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2,3-anti-selective aldol of benzylic nitriles

Two, three-anti-selective aldol of benzylic nitriles

Authors Lo, Kam Moon
Issue Date 1995
Summary It is suprising that very little attention has been given to the stereochemistry of addition of metallated nitriles to aldehydes, given that βP-hydroxynitriles are versatile synthetic intermediates. One useful transformation of these compounds that immediately presents itself is reduction to the corresponding γ-aminoalcohols, which are potential antidepressant candidates. After a general introduction recounting the development of aldol reaction of carbonyl compounds, the first portion of this thesis describes attempts to mediate the nitrile aldol reaction with boron reagents. This work can be further divided into two different approaches, the "Hydroboration Approach," and the "Lewis Acid/Tertiary Amine Base" approach. The regio- and chemoselectivity of addition of boranes to cinnamonitrile was studied. The hydroboration product was found to be unexpectedly reactive towards protonolysis in dilute mineral acid, suggesting the intermediacy of an [italic]N-boronated ketenimine. However attempts to achieve aldol reaction via this approach were unsuccessful. Several chiral and achiral boron Lewis acids were prepared and screened for ability to mediate aldol reaction of acetonitrile and benzaldehyde in the presence of a tertiary amine base. Unfortunately in each of these cases low yields of the aldol dehydration products (α, β-unsaturated nitrile) are obtained; the desired aldol products could not be isolated. These results have been interpreted in terms of 1) slow "enolization" of the nitrile, and 2) fast elimination of the boronated aldol product. Secondly, the first systematic investigation of the relative stereochemistry of addition of lithiated benzylic nitriles to aldehydes has been carried out. Moderate ([italic]anti : [italic]syn = 3 : 1) to excellent ([italic]anti : [italic]syn [is greater than] 50 : 1) 2,3-[italic]anti selectivity was obtained, which increases with increasing size of the aldehyde substituent. Assignment of relative configuration of the aldols was carried out by single crystal X-ray determinations, chemical correlations, and by means of a new [supperscript 1]H NMR correlation method developed in this study. The effects of the of metal, solvent, base, and reaction concentration on diastereoselectivity were studied in detail to establish optimum conditions for nitrile aldol reaction. Control experiments demonstrated that nitrile aldol reaction is irreversible under the optimum conditions, thereby establishing that the observed selectivities are kinetic, not thermodynamic in origin. In addition, a preliminary study of the relative stereochemistry of aldol reaction of lithiated isovaleronitrile was undertaken. This study indicates that lithiated aliphatic nitriles are not uniformly anti-selective in their addition to aldehydes. Finally, a six-membered cyclic transition state model featuring the intermediacy of a monomeric [itlaic]N-lithiated nitrile anion is put forward to account for the observed anti-selectivity of lithiated benzylic nitriles. This model rationalizes diastereoselection purely in terms of non-bonded steric interactions of the nitrile and aldehyde substituents. The effect of electronic perturbation of the aldehyde on diastereoselectivity is then rationalized in terms of the "reactivity-selectivity principle." In contrast, the effect of electronic perturbation of the benzylic nitrile on diastereoselectivity is rationalized in terms of changes in the ground state structure of the lithiated nitrile.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 1995
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Language English
Format Thesis
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