“…The first step in the synthesis and resolution of the new chiral building blocks involves a regioselective ortho -lithiation of the pentamethylated azaferrocene 1 17 or azaruthenocene 2 21 giving solely the 2-lithiated isomer. Early reports on the selective ortho -lithiation of azaferrocenes describe problems with concomitant 1‘,2-dilithiations as well as monolithiation of the Cp ring. − By using pentamethylated azaferrocenes, these problems are conveniently avoided, since only monolithiation should be possible. The anion is then trapped by the addition of (−)-(1 R ,2 S ,5 R )-menthyl-( S )- p -toluenesulfinate ( 3), giving a mixture of two enantiopure sulfoxide diastereomers.…”
Section: Resultsmentioning
confidence: 99%
“…Early reports on the selective ortho-lithiation of azaferrocenes describe problems with concomitant 1′,2-dilithiations as well as monolithiation of the Cp ring. [24][25][26] By using pentamethylated azaferrocenes, these problems are conveniently avoided, since only monolithiation should be possible. The anion is then trapped by the addition of (-)-(1R,2S,5R)-menthyl-(S)-p-toluenesulfinate (3), giving a mixture of two enantiopure sulfoxide diastereomers.…”
A methodology for the enantioselective synthesis of planar chiral 2-substituted 1',2',3',4',5'-pentamethylazaferro- and azaruthenocenes is reported. The key step is the introduction of an enantiopure chiral sulfoxide auxiliary via a selective ortho-lithiation with subsequent chromatographic separation of the resulting diastereomers. Cleaving off the sulfoxide auxiliary by treatment with t-BuLi generates an optically pure anion which may be quenched with a variety of electrophiles to give the optically pure azametallocene derivatives. In addition, it was attempted to extend the methodology to encompass 1',2',3',4',5'-pentaphenyl derivatives. It was, however, not possible to attach the chiral sulfoxide auxiliary onto the pentaphenylated azaferrocene, and for the azaruthenocene case, only one diastereomer could be isolated. Most importantly, the sulfoxide group could be cleaved off and the resulting chiral azaruthenocenyl anion was quenched with paraformaldehyde and iodine, resulting in products with ee values of 85% and 99%, respectively.
“…The first step in the synthesis and resolution of the new chiral building blocks involves a regioselective ortho -lithiation of the pentamethylated azaferrocene 1 17 or azaruthenocene 2 21 giving solely the 2-lithiated isomer. Early reports on the selective ortho -lithiation of azaferrocenes describe problems with concomitant 1‘,2-dilithiations as well as monolithiation of the Cp ring. − By using pentamethylated azaferrocenes, these problems are conveniently avoided, since only monolithiation should be possible. The anion is then trapped by the addition of (−)-(1 R ,2 S ,5 R )-menthyl-( S )- p -toluenesulfinate ( 3), giving a mixture of two enantiopure sulfoxide diastereomers.…”
Section: Resultsmentioning
confidence: 99%
“…Early reports on the selective ortho-lithiation of azaferrocenes describe problems with concomitant 1′,2-dilithiations as well as monolithiation of the Cp ring. [24][25][26] By using pentamethylated azaferrocenes, these problems are conveniently avoided, since only monolithiation should be possible. The anion is then trapped by the addition of (-)-(1R,2S,5R)-menthyl-(S)-p-toluenesulfinate (3), giving a mixture of two enantiopure sulfoxide diastereomers.…”
A methodology for the enantioselective synthesis of planar chiral 2-substituted 1',2',3',4',5'-pentamethylazaferro- and azaruthenocenes is reported. The key step is the introduction of an enantiopure chiral sulfoxide auxiliary via a selective ortho-lithiation with subsequent chromatographic separation of the resulting diastereomers. Cleaving off the sulfoxide auxiliary by treatment with t-BuLi generates an optically pure anion which may be quenched with a variety of electrophiles to give the optically pure azametallocene derivatives. In addition, it was attempted to extend the methodology to encompass 1',2',3',4',5'-pentaphenyl derivatives. It was, however, not possible to attach the chiral sulfoxide auxiliary onto the pentaphenylated azaferrocene, and for the azaruthenocene case, only one diastereomer could be isolated. Most importantly, the sulfoxide group could be cleaved off and the resulting chiral azaruthenocenyl anion was quenched with paraformaldehyde and iodine, resulting in products with ee values of 85% and 99%, respectively.
“…It is worthy noting that similar behaviour was reported for lithiated parent azaferrocene. The treatment of this compound with n-BuLi in THF at À50°C, followed by quenching with iodomethane afforded a mixture of 2-and 1 0 -methylazaferrocene and 1 0 ,2-dimethylazaferrocene, suggesting that 2-and 1 0 -lithioazaferrocenes are formed in comparable amounts along with some amount of 1 0 ,2-dilithioazaferrocene [8]. However, when benzophenone, benzaldehyde, dibenzyl ketone or diisopropyl ketone were used as quenchers the exclusive formation of 2-substituted azaferrocenes was observed [9,10].…”
Section: Discussion and Concluding Remarksmentioning
confidence: 97%
“…Up to now the most promising synthetic strategy for introduction to azaferrocene of desired carbon chains and/or functional groups is based on its lithiation. It has been found that azaferrocene is lithiated by n-BuLi to comparable extent on the Cp ring and at the a-position of the pyrrolyl ring [8]. Moreover, reactions of lithiated azaferrocene with electrophiles show unexpected (and yet inexplicable) selectivities, giving (depending on the nature of the electrophile) either Cp-or pyrrolyl-substituted products [9,10].…”
“…Such complexes are chiral by virtue of the presence of ML n and the differentiation of R from H, and the asymmetric environment around the Lewis-basic heteroatom can readily be “tuned” by varying the steric bulk either of the metal fragment or of R. We recently demonstrated that enantiopure planar−chiral heterocycles function as efficient nucleophilic acylation catalysts for the kinetic resolution of secondary alcohols . In this paper, we report that they also serve as effective chiral ligands, catalyzing the enantioselective addition of organozinc reagents to aldehydes. …”
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