Complexing groups, lead

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

Preliminary experiments prove that the substitution pattern of the /V-aryl moiety of imine 1 is crucial for the stereoselectivity of this reaction. The 2-substituent on the aryl group is of special importance. Namely, introduction of a methoxy group leads to a considerable decrease of enantioselectivity compared to the corresponding 2-H derivative, probably due to disfavor-able coordination with the organolithium complex. In contrast, alkyl groups show the reverse effect along with increased bulkiness (e.g., Tabic 1, entries l-3a) but 2,6-dimethyl substitution provides lower ee values. Furthermore, the 4-substituent of the TV-aryl moiety is of minor importance for the stereoselectivity of the reaction [the Ar-phcnyl and the /V-(4-methoxyphenyl) derivatives give similar results], whereas a substituent in the 3-position results in lower stereoselectivities (e.g., Et, Cl, OCHj)41. 

Although oxime complexes of Co share many of the physical properties of their imine relatives, the presense of an ionizable OH group attached to the coordinated N=C group leads to these ligands binding in their anionic forms. For this reason, the trivalent oxidation state is preferred in the Co coordination chemistry of oximes. 

With RCH2CN, however, there is a tendency for Grignard reagents to remove a proton from the CH2 group, leading to more complex reactions. Reduction with Li AlH4e (c/. p. 214) yields RCH2NH2, NH3 adds to (193), in the presence of NH4 Cle to yield salts of amidines, RC(NH2)=NH2 Cle. Acid-catalysed addition of alcohols,... 

Further reduction and protonation of the imido group leads to the formation of a Mo(I) ammine complex (8b) in a mechanism similar to the fluoro system in the classic Chatt cycle. The Mo(I) ammine complex 8b is first reduced to the corresponding Mo(0) complex 8c at which stage the ammine ligand is exchanged with... 

Reversible inhibition caused by materials that can function as ligand. Many compounds will bind to a metal this might be the solvent or impurities in the substrate or the solvent. It can also be a functional group in the substrate or the product, such as a nitrile. Too many ligands bound to the metal complex may lead to inhibition of one of the steps in the catalytic cycle. Likely candidates are formation of the substrate-catalyst complex or the oxidative addition of hydrogen. Removal of the contaminant will usually restore the catalytic activity. 

Yang described the Pd-induced cyclization of an aryl bromide onto a pendant cyano group leading to y-carbolines and related compounds [488], Genet studied the use of chiral palladium complexes in the construction of the C-ring of ergot alkaloids, a study that culminated in a synthesis of (-)-chanoclavine I [489-491]. For example, nitroindole 388 is cyclized to 389 in 57% yield and with enantioselectivities of up to 95% using Pd(OAc)2 and (S)-(-)-BINAP. 

Andrus et al. (109) proposed a stereochemical rationale for the observed selec-tivities in this reaction. The model is based on the Beckwith modification (97) of the Kochi mechanism, suggesting that the stereochemistry-determining event is the ally lie transposition from Cu(III) allyl benzoate intermediates 152 and 153, Fig. 13. Andrus suggests that the key Cu(III) intermediate assumes a distorted square-planar geometry. Steric interactions are decreased between the ligand substituent and the cyclohexenyl group in Complex 152 as opposed to Complex 153 leading to the observed absolute stereochemistry. 

To close on a more positive note, we observe that the computed geometry of the enzyme-dienolate complex in the vicinity of the 3-carbonyl is insensitive to the assumed dielectric constant and is in close agreement with X-ray structures of enzyme-inhibitor complexes (see Table 4.11 and Fig. 4.15). It is really quite remarkable that 4 billion years of random walk by mother nature and a few hours of optimization with a quantum chemistry program such as Gaussian (starting with the correct functional groups) lead to the same structure for the active... 

Compounds 81 and 83-86 were linked to P-CD, the ruthenium p-complexes prepared in situ and reductions carried out with the standard substrate 63 (Fig. 26). Comparing the results of ruthenium complexes with ligands 87-91 reveals that any substituent adjacent to the tosyl group leads to modest to good ee values but reduces the reactivity of the catalyst considerably, see 87-89 (Fig. 26), improvement of the 5delds between 33% and 53% was only achieved at elevated temperatures (50°C). In contrast, ruthenium complexes with ligands... 

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