Benchrotrene complexes

The preparation of enantiopure or enriched complexes possessing planar chirality has been accomplished either by resolution of racemic mixtures or by asymmetric syntheses. Reported methods for the resolution of planar chirality include both chemical and kinetic resolution procedures, whilst reported asymmetric syntheses of enantiomerically pure or enriched benchrotrenic complexes include enantioselective ort/io-deprotonations with chiral lithium amide bases, and the transfer of side chain chirality onto the arene ring mediated by diastereoselective orf/io-nucleophilic additions and o/tfeo-metalations. 

Caution When using chromium tricarbonyl complexes and t-BuLi, all procedures should be conducted in a well-ventilated hood, using standard vacuum/nitrogen line and Schienk tube techniques. Disposable vinyl or latex gloves and safety glasses should be worn. As benchrotrenic complexes are unstable in solution when exposed to air and light, the deprotonation reaction and subsequent work-up are carried out in giassware covered with foil. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

Carborane-metal complexes, 8, 87-113 Catalysis by organometallics carbonylation, 6, 158-163 hydroformylation, 6, 128-136 olefin reactions, 7, 199-202 organic syntheses by nickel compounds, 8, 48-83 reviews, 10, 331-336 Cationic metal carbonyls, 8, 117-159 Chromium carbonyls, 8, 133-159, see also Benchrotrenes Cobaloximes, 7, 161, 203 Cobalt carbonyl, 6, 119-163 8, 152-155 Cotton-Kraihanzel method, 10, 213-214 Coupling reactions, on nickel, 8, 30-39, 82-83... 

For the alkylation of enolates, chromium tricarbonyl complexes of aromatic compounds (benchrotrenes) are useful, as they make simple aromatic compounds chiral. Thus, enantiomer-ically pure (indanone)tricarbonylchromium (2R)-25 has been prepared by resolution of the racemic benchrotrene derivative with cinchonidine and oxidation of the alcohol to the ketone with manganese dioxide60. The chiral ketone is alkylated diastereoselectively via the enolate, leading to the f.vo-2-methyl derivative (2/ )-25 which has been used in enolate alkylations and annulation reactions (Section D.1.5.2.4.). If necessary, complete isomerization to the endo-methyl compound can be achieved by treatment with base. 

This concept of the enantiotopic faces of a prochiral molecule is very important as it allows us to show that the introduction of an atom or group of atoms in the third dimension leads to a chiral molecule. This is the case for all such complexes of benchrotrene, ferrocene, cymanthrene, and so on (Figure 2.18). 

Starting from meso-complexes (achiral species), benchrotrenic planar chirality can be generated. The differentiation of the enantiotopic substituents on the aromatic ring has been achieved using enzymes or a chiral palladium catalyst. 

Chromium, Molybdenum, and Tungsten.— Applications of benchrotrenes and analogous Mo complexes in organic synthesis have been reviewed, and studies of the use of such complexes as catalysts for alkene polymerization, isomerization, and hydrogenation have been reported. Decomplexation of the arene ligand of benchrotrenes can be conveniently effected by treatment with boiling pyridine. Spectroscopic studies of ( -arene)M(CO)3 complexes have been described, including discussions of carbonyl vibrational spectra, ih 2 and... 

Fulvene Complexes.—Several ( -fulvene)Cr(CO)3 complexes have been obtained by photochemical ligand-exchange reactions of benchrotrenes with 6,6-disubsti-tuted pentafulvenes. The crystal structure and C n.m.r. spectrum of the 6-(dimethylamino) complex have shown that it is best formulated as a... 

Chromium, Molybdenum, and Tui isten.—A review of substituent effects in organo-transition-metal complexes includes discussion of the properties of benchrotrene derivatives. There have been studies of charge-transfer complexation of [( -arene)M(CO)3] (M=Cr, Mo, and W) with -acceptor molecules, ... 

New benchrotrene derivatives have been prepared by reactions of [Cr(CO)s], etc., with oestrone and related steroids, phenalene, 2,6,15,19-tetrathia-[7.7]paracyclophane, PhjAs, and diphenic acid and related compounds. Ring-substituted derivatives of [( benzaldehyde)Cr(CO)3] have been resolved into enantiomeric forms. Reactions of carbene complexes of the type [(OQs-CrC(OMe)Ar] with alkynes have afforded derivatives of [(jy -naphthalene)-Cr(CO>3] (see Vol. 8, refs. 393, 394). 

Complexes of the type [( -arene)Cr(CO)(dppe)] have been obtained by photochemical CO-displacement reactions of the benchrotrenes. ... 

MO calculations suggest that, in nucleophilic addition reactions of substituted benchrotrenes [( -PhX)Cr(CO)3] (see Vol. 9, ref. 412), the preferred orientation of addition is influenced more by the conformation of the Cr(CO)a residue than by the electronic character of the substituent X. Whereas lithiation (with BuLi-TMED) of 3-methoxybenzyl alcohol occurs very predominantly at the 2-position of the arene ring, similar treatment of the corresponding n-Cr(CO)3 complex gives a mixture of the 2- and 4-lithio derivatives in the ratio 23 77, respectively these products were characterized by isolation of carboxylic acid derivatives following carbonation (COg) and metal decomplexation hv 0. Experimental details for the preparation of [(iy-PhX)Cr(CO)3] (X=I and SiMes) from [( -PhLi)Cr(CO)3] have been reported. This lithio derivative has also been used to prepare the carbene complexes [(OC)3Cr(i7-Ph)C(OEt)M(CO)5] (M=Cr, Mo, and W), which react with BX, (X=C1 and Br) to give the carbyne products [(OC)3Cr( Ph)C=M(CO)4X] (M=Cr and W only). ... 

Diazadiborab izene Complexes.— Various ring-substituted l,2-diaza-3,6-dibora-benchrotrenes have been synthesized. ... 

---