Reversal Diastereofacial Selectivity in then-Butyllithium Addition toO-ProtectedN-Trimethylsilylimines of (2S)-Lactal: Enthalpic versus Entropic Contributions

Cainelli, Gianfranco; Giacomini, Daria; Galletti, Paola

doi:10.1002/(sici)1099-0690(199901)1999:1<61::aid-ejoc61>3.0.co;2-p

Cited by 25 publications

(13 citation statements)

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“…[16] When ∆∆S ϶ Ͼ 0 (as for distilled imine 1a and for distilled imine 1e at T Ͻ T inv ), the syn transition state is more ordered. [17] The thermodynamic data in Table 3 show that the main difference between the imines is in the sign of the entropic term ∆∆S ϶ : in the case of the distilled imine 1a the anti isomer is always favoured by entropy, while the presence of the Me 3 SiOLi entropically favours the syn isomer and, in fact, the diastereomeric ratio in this case is lower or, in other terms, the amount of the syn diastereoisomer becomes higher with increasing reaction temperature.…”

Section: Effect Of Reaction Temperaturementioning

confidence: 93%

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Diastereoselectivity in the Allylation of N-Trialkylsilylimines of O-Protected (2S)-Lactal − Some Unexpected Results

et al. 2002

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A comparison of the diastereoselective addition of allylMgCl to differently O-protected N-trialkylsilyllactaldimine and to N-benzyllactaldimines is reported. The stereoselectivity strongly depends on the O-and N-protecting groups, on the metal cation, and on the reaction temperature, and varies from predominance of the anti diastereoisomer to predominance of the syn one. O-TBS-and O-TIPS-silyl imines displayed better anti diastereoselectivity than their O-alkyl counterparts. The N-protection produced unexpected results:

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Section: Effect Of Reaction Temperaturementioning

confidence: 93%

“…The opposite applies for ∆∆S ϶ Ͼ 0: the entropy now favours the anti isomer. [17] For imine 1e*, the same signs of enthalpy and entropy dictate a reversal of diastereoselectivity with temperature and for imine 1e a strong decrease in diastereoselectivity at high temperatures.…”

Section: Effect Of Reaction Temperaturementioning

confidence: 99%

Diastereoselectivity in the Allylation of N-Trialkylsilylimines of O-Protected (2S)-Lactal − Some Unexpected Results

et al. 2002

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“…In contrast to in hexane, in THF and the mixture no inversion temperature (T inv ) is observed, indicating the presence of just one solvation cluster in the temperature range explored (for a review, see [8]). Temperature-dependent measurements give insight into the interplay of differential activation enthalpy and entropy in determining the stereoselectivity [9].…”

Section: Methodsmentioning

confidence: 96%

Butyllithium Addition toα-Chiral Compounds: Solvent Mixture Effects on Diastereofacial Selectivity

Cainelli

Galletti

Giacomini

et al. 2000

HCA

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Dedicated to Professor Albert Eschenmoser on the occasion of his 75th birthday Temperature-dependent selectivity in nucleophilic additions is affected by the solvent. In this context, we investigated the effect on diastereoselectivity of solvent mixtures with respect to pure solvents. Binary systems of THF/hexane and of four different hydrocarbon mixtures were employed in BuLi addition to 2-phenylpropanal, (2S)-2-[(tert-butyl)dimethylsilyloxy]-2-phenylethanal, and (2S)-2-[(tert-butyl)dimethylsilyloxy]-N-(trimethylsilyl)propan-1-imine. A 5-mol-% of THF in hexane affects the isomer ratio by reducing both DDH = and DDS = contributions, and suppresses T inv . On the contrary, in hydrocarbon binary mixtures, the T inv is still observed and occurs at a higher temperature than in pure solvents. Studying the dependence of T inv on the hexane/decane mixture composition, we propose the formation of a peculiar solvation cluster that is unaffected by the composition of the bulk reaction solvent.

