Chromium, arene complexes

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

Recently, Lacour, Sauvage and coworkers were able to show that the association of chiral [CuL2] complexes (L=2-R-phen,6-R-bpy and2-iminopyridine) with TRISPHAT 8 leads to an NMR enantiodifferentiation, which allows the determination of the kinetics of racemization of the complexes (bpy=2,2 -bipyri-dine phen=l,10-phenanthroline) [119]. This type of application has recently been reported in conjunction with chiral sandwich-shaped trinuclear silver(l) complexes [122]. Several reports, independent from Lacour s group,have confirmed the efficiency of these chiral shift agents [123-127]. Finally, TRISPHAT can be used to determine the enantiomeric purity of (r] -arene)chromium complexes. These results broaden the field of application of 8 to chiral neutral, and not just cationic, species [114,128,129]. 

Uemura and coworkers utilized an arene-chromium complex ligand to form the C—N bond on a pyridine ring [146]. In the presence of 3 mol% of the monophosphine(dicarbonyl)chromium... 

Tipson-Cohen reaction, 30, 2 Tosylhydrazones, 23, 3 39, 1 Tosyhnethyl isocyanide (TosMIC), 57, 3 Transmetallation reactions, 58, 2 Tricar bonyl(r -arene)chromium complexes, 67, 2... 

A further chiral auxiliary-based tactic exploited tricarbonyl( 76-arene)chromium complexes of aromatic imines 71, which reacted under ultrasound (US) irradiation with a-bromoesters in a predictable stereochemical course to give comparable amounts of /S-aminoesters and / -lactams, as outlined in equation 44127. Chromium decomplexation is eventually achieved by photochemical oxidation under air. 

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

Bis(arene)chromium complexes applications, 5, 345 in engineered materials, 5, 344 Bis(arene) complexes... 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

Suzuki biaryl coupling with (arene)chromium complexes... 

Uemura and Kamikawa have presented a review on the stereoselective synthesis of axially chiral biaryls utilizing planar chiral (arene)chromium complexes [44]. 

General reviews have appeared in the literature [1, 2] concerning arene chromium complexes as well as iron [2] and manganese [3,4] complexes, and nucleophilic additions followed by iodine oxidation and loss of the metal have been well reported, for example by Semmel-hack [lc] and by Morris in 1995 [le]. The purpose of this chapter is to focus only on ipso,... 

The idea that complex formation may be important in the catalytic process can be carried further. It has been found, for example, that bis-arene chromium complexes supported on silica-alumina are active for ethylene polymerization. These catalysts are prepared by activating the support alone in the usual manner and then impregnating with a hydrocarbon solution of the bis-arene compound at room temperature in the absence of air or other oxidizing agent. 

For a detailed discussion of the [Pg.143]

The variously substituted bis(arene)chromium(0) compounds are generally easy to purify by crystallization or sublimation. Bis(arene)chromium complexes with substituents like Cl, F, OMe, or NMe2 are not preparable by classical means... 

The same chiral auxiliary has also been used for the stereoselective synthesis of arene-chromium complexes treatment of an aromatic aminal with chromium hexacarbonyl gives the corresponding complex with high diastereomeric excess. This protocol was recently applied in a total synthesis of (—)-lasubine (eq 4). A successful application of 1,2-diaminocyclohexane (as its IR,2R enantiomer) as a chiral auxiliary is illustrated by the di-astereoselective alkylation of the potassium enolate of bis-amide (3) with electrophiles such as benzyl bromide to give bis-alkylated products with excellent diastereoselectivity (eq 5). Lower levels... 

A25. H. Zeiss, P. J. Wheatley, and H. J. S. Winkler, Benzenoid Metal Complexes. Ronald Press, New York, 1966. Excellent account of the chemistry of arene-chromium complexes, now dated. 

Catalytic asymmetric induction of planar chirality in an (arene)chromium complex has been reported in the cross-coupling of tricarbonyl (o-dichlorobenzene)chromium 30 with vinylic metals, where one of the meso chlorine atoms undergoes the coupling to give the monovinylation product 31 with up to 44% ee (Scheme 2-19) [38]. 

Two other groups investigated P,N ligands for amination chemistry with secondary amines. Uemura and coworkers have prepared ligands similar to those of Kumada, but based on arene chromium complexes rather than ferrocenes64. Catalysts containing ligand... 

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