Non chelation

Noyori "Open" Transition State for non-Chelation Control Aldols... 

Non-chelation aldol reactions proceed via an "open" transition state to give syn aldols regardless of enolate geometry. 

Enantiomerically pure /J-keto sulfoxides are prepared easily via condensation of a-lithiosulfinyl carbanions with esters. Reduction of the carbonyl group in such /J-keto sulfoxides leads to diastereomeric /J-hydroxysulfoxides. The major recent advance in this area has been the discovery that non-chelating hydride donors (e.g., diisobutylaluminium hydride, DIBAL) tend to form one /J-hydroxysulfoxide while chelating hydride donors [e.g., lithium aluminium hydride (LAH), or DIBAL in the presence of divalent zinc ions] tend to produce the diastereomeric /J-hydroxysulfoxide. The level of diastereoselectivity is often very high. For example, enantiomerically pure /J-ketosulfoxide 32 is reduced by LAH in diethyl ether to give mainly the (RR)-diastereomer whereas DIBAL produces exclusively the (.S R)-diastereomer (equation 30)53-69. A second example is shown in... 

Of the various results, these appear to the reviewer to be the most meticulously obtained and they do not fit the other two rate laws suggested. At 25 °C n =. 0M perchlorate), = 5.10x10 l mole. sec" and = 4.78x10 l .mole" . sec . The corresponding reaction intermediates are a neutral chelate mono-oxalato complex and a (non-chelate) bis-oxalato complex of Cr(VI). 

Bipyridine (44-BP) is a bifunctional, non-chelating molecule, which consists of two planar pyridyl rings connected by a C-C bond in 4 and 4 positions. It has been extensively employed as a bridging ligand in coordination chemistry [291]. 

Ion-association complexes may be classified into three types non-chelated complexes chelated complexes oxonium systems. 

Non-chelated ion-association systems tetraphenylarsonium and tetraalkyl-ammonium salts... 

The use of oxonium and other non-chelated systems can be advantageous where relatively high concentrations of metals are to be extracted as solubility in the organic phase is not likely to be a limiting factor. Metal chelates, on the other hand, have a more limited solubility and are more suited to trace-level work. 

Comparisons of catalysis of these C2-tethered P,C chelate complexes with the non-chelate analogues in CD2C12 solvent showed that the former were somewhat slower catalysts than the latter, but the compelling advantage of the chelate complexes is that they had much longer lifetimes. 

It is also noteworthy that complexes containing ligands such as TAP, HAT, bpz (2,2 -bipyrazine) or bipym (2,2 -bipyrimidine) (Fig. 2), have free non-chelated nitrogen atoms. It has been shown that all these compounds are all more basic in the excited state than the ground state [75,93,94], so that the excited states are already protonated at pH 5-6 on the non-chelated nitrogen atoms. 

Darzens reaction of (-)-8-phenylmethyl a-chloroacetate (and a-bromoacetate) with various ketones (Scheme 2) yields ctT-glycidic esters (28) with high geometric and diastereofacial selectivity which can be explained in terms of both open-chain or non-chelated antiperiplanar transition state models for the initial aldol-type reaction the ketone approaches the Si-f ce of the Z-enolate such that the phenyl ring of the chiral auxiliary and the enolate portion are face-to-face. Aza-Darzens condensation reaction of iV-benzylideneaniline has also been studied. Kinetically controlled base-promoted lithiation of 3,3-diphenylpropiomesitylene results in Z enolate ratios in the range 94 6 (lithium diisopropylamide) to 50 50 (BuLi), depending on the choice of solvent and temperature. ... 

SCHEME 23. Non-chelate controlled addition of l-bromo-l-hthioalkene 164 to O-protected lac-taldehydes 162. Introduction of an umpoled formyl d synthon CHO... 

A stereochemical behavior similar to that of the 1-bromo-l-lithio aUcene 164 with regard to chiral aldehydes is shown by the hthiated methoxyallene 183. When added to iV,iV-dibenzylated a-aminoaldehydes 188, it reacts with non-chelate control so that awh -carbinols 189 are obtained predominantly. Diastereomeric ratios of 189 190 range from 80 20 to 95 5. As outlined above, the hydroxyalkylated allenes 189/190 can be converted into furanones 191/192 upon treatment with potassium f-butoxide and subsequent acid hydrolysis" . When, on the other hand, the adducts of 183 to the aldehydes 193 are submitted to an ozonolysis, A-protected a-hydroxy-/3-amino esters 194/195 result (Scheme 25)"" . 

For the compounds M(o-nitrophenolate) (l,10-phenanthroline)2 (10), however, with M = K, Rb, Cs, the isomorphous crystals contain dimeric molecules in which the metal is 7-coordinated, being chelated by o-nitrophenolate and by two molecules of base, and having as the seventh contact the non-chelating nitro oxygen from the other molecule of the dimer no donor atom is shared by cations. There is no such seventh contact for M = Na, the metal being in a discrete 6-coordinated monomer (11). 

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