Absolute asymmetric synthesis in asymmetric autocatalysis and kinetic isotope effect studies of organometallic reactions /
Absolute asymmetric synthesis is the formation of optically active materials from achiral starting materials in the absence of optically active reagents or catalysts. This would appear to break a widely assumed rule of chemistry - that optically active products can only arise in the presence of opt...
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| Format: | Thesis Book |
| Language: | English |
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[Place of publication not identified] :
[publisher not identified] ;
2004.
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| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=790270741&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | Absolute asymmetric synthesis is the formation of optically active materials from achiral starting materials in the absence of optically active reagents or catalysts. This would appear to break a widely assumed rule of chemistry - that optically active products can only arise in the presence of optically active materials. Indeed, absolute asymmetric synthesis has never been observed in a closed homogeneous system. We have been attempting absolute asymmetric synthesis by the repeated asymmetric amplification of the small enantiomeric excess generated by random chance in the initial racemate. By an adaptation of the asymmetric autocatalysis reactions of Soai, it was found that small enantiomeric excesses may be amplified to an arbitrary extent. When this process was applied to reactions in the absence of discrete optically active additives, all ultimately afforded substantial optical activity in the product. However, reactions conducted in either toluene or benzene afforded a decidedly nonrandom distribution of enantiomers. Observations strongly suggest that the enantiomeric outcome of the reactions has been dominated by trace optically active impurities. Interestingly, distribution of optical activity of the product exhibits the so called stochastic behavior when reactions were conducted in diethyl ether. It was found that only a few molecules (approximately 60,000) of chiral products were enough to sufficiently control the enantiomeric outcome of the reaction. The mechanisms of two organometallic reactions were studied by a combination of experimental studies and theoretical calculations. The epoxidation of olefin catalyzed by molybdenum(VI)-oxo-diperoxo complex exhibits two small ¹³C isotope effects (1.008 and 1.011) at the two olefinic carbons. This supports the concerted mechanism. The calculations predict isotope effects that match experimental values and suggest a considerably low energetic path for the concerted transfer of the oxygen from the molybdenum complex to the olefin. The hydroamination of styrene with aniline catalyzed by phosphine-ligated palladium triflates exhibits a substantial ¹³C isotope effect (1.030) at the benzylic carbon and a small isotope effect (1.006) at the terminal olefinic carbon. This supports rate-determining nucleophilic attack of amine on a η³-phenethyl palladium complex, and it is not consistent with rate-limiting or selectivity-determining hydropalladation of the alkene. Deuterium exchange observations and predicted isotope effects support this mechanism. |
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| Item Description: | Vita. "Major Subject: Chemistry". |
| Physical Description: | xii, 194 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilm Inc. |
| Bibliography: | Includes bibliographical references (leaves 119-126). |