Download PDF by D. Bethell (ed.): Advances in Physical Organic Chemistry, Vol. 23

By D. Bethell (ed.)

ISBN-10: 0120335239

ISBN-13: 9780120335237

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Extra resources for Advances in Physical Organic Chemistry, Vol. 23

Sample text

Using a much improved preparation of ethyl-(R)-( )-a-bromopropionate, the reaction with trans-[IrCl(CO)(PMe,),] yielded a product in which complete racemisation occurred at the carbon centre. These reactions were all repeated with a series of complexes of formula, trans-[IrCI(CO)L,] (L = PMe,Ph, PPh,Me or AsMe,Ph) and in all cases complete loss of stereochemical integrity at the chiral centre was observed. One possible means of racemisation of the alkyl complex containing an a-hydrogen, is rapid proton exchange as shown in (18).

Irrespective of the leaving group, the reaction rates of alkyl compounds with various nucleophiles are perturbed by similar amounts with increasing congestion at the carbon centre. Substitution of one or two hydrogen atoms for methyl groups at the reaction centre retards the rate of the reaction with vitamin B,,, whereas increasing the carbon chain length by similar amounts has little effect (Schrauzer and Deutsch, 1969). If however the hydrogen atoms are replaced by a group which can delocalise the incipient electron density on the metal, thus increasing the stability of the transition state, the reaction rate is increased.

More recently, studies of the reactions of (R)-2-C8H,,X (X = C1, Br or OTs) with Me,SnLi have shown that the stereoselective inversion of configuration decreases along the series OTs > CI > Br and, furthermore, the temperature and mode of addition affects the selectivity (San Filippo Jr and Silbermann, 1981). There has also been criticism of earlier work in which trapping agents were used to detect radical reactions; it has been claimed that the trapping agents perturb the reaction and give rise to radical mechanisms.

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Advances in Physical Organic Chemistry, Vol. 23 by D. Bethell (ed.)

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