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“…A break in the Hammett plot can be interpreted as either a change in the stereochemistry-determining step as substrates change from electron-rich to electron-poor aldehydes, or a change in factors that control the two selectivity regimes (50)(51)(52). This implication poses a mechanistically interesting question as to whether the configurations of the hydroxyl-bearing methine carbons in the aldol products at both ends of the electronic spectrum are the same.…”

Section: Preparation Of Trichlorosilyl Enolatementioning

confidence: 99%

Chiral phosphoramide-catalyzed, enantioselective, directed cross-aldol reactions of aldehydes

Denmark

Bui

2004

Proc. Natl. Acad. Sci. U.S.A.

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Catalytic, enantioselective, directed cross-aldol reactions of aldehydes are described. The addition of isobutyraldehyde trichlorosilyl enolate 2 to various aldehydes in the presence of 10 mol % bisphosphoramide 4 provides aldol products in high yields with moderate to good enantioselectivities. The reaction works well with a wide range of aromatic, olefinic, and aliphatic aldehydes. Enantioselectivities are highly dependent on the electronic nature of the aldehyde substituent. Hammett studies reveal that enantioselectivity increases as aldehydes become either more electron rich or more electron poor. T he catalytic enantioselective aldol reaction has achieved ''strategy level'' status in organic synthesis (1-6). In this process, a new carbon-carbon bond is formed with the creation of up to two new stereogenic centers. The relative and absolute configurations of the newly created stereocenters are usually controlled through the use of chiral Lewis acids (7) or through enzymatic methods (3,8). Chiral Lewis acid-catalyzed aldol reactions generally afford aldol products from enoxysilane derivatives of ketones or esters [directed aldol reaction (9)] in high yields with good to excellent selectivity. A few notable examples of chiral Lewis acids used in these catalytic aldol processes include bisoxazoline copper(II) complexes (10, 11) and Schiff base titanium(IV) complexes (12, 13), which have been developed by Evans et al. and Carreira et al., respectively. Recent advances also include the development of direct catalytic enantioselective aldol reactions that employ the two unmodified carbonyl components. Early reports on this type of aldol reaction using LaLi 3 tris(binaphthoxide) as a catalyst were disclosed by . Moderate to good yields and selectivities were obtained in these studies, albeit at long reaction times. Further, Trost et al. (17,18) have developed dinuclear zinc catalysts for direct catalytic enantioselective aldol reactions. These catalysts, which mimic class II aldolases, have been shown to catalyze aldol reactions of unmodified ketones with aldehydes in moderate to good yields with excellent levels of stereocontrol. Parallel to these successful developments is the finding of proline and its derivatives as effective catalysts for direct enantioselective aldol reactions (19-21). These simple cyclic amine catalysts are capable of mimicking class I aldolases, and they provide another approach to asymmetric aldol reactions (22,23).Besides these methods, the use of enzymes (3, 24) and monoclonal antibodies (8) in enantioselective aldol reactions has also been described with success. Wong et al. (24) have demonstrated the use of natural aldolase-catalyzed aldol reactions as a means to effectively and selectively synthesize highly oxygenated carbon chains, allowing a rapid access to various carbohydrates. However, broadening enzyme specificities and being able to perform enzymatic transformations on a preparative scale remain challenges for further development.For the directed aldol reaction, silyl enol et...

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Reversal Diastereofacial Selectivity in then-Butyllithium Addition toO-ProtectedN-Trimethylsilylimines of (2S)-Lactal: Enthalpic versus Entropic Contributions

Cited by 25 publications

References 35 publications

Diastereoselectivity in the Allylation of N-Trialkylsilylimines of O-Protected (2S)-Lactal − Some Unexpected Results

Diastereoselectivity in the Allylation of N-Trialkylsilylimines of O-Protected (2S)-Lactal − Some Unexpected Results

Butyllithium Addition toα-Chiral Compounds: Solvent Mixture Effects on Diastereofacial Selectivity

Chiral phosphoramide-catalyzed, enantioselective, directed cross-aldol reactions of aldehydes

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