Ligands nucleophilic

A very few coordination complexes of tetramethylene sulphone [(CH2)4S02] with transition metal ions have been prepared, and the coordinative ability of sulpholane is generally regarded as quite weak224,225. Sulpholane metal complexes should therefore serve as excellent precursors of the coordination compounds containing other weakly nucleophilic ligands. 

Direct evidence for the presence of the coordinated complex has been sought from a study of the interaction of various nucleophilic ligands with zirconium benzyl. 

The first attempt to effect the asymmetric cw-dihydroxylation of olefins with osmium tetroxide was reported in 1980 by Hentges and Sharpless.54 Taking into consideration that the rate of osmium(VI) ester formation can be accelerated by nucleophilic ligands such as pyridine, Hentges and Sharpless used 1-2-(2-menthyl)-pyridine as a chiral ligand. However, the diols obtained in this way were of low enantiomeric excess (3-18% ee only). The low ee was attributed to the instability of the osmium tetroxide chiral pyridine complexes. As a result, the naturally occurring cinchona alkaloids quinine and quinidine were derived to dihydroquinine and dihydroquinidine acetate and were selected as chiral... 

In summary, the reaction of osmium tetroxide with alkenes is a reliable and selective transformation. Chiral diamines and cinchona alkakoid are most frequently used as chiral auxiliaries. Complexes derived from osmium tetroxide with diamines do not undergo catalytic turnover, whereas dihydroquinidine and dihydroquinine derivatives have been found to be very effective catalysts for the oxidation of a variety of alkenes. OsC>4 can be used catalytically in the presence of a secondary oxygen donor (e.g., H202, TBHP, A -methylmorpholine-/V-oxide, sodium periodate, 02, sodium hypochlorite, potassium ferricyanide). Furthermore, a remarkable rate enhancement occurs with the addition of a nucleophilic ligand such as pyridine or a tertiary amine. Table 4-11 lists the preferred chiral ligands for the dihydroxylation of a variety of olefins.61 Table 4-12 lists the recommended ligands for each class of olefins. 

The addition of Grignards and organolithium reagents proceeds by attack at the metal center in ir-allylpalladium complexes. The regiochemical selectivity exhibited by these hard carbon nucleophiles with ir-allyl complexes substituted at the termini with alkyl or aryl groups is comparable to the soft carbon nucleophiles (ligand attack) in most cases, with addition occurring predominantly at the less substituted terminus (equations 248 and 249).1591387... 

Possible unidentate reagents are atomic cations and other simple electrophiles as well as atomic anions and other simple nucleophiles. They can react with the cluster with or without change in the electron count as well as with or without change in cluster core shape. Not considered here are nucleophilic ligand substitutions or reactions in which the number of metal atoms in the cluster changes (cf. Section V). 

This is an important mechanism, and we have seen the consequences of attack by an intramolecular nucleophile (ligand) in earlier chapters. A particularly interesting example is seen in the intramolecular Michael addition of a co-ordinated amide at a cobalt(m) centre to yield an amino acid derivative (Fig. 5-44). 

The polymerisation of norbornene occurs in the presence of cationic Pd(II) complexes with weakly nucleophilic ligands via the cis insertion involving predominantly the exo face of the monomer (the diastereotopic endo face of the monomer is much less reactive) [10] ... 

The much higher stability of onium ions, compart to that of the mqority of carbocations, reflrets the fact that, formally, onium ions can be considared to be equivalent to carbocatkrns bonded to (and therefore stabilized by) nucleophilic ligands such as amines, ethers or sulfides. For instance, trimethyloxonium ion lb can be treated as a methylium cation CH la attached to methyl ether and using... 

The other onium salts (sulfonium or ammonium salts) are less often applied due to their lower reactivity. They can neither initiate the polymerization of cyclic ethers or acetals nor be displaced by less nucleophilic ligands (cyclic sulfides and amines respectively). 

The electronegativity of Sn(II) and Sn(IV) is shown in Table 1 [3,4]. Sn(II) is more electropositive and hence cationic than Sn(IV), and is expected to coordinate with nucleophilic ligands. The covalent and ionic radii of Sn(II) are, on the other hand, larger than those of Sn(IV) (Tables 1 and 2) [3-11]. This is because electronic repulsion of the unpaired electrons of Sn(II) weakens the <5-bond because Sn(II) uses the p-orbital for bonding. 

Another impetus to mechanistic studies arose from the recognition that compounds of these d ions were those on the energy borderline between stable 18-electron and 16-electron molecules (1) and that the reactions involving transitions between these states are those encountered in catalytic cycles based on these compounds. Nucleophilic ligand substitution, involving association of an entering nucleophile with a square-planar compound, is just one example of the easy 16- 18- 16... 

Despite the richness of this field of study, however, a number of anomalies and some major uncertainties remain. One curious feature, which is probably no more than an historical accident, is that though most studies of nucleophilic ligand substitution have been carried out on complexes of platinum(II) and palladium(II) and few on complexes of rhodium(I) and iridium(I), the reverse distribution is apparent for studies of oxidative additions. The scope for rectifying this imbalance is vast. On the other hand, a fundamental and persistent uncertainty in this field of study concerns the very nature of square-planar compounds in solution. We address this problem in some detail. 

A main objective of this work is to develop the relationship between the many reaction pathways leading to ligand substitution at square-planar molecules. Concentrating on more recent results to illustrate the processes under discussion, we examine in detail the evidence for operation of the less common and sometimes controversial routes such as dissociative ligand exchange (6). It cannot be stressed too much, however, that the field is still dominated by associative reactions, so to maintain a balance, as well as to provide the now necessary comparative evidence, we also cover the essential features of nucleophilic ligand replacements. 

The first consequence of relating these structures to possible intermediates for nucleophilic ligand replacements at square-planar complexes is that the simple reaction profile of Fig. 2 is inadequate. A... 

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