Synthesis applications with chromium complexes

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... 

Substitution reactions also proceed well with cationic t/ -cycloheptadienyliron complexes such as 105 [81] and related chromium complexes [82], and have found applications in natural product synthesis (Scheme 2.38). 

The first carbene compound to be well characterized was prepared in 1966 and was one of many Fischer-Type Carbene Complexes to be reported (see equation 7). Fischer carbenes are characterized by heteroatom substituents at the carbene carbon, stabilization by a low-valent metal center, and a partial positive charge at the carbene carbon. In contrast, Schrock-Type Carbene Complexes, or alkylidenes," that have alkyl substituents, are found on metal centers in higher oxidation states, and are nucleophihc at carbon. Many Fischer carbenes are known for chromium, whereas chromium alkylidenes are much less common. Monohalocarbenes of chromium, for example, (OC)5Cr=C(F)NEt2, have also been extensively investigated." Two carbene reactions of note for their application to organic synthesis are the cycloaddition of alkenes with carbene complexes and the reaction of aromatic carbenes with aUcynes to yield complexed naphthols (the Dotz reaction ). ... 

The cross coupling reactions of aryl halides or aryl triflates with arylmetals catalysed by palladium(O) has found extensive use in the synthesis of unsymmetrical biaryls (ref. 27). A recent application of this coupling approach in the presence of chiral planar chromium complexes (ref. 25) has proved fruitful in the synthesis of chiral biaryls. Furthermore, Cr(CO)3-complexed biaryls can be isomerised under thermodynamic control, providing access to both atropisomers. Heating a solution of the complexed biaryl 37 in xylene under reflux resulted in a 1 99 ratio favouring the isomerised biaryl chromium complex 38 (ref. 28)... 

The finding (ref. 126) that methoxyarenes complexed with chromium hexacarbonyl, thus forming arene chromium tricarbonyls, then undergo 3- rather than the anticipated 2-/4- substitution with certain nucleophiles has a potential application in the synthesis of cardanols and cardols. The method has a number of variants (refs. 127, 128) and is shown in the following scheme. Salicylates formed two isomers. 

Another example of the application of the chromium carbene complex for the synthesis of benzannulated compounds was described by Herndon and Wang." The coupling of substituted carbene chromium complex 27 with conjugated enediyne 28 results in the formation of intermediate enyne-ketene 29, which undergoes the Moore cyclization to produce the intermediate chromium-complexed diradical 30. The... 

Another interesting application of arene group activation by chromium complex formation is found in the facile metallation of coordinated arene moieties with lithium, the lithiated complex being activated towards subsequent substitutions by electrophilic reagents. This principle has been applied to the synthesis of anthracyclone analogues [5e]. 

Chiral Lewis acid-catalyzed HDA reactions have foimd application in the asymmetric synthesis of THP-containing natural products. Chiral chromium complexes, especially the adamantyl-Cr(III) complexes discovered by Jacobsen et al., have been applied to the use of unactivated aldehyde dienophiles with various diene partners [111]. Paterson and coworkers employed this variation in the synthesis of... 

Heavy metals are widely used as catalysts in the manufacture of anthraquinonoid dyes. Mercury is used when sulphonating anthraquinones and copper when reacting arylamines with bromoanthraquinones. Much effort has been devoted to minimising the trace metal content of such colorants and in effluents from dyemaking plants. Metal salts are used as reactants in dye synthesis, particularly in the ranges of premetallised acid, direct or reactive dyes, which usually contain copper, chromium, nickel or cobalt. These structures are described in detail in Chapter 5, where the implications in terms of environmental problems are also discussed. Certain basic dyes and stabilised azoic diazo components (Fast Salts) are marketed in the form of tetrachlorozincate complex salts. The environmental impact of the heavy metal salts used in dye application processes is dealt with in Volume 2. 

The addition-protonation procedure maintains the arene-chromium bond and allows further application of the activating effect of the metal. In an approach to the synthesis of anthraquinone antibiotics, the dihydronaphthalene complex (79) was allowed to react with a cyanohydrin acetal anion and then quenched with acid.129 The resulting tetralin complex (80) could be metallated effectively and carried on to a key intermediate (81) in anthraquinone construction (equation 54)... 

Three types of reaction systems have been designed and applied for the enantioposition-selective asymmetric cross-coupling reactions so far. First example is asymmetric induction of planar chirality on chromium-arene complexes [7,8]. T vo chloro-suhstituents in a tricarhonyl("n6-o-dichlorobenzene)chromium are prochiral with respect to the planar chirality of the 7t-arene-metal moiety, thus an enantioposition-selective substitution at one of the two chloro substituents takes place to give a planar chiral monosubstitution product with a minor amount of the disubstitution product. A similar methodology of monosuhstitution can be applicable to the synthesis of axially chiral biaryl molecules from an achiral ditriflate in which the two tri-fluoromethanesulfonyloxy groups are enantiotopic [9-11]. The last example is intramolecular alkylation of alkenyl triflate with one of the enantiotopic alkylboranes, which leads to a chiral cyclic system [12], The structures of the three representative substrates are illustrated in Figure 8F.1. 

FT-ICR, see Fourier-transform ion cyclotron resonance Fullerene[60], germanium-germanium addition, 10, 748 Fullerenes with cobalt, 7, 51 on cobalt Cp rings, 7, 73 inside metallodendrimers, 12, 401 microwave applications, 1, 334 Pd rc-complexes, 8, 348 Ru—Os complexes, 6, 830 with tungsten carbonyls, 5, 687 )2-Fullerenes, with platinum, 8, 634 Fulvalene actinide complex, synthesis, 4, 232 Fulvalene chromium carbonyls, synthesis and characteristics, 5, 264... 

Alkynes too have found application in the synthesis of oxepanes. The A/B ring system (45) of ciguatoxin has been formed in a stereoselective manner by cyclisation of the alkyne cobalt complex (44) with boron trifluoride followed by reductive removal of the chromium with... 

Arenes form ri -complexes with a number of transition metals (e.g. Cr, Mo, W, Fe). Complexes of chromium have found widespread application because of their ease of synthesis, stability, easy removal of the ligands and usefulness in synthesis. 

Abstract Planar-chiral ri -arene-Cr(CO)3 complexes represent highly valuable buUdlng blocks for the dlastereo- and enantloselectlve synthesis of complex natural products and related bloactlve compounds. Highly original and competitive overall syntheses of various classes of natural products, such as sesquiterpenes, diterpenes, alkaloids and compounds with axial chirality, have been developed. In certain cases, the whole strategy is based on arene chromium chemistry and the various chemical and stereochemical effects of the metal unit are exploited In several subsequent transformations. Cationic Cp-ruthenium complexes allow arylether formation by Sj Ar reactions and have found application in the synthesis of glyco-peptide antibiotics. 

---