Chromium complexes arenes

Keywords Arene-chromium complexes Arene-manganese complexes Dearomatization Cyclohexadiene Organo lithium reagents Asymmetric arene transformations... 

A plausible reaction mechanism is shown in Scheme 2. The generated ketyl radical 17 adds to the chromium-complexed arene ring from the face opposite to the Cr(CO)3 fragment. Further electron reduction and protonation give an anionic q -intermediate 19, which lead to the cyclization product 16 by elimination ofMeOH. 

Keywords Arene chromium complexes Arene ruthenium complexes Enantioselective total synthesis Planar-chiral metal complexes Natm-al products... 

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]. 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

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... 

Another example of an unusual reaction occurring in the gas phase is ammonia in a chromium complex ion being substituted by arenes such as benzene (4). It is important to note the uncommon oxidation state of the chromium. 

Arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal, or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-1-naphthols, with the sterically more demanding substituent of the alkyne (Rl Figure 2.24) ortho to the hydroxy group. Acceptor-substituted alkynes can also be used in this reaction (Entry 4, Table 2.17) [331]. Donor-substituted alkynes can however lead to the formation of other products [191,192]. Also (diarylcarbene)pentacarbonyl chromium complexes can react with alkynes to yield phenols [332]. 

Unlike the ferrocenes, the arene complexes of chromium, in particular the arenechromi-umtricarbonyls, have seen much less use in asymmetric catalysis. This is beginning to change, however , and a number of synthetic transformations of arenechromiumtricar-bonyls owe their existence to the formation of planar chiral chromium complexes by... 

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 extensive organometallic chemistry of chromium, i.e. the hexacarbonyl and its derivatives, organochromium compounds without carbonyl ligands, cyanide and isocyanide complexes, alkene, allyl, diene, cyclopentadiene and arene derivatives, and complexes of a-donor carbon ligands, has been recorded in Chapters 26.1 and 26.2 of Volume 3 of Comprehensive Organometallic Chemistry .1 In the present section, chromium complexes... 

The carbonyl complexes listed in Table V are of two types tricarbonyl-chromium rj6-arene 77-complexes, and pentacarbonyltungsten <7-pyridine complexes, with both complex types having relatively low y. Nonlinearities increase on arene or pyridine 77-system lengthening, and on proceeding from acceptor to donor substituent on the tricarbonylchromium-coordinated arene ring. Relative magnitudes and trends thus mirror those observed with quadratic nonlinearities of these complexes (see Ref. 1)... 

Arene(tricarbonyl)chromium complexes, 19 Nickel boride, 197 to trans-alkenes Chromium(II) sulfate, 84 of anhydrides to lactones Tetrachlorotris[bis(l,4-diphenyl-phosphine)butane]diruthenium, 288 of aromatic rings Palladium catalysts, 230 Raney nickel, 265 Sodium borohydride-1,3-Dicyano-benzene, 279 of aryl halides to arenes Palladium on carbon, 230 of benzyl ethers to alcohols Palladium catalysts, 230 of carboxylic acids to aldehydes Vilsmeier reagent, 341 of epoxides to alcohols Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281 of esters to alcohols Sodium borohydride, 278 of imines and related compounds Arene(tricarbonyl)chromium complexes, 19... 

Substitution reactions at aromatic carbon (see also Reduction reactions, Ullmann ether coupling, specific reactions such as Nitration) Arene(tricarbonyl)chromium complexes, 19... 

Hydrogenation catalysts Arene(tricarbonyl)chromium complexes, 19... 

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