Ligand reorganization

Compound 233 was prepared from 144 and 234, and its intramolecular ligand reorganization was investigated by variable temperature NMR spectroscopy (80JA5073). 

The and F n.m.r. spectra of the phosphorane (47) indicate rapid positional exchange of the ligands attached to phosphorus. This has been quoted as evidence for a (2 + 3)-turnstile process of ligand reorganization, the normal Berry (1 + 4)-pseudorotation being held to be impossible in (47) because of increased strain. Details of this work are awaited with interest. 

Perola, E., Charifson, P. S. Conformational analysis of drug-like molecules bound to proteins an extensive study of ligand reorganization upon binding. /. Med. Chem. 2004, 47, 2499-2510. 

In addition to the foregoing explanation of retention of configuration, two possibilities may be taken into account. The first is that apical entry is followed by basal departure, and vice versa. Second, the sulfurane intermediate formed may have the structure of a square basal pyramid. In both cases nucleophilic substitution reactions may occur with retention of configuration without ligand reorganization. It appears that the apicophilicity of substituents in sulfurane species... 

Finally, although sulfurane intermediates have been proposed in many cases, they have not been isolated from nucleophilic substitution reactions. However, the concept of an addition-elimination mechanism is supported by the independent syntheses of a number of stable sulfuranes these compounds have a trigonal-bipyramidal structure and in some cases the ligand reorganization was found to occur very easily (189-191). 

Whereas a coordination geometry with the ligand in a K -coordination mode is preferred for the M(I) species, a square-pyramidal structure is preferred for the M(ll) analogues (M = Rh, Ir). As a result, reorganization takes place upon oxidation. For small ligands, reorganization is fast on the cyclic voltammogram... 

Structure. Bonding and Ligand Reorganization - Several pentaaryl bismuth compounds (3) have been synthesised by the conventional reaction of (1) with arylithium (2) and three were characterised by single-crystal > ray structure analysis. In contrast to pentaphenylphosphorus (and AsPhs) which are trigonal bipyramids, the bismuth compounds exhibit almost ideal square pyramidal geometry. 

The alternative explanation of zwitterionic hexaco-ordinated intermediates (or transition states) to account for the faster pseudorotation rates with X=0, appears to be negated by studies of pseudorotational barriers in (16ab). In these molecules, no conformational transmission effect is possible but the presence of the tetrahydrofuran oxygen atoms would permit a zwitterionic T.S. to accelerate ligand reorganization. In fact the pseudorotational barriers for (16a) and (16b) at 55.2 and 71.9 kjmol respectively, are in excellent agreement with those found for (11a) and (11c) where no hexaco-ordinate species are possible along the isomerisation pathway. 

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