- Mechanism and optimisation of the homoboroproline bifunctional catalytic asymmetric aldol reaction: Lewis acid tuning through in situ esterification
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The use of homoboroproline as a bifunctional catalyst in the asymmetric aldol reaction has been investigated mechanistically, particularly with respect to tuning the Lewis acidity of boron by in situ esterification with mildly sigma-electron withdrawing diols such as hydrobenzoin and tartrate esters. The stability of simple cyclohexyl and cyclopentyl boronate diol esters shows that the 5-ring boronate esters are more stable, which sheds light on the mode of action of esterified homoboroproline catalyst in the enamine-mediated aldol reaction, which is also studied by NMR. The result is reaction optimisation to provide an efficient aldol reaction and a proposed mechanistic proposal. The Royal Society of Chemistry 2012.
- Georgiou, Irene,Whiting, Andrew
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p. 2422 - 2430
(2012/04/17)
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- A comparative study of the relative stability of representative chiral and achiral boronic esters employing transesterification
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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 bicyclo[2.2.1]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.
- Roy, Chandra D.,Brown, Herbert C.
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p. 879 - 887
(2008/02/11)
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