Chemoselective Functionalization of Different Carbonyl Group

Chemoselective Function(iUz.ution of Different Carbonyl Group... 

The tendency of Ti reagents like Ti(NR2)4 to release the yVJV-dialkylamino group is conveniently exploited to temporarily inactivate a carbonyl function which is fairly reactive (e.g. an aldehyde),thus enabling a second, less reactive carbonyl fiinction e.g. a ketone), to react with another nucleophilic reagent. This leads to a sort of reverse chemoselectivity which is highly effective in discriminating between different carbonyl compounds, as well as between carbonyls having different steric environments. 

Figure 4.3 TGA-FTIR A schematic diagram of a TGA-FTIR interface B (a), weight-loss curve of a plasticised PVC ( 20 mg) by TGA (b), first derivative of weight-loss curve (c), Gram-Schmidt reconstruction calculated from spectral data acquired by FTIR analysis C Thermograms , specific reconstruction for three different functional groups, which are a chemoselective record of the evolving gas (a) carbonyl (1760-1740 cm ) (b) HCl (2831-2785 cm ) (c) benzene (684-664 cm ) D spectrum obtained from the middle of the first weight-loss region. Reproduced from R. C. Wieboldt, S. R. Lowry and R. J. Rosenthal (1988) Mikrochim. Acta /, Recent Aspects of Fourier Transform Spectroscopy, Vol. 2, Proc. 6th Int. Conf Four. Trans. Spec.), pp. 179-82, by...

Second, functional group selectivity is observed - i.e. reactions with aldehydes proceed at —78 °C whereas those with ketones proceed at elevated temperatures (ca. 0 °C). Chemoselectivity is observed with acceptors having two different kinds of carbonyl function, for example aldehyde and ketone or ester, in the same molecule. Treatment of phenylglyoxal with silyl enol ether 38 at —78 °C affords a-hydroxy-y-diketone 39 (Eq. (20)) [20b]. The reaction of ketoesters 40 other than j5-ketoesters with silyl enol ether 38 gives hydroxyketoesters 41 as sole products (Eq. (21)) [23]. 

According to low temperature C-NMR study on the affinity of MAD with oxygen containing functional groups, such as carbonyl compounds and ethers, coordination of amides to MAD is stronger than that of aldehydes, ester, and ketones, while coordination of ethers is rather weak [46). One of the most important synthetic utilities of MAD is that in the hydride reduction of two different ketones MAD can protect the less hindered ketone or more basic ketones by forming complexes with these ketones, that is, selective reduction of more hindered free ketones can realize. As shown in Scheme 6.38, for example, DIBAL reduction (1 equivalent) of a 1 1 mixture of acetophenone and tert-butylphenylketone provide two carbinols with low chemoselectivity. On the other hand, in the presence of... 

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