Benchrotrene

For general syntheses applied to fused ferrocene, tricarbonyl cyclobuta-dieneiron, and benchrotrene derivatives, see Tables II, IV, V, and VIII.)... 

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). 

It has also been established (see Volume 12, refs. 285-288 Volume 13, refs. 200-206) that the orientation of lithiation of the arene ligand ring in substituted benchrotrenes is strongly influenced by the nature of the substituent groups already attached to the ring. The regioselectivity of such reactions has been utilised in... 

Bacteriochlorophyll, Y23 Baikiain, AlO Balchanolide, T21 Balfourodine, K31 Balfourolone, K30 Baptifoline, K21 Batatic acid, T17 Batrachotoxinin A, K36 Baurenol, T42 Baurenone, T42 Benchrotrenes, X9 Benzoin, A23 Benzoquinolizines, A5 Benzoyl allylhippuric acid, AlO Benzylglutamic acid, A32 3-O-benzylglycerol, A14 Betanidin, K17 Betanin, K17 Betulafolienetriol, T51 Betulenols, T28 Betulinic acid, T43 Betulin, T43 Beyerane, T35 Beyerol, T35 Bianthryls, X0 7 Biaryls, K2-4, K20, K27, Y2, YIO, Y15, Y17-18, X3, X6, XIO... 

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. 

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. 

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. 

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... 

This last fact, coupled with the observation that significant quantities of benchrotrene itself are present in the reaction medium (and thus susceptible to nucleophilic attack) has led to the synthesis of substituted arenes and... 

This new method of carbanion phenylation would seem to be quite useful, since only mild conditions (0°) are necessary, and recently a much improved synthetic method for benchrotrene has been reported (Rausch, 1974). It may... 

It has previously been stated that attack by nucleophiles on cationic arene rings occurs stereospecifically in an exo fashion. In fact, coordination by a transition metal confers a third dimension on the molecule which has several stereochemical consequences. For example, electrophilic or nucleophilic attack at the reactive center of an alicyclic ring ortho-condensed to an aromatic moiety always occurs stereospecifically in an exo fashion. Arene derivatives containing different ortho or meta substitutents are chiral, and numerous examples exist of the resolution of benchrotrene derivatives... 

Photochemical decomplexation of the two meso pseudoasymmetric benchrotrene stereoisomers yields the same meso benzene glycol (LXXX-VIII), while decomplexation of the racemic benchrotrene glycol (LXXXVII) yields a racemic benzene glycol (LXXXIX). The overall yield using this procedure is not particularly high, but it may be generalized to the synthesis of other aromatic glycols. 

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, ... 

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