Other Assisted Ligand Substitutions

Many other reagents and reaction conditions accelerate ligand substitution, but are less defined than the examples described above. A variety of nucleophilic reagents, such as phosphine oxides, accelerate ligand dissociation from stable octahedral carbonyl complexes, but the mechanism that accounts for this acceleration is not well imderstood.  

The site of initial attack by the phosphine oxide is although attack at the 

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There is a review of selected topics in organometallic ion chemistry and evidence for the hollow cage structure of met-cars is collected together with a discussion of cluster-assisted O-bQnd activation.Metal containing ions, [ML]+ (M = Cr, Mn, La, Pr, Yb, Nd and others L = Ph) react with crown ethers in the gas phase by adduct formation or ligand substitution. s... 

The effects predicted are qualitative at best. There are other factors that must be taken into account when predicting how various characteristics of the metal and ligand affect substitution reactions. For example, increasing the size of the metal ion is predicted to assist the formation of the transition state in SN1,... 

The kinetics of aquation of a number of azidochromium(III) complexes have been investigated.303,655 Compared with other acidochromium(III) complexes, the chromium-azide bonds in these species seem remarkably stable to thermal substitution. Hence in the base hydrolysis of [CrN3(NH3)s]2+ a pathway involving initial loss of NH3 concurs with the usual base hydrolysis pathway involving loss of Nj. The aquation of azidochromium(III) complexes is H+-assisted with protonation of the azido ligand accounting for the enhanced reactivity. 

Pyridine and related aromatic (quinoline, quinazoline) P,N derivatives (11, 12) have been created for Rh-catalyzed hydroboration-oxidation [44] or -amination [45]. Other pyridine-related auxiliaries have been synthesized for Pd-assisted allylic alkylation [46] in test conditions furnishing the substitution product in up to 93 % ee. The QUIPHOS ligand 13 has been tested in Pd-assisted allylic amination (up to 94 % ee) [47], allylic alkylation of -ketoesters (up to 95 % ee) [48], and Cu-catalyzed Diels-Alder reaction between an acryloyl derivative and cyclopentadiene [49]. 

This section covers cyclizations to the pyrrole nucleus catalyzed by other metals (Ti, Mn, Ru, Pd, Pt, Zn, In). Dembinski and co-workers used zinc(II) chloride as ligand-free catalyst for the microwave-assisted cyclization of homopropargyl azides 26 to afford substituted pyrroles 27 (Scheme 8) [62]. A similar methodology for the synthesis of 2,4,5-trisubstituted pyrroles was described by Driver et al. employing substituted 1-azidobuta-l,3-dienes in a cyclization reaction using catalytic amounts of zinc(ll) iodide [63]. A three-component zinc-catalyzed one-pot cyclization of aromatic and aliphatic propargylic acetates, silyl enol ethers, and primary amines to substituted pyrroles has been described by Zhan et al. The reaction sequence includes propargylation of the silyl enol ether, amination, 5-exo-(7ig-cyclization, and isomerization [64]. Hiroya and co-workers have shown... 

Of note, substitution of unsaturated NHC ligands by their saturated analogues, such as 19, doubled the activity but left the branched/linear ratios unaffected." An increase in activity was also observed upon addition of one equivalent of triethyl-amine with no appreciable loss of selectivity. This increase in activity might be attributed to a base-assisted generation of the active catalyst, which is thought to be a rhodium hydride species. Many other rhodium complexes of the general formula [(NHQRh(COD)X] 20 were tested and usually displayed reasonable activity." " ... 

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