Chelating substituents

Potential C-hydroxy compounds usually exist as the 0x0 tautomers, unless the hydroxy tautomer is appreciably stabilized by electron withdrawing or chelating substituents. The... 

A synthetic strategy which ensures retention of the monomeric form of SnR2 even in the crystalline state is to use functionalized R groups which contain a chelating substituent, e.g. by replacing the H atom in -CH(SiMe3)2 with a 2-pyridyl group. 

Azoles with Potentially Tautomeric Chelated Substituents. 292... 

Hydroxyalkylation of these allyltitanium derivatives by carbonyl compounds proceeds at the more highly substituted terminus. Exceptions are observed for those hetero-substitutcd derivatives in which the chelating substituent Y ties the cation to the 1-position. 

If the amine carries a chelating substituent, as for 2-methoxyethylamine, the rate of deprotonation is accelerated. For any specific imine, ring substituents also influence the imine conformation and rate of deprotonation. These relationships reflect steric, stereoelectronic, and chelation influences, and sorting out each contribution can be challenging. 

Chelation Control. The stereoselectivity of reduction of carbonyl groups can be controlled by chelation when there is a nearby donor substituent. In the presence of such a group, specific complexation among the substituent, the carbonyl oxygen, and the Lewis acid can establish a preferred conformation for the reactant. Usually hydride is then delivered from the less sterically hindered face of the chelate so the hydroxy group is anti to the chelating substituent. 

Association of Sulfenyl, Selenenyl and Tellurenyl Halides with Chelating Substituents... 

When the reaction was applied to a chiral cyclic ketal instead, very low selectiv-ities were obtained. Introduction of chelating substituents into the ketal made improvement possible, though (Scheme 8.14) [23, 26]. 

Internal complexation has to be considered additionally as chelating substituents may alter the situation completely. For example, in the lithiated allyl A,A-diisopropylcarbamate 3 possessing a delocalized structure, it is clearly a 1-lithiated hetero-substituted allyl compound despite the fact that oxygen is a donor and the silyl group can be regarded more as an acceptor13. 

In an approach to direct the alkylation of (l-silyl-2-propenyl)lithium into the a-position by chelating substituents on the silicon atom, chiral pyrrolidinyl-substituted allylsilane 1 was deprotonated and alkylated with iodomethane (94%)51. Regiocontrol was moderate, the a-product 2 dominating (85%). 

Pyridazine resembles pyrazole and the triazoles with regard to bonding, as it has the ability to bridge between two metal ions. Unsubstituted pyridazine, however, yields few stable compounds, and only when chelating substituents are present at positions 3 and 6 can stable compounds (10) be obtained with transition metal ions.60... 

A series of catechol ligands 949 951 functionalized with N-donor chelating substituents was developed [185,186] ... 

The stereochemical outcome was rationalized by a Zimmerman-Traxler type transition state 45.64 Assuming the titanium enolate of 42 has a Z-geometry and forms a 7-membered metallacycle with a chairlike conformation, a model can be proposed where a second titanium metal coordinates to the indanol and aldehyde oxygens in a 6-membered chairlike conformation. The involvement of two titanium centers was supported by the fact that aldehydes that were not precomplexed with titanium tetrachloride did not react (Scheme 24.7).63 Ghosh and co-workers further hypothesized that a chelating substituent on the aldehyde would alter the transition state 46 and consequently the stereochemical outcome of the condensation, leading to. vyn-aldol products 47.64 Indeed, reaction of the titanium enolate of 42 with bidentate oxyaldehydes proceeded with excellent. s v -diastereo-selectivity (Scheme 24.8).65... 

The diastereoselective allylindium addition to norbornyl cr-diketones leads to acyloins. The diastereoselection in the case of monoallylation products greatly depends on the nature of 5-endo substituents. Non-chelating groups direct the addition from the sterically less congested exo-face, diagonal to the substituent, whereas chelating substituents, such as an alkoxy or acetoxy unit, induce a complete reversal in the selectivity (Equation (16)).169... 

Hydroxy derivatives. 2-Hydroxy derivatives usually exist as the oxo tautomers, unless the hydroxy tautomer is appreciably stabilized by electron-withdrawing or chelating substituents. The tendency for enolic hydroxy compounds to revert to the oxo form can be understood by reference to simple aliphatic ketones where the keto-enol equilibrium constants are of the order of 108. In the five-membered heterocycles under consideration, this tendency will be in opposition to the loss of aromatic resonance energy that increases in the order furan << thiophene < pyrrole. For the 2-hydroxy compounds 217 some extra stabilization of the oxo tautomers 218 and 219 is derived from the resonance energy of the X-C=0 group, which by analogy with open-chain compounds should increase in the sequence thiolester, ester << amide. 

Salts of mercury(II) and thallium(HI), such as mercury(II) acetate, Hg(OAc)2, and thallium trifluoroacetate, Tl(OCOCF3)3, are reactive electrophilic metallating species which attack benzene directly. Electron-donating groups in the aromatic ring accelerate the reaction in the conventional manner and direct attack to the ortho and para positions. In addition, metal-chelating substituents, such as amide, promote the reaction and direct attack to the 2-position in a manner similar to that seen in the directed orthometallation reaction. 

Keywords Aryllithiuiti Compounds / Chelating Substituents / Highly Coordinated Silicon Compounds / Sterically Hindered Silicon Centers... 

The ability of the 1,2-diazole ring unit of pyrazolate to bridge two metal ions is well documented.99 Meyer reported some pyrazolyl-based ligands possessing additional chelating substituents in the 3- and 5-positions.100 101... 

C2-symmetric bis(oxazolinyl)pyridine (pybox)-Cu (II) complex 27 has been shown to catalyze highly enantioselective Mukaiyaraa aldol reactions between (benzyloxy)acetaldehyde and silyl ketene acetals by Evans and co-workers as exemplified in Scheme 1-9 [38]. Here, the requirement for a chelating substituent on the aldehyde partner is critical to catalyst selectivity, as a-(terl-butyldimethylsil-oxy)acetaldehyde gave lower enantioselectivity (56% ee). In addition, P-(benzyl-oxy)propionaldehyde provided the racemic product, indicating a strict requirement for a five-membered catalyst-substrate chelate. 

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