Facial geometry

Bis(pyrazolylethyl)ether derivatives (106) have been coordinated to zinc providing an N20 donor set. The structural data shows that the ligand coordinates in a meridional rather than facial geometry limiting the application for the modeling of N20 zinc enzyme sites, (derivatives R = i-Pr or Me).161... 

Table 4 lists properties of the neutral homoleptic complexes with facial geometry, Ir(CAN)3, whose ligands are illustrated in Fig. 6 [35]. 

Tp is often compared with Cp (cyclopentadienyl) and Cp (the pentamethyl derivative) due to the same charge and number of electrons donated (anionic 6e donors) as well as the facial geometry typically adopted. Accordingly, in the case of... 

In order to smdy the effect of perturbation arising from spiro-conjugation on the chemical reactivities, in particular the facial selectivities, sterically unbiased dienes (96 and 97) based on fluorenes in spiro geometry have been synthesized [165]. These dienes react as Diels-Alder dienes with several dienophiles (maleic anhydride (MA), A-phenylmaleimide (PMI), A-phenyl-l,3,5-triazoUne-2,4-dione (PTD) and iV-methyl-l,3,5-triazoline-2,4-dione (MTD)). 

The cycloaddition of ketone 54 could be effected in a sealed glass tube in a modified microwave oven to afford the tricyclic system stereoselectively. This major adduct arose via the preferred transition state, in which the nonbonded interactions were minimized, because of the alignment of the dienophile beneath the triene unit furthest from the MOM substituent. This pattern of n-facial selectivity implies that, with the natural C2 stereoselectivity, the preferred geometry should provide the relative stereochemistry required for taxol itself. 

In entries 10-13 (Table 21.8) of trisubstituted alkenes, very high diastereo-selectivity is realized by the use of a cationic rhodium catalyst under high hydrogen pressure, and the 1,3-syn- or 1,3-anti-configuration naturally corresponds to the ( )- or (Z)-geometry of the trisubstituted olefin unit [48, 49]. The facial selectivity is rationalized to be controlled by the A(l,3)-allylic strain at the intermediary complex stage (Scheme 21.2) [48]. 

When a nucleophilic reagent, Nu X+ (or Nu—X), is reacted with a ketone, com-plexation of oxygen by X+ may precede attack at carbon. Geometric changes associated with such complexation have been calculated for a series of 4-substituted cyclohexanones. The results allow the facial selectivity of the subsequent nucleophilic attack to be predicted, and without the need to calculate the transition-state geometry. 

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