Chromium complexes acetates

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

The malic acid-derived auxiliary which gives good results in the ferrocene series also looks promising among the chromium complexes, and the six-membered acetal of 400 is much more easily hydrolysed than the tartrate-derived acetals of Scheme 164 . Lithiation and bromination of 400 gives, after hydrolysis of the acetal, the complex 401 in 90% ee, increasing to >99% after recrystallization (Scheme 165). 401 is an intermediate in a formal synthesis of (—)-steganone (Scheme 182, Section III.B.2.b). 

The reaction is catalyzed by a group VIII metal species, particularly that of rhodium or palladium. The initial metal species may be any variety of complexes (e.g., PdCl2 Pd acetate, etc.). A source of halide is necessary iodide is especially effective. The most convenient source is methyl iodide, since it is likely a reaction intermediate. In addition, an organic promoter must be included for catalytic activity. These promoters are generally tertiary phosphines or amines. Also, chromium complexes were found to have an important promotional effect. 

C H402, Acetic acid palladium complex, 26 208 tungsten complex, 26 224 C2H7PS, Phosphine sulfide, dimethyl-manganese complex, 26 162 C H,N2, 1,2-Ethanediamine chromium complex, resolution of, 26 24, 27... 

Dehalogenation. Barton et at. (1, 148) effected dehalogenation of steroidal /i-hydroxy halides with chromium(II) acetate and butancthiol as the proton donor in DMSO. The method is only useful with tertiary halides. A recent improvement that permits reduction of halides of all types uses the ethylenediamine complex of CrtCIOzh and the tetrahydropyranyl ethers of the /J-hydroxy halide. Catalytic amounts of the reducing agent can be used in "indirect electrolysis." The reaction is convenient for preparation of deoxynucleosides.1... 

Typical TLC data (silica gel, 6 1 hexanesiethyl acetate) include R - 0.61 (1-acetoxy-1,3-butadiene) 0.51, a red spot [tricarbonyl(cycloheptatriene)chromium] 0.45 a yellow spot (side product that often overlaps with the starting complex) and 0.31 a yellow spot (main intermediate chromium complex). 

Attempts to isolate pure cyanochromium(II) complexes from the deep red solutions containing [Cr(CN)6]4 ions formed when chromium(II) acetate is added to aqueous potassium cyanide in excess, and in other ways, have not been very successful. Now, through careful control of concentration and of pH, the known but poorly characterized green crystalline hexacyanochromates(II) M4[Cr(CN)6]-2H20 (where M = Na or K), the new dark green simple cyanide Cr(CN)2-2H20, and their anhydrous forms have been prepared pure (Scheme 1 and Table 2).10... 

Complexes of acetylacetone (acacH), benzoylacetone (bzacH) and dipivaloylmethane (dpmH) have been reported. The acetylacetonate [Cr(acac)2] has been prepared from chromium(II) acetate and acetylacetone.142,143 It can also be obtained by the addition of aqueous sodium acetylacetonate to an aqueous solution of chromium(II) chloride, but in any preparation the yellow solid must be filtered off and dried as rapidly as possible, otherwise the chromium(III) compound is obtained. Its magnetic moment is 4.99 BM at room temperature consistent with a high-spin d4 configuration. 142The powerful reducing ability of [Cr(acac)2] has been used to prepare iron(II) and chromium(II) complexes80 of porphyrins and related ligands. 

The addition of en or 1,3-diaminopropane to a mixture of chromium(III) acetate hexahy-drate and diacetyl under reflux, followed by addition of NaC104, has given the complexes [Cr(diacen)(H20)2]C104 and [Cr(diacpn)(H20)2]C104, which contain the tetradentate Schiff bases (234).1119 The thiocyanates [CrL(H20)(NCS)](NCS)2 were also isolated. From their IR spectra, and because the coordinated Schiff base would not react with another molecule of diacetyl to form a macrocycle, they have been assigned cis configurations. 

III. Chromium VI Reagents / 227 Chromium Vi-acetic acid / 227 Chromium Vl-acetone / 228 Chromium trioxide-pyridine complex / 229 Chromic acid-dimethylformamide / 231 Two-phase oxidations / 232... 

Stereoselective benzylic alkylation.6 Benzylic acetates of arenechromium tricarbonyls react with R3A1 or (QHs Zn-TiCU to form exo-alkyl chromium complexes. 

Sodium hydrogen telluride, 282 Tin(II) chloride, 298 By other methods Acetic-formic anhydride, 1 Arene(tricarbonyl)chromium complexes, 19... 

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). 

Stereoselective a-alkylation of ketones. This reaction can be effected by reaction of silyl enol ethers with benzyl acetates complexed with Cr(CO), in the presence of ZnCh (I equiv.). This methodology is particularly useful because only the adduct anti to the metal is obtained. Use of an optically active chromium complex such as 1 results in 100% stereoselective alkylation. 

Direct nucleophilic addition of potassium enolates derived from bis(trimethylsilyl)ketene acetals to aromatic chromium-complexed aromatic ethers affords meta substituted products (Scheme 124). A very high degree of asymmetric induction is obtained upon reaction of chiral arene chromium tricarbonyl complexes. For example, alkylation of complex (80)gave (81)afterdecomplexation(Scheme 125). ... 

The reaction between chromium(II) acetate and bipyridyl has been extensively studied. If a metal-to-ligand ratio of 2 3 is employed, disproportionation occurs and Cr(bipy)3 is the product (359). A 1 2 ratio of reactants affords the neutral monomeric complex Cr(bipy)2(OAc)2... 

Methanol, platinum complex, 26 135 tungsten complex, 26 45 Methyl, iridium complex, 26 118 manganese complex, 26 156 osmium complex, 27 206 rhenium complexes, 26 107 Methyl acetate, iron complex, 27 184 osmium complex, 27 204 Methyl benzoate, chromium complex, 26 32 Methylene, osmium complex, 27 206 Molybdate(l -), (acetato)pentacarbonyl-, M.-nitrido-bis(triphenylphosphorus) (I-h), 27 297... 

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