“…Ferrocenyloxazoline 8 [59,60] was regarded as a promising substrate for catalytic C-H activation, because oxazolines have a long successful record as efficient ODGs. [ An ODG closely related to the oxazolinyl system is the 1H-pyrazolyl group, which can be attached at ferrocene by a copper(I)-or copper(II)-mediated coupling reaction.…”
Transition‐metal‐catalyzed C–H functionalization is an emerging powerful alternative to ortho‐lithiation protocols. For ferrocene systems almost exclusively noble‐metal catalysts have so far been used. Herein we present the first examples of cobalt‐catalyzed ortho‐methylations by C–H activation at ferrocenes bearing various ortho‐directing groups (ODGs). The cobalt‐catalyzed ortho‐methylation results in the formation of racemic 1,2‐disubstituted ferrocenes in 21–89 % yield. In some cases symmetric 2,5‐dimethylated ferrocenes prevailed or even were the only isolated products.
“…Ferrocenyloxazoline 8 [59,60] was regarded as a promising substrate for catalytic C-H activation, because oxazolines have a long successful record as efficient ODGs. [ An ODG closely related to the oxazolinyl system is the 1H-pyrazolyl group, which can be attached at ferrocene by a copper(I)-or copper(II)-mediated coupling reaction.…”
Transition‐metal‐catalyzed C–H functionalization is an emerging powerful alternative to ortho‐lithiation protocols. For ferrocene systems almost exclusively noble‐metal catalysts have so far been used. Herein we present the first examples of cobalt‐catalyzed ortho‐methylations by C–H activation at ferrocenes bearing various ortho‐directing groups (ODGs). The cobalt‐catalyzed ortho‐methylation results in the formation of racemic 1,2‐disubstituted ferrocenes in 21–89 % yield. In some cases symmetric 2,5‐dimethylated ferrocenes prevailed or even were the only isolated products.
“…Starting from achiral 2-ferrocenyloxazoline 13 that was synthesized from ferrocene carboxylic acid 11 and 2-amino-2-methylpropanol in the common two-step procedure following Richard's route, 12b, planar-chirality-inheriting ferrocene 14 was obtained as a racemate (Scheme 2). …”
Employing a directed ortho-metalation route, chiral ferrocene-based hydroxyloxazolines have been synthesized, and their capability to serve as bidentate ligand precursors in the diethylzinc addition to benzaldehyde has been investigated. On the basis of the X-ray crystal structures of two ferrocenes, (S,R p )-6 and (S,S p )-10, it became possible to determine the general importance of the element of planar chirality for diastereomeric ferrocene complexes in this process. Comparison of the assumed transition states revealed a preferential catalyst conformation which led to the design of an adequate arene compound [(S)-20] that displayed catalytic activity similar to that of (S,R p )-6.
“…Compound 149 could be regioselectively lithiated at the ortho position to produce after subsequent reaction with an electrophile racemic, planar-chiral ferrocene 150 in about 70% yield (Scheme 43). 262 The asymmetric variant of the reaction described before was not explored until 1995. At that time Richards, Sammakia, and Uemura independently reported on diastereoselective orthodirected lithiations of 4-substituted 2-ferrocenyl-2-oxazolines.…”
“…Schmitt et al synthesized 5-substituted 2-ferrocenyl-2-oxazoline 149 by acid-catalyzed rearrangement of 2-substituted 1-ferrocenoyl aziridines, obtained by condensation of ferrocenoyl chloride and the respective aziridine. Compound 149 could be regioselectively lithiated at the ortho position to produce after subsequent reaction with an electrophile racemic, planar-chiral ferrocene 150 in about 70% yield (Scheme ) …”
Planar-chiral
ferrocenes are widely applied in catalytic asymmetric
transformations in scientific research as well as in industry. A plethora
of different methodologies have been developed to access these molecules
with a high degree of regio- and stereoselectivity. The aim of this
contribution is to give a comprehensive overview of this topic. The
synthesis of 1,2- and 1,3-substituted ferrocenes by electrophilic
aromatic substitution, ortho-directed metalation, kinetic resolution,
and desymmetrization is discussed. Advantages and disadvantages are
highlighted.
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