Chromium, -, 8:6 complex with

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

The chiral diol 17 derived from tartaric acid is exploited in the titanium-catalyzed asymmetric pinacol coupling in the presence of Zn and MesSiCl to give the corresponding diol in 11-71 ee % [44], The chiral salen ligands 18-20 are used in the titanium-catalyzed enantioselective coupling reaction, which achieves the higher selectivity [45-47]. The chromium complex with TBOxH (21) efficiently catalyzes the asymmetric coupling reaction of both aromatic and aliphatic aldehydes [48]. 

Fluoro- and chloroarenes activated by complexation with chromium tricarbonyl react readily with primary and secondary alcohols under basic conditions in the presence of a quaternary ammonium catalyst [46, 47]. Reaction of the chromium complexes with acetone oxime produces O-aryloximes in good yields (55-80%) [48]. 

Table 6 compiles the electrode potentials of the chromium complexes with phenantroline, bipyridine and terpyridine ligands, respectively25-27. 

Figure 16 shows the octahedral geometry of two chromium complexes with N204 coordination. The first with the Schiff base [iV,./V-ethylene-bis(salicylideneimine)] + two water molecules, [Cr(salen) (H202)2]+ 29 the second with two molecules of [2,2 -bipyridine-6-phenolate], [Cr 6-(2-C6H40)bipy 2]+.3°... 

The thermal reaction of vinyl- or aryl(alkoxy)carbene chromium complexes with alkynes can give high yields of 4-alkoxyphenols (Figure 2.24). This elegant synthetic procedure was reported for the first time by Dotz in 1975 [326] and has proven robust and of broad scope. 

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

The reaction of alkoxy(alkyl)carbene chromium complexes with alkynes has been reported to give modest yields of cyclopentenones [368] and a few examples of intramolecular carbene C-H insertions of Fischer-type carbene complexes, leading to five-membered heterocycles, have been reported [369,370] (Table 2.22). 

Fig. 19. Spectra of two chromium complexes with double octahedral coordination sphere (type 11) and of a complex with triple octahedral coordination sphere.

Fig. 1. Simulated room-temperature CW-EPR spectrum for an oxo-chromium complex with giso = 1.9803, Craiso= 15.9 x 10-4 cm-1 and H iso,i = 0.84 x 10-4 cm-1 and H<2iso,2 = 0.87 x 10-4 cm-1. (A) Full-field scan and (B) central detail of spectrum. The microwave frequency was set to 9.48 GHz.

Relatively few chromium complexes with phthalocyanine ligands have been reported and, ir view of the chemical relationship between the few known compounds, all of them, irrespectivt of the metal oxidation state, are dealt with in this section. In the following discussion Pi represents the dianionic phthalocyanine ligand. 

Complexation of indoles with chromium hexacarbonyl, which reduces the electron density of the heterocyclic system, promotes nucleophilic attack at the 7-position and, to a lesser extent, also at the 4-position of the indole ring and provides a viable synthetic route to 7-formyl-l-methylindole (78CC1076). Curiously, although the benzenoid ring is rendered susceptible to nucleophilic attack, the reaction of the chromium complex with butyllithium results in abstraction of the proton from the 2-position. However, if this position is... 

Bis( j5-cyclopentadienyl)chromium complexes with bulky alky substituents, 5, 330 chromocenes with substituted cyclopentadienyl rings,... 

Unsymmetrical 1 2 complex dyes containing only one sulfo group 1970 1 2 chromium complexes with two sulfo groups... 

Chromium Complexes with One Sulfonic Acid Group. By definition, l 2-metal complexes with one sulfonic acid group must be unsymmetrical, because only one of the two azo dye ligands bears a sulfonic acid group. In 1962 the first metal-... 

Chromium Complexes with Two or More Sulfonic Acid Groups. The monosul-fonated 1 2 chromium complex dyes were followed by those with two or more sulfonic acid groups. These more highly sulfonated complex dyes had been known since 1932 [24], but were introduced only in 1970 after detailed studies [25], An example is C.I. Acid Blue 193, 15707 [ 75214-58-3] (15). 

In practice, and despite much study of Cr(III) systems, there is only a little definitive information about any of these factors. In this report we discuss some current studies of chromium complexes with sterically constrained ligands. These studies are providing information mostly about the fourth factor listed above. 

In humans and animals, chromium(III) is an essential nutrient that plays a role in glucose, fat, and protein metabolism by potentiating action of insulin. Chromium picolinate, a trivalent form of chromium complexed with picolinic acid, is used as a dietary supplement, because it is claimed to speed metabolism... 

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... 

Chromium complexes with crown ethers as ligands 90MI43. Cryptands as lipophilic cell ligands 86PAC1503. 

The aromatic nucleqrhilic substitution on tricarbonyl(halogenoarene)chromium complexes with N-t-butyloxycarbonylhydroxylamine provides a convenient route to 0-arylhydroxylamine derivatives (Scheme 14). ... 

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