavorable isotherm leads to a broad one. 

The interpretation and prediction of the relationship between the configuration of the newly formed chiral center and the configuration of the amine are usually based on steric differentiation of the two faces of the imine anion. Most imine anions that show high stereoselectivity incorporate a substituent which can hold the metal cation in a compact transition state by chelation. In the case of entry 2 in Table 1.3, for example, the observed enantioselectivity is rationalized on the basis of transition state L. 

Boron Enolates. Another important version of the aldol reaction involves the use of boron enolates. A cyclic transition state is believed to be involved, and, in general, the stereoselectivity is higher than for lithium enolates. The O—B bond distances are shorter than the O—Li bond in the lithium enolates, and this leads to a more compact transition state, which magnifies the steric interactions that control stereoselectivity. 

The less compact transition aluminas (y-type) are highly porous, more reactive and do not occur in nature. They are prepared by the heat treatment of Al(OH)3 or AlOOH at intermediate temperatures and undergo an irreversible change to a-Al203 at high temperature. Their BET areas are typically 300-400 m2 g 1 and they are widely used as catalysts and catalyst supports. 

Due to this compact transition state, the introduction of substituents on the carbon skeleton leads to only one diastereomer after intramolecular carbometallation [60]. For example, metallation of the propargyl ether 64 with ec-BuLi followed by transmetallation with ZnBr, resulted in the formation of a cyclic propargyl ether as a single diastereomer in which the remaining metal had been trapped by different electrophiles [60] (Scheme 7-55). According to this approach, a quaternary and a tertiary stereogenic center are linked to form a cyclopentane ring with total diastereoselection. 

Dumas et al. noted the good yields and syn diastereoselectivities obtained in a high-pressure aldol reaction of bis-silyl ketene acetals 154 with benzaldehyde (155) (Scheme 7.39). The syn aldol 156 was obtained with a diastereoselectivity that was significantly correlated with the steric bulkiness of the R-substituent in the acetals 154. The preference for syn bis-silyl aldols 156 has been attributed to the reaction pathway that involves compact transition states in which steric interactions between the R substituent of 154 and the phenyl group of benzaldehyde are minimized. The authors also studied the condensation of unsaturated bis-silyl ketene acetal as a model for the synthesis of retinoid compounds. ... 

High pressure stopped-flow experiments upon the reaction of hexaaquairon(III) and promazine (R = CH3CH2CH2N(CH3)2, see reaction given below) yielded AV values of -6.3 and -12.5 cm mof for the forward and reverse reactions, respectively. [139] The derived reaction volume of 6.2 cm mof has been confirmed by calculation from the pressure dependence of the equilibrium constant obtained from the variation of the UV/Visible spectra as a function of pressure. The volume profile (see Figure 9) displays the fact of a compact transition state, and the positive reaction volume was suggested to arise from charge dilution in proceeding from the reactants to Fe(aq) and the promazine cation radical. 

The second case is more complicated. Again, it can be said that the more compact transition structure is preferred under high pressure. In cycloadditions the endo transition structure (Scheme 8.8) usually has a smaller volume than the exo transition stmeture and is therefore stabilized under high pressure. 

Key words Polymerized membrane - poor solvent - flat-compact transition - critical state - folding... 

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