Metallation-directing group

In his review of 1990,104 Snieckus divided the typical ortholithiation-directing groups into classes, according to the ease with which they may be lithiated and the practicalities of their application in synthesis. It is clear that in the meantime certain metallation-directing groups have remained little more than curiosities, while others have become widely used. In this section, we shall survey metallation-directing groups in broad assemblies which share certain features. 

MDG, metallation-directing group EWG, electron-withdrawing group. 

A more recent method for the synthesis of isatins is based upon the directed ortho-metalation (DoM) of A-pivaloyl- and 7V-(/-butoxycarbonyl (-anilines. The corresponding dianions are treated with diethyl oxalate and the isatins are obtained after deprotection and cyclisation of the intermediate a-ketoesters. This method has the advantage of being regioselective for the synthesis of 4-substituted isatins from me to-substituted anilines where the substituent is a metalation directing group (e.g. OMe) 49 (Scheme 12). 

As is apparent from Tab. 3.8, an ortho methoxy group acts as a metal-directing group. Thus selective exchange occurs at the ortho position of a methoxy group. More than two bromines are necessary to complete the reaction. 

Keywords 8-Acylquinoline 8-Quinolinyl ketone All-carbon quaternary centers Bond insertion Carboacylation Carbon-carbon bond activation Chelation Cyclometalation Directed metalation Directing group Metallacycle Oxidative addition Rhodium catalyst... 

As indicated by the title, these processes are largely due to the work of Ziegler and coworkers. The type of polymerisation involved is sometimes referred to as co-ordination polymerisation since the mechanism involves a catalyst-monomer co-ordination complex or some other directing force that controls the way in which the monomer approaches the growing chain. The co-ordination catalysts are generally formed by the interaction of the alkyls of Groups I-III metals with halides and other derivatives of transition metals in Groups IV-VIII of the Periodic Table. In a typical process the catalyst is prepared from titanium tetrachloride and aluminium triethyl or some related material. 

The metals in Groups I and 2 of the periodic table react directly with hydrogen to form white, crystalline, stoichiometric hydrides of formula MX and MX2 respectively. The salt-like character of these compounds was recognized by G. N. Lewis in 1916 and he suggested that they contained the hydride ion H". Shortly thereafter... 

Ziegler-Natta catalyst for polymerization of alkenes. Considerable attention has been directed to double-bonded Fischer carbenes of Cr and W, the Schrock carbenes of Ta and Ti, and cyclic polyene ligands of Fe, Co, Cr, and U. Carbonyls of transition metals from groups 6 to 10 of the periodic table include both the monomeric compounds such as Cr(CO)g, Fe(CO)5, Ni(CO)4 and those with two metal groups such as Mn2(CO)io and Co2(CO)s, which is used industrially for hydroformylation. Although their source has not been identified, it has been shown that volatile compounds from landfills contain carbonyls of Mo and W (Feldmann and Cullen 1997). 

Presumably, the stereoselectivity in these cases is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released by hydrogenation, permitting coordination of the reactant and reaction with hydrogen. 

Structural studies on the nature of the organometallic intermediates following chelation-assisted CH additions of pincer iridium complexes have been carried out. The product was found to have an unexpected /ram-disposition of the hydride with respect to the metallated aromatic group. This is not the expected direct outcome of a chelation-assisted reaction since coordination of oxygen to iridium prior to C-H activation would be expected to afford the m-isorner (Equation (97)). 

The reason that directing effects are so efficient is related to the low PR3 to Ir ratio, which allows the directing group, the H2 and the C=C bond all to bind to the metal at the same time. This was suggested by the detection of 5 at low temperature in the reaction of Eq. (3) [9]. 

The hammerhead ribozyme and leadzyme belong to the second class of ribozymes. The short extra sequences of the ribozymes form the so-called catalytic loop which acts as the enzyme. There are two likely functions for metal ions in the mechanism of action of hammerhead ribozymes formation of metal hydroxide groups or direct coordination to phosphoryl oxygens. 

Metallation of simple iodopyridines faUs °, but iodopyridines containing additional halogens metallate and rearrange to place the lithium between the two directing groups, as shown in Scheme 103. ... 

Brummond [28] was the first to illustrate that cross-conjugated trienes could be obtained via an allenic Alder-ene reaction catalyzed by [Rh(CO)2Cl]2 (Eq. 14). Selective formation of the cross-conjugated triene was enabled by a selective cycloisomerization reaction occurring with the distal double bond of the aUene. Typically directing groups on the allene, differential substitution of the aUene termini, or intramolecularization are required for constitutional group selectivity. However, rhodium(f), unlike other transition metals examined, facihtated selective cyclization with the distal double bond of the allene in nearly aU the cases examined. 

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