Chiral oxidation

G. Bellucci, C. Chiappe, G. Ingrosso, Kinetics and Stereochemistry of the Microsomal Epoxide Hydrolase-Catalyzed Hydrolysis of cw-Stilbene Oxides , Chirality 1994, 6, 577 - 582. 

Using a different dimerization method, namely phenolic oxidation, chiral substrates react in a more stereoselective manner than under reductive conditions. The choice of oxidizing reagent may drastically affect the stereochemical outcome of the reaction. Thus, when potassium hexacyanoferrate(III) is used (17 )-l,2,3,4-tetrahydro-6-methoxy-l,2-dimethyl-7-isoquino-linol couples to give a mixture of atropisomers 3 in 38 % yield and with a d.r. (M)I(P) of 45 553,4. Only one single atropisomer, namely (A/)-3, is formed, in a 66% yield by anodic oxidation, which is attributed to electrode surface effects3. 

Chiral homoallylic alcohols. The borane 1 reacts with aldehydes in ether at - 78° to furnish, after oxidation, chiral secondary homoallylic alcohols (2) with enantiomeric excesses of 83-96%. The addition occurs in all six cases examined in the same stereochemical sense. 

Asymmetric Hydroboration. For reaction with a prochiral alkene, (/ ,/ )-2,5-dimethyl-B-methoxyborolane is liberated from (1) and a standard solution of the corresponding lithium dihydri-doborate in ether is prepared (eq 5). Hydroboration is effected by addition of lodomethane to the solution of dihydridoborate and alkene (eq 6). After oxidation, chiral secondary alcohols of high enantiomeric purity and predictable configuration are obtained from cis, trans, and (risubstituted alkenes. As is the case with other known asymmetric hydroborating agents, 2-methyl-1-alkenes react with low asymmetric induction. 

One feasible method for the exploration of chiral open-framework compounds is the use of chiral chemical units as primary building blocks by coordinating with metal or other assembly methods to form 2-D layer or 3-D open-framework structures with optical activity. A notable example is the enantiomerically pure zinc phosphonate based on a mixed phosphonic acid-phosphine oxide chiral building block reported by Bujoli and coworkers in 2001.[91] The reaction procedures are shown as follows. 

F. Fredoueil, M. Evain, D. Massiot, M. Bujoli-Doeuff, and B. Bujoli, Enantiomerically Pure Zinc Phosphonates Based on Mixed Phosphonic Acid-phosphine Oxide Chiral Building Blocks. J. Mater. Chem., 2001, 11, 1106-1110. 

Chiral . For the purist, this is an especially irritating misuse. A worker who has been using racemic stilbene oxide is about to move onto using enantiomerically pure stilbene oxide. He describes the enantiomerically pure stilbene oxide as chiral stilbene oxide or more probably, in the abreviated colloquial language of the lab as my chiral compound . Racemic stilbene oxide is no less chiral than optically pure stilbene oxide and so chiral stilbene oxide says no more than stilbene oxide . Chiral is sometimes used in this way even by experienced and distinguished chemists ... 

The third class of systems to be discussed here appeared in the last years, reflecting the growing need in simple and non-toxic systems for sulflde oxidation. Chiral [Fe "(salen)Cl] complexes were synthesized and used as catalysts for asymmetric oxidation of several alkylaryl sulfides with PhlO. Surprisingly, the reactive intermediate in this system was found different from those detected in Mn-salen and Cr-salcn based catalytic systems. Namely, the intermediate was shown to be a iodosylbcnzcnc-irondata obtained are analyzed to understand some catalytic properties of these systems. 

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