Match/mismatch effect

Diastereoselective hydrogenations of this type have been reported by Burgess and coworkers [54—59] using chiral-protected and -unprotected allylic and homo allylic alcohols as substrates with their carbene catalyst lr(9). Catalyst control was found to be dominant, but depending on the position and nature of the oxygen substituents, moderate to strong match/ mismatch effects were observed. 

By contrast, lithium enolates derived from tertiary amides do react with oxiranes The diastereoselectivity in the reaction of simple amide enolates with terminal oxiranes has been addressed and found to be low (Scheme 45). The chiral bicyclic amide enolate 99 reacts with a good diastereoselectivity with ethylene oxide . The reaction of the chiral amide enolate 100 with the chiral oxiranes 101 and 102 occurs with a good diastereoselectivity (in the matched case ) interestingly, the stereochemical course is opposite to the one observed with alkyl iodides. The same reversal is found in the reaction of the amide enolate 103. By contrast, this reversal in diastereoselectivity compared to alkyl iodides was not found in the reaction of the hthium enolate 104 with the chiral oxiranes 105 and 106 °. It should be noted that a strong matched/mismatched effect occurs for enolates 100 and 103 with chiral oxiranes, and excellent diastereoselec-tivities can be achieved. 

Both the selectivity and the activity of the diastereomeric ligands, (Rp,S)-5a and (Sp,S)-5a, were nearly identical and both showed positive chiral cooperativity (there was no observation of a mismatched pair). Through introduction of a phenyl residue in the a-position of the side-chain as shown in ligand pair 6a, there was again a small matched/mismatched effect, which was not as strong as in the unsubstituted case in ligand pair 4a. Overall, we could derive a simple rule for the ligands ... 

A remarkable match-mismatch effect is observed. The difference in reactivity of the matched catalyst—for example, Ru(R-BINAP)(S,S-DPEN)Cl2—can be 120 times more reactive than the mismatched catalyst, Ru(S-BINAP)(S,S-DPEN)Cl2.196 197... 

The asymmetric approaches include the preparation of the configurationally stable chiral aUenyltin starting from enantio-enriched propargyUc precursors. When submitted to transmetaUation with Sn, Bi or In Lewis acids before addition to the aldehyde, the homopropargyl alcohol is obtained in a 95 5 anti/syn ratio and in a 90% ee . On the other hand, the use of a chiral aUenyltin reagent, without prior transmetaUation, gives the syn adduct selectively (95 5) (equation 12) . However, the use of chiral aUenyltin and chiral aldehydes may lead to the same match/mismatch effect that was observed with allyltins . Both approaches were applied to the synthesis of macrolides subunits ° i . 

A -allylanilides (eq 63). A match/mismatch effect was observed as the (SJty(SJS) ratio increased from 3.2 to 15.9 using (/f)-BINAP instead of (5)-BINAP. The major diastereomer could be easily transformed to optically pure (ee >98%) material via a MOM deprotection reaction. 

The ruthenium catalyst generated in situ from H2Ru(CO)(PPh3)3, (S)-SEGPHOS and the TADDOL-derived phosphoric acid (169) promoted butadiene hydrohydroxyalkylation to form enantiomerically enriched products (168) (Scheme 61). Match/mismatch effects between the chiral ligand and the chiral TADDOL-phosphate counterion have been described. For the first time, single-crystal X-ray diffraction data for a ruthenium complex modified by a chiral phosphate counterion has been reported. ... 

Cycloadditions to racemic mixmres of chiral alkenes also show that there is virtually no induction from the nitrile oxide part, meaning that there is no effect of matching-mismatching of the partners in the transition state. In the reaction with glyceraldehyde, and threose- and glucose-derived nitrile oxides, only the usual ... 

Consequently, matched/mismatched cases [25] can result, and indeed our investigations on cooperative effects of stereogenic elements in such systems revealed 9 be the matched case and 23 (which is also easily prepared by following a directed deprotonation-silylation-deprotonation-trapping-desilylation sequence [11]) to be the mismatched case in diethylzinc additions to aldehydes [26]. Later, these investigations were extended to more complex systems such as 24 [27], but ferrocene 9 still remains superior to all other compounds. 

Later, the oxazolines 25 were examined to study the effects of matched/mismatched combinations of stereogenic centers on catalyzed aryl transfer reactions to aldehydes. Of these mandelic acid-derived catalysts, 25b gave the best results in terms of enantioselectivity (up to 35% ee), while diastereomer (l ,S)-25b proved to be superior to (S,S)-25b with respect to catalyst activity [29]. With both compounds, the absolute configuration of the product was determined by the oxazo-line moiety. 

Additions to the anti,anti and anti,syn aldehydes exhibited a more typical matched/mismatched profile (equations 28 and 29). It was concluded that dipole effects of the /S-OTBS group must play an important role in the mismatching observed with these latter two aldehydes. 

It should be noted that when the reaction is conducted with a chiral aldehyde and a chiral y-substituted allyltins, a matching effect when both partners impose a convergent selectivity, or a mismatching effect when the facial selectivity is divergent, may happen . Such a stereoconvergent effect was used in the synthesis of the antitumour agent azinomycin (equation 4). 

The principle underlying the use of oligonucleotides and DNA microchips is the distinction between perfect and mismatched duplexes. The effectiveness of distinction depends on many parameters, such as the length of the probe, the position of the mismatch in the probe, the AT content, and the conditions of the hybridisation reaction [19, 20]. Because of the aforementioned parameters, centrally located mismatches are easier to locate than terminal ones, and shorter probes allow for easier match/mismatch discrimination. The only drawback of using shorter probes is that overall duplex stability decreases as the length of the ohgomer deaeases, a phenomenon that is suspected to lead to low hybridisation signals with shorter probes. 

In 2005, Bolm and coworkers disclosed a new planar-chiral imidazolium salt 30 incorporating a chiral Al-substituent (Scheme 3.16) [35]. Its apphcation in rhodium-catalyzed additions of arylboronic acids to aromatic aldehydes was demonstrated, and the corresponding diarylmethanols were obtained with up to 38% ee. Since the hgand bears two chiral elements, match and mismatch effects were observed by showing that NHC 31, which is the diastereomer of 30, did not give any enantioinduction. 

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