Starting from (S)-S-ferrocenyl-S-4-tolyl sulfoxide the planar-chiral imidazolium salt (R
p
)-1-[2-(trimethylsilyl)ferrocenylmethyl]-3-methylimidazolium iodide was synthesized utilizing
the directed ortho-metalation strategy. Deprotonation thereof yielded the first planar-chiral
carbene, (R
p
)-3-methyl-1-[2-(trimethylsilyl)ferrocenylmethyl]imidazolin-2-ylidene, which is
stable in tetrahydrofuran at room temperature for several hours. Conversion with sulfur
resulted in the formation of the corresponding thiourea derivative. Reaction with the metal
precursors [Rh(COD)Cl]2 and Cr(CO)6 gave air-stable complexes. For the latter product an
X-ray structural analysis was conducted.
The capability of the sulfonimidoyl moiety to serve as ortho-directing group for the diastereoselective lithiation of enantiopure ferrocenylsulfoximines is demonstrated. (S)-Np-Tolylsulfonyl-S-ferrocenyl-S-tert-butylsulfoximine showed excellent diastereoselectivity concerning planar chirality of the products obtained after quenching of the lithium species with several electrophiles. Various electrophiles (MeI, Me 2 S 2 , Me 3 SiCl, p-anisaldehyde, acetone, n-Bu 3 SnCl, I 2 ) were introduced in yields from 36 to 78%. Only up to 10% of product substituted at both the cyclopentadienyl ring and the phenyl group of the tosyl moiety was isolated as byproduct. The relative configuration of the products was determined by chemical correlation and X-ray crystallographic analysis of (S,R p )-N-p-tolylsulfonyl-S-tert-butyl-S-(2iodoferrocenyl)sulfoximine. Taking into account the structure of the starting material, the ortho-directing effect can be ascribed to the oxygen atom of the sulfoximine.
An oxazolinyl ferrocenyl diselenide was synthesised by directed ortho-metalation and used as catalyst precursor in the asymmetric addition of diethyl-and diphenylzinc to various aldehydes, yielding synthetically useful secondary alcohols, of which some are difficult to access using other catalytic methodologies. Enantioselectivities of up to 44% for the former and up to 85% for the latter transformations were obtained.
A novel planar chiral cyrhetrene 1 has been identified as an efficient catalyst precursor for the asymmetric synthesis of diarylmethanol compounds 2 by phenylation of aldehydes. Compared to the corresponding ferrocene derivative, the performance of the cyrhetrene is superior or at least equally as good and gives excellent enantioselectivities even with catalyst loadings as low as 2 mol %.
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