A Comparative Study of the Relative Stability of Representative Chiral and Achiral Boronic Esters Employing Transesterification |
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Authors: | Chandra D Roy and Herbert C Brown |
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Institution: | (1) Laboratoire de glycochimie et de synth?se asym?trique, Ecole Polytechnique F?d?rale de Lausanne (EPFL), BCH, 1015 Lausanne-Dorigny, Switzerland;(2) Departamento de Qu?mica Org?nica, Universidad de Sevilla, 41071 Sevilla, Spain |
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Abstract: | A comparative study of the transesterification of five representative chiral and achiral boronic esters with various structurally
modified diols was undertaken to qualitatively understand the factors influencing the relative stability of these boronic
esters. Several factors such as chelation, conformation, steric bulk of the substituents, size of the heterocycle, and entropy
influence the relative rate of transesterification as well as the stability of the boronic esters. Amongst these boronic esters,
pinanediol phenylboronic ester was found to be the most stable boronic ester whereas DIPT boronic ester appeared to be thermodynamically the least stable one. The transesterification with sterically hindered diols
was observed to be relatively slow, but afforded thermodynamically more stable boronic esters. Boronic esters derived from
cis-cyclopentanediols and the bicyclo2.2.1]heptane-exo,exo-2,3-diols are relatively more stable. This study not only presents the qualitative picture of relative stability of various
boronic esters, but also provides helpful hints regarding the possible recovery of chiral auxiliaries. Many C
2-symmetric chiral auxiliaries, such as 2,3-butanediol, 2,4-pentanediol, DIPT, and cis-cyclohexane-1,2-diol, can be retrieved by simple transesterification of the corresponding boronic esters with commercial
inexpensive diols, such as pinacol, 1,3-propanediol, and neopentyl glycol. |
